U.S. patent number 3,733,845 [Application Number 05/218,870] was granted by the patent office on 1973-05-22 for cascaded multicircuit, multirefrigerant refrigeration system.
Invention is credited to Daniel Lieberman.
United States Patent |
3,733,845 |
Lieberman |
May 22, 1973 |
CASCADED MULTICIRCUIT, MULTIREFRIGERANT REFRIGERATION SYSTEM
Abstract
A refrigeration system having two or more closed refrigerant
circuits in which each circuit has a compressor, a condenser
connected to the compressor outlet, a high-pressure line connecting
the condenser to an evaporator and a low-pressure line returning
from the evaporator to the compressor inlet and in which each
circuit after the first has at least one vapor-liquid separator in
the high-pressure line and is charged with a mixture of
refrigerants. The evaporator of each circuit is in heat exchange
with the high-pressure line of the next successive circuit at a
point downstream of the first separator therein. The system enables
very low temperatures to be reached in the evaporator of the final
circuit with conventional, relatively low-pressure, oil-lubricated
compressors being used throughout.
Inventors: |
Lieberman; Daniel (Berkeley,
CA) |
Family
ID: |
22816818 |
Appl.
No.: |
05/218,870 |
Filed: |
January 19, 1972 |
Current U.S.
Class: |
62/335; 62/95;
62/120; 62/502; 62/512; 62/114; 62/175; 62/510 |
Current CPC
Class: |
F25B
7/00 (20130101); F25B 9/006 (20130101) |
Current International
Class: |
F25B
7/00 (20060101); F25B 9/00 (20060101); F25b
007/00 () |
Field of
Search: |
;62/175,335,79,114,95,120,332,502,510 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wye; William J.
Claims
Having thus described my invention, I claim:
1. A multicircuit, multirefrigerant refrigeration system
comprising:
a. first and second closed refrigerant circuits, each circuit
having
i. a compressor,
ii. a condensor connected to receive refrigerant vapors under
pressure from said compressor,
iii. an evaporator,
iv. a high-pressure line connecting from the condenser outlet to
the evaporator inlet,
v. a low-pressure line connecting from the evaporator outlet to the
compressor inlet,
b. a vapor-liquid separator in said second circuit having its inlet
and vapor outlet connected in the high-pressure line and its liquid
outlet connected to the low-pressure line,
c. said second circuit being charged with a mixture of at least two
refrigerants having different boiling points,
d. means forming a heat exchanger between the evaporator of said
first circuit and the high-pressure line of said second circuit at
a point therein downstream of said separator.
2. A cascaded, multicircuit, multirefrigerant refrigeration system
comprising:
a. a plurality of closed refrigerant circuits, each circuit
having
i. a compressor,
ii. a condenser connected to receive refrigerant vapors under
pressure from said compressor,
iii. an evaporator,
iv. a high-pressure line connecting from the condenser outlet to
the evaporator inlet,
v. a low-pressure line connecting from the evaporator outlet to the
compressor inlet,
vi. a plurality of separator stages successively connected between
said condenser and evaporator, each stage having a vapor-liquid
separator having its inlet and vapor outlet connected in said
high-pressure line and means connecting between the fluid outlet of
said separator and said low-pressure line to expand liquid
refrigerant and to exchange heat between the high-pressure line
downstream of said separator and the expanded refrigerant,
b. means forming a heat exchanger between the evaporator of a
circuit and the high-pressure line of the next successive circuit
at a point therein downstream of the first vapor-liquid separator
therein,
c. each circuit having a mixture of refrigerants therein, said
refrigerants in circuit being one more in number than the number of
separator stages therein and having different boiling points at the
same pressure,
d. i. the highest-boiling-point refrigerant in each circuit having
a boiling point sufficiently high to enable said refrigerant to
condense prior to delivery to the first vapor-liquid separator in
said circuit,
ii. the difference between the highest and lowest boiling point of
the refrigerants in each circuit being progressively greater for
each circuit,
iii. the boiling point of the lowest-boiling-point refrigerant
being progressively lower for each circuit.
3. A refrigeration system as set forth in claim 2 and further
including means forming a heat exchange between the low-pressure
line of one circuit and the high-pressure line of another
circuit.
4. A refrigeration system as set forth in claim 2 wherein the
lowest-boiling-point refrigerant of the last circuit is selected
from the group consisting of neon, hydrogen and helium.
5. A refrigeration system as set forth in claim 2 wherein all of
the compressors are of the hermetic oil-lubricated type.
6. A refrigeration system as set forth in claim 5 wherein the
highest-boiling-point refrigerant in each circuit is a
halocarbon.
7. A refrigeration system as set forth in claim 5 wherein the
lowest-boiling-point refrigerant of the last circuit is selected
from the group consisting of neon, hydrogen and helium.
8. A refrigeration system as set forth in claim 7 wherein the
highest-boiling-point refrigerant in each circuit is a
halocarbon.
9. A refrigeration system as set forth in claim 5 and further
including means for exchanging heat between the low-pressure line
of one circuit and the high-pressure line of another circuit.
10. A refrigeration system as set forth in claim 9 wherein said
means for exchanging heat is located in the high-pressure line of
the next successive circuit, upstream of said first separator
therein and is located in said low-pressure line at a point therein
where the temperature in said low-pressure line is above the
freezing point of the oil used to lubricate the compressor in said
next successive circuit.
Description
BACKGROUND OF THE INVENTION
This invention relates to compression refrigeration systems. More
particularly, the present invention is concerned with a novel
system for achieving a wide range of extremely low refrigeration
temperatures.
In a typical single refrigerant system of compression
refrigeration, the refrigerant vapors are compressed, the vapors
are condensed by heat exchange with ambient air or water, the
condensate is cooled by expansion, the liquid refrigerant is
evaporated at low pressure to produce refrigerating temperatures,
with the refrigerant vapors being returned to the compressor so
that the cycle may be repeated. When commercially available
single-stage hermetic and semi-hermetic oil-lubricated compressors
are employed in refrigeration systems of the above type, such
systems have been limited to achieving temperatures on the order of
-40.degree. F. Where lower temperatures have been required,
particularly temperatures approaching the cryogenic range (i.e.,
-250.degree. F. and below), the simple refrigeration cycle has
required substantial modification, including the use of
high-pressure gas systems, expendable refrigerants or specially
designed multi-stage compressors, turbo-compressors or
high-pressure oilless compressors. Such systems are expensive to
manufacture, operate and maintain and often require skilled
personnel in constant attendance.
Relatively low refrigeration temperatures have been achieved by
employing two or more closed refrigeration systems in conventional
cascade connection wherein the final evaporator of one stage forms
a heat exchanger with the initial condenser of the next lower
stage. Such systems have not been effective in producing practical
systems operating at or near cryogenic temperatures, particularly
due to freezing problems with compressor lubricating oils in the
low-temperature stage, unless the lowest stage included the use of
high-pressure hazardous hydrocarbon refrigerants and special
low-temperature compressors.
A further proposal for achieving low refrigeration temperatures
involves the use of a mixture of refrigerants in a single
refrigeration cycle. In such a system, a compressed mixture of
refrigerant gases undergoes a partial condensation in a first
condensation stage so that a liquid fraction which is rich in the
higher boiling refrigerant is formed. The liquid fraction is
separated from the remaining vapors and the vapors are transferred
to a second condensation stage where they are condensed by the
evaporation of the expanded and cooled liquid component from the
first condensation stage. The final refrigeration temperature is
achieved by expanding and evaporating the condensate from the
second stage of condensation. The refrigeration cycle is closed by
mixing the vapor leaving the final evaporator with the vapor formed
by evaporation of the higher boiling refrigerant in the second
condensation stage and returning the vapor mixture to the
compressor.
A further proposal is that disclosed in my copending United States
application Ser. No. 87,423, filed Nov. 6, 1970, now abandoned and
entitled "Low-Temperature Refrigeration System," wherein a single,
closed-cycle compression refrigeration system is shown which
employs a mixture of non-flammable, non-explosive and non-toxic
refrigerants having different boiling points, and which includes at
least two vapor-liquid separation stages and at least two heat
interchanges in which an expanded liquid refrigerant condensed in
an earlier stage of the cycle is evaporated to cause further
condensation of remaining refrigerant vapors.
It was discovered that by employing successive stages of
vapor-liquid separation and utilizing the expanded liquid phase to
provide cooling for the next successive stage of condensation, a
refrigeration system which is capable of achieving a wide range of
low temperatures, including temperatures in the cryogenic range, at
unexpectedly low discharge pressures and compression ratios was
achieved. This, unlike prior art refrigeration processes, made it
possible to achieve extremely low refrigeration temperatures
utilizing conventional single-stage hermetic or semi-hermetic
compressors.
The refrigeration process described in my copending application is
applicable to a wide combination of refrigerants, partial
condensation stages and separation stages. Thus, for example, the
process therein could be employed to achieve low temperatures at
low discharge pressures and compression ratios with a binary
mixture of refrigerants utilizing two or more vapor-liquid
separation stages and an equal number of heat interchangers.
Alternatively, refrigeration systems comprising a mixture of three
or four refrigerants and two, three or more stages of vapor-liquid
separation in a single, closed refrigeration cycle operated by a
single compressor are possible. In general, the process
contemplates compression refrigeration systems employing a mixture
of any number of refrigerants and at least two distinct
vapor-liquid separation stages. In the preferred embodiments the
system comprises a mixture of N refrigerants and N-1 vapor-liquid
separators and heat interchangers where N equals 3 or more
refrigerants. However, the invention is equally applicable where
the number of separators is equal to or exceeds the number of
refrigerants in the mixture.
The ultimate low temperatures achieved by the refrigeration system
of my previous system is governed by the boiling points of the
respective refrigerants employed and is limited by the evaporation
pressure of the lowest boiling refrigerant in the mixture. It was
discovered that temperatures of less than -290.degree. F. might be
achieved by employing nitrogen as one of the refrigerants and the
principles of the invention would be equally applicable to lower
boiling refrigerants which would achieve even lower
temperatures.
However, the multirefrigerant system described in my copending
application has inherent pressure and temperature limitations which
make it difficult to achieve very low, cryogenic temperatures by
the use of conventional compressors. More particularly, in each
stage, a refrigerant which has been condensed is allowed to
evaporate by reducing the pressure thereon from the high pressure
in the separator to the low pressure in the return line to the
compressor. As such refrigerant evaporates, it absorbs heat from
the gaseous refrigerant mixture leaving the separator, and reduces
the temperature thereof to a level below the boiling point (at high
pressure) of the next refrigerant. Thus, the pressure in the
high-pressure line must be sufficiently high so that the boiling
point of the next refrigerant is sufficiently low so that it will
be liquefied by the evaporation of the first refrigerant. The
greater the spread in boiling points (at low pressure) of the
refrigerants, the higher must be the head pressure of the system.
As a result, for a given number of refrigerants in the refrigerant
mixture, higher and higher head pressures must be utilized to
achieve lower and lower final temperatures. To achieve a desired
low temperature, the number of refrigerants may be increased, in
order to reduce the spread in boiling points between adjacent
refrigerants. However, an increase in the number of refrigerants
will increase the head pressure necessary to condense the added
refrigerants.
Thus, in order to achieve cryogenic temperatures with a closed,
single-loop multirefrigerant circuit, it is necessary to use head
pressures that are not obtainable with conventional commercial
oil-lubricated single-stage compressors, such compressors having a
maximum working capacity of about 350 psi.
SUMMARY OF THE INVENTION
It is the principal object of the present invention to provide a
multirefrigerant refrigeration system capable of achieving very low
temperatures with no external cooling except conventional
ambient-temperature air or water condensers, with relatively low
head pressures, and with provision whereby any oil introduced into
the refrigerant mixture will be prevented from freezing. Although
the invention is not so restricted, it has the significant
advantage that it will enable the system to operate at room
temperatures using conventional commercially available hermetic or
semi-hermetic oil-lubricated single-stage compressors.
In general, the objects of the invention are achieved by using a
plurality of closed-circuit refrigeration circuits each having a
compressor and an initial condenser cooled by ambient temperature.
The second circuit is of the type described in my copending
application Ser. No. 87,423, filed Nov. 6, 1970, now abandoned and
entitled "Low-Temperature Refrigeration System" wherein one or more
separator stages and two or more refrigerants are used. The final
evaporator of the first circuit is in heat exchange relation with
the high-pressure line of the next circuit downstream of the first
separation therein. In this way, the oil separation function of the
first separator in each circuit is unaffected by the other circuits
and there is no danger of oil passing through the circuits to a
point wherein the oil will freeze. With the cooling of one circuit
by the preceding circuit, the spread between boiling points of the
refrigerants in a circuit may be increased without a corresponding
increase in head pressure. Increasing the spread between boiling
points of the refrigerants will thus enable a lower final
temperature to be achieved without increasing the number of
refrigerants in a circuit.
The multicircuit system has a distinct advantage over a single
circuit system designed to reach the same low temperature in that
the circuits of the multicircuit system can operate at lower head
pressures. Accordingly, the various heat exchangers throughout may
be designed for lower-pressure operation, with thinner walls for
more efficient heat exchange. Correspondingly, the other system
components may be of more inexpensive construction.
Other objects and advantages will become apparent in the course of
the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings accompanying this application,
FIG. 1 is a schematic representation of a two-circuit system
wherein the high-temperature circuit is charged with a single
refrigerant and the low-temperature circuit is charged with a
mixture of two refrigerants and has one separator stage therein,
there being one thermal interchange between the two circuits.
FIG. 2 is a schematic representation of a two-circuit system, each
circuit having two separator stages and being charged with a
mixture of three refrigerants, there being one thermal interchange
between the circuits, the system being designed to operate at
approximately -300.degree. F.
FIG. 3 is a schematic illustration of a three-circuit system, each
circuit having two separator stages and a mixture of three
refrigerants, there being a single thermal interchange between
successive circuits, the system being designed to operate at
approximately -345.degree. F.
FIG. 4 is a schematic illustration of a three-circuit system, the
high-temperature circuit having three separator stages and four
refrigerants, the intermediate-temperature circuit having two
separate stages and three refrigerants and the low-temperature
circuit having three separator stages and four refrigerants, there
being one thermal interchange between the high- and
intermediate-temperature circuits and two thermal interchanges
between the intermediate- and low-temperature circuits, the system
being designed to operate at approximately -440.degree. F.
FIG. 5 is a schematic illustration of a typical single-stage
oil-lubricated hermetic compressor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a simplified embodiment of the present invention
having a high-temperature closed circuit 10, a low-temperature
closed circuit 11, and a single cascade heat exchanger 12
therebetween. This system will use a single refrigerant in the
high-temperature circuit and a binary mixture in the
low-temperature circuit. In operation, the refrigerant in the
high-temperature circuit 10 is aspirated by compressor 13 and
compressed and discharged through line 14 to condenser 15.
Condenser 15 may be cooled by air or water, as desired. The
refrigerant condenses and flows through the high-pressure line 16
to heat exchanger 17, wherein the fluid refrigerant is subcooled by
returning cold refrigerant gas in the low-pressure suction line 18.
The subcooled liquid continues through high-pressure line 16 to
expansion valve 19. Such valve is illustrative of expansion devices
such as thermal expansion valves, capillary tubes, etc. The
pressure is reduced through the valve and the fluid temperature
drops. This fluid flows through line 20 to the evaporating coil 21
of cascade heat exchanger 12 to condense the low-temperature
fraction of the low-temperature circuit 11. In so doing, the
high-temperature circuit refrigerant absorbs thermal energy and in
general will become completely evaporated to gas in the evaporating
coil 21. The gas proceeds through low-pressure line 18 to heat
exchanger 17 where it subcools the liquid refrigerant and in turn
becomes superheated. This gas then continues through line 18 back
to the inlet of compressor 13 to complete the
high-temperature-stage portion of the cycle.
In the low-temperature circuit, a mixture of two refrigerants is
used, the refrigerants having different boiling points. The
higher-boiling-point refrigerant is one in which lubricating oil is
highly miscible. The binary refrigerant mixture in the
low-temperature circuit 11 is aspirated by compressor 22, is
compressed thereby and discharged through line 23 to condenser 24.
Condenser 24 may be air- or water-cooled as required and condenses
a portion of the refrigerant mixture to liquid. The liquid consists
primarily of the high-boiling-point refrigerant of the binary
mixture, entrained oil which is scrubbed out of the vapor by the
condensing refrigerant, and a fraction of the low-boiling-point
refrigerant of the binary mixture. The composition of the
vapor-liquid mixture leaving the condenser 24 will vary with choice
of refrigerants, pressure and condenser temperatures. The mixture
goes through high-pressure line 25 to heat exchanger 26 wherein
supercooling and some additional condensation may take place. The
vapor-liquid mixture continues through high-pressure line 25 to
separator 27. Here, by means of gravity and velocity changes, the
vapor and liquid portions are separated. The liquid portion
containing the oil flows from the liquid outlet of separator 27
through line 28 to expansion valve 29. The liquid mixture is
reduced in pressure going through the valve and experiences a drop
in temperature. The high-boiling-point refrigerant is chosen so
that its temperature will not drop below the freezing point of the
oil used in lubricating the compressor. Typically such oils will
not freeze unless their temperature is reduced to about
-175.degree. F.
The vapor portion of the low-boiling-point refrigerant leaves the
vapor outlet of separator 27 and continues through the
high-pressure line 25 to the cascade heat exchanger 12. Here it is
condensed into a liquid by thermal exchange with the
high-temperature circuit refrigerant. The cooled liquid refrigerant
in the low-temperature circuit leaves cascade heat exchanger 12,
continues through high-pressure line 25 and enters heat exchanger
30 where it is subcooled by returning gases from the final
evaporator coil 31. The subcooled liquid flows through
high-pressure line 25 to expansion valve 32 (again representative
of any applicable expansion device). In going through expansion
valve 32 it is reduced in pressure and its temperature drops. The
low-pressure fluid goes through line 33 to evaporator 31. Here it
is evaporated and draws thermal energy from its surroundings,
producing the desired refrigeration effect. The vapor flows back
through low-pressure line 34 to heat exchanger 30 where it subcools
the liquid and is itself superheated. This vapor then continues
through low-pressure line 34 and is mixed with the fluid from line
35 entering the low-pressure return line 30. The mixture is used to
subcool liquid in heat exchanger 26 and returns in vapor form to
compressor 22 through the low-pressure line 34 to complete the
cycle.
FIG. 2 shows an embodiment of the invention having two
multirefrigerant circuits with their initial condensers cooled at
ambient temperature and designed to produce a temperature in the
final evaporator of approximately -300.degree. F. The
high-temperature circuit 50 is charged with a mixture of three
refrigerants: R21 (dichloromonofluoromethane, also known as "Freon"
21, boiling point = -22.degree. F.), R13B1 (bromotrifluoromethane,
boiling point = -72.degree. F.) and R14 (carbontetrafluoride,
boiling point = -195.degree. F.). The low-temperature circuit 51 is
charged with a mixture of the following three refrigerants: R21,
R14 and R728 (nitrogen, boiling point = -320.degree. F.).
The system of FIG. 2 operates as follows. The refrigerant mixture
in the high-temperature circuit 50 will be compressed, in a vapor
state, by compressor 52, which may be a single-stage, hermetic or
semi-hermetic, oil-lubricated compressor, with the compressed gas
mixture being delivered to the receiver tank 53. This tank is
utilized to provide storage of gas to prevent excessive refrigerant
pressure during such time as the system is turned off and is not in
operation, although other conventional techniques may be used to
obtain this result. From the receiver tank 53 the compressed gas
mixture flows to the inlet of condenser 54, which may be cooled by
ambient air or water as desired. As the refrigerant mixture is
cooled in the condenser 54, the highest-boiling-point refrigerant
R21 condenses to liquid, with any oil in the mixture which may have
been introduced into the mixture as it passed through the
compressor 52 being scrubbed out of the mixture by the condensation
of refrigerant R21. Most of the refrigerants R13B1 and R14 will
remain in their vapor phase since they will be at a temperature
considerably above their boiling points.
The mixture of liquid and vapor leaves condenser 54 and flows
through the high-pressure line 55 to heat exchanger 56 wherein the
mixture is further cooled by the refrigerant mixture returning
through the low-pressure line 57. The cooled vapor-liquid mixture
continues through high-pressure line 55 to the inlet of the
vapor-liquid separator 58 wherein separation occurs between the
liquid and vapor fractions of the mixture. The liquid fraction,
containing the entrained oil which had been scrubbed during the
condensing process, flows from the liquid outlet of the separator,
through drier-strainer 59 to capillary tube 60. Here the pressure
on the liquid fraction is reduced, thereby lowering the temperature
thereof. If desired, a portion 61 of capillary tube 60 may be in
heat-exchange relation with the low-pressure line 57 to provide
additional cooling of the liquid fraction in the capillary tube by
the returning refrigerant mixture. The cooled liquid fraction with
lubricating oil mixed therein enters the low-pressure line 57 at
junction 62 to return to the compressor. As the cooled liquid
fraction passes through heat exchange 63 it will evaporate and
absorb heat.
The vapor fraction in separator 58 will leave the vapor outlet of
the separator and will flow through the high-pressure line 55 to
heat exchanger 63. Here a fraction of the vapor is condensed to
liquid by thermal interchange with the low-temperature vapor-liquid
mixture coming into the heat exchanger 63 from the junction 62.
The vapor-liquid mixture in the high-pressure line then flows to
and through auxiliary heat exchanger 64 to be further cooled
therein by the returning refrigerant vapors in low-pressure line
57. From heat exchanger 64, the vapor-liquid mixture in the
high-pressure line flows to separator 65 wherein separation of the
vapor and liquid fractions again occurs. The liquid fraction is
predominantly the middle-boiling-point refrigerant R13B1 but will
contain some of the higher- and lower-boiling-point refrigerants
R21 and R14.
The liquid fraction flows through drier-strainer 66 and capillary
tube 67 to join the low-pressure line 57 at junction 68. As before,
the pressure on the liquid fraction will be reduced and the
temperature thereof lowered, and the liquid fraction will then
evaporate in heat exchanger 69.
The vapor fraction in separator 65 is predominantly the
lowest-boiling-point refrigerant R14 with some refrigerant R13B1
and a very small amount of refrigerant R21 mixed therewith. The
vapor fraction leaves the vapor outlet of separator 65 and flows
through high-pressure line 55 to heat exchanger 69 where the vapor
fraction is cooled and condensed to liquid. The liquid refrigerant
then flows through drier-strainer 70 and capillary tube 71 wherein
the pressure is reduced, thus lowering the temperature of the
liquefied refrigerant. The refrigerant then flows through the
evaporating coil 72 of cascade heat exchanger 73 wherein it
evaporates and absorbs heat from the low-temperature circuit 51.
The resulting vapor then flows through the low-pressure line 57
back to the compressor 52, mixing with other fractions from
junctions 68 and 62 and rising in temperature as it passes through
heat exchangers 69, 64, 63 and 56.
In the low-temperature circuit 51, the compressor 74 compresses the
refrigerant vapors coming from the low-pressure return line 75 and
delivers the compressed vapors through storage receiver 76 to
condenser 77 wherein the highest-boiling-point refrigerant R21
condenses to liquid, scrubbing out any oil from the compressor 74
which may have been entrained with the refrigerant vapors. Again,
the compressor is preferably of the type as compressor 52 used in
the high-temperature circuit 50, and the condenser 77 may be cooled
by ambient air or water.
The vapor-liquid refrigerant flows through the high-pressure line
78 and heat exchanger 79 to separator 80. As before, the liquid
fraction, containing the scrubbed oil, exits the liquid outlet of
the separator, passes through the drier-strainer 81 and the
capillary tube 82 wherein the pressure and temperature of the
liquid fraction is reduced, the liquid fraction joining the
low-pressure return line 75 at junction 83 and then passing through
heat exchanger 84 wherein the liquid fraction evaporates and
absorbs heat. The vapor fraction leaving separator 80 is
predominantly the R14 and R728 refrigerants with a small amount of
R21 mixed therein, and this vapor fraction is cooled as it flows
through heat exchanger 84. It is not necessary to operate the
low-temperature circuit at a head pressure wherein the boiling
point of the refrigerant mixture in the high-pressure line is
raised to a point wherein condensation of the mixture will occur in
heat exchanger 84, as was the case in heat exchanger 63 of the
high-temperature circuit. Thus refrigerants may be used in the
low-temperature circuit which have a considerably greater
difference in boiling points without the attendant need for
extremely high-pressure operation.
The heat exchanger 84 will condense any of the R21 vapors still in
the mixture, and, if desired, the heat exchanger 84 may be operated
as a reflux condenser by opening valve 85 to allow condensed liquid
to flow back through line 86 to separator 80 and mix with the
liquid fraction therein. Such operation would further ensure that
all oil is removed from the refrigerant mixture before such mixture
is reduced in temperature to a point wherein the oil could freeze.
In this regard, it is important that the high-boiling-point
refrigerant be chosen so that it will not produce low temperatures
such that the oil entrained therewith can freeze.
The cooled vapor mixture leaving heat exchanger 84 flows through
heat exchanger 87 wherein it is further cooled and then through the
high-pressure line 78 to cascade heat exchanger 73. In this heat
exchanger the evaporation in coil 72, at low pressure, of the
refrigerant R14 in the high-temperature circuit will condense a
fraction of the higher-pressure refrigerant R14 in the
low-temperature circuit, the condensed fraction being predominantly
the refrigerant R14 in the mixture.
The vapor-liquid refrigerant mixture will then flow from cascade
heat exchanger 73 to separator 88 wherein separation of the liquid
and vapor fractions again takes place. The liquid fractions,
predominantly R14, passes through drier-strainer 89 and capillary
tube 90 to junction 91 in the low-pressure return line 75 and then
through heat exchanger 92 wherein the liquid fraction evaporates.
The vapor fraction, predominantly R728, passes from separator 80,
and flows through the high-pressure line 78 to heat exchanger 92
wherein the vapor condenses to liquid. The head pressure, of
course, in the high-pressure line must be sufficiently high so that
the boiling point of the vapor fraction is sufficiently high that
the vapor fraction will be condensed in the heat exchanger 92.
The liquid refrigerant then flows through heat exchanger 93 for
further cooling by the returning refrigerant and then flows through
drier-strainer 94 and capillary tube 95 wherein the pressure and
temperature are reduced. The low-pressure liquid is then evaporated
in the final evaporator coil 96 to absorb thermal energy from the
surroundings and perform useful refrigeration. The stabilized
temperature at this point will be approximately -300.degree. F.
As before, the refrigerant vapor from evaporator coil 96 will flow
back through the low-pressure return line 75, mixing with the other
fractions and rising in temperature as the refrigerant passes
through the various heat exchangers until the entire mixture
reaches the compressor inlet to complete the cycle.
FIG. 3 shows yet another embodiment of the invention, wherein all
initial condensers are cooled by ambient water or air and where
conventional oil-lubricated compressors may be used to achieve a
final temperature of approximately -345.degree. F.
The FIG. 3 system utilizes three closed multirefrigerant circuits,
cascaded together in accordance with the invention. If desired, the
two circuits of FIG. 2, i.e., 50 and 51, may be used as the low-
and intermediate-temperature circuits, and with the evaporator coil
96 of circuit 51 being employed in the cascade heat exchanger 100
between the intermediate-temperature circuit and the
low-temperature circuit 101. Preferably, the low-temperature
circuit 101 is charged with a refrigerant mixture of R21 (Freon),
R728 (nitrogen) and R720 (neon). The entire system would thus be
charged with refrigerant mixtures as follows (the boiling points
being at atmospheric pressure):
High Intermediate Low Temperature Temperature Temperature Circuit
50 Circuit 51 Circuit 101 Refri- Boiling Refri- Boiling Refri-
Boiling gerants Point gerants Point gerants Point R21 -22.degree.
F. R21 -22.degree. F. R21 -22.degree. F. R13B1 -72.degree. F. R14
-195.degree. F. R728 -320.degree. F. R14 -195.degree. F. R728
-320.degree. F. R720 -406.degree. F.
the low- and intermediate-temperature circuits will operate as
previously described, resulting in a temperature in the evaporating
coil 96 of approximately -300.degree. F., produced by the
evaporation of a refrigerant fraction which is predominantly R728
(nitrogen).
The low-temperature circuit 101 operates in the same manner as
previously described in connection with circuit 51. Any oil in the
refrigerant mixture, introduced from compressor 102, will be
scrubbed from the mixture as the highest-boiling-point refrigerant
condenses in condenser 103 and separated out with the liquid
fraction in separator 104. The nitrogen and neon vapor fraction
from separator 104 will be cooled but not condensed in heat
exchangers 105 and 106. As the vapor fraction passes through the
cascade heat exchanger 100, a fraction (predominantly nitrogen)
will be condensed by the evaporating nitrogen in evaporator coil
96. The vapor-liquid mixture will be separated in separator 107,
with the liquid fraction being used to condense the vapor fraction
(predominantly neon). This fraction then has its pressure and
temperature lowered and it then evaporates in the final evaporator
coil 108 to perform useful refrigeration at approximately
-345.degree. F.
FIG. 4 illustrates yet another embodiment of the invention,
utilizing three closed multirefrigerant circuits and designed to
produce a final temperature of approximately -440.degree. F.
resulting from the evaporation of a refrigerant fraction which is
predominantly R704 (helium).
The high-temperature circuit 125 has three separator stages 126,
127 and 128, the intermediate-temperature circuit 129 has two
separate stages 130 and 131, and the low-temperature circuit 132
has three separator stages 133, 134 and 135. As is apparent from
the preceding description, a separator stage includes a
vapor-liquid separator having its inlet and vapor outlet connected
in the high-pressure line and its liquid outlet connected to the
low-pressure line, the stage operating so that the liquid fraction
from the separator is reduced in pressure and temperature and
evaporated to cool the vapor fraction from the separator.
The system of FIG. 4 is charged with the following refrigerant
mixtures:
High Intermediate Low Temperature Temperature Pressure Circuit 125
Circuit 129 Circuit 132 Refri- Boiling Refri- Boiling Refri-
Boiling gerants Point gerants Point gerants Point R21 -22.degree.
F. R 21 -22.degree. F. R21 -22.degree. F. R13B1 -72.degree. F. R728
-320.degree. F. R728 -320.degree. F. R14 -195.degree. F. R720
-406.degree. F. R720 -406.degree. F. R728 -320.degree. F. R704
-453.degree. F.
in the high-temperature circuit 125, the various refrigerants will
be progressively condensed, resulting in a refrigerant fraction
which is predominantly liquid R728 (nitrogen) at the end of
separator stage 128. This refrigerant is then evaporated at low
pressure in evaporator coil 136 in cascade heat exchanger 137.
In the intermediate-temperature circuit 129, refrigerant R21 with
any entrained oil therein will be separated out and returned to the
compressor in separator stage 130. The vapor fraction -- nitrogen
and neon -- will pass through cascade heat exchanger 137 and a
fraction thereof, predominantly nitrogen, will be condensed. The
condensed liquid fraction is separated out and used to condense the
remaining refrigerant which is predominantly neon. This refrigerant
is then evaporated at low pressure in evaporator coil 138 of
cascade heat exchanger 139.
In the low-temperature circuit 132, refrigerant R21 is again
liquefied to scrub out any entrained compressor oil and is
separated and returned to the compressor in separator stage 133.
The vapor fraction from this state, a mixture of nitrogen, neon and
helium vapors is passed through cascade heat exchanger 139 to
liquefy the nitrogen and some of the neon and to cool the
liquid-vapor mixture to a very low temperature. The liquid,
predominantly nitrogen, separates out in separation stage 134 and
is used to condense essentially all of the neon, which is then
separated out in separator stage 135 and used to condense the
remaining refrigerant, which is neon and helium. This refrigerant
is then reduced in pressure and temperature and evaporated in the
final evaporating coil 140. If it is desirable to operate above the
helium liquefaction point the final high pressure circuit may use a
Joule Thomson expansion device rather than a liquid expansion
valve.
FIG. 4 also shows the use of an intercircuit heat exchanger 141
connected to exchange heat between the high-pressure line of one
circuit, in this case high-pressure line 142 of low-temperature
circuit 132, and the low-pressure return line of another circuit,
in this case low-pressure line 143 of intermediate circuit 129.
Such heat interchange may be employed to balance compressor
operations and increase the efficiency of the system. For example,
if, without such heat exchanger, the compressor in the low-pressure
circuit 132 were operating at a higher pressure than the compressor
in circuit 132 were operating at a higher pressure than the
compressor in circuit 129, then the heat exchanger 141 could be
employed so that the refrigerant mixture in the low-pressure line
143 of circuit 129 would absorb heat from, and thereby cool, the
refrigerant mixture in the high-pressure line 142 of circuit 132.
To compensate for the additional heat being absorbed, the
compressor in circuit 129 would have to be operated at a higher
pressure, but, with the additional cooling provided, the compressor
in circuit 132 could then be run at a lower pressure.
Correspondingly, if the head pressure in circuit 129 were higher
than that in circuit 132, an intercircuit heat exchanger could be
employed whereby the refrigerant mixture in the low-pressure line
of circuit 132 cools the refrigerant mixture in the high-pressure
line of circuit 129. Similarly, such heat interchange can be
provided, if required, between circuits 132 and 125 or between 129
and 125. The exact location in the circuits of such heat
interchange is not critical except that is must be located at such
point wherein the loss or gain of heat by the refrigerants passing
therethrough will not be sufficient to interrupt normal operation
of the circuits. Also, such heat interchange must not be located
such that if a refrigerant mixture having entrained oil passes
therethrough, such mixture will not be lowered in temperature to a
point whereby the oil will freeze.
It is to be understood that the foregoing recitals of specific
refrigerants in connection with the circuits of FIGS. 2, 3 and 4 is
for the purpose of illustration. Other refrigerants may of course
be used and a wide variety of combinations will readily occur to
those persons skilled in the art by inspection of standard
refrigerant tables and charts.
The relative amounts of each refrigerant in a system is not
critical. However, changing relative quantities will affect final
temperatures. Sufficient amounts of each refrigerant, of course,
must be present to ensure an adequate flow of liquid from each
stage of the process when the system is in full operation. The
optimum weight ratio of refrigerants in any particular circuit will
depend upon their respective molecular weight, which influences
their individual partial pressures, their liquid densities and the
amount of liquid required at each stage of separation. In general,
the amount of lowest-boiling-point refrigerant in a circuit should
be maintained at the minimum necessary to achieve the required
refrigeration effect of the circuit, since greater amounts of the
lower-boiling-point refrigerant will tend to increase the necessary
head pressure of the circuit.
As set forth in the foregoing specification, although the present
invention is not so limited, it has a significant advantage in that
it enables standard single-stage oil-lubricated hermetic
compressors to be used in achieving very low temperatures. FIG. 5
illustrates schematically a typical compressor of this type. A
piston compressor unit 151 and the actuating motor 152 are both
disposed within a hermetically sealed housing 153. The incoming
refrigerant vapors from the return line of the refrigeration
circuit enter the housing through inlet 154 and flow across the
exterior of the motor and compressor unit to cool them before the
vapors enter the compressor unit. The vapors are then compressed
within the unit and leave through the housing outlet 155 to the
high-pressure line of the refrigeration circuit. Oil from the sump
formed by the lower part of the housing is drawn up into the motor
to lubricate the motor, the oil also being used to lubricate the
piston in the compression unit. Such compressors provide
trouble-free operation for years since the oil is not exposed to
air and cannot leak from the housing. Similarly, there is no
problem of refrigerant leakage from the unit since there are no
seals required in the compression unit to seal against escape of
refrigerant to atmosphere as would be necessary in a non-hermetic
unit. The contact of refrigerant vapor and oil in the compressor
will cause some entrainment of oil in the refrigerant vapor. A
semi-hermetic compressor operates in the same manner and differs
only in that portions of the housing are removable in the event
repair of the internal parts is required. Compressors of the type
described may be used for the compressors 13 and 22 of FIG. 1,
compressors 52 and 74 of FIG. 2, compressor 102 of FIG. 3, and the
compressors of FIG. 4.
* * * * *