U.S. patent number 3,779,694 [Application Number 05/197,207] was granted by the patent office on 1973-12-18 for heat gun.
Invention is credited to Dimiter S. Zagoroff.
United States Patent |
3,779,694 |
Zagoroff |
December 18, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
HEAT GUN
Abstract
Hand held heat gun producing heated air in 250.degree. to
1,000.degree. F range employs high performance internal combustion
burner discharging exhaust gas at velocity above 4,000 feet per
minute and temperature on order to of stoichiometric burning
temperature, i.e. 3,450.degree. F for propane fuel. High velocity
exhaust gases enter mixing zone preferably in a divergent manner
and with perimeter of gas flow cross-section at least 25 percent
greater than the perimeter of a circle of equal area, to provide an
extended gas-air interface The exhaust gas has a high volume
pumping and mixing action upon ambient air, producing in a
practical small distance a useful flow of treating air at the
desired temperature. Preferably the burner outlet is of elongated
form with decreasing cross-section towards outlet, e.g. a multiple
legged outlet cross-section. Another form has a shower-head like
series of small outlets, with diverging axes. Preferably a
positioning means set a minimum distance between work piece and
outlet, to ensure mixing. Where the exhaust gas stream is exposed
to admission of increasing air along the length, the positioning
means sets the working temperature. Preferably a shield extends
about the exhaust gas stream, preferably in the form of a tube with
space for ambient air. With apertures along the tube length, the
mass of air increases in mass and decreases rapidly in temperature
there along, so length sets discharge temperature. With a closed
wall tube the tube cross-section and its inlet in the vicinity of
the burner outlet defines the amount of ambient air entrained and
thereby sets the discharge temperature. Burners useful in this heat
gun are of high capacity type with jet pump feed, pressure recovery
passage and flame holder positioned at entry of fuel mixture into
the burner chamber.
Inventors: |
Zagoroff; Dimiter S.
(Marblehead, MA) |
Family
ID: |
22728466 |
Appl.
No.: |
05/197,207 |
Filed: |
November 10, 1971 |
Current U.S.
Class: |
431/347; 431/158;
431/353; 431/352 |
Current CPC
Class: |
B65B
53/06 (20130101); F23D 14/02 (20130101); F23D
14/38 (20130101) |
Current International
Class: |
F23D
14/38 (20060101); F23D 14/02 (20060101); F23D
14/00 (20060101); B65B 53/06 (20060101); B65B
53/00 (20060101); F23d 013/12 (); F23d
015/02 () |
Field of
Search: |
;431/158,347,351-355
;126/226,229,231,233,237,238,401,403,406 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dority, Jr.; Carroll B.
Claims
I claim:
1. A hand held gun for providing a flow of heated air in the
250.degree. F to 1,000.degree. F range against a work object
relying upon fuel alone without assistance of blowers or
compressors, said gun comprising the combination of a gaseous fuel
jet adapted for connection to a conventional fuel gas source such
as propane having a stoichiometric burning temperature above
3,000.degree. F, a jet pump activated by said gas jet and having an
opening for drawing atmospheric air for combustion into a
subatmospheric pressure region produced by said jet, said jet pump
constructed to impart velocity to said combustion air by mixing, an
enlarged pressure recovery passage into which the mixture of
gaseous fuel and combustion air proceeds, said recovery passage
constructed to convert velocity head of said gases to a pressure
head exceeding atmospheric pressure, an internal combustion
chamber, said chamber having an entry into which said pressure
recovery passage discharges, a flame holding means at said entry
and an outlet discharging combustion gases, the effective wetted
perimeter of the flow cross-section of said outlet being at least
25 percent greater in length than the perimeter of a single circle
of identical cross-sectional area, providing an extended interface
between combustion gases discharging from said outlet and
atmospheric air, and a temperature-limiting structure extending
downstream of said outlet, said structure preventing direct access
of the work object to said outlet and to the combustion gases
emitting therefrom and defining an ambient air propelling and
mixing zone through which said combustion gases flow preceding said
work object, said temperature-limiting structure being open to
admit atmospheric air freely, progressively to the stream of
combustion gases as they proceed through said structure, the
respective parts of said gun constructed to burn said fuel in
substantially stoichiometric conditions and discharge said
combustion gases through said outlet at a temperature exceeding
3,000.degree. F and a velocity in excess of 4,000 feet per minute,
said gun, through the cooperation of said extended combustion
gas-air interface, effectively producing a flow comprised in major
part of ambient air propelled and heated by said combustion gases,
the effective length of said temperature-limiting structure
downstream of said outlet determining the temperature of the
resultant flow at the output end.
2. The gun of claim 1 having a multiplicity of elongated outlet
aperture portions for combustion gases, and aperture portions
arranged to discharge in the same general direction into different
portions of said mixing and propelling zone.
3. The gun of claim 1 wherein said temperature limiting structure
comprises a shield member extending about said outlet in a spaced
relation defining an axial inlet for atmospheric air adjacent said
outlet and having wall portions extending downstream providing a
multiplicity of lateral atmospheric air openings along the length
of said temperature-limiting structure through which air may freely
enter said shield, there to be propelled and heated by said
combustion gases.
4. The gun of claim 1 including means fixing the downstream end of
said temperature-limiting device at a predetermined position from
said outlet thereby for a given fuel flow, enabling the temperature
at said downstream end to be set at a predetermined level.
5. A hand held gun for providing a flow of heated air in the
250.degree. F to 1,000.degree. F range against a work object
relying upon fuel alone without assistance of blowers or
compressors, said gun comprising the combination of a gaseous fuel
jet adapted for connection to a conventional fuel gas source such
as propane having a stoichiometric burning temperature above
3,000.degree. F, a jet pump activated by said gas jet and having an
opening for drawing atmospheric air for combustion into a
subatmospheric pressure region produced by said jet, said jet pump
constructed to impart velocity to said combustion air by mixing, an
enlarged pressure recovery passage into which the mixture of
gaseous fuel and combustion air proceeds, said recovery passage
constructed to convert velocity head of said gases to a pressure
head exceeding atmospheric pressure, an internal combustion
chamber, said chamber having an entry into which said pressure
recovery passage discharges, a flame holding means at said entry
and an outlet discharging combustion gases, the effective wetted
perimeter of the flow cross-section of said outlet being at least
25 percent greater in length than the perimeter of a single circle
of identical cross-sectional area, providing an extended interface
between combustion gases discharging from said outlet and
atmospheric air, and a temperature-limiting structure extending
downstream of said outlet, said structure preventing direct access
of the work object to said outlet and to the combustion gases
emitting therefrom and defining an ambient air propelling and
mixing zone through which said combustion gases flow preceding said
work object, said temperature -limiting structure comprising an
elongated tube extending about said outlet in a spaced relation
defining an axial inlet for induced flow of atmospheric air
adjacent said outlet, said tube having a closed wall along its
length and the flow cross-section and capacity of said air inlet
and said tube being more than five times greater than the flow
cross-section and flow capacity of said burner, the respective
parts of said gun constructed to burn said fuel in substantially
stoichiometric conditions and discharge said combustion gases
through said outlet at a temperature exceeding 3,000.degree. F and
a velocity in excess of 4,000 feet per minute, said gun through the
cooperation of said extended combustion gas-air interface
effectively producing a flow comprised in major part of ambient air
propelled and heated by said combustion gases, the effective flow
capacity of said air inlet determining the temperature of the
resultant flow at the output end.
6. The head gun of claim 1 wherein said burner has a constant or
decreasing flow cross-section area leading to said outlet.
7. The head gun of claim 1 wherein said outlet comprises an
elongated outlet aperture, said burner having walls diverging in
the direction of elongation of said aperture.
Description
Numerous applications in industry and home require low temperature
heating, for example heating of plastics, to shrink film and to
weld and soften tubing, and putty and paint, to soften and remove
or dry. Low temperature, in the range of 250.degree. to
1,000.degree. F, is extremely important since higher temperatures
lead to blistering, cracking and charring of these inherently low
temperature materials. The most widely used tool for this purpose
is the electric heat gun. An electric blower passes cold air over a
resistance heating element, and the hot air is directed at the work
piece. Two disadvantages are that the power is limited to 3 kw
using common electric outlets rated for 30 amp fuses and the tool
is not usable in the field where electricity is not available.
To get around the first limitation, units have been built in which
a gas flame supplies the heat and a blower is used to mix in
tempering air. These units, incorporating two different power
systems, are relatively complicated, bulky, and expensive. A
typical 25 kw unit intended for hand held use weighs 12 lbs.
Units that rely on fuel alone, such as hand held torches have the
problem that the flame temperature of common fuels such as natural
gas or propane are quite high, above 3,000.degree. F, many more
times the desired temperature. In an attempt to avoid overheating
the product, efforts have been made to slow down the flame being
applied to the work piece by means such as spreaders, or by
employing fuel rich, so-called yellow flames, but still hot spots,
overheating, scorching and charring problems persist.
I have discovered that I can achieve a much more satisfactory low
temperature heating device capable of use as a tempered air heat
gun using a flame alone by employing high velocity, intense burning
rather than the common low velocity, diffused burning pattern. This
finding appears at first sight contradictory. Commonly, higher
velocity burners are employed to achieve faster, more intense
heating rates. This behaviour can be illustrated by plotting the
time needed to start melting the end at e.g. a copper piece using
the same energy input but varying the exhaust gas velocity directed
against the copper. The inference here is that when more gentle
heating is sought, low velocities should be employed. The gas
torches described above attempt to employ this principle to achieve
gentle heating.
I have realized the practical importance in this context of the
fact that, provided the work piece is held some distance away from
the burner, quite beyond the flame, extremely high velocities lead
to more gentle and uniform heating which can be controlled to the
degree required. This behaviour can be illustrated as a plot of the
peak temperature of the products measured a distance away from a
15,000 Btu/hr burner as a function of burner exhaust gas velocity.
If a temperature of 1,000.degree. F is sought within a distance of
9 inches velocity of 100 ft/sec will succeed. A high velocity
burner according to my invention will reach a desired temperature
within a shorter distance than prior art low velocity burners of
the same fuel consumption.
These good results are attibutable, it is believed, to a
combination of factors. The much higher velocity (for a burner of
given fuel consumption) leads to a smaller area outlet aperture,
which leads to a larger ratio of cross-section perimeter to
cross-section area of the stream, which leads to a more effective
pumping rate and mixing rate for a given length of the mixing zone.
This can be enhanced by flattening the outlet area or otherwise
shaping to get a large perimeter of gas-to-air interface. As more
and more cold air is drawn in, the momentum of the exhaust gases is
spread over a greater air mass however the velocity at the work
piece remains sufficiently high to achieve good heat transfer.
Furthermore the short time it takes for the exhaust gases from my
system to reach treating temperature means that they are not
subject to detrimental buoyancy forces. Note that for slower burner
output velocities, and slower cooling found in prior devices, the
stream of gaseous products is exposed for a quite long time to the
effects of the general surroundings as it moves to the work piece
and it starts curving upwards due to buoyancy forces and becomes
seriously prone to being deflected by drafts of air, becoming
uncontrollable.
I thus propose to construct a heat gun with a high velocity burner
to entrain air to produce an air blast of intermediate temperature.
In one preferred embodiment the air entrainment can take place as
in a free jet with a predetermined distance between burner and work
piece. The entrainment zone preferably is within an open metal
cage, one function of which is to space the burner predictably from
the work piece to assure a predetermined blast temperature
delivered to the work piece and another function is the admission
of additional air along the length. The peak gas product
temperature is a predictable function of spacing to the work piece,
thus an adjustable cage or other standoff or positioning means can
be preset and precalibrated for different output temperature
requirements.
Another preferred embodiment employs a closed mixing tube of bigger
(by at least 5 time) cross-sectional area than the burner outlet
area. Given a sufficient length of mixing tube, generally 3 to 7
diameters, or shorter where highly dispersive burner outlets are
used, this construction assures complete mixing of the burner
products with the entrained air resulting in high uniformity in
temperature of the resulting stream. The degree of temperature
attenuation, (or mixing ratio) is here governed by the ratio of the
mixing tube and burner outlet cross-sectional area, and thus a
desired temperature can be reproduced repeatedly by the
predetermined sizing.
In preferred embodiments the outlet of the combustion chamber is
shaped such that the outlet cross-sectional area and the down
stream cross-section assumes a shape with a perimeter substantially
greater than the radius of a single circle of the same area. Such
outlets typically take the shape of slits, or multiple rounds. By
this means the mixing length to achieve a desired temperature
attenuation is reduced in direct proportion to the
ambient-to-exhaust gas interface, defined by the exposed perimeter
of the stream cross-section. This behaviour can be illustrated by
comparing the mixing length of two burners having the same capacity
and same exhaust gas velocity but different combustion chamber
outlet configurations. An outlet that has at least 25 percent more
flow perimeter than a round outlet achieves a desired temperature
such as 600.degree. F in 12 inches, contrasted with 16 inches for
the circular outlet. For success as a practical hand held burner
for e.g. applications by power line repairmen, where too long a
device is unwieldy, it is important that the perimeter thus be
greater by 25 percent than the circle of equal flow area.
Further reduction in mixing length can be achieved if the
combustion chamber is shaped such that the streamlines of the
exhaust gases assume a divergent pattern from the centerline, so
that the perimeter of the stream cross-section downstream of the
outlet is greater than at the outlet and to separate the exhaust
gas molecules from each other as much as possible to maximize
exposure to and mixing with ambient air. In the case of multiple
round outlets such a pattern can be achieved by inclining the axis
of the outlet nozzles away from the center line. In the case of
slits, such a pattern can be achieved by tapering the walls of the
combustion chamber away from the center line but maintaining a
constantly decreasing cross-sectional area of the chamber to avoid
diffusion or separation of the flow inside the chamber.
For other feature of the invention reference is made to the
abstract, which is incorporated herein, and to the following
description of preferred embodiments, the drawings and the
claims.
FIG. 1 is a partially diagrammatic vertical cross-sectional view of
a preferred embodiment having a ducted mixing chamber and a
flattened and diverging burner outlet.
FIGS. 1a, 2, 3 and 4 are transverse cross-sections taken on lines
12, 2, 3 and 4 respectively in FIG. 1.
FIG. 5 is a downward view taken on line 5 of FIG. 1.
FIG. 1b is a temperature profile taken across the end of the
duct.
FIG. 6 is a view similar to FIG. 1 of a second preferred
embodiment;
FIG. 6a is a temperature profile taken across the end of the cage
and
FIGS. 7, 8 and 9 are transverse cross-sectional views taken on
lines 7, 8 and 9 respectively of FIG. 6.
FIG. 10 is a series of plots illustrating temperature, velocity and
mass flow of the embodiment of FIG. 1, with and without the
duct;
FIGS. 11 and 12 are cross-sectional and end views of another
preferred embodiment employing multiple burner outlet passages.
Referring to FIGS. 1-5 pressurized gas G passes through nozzle 1.
The nozzle aims into a duct 3. The nozzle-duct combination is
commonly known as a jet pump and its function is to entrain air A
from openings O around the nozzle, between struts 2, see FIG. 1a.
The duct comprises a first section of rounded form 3a, then a
straight section 3b followed by divergent section 3c and then a
short length of straight section 3d. The pump formed by rounded
inlet, and subsequent straight, divergent and straight sections
provide a fuel-air mixture at as high a pressure as possible,
typically 2 inches water column, up to 4 inches water column,
assuming a pumping pressure of 20 psi for gas G. Handle 4 supports
the duct 3 which supports all else.
The mixture is directed into the burner. The burner consists of an
internal combustion chamber 5 and the bluff body flameholder 8. Gas
is burned in the combustion chamber. Flame is prevented from
flashing back into the jet pump because of the design of the
flameholder. Passages, dimension e, are so small that the gas
velocity therethrough is greater than the burning velocity so the
flame simply cannot travel upstream.
At section 2 the combustion chamber is cylindrical, and then
flattens out, FIG. 3. In this embodiment, with a spreading flow of
the exhaust gases it is important that in the latter part of the
burner, after combustion has occurred, the passage has equal or as
shown, decreasing cross-sectional area while it fans out in one
direction to increase the wetted perimeter. The flow cross-section
area of FIG. 3 is larger than the outlet 5.sub.0, FIG. 4. The gases
are thus accelerated as they come out of the burner. The geometry
is particularly important in this latter half of the burner, to
maintain velocity and avoid separation of the stream from the
diverging walls.
In this particular burner embodiment there is first a cylindrical
section 5a less than one diameter in length from the flameholder
and then a transition section 5b of conical form terminating less
than one diameter length from the cylindrical section. From that
position which is the widest cross section of the burner, the
passage 5c cross section area decreases. In operation combustion
initiates at the flameholder and spreads downstream.
Referring to the velocity profile FIG. 10 of air A, the air enters
the rounded inlet 3a at a slow velocity and speeds up to a very
high velocity inside pump 3 reaching a maximum around 8,000 fpm in
section 3b. In the diffuser 3c it slows, the velocity energy
converting to static pressure head. When the gas enters the burner
5 and is heated it tends to expand and it increases in velocity
again to a maximum in the outlet 5.sub.0 of the burner at dimension
g, to around 6,000 fpm. From then on the gas starts to entrain
large quantities of treating air A.sub.2 and the mixture slows
down. At the outlet 7.sub.0 the air velocity will generally be
greater than 75 feet per second, ranging from 100 to 200 feet per
second. Graph lines A represent performance of a free jet, i.e.
where free flow of air occurs into the exhaust stream at all points
along jet length. Dashed lines B represent use of the closed wall
tube, 7.
In effect the hot gases from the burner 5 drive a second jet pump,
to pump, mix and heat ambient air A.sub.2. The duct 7 of this
second jet pump in FIG. 1 has a cross section area which as is
shown in FIG. 4 is substantially larger than the outlet area
5.sub.0 of the burner, with an order of magnitude from 5 to 50. The
air inlet 7a to duct 7 is of corresponding size, due to its flared
form, positioned by struts 6 concentrically about the burner 5. The
velocity of the hot gases entrains cold air, and this stream mixes
in the duct, the reason for the duct being to equalize the
velocities and the temperature of the mixture. If the duct were cut
too short a hot core and a cold outside would be found. Complete
mixing occurs so that after a length of more than about 3 diameters
up to 7 depending upon design, equal temperature and equal velocity
come out. The amount of air entrained is governed primarily by the
area ratio of duct 7 to the burner outlet area.
In a typical construction in accordance with the embodiment of FIG.
1 the dimensions may be selected as follows:
a 0.0187 in. 1.sub.1 0.250 in. b 0.235 in. 1.sub.2 1.500 in. c
0.575 in. 1.sub.3 2.800 in. d 0.670 in. 1.sub.4 1.000 in. e 0.134
in. 1.sub.5 1.500 in. f 0.890 in. 1.sub.6 4.000 in. g 0.340 in. h
1.300 in. Angle .alpha. 7.degree. i 1.250 in. B 20.degree. j 3.500
in.
With this particular construction with propane introduced at 221/2
psig, at a fuel rate of 0.0116 lb/min, corresponding to 13,500
BTU/hour, the hot air gun will deliver 28.6 cfm of air at
1,000.degree. F, velocity 1,200 fpm.
In accordance with the embodiment of FIG. 6 the mixing process can
be obtained without the closed-wall duct in what is called a free
jet in which air can enter the mixing stream at any point
downstream. FIG. 6 has the same jet pump in common as FIG. 1. It
shows different burner geometry 9. The burner in FIG. 6 has three
outlet slits 9.sub.0 arranged in clover-leaf formation rather than
one slit, with transition from cylindrical to that form, compare
FIGS. 7, 8 and 9. Again the first half of the combustion chamber
shape or cross sectional area is not critical but the latter half
has ever decreasing cross-sectional areas. On the basis of wanting
at least 75 feet per second outlet gas velocity, the cross-sections
become calculable after the heating capacity is established. A
15,000 BTU/hour burner would typically have a 3/10 square inch of
outlet area, FIG. 7, arrived at taking into consideration the
maximum velocity of the generator and the amount of combustion
gases.
The mixing of FIG. 6 is very length-dependent. The further
downstream from the burner the more air is drawn in, the lower the
temperature has dropped, see FIG. 10. This temperature attenuation
curve is very predictable for each size of outlet and velocity
through it. Due to the fact that there is an ever decreasing
temperature, one can select the temperature wanted. To assure
constant spacing, a device such as shown in FIG. 6 is employed
where cage 11 serves to position the burner relative to the
workpiece and still admit air for mixing. This cage can be
adjusted. It consists of a strut 12 that mounts in a support post
13. It is fixed with a screw 14. This strut has indentations shown
so it can be calibrated for various temperatures. This whole
structure holds the cage in the calibrated position relative to the
burner.
In the embodiment of FIG. 6 one needs to move the cage back and
forth to establish the heated air temperature at the end of the
cage. It sets a maximum temperature deliverable to the workpiece
and by moving away one can set lower temperatures. Typically, to
achieve the same low temperature with same burner design, the
length of cage 11 beyond the burner will be less than the length of
tube 7, and hence may be more convenient for certain applications.
Where the uniformity of the temperature of all air emitted from the
outlet is important, one may choose however the embodiment of FIG.
1 over that of FIG. 6, compare FIGS. 1b and 6a and 10. Another
advantage of FIG. 1 is that it is windproof in high cross winds,
useful for instance in airports, railroads and power and telephone
line repair.
Referring to FIGS. 10 and 11 another embodiment employs chamber
outlet openings like a shower head, with axes of openings divergent
from one another to provide a downwardly expanding stream.
Numerous other embodiments will be recognized to be within the
scope of the invention.
* * * * *