U.S. patent application number 12/092443 was filed with the patent office on 2008-12-18 for dual temperature refrigeration circuit.
Invention is credited to Neelkanth S. Gupte.
Application Number | 20080307805 12/092443 |
Document ID | / |
Family ID | 36590237 |
Filed Date | 2008-12-18 |
United States Patent
Application |
20080307805 |
Kind Code |
A1 |
Gupte; Neelkanth S. |
December 18, 2008 |
Dual Temperature Refrigeration Circuit
Abstract
Combined medium and low temperature refrigeration circuit for
circulating a refrigerant in a predetermined flow direction
includes, in flow direction, a heat-rejecting heat exchanger, a
medium temperature refrigeration consumer, a medium pressure vapor
separator having a vapor portion and a liquid portion connected to
the medium temperature refrigeration consumer unit, a low
temperature refrigeration consumer connected to the liquid portion
of the medium pressure vapor separator, and a compressor unit
having an inlet connected to the vapor portion of the medium
pressure vapor separators and the low temperature refrigeration
consumer.
Inventors: |
Gupte; Neelkanth S.; (Katy,
TX) |
Correspondence
Address: |
CARLSON, GASKEY & OLDS, P.C.
400 WEST MAPLE ROAD, SUITE 350
BIRMINGHAM
MI
48009
US
|
Family ID: |
36590237 |
Appl. No.: |
12/092443 |
Filed: |
November 4, 2005 |
PCT Filed: |
November 4, 2005 |
PCT NO: |
PCT/US05/40047 |
371 Date: |
July 22, 2008 |
Current U.S.
Class: |
62/117 ; 62/225;
62/498; 62/510 |
Current CPC
Class: |
F25B 2400/075 20130101;
F25B 2400/13 20130101; F25B 5/04 20130101; F25B 2400/23 20130101;
F25B 2309/061 20130101; F25B 9/008 20130101; F25B 41/39 20210101;
F25B 1/10 20130101; F25B 2400/22 20130101 |
Class at
Publication: |
62/117 ; 62/510;
62/225; 62/498 |
International
Class: |
F25B 5/00 20060101
F25B005/00; F25B 1/10 20060101 F25B001/10; F25B 41/04 20060101
F25B041/04; F25B 1/00 20060101 F25B001/00 |
Claims
1-20. (canceled)
21. A combined medium and low temperature refrigeration circuit for
circulating a refrigerant in a predetermined flow direction
comprising, in flow direction: a heat rejecting heat exchanger; a
medium temperature refrigeration consumer; a medium pressure vapor
separator having a vapor portion and a liquid portion, the medium
pressure vapor separator being connected to the medium temperature
refrigeration consumer; a low temperature refrigeration consumer; a
compressor unit having an inlet connected to the vapor portion of
the medium pressure vapor separator and to the low temperature
refrigeration consumer, a medium pressure expansion device; and an
intermediate pressure vapor separator, with the medium pressure
expansion device connected to the liquid portion of the medium
pressure vapor separator, the liquid portion of the intermediate
pressure vapor separator connected to the low temperature
refrigeration consumer, and the vapor portion of the intermediate
pressure vapor separator connected to the compressor unit.
22. The refrigeration circuit according to claim 21, further
comprising a high pressure vapor separator having a vapor portion
and a liquid portion between the heat rejecting heat exchanger and
the medium temperature refrigeration consumer, wherein the vapor
portion is connected to the inlet of the compressor unit and the
liquid portion is connected to the medium temperature refrigeration
consumer.
23. The refrigeration circuit according to claim 22, further
comprising an expansion device between the heat rejecting heat
exchanger and a high temperature vapor separator.
24. The refrigeration circuit according to claim 22, further
comprising a pressure regulated valve in a line between the vapor
portion of the high pressure vapor separator and the compressor
unit.
25. The refrigeration circuit according to claim 22, further
comprising a refrigeration consumer comprising at least one
expansion device and at least one evaporator.
26. The refrigeration circuit according to claim 21, wherein the
compressor unit comprises a low pressure compressor set, a medium
pressure compressor set, and a high pressure vapor compressor, with
the low pressure compressor set being connected to the low
temperature refrigeration consumer, the medium pressure compressor
set being connected to the vapor portion of the medium pressure
vapor separator, and the high pressure vapor compressor being
connected to a vapor portion of a high pressure vapor
separator.
27. The refrigeration circuit according to claim 21, wherein the
compressor unit further comprises an intermediate compressor
between the vapor portion of the intermediate pressure vapor
separator and the medium pressure compressor set.
28. The refrigeration circuit according to claim 21, further
comprising a pressure regulating valve between the vapor portion of
the intermediate pressure vapor separator and the compressor
unit.
29. The refrigeration circuit according to claim 21, further
comprising a superheat sensor associated to an exit of the low
temperature refrigeration consumer and connected to a control for
securing superheat of the refrigerant.
30. The refrigeration circuit according to claim 21, wherein the
refrigerant is carbon dioxide.
31. A refrigeration apparatus comprising: a refrigeration circuit
including: a heat rejecting heat exchanger; a medium temperature
refrigeration consumer; a medium pressure vapor separator having a
vapor portion and a liquid portion, the medium pressure vapor
separator being connected to the medium temperature refrigeration
consumer; a low temperature refrigeration consumer; a compressor
unit having an inlet connected to the vapor portion of the medium
pressure vapor separator and to the low temperature refrigeration
consumer, a medium pressure expansion device; and an intermediate
pressure vapor separator, with the medium pressure expansion device
connected to the liquid portion of the medium pressure vapor
separator, the liquid portion of the intermediate pressure vapor
separator connected to the low temperature refrigeration consumer,
and the vapor portion of the intermediate pressure vapor separator
connected to the compressor unit.
32. A method for operating a combined medium and low temperature
refrigeration circuit for circulating a refrigerant in a
predetermined flow direction, the refrigeration circuit comprising,
in flow direction, a heat rejecting heat exchanger, a medium
temperature refrigeration consumer, a low temperature refrigeration
consumer, and a compressor unit, each thereof having an inlet and
an outlet, respectively, wherein the method comprises the steps of:
a) separating liquid medium pressure refrigerant leaving the outlet
of the medium temperature refrigeration consumer from gaseous
medium pressure refrigerant leaving the outlet of the medium
temperature refrigeration consumer; b) directing the gaseous medium
pressure refrigerant towards the inlet of the compressor unit; c)
expanding the liquid medium pressure refrigerant to an intermediate
pressure; d) separating liquid intermediate pressure refrigerant
from gaseous intermediate pressure refrigerant; e) directing the
gaseous intermediate pressure refrigerant towards the inlet of the
compressor unit; and f) directing the liquid intermediate pressure
refrigerant towards the low temperature refrigeration consumer.
33. The method according to claim 32, further comprising the steps
of: g) separating liquid high pressure refrigerant leaving the
outlet of the heat rejecting heat exchanger from gaseous high
pressure refrigerant leaving the same outlet; h) directing the
gaseous high pressure refrigerant towards the inlet of the
compressor unit; and i) directing the liquid high pressure
refrigerant towards the inlet of the medium temperature
refrigeration consumer.
34. The method according to claim 33, further comprising the steps
of expanding the high pressure refrigerant leaving the heat
rejecting heat exchanger before step g).
35. The method according to claim 33, further comprising the step
of regulating the pressure of the gaseous high pressure
refrigerant.
36. The method according to claim 32, further comprising the step
of regulating the pressure of the gaseous intermediate pressure
refrigerant.
37. The method according to claim 32, further comprising the step
controlling the superheat of gaseous low pressure refrigerant
entering the inlet of the compressor unit.
38. The method according to claim 32, wherein the compressor unit
comprises a low pressure compressor set, a medium pressure
compressor set, a high pressure vapor compressor, and an
intermediate compressor, and wherein gaseous refrigerant leaving
the low temperature refrigeration consumer is directed to the low
pressure compressor set, the gaseous medium pressure refrigerant is
directed to the medium pressure compressor set, gaseous high
pressure refrigerant is directed to the high pressure vapor
compressor, and the gaseous intermediate pressure refrigerant is
directed to the intermediate compressor.
Description
[0001] The present invention relates to a combined medium and low
temperature refrigeration circuit and particularly to a respective
CO.sub.2 refrigeration circuit and a corresponding method.
[0002] Such dual temperature refrigeration circuits are known to
circulate a refrigerant in a predetermined flow direction through a
heat-rejecting heat exchanger, a medium temperature refrigeration
consumer, a low temperature refrigeration consumer and a compressor
unit which returns the refrigerant pressure to the high pressure
present in the heat rejecting heat exchanger. Generally, the medium
and low temperature refrigeration consumers are arranged in
parallel in the circuit, i.e. the refrigerant leaving the
heat-rejecting heat exchanger is branched so that part thereof
flows through the medium temperature refrigeration consumer and the
remainder through the low temperature refrigeration consumer. The
compressor unit generally is a two stage compressor unit with a low
pressure compressor set and a medium pressure compressor set with
the low pressure compressor set being connected to the exit of the
low temperature refrigeration consumer and compresses the
refrigerant leaving the same up to a pressure level which
corresponds to the pressure level present at the outlet of the
medium temperature refrigeration consumer. Both the refrigerant
leaving the outlet of the medium temperature refrigeration consumer
as well as the refrigerant leaving the outlet of the low pressure
compressor are directed to the inlet of the medium pressure
compressor which compresses the refrigerant up to the required high
pressure as present in the heat rejecting heat exchanger. While
such dual temperature refrigeration circuit is satisfying the
refrigeration needs, there still is a requirement for improving the
efficiency.
[0003] Accordingly, it is an object of the present invention to
provide a dual temperature refrigeration circuit and a
corresponding method which satisfies the cooling requirement and
which is relatively simple and inexpensive, but of high
efficiency.
[0004] In accordance with the present invention, this problem is
solved by a combined medium and low temperature refrigeration
circuit for circulating a refrigerant in a predetermined flow
direction, comprising in flow direction [0005] (a) a heat-rejecting
heat exchanger; [0006] (b) a medium temperature refrigeration
consumer; [0007] (c) a medium pressure vapor separator having a
vapor portion and a liquid portion connected to the medium
temperature refrigeration consumer unit; [0008] (d) a low
temperature refrigeration consumer connected to the liquid portion
of the medium pressure vapor separator; and [0009] (e) a compressor
unit having an inlet connected to the vapor portion of the medium
pressure vapor separator and the low temperature refrigeration
consumer.
[0010] As compared to the prior art dual temperature refrigeration
circuits, the medium and low temperature refrigeration consumers
are arranged in line so that the refrigerant will initially be used
in the medium temperature refrigeration consumer and subsequently
the remaining liquid portion of the refrigerant will be used in the
low temperature refrigeration consumer. The medium temperature
refrigeration consumer may be arranged and/or operated so that the
exiting refrigerant is a two-phase refrigerant with part thereof
being liquid and the remainder gaseous. Such two-phase refrigerant
is separated in the medium pressure vapor separator with the
gaseous portion thereof being compressed and returned to the heat
rejecting heat exchanger and the liquid portion thereof being used
for the low temperature refrigeration consumer.
[0011] As compared to the conventional parallel arrangement of
medium and low temperature refrigerations consumers, the serial
arrangement directs substantially cooler liquid refrigerant but
with a reduced pressure towards the low temperature refrigeration
consumer. The refrigerant leaving the low temperature refrigeration
consumer will be compressed in the compressor unit either directly
to the pressure level as required for the heat rejecting heat
exchanger or in a first stage up to an intermediate pressure level,
for example the pressure level of the gaseous medium pressure
refrigerant, and in a second step up to the pressure level as
pre-sent in the heat rejecting heat exchanger. Accordingly, the
compressor unit may comprise a plurality of individual compressors,
it may also comprise one or two compressor sets, for example a low
pressure compressor set and a medium pressure compressor set. Each
of the low and medium compressor sets may also comprise one or a
plurality of individual compressors.
[0012] It is to be noted that due to the lower temperature and the
lower pressure as pre-sent in the serially connected low
temperature refrigeration consumer as compared to the parallel
arrangement, it is possible to substantially reduce the dimensions
of the low temperature refrigeration consumer, i.e. the reduced
pressure for example requires far less wall thickness, etc.
[0013] The refrigeration circuit may further comprise a high
pressure vapour separator having a vapor portion and a liquid
portion, which is arranged between the heat rejecting heat
exchanger and the medium temperature refrigeration consumer, and
having its vapor portion connected to the inlet of the compressor
unit and its liquid portion connected to the medium temperature
refrigeration consumer.
[0014] The high pressure vapor separator also allows to reduce the
temperature and pressure in advance of the medium temperature
refrigeration consumer. This increases the efficiency of the medium
temperature refrigeration consumer and allows smaller dimensions
thereof.
[0015] The refrigeration circuit may further comprise an
intermediate expansion device between the heat rejecting heat
exchanger and the high temperature vapor separator. By means of the
intermediate expansion device, a two-phase refrigerant can be
generated even if the refrigerant leaving the heat rejecting heat
exchanger is purely gaseous. The expansion device can be a
controllable expansion device in order to control the conditions in
the high pressure vapor separator, like temperature, pressure,
proportion of the liquid to gaseous refrigerant, etc.
[0016] The refrigeration circuit may further comprise a pressure
regulated valve in a line between the vapor portion of the high
pressure vapor separator and a compressor unit. For example the
proportion between the gaseous and liquid refrigerant in the high
pressure vapor separator can be controlled by means of such
pressure regulated valve. Another type of controllable valve may
also be provided for. It is possible to connect this controllable
valve as well as any other controllable element in the circuit to
an individual control or alternatively to the overall control of
the circuit.
[0017] A refrigeration consumer may comprise at least one expansion
device and at least one evaporator. The expansion device can be a
controllable expansion device for controlling the condition in the
evaporator and particularly the condition of the refrigerant at the
outlet of the evaporator. By controlling the expansion device, the
velocity of the refrigerant flow through the evaporator can be
controlled so that the refrigerant at the outlet can have any
condition between two-phase refrigerant and super heated
refrigerant. Also, the refrigeration performance and thus the
temperature next to the refrigeration consumer can be controlled
thereby.
[0018] The compressor unit may comprise a low pressure compressor
set, a medium pressure compressor set and a high pressure vapor
compressor. The low pressure compressor set can be connected to the
low temperature refrigeration consumer, the medium pressure
compressor set can be connected to the liquid portion of the medium
pressure vapor separator and the high pressure vapor compressor can
be connected to the vapor portion of the high pressure vapor
separator. The low pressure compressor set and the medium pressure
compressor set can form a two stage compressor with the outlet of
the low pressure compressor set being connected in the inlet of the
high pressure compressor set. While the high pressure compressor
set compresses gaseous refrigerant from the medium pressure to the
high pressure as present in the heat rejecting heat exchanger, the
pressure difference over the high pressure vapor compressor will
typically be substantially less. There may be one or a plurality of
high pressure vapor compressors. Typically, a single high pressure
vapor compressor will be sufficient. The high pressure vapor
compressor can be a controllable compressor.
[0019] The refrigeration circuit may further comprise a medium
pressure expansion device and an intermediate pressure vapor
separator with the medium pressure expansion device connected to
the liquid portion of the medium pressure vapor separator, the
liquid portion of the intermediate pressure vapor separator
connected to the low temperature refrigeration consumer, and the
vapor portion of the intermediate pressure vapor separator being
connected to the compressor unit. Such additional medium pressure
expansion device and a joint intermediate pressure vapor separator
may further reduce the temperature and the pressure of the
refrigerant before it will finally be directed to the low
temperature refrigeration consumer and the dimensions of the low
temperature refrigeration consumer can further be reduced. It is to
be noted that together with the reduction of the dimension of the
respective refrigeration consumer, the temperature and pressure
reduction as provided through the vapor-liquid separation also
allows substantially smaller dimensions for the conduits leading
towards the respective refrigeration consumer.
[0020] The compressor unit may further comprise an intermediate
compressor between the vapor portion of the intermediate pressure
vapor separator and the medium pressure compressor set. Similar to
the high pressure vapor compressor, the intermediate compressor is
compressing only over a reduced pressure difference as compared to
the low pressure compressor set. This applies particularly, if the
intermediate compressor operates merely between the intermediate
pressure and the medium pressure levels. Again, a plurality of
intermediate compressors can be provided. It is possible to use a
controllable intermediate compressor.
[0021] The refrigeration circuit may further comprise a pressure
regulating valve between the vapor portion of the intermediate
pressure vapor separator and the compressor unit and the
intermediate compressor, respectively.
[0022] The refrigeration circuit may further comprise a superheat
sensor associated to the exit of the low temperature refrigeration
consumer and connected to a control for securing superheat of the
refrigerant. Control can be a local superheat control which
controls the refrigeration consumer's expansion valve, etc. but can
also be the general refrigeration circuit control.
[0023] The refrigeration circuit may use a refrigerant which is
working also in a super critical condition, for example
CO.sub.2.
[0024] Another embodiment of the invention relates to a
refrigeration apparatus comprising a refrigeration circuit in
accordance with the embodiment of the present invention. The
refrigeration apparatus can be a refrigeration system for a
supermarket, and industrial refrigeration system, etc. In case of a
supermarket refrigeration system, the medium temperature
refrigeration consumer(s) can be display cabinet and the likes, for
example for milk products, meat, vegetables and fruits, with a
medium refrigeration level of less than 10.degree. C. down to
around 0.degree. C. The low temperature refrigeration consumer(s)
can be freezers with a refrigeration level of 20.degree. C. and
below.
[0025] Another embodiment of the present invention relates to a
method for operating a combined medium and low temperature
refrigeration circuit for circulating a refrigerant in a
predetermined flow direction, the refrigeration circuit comprising
in flow direction a heat-rejecting heat exchanger, a medium
temperature refrigeration consumer, a low temperature refrigeration
consumer and a compressor unit each thereof having an inlet and an
outlet, respectively, wherein the method comprises the following
steps: [0026] (a) separating the liquid medium pressure refrigerant
leaving the outlet of the low temperature refrigeration consumer
from the gaseous refrigerant leaving the same outlet; [0027] (b)
directing the gaseous medium pressure refrigerant towards the inlet
of the compressor unit; and [0028] (c) directing the liquid medium
pressure refrigerant towards the inlet of the low temperature
refrigeration consumer.
[0029] The method may further comprise the following steps: [0030]
(d) separating the liquid high pressure refrigerant leaving the
outlet of the heat-rejecting heat exchanger from the gaseous
refrigerant leaving the same outlet; [0031] (e) directing the
gaseous high pressure refrigerant towards the inlet of the
compressor unit; and [0032] (f) directing the liquid high pressure
refrigerant towards the inlet of the low temperature refrigeration
consumer.
[0033] The method may further comprise in advance of step (d) the
step of expanding the high pressure refrigerant leaving the heat
rejecting heat exchanger and preferably regulating the pressure of
the gaseous high pressure refrigerant.
[0034] The method may further comprise the following steps: [0035]
(g) expanding the liquid medium pressure refrigerant to an
intermediate pressure; [0036] (h) separating the liquid
intermediate pressure refrigerant from the gaseous intermediate
pressure refrigerant; [0037] (i) directing the gaseous intermediate
pressure refrigerant towards the inlet of the compressor unit; and
[0038] (j) directing the liquid gaseous intermediate pressure
refrigerant towards the low temperature refrigeration consumer, and
preferable regulating the pressure of the gaseous intermediate
pressure refrigerant; and preferably [0039] (k) controlling the
superheat of the gaseous low pressure refrigerant entering the
inlet of the compressor unit.
[0040] Embodiments of the present invention are described in
greater detail below with reference to the figures wherein
[0041] FIG. 1 is a combined medium and low temperature
refrigeration circuit in accordance with a first embodiment of the
present invention; and
[0042] FIG. 2 is a combined medium and low temperature
refrigeration circuit in accordance with a second embodiment.
[0043] FIG. 1 shows a combined medium and low temperature
refrigeration circuit 2 for circulating a refrigerant in a
predetermined flow direction as indicated by the arrows, comprising
in flow direction a heat rejecting heat exchanger 4, a plurality of
medium temperature refrigeration consumers 6, a medium pressure
vapor separator 8 having a vapor portion 10 and a liquid portion
12, a plurality of low temperature refrigeration consumers 14 as
well as a compressor unit 16. The compressor unit 16 comprises a
plurality of individual compressors 18 some of which are grouped
together to compressor sets, like a low pressure 2-stage compressor
set comprising a first stage low pressure compressor set 20 and a
second stage low pressure compressor set 22. There also is a medium
pressure compressor set 24, the input 26 thereof being connected to
the vapor portion 10 of the medium pressure vapor separator 8. The
compressors and compressor sets may all be located at the same
location, but may also be located at different places within the
circuit 2. The outlets 28 and 30 of the second stage low pressure
compressor set 22 and the medium pressure compressor set 24 are
connected with the inlet 32 of the heat rejecting heat exchanger 4.
The heat rejecting heat exchanger 4 can be a conventional condenser
in case of a conventional refrigerant and can be a gas-cooler in
case of a refrigerant which is at least partially operated in a
supercritical condition.
[0044] A high pressure vapor separator 34 having a vapor portion 36
and a liquid portion 38 is provided between the heat rejecting heat
exchanger 4 and the medium temperature refrigeration consumers 6.
Particularly, the vapor portion 36 of the high pressure vapor
separator 34 is connected with the outlet 40 of the heat rejecting
heat exchanger 4 via heat exchanger outlet line 42. An expansion
device 44 is located in the heat exchanger outlet line 42. The
liquid portion 38 of the high pressure vapor separator 34 is
connected to the medium temperature refrigeration consumers 6 by
means of a high pressure liquid line 46. The vapor portion 36 of
the high pressure vapor separator 34 is connected by means of the
high pressure vapor line 48 to a high pressure vapor compressor 50.
A pressure regulated valve 52 is arranged in the high pressure
vapor line 58. Each of the medium and low temperature refrigeration
consumers 6 and 14, respectively, may comprise at least one
expansion device 54 and 56, respectively, and at least one
evaporator 58 and 60, respectively. Of course, there may be a
single as well as a plurality of low and medium temperature
refrigeration consumers, respectively.
[0045] As mentioned above, the compressor unit 16 comprises a
plurality of compressor sets 20, 22 and 24 as well as individual
compressors 18, 50. The compressor unit 16 comprises a plurality of
inlets at different pressure levels with the inlets 62 to the first
stage compressor set 20 of the low pressure compressor set 20, 22
being at the lower most pressure level, the inlet 26 to the medium
pressure compressor set 24 being at a higher medium pressure level
and the inlet 64 to the high pressure vapor compressor 50 being as
compared therewith at the highest level.
[0046] Subsequent, the operation of a refrigeration circuit in
accordance with FIG. 1 will be described with reference to a
CO.sub.2 refrigeration circuit 2. In use, the pressure of the
refrigerant in the heat rejecting heat exchanger 4 can be up to 120
bar and is typically approximately 85 bar in "summer mode" and
approximately 45 bar in "winter mode". The refrigerant flowing
through the heat exchanger outlet line 42 is expanded in expansion
valve 44 which reduces the pressure to between approximately 30 and
40 bar and preferably 36 bar with such pressure being typically
independent from a "winter mode" and "summer mode".
[0047] The high pressure receiver or high pressure vapor separator
34 collects and separates liquid and gaseous refrigerant in the
liquid and vapor portions 38 and 36, respectively. The high
pressure liquid line 46 directs the liquid refrigerant from the
liquid portion 38 to the expansion devices 54 of the medium
temperature refrigeration consumers 6.
[0048] The medium temperature refrigeration consumers 6 cool down
to approximately 1 to 10.degree. C. They can be arranged or
controlled so that there is a two-phase refrigerant present at the
outlet thereof. Such two-phase refrigerant is fed to the medium
pressure vapor separator 8 where it is collected and separated in a
vapor portion 10 and liquid portion 12, respectively. Gaseous
refrigerant from the vapor portion 10 is directed to the inlet 26
of the medium pressure compressor 24. The input pressure is
typically between 20 and 30 bar and approximately 26 bar, which
results in a temperature of the refrigerant of approximately
-10.degree. C. in the medium temperature refrigeration consumers 6.
A high pressure line 66 returns the compressed, hot, gaseous
refrigerant to the heat rejecting heat exchange 4. Similar, the
gaseous refrigerant from the vapor portion 36 of the high pressure
vapor separator 34 is directed through high pressure vapor line 48
and pressure controlled valve 52 to the inlet 64 of the high
pressure vapor compressor 50 and returned to high pressure line 66
to the heat rejecting heat exchanger 4.
[0049] The liquid refrigerant from the liquid portion 12 of the
medium temperature vapor separator 8 is directed to and through the
low temperature refrigeration consumers 14. The low temperature
refrigeration consumers 14 are arranged or controlled so as to
provide super heated gaseous refrigerant only to the inlet 68 of
the first stage low temperature compressor set 20. A superheat
sensor (not shown) can be associated to the exit of the low
temperature refrigeration consumers 14 or the inlet 68 to the first
stage low temperature compressor set 20 in order to ensure that no
liquid refrigerant may enter the first stage low temperature
compressor set 20. Additionally and/or alternatively, an internal
heat exchanger (not shown) can be provided for between the low
temperature refrigeration consumers 14 and the inlet 68.
[0050] The pressure at the inlet 68 is typically between 8 and 20
bar and preferably approximately 12 bar which results in a
temperature of the refrigerant of approximately -37.degree. C. in
the low temperature refrigeration consumers 14.
[0051] Again, the gaseous refrigerant is returned through the low
pressure compressor 26 and 22, respectively and the high pressure
line 66 to the heat rejecting heat exchanger 4.
[0052] The embodiment of FIG. 2 corresponds substantially to the
embodiment of FIG. 1. Accordingly, corresponding elements are
indicated with corresponding reference numbers. The main difference
between the two embodiments results in a different routing of the
liquid refrigerant leaving the liquid portion 12 of the medium
temperature vapor separator 8 as compared to FIG. 1. Particularly,
a medium pressure expansion device 70 and an intermediate pressure
vapor separator 72 having a vapor portion 74 and a liquid portion
76 are additionally arranged subsequent to the medium temperature
vapor separator 8. Moreover, a pressure regulated intermediate
valve 78 as well as an intermediate compressor 80 connect the vapor
portion 74 of the intermediate pressure vapor separator 72 to the
intermediate pressure level between first stage low pressure
compressor set 20 and second stage low pressure compressor set 22.
Again, the intermediate compressor 80 can be a single compressor or
a plurality of compressors and may further be a controllable
compressor. The pressure difference over intermediate compressor 80
is substantially less than the pressure difference over the first
stage low pressure compressor set 20. The pressures and
temperatures are by and large the same as with the embodiment of
FIG. 1 and for the intermediate vapor separator 72 the saturation
temperature is approximately half way between the low temperature
and medium temperature evaporator temperatures.
[0053] The pressure regulated valves 52 and 78 allow to hold or
control the back pressure feeding into the expansion valves for the
medium and low temperature cases.
* * * * *