U.S. patent application number 15/367017 was filed with the patent office on 2017-08-24 for heater for liquefied petroleum gas storage tank.
The applicant listed for this patent is Algas-SDI International LLC. Invention is credited to Jeffrey R. Ervin, Michael J. Kirby, George M. Zimmer.
Application Number | 20170241639 15/367017 |
Document ID | / |
Family ID | 44627524 |
Filed Date | 2017-08-24 |
United States Patent
Application |
20170241639 |
Kind Code |
A1 |
Zimmer; George M. ; et
al. |
August 24, 2017 |
HEATER FOR LIQUEFIED PETROLEUM GAS STORAGE TANK
Abstract
A catalytic tank heater includes a catalytic heating element
supported on an LPG tank by a support structure that holds the
element in a position facing the tank. Vapor from the tank is
provided as fuel to the heating element, and is regulated to
increase heat output as tank pressure drops. The heating element is
internally separated into a pilot heater and a main heater, with
respective separate fuel inlets. The pilot heater remains in
continual operation, but the main heater is operated only while
tank pressure is below a threshold. Operation of the pilot heater
keeps a portion of the catalyst hot, so that, when tank pressure
drops below the threshold, and fuel is supplied to the main heater,
catalytic combustion quickly expands from the area surrounding the
pilot heater to the remainder of the catalyst.
Inventors: |
Zimmer; George M.; (Kent,
WA) ; Ervin; Jeffrey R.; (Bellevue, WA) ;
Kirby; Michael J.; (Kent, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Algas-SDI International LLC |
Seattle |
WA |
US |
|
|
Family ID: |
44627524 |
Appl. No.: |
15/367017 |
Filed: |
December 1, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
14604547 |
Jan 23, 2015 |
9523498 |
|
|
15367017 |
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|
13162363 |
Jun 16, 2011 |
8951041 |
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14604547 |
|
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61355463 |
Jun 16, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F17C 2223/0153 20130101;
F17C 2225/0123 20130101; F17C 7/04 20130101; F17C 2201/035
20130101; F17C 2223/0161 20130101; F23C 13/02 20130101; F17C
2201/054 20130101; F17C 2227/0107 20130101; F17C 2227/0306
20130101; F17C 2227/0332 20130101; F17C 2221/035 20130101; F17C
2205/018 20130101; F17C 13/025 20130101; F17C 2221/033 20130101;
F17C 2201/0109 20130101; F17C 2227/0386 20130101 |
International
Class: |
F23C 13/02 20060101
F23C013/02; F17C 7/04 20060101 F17C007/04 |
Claims
1. A device, comprising: a housing having a face and a back panel,
and being defined around a perimeter by sidewalls, the back panel
and sidewalls being substantially gas-tight, and the face being
substantially open; a catalyst layer substantially coextensive with
the face of the housing; an open space between the catalyst layer
and the back panel defining a plenum chamber; a main fuel inlet
traversing the back panel and configured to deliver fuel to the
plenum chamber; a pilot heater positioned entirely within the
perimeter of the housing, defined and enclosed by pilot sidewalls
extending from the back panel toward the face at least a depth of
the plenum chamber, the back panel and the pilot sidewalls being
substantially gas-tight, and including a portion of the plenum
chamber as a pilot plenum chamber, and configured to deliver fuel
to a portion of the catalyst layer positioned in front of the pilot
heater; and a pilot fuel inlet traversing the back panel and
configured to deliver fuel to the pilot plenum chamber.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/604,547, filed Jan. 23, 2015, which is a
continuation of U.S. patent application Ser. No. 13/162,363, filed
Jun. 16, 2011, which claims the benefit of U.S. Provisional Patent
Application No. 61/355,463, filed Jun. 16, 2010, which applications
are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
Technical Field
[0002] Embodiments described in the present disclosure are directed
generally to catalytic heaters and heaters for warming storage
tanks containing fluids that are normally gaseous at normal
atmospheric pressure and typical ambient temperatures, and in
particular to catalytic heaters configured to be coupled to such
storage tanks, and including pilot heaters to enable rapid
activation of the heaters.
Description of the Related Art
[0003] A number of fluids that are normally found in gaseous form
are commonly stored and transported under pressure as liquids,
including, for example, methane, butane, propane, butadiene,
propylene, and anhydrous ammonia. Additionally, fuel gasses
comprising one or more constituent gasses are also stored and
transported under pressure as liquids, including, e.g., liquefied
petroleum gas (LPG), liquefied natural gas (LNG), and substitute
natural gas (SNG). Of these, LPG is perhaps the most commonly used.
Accordingly, the discussion that follows, and the embodiments
described, refer specifically to LPG. Nevertheless, it will be
understood that the principles disclosed with reference to
embodiments for use with LPG tanks can be similarly applied to
tanks in which other liquefied gases are stored or transported, and
are within the scope of the invention.
[0004] LPG is widely used for heating, cooking, agricultural
applications, and air conditioning, especially in locations that do
not have natural gas hookups available. In some remote locations,
LPG is even used to power generators for electricity. LPG is
typically held in pressurized tanks that are located outdoors and
above ground. Under one atmosphere of pressure, the saturation
temperature of LPG, i.e., the temperature at which it boils, is
around -40.degree. C. As pressure increases, so too does the
saturation temperature. LPG is held in a liquid state by gas
pressure inside the tank. As gas vapor is drawn off from the tank
for use, the pressure in the tank drops, allowing more of the
liquefied gas to boil to vapor, which increases or maintains
pressure in the tank.
[0005] As the gas boils, the phase change from liquid to gas draws
thermal energy from the remaining liquid, which tends to reduce the
temperature of the LPG in the tank. If LPG temperature drops, the
boiling slows or stops, as the LPG temperature approaches the
saturation temperature. Thus, boiling LPG tends to increase
pressure and saturation temperature, while at the same time tending
to decrease the actual temperature of the LPG in the tank, until an
equilibrium temperature is reached, at which the saturation
temperature is equal to the current temperature of the LPG.
Provided the energy expended to vaporize the gas does not exceed
the thermal energy absorbed by the tank externally, from, for
example, sunlight and the surrounding air, the LPG will continue to
boil as vapor is drawn off, until the tank is empty. On the other
hand, if more energy is expended to vaporize the gas than is
replaced by external sources, the temperature in the tank will drop
toward the equilibrium temperature, resulting in less energetic
boiling, and a drop in tank pressure. If tank pressure drops too
low, it can interfere with the operation of appliances and
equipment that draw gas for use, such as furnaces, ovens, ranges,
etc.
[0006] For purposes of the following disclosure, the maximum
continuous rate at which gas can flow from a supply tank using only
ambient energy to vaporize the LPG, without causing the tank
pressure to drop below an acceptable level, will be referred to as
the maximum unassisted flow rate. It will be recognized that this
rate will vary according to the ambient temperature near the
tank.
[0007] Low tank pressure is a particular concern in regions where
ambient temperature can drop to very low levels, such as during the
winter at high latitudes, or at very high altitudes. For example,
when ambient temperature drops very low, the heat energy available
to warm an LPG storage tank is reduced, while at the same time, the
cold temperature prompts an increased draw of gas to fuel furnaces
to warm homes and other buildings. As gas pressure drops below the
regulated pressure of the gas line, flames in furnaces, water
heaters, and other gas consuming appliances reduce in size,
producing less heat and prompting users to open gas valves further,
which only accelerates the pressure drop. Eventually, tank
temperature can drop below the boiling point of unpressurized gas,
at which point, no gas will flow. It can be seen that, as ambient
temperature drops, the potential for unacceptable loss of pressure
increases, as does the potential demand for gas, for heating.
[0008] To prevent such a pressure reduction, there are a number of
measures that can be taken, which fall into three general
categories, each with its own advantages and disadvantages.
[0009] In the first category, LPG is drawn from the bottom of a
tank as a liquid, and passed through a separate vaporizer in the
supply line, to meet demand. The volume of liquid flow has
relatively little effect on tank--or system--pressure, because the
liquid in the tank boils only to the extent necessary to replace
the volume of fluid drawn from the tank. Thus, the limiting factor
is more frequently the capacity of the vaporizer. In some limited
situations, where, for example, the ambient temperature is very
low, and the draw by the load is very high, tank pressure can still
drop. In such cases, a vapor return line is frequently employed
from the outlet of the vaporizer to the tank to increase the tank
pressure.
[0010] There are a number of types of LPG vaporizers, including
direct gas-fired and electrically heated. Some electric vaporizers
with explosion-proof electrical connections can be mounted on or
near the storage tank. However, safety regulations in most
jurisdictions require that sources of combustion, such as an open
flame, or heat sources that exceed the auto-ignition temperature of
LPG, cannot be located in a same enclosure with an LPG storage
tank, or within some minimum distance. Thus, a gas fired vaporizer
must be positioned away from the storage tank, which adds cost and
complexity, and increases maintenance requirements. Nevertheless,
gas-fired vaporizers are more commonly used with large LPG storage
systems, because the heating cost is generally lower than with
electrically heated vaporizers. Additionally, gas-fired units can
be used in locations where electricity is unavailable. A
disadvantage of in-line vaporizers in general is that because they
draw liquid from the bottom of the tank, they are always in
operation, even when the maximum unassisted flow rate exceeds the
current vapor demand.
[0011] In a second system configuration, gas for normal use is
drawn from the top of the tank, but when pressure drops below a
threshold, liquid is drawn from the bottom and boiled to vapor in a
vaporizer and returned to the top of the tank to re-pressurize the
tank. On one hand, such systems have more complex control,
plumbing, vapor, and fluid circuits. On the other hand, these
systems employ the vaporizer only when tank pressure drops below
the threshold, so they tend to be more fuel efficient than in-line
vaporizer systems.
[0012] In a third configuration, a tank heater is activated to warm
the tank and its contents when tank temperature or pressure drops
below a threshold. One type of tank heater comprises an electric
element strapped to the tank. In another type, indirect heat is
used, in which a medium, such as water or steam, is heated at a
remote location, then piped to a heat exchanger in contact with the
tank walls. Indirect heat is advantageous in situations where waste
heat is available, such as where water is used to cool industrial
machinery, etc.
[0013] Generally, disadvantages of many of the systems available
are often related to the difficulty of providing heat in the close
vicinity of an LPG tank without creating a condition that would be
dangerous in the event of a tank leak or tank over-pressure. The
complexity of systems in which a heat source is remotely located
not only increases the cost, but also the likelihood of
malfunction. Additionally, vaporizers and heaters that employ
electric heating elements, or that are electrically controlled, are
impractical for use in applications where electrical power is not
available. In such cases, an electric generator is required to
provide the electricity, resulting in costly efficiency losses.
[0014] One problem associated with electric tank heaters, in
particular, is that the heating element is in direct contact with
the tank wall. Temperature differentials between the element and
the tank can promote water condensation, which can be trapped
between the heating element and the surface of the tank, resulting
in deterioration of the paint and subsequent corrosion of the steel
tank wall.
[0015] Most jurisdictions have stringent regulations regarding the
use of combustion sources near LPG tanks and gas transmission
lines. These regulations dictate explosion-proof requirements for
electrical connections, minimum distances to open flames, etc. The
restrictions vary according to the size of a tank and proximity to
public areas.
BRIEF SUMMARY
[0016] According to an embodiment, a catalytic heater system
includes a catalytic heating element supported on an LPG storage
tank by a support structure that holds the element in a position
facing the tank. When a load draws sufficient vapor to cause the
tank to self refrigerate and lose pressure, the catalytic heating
element is operated to warm the tank and restore pressure. Vapor
from the tank is provided as fuel to the heating element, and can
be regulated to increase heat output as tank pressure drops.
[0017] According to an embodiment, the catalytic heating element is
internally separated into a pilot heater and a main heater, with
respective separate fuel inlets. In use, the pilot heater remains
in continual operation, but the main heater is operated only as
required. Operation of the pilot heater keeps a portion of the
catalyst hot, so that, when fuel is supplied to the main heater,
catalytic combustion quickly expands from the area surrounding the
pilot heater to the remainder of the catalyst in the main
heater.
[0018] According to an embodiment, a catalytic heating system is
provided, including a catalytic heating element separated into a
pilot heater and a main heater, with respective separate fuel
inlets. A pressure regulator controls fuel flow to the main heater,
and a shut-off valve controls fuel to both the pilot and main
heaters. A heat sensor positioned in or near the pilot heater
operates to hold the shut-off valve open. If the pilot heater stops
producing heat, the shut-off valve closes, terminating all fuel
flow to the heating element. Where this catalytic heating system is
employed to warm an LPG storage tank, a control terminal of the
pressure regulator is coupled to a direct tank pressure feedback
line, and configured to control fuel flow to the main heater in
inverse relation to the tank pressure. If tank pressure drops below
a threshold, the regulator permits fuel to flow to the main heater,
and as tank pressure drops further, the flow increases, to produce
more heat. One or more temperature sensors positioned on the tank
wall near the heating element detect reduced levels of liquid in
the tank, and signal a fuel interrupt to the main heater or to the
main and pilot heaters, according to the embodiment and specific
conditions.
[0019] According to an embodiment, a catalytic heating element is
coupled to a mounting structure configured to be coupled to a
cylindrical tank, and to support the heating element facing the
tank wall. The mounting structure includes a shroud that extends
around at least a portion of the heating element and that conforms,
on one side, to the contour of the cylindrical tank. The shroud can
be in the form of a cabinet that substantially encloses the heating
element against the tank wall, or can be an extension of a housing
of the heating element. The shroud can also be configured to
enclose heater controls as provided in other embodiments.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0020] FIG. 1 is a perspective view of an LPG storage system
according to an embodiment, including an LPG storage tank and a
tank heater system.
[0021] FIG. 2 is an end view of the system of FIG. 1.
[0022] FIG. 3 is a schematic diagram of a catalytic tank heater
control circuit according to an embodiment.
[0023] FIG. 4 is a diagrammatic plan view of a catalytic heater
according to an embodiment, showing configurations and positions of
various features as viewed from the back of the device.
[0024] FIG. 5 is a diagrammatic view of the heater of FIG. 4
showing configurations and positions of various features, the view
taken from a side of the device along lines 5-5 of FIG. 4.
[0025] FIG. 6 is a diagrammatic view of the catalytic heater of
FIG. 4 showing configurations and positions of various features,
the view taken from an end of the device along lines 6-6 of FIG.
4.
[0026] FIG. 7 is a schematic diagram of a catalytic tank heater
control circuit according to an embodiment.
[0027] FIGS. 8-10 are end view diagrams showing selected features
of catalytic tank heater systems according to respective
embodiments.
[0028] FIG. 11 is a schematic diagram of a circuit for controlling
a catalytic tank heater that includes multiple heater units,
according to an embodiment.
[0029] FIG. 12 is a perspective view of an LPG storage system
according to an embodiment, including an LPG storage tank and a
tank heater system.
[0030] FIG. 13 is a section end view of the LPG storage system of
FIG. 12.
[0031] FIG. 14 is a diagrammatic plan view of a catalytic heater
according to an embodiment, showing configurations and positions of
various features as viewed from the back of the device.
[0032] FIG. 15 is a diagrammatic view of the heater of FIG. 14
showing configurations and positions of various features, the view
taken from a side of the device along lines 15-15 of FIG. 14.
[0033] FIG. 16 is a schematic diagram of a catalytic tank heater
control circuit according to an embodiment.
[0034] FIG. 17 is a diagrammatic view of a catalytic heater
according to an embodiment, showing configurations and positions of
various features as viewed from the back of the device.
[0035] FIG. 18 is a diagrammatic view of the heater of FIG. 17
showing configurations and positions of various features, the view
taken from a side of the device along lines 18-18 of FIG. 17.
[0036] FIG. 19 is a schematic diagram of a heater control circuit
according to an embodiment.
[0037] FIG. 20 is a diagrammatic view of a catalytic heater
according to an embodiment, showing configurations and positions of
various features as viewed from an end of the device.
[0038] FIG. 21 is a detail of a tank heater system in a
diagrammatic end view according to an embodiment.
DETAILED DESCRIPTION
[0039] FIGS. 1 and 2 show an LPG storage system 100 according to an
embodiment, which includes an LPG tank 102 and a catalytic tank
heater system 104. The heater system 104 includes a catalytic
heater element 106, a heater control 118, a shroud 108, mounting
brackets 141, support frames 110, and straps 112. The support
frames 110 are coupled to the tank 102 by the straps 112. The
catalytic element 106 is coupled to the mounting brackets 141,
which extend between the support frames 110, and are coupled
thereto by first fasteners 111 via slot apertures 114 of the
support frames. The slot apertures 114 permit adjustment of the
position of the catalytic element 106 relative to the wall of the
tank 102, to provide for appropriate air circulation and transfer
of radiant heat from the element to the tank. The support frames
110 hold the catalytic element 106 spaced from and facing the wall
of the tank. Along a line where the catalytic element 106 lies
closest to the tank, the distance between the element and the tank
is preferably between one-quarter inch and eight inches, more
preferably between one-quarter inch and five inches, and most
preferably, about one-half inch. The shroud 108 is coupled to the
support frames 110 by second fasteners 113, and serves to shield
the catalytic element 106 from debris and unintentional contact,
and also to control air flow around the element. The shroud 108 is
shown in FIGS. 1 and 2 with a portion cutaway so that the catalytic
element is visible.
[0040] The heater control 118 is in fluid contact with the interior
of the tank via an input line 115, and controls operation of the
catalytic element 106 via output line 117. The catalytic element
106 is configured to operate by oxidation of vaporized gas from the
tank 102 in accordance with known principles of catalysis, as
regulated by the heater control 118.
[0041] The heater control 118 is configured to monitor the pressure
in the tank 102, to control operation of the catalytic heater
element 106 in response to variations in the tank pressure, in
order to maintain supply pressure above a selected threshold. The
pressure threshold is selected according to the requirements of the
particular application, and will generally be higher than an
anticipated maximum load pressure requirement, so that the tank
heater system can come on line and begin to restore the pressure
before it drops to a critical level.
[0042] Accordingly, when the tank pressure drops below the selected
threshold, the heater control 118 detects the drop and initiates
activation of the catalytic element 106. While the element 106 is
in operation, vaporized gas from the tank is fed to the catalytic
element 106, where it undergoes catalytic combustion, i.e.,
flameless oxidation of the fuel in the presence of a catalyst,
which is accompanied by the release of heat. The heat is
transmitted by radiation from the front face of the catalytic
element 106 to the wall of the LPG storage tank 102, where it is
absorbed and conducted to the liquefied gas inside, offsetting the
temperature and pressure drop caused by self-refrigeration as gas
is drawn from the tank.
[0043] FIG. 3 shows a schematic drawing of a heater control circuit
119 according to one embodiment, which can operate, for example as
the heater control 118 described with reference to FIG. 2. The
heater circuit 119 includes a catalytic heater element 106, and
first and second pressure regulator valves 163, 166. The catalytic
heater element 106 includes a gas supply port 136. Gas supply lines
176 extend from an outlet 173 of the tank 102 to the first pressure
regulator valve 163, from the first pressure regulator to the
second pressure regulator valve 166, and from there to the
catalytic heater element 106. A pressure feedback line 177 is
coupled to provide direct tank pressure to a control terminal 167
of the second pressure regulator valve 166. The first pressure
regulator valve 163 is configured to regulate pressure from the
tank to an appropriate supply pressure, such as, e.g., 5 psi, which
is provided to the second pressure regulator. Although not part of
the heater control circuit 119, a third pressure regulator valve
172 is shown, coupled to regulate pressure in a gas supply line 174
to supply the load of the system. In embodiments where the supply
pressures of the control circuit 119 and the load can be
substantially equal, the third pressure regulator 172 may not be
required. Instead, the first pressure regulator may be configured
to provide regulated gas to both the heater control circuit 119 and
the load, in which case, the supply line 174 will be coupled to
draw from the line 176 downstream from the first pressure regulator
163.
[0044] In operation, the tank 102 supplies vaporized gas to the
load as required, according to known processes, absorbing heat from
its environment to boil the liquefied gas as it is drawn. As long
as the gas pressure remains above a selected threshold, the
pressure at the control terminal 167 of the second regulator valve
166 is sufficient to hold the valve closed. However, in the event
the pressure drops below the threshold, the valve 166 opens and the
catalytic heater element 106 is activated to produce radiant heat
by catalytic oxidation of the gas. As pressure drops in the tank
102, the reduction of pressure, as transmitted by the feedback line
177 to the control terminal 167 of the second regulator valve 166,
opens the valve further, increasing the gas flow to the heater
element 106, and thereby increasing the amount of heat produced. As
heat from the catalytic heater element 106 is absorbed by the tank
102, it is conducted to the interior of the tank, and transferred
to the liquefied gas inside, warming the gas and increasing the
equilibrium temperature, resulting in an increased rate of boiling,
thereby increasing tank pressure. The increased tank pressure is
fed back, via the feedback line 177, to the second regulator valve
166, which reduces gas flow as the pressure rises, thereby
regulating the tank pressure.
[0045] There are a number of parameters associated with operation
of the second regulator valve 166 including the threshold at which
the valve opens as tank pressure drops, the threshold at which the
valve closes as tank pressure rises, and the change in aperture
size per unit of change in control pressure (.DELTA.a/.DELTA.p),
i.e., the degree to which the valve opens or closes in response to
a given change in pressure at the control terminal 167.
Additionally, the .DELTA.a/.DELTA.p may in some cases be
non-linear, so that, for example, at a relatively high level of
tank pressure, a change of one psi at the control terminal 167 may
produce one change in aperture, while at a lower tank pressure, a
one psi change may produce a larger or smaller change in aperture.
The values may also be selected to include hysteresis, so that
drops in pressure produce one value of .DELTA.a/.DELTA.p, while
rises in pressure produce a different value. Values for such
parameters can be selected according to the particular
application.
[0046] For example, in an application where the load requirements
and the ambient temperature are such that the rate of draw by the
load normally exceeds the maximum unassisted flow rate by a small
amount, the tank heater system, if configured with typical
parameter settings, will turn on as the tank pressure drops,
warming the tank and bringing the pressure up to an acceptable
level, at which point the system will shut off, whereupon the tank
pressure will immediately begin to drop again, until the heater
system is again required to turn on, to repeat the cycle. To avoid
the continual cycling of the system, and improve efficiency,
parameters of the second regulator valve 166 can be selected so
that the catalytic heater element is always in operation, but at a
lower average output. This might involve reducing the
.DELTA.a/.DELTA.p at pressure levels close to the thresholds, but
increasing the .DELTA.a/.DELTA.p at lower tank pressures. In this
way, the heater output initially increases by very small amounts as
the tank pressure drops below the turn-on threshold, then increases
by larger amounts if the tank pressure drops significantly below
the threshold. As a result, the average tank pressure is lowered
slightly, preferably to a value below the turn-off threshold.
However, the more continual operation avoids constant repetition of
the relatively less efficient warm up period during which the
catalytic heating element is warmed to its light-off
temperature.
[0047] For most applications, it is preferable that the turn-on
threshold be set to a pressure corresponding to an equilibrium
temperature that is greater than 32.degree.. This will prevent the
formation of ice on the outside of the tank, which might otherwise
interfere with proper and efficient operation of the heater.
[0048] Also shown in FIG. 3 is an optional alternate fuel source
182, coupled to the first regulator valve 163 via alternate gas
supply line 176b, shown in dotted lines. In the case where a
storage tank similar to the tank 102 of FIG. 3 is used to store
liquefied gas that is not flammable, or is otherwise not
appropriate for use in a catalytic heater system, such as, e.g.,
anhydrous ammonia, vapor from the storage tank cannot be used to
operate the catalytic heater 106. In such a case, the feedback line
177 is coupled directly to the outlet 173 of the tank 102, and the
alternate supply line 176b replaces the portion 176a of the supply
line 176. The heater control circuit 119 operates substantially as
described above to control the catalytic heater 106 to warm the
tank 102, but draws fuel from the alternate fuel source 182.
[0049] Additional heater control circuits are described later
according to respective embodiments. While they are not shown as
having optional alternate fuel sources, it will be recognized that
an alternate fuel source can be provided for such control circuits
as necessary, and can be configured substantially as shown with
reference to FIG. 3.
[0050] Turning now to FIGS. 4-6, a catalytic heater element 106 is
shown, according to one embodiment. FIG. 4 shows the element in a
bottom plan view showing selected features as viewed from the back,
with the back panel and additional details omitted to better show
the arrangement of the selected features. FIG. 5 is a sectional
view of the catalytic heater element 106 of FIG. 4, taken along
lines 5-5, and FIG. 6 is a sectional view of a portion of the
catalytic heater element of FIG. 4, taken along lines 6-6. The
heater element 106 comprises a housing 120 that includes a back
panel 122, sides 124 and a front grille 134. The interior of the
heater element 106 is divided horizontally (as viewed in FIG. 5)
into a plenum chamber 128, a gas-permeable diffusion and insulation
layer 130, and a catalyst layer 132. The diffusion/insulation and
catalyst layers 130, 132 are supported and separated from the back
panel 122 by an internal grid or perforated panel, creating a gas
plenum chamber 128, such as are well known in the art. A fuel
supply port 136 is positioned to provide fuel to the plenum chamber
128. The sides 124 and back panel 122 of the housing 120 are
substantially gas tight, so that gas flowing into the plenum
chamber 128 from the fuel supply port 136 flows into the plenum
chamber 128 and rises through the diffusion/insulation layer 130
and the catalyst layer 132.
[0051] Mounting brackets 141 are coupled to the back panel 122 of
the housing 120, and, in the embodiment shown, extend the length of
the housing, although most of the central portions are cut away so
as not to obscure other details of the drawings. Tabs 143 extend
from the mounting brackets toward the front of the housing 120, and
provide means for mounting the heater element 106 to additional
support structure. Where the catalytic element 106 is employed in a
tank heater system like that described with reference to FIGS. 1
and 2, apertures can be provided in the tabs 143, through which the
fasteners 111 pass to couple the element to the mounting frames
110. The mounting brackets 141 can be coupled to the housing 120 by
any appropriate means, such as, e.g., screws, rivets, or adhesive.
Additionally, the shape and form shown are merely exemplary.
Mounting brackets can be attached to extend from the top to the
bottom to the housing, as viewed in FIG. 4, rather than side to
side, or can be attached only to the sidewalls 124, rather than
across some portion of the back panel 122. Furthermore, the
mounting brackets can be omitted entirely and other appropriate
means for mounting the heater element 106 used, as required for the
particular application.
[0052] The catalytic heater element 106 is divided into a main
heater 139 and a pilot heater 140 by sidewalls 142, coupled to the
back panel 122 in a substantially gas-tight fashion. The pilot
heater 140 includes a pilot supply port 144 and a thermocouple 146.
In FIGS. 5 and 6, the sidewalls 142 are shown extending from the
back panel through the plenum chamber 128 and the
diffusion/insulation layer 130 to the back of the catalytic layer
132, defining a separate pilot plenum chamber 129. However,
according to other embodiments, the sidewalls 142 can extend only
as far as the back of the diffusion/insulation layer 130, or as far
as the front of the catalytic layer 132. The pilot supply port 144
includes an orifice 145 which limits the volume of fuel that can
enter the pilot heater 140. The thermocouple 146 is positioned to
sense the temperature of the catalyst layer 132 within the
perimeter of the pilot heater 140.
[0053] To initiate combustion, the temperature of the catalyst must
be raised above the activation temperature, i.e., the temperature
at which catalysis of the particular fuel and catalyst combination
is self-sustaining. In the case of petroleum gas, the reaction
temperature is about 250.degree.-400.degree. F. (about
120.degree.-200.degree. C.), depending on factors that include the
formulation of the gas and the catalyst employed. In the embodiment
of FIGS. 4-6, an electric heating element 148 is embedded in the
catalyst layer 132, which can be used to heat the catalyst and
initiate combustion. Portions of the electric heater element 148
extend across the pilot heater 140 via slots 141 in the sidewalls
142 of the pilot element 140, as shown in FIG. 6.
[0054] For initial operation, an electrical power source 152 is
coupled to terminals 150 of the heating element 148, which heats to
a temperature above the light-off temperature of the fuel supplied
to the element 106. As the temperature of the catalyst in the
catalyst layer 132 rises, the thermocouple 146 begins to produce a
small electric current. When the temperature reaches a selected
threshold, the heater control 154 begins to supply fuel at least to
the pilot heater 140, and catalytic combustion is thereby initiated
in the pilot heater. The power to the electric element 148 is then
removed. The fuel supplied to the pilot heater 140 via the pilot
supply port 144 is controlled by the heater control 154 to continue
flowing as long as the current from the thermocouple 146 is greater
than a selected value. Thus, once the pilot is initially activated,
absent a system malfunction or complete exhaustion of the available
fuel, the pilot heater will continue to operate perpetually.
[0055] Once the pilot heater 140 is initially activated, any time
thereafter that the main heater 139 is operated, combustion will be
initiated by heat from the pilot heater, as described below. Thus,
there is generally no requirement for a permanent connection of the
system to an electric power source for operation of the electric
heating element 148. Instead, electric power can be provided via a
temporary connection or source. In a preferred embodiment, the
catalyst layer 132 extends unbroken across the entire housing 120,
including the pilot heater 140. During pilot operation, fuel that
enters via the pilot supply port 144 is constrained by the
sidewalls 142 to the pilot plenum chamber 129. As fuel rises
through the catalyst layer 132, it dissipates beyond the perimeter
of the pilot heater 140 to a small degree, but is largely
constrained to that portion of the heating element, where it reacts
with the catalyst layer to oxidize, and release heat, thereby
maintaining that part of the catalyst layer at a temperature well
above the reaction temperature of the fuel.
[0056] According to an embodiment, the pilot heater 140 consumes
less than about 20% of the fuel consumed by the heater element 106
when the heater element is operating at full power. According to
another embodiment, the pilot heater 140 consumes less than about
15% of the fuel consumed by the heater element 106 when the heater
element is operating at full power. According to a further
embodiment, the pilot heater consumes about 10% or less than of the
fuel consumed by the heater element 106 when the heater element is
operating at full power.
[0057] When the heater control 154 initiates operation of the main
heater 139, fuel is supplied to the fuel supply port 136, from
which it flows into the plenum chamber 128, and rises through the
diffusion/insulation layer 130 to the catalyst layer 132. In the
area immediately surrounding the pilot heater 140, the catalyst
layer 132 is already at or above the activation temperature, so
fuel immediately begins catalytic combustion, releasing additional
heat and quickly bringing the remainder of the catalyst layer
beyond the activation temperature. Thereafter, the heat produced by
the main heater 139 is controlled by regulation of the fuel to the
fuel supply port 136. When heat is no longer required, the supply
to the fuel supply port 136 is shut off, after which the main
heater 139 shuts down, leaving only the pilot heater 140 in
operation.
[0058] In the embodiment of FIGS. 4-6, the electric element 148
extends across the entire housing 120. Thus, while the pilot heater
140 is in operation, the electric element 148 is kept hot in the
immediate area of the pilot heater. Heat from the pilot heater 140
is transmitted by conduction in the electrical element 148 to the
area surrounding the pilot heater, so that portions of the catalyst
layer 132 along the paths of the electric element 148 are
continually maintained above the light-off temperature. When fuel
is supplied to the main heater 139, those heated portions of the
catalyst layer 132 immediately begin catalytic combustion, which
accelerates activation of the remainder of the catalyst layer.
[0059] If the requirement for heat from the catalytic element 106
is seasonal, the pilot heater can be shut down once the likely need
has passed, in order to conserve the small amount of fuel consumed
by the pilot heater.
[0060] In the embodiment of FIGS. 4-6, the electric element 148 is
shown as comprising separate electric element sections 148a and
148b, with respective terminals 150a and 150b. This arrangement is
not essential, but provides some advantages. For example, each
section can be configured to produce a requisite level of heat when
connected to a 110-120 volt AC power supply, which is standard in
many parts of the world, including the U.S. In that case, the
sections 148a and 148b can be connected in parallel to produce the
necessary heat. On the other hand, where the same system is to be
used in a location where the available power is at a 220-240 volt
level, which is also very common, the sections can be coupled in
series, so that each drops half the available voltage, thereby
producing the same heat output. Alternatively, one of the sections
can be configured to operate from a standard power supply, while
the other is configured to operate at another power level, such as,
e.g., 12 volts. In this way, where municipal power is not
available, a single section can be powered by a portable source,
such as a car battery, to initiate combustion. Thereafter, as
previously discussed, the pilot heater 140 will continue to operate
for normal use.
[0061] In some embodiments, heat conductors, such as, for example,
steel or aluminum rods, are provided, embedded in the catalyst
layer and extending through the pilot heater and into the main
heater, substantially as shown with reference to the electric
element 148. The heat conductors conduct heat from the pilot heater
to the catalytic material of the main heater, maintaining a portion
of the catalytic material above the light-off temperature, to
quickly initiate catalytic combustion when the main heater is
activated. Heat conductors are particularly useful in embodiments
that do not include an electric heating element like the element
148 described above, which otherwise serves a similar purpose.
[0062] Turning now to FIG. 7, a schematic drawing of a tank heater
system 160 is shown, according to an embodiment. The system 160
includes a catalytic heater element 106, substantially as described
with reference to FIGS. 4-6, and a heater control circuit 161 that
includes a number of components previously described with reference
to the heater control 119 of FIG. 3, which components are provided
with identical reference numbers. In addition to previously
described components, the heater control circuit 161 includes a
pressure limit switch 168, a heater shut-off valve 162, a solenoid
164 arranged to control operation of the heater shut-off valve, and
a temperature-controlled switch 116. The pressure limit switch 168
is configured to open if tank pressure exceeds a maximum pressure
threshold. The temperature-controlled switch 116 is coupled to the
wall of the tank 102 near the level of, or slightly above the
uppermost part of the catalytic heater element 106, and is
configured to open when the temperature of the tank wall rises
above a switching threshold, such as, e.g., 125.degree. F.
[0063] A pilot supply line 179 is coupled to the gas supply line
176 at a point between the shut-off valve 162 and the second
regulator valve 166, and extends to the pilot supply port 144.
Accordingly, fuel for the pilot heater 140 is regulated by the
first regulator valve 163 and controlled by operation of the
shut-off valve 162, but is not subject to control by the second
regulator valve 166. Because the first regulator valve is
configured to supply fuel at a volume and pressure appropriate for
operation of the main heater element 139, an orifice 170 is
provided to limit the flow of fuel to the pilot element, which
requires much less fuel for operation. While shown as a separate
component, such an orifice may be incorporated into the pilot
supply port 144, or its function may be accomplished simply by
selection of the bore size of the pilot supply line.
[0064] The thermocouple 146 of the pilot element 140 is coupled in
series, via electrical lines 178, with the temperature-controlled
switch 116, the pressure limit switch 168, and the solenoid 164,
with ends of the resulting circuit coupled to circuit ground 180.
The feedback line 177 is coupled to the control terminal 167 of the
regulator valve 166, as previously described, and also to a control
terminal 169 of the pressure limit switch 168.
[0065] When the pilot heater 140 is in operation, the thermocouple
146 produces an electric current that is transmitted to the
solenoid 164 via the temperature-controlled switch 116 and the
pressure limit switch 168. When sufficient current is provided, the
solenoid 164 acts to move or hold the shut-off valve 162 open so
that gas can flow through the valve to the catalytic heater element
106. If combustion in the pilot heater 140 stops, the thermocouple
will stop producing current, and the solenoid 164 will permit the
shut-off valve 162 to close, shutting off fuel supply to the heater
element 106. Likewise, if the temperature of the tank wall rises
above the switching threshold, the temperature-controlled switch
116 will open, the current will be interrupted, and the shut-off
valve will close. Finally, if tank pressure at the control terminal
169 rises above a maximum pressure threshold, the pressure limit
switch 168 will open, interrupting the current and closing the
shut-off valve 162. In other respects, the heater control circuit
161 operates substantially as described with reference to the
heater control circuit 119 of FIG. 3.
[0066] As the level of liquefied gas in the tank 102 drops,
eventually, the liquid level inside the tank drops into a region
directly opposite the catalytic element 106 outside the tank. As
the liquid level continues to drop, an increasing portion of the
heat produced by the element 106 heats the outside of the tank
above the fluid level inside the tank. Efficiency of heat transfer
from the tank wall to the liquid LPG drops significantly as more
and more of the tank wall is exposed to heat from the element 106,
without liquid on the opposite side to which heat can be directly
transmitted. Accordingly, the temperature of the tank wall at the
level of the temperature-controlled switch 116 begins to rise. At
the same time, because the surface area of the remaining liquefied
gas in contact with the tank wall diminishes significantly as the
tank nears empty, less of the heat from the tank wall is
transmitted to the liquid, and the rate of self refrigeration
increases. This further reduces tank pressure, causing the second
regulator valve 166 to open further, and resulting in an increase
of fuel to the heater element 106 to restore tank pressure. In such
a case, there is a potential danger of damage to the painted
surface of the tank by the excessive heat produced. To prevent the
possibility of such damage, the temperature threshold at which the
switch 116 opens is selected to interrupt the current from the
thermocouple before the tank wall temperature reaches a dangerous
level. When the switch 116 opens, current to the solenoid 164 is
interrupted, permitting the shut-off valve 162 to close. This shuts
off not only the main heater 139, but also the pilot heater 140. If
the rate of draw by the load continues, it is likely that tank
pressure will shortly thereafter drop below the regulated pressure,
affecting operation of the gas-powered devices of the load.
[0067] Ideally, the tank 102 is refilled before the level drops to
this point, but loss of function of gas appliances can at least
serve as a reminder that the tank should be filled. Nevertheless,
even if the tank is not refilled, the pilot heater can be restarted
once the temperature of the tank wall has dropped below the
threshold. Thus, in exigent circumstances, the remaining fuel in
the tank can be accessed, although unless the load demand is
reduced, the same outcome will eventually occur.
[0068] FIGS. 8-10 show, in side views, catalytic heater elements
according to respective embodiments. As shown in FIG. 8, a heater
element 190 is provided, in which the element is curved to conform
to the contour of the tank 102. The catalytic heater element 190 is
in the form of a segment of a cylinder whose radius, at least at
the face of the element, preferably exceeds a radius of the tank by
an amount substantially equal to the distance between the element
and the outer surface of the tank, so that the face of the element
is substantially equidistant from the tank wall across its entire
surface. This arrangement permits a more efficient transfer of
heat, as compared to the rectangular elements of previous
embodiments.
[0069] A rectangular element has one line, lying parallel to a
longitudinal axis of the tank, along which it lies closest to the
tank, and along which heat is most effectively transferred to the
tank. In contrast, the catalytic heater element 190 of FIG. 8 is
equidistant from wall of the tank 102 across the entire face of the
element, so that heat is more efficiently transferred to the tank
over the entire surface of the element. The heater element 190
includes a plenum chamber 196, a diffuser/insulation layer 198, and
a catalyst layer 200, each of which conforms to the contour of the
face of the element, as shown in dotted lines in FIG. 8. Other
features of the element are substantially similar to features
described with reference to previous embodiments are not shown in
detail, but can be provided as required for a particular
application. For example, the element 190 can be provided with a
pilot heater and an electric element, can be mounted to the tank
102 by appropriate means, and can be coupled to a heater control
such as described elsewhere in this disclosure.
[0070] FIG. 8 also shows a shroud, or cabinet 194, enclosing the
heater element 190. The cabinet 194 provides protection for the
heater element 190 from weather and small animals, and also
prevents unintentional contact with the element during operation.
Louvers or perforations 202 and 204 are provided to permit entry
and exit of air into the cabinet 194, so that oxygen necessary for
catalytic combustion can be continually provided, and a baffle 205
extends from an uppermost side of the element 190 to an inner
surface of the cabinet 194 and along the length of the element, to
prevent passage of air at that point. Air passing between the
heater element 190 and the wall of the tank 102 is heated by the
heater element so that it rises, and flows out of the cabinet 194
via louvers 202. Heated air rising at the upper side of the cabinet
194 close to the tank creates a chimney effect, which draws
replacement air into the cabinet via louvers 204 to circulate
around the element 190 as shown by the arrows in FIG. 8. Much of
the heat that inevitably passes to the back of the element 190 is
transferred to the air as it enters the cabinet, where it is
carried to the front and combined with the heat from the catalytic
reaction. This also permits the element 190 to be positioned nearer
to the bottom of the tank, because the chimney effect provides
sufficient air circulation to maintain catalytic combustion. In
contrast, a planar catalytic heater tends to operate at lower
efficiency when positioned with the face at an angle that is much
closer to horizontal than about 45 degrees.
[0071] FIG. 9 shows a catalytic heater element 210 according to
another embodiment, in which the element is divided by internal
walls 220 into three sections 214, 216, and 218 each provided with
a respective supply port 136a, 136b, and 136c. In other respects,
the heater element 210 is substantially similar to the element 190
of FIG. 8. According to the embodiment of FIG. 9, each of the
sections is separately controllable, so that as the level of LPG
inside the tank 102 drops, the sections can be shut down in
sequence, so that less heat is radiated to portions of the tank
wall above the level of the LPG inside. In this way, the remaining
LPG can be more efficiently heated, while avoiding, to at least
some extent, overheating the tank wall. A pilot heater is
preferably provided as part of the third section 218 so that the
bottommost section can be activated, even when the remaining
sections remain shut down. Heat conductors can be provided,
extending between the sections, to assist in initial combustion.
Control of the fuel supply to each of the supply ports 136a, 136b,
and 136c can be provided with respective temperature controlled
switches, which are attached to the tank wall adjacent to the
respective section of the heater element. The switches controlling
the separate sections are set to a lower temperature than the
switch 116, and are able to detect the rise in temperature as the
fluid level inside the tank drops below that switch. An exemplary
circuit is described below with reference to FIG. 11.
Alternatively, control of the respective sections can be on the
basis of a signal from a tank level sensor. Such sensors are well
known in the art, and are commonly used to indicate the level of
liquid in an LPG storage tank. Here, a circuit can be configured to
close a shut-off valve supplying fuel to the section 214, for
example, when the level of liquid in the tank drops into the range
in which the heat generated by that section strikes the tank,
etc.
[0072] FIG. 10 shows a catalytic heater element 230 according to
another embodiment, in which the element comprises first, second,
and third separate catalytic elements 232, 234, 236, linked
side-by-side, each having a respective supply port 136d, 136e,
136f. Heat conductors 238, such as, e.g., steel rods, extend in the
catalyst layer from the third element 236 to the second and first
elements 234, 232, to conduct heat from one to the next during
initiation of combustion. In embodiments that include a pilot
heater, it is positioned in the third element 236.
[0073] According to one method of operation, the first, second, and
third elements 232, 234, 236 collectively function substantially as
the catalytic element 106 described with reference to FIGS. 1-7,
with each element being supplied from a common fuel line controlled
by a single valve and distributed via a distribution head, for
example. Because each element 232, 234, 236 is narrower than the
single element 106, and is rotated along a longitudinal axis to
directly face the tank wall, the overall transfer of energy to the
tank is more efficient, and may approach the efficiency of the
catalytic element 190 of FIG. 8. However, the catalytic element 230
of FIG. 10 is less costly to manufacture than either of the
elements 190 or 210 because, to a large extent, it can be assembled
from commercially available components using common procedures.
[0074] According to another method of operation, the first, second,
and third elements 232, 234, 236 collectively function
substantially as the three sections 214, 216, 218 of the catalytic
heater element 210, as described above with reference to FIG. 9, so
that each element is independently controlled, and can be shut off
if the liquid in the tank drops below the level of the respective
element.
[0075] Turning to FIG. 11, a schematic diagram of a heater control
circuit 240 is shown, according to an embodiment. The heater
control circuit 240 is configured to control multiple heater units
of a catalytic heater element, as described, for example, with
reference to FIGS. 9 and 10. FIG. 11 shows first, second, and third
heater units 242, 244, 246 that collectively form a catalytic
heater element 258. The first heater unit 242 comprises a catalytic
heater element 250, a temperature-controlled switch 252, and a
shut-off valve 254. A thermocouple 256 is positioned in the heater
element 250 and is electrically coupled in series with the switch
252 and a solenoid 257 of the shut-off valve 254. A fuel supply
port 259 of the heater element 250 is coupled to the supply line
176 via the shut-off valve 254.
[0076] The second heater unit 244 comprises a catalytic heater
element 260, a temperature-controlled switch 262, and a shut-off
valve 264. A thermocouple 266 is positioned in the heater element
260 and is electrically coupled in series with the switch 262 and a
solenoid 268 of the shut-off valve 264. A fuel supply port 269 of
the heater element 260 is coupled to the supply line 176 via the
shut-off valve 264. Fuel entering the catalytic heater element 260
first passes through an orifice 267.
[0077] The third heater unit 246 comprises a catalytic heater
element 270, including a thermocouple 276, a fuel supply port 279,
and an orifice 277. The thermocouple 276 is electrically coupled in
series with the temperature-controlled switch 116 and the solenoid
164 of the shut-off valve 162. The fuel supply port 279 is coupled
to the supply line 176 via the orifice 277.
[0078] The first, second, and third heater units 242, 244, 246 are
positioned in the order shown, with the first heater unit
positioned above the second heater unit, and the first and second
heater units positioned above the third heater unit. The
temperature controlled switch 252 is positioned against the wall of
an LPG storage tank at a height that corresponds to the position of
the catalytic heater element 250, and similarly, the temperature
controlled switch 262 is positioned against the wall of the storage
tank at a height that corresponds to the position of the catalytic
heater element 260. The temperature controlled switch 116 is
positioned against the wall of the storage tank at or above the
height of the temperature controlled switch 252.
[0079] FIG. 11 does not show a pilot heater or other means for
initiating combustion, but it will be understood that such means
can be provided as described with reference to any of the
embodiments. For example, if the heater units are arranged in
physical contact with each other, a single pilot heater can be used
to initiate combustion in all of them, as described with reference
to FIGS. 10 and 11, in which case the pilot heater will be
positioned in the catalytic heater element 270, which is lowermost
of the heater elements.
[0080] The first, second, and third heater units 242, 244, 246
normally operate together as a single heater element controlled by
the second regulator valve 166. If the liquid level within the tank
drops into the range that is directly heated by the first heater
unit 242, so that a portion of the heat from the catalytic heater
element 250 strikes the tank wall above the level of the liquid in
the tank, the tank wall above the liquid will become warmer than
below the liquid level. The switching temperature of the
temperature controlled switch 252 is selected so that the switch
will open once the liquid level drops a small distance below the
switch, thereby interrupting the current to the solenoid 257 and
closing the shut-off valve 254. The heater unit 242 is thus shut
down when the liquid level drops below that unit. Similarly, the
second heater unit 244 is configured to shut down when the liquid
level drops below its position. When a tank is heated at a point
that is above the level of the liquid inside, a much greater
portion of the heat is lost to the environment, which can
significantly reduce efficiency of the heating system. Shutting
down the first and second heater units 242, 244 when the liquid
level drops below their respective positions therefore improves the
overall efficiency of the system, in particular when such a heater
system is used with LPG supply systems that are routinely drawn
down below about 25% of tank capacity.
[0081] The temperature controlled switch 116 is configured to open
at a much higher temperature threshold than the thresholds at which
the temperature controlled switches 252 and 262 are configured to
open, and acts as a safety device to protect the tank. If for any
reason the tank temperature rises excessively, such as, for
example, due to a malfunction in which one or both of the first and
second heater units 242, 244 fail to shut down when the liquid
drops below their respective levels, the temperature controlled
switch 116 will open, interrupting the current to the solenoid 164,
closing the shut-off valve 162, and shutting down the entire
system.
[0082] When the first heater unit shuts down, as described above,
the volume of fuel passing through the second regulator valve 166
is not proportionately reduced, so it is possible that the volume
could exceed the combined capacities of the second and third heater
units. The orifices 267 and 277 are provided to prevent a flow that
exceeds the capacity of the respective catalytic heater element,
but do not significantly limit normal levels of flow. This function
may also be served by selection of the diameter of the individual
supply lines or the size of the respective supply ports, or by
other appropriate means.
[0083] The inventors built a prototype tank heater system
substantially as described with reference to FIGS. 1, 2, and 4-7,
which was installed on a 500 gal. LPG storage tank, and using the
following commercially available components: for the regulator
corresponding to the first pressure regulator 163, a Fisher.RTM.
type 912, set to regulate pressure to 12-14 inches of water column
(InWC), or about 5 psi; for the regulator corresponding to the
second pressure regulator 166, a Mooney.RTM. Series 20.TM.
regulator; for the switch corresponding to the pressure limit
switch 169, a Barksdale.TM. Series 9692X pressure switch, set to
open at 220 psi; for the valve corresponding to the shut-off valve
162, a BASO.RTM. H15 Series pilot valve; and for the catalytic
heater element, a modified Cata-Dyne.TM. WX Series 18.times.48
infrared catalytic heater, with a maximum output of 25,000 btu/hr.
The switch corresponding to the temperature limit switch 116 was
set to open at 115.degree. F. (about 46.degree. C.).
[0084] Modifications and other components of the prototype
embodiment were purpose built. These included components
corresponding to the pilot heater 140, the mounting brackets 141,
support frames 110, and shroud 108. The dimensions of the pilot
heater, as defined by the sidewalls, was about 6 inches by 10
inches, or about 7% of the total area of the heating element, and
in operation produced about 200-2000 btu/hr. In addition to the
elements described with reference to FIGS. 1-7, the prototype
system included access ports at various locations to enable
pressure and temperature readings to monitor the systems
operation.
[0085] In initial testing of the prototype tank heater system, the
system performed exactly as anticipated. The system was configured
to turn on when tank pressure dropped below 25 psi, and to turn off
when tank pressure reached 35 psi. Total activation time, i.e., the
period from the moment the second regulator valve opened to send
fuel to the main heater, to the moment the entire main heater was
at or above the light-off temperature, was about 15 minutes. Fuel
consumption of the pilot heater was about 1 cf/hr. Or approximately
10% of the overall heater output.
[0086] FIGS. 12 and 13 depict an LPG storage system 300 according
to another embodiment. The system 300 includes an LPG storage tank
102 with a tank heating system 304. The tank heating system 304
includes a catalytic heater element 306 and a shroud, or cabinet
308. Various details, including heater control components, pilot
element, etc., are omitted to simplify the drawings, but it will be
understood that features not shown, but necessary for proper
operation, including any of the features described with respect to
other disclosed embodiments, can be incorporated as
appropriate.
[0087] Straps 312 are attached to the tank 102 by buckles 302. Each
of the straps 312 includes first and second connectors 311, 317
configured to engage corresponding first and second attachment
features 313, 319 of the cabinet 308. As shown in FIGS. 12 and 13,
the first connector 311 is a hook and the first attachment feature
313 is a slotted aperture in the cabinet 308. The second connector
317 is shown as a toggle buckle configured to engage a hook coupled
to a lower portion of the cabinet and serving as the attachment
feature 319. The connectors and attachment features shown are
provided as examples, only. Any of a wide variety of mechanisms,
including many that are commonly available for similar
applications, can be employed to couple the tank heating system 304
to the tank 102. For example, straps 301, shown in dashed lines,
can be attached to the straps 312 and positioned to extend so as to
engage the back of the cabinet 308 to hold it tightly against the
tank. Buckles, attachment hardware, and tightening mechanisms are
not shown, but are well known in that field of art.
[0088] End walls 307 of the cabinet 308 can be shaped to conform to
the curvature of the tank so that when installed, sidewalls 305,
which extend between the end walls 307, can be positioned against
the tank wall, so that substantially the entire perimeter of the
cabinet contacts the tank wall. Alternatively, as shown in FIG. 12,
the end walls 307 include conformable panels 309 made from a
resilient material such as, e.g., an elastomeric polymer like
silicone, or synthetic rubber. When the cabinet 308 is positioned
against the tank 102, the conformable panels 309 stretch to
accommodate the curvature of the tank, thereby forming a
substantially gas-tight seal. The conformable panels enable the
tank heating system 304 to be mounted to tanks having a wide range
of diameters and capacities. The curvature of the forward edge 315
of the rigid portion of the end walls 307 is selected to
accommodate a tank having the smallest diameter to which the
heating system 304 can be mounted, with full contact around the
perimeter of the cabinet, without permitting contact between the
tank wall and the face of the heating element 306.
[0089] A door 314 provides access through a back panel 303 to the
interior of the cabinet 308. Inlet vents 318 provide passage of air
through the back panel 303, and outlet vents 316 provide passage of
air through the upper sidewall 305.
[0090] The catalytic element 306 is mounted to the cabinet 308 by
fasteners 310, extending from the element to mounting apertures in
the end walls 307 of the cabinet. A heat exchanger 327 is
positioned between the heating element 306 and an inner surface of
the cabinet 308, along the length of the element.
[0091] During installation on the tank 102, the cabinet 308 is
positioned so that the hook 311 of each strap 312 engages the
respective aperture 313, so that the cabinet hangs from the two
hooks. The cabinet 308 is then rotated so that the lower portion of
the cabinet swings under the tank 102 until bails of the toggle
buckles 317 can engage the lower hooks 319. The toggle buckles 317
are then rotated to their locked positions, pulling the cabinet
tightly against the tank, and securely coupling the cabinet to the
tank. According to an embodiment, a resilient insulator material is
provided along the front edges of the sidewalls 305 of the cabinet
308 to provide a substantially complete seal between the cabinet
and the wall of the tank.
[0092] Referring to FIG. 13, in which the heat exchanger 327 is
shown diagrammatically, airflow is indicated by arrows
A.sub.1-A.sub.4. Because catalytic combustion requires oxygen, a
source of oxygen is required for proper operation of the catalytic
heating element 306. Thus, an air space is provided between the
heater element 306 and the wall of the tank 102. As the oxygen in
the air in front of the heating element is depleted, the air is
heated by the operation of the element, so that it rises across the
face of the element, pulling fresh air into its place. A resilient
baffle 323 is positioned to press against the tank wall and fills
the space between the heat exchanger and the tank. The baffle 323
blocks direct passage from the heating element 306 to the outlet
vents 316, leaving passage through the heat exchanger as the only
path to the outlet vents. Rising exhaust air therefore enters the
heat exchanger 327 via an exhaust air inlet, as indicated at arrow
A2, and exits via an exhaust air outlet, as indicated at arrow A4.
Internal ducting 329 can be provided to reduce resistance to air
passing to and from the heat exchanger 327 inside the cabinet
308.
[0093] As hot air rises in front of the heating element 306, air
pressure inside the cabinet is reduced, which creates a vacuum to
draw fresh air into the inlet vents 318 of the cabinet. Outside air
is pulled into the inlet vents 318 and into a fresh air inlet of
the heat exchanger 327 as indicated by arrow A1. As the fresh air
passes through the heat exchanger, heat from the exiting exhaust
air is transferred to the incoming fresh air, thereby conserving a
portion of the heat that would otherwise be lost with the exiting
exhaust air. The preheated fresh air exits the heat exchanger 327
by a fresh air outlet to the interior of the cabinet, as indicated
at arrow A3. The fresh air is then drawn down across the back of
the heating element 306, where it is further heated, until it
passes under the element and begins to rise across the face of the
heating element, continuing the cycle. Insulating 325 can be
provided in the interior of the cabinet 308 to reduce the amount of
heat lost through the back and sides of the cabinet.
[0094] Turning now to FIGS. 14 and 15, a catalytic heater element
320 is shown, according to another embodiment, in views that
substantially correspond to the views of the element 106 of FIGS. 4
and 5. FIG. 14 shows the element 320 in a bottom plan view, and
FIG. 15 is a sectional view of the catalytic heater element 320 of
FIG. 14, taken along lines 15-15. Features that are substantially
identical in function to corresponding features of previously
described embodiments are identically numbered, and will not be
described in detail.
[0095] The catalytic heater element 320 is divided into a main
heater 331 and a pilot heater 322 by sidewalls 332, coupled to the
back panel 122 in a substantially gas-tight fashion. The pilot
heater extends lengthwise for a substantial portion of the housing,
although portions are shown larger than in practice, to better
illustrate the various components. Preferably, the pilot heater 322
occupies about 3% to 25% of the area of the housing 120, and most
preferably between about 8% and 20%. According to one embodiment,
the pilot heater 322 occupies about 10% of the area of the housing
120.
[0096] The pilot heater 322 includes a pilot supply port 330 and an
electric heating element 334. The heating element 334 is contained
entirely within the perimeter of the pilot heater 322. In
operation, the pilot heater achieves light-off much more quickly
and efficiently, because all the heat produced by the electric
element 334 serves to heat only the portion of the catalyst layer
132 that operates with the pilot heater. While the electric heating
element 334 is shown extending through much of the pilot heater
322, according to an alternative embodiment, the electric element
334 occupies only a very small portion of the pilot heater, and
requires a relatively much smaller amount of power to reach an
adequate activation temperature. Accordingly, when the pilot heater
322 is initially placed in operation, the electric heater 334 is
energized to heat a small portion of the catalyst over the pilot
heater 322 to the activation temperature, using a small battery
supply, and that small portion begins catalytic combustion. Within
a short time, as heat spreads from the small portion, the entire
pilot heater comes into operation, and continues as described with
reference to previous embodiments.
[0097] A fuel distribution header 324 is provided to more evenly
distribute fuel to the heating element, and includes fuel ports 326
through which fuel is supplied from the distribution header to
respective portions of the housing 120. The fuel distribution
header 324 includes a fuel supply port 328 to which fuel is
supplied from the heater control 335.
[0098] A thermoelectric device 336 is coupled to an outer surface
of the back panel 122 opposite the pilot heater 322, and includes
one or more thermoelectric modules 340 sandwiched between a first
heat sink 341 and a second heat sink 342. The first heat sink 341
is coupled to the back panel 122 to provide a rigid mounting
surface for the modules 340. When the catalytic heater element 320
is used in an enclosure like the cabinet 308 of FIGS. 12 and 13, an
aperture 344 is preferably provided in the back panel 303 of the
cabinet in a location that corresponds to the position of the
thermoelectric device so that the second heat sink 342 extends
through the aperture to the exterior of the cabinet.
[0099] Operation of thermoelectric devices are well known, and are
commonly used to perform various functions, according to
thermoelectric principles. For example, the Peltier effect refers
to a phenomenon that occurs when an electrical potential is applied
across a junction of two different conductive materials, in which
heat is absorbed at one part of the circuit and released at
another. This effect is often employed to cool microprocessors
within a computer cabinet, by affixing a thermoelectric module
similar to the modules 340 of FIG. 15 to the outer surface of a
microprocessor, and coupling a heat sink to the opposite side of
the panel, also as shown in FIG. 15. When a potential of the
correct polarity is applied to the thermoelectric module, it
transfers heat energy from the side in contact with the
microprocessor to the opposite side. A heat sink is typically
positioned on the opposite side, and carries the heat out to
radiator fins where it can be dissipated by convection. According
to another thermoelectric principle, if separate junctions of the
circuit are placed at different temperatures, an electric current
is generated, according to the Seebeck effect. The greater the
temperature differential between the junctions, the stronger the
electrical current. This is the principle of operation of the
thermocouple 146 described with reference to Figured. 4-7. A heat
differential between the thermocouple probe and other portions of
the circuit produce a small electric current that controls the
shut-off valve 162, so that if the pilot heater 140 goes out, the
current stops and the valve closes.
[0100] In the present embodiment, the thermoelectric device 336 is
positioned on the back panel 321 of the housing 120, opposite the
pilot heater 322. However, rather than operating the thermoelectric
modules 340 as Peltier devices, to transfer heat from one location
to another, as is typical with such devices, they are operated as
Seebeck devices, to generate electricity to power the control
circuit, using waste heat produced by the pilot heater 322. Because
Seebeck operation relies on a temperature differential, it is
important that the second heat sink 342 be cooled as efficiently as
possible, so that the outer face of the thermoelectric moduled 336
are cooler than the opposite face, in contact with the first heat
sink 341. Cooling of the heat sink 342 is generally greatly
enhanced by extending the heat sink through the aperture 344 out of
the cabinet 308.
[0101] While the thermoelectric device 336, like the thermocouple,
operates on the Seebeck principle, it provides a couple of
advantages over the thermocouple. First, better safety and
efficiency: an opening must be made in the back panel 122 of FIGS.
4-6 to permit the thermocouple to penetrate into the catalytic
element 106. In contrast, the thermoelectric panel 340 is surface
mounted to the back panel 321 housing 120, so the possibility of a
gas leak at that location is eliminated. Second, higher power
capacity: the thermocouple typically operates on a single junction
between a copper tube that forms the probe of the device, and a
wire that extends down the tube. The result is a relatively weak
current, with a very low power capacity. In contrast, a
thermoelectric panel can have dozens or hundreds of individual
junctions, each producing a small current, so that collectively, a
much more powerful current is produced, which affords the designer
a wider choice of components to use in a control circuit.
Furthermore, if additional power is required, additional
thermoelectric devices can be added.
[0102] Turning now to FIG. 16, a heater control circuit 350 for
operating the catalytic heater 320 is schematically illustrated,
according to one embodiment. In addition to components previously
described, the circuit 350 includes first and second tank wall
temperature sensors 352, 354, a second shut-off valve 356, and a
second regulator valve 358. The thermocouple device 336 of the
catalytic element 320 is coupled to the shut-off valve 162 in
series with the first tank wall temperature sensor 352 via a first
electrical line 362. The thermocouple device 336 is coupled to the
second shut-off valve 356 in series with the second tank wall
temperature sensor 354, and the pressure switch 168 via a second
electrical line 364. Finally, the thermocouple device 336 is
coupled to the second regulator valve 358 via a third electrical
line 366. Operation of the second regulator valve 358 is controlled
by the pressure feedback signal at its control terminal, but the
valve is powered electrically by the thermoelectric device 336.
[0103] All of the electrically operated functions are shown as
being powered by the thermoelectric device 336. However, as
mentioned above, in systems that require more power than is
available from a single thermoelectric device, additional such
devices can be added. The pilot heater 322 remains in operation
continually, and its heat, especially the heat emanating from the
back side of the catalytic element 320, is usually waste heat, so
placing two or more thermoelectric devices has no appreciable
impact on the system's operation.
[0104] During normal operation, the heater control circuit 350
operates much as described with reference to previous embodiments.
The first regulator valve 163 regulates supply pressure to the
system; pressure feedback line 177 provides direct tank pressure to
control terminals of the pressure switch 168 and the second
regulator valve 358, which regulates operation of the main heater
of the catalytic heater element 320, to maintain tank pressure
above a threshold; and the pilot heater 322 draws fuel via the
pilot supply line 179 from a point between the shut-off valve 162
and the second regulator valve 358. These operations are discussed
in more detail above.
[0105] The first tank wall temperature sensor 352 is positioned at
a point that is below the heater element 320, and preferably near
the bottom of the tank 102, and the second tank wall temperature
sensor 354 is positioned near or above the uppermost portion of the
heater element as described elsewhere.
[0106] In operation, when the liquid level inside the tank drops
into the region where heat from the catalytic element 320 directly
impinges on the tank wall, the wall heats up, because of the less
efficient heat transfer. When the temperature of the tank wall
exceeds a selected threshold, the switch of the second temperature
sensor 354 opens, removing power to the second shut-off valve 356,
which closes, shutting off fuel to the main heater. However, the
pilot supply line 179 is coupled to the fuel supply line upstream
from the second shut-off valve 356, in contrast to the embodiment
of FIG. 7, and so is not controlled by this action. Thus, the pilot
heater 322 remains in operation when the main heater is shut-down.
Accordingly, when the tank temperature drops again, the main heater
can relight, to continue operation.
[0107] This operation continues until the tank level drops to below
the first tank wall temperature sensor 352, positioned near the
bottom of the tank. This portion of the tank wall will not begin
heating until the tank is nearly or completely empty. Accordingly,
when the first sensor reaches its threshold, it shuts of power to
the shut-off valve 162, which is upstream from the pilot heater as
well as the main heater. Therefore, when the shut-off valve 162
closes, the entire heater system shuts down, so that it cannot
return to operation until it is manually relighted.
[0108] FIGS. 17 and 18 show a catalytic heater element 370,
according to another embodiment, in diagrammatic views that
substantially correspond to the views of the element 106 of FIGS. 4
and 5. FIG. 17 shows the element 370 in a bottom plan view, and
FIG. 18 is a side view of the catalytic heater element 370 of FIG.
17, taken along lines 18-18. Many features that are not essential
to an understanding of the embodiment are omitted for
simplicity.
[0109] Features that distinguish the catalytic element 370 from
elements of previously disclosed embodiments include a fuel
distribution header 372 and a pilot heater 374. In particular, the
pilot heater is positioned at the bottom of the housing 120, as
viewed in FIG. 17. When the catalytic element 370 is mounted to an
LPG storage tank, the pilot heater is positioned below the main
heater 378 and extends substantially the full width of the housing.
When the main heater is engaged, all portions of the main heater
can be warmed by the rising heat from the pilot element. Thus,
total activation time is significantly shortened, as compared to
other embodiments.
[0110] Additionally, the fuel distribution header 372 is positioned
inside the housing 120, in the plenum chamber 376, rather than
outside the housing, as described with respect to previous
embodiments. While this may require a slight increase in the depth
of the plenum chamber, relative to other embodiments, the overall
dimensions of the heating element, including the header, are
reduced. Additionally, with the distribution header 372 positioned
inside the housing 120, clutter is reduced, as well as the number
of apertures that are required to penetrate through the back of the
housing, thereby also reducing the number of seals necessary, and
improving safety and economy.
[0111] FIG. 19 is a schematic diagram of a heater control circuit
410 according to another embodiment. The circuit is shown to
include the catalytic heating element 370 described with reference
to FIGS. 17 and 18, but this is exemplary, only. Any appropriate
heating element can be used with the circuit. The circuit of FIG.
19 is similar in structure and operation to the circuit of FIG. 16.
Features that distinguish the circuit of FIG. 19 include a second
pressure switch 412, and the absence of a second regulator
valve.
[0112] In the circuit of FIG. 19, the first pressure switch 168
acts to control normal operation of the heating element 370. The
first pressure switch 168 is set to close when tank pressure drops
below a selected minimum tank pressure threshold, i.e., the turn-on
threshold of the system. Because the regulator valve 163 is
configured to maintain a fixed pressure in the supply line 176, and
there is no other intervening regulator valve, the main element of
the catalytic heater 370 always operates at the same output level,
preferably near its maximum output level. The appropriate fuel
volume can be controlled by providing an orifice 414 or its
equivalent, to limit fuel flow, in combination with selecting the
pressure maintained by the regulator valve 163.
[0113] The second pressure switch 412 is connected in series with
the first tank wall temperature sensor 352 and the shut-off valve
162, and acts as an over-pressure shut-off. The switch is set to
open if tank pressure rises above a selected maximum tank pressure
threshold. When the second pressure switch opens, power is removed
from the shut-off valve 162, which closes, thereby shutting off
both the main and the pilot elements of the heater 370. As
described above with reference to the circuit of FIG. 16, the first
tank wall temperature sensor 352 is positioned to detect a rise in
temperature indicating that the liquid in the tank is substantially
exhausted. Thus, according to the embodiment of FIG. 19, a complete
system shut down can be triggered either by excessive temperature,
via temperature switch 352, or by excessive tank pressure,
triggered by the second pressure switch 412.
[0114] Turning now to FIG. 20, a tank heater system 380 is shown in
a side diagrammatic view, coupled to an LPG tank 102, according to
another embodiment. The system 380 includes a catalytic heater
element in a housing 381 that combines the functions of the housing
of a heating element, as previously disclosed, and those of a
cabinet or shroud, also as previously disclosed. In particular, the
housing 381 includes sidewalls 383 that extend beyond the face of
the catalyst layer 132 to contact the wall of the tank 102,
enclosing a space between the catalyst layer and the tank wall for
efficient transfer of heat from the element to the tank, without
requiring a separate shroud.
[0115] Connectors 390 are provided near the outer edges of the
sidewalls 383 for coupling the tank heater system 380 to the tank
102. In the illustrated embodiment, the connectors 390 are shown as
hooks, which are engaged by toggle buckles 317 substantially as
described with reference to the connectors 319 of the embodiment of
FIG. 13.
[0116] The tank heater system 380 is shown positioned at the bottom
of the tank 102, so that the face of the catalyst layer 132 is
lying in a horizontal plane. In a typical catalytic heating
element, such an orientation will permit combustion only around the
perimeter of the heating element, as heated gas rising from the
perimeter prevents oxygen from reaching much of the catalyst layer
inside the perimeter. However, according to the embodiment of FIG.
20, a fuel supply port 400 and a pilot supply port 398 are each
provided with venturi-type fuel inlets 402 and nozzles 404. Thus,
for example, as fuel passes from the fuel supply line 176 through
the nozzle 404 and into the inlet 402 of the fuel supply port 400,
the flow of gas is accelerated by a reduced aperture of the venturi
nozzle. The accelerated gas flow entrains air in the vicinity,
which is drawn with the fuel into the inlet 402. The mixture passes
from the inlet 402 to a distribution header 388 and thence to a
plenum chamber 392. A pilot element 394 is similarly supported by
the pilot supply port 398.
[0117] The relative sizes of the apertures of the nozzles 404 and
the inlets 402 are selected to admit an appropriate volume of fuel
to operate the catalytic element, and to entrain a volume of air
sufficient to provide the oxygen necessary for its operation.
Because the necessary oxygen is premixed with the fuel, there is no
requirement for air flow across the face of the catalytic element.
The sidewalls 383 are provided with exhaust vents 386 to permit the
escape of exhaust gas from the housing 381.
[0118] A particular advantage of the embodiment of FIG. 20 is that
it can be mounted at the bottom of the tank. This permits heating
of the tank wall at a location where liquefied gas is present until
the tank is completely empty. This is in contrast to other
embodiments, in which heating elements are mounted to the side of a
tank, so that the liquid in the tank can drop below a level of the
element, reducing heat transfer efficiency.
[0119] It should be noted that the tank heating system 380 of FIG.
20 is not limited to the position or angle shown, but can be
mounted at any angle. Additionally, more than one tank heating
system can be mounted to a single tank, especially where the tank
capacity is very large, relative to the heat output of a single
heating system.
[0120] FIG. 21 is a detail of a tank heater system in a
diagrammatic end view, according to an embodiment, showing
alternative configurations of features disclosed with reference to
previous embodiments. The embodiment of FIG. 20 is shown with a
housing 381 with sidewalls 383 that extend, as viewed in the
drawing, in substantially straight lines from the back of the
housing to the front edges that contact the tank 102. In the
embodiment of FIG. 21, a housing 382 includes first sidewall
portions 384a that extend from the back of the housing
substantially perpendicular to the back as far as the front of the
catalytic layer 132. Second sidewall portions 384b are coupled to
the first sidewall portions 384a and extend forward at an angle
until they contact the wall of the tank 102. One advantage of this
configuration, is that it permits the use of commercially available
catalytic heating elements, which are generally rectangular in
shape, and to which the second portions 384b of the sidewalls are
coupled for operation as described with reference to the embodiment
of FIG. 20.
[0121] Also shown in FIG. 21 is an alternative mounting structure
406 for mounting a catalytic heater to an LPG tank. The mounting
structure 406 includes a mounting post 407 welded or otherwise
coupled to the wall of the tank 102. The mounting post 407 includes
a threaded rod 409 that extends therefrom. A mounting bracket 408
that includes an aperture 405 is coupled to the catalytic heater.
The heater is positioned so that the threaded rod 409 extends
through the aperture 405 and is fixed in place by a nut threaded
onto the bolt 409. A catalytic heater may employ four or more such
mounting structures to securely couple the heater to the tank.
[0122] The mounting structure 406 can be used as an alternative to
the various structures that employ straps around the tank 102, as
disclosed with reference to other embodiments.
[0123] In the embodiment shown, the aperture 405 is in the form of
an elongated slot that permits some adjustment of the angle of the
heater around a longitudinal axis of the tank 102. This is
particularly useful when the mounting bracket is used to mount a
heater that does not include venture-type inlet ports, and that
therefore requires a flow of air across the face of the catalytic
layer. The slot 405 in the bracket 408 permits angular adjustment
of the heater, upward to improve airflow, or downward to apply heat
closer to the bottom of the tank.
[0124] In embodiments that include a pilot heater, the size of the
pilot heater relative to the total size of the catalytic element is
a design consideration that will be influenced by a number of
factors, including the overall size and output of the heating
element, the expected frequency and duty cycle of operation of the
system, the cost and availability of LPG fuel, etc. For example, a
relatively larger pilot heater will consume more fuel than a
smaller one, but will bring the main heater to full operation more
quickly. During the activation period between the time fuel begins
to enter the main heater and the time the main heater reaches full
operation, some amount of fuel will flow through portions of the
catalyst that have not yet reached the activation temperature, and
will thus be wasted. If the system cycles on and off at a
relatively high frequency, it may be more efficient to use a larger
pilot heater so that the system reaches full operation more quickly
and with less loss of unburned fuel. On the other hand, in a system
that requires supplemental heat only infrequently, a small pilot
heater may be preferable, so as to consume less fuel while the
system is not in active operation.
[0125] In view of the difficulties associated with known systems
for assisting in the vaporization of liquefied gas, the inventors
have recognized that a catalytic tank heater can resolve many of
the problems, and can provide additional benefits that are not
available from prior art systems. First, a catalytic heating
element operating on LPG gas cannot raise the temperature of LPG
gas in its environment to the auto-ignition temperature of the gas,
so there is no ignition or explosion danger in the event of a gas
leak. The catalytic heater systems can meet or exceed the
requirements for operation within a Class I, Division 1, Group D,
hazardous location as governed by NFPA (National Fire Protection
Agency) 58 and NEC (National Electrical Code) 70, and thus, in the
U.S. can be used in close proximity to an LPG storage tank in any
location where a storage tank is permitted. More expensive and
complex systems can thus be eliminated, and the overall footprint
of many LPG supply systems reduced by elimination of remotely
located vaporizers and plumbing connections. Similarly, catalytic
heaters can meet the requirements of equivalent regulations in many
countries outside the U.S.
[0126] Because the catalytic heater element of the disclosed
embodiments is not in physical contact with the tank, condensation
is not trapped against the tank, but is permitted to evaporate,
which substantially eliminates the corrosion problems associated
with prior art tank heaters.
[0127] Many consumers of LPG are in locations that are remote from
an electric grid, so any electric power must be generated at the
site. The catalytic tank heater systems disclosed above do not
require a regular source of electric power. Once the pilot heater
is operating, no external power source is required, and the pilot
heater can be started in a few minutes using a generator, a car
battery, or even a smaller battery, depending on the configuration
of the system.
[0128] In most jurisdictions, where permanent electrical
connections are necessary within a specified distance from an LPG
storage tank, those connections must be installed and serviced by
electricians who are certified to perform the work, because of the
potential dangers that could arise if the work is done improperly.
Similarly, work that entails servicing or modifying gas connections
within the same distance must be done by personnel who are
certified to perform that work. This means that with prior art
systems that employ an electric tank heater or vaporizer,
installation and maintenance generally requires the services of at
least two people: one to perform the electrical work, and another
to perform the work on the gas equipment. In contrast, systems
configured according to many of the present embodiments can be
installed and serviced by one individual, because there are no
permanent electrical connections required.
[0129] The term psi is commonly understood as referring, broadly,
to pounds per square inch, but technically defines pounds per
square inch relative to a vacuum. Where psi is used in the present
specification or claims, it is to be understood as referring, more
specifically, to psig, or psi gauge, which defines the pressure
being measured relative to the ambient pressure, rather than to a
vacuum.
[0130] In describing the embodiments illustrated in the drawings,
directional references, such as right, left, top, bottom, above,
below, etc., are used to refer to elements or movements as they are
shown in the figures. Such terms are used to simplify the
description and are not to be construed as limiting the claims in
any way.
[0131] Where front and back are used in the specification and
claims with reference to catalytic heater elements and associated
features, front refers to the face of the element where the
catalyst is located, and from which most of the heat is radiated
when a fuel is catalyzed. Back, therefore, refers to the surface of
the element opposite the front. In this context, front and face are
used synonymously. Sidewall refers to the portions of a catalytic
heater element housing that extend from the back of the element
toward the front, and that define the perimeter of the element or
portion of the element, as viewed in front or back plan view. The
claims are not limited by the use of these terms in the
specification to describe the disclosed embodiments.
[0132] A feature described as being gas-tight is one that will
generally not permit passage of gas at that location at the
pressure range that the described feature would be expected to be
normally subjected to. For example, during operation, the gas
pressure in the plenum chamber of a catalytic heater is normally
equal to, or only slightly above ambient pressure, so where the
sides and back panel of a housing of a heater element are described
as being gas-tight, those features need only be capable of
substantially preventing passage of gas at slightly above the
ambient pressure. Thus, unnecessary gaps or openings or loose
joints where gas could easily pass are not present, but special
seals, hermetic sealing materials, or joints, such as would be
necessary at higher pressure differentials are not generally
required.
[0133] Ordinal numbers, e.g., first, second, third, etc., are used
according to conventional claim practice, i.e., for the purpose of
clearly distinguishing between claimed elements or features
thereof. The use of such numbers does not suggest any other
relationship, e.g., order of operation or relative position of such
elements, nor does it exclude the possible combination of the
listed elements into a single component, structure, or housing.
Furthermore, ordinal numbers used in the claims have no specific
correspondence to ordinal numbers used in the specification to
refer to elements of disclosed embodiments on which those claims
might read.
[0134] Where a claim limitation recites a structure as an object of
the limitation, that structure itself is not an element of the
claim, but is a modifier of the subject of the limitation. For
example, in a limitation that recites "a shroud configured to
conform to the wall of a cylindrical tank," the cylindrical tank is
not an element of the claim, but instead serves to define the scope
of the term shroud. Additionally, subsequent limitations or claims
that recite or characterize additional elements relative to the
tank do not render the tank an element of the claim, except where
the tank is recited as the subject of the limitation, rather than
an object.
[0135] The term coupled, as used in the claims, includes within its
scope indirect coupling, such as when two elements are coupled with
one or more intervening elements, even where no intervening
elements are recited. Coupled can also refer to a direct coupling,
in which elements are directly coupled or are formed from a same
piece of material so as to be monolithic or integral.
[0136] The abstract of the present disclosure is provided as a
brief outline of some of the principles of the invention according
to one embodiment, and is not intended as a complete or definitive
description of any embodiment thereof, nor should it be relied upon
to define terms used in the specification or claims. The abstract
does not limit the scope of the claims.
[0137] Features of the various embodiments described above are
generally disclosed with reference to particular embodiments as a
matter of convenience. Individual features of one embodiment can be
omitted, exchanged with corresponding features of another
embodiment, or otherwise combined therewith, and further
modifications can be made, to provide further embodiments, without
deviating from the spirit and scope of the invention. All of the
commercial devices and structures referred to in this
specification, are incorporated herein by reference, in their
entirety. Aspects of the embodiments can be modified, if necessary
to employ concepts of the various patents, applications and
publications to provide yet further embodiments.
[0138] These and other changes can be made to the embodiments in
light of the above-detailed description. In general, in the
following claims, the terms used should not be construed to limit
the claims to the specific embodiments disclosed in the
specification, but should be construed to include all possible
embodiments along with the full scope of equivalents to which such
claims are entitled. Accordingly, the claims are not limited by the
disclosure.
* * * * *