U.S. patent number 3,662,561 [Application Number 05/059,643] was granted by the patent office on 1972-05-16 for cooling apparatus.
This patent grant is currently assigned to Veskol, Inc.. Invention is credited to Michael J. Schroeder.
United States Patent |
3,662,561 |
Schroeder |
May 16, 1972 |
COOLING APPARATUS
Abstract
Apparatus for use in a cryogenic gas cooling system for
conditioning liquid cryogenic gas prior to introducing the same to
an evaporator where the conditioned gas is vaporized, and an
improved cryogenic gas cooling system utilizing the conditioning
apparatus to provide a more efficient cooling system and to provide
improved temperature control of a space to be cooled.
Inventors: |
Schroeder; Michael J.
(Carrollton, TX) |
Assignee: |
Veskol, Inc. (Garland,
TX)
|
Family
ID: |
22024298 |
Appl.
No.: |
05/059,643 |
Filed: |
July 30, 1970 |
Current U.S.
Class: |
62/119; 62/50.2;
62/51.1; 62/56; 62/208; 62/212; 62/217 |
Current CPC
Class: |
F25D
3/10 (20130101); F24F 5/0007 (20130101) |
Current International
Class: |
F24F
5/00 (20060101); F25D 3/10 (20060101); F25d
015/00 () |
Field of
Search: |
;62/52,53,514A,514,208,209,212,217,45,119 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wye; William J.
Claims
What is claimed is:
1. Apparatus for cooling a defined space, comprising:
a supply of super-cooled liquid gas;
evaporator means for substantially vaporizing said liquid gas;
conditioning means coupled to the supply and to the evaporator
means for atomizing the liquid gas and for creating turbulence in
said atomized gas prior to the introduction of the gas into said
evaporator means;
discharge means for disposing of the vaporized gas; and
control means for regulating the flow of the vaporized gas to
provide a temperature control of the space to be cooled.
2. The apparatus claimed in claim 1 wherein the conditioning means
includes second means for providing a smooth and continuous flow of
atomized gas to the evaporator means.
3. The apparatus claimed in claim 2 wherein the evaporator means
includes a plurality of evaporator coil circuits, and the second
means includes means for providing a smooth and continuous flow of
atomized gas to each of the plurality of evaporator coil
circuits.
4. Apparatus for air-conditioning a defined space by vaporizing a
liquid gas at a controlled rate, comprising:
a source of liquid gas;
evaporator means for substantially vaporizing the liquid gas;
first controlling means for regulating the flow of the vaporized
gas to provide a temperature control of the space to be
refrigerated;
a system of conduits coupling the source of liquid gas to the
evaporator means, the evaporator means to the controlling means,
and for disposing of the substantially vaporized gas; and
second controlling means for balancing the heat loading of the
evaporator means to the pressure in the evaporator means to
increase the cooling efficiency of the apparatus.
5. Apparatus for cooling a defined space, comprising:
a supply of super-cooled liquid gas;
evaporator means for substantially vaporizing said liquid gas;
a conduit coupling the supply to the evaporator means;
a baffle fixedly positioned in the conduit perpendicularly of the
longitudinal axis thereof, and having an orifice located in said
baffle substantially at the center thereof;
discharge means for disposing of the vaporized gas; and
control means for regulating the flow of the vaporized gas to
provide a temperature control of the space to be cooled.
6. The apparatus claimed in claim 5 wherein the conduit is 3/8 in.
conduit, the baffle is a metal plate 0.030 in. thick, and the
orifice is 0.075 in. in diameter.
7. The apparatus claimed in claim 6 wherein the baffle is
positioned less than 6 inches from the evaporator means.
8. The method for improving the efficiency of cooling a defined
space with a predetermined amount of super-cooled liquid gas to be
substantially vaporized in an evaporator to provide cooling for
said space, the method comprising:
flowing super-cooled liquid gas to said evaporator;
atomizing said liquid gas to a predetermined degree before it
reaches said evaporator; and
creating a controlled amount of turbulence in said gas before it
reaches said evaporator.
9. The method claimed in claim 8 including the step of providing a
smooth, continuous and unobstructed flow of atomized and turbulent
gas into the evaporator.
10. The method claimed in claim 8 wherein when the temperature of
the evaporator exceeds a predetermined amount, as the super-cooled
liquid gas initially flows toward the evaporator, the method
includes the intermediate step of flowing the super-cooled liquid
gas directly into the evaporator until the evaporator cools to a
second predetermined temperature at which time the steps of
atomizing and creating turbulence are resumed.
Description
This invention relates to air-conditioning and refrigeration
systems and, more particularly, to air-conditioning and
refrigeration systems utilizing cryogenic gases.
Air-conditioning and refrigeration systems are known which operate
to cool a defined space by vaporizing cryogenic or super-cooled
liquid gas, such as liquid nitrogen, through an evaporative coil
and thereafter disposing of the vaporized liquid gas into the
atmosphere or, where the cooled space contains only vegetable
produce or the like, into the defined space. Ventilating means are
utilized in a conventional manner and are arranged with respect to
the evaporative coils such that air is circulated across the coils
and through the defined space to provide the cooling desired.
While cryogenic cooling systems appear to have substantial utility
for cooling applications where there is little or no power
available for driving compressors, such as would be required in the
well known closed-loop, recirculating type of air-conditioning or
refrigeration system in use today, they have not come into common
use.
The cost of certain cryogenic gases, for instance liquid nitrogen,
has been reduced to a point where if the gas is used efficiently,
then the cost of operating a cryogenic cooling system is within the
means of high-volume consumers. Boat, aircraft, and automobile
owners can utilize this type of cooling system without wasting
valuable engine power driving compressors. Known cryogenic cooling
systems do not operate efficiently because of two basic reasons:
inefficient utilization of the coil surfaces and "surging" in the
coil.
In a cryogenic cooling system, the cryogenic gas is stored in a
liquid state, and in order to improve the safety of the system, the
liquid gas is stored at a low-pressure. In the past, the liquid gas
has been allowed to flow directly into the evaporative coils as the
cooling demand is increased. As with water in a pipe under
low-pressure, the liquified gas flows along one portion of the side
of the conduit, concentrating the cooling to that portion until
vaporization is substantially complete. As a result, icing occurs
along the outside of the coil corresponding to the super-cooled
side, and the convectional efficiency between coil and air if
substantially reduced. The remaining portions of the conduit side
are not efficiently utilized. It is desirable and would
substantially increase the operating efficiency of the unit if the
cold from the cryogenic gas could be applied uniformly to all
portions of the interior of the coil.
The more significant and related problem of "surging" occurs in a
system of the type described when vaporization inside the
evaporative coil takes place at a rate which exceeds the rate at
which the substantially vaporized gas can be discharged from the
system, thus the internal pressure of the evaporative coil exceeds
the discharge pressure and a back-pressure results. The
back-pressure forces vaporized gas back through the evaporative
coil, the connecting conduits, and to the storage tank, where the
tank pressure-regulator means allows gas to escape until the
internal pressure of the evaporator is reduced sufficiently to
allow the flow of liquid gas into the evaporator in the initial
manner. As soon as the liquid gas begins to vaporize again, the
internal pressure of the evaporator coil again begins to increase
until the cycle is repeated. As gas in the system cycles in the
manner described, the temperature of the evaporative coil and thus
the air passing over the coil to be cooled oscillates or varies
over a wide range, making it virtually impossible to obtain uniform
cooling, and the warmer, vaporized gas flowing back on or through
the liquid gas warms the previously cooled coil and accelerates
vaporization in the tank and the conduits coupling to the
evaporator such that a substantial decrease in efficiency is
experienced, cryogenic gas is wasted, and temperature control is
detrimentally affected. Attempts to overcome the effects of
"surging" have not resulted in a satisfactory solution to the
problem.
Thus, it is an object of this invention to provide improvements in
cryogenic gas air-conditioning and refrigeration systems.
Another object is to provide a means for improving the efficiency
of cryogenic gas refrigeration and air-conditioning systems.
Still another object is to provide means for accurately controlling
the operation of cryogenic gas air-conditioning and refrigeration
systems to provide improved temperature control of the space cooled
by said systems.
A further object is to provide means for use in cryogenic gas
air-conditioning and refrigeration systems for balancing the heat
loading of the evaporative coil of the system to the pressure
generated in the coil to provide a smooth and continuous flow of
gas through the systems.
Other objects and advantages will be apparent from the
specification and claims and from the accompanying drawing
illustrative of the invention.
FIG. 1 is a diagrammatic view of an air-conditioning system
embodying this invention.
FIG. 2 is a partially sectioned side view of the cryogenic gas
conditioning device of this invention.
FIG. 3 is a partially sectioned front view of the conditioning
device illustrated in FIG. 2.
FIG. 4 is a top view of a portion of the air-conditioning system of
this invention and illustrating the connection of the
gas-conditioning device to an evaporator assembly having two
circuits.
Refer now to FIG. 1. A cooling system embodying the features of
this invention is shown generally at 10. For purposes of
illustration, the defined space to be cooled 23 can be taken as the
passenger cabin of a small aircraft, and the line 36 can be taken
as at least a portion of the body of the aircraft. The location of
components between housing 36 and cabin 23 is not critical and
several acceptable arrangements are possible, except, of course,
the vents 34 must communicate with the space 23.
The system 10 includes a source of liquified, cryogenic gas 11,
which in the embodiment described is a well-insulated, low-pressure
gas storage bottle. Liquid nitrogen is stored in this tank at a
temperature of -320.degree. F., thus the tank is insulated both to
help retain the low temperature of the liquid gas stored therein
and to prevent injury to personnel who might come into contact with
its exterior surface. Since low-pressure gas is used in the system
described, the tank 11 may be shaped to fit any convenient space,
and for aircraft applications can be shaped to fit under one or
more seats in the passenger cabin.
Any one of several gases can be used in the system; however, liquid
nitrogen is most often used and is chosen here for illustration
purposes because it is easy to handle and is relatively
inexpensive. Other liquid gases which might be used would include
liquid argon, oxygen, helium and hydrogen. These gases are
characterized by the fact that they boil and vaporize under
standard atmospheric conditions at very low temperatures. For
instance, liquid nitrogen boils and beings to vaporize at a
temperature of approximately -319.degree. F., at standard
conditions.
The storage tank 11 is provided in a conventional manner with a
pressure gauge 22 and a regulator of "pop-off" valve 19, such that
when the pressure in the tank exceeds a preselected value, the
valve 19 opens momentarily to bleed gas out of the tank. In the
embodiment described, the tank pressure is maintained at 22.+-.3
psig, and the gas is vented to the atmosphere via conduit 36. In a
refrigeration system or air-conditioning system of the type
described, it is often desirable to vent the vaporized gas into the
space being cooled where it aids in the cooling process and will
retard spoilage of vegetable produce or the like. Venting into the
space to be cooled is not recommended where humans or animals are
exposed to the vaporized gas.
The tank 11 is provided with a second fitting which couples through
conduits 27, 46, respectively, to a conditioning device shown
generally at 25 and through a valve 20 to a quick-disconnect
coupling 21. A second valve 37 is positioned in the conduit 27
between tank 11 and the conditioning device 25 and is also
positioned between the conditioning device and the coupling between
the tank 11 and the valve 20. This arrangement of valves 20 and 37
provides a means for charging the tank 11 with liquid gas.
Valves 20 and 37 are manually operated valves, and when it is
desired to charge the tank 11 with liquid gas, valve 37 is closed,
valve 20 is opened, and an external supply of liquid gas (not
shown) is fed through the quick-disconnect coupling 21 to the tank
11 until the tank is filled to the desired level. Valve 20 is
normally closed during operation of the system and valve 37 is
normally open.
A conduit 26 couples the conditioning device 25 to an evaporator
assembly 12, and particularly to the evaporator coil 39 of that
assembly. As will be fully described in the following disclosure,
the conduit 26 is constructed of a relatively short piece of
conduit, and in the embodiment illustrated will not exceed 6 inches
in length.
A means, shown generally at 38, for providing quick cooling of the
evaporator coil 39 includes a conduit 45 coupled to the conduit 27,
between valve 37 and device 25, and is coupled through a
thermostatically controlled valve 29 to the conduit 26. When the
valve 29 is open, liquid gas flowing past valve 37 will by-pass the
conditioning device 25 and is communicated to the conduit 26, and
thus directly into the evaporator coil 39.
The remaining or exit end of the evaporator coil 39 is coupled
through a conduit 49 to a second thermostatically controlled valve
14 which is in turn coupled through a conduit 24 to the atmosphere
or, when desired, to the defined space 23.
Thermostatically controlled valves 29 and 14 are electrically
operated, respectively, by thermostats 35 and 16. Power is supplied
to thermostats and valves by power supply 17, which in the
embodiment illustrated, may be the standard aircraft or vehicle
power supply. Each thermostat is provided, respectively, with
temperature sensors 28, 15. The sensors 28, 15 are preferably in
physical contact with the coil 39 of evaporator 12.
A check-valve 51 is located in the conduit 27 which couples the
tank 11 to the evaporator assembly 12 and is positioned between the
valve 37 and the conditioning device 25. The check-valve 51 may be
located on either side of the junction between conduit 27 and the
input side of by-pass circuit 38, but is preferably located between
this junction and valve 37. The check-valve 51 is preferably a
ball-type check valve, such as those which are commonly known for
use with cryogenic systems and which will pass fluid freely as it
flows from the tank 11 toward the evaporator 12, but will operate
to prevent the flow of fluid in a reverse direction.
A second pressure-relief valve 50 is positioned in a conduit 52
which communicates with the conduit 49 at a point between the
evaporator assembly 12 and the thermostatically controlled valve
14. The conduit 52 on the opposite side of the relief valve 50 from
the evaporator assembly 12 either communicates with the exit
conduit 24 or is itself vented to the atmosphere. The pressure
relief valve 50 is similar to pressure-relief valve 18 and can be
adjusted to provide the venting of pressures in the evaporator
assembly 12 in excess of a given amount, for instance 26 psig, to
the atmosphere. The pressure relief valve 50 and the check-valve 51
provide still another means for improving the efficiency of the
air-conditioning system 10, as will hereinafter be described.
A conventional blower assembly 18 is provided to ventilate air
across the evaporator assembly 12. In the embodiment described, the
blower 18 circulates the air across the evaporator 12 at a rate of
about 300 ft. per minute. The blower assembly 18 is powered by the
power supply 17, and blower speed is controlled by means of an
external speed controller such as 55. Ducting 32 provides a means
for directing the air across the evaporator assembly 12 in the
conventional manner. The ducting 32 is provided with an air inlet
33 and at least one outlet 34. The at least one outlet 34 is
directed into the defined space 23 and in the embodiment
illustrated, the inlet 33 communicates with atmosphere surrounding
the vehicle 36.
It is possible to position the inlet 33 in the defined space 23
such that the air in the space 23 may be circulated, and with this
arrangement, the blower 18 can either blow air across the
evaporator 12 or can circulate the air in a reverse manner by
drawing air from the defined space across the coil and then venting
the air passing the evaporator back into the space. It is also
possible to draw air through vent 33 from the exterior of the
vehicle, i.e. the ambient air, and to blow this ambient air across
the evaporator 12 and into the defined space 23.
The evaporator assembly 12 includes coils 39 which are constructed
in an elongated arrangement with five tubes or passes of the tubing
per layer, in six layers. The stack of coils thus arranged are
equipped with fins 48 in a conventional manner to vastly improve
the heat transfer or convection from the air to the fluid in the
coil as compared to the transfer of heat from the air to the coil
where there are no fins.
The conduits used in the preferred embodiment, including the coils
of the evaporator 12, are all standard copper tubing having
substantially a 3/8 in. outside diameter. Leakproof fluid
connections are made with sealing clamps of the type hereinafter
described.
Refer now to FIGS. 2 and 3. The conditioning device 25 includes a
baffle 31 having an orifice 30 with a circular cross-section
located substantially at the center thereof. The baffle 31 is
constructed of a substantially flat, rigid material, preferably
steel, which is preferably 0.030 in. thick; although, the exact
thickness is not critical, except that the thickness of the baffle
31 should not exceed the diameter of the orifice 30 by any
substantial amount. In the embodiment described, the orifice is
0.075 in. in diameter.
For ease of installation, the baffle 31 is cut in a circular shape,
with an outside diameter substantially equal to the outside
diameter of the standard 3/8 inch copper tubing, and is then
mounted in one end of an Gyro-Lock fluid coupling of the model and
type described in the material that follows. Essentially, the
Gyro-Lock coupling has a center portion 40 which is characterized
by the fact that it has first and second ends which have a
substantially circular cross-section and have standard screw-type
threads on the exterior surface of each respective end. The
coupling has a passageway communicating with the respective ends
along a longitudinal axis of the coupling center portion 40. At
approximately the center of the passageway is a passageway portion
44 having a reduced cross-section with respect to the remainder of
the passageway, such that the portion of the passageway joining the
reduced portion 44 with the larger passageway on either end of the
center portion 40 forms shoulders. The larger portion of the
passageway is sized to accommodate standard 3/8 inch outside
diameter copper tubing. From a predetermined point near each
respective end, the passageway is machined progressively larger in
diameter out to the respective end of the coupling to form a swage
as hereinafter described. The baffle 31 is placed in a
tight-fitting relationship in one of the larger portions of the
passageway against the shoulder adjacent to the coupling center
portion 40 and located in that side of the passageway. Copper
tubing inserted in the passageway and secured in the manner
described in the following material secures the baffle 31 in place.
In this manner, the orifice 30 is fixedly positioned substantially
in the center of the passageway, and all fluid passing through the
coupling passageway must pass through the orifice.
The coupling is completed by first placing a retainer 41 on each
piece of conduit to be joined. The retainer 41 has an interior
threaded surface which is designed to mate with the exterior
threaded surface on each end of the coupling center portion 40. The
retainer 41 has a passageway interiorally of the threaded portion
which accommodates the conduit, and at one end of the passageway,
opposite the threaded portion, the passageway has a flat surface,
positioned substantially perpendicularly to the longitudinal axis
of the passageway.
Also placed on the conduit is a circularly shaped metal seal 43,
preferably brass or aluminum, having a shoulder around the
periphery of a first end thereof for mating with the flat surface
of the passageway of the retainer 41. Additionally, a cylindrically
shaped swage or second seal portion 42 is placed on the conduit in
front of the seal 43, such that beginning at the end of the
conduit, in order, there is first the swage 42, then the seal 43
and then the retainer 41. The remaining end of the swage 42 is
designed to accommodate the seal 43 and to operate in cooperation
therewith and with the retainer 41 and the respective of center
portion 40 to make a secure fluid-tight seal as the retainer
threads are screwed tightly on the threads of the coupling center
portion 40. Before the retainers 41 are finally tightened, the
conduits, such as 26 and 27, are forced into the passageway and
against the respective shoulder of passageway portion 44. Since the
baffle 31 is normally placed adjacent to the conduit 27, on the
up-stream side of the conditioning device 25.
Referring to FIG. 4, a portion of the air-conditioning system 10 is
shown. It is well known that the evaporator assembly 12 may have
several circuits in order to improve the efficiency of the cooling
to be obtained from the unit. For instance, where a large air
volume is required, as for cooling a larger space, there will be
several primary circuits in order to increase the effective frontal
area of the evaporator which is subjected to the flow of air. In
cryogenic gas cooling systems, the gas is introduced directly into
the input of each of these separate circuits directly from the
storage tank. In this invention, it is important that the cryogenic
gas from the tank 11 be introduced to the evaporator assembly 12 in
the proper manner. The apparatus shown in FIG. 4 is suitable for
accomplishing this proper introduction when several circuits are
necessary in the evaporator. As in FIG. 1, conduit 27 couples the
tank 11, through valves 37 and 51, to the conditioning device 25.
The conduit 26 now couples conditioning device 25 into a coupler
53. Coupler 53 also is adapted to connect, at an opposite end from
conduit 26, to each of the three evaporator coils 39a, 39b, 39c,
and provides a means for introducing the atomized gas out of
conditioning device 25 into the evaporator coils without
restrictions in the flow and in a continuous, substantially
uninterrupted stream into the respective coils. The coupler 53 is a
metal, preferably copper, device having a first end adapted to
accept conduit 26 and is attached thereto, as by soldering. The
second end of the coupler 53 is adapted to accept the three input
portions of coils 39a, 39b, and 39c, with these input portions
arranged in a plane and the second end of the coupling flared
accordingly, in that same plane. The ends of each input portion of
coils 39a-39c are fitted into the coupler 53 and are spaced therein
a short distance, for instance 1 inch, from the end of conduit 26,
which is also secured to the coupler. The flared end of coupler 53
is crimped or otherwise secured to the respective ends of coils
39a-39c, and an appropriate sealant or soldering is used such that
a fluid-tight coupling is made to the coil ends. The interior of
the coupler 53 has a short passageway, and fluid is communicated
from conduit 26, through the passageway, to the coils 39a-39c. The
path provided into each conduit 39a-39c is substantially smooth and
has no substantial edges of sharp turns in the conduit to impede or
disrupt the flow of fluid.
Examples of components utilized in the embodiment described are as
follows:
1. Tank 11--Model VHT-25, manufactured by Minnesota Valley
Engineering Co., of New Prague, Minnesota.
2. Valves 20 and 37--Model 580, 3/8 inch valve, manufactured by
Walworth Co. of Raintree, Massachusetts.
3. Regulator valve 19 --Model 559B3MP-22 manufactured by Circle
Seal Co., of Anaheim, California.
4. Thermostatically controlled valves 29, 14--Model B651
manufactured by Sporlan, Inc., of St. Louis, Missouri.
5. Thermostats 16, 35 (including sensors 15, 28)--Model A10
manufactured by Ranco, Inc., of Columbus, Ohio.
6. Evaporator assembly 12--5 .times. 6 coil (3/8 in.) evaporator
manufactured by The Dankard Company, Dallas, Texas.
7. Pressure Relief Valve 50--Model V06-100-N6KA, manufactured by C.
A. Norgren Co., Denver, Colorado.
8. Check Valve 51--Model 403 manufactured by Republic Valve Co., of
Cleveland, Ohio.
In operation, super-cooled liquid nitrogen (LN.sub.2) is released
from the storage tank 11, FIG. 1, into the evaporator 12 where it
is vaporized and discharged through conduit 24, either into the
atmosphere or, in the case where vegetable produce or the like will
occupy the define space 23, into the space 23.
In a conventional manner, the blower 18 circulates air across the
evaporator 12 and into or through the space 23, such that heat is
absorbed by the liquid refrigerant in the evaporator, causing the
refrigerant to vaporize. The heat in the air is conducted to the
refrigerant by the fins 48 and the coils 39, and the air cooled in
this manner is circulated through the defined space 23. The
refrigerant, for instance the LN.sub.2, is stored at -320.degree.
F., and as it absorbs heat, it begins to vaporize and become a
gaseous vapor as it passes through the coil 39.
Also in a conventional manner, a sensor 15 detects the temperature
of the coils 39 of the evaporator 12 and provides an indication of
the temperature detected to the thermostat 16. The thermostat 16 is
set for a desired temperature of the coils 39, for instance
50.degree. F. When the temperature detected by sensor 15 is less
than the desired temperature by a predetermined amount, for
instance, 2.degree., the thermostat provides an electrical signal
to solenoid valve 14 causing it to open. As the valve 14 opens, the
LN.sub.2 again flows through the system, and the coil begins to
cool down accordingly. As the temperature of the coils 39 lower and
become cooler, the change is detected by sensor 15. When the
temperature exceeds the desired temperature by 2.degree. the
thermostat 16 switches off, cutting off the current to valve 14,
thereby causing the valve to close until the cycle is repeated.
When the valve 14 closes, circulation of the gas in the evaporator
12 stops, after which time additional cooling of the air passing
the evaporator 12 occurs only to the extent that the temperature of
the metal and gas contained therein attempts to neutralize itself
to a temperature corresponding to that of the surrounding air. Air
passing the evaporator 12 is then directed into the space 23 to be
cooled through vents 34 which provide, in cooperation with the
blower 18, the circulation of the refrigerated air through the
space, as desired.
The solenoid valve 29 and high-temperature by-pass circuit 38
provide a means to quickly cool the evaporator 12. The operation of
this portion of the system 10 is more fully described in the
material that follows, but for the purpose of the immediately
following disclosure should be taken to be a closed circuit, i.e.
the liquid gas cannot pass by this alternative path.
The device 25 provides a means for conditioning the cryogenic
liquid gas to prevent "surging" in the evaporator 12 and to thereby
enable the system 10 to be operated in an efficient manner. As
previously explained, pressure in the evaporator coil 39 builds up
rapidly as air is circulated across the coils 39 and fins 48. Since
the exhaust outlet 14 is of a fixed size, the pressure at that
point is easily exceeded by the pressure in the coil 39. In this
condition, the gas cannot be completely exhausted through the
outlet 24, and vaporized gas will flow to any point of lower
pressure available to it, until the pressures in the system are
equalized. It has been explained that it is important that the
pressure in a cryogenic gas air-conditioning or refrigeration
system should be kept at a low-pressure to make the system safe by
reducing the possibility of explosion. In the system herein
described, the cryogenic gas in the storage tank 11 is maintained
at a pressure of 22.+-.3 psig by means of the pressure regulator
19, thus the higher pressure gasses generated in the coil 12 as a
result of vaporization of the cryogenic gas therein, flow back into
the tank 11, and to the extent that the pressure exceeds 22.+-.3
psig is vented to the atmosphere via vent conduit 36. While the gas
flows in a reverse manner through the system 10, as described, the
super-cooled liquid gas is cut off from the coil 12, and the
transfer of refrigerated air to the space 23 is momentarily cut
off. Additionally, the warmed, vaporized gas returned to the
storage tank 11 results in excessive vaporization of the liquified
cryogenic gas in the tank 11, thereby causing a higher percentage
of waste from the storage tank and resulting in a higher degree of
inefficiency.
The conditioning device 25 operates to prevent this waste and
inefficiency by balancing the heat loading and transfer
characteristics of the coil to the pressure in the coil to thereby
produce a smooth and continuous flow of gas through the evaporator
12, without "surging." The conditioning device 25 achieves this
result in cooperation with the system 10, by atomizing the
low-pressure, liquified, super-cooled gas and by creating a
turbulent flow of the atomized gas into and at least partially
through the evaporator 12.
Atomization of the gas is accomplished by sizing the orifice 30
(FIGS. 2 and 3) with respect to the pressure of the gas in the tank
11 and the ambient operating temperature of the system to obtain a
controlled, very-slight amount of vaporization of the gas and
atomization before it is introduced to the coil 12. For instance,
for one set of conditions in the embodiment described, the
temperature of the gas in the conduit 27 is -320.degree. F., while
in the conduit 26, the temperature of the gas is substantially
-319.5.degree. F.; thus, a slight, controlled amount of
vaporization has occurred since a nominal pressure drop must
accompany the temperature change. Additionally, the pressure on the
orifice 30 causes the liquified gas to spray into conduit 26. The
combination effect of the spray and partial vaporization results in
the atomization of the gas which is desired. Heat transferred to
the liquid, cryogenic gas as it passes through conduit 27 and heat
generated by the passage of the liquid cryogenic gas through the
orifice 30 reduces the temperature of the liquid gas as described,
but does not reduce the temperature to the boiling point of the
gas, thus the atomized gas having a turbulent flow exists in the
conduit 26, and rapid vaporization of the liquid gas does not occur
until the atomized gas reaches the evaporator coils 39.
The embodiment described is designed to operate in an ambient
operating temperature range of between 80.degree. and 120.degree.
F. For standard 3/8 in. copper then, the orifice 30 which will
accomplish the most effective result in this range and at these
conditions is 0.075 in. in diameter. The orifice 30 will, of
course, be sized differently if the operating ambient temperature
range varies somewhat from that indicated above. In any case, the
orifice 30 is sized to provide an atomized condition downstream of
the orifice, with only a nominal loss in the temperature of the
gas. The amount of vaporization which occurs will be proportional
to the temperature change, thus a substantial physical change in
the gas, i.e., substantially complete vaporization does not occur
until the atomized gas reaches the evaporator 12. The liquified
gas, conditioned as described, cools the sides of the coils 39 in
the evaporator 12 uniformly, since the turbulence of the gas
introduced by the action of the orifice 30 causes the atomized gas
to be uniformly applied to all portions of the inside wall of the
coils, thus the maximum degree of cooling per unit of gas flowing
through the unit is achieved. Further, "surging" is substantially
eliminated over the operating range of the system 10, since the
coil 39 is not charged fully in one rush of super-cooled liquid.
Having properly selected the size of the orifice 30 with respect to
the pressure of the storage tanks and the operating ambient
temperature of the system, the gas is fed through the conditioning
device at a rate and in a form which can be vaporized in the coil
39 at a constant and continuous rate, thus the orifice 31 also
serves to meter the gas to the coil at a rate which provides the
maximum amount of cooling to the coil in a form in which the gas
can be vaporized in the most efficient manner. With the gas
pressurized at 22 psig, as described, the internal pressure of the
evaporator coil 39 normally rises to a point where the pressure in
the coil either equals or only slightly exceeds the pressure of
vaporized gas at the outlet 14. The physical presence of the baffle
31 reduces the amount of back-pressure due to excess pressure in
the coil 39 that reaches the storage tank 11, and since any
pressure differential between the pressure in the coil and the
outlet 24 will be slight, the pressure differential is easily
absorbed by the conditioned gas without creating an excessive
safety hazard or affecting the efficiency of the operation of the
system 10. This performance is further improved, however, through
the use of the check-valve 51 and the pressure regulator 50, as
hereinafter described.
It has previously been stated that the thickness of the baffle 31
should not exceed the diameter of the orifice 30 by any substantial
amount. This is necessary in order to assure that the effective
size of the orifice 30 is not reduced by boundary layer effect, and
to assure that the proper spraying effect is obtained at the outlet
of the orifice to create the turbulence and atomization
required.
If the atomized gas is sprayed into a conduit, and the distance it
must travel is too great, the droplets of the atomized, liquified
gas will re-combine into one fluid stream, therefore, it is
necessary to assure that the conduit 26 is kept short enough in
length such that the atomized gas reaches the evaporator 12 in the
atomized state. In the embodiment described, the conduit 26 is kept
less than 6 inches in length and the desired result is
achieved.
One additional result is obtained by using the techniques described
above. When "surging" is present in a particular cryogenic gas
air-conditioning or refrigeration system or the system is otherwise
operated inefficiently, it is virtually impossible to precisely
control the temperature of the vaporized gas emitted from the
discharge conduit 24; however, utilizing the invention described
herein, the discharge temperature can be controlled to within
.+-.3.degree. of a desired mean, thus another important result is
achieved. In a typical aircraft cryogenic cooling system utilizing
this invention, the tank 11 holds about 40- of the cryogenic gas
LN.sub.2. Where the discharge temperature is erratic, as in a
"surging" system, the outlet temperature during a cooling cycle
will vary about .+-.15 percent with respect to the temperature of
the air being discharged out of outlet 34 into space 23. At an
outlet or discharge temperature of 70.degree. F., the LN.sub.2,
when vaporized, absorbs about 186 BTU/lb., thus, the maximum system
cooling capability is 40.times.186 or about 7,440 BTU. When the
discharge temperature can be accurately controlled in accordance
with this invention, a more complete vaporization of the liquified
gas can be achieved, thus adding even more to the efficiency of the
system. For instance, by increasing the volume of air passed across
the evaporator 12, the discharge temperature at conduit 24 of the
system described herein and embodying this invention, can be
accurately controlled to within .+-.2.degree. of the temperature of
the air passing through vents 34 into space 23. Where the discharge
temperature is controlled in this manner, more cooling can be had
from the same gas such that the outlet temperature of the discharge
gas is reduced to approximately 90.degree. F. At this temperature,
211 BTU of cooling are available from each pound of LN.sub.2,
therefore the system capacity is increased to 40.times.211 or 8,440
BTU, or an increase of 13-14 percent. This is an important feature
in all cryogenic gas cooling systems in that it reduces the cost of
operation, but is of special importance in applications such as
aircraft where weight is an important feature, since more cooling
is available from each pound of the expendable refrigerant. The
discharge temperature of the system described will not vary more
than 2-3.degree. in normal operation.
In a typical application, the system described herein has been used
to air condition a 150 cu. ft. aircraft cabin, and, in this case,
operating in an ambient temperature of 90.degree. F., the
temperature of the cabin has been maintained at 72.+-.2.degree. for
in excess of 3 hours with a 40 pound tank of liquid nitrogen gas.
This system exceeds by a substantial factor the cooling time
available from a 40 pound tank of liquid nitrogen used with
previously known systems, and in these earlier systems the
temperature of the equivalent space has been known to vary as much
as 15.degree. and has never equaled the 4.degree. of control
available with the system described herein. While cryogenic gas
cooling systems of the type described herein have not been
extensively used for the reasons previously mentioned, data
available indicates that 15-20 percent more cooling time is
available from the same quantity of cryogenic gas from a cooling
system embodying the invention described herein than has previously
been available from the "surging" systems.
When the space 23 to be cooled is at a very high initial
temperature, for instance, in excess of 100.degree., the structure
of the evaporator 12 is normally warmed to a correspondingly high
temperature. In this case, it is desirable to quickly cool the
evaporator 12 to increase the rate at which the space 23 can been
cooled by the air-conditioning system; therefore, an automatic,
quick-cooling circuit 38 is provided which by-passes the
conditioning device 25 and allows a large quantity of the cryogenic
gas to be transferred to the evaporator 12 at a faster rate than
can be provided through the conditioning device 25. This apparatus
provides a means for quickly filling the evaporator 12 with the
liquid gas, thus rapidly cooling the coil 39 to operating
temperature.
The thermostat 35 and sensor 28 control the operation of the
circuit 38. The thermostat 35 is set for a temperature in coils 39
of 90.degree. F., such that when the temperature of the coils 39
sensed by sensor 28 exceeds this level, the solenoid valve 29 is
opened in response to electrical signal from the thermostat 35.
When the temperature of coils 39 is reduced below the predetermined
level, i.e. 90.degree. F., the electrical signal is removed and the
valve 29 returns to its normally closed position, and the system
operation returns to that described above before oscillations in
the system caused by "surging" have time to occur.
The check-valve 51 operates to assure that any high-pressures
created in the coils 39 are not transmitted back into the storage
tank 11. When any back-pressure from the coil 39 reaches the
check-valve 51, the valve closes in a manner well known in the art,
and the gas cannot, therefore, reach the tank, where it would
result in heating, vaporization and waste of the stored cryogenic
gas. In the system described, back-pressure may occur when the
valve is closed, since at that time the gas in the coils 39
continues to vaporize and expand as air continues to flow across
the evaporator 12. As previously stated, the coil 39 can withstand
some back-pressure; however, as a safety measure and to assure, in
cooperation with check-valve 51 that no reverse-flow gas reaches
the storage tank 11, the pressure-relief valve is set to open at a
pressure of 26 psig, such that whenever the pressure in the coil 39
reaches that value, it will be discharged to the atmosphere rather
than flowing back into the tank 11. This then results in even
greater efficiency in the utilization of the cooling available from
each pound of stored refrigerant.
It has been found that restrictions effecting the flow of the
atomized gas out of the conditioning device 25 will cause the
atomized gas to re-form into a liquid and that with such
restrictions downstream of the conditioning device 25, the
advantageous results obtained from the conditioned gas are lost. By
restrictions are meant sharp turns in the conduits where the
interim of the conduit has edges or T-connections or the like where
obstructions or partial obstructions are placed in the path of the
atomized gas. Such restrictions cause boundary-layer and
eddy-current effects in the fluid which restrict the effective
cross-sectioned area of fluid stream and cause points of increased
pressure in the conduit which will break down the atomized
condition of the gas and will cause the atomized gas to return to
its original liquid state. This is particularly a problem where the
atomized gas is to be introduced into a plurality of coils in the
evaporator 12. The fluid flow into the evaporator coils 39 from the
conditioning device 25 must be kept continuous and smooth.
90.degree. or even 180.degree. turns can be made if the flow is
kept smooth and continuous by omitting restrictions from the path
of the fluid stream. For a single coil evaporator, this does not
present much of a problem, because restrictions are easily avoided.
FIG. 4 illustrates a suitable means for providing a smooth and
continuous flow of atomized fluid into the plurality of evaporator
coils 39a-39c. The atomized gas out of conditioning device 25 flows
smoothly and continuously into each respective input portion of
coils 39a-39c, since the flow path does not include any
restrictions. Additionally, the input portions of the coils 39a-39c
are formed without restrictions and the coils themselves do not
normally include restrictions of the type described.
It will be evident, too, that various further modifications are
possible in the manner of practicing the invention and in the
arrangement and construction of the invention herein described.
Accordingly, it should be understood that the forms of the present
invention described above and shown in the accompanying drawing are
illustrative only and not intended to limit the scope of the
invention.
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