U.S. patent application number 14/372396 was filed with the patent office on 2014-12-04 for cascade refrigeration system.
The applicant listed for this patent is ARKEMA FRANCE. Invention is credited to Wissam Rached.
Application Number | 20140352336 14/372396 |
Document ID | / |
Family ID | 47628367 |
Filed Date | 2014-12-04 |
United States Patent
Application |
20140352336 |
Kind Code |
A1 |
Rached; Wissam |
December 4, 2014 |
CASCADE REFRIGERATION SYSTEM
Abstract
A method for cooling a fluid or a body by means of at least a
first vapour compression circuit containing a first heat transfer
fluid and at least a second vapour compression circuit containing a
second heat transfer fluid, the method including: measuring the
temperature of the external surroundings; and setting the
temperature of the second heat-transfer fluid to evaporation,
according to the temperature of the external surroundings. Also, an
installation suited to implementing this method.
Inventors: |
Rached; Wissam; (Chaponost,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ARKEMA FRANCE |
Colombes Cedex |
|
FR |
|
|
Family ID: |
47628367 |
Appl. No.: |
14/372396 |
Filed: |
January 8, 2013 |
PCT Filed: |
January 8, 2013 |
PCT NO: |
PCT/FR2013/050034 |
371 Date: |
July 15, 2014 |
Current U.S.
Class: |
62/115 ;
62/190 |
Current CPC
Class: |
F25D 2500/04 20130101;
F25B 2700/2106 20130101; F25B 7/00 20130101; F25B 2400/121
20130101; F25B 49/02 20130101 |
Class at
Publication: |
62/115 ;
62/190 |
International
Class: |
F25B 7/00 20060101
F25B007/00; F25B 49/02 20060101 F25B049/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2012 |
FR |
12.50746 |
Claims
1. A process for cooling a fluid or a body by means of at least one
first vapor compression circuit comprising a first heat-transfer
fluid and of at least one second vapor compression circuit
comprising a second heat-transfer fluid, the process comprising: in
the first vapor compression circuit: at least partial evaporation
of the first heat-transfer fluid by exchange of heat with said
fluid or body; compression of the first heat-transfer fluid; at
least partial condensation of the first heat-transfer fluid by
exchange of heat with the second heat-transfer fluid; reduction in
pressure of the first heat-transfer fluid; in the second vapor
compression circuit: at least partial evaporation of the second
heat-transfer fluid by exchange of heat with the first
heat-transfer fluid; compression of the second heat-transfer fluid;
at least partial condensation of the second heat-transfer fluid by
exchange of heat with an external medium; reduction in pressure of
the second heat-transfer fluid; the process additionally
comprising: measurement of the temperature of the external medium;
and adjustment of the temperature of the second heat-transfer fluid
at the evaporation, as a function of the temperature of the
external medium.
2. The process as claimed in claim 1, in which the adjustment of
the temperature of the second heat-transfer fluid at the
evaporation is carried out continuously or is carried out at least
once per hour.
3. The process as claimed in claim 1, comprising the detection of
variations in the temperature of the external medium and in which
the adjustment of the temperature of the second heat transfer fluid
at the evaporation comprises an increase in the temperature of the
second heat-transfer fluid at the evaporation if an increase in the
temperature of the external medium is detected and a decrease in
the temperature of the second heat-transfer fluid at the
evaporation if a decrease in the temperature of the external medium
is detected.
4. The process as clamed in claim 1, comprising the calculation of
an optimum evaporation temperature as a function of the measurement
of the temperature of the external medium.
5. The process as claimed in claim 4, in which the temperature of
the second heat-transfer fluid at the evaporation is adjusted to
the optimum evaporation temperature.
6. The process as claimed in claim 4, in which the optimum
evaporation temperature corresponds to the evaporation temperature
for which the overall coefficient of performance of the first vapor
compression circuit and of the second vapor compression circuit is
at a maximum.
7. The process as claimed in claim 4, in which the optimum
evaporation temperature is defined by the formula T.sub.opt=25
A.times.T.sub.ext+B, in which T.sub.ext is the temperature of the
external medium in degrees Celsius, A is a dimensionless constant
and B is a constant in degrees Celsius.
8. The process as claimed in claim 7, in which the constant A has a
value from 0.3 to 0.6 and the constant B has a value from
-50.degree. C. to 0.degree. C.
9. The process as claimed in claim 1, in which the fluid or body is
cooled to a temperature of -50 to -15.degree. C.
10. The process as claimed in claim 1, in which: the first
heat-transfer fluid is chosen from carbon dioxide, hydrocarbons,
hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and
mixtures thereof; and/or the second heat-transfer fluid is chosen
from ammonia, hydrocarbons, hydrofluorocarbons, ethers,
hydrofluoroethers, fluoroolefins and mixtures thereof.
11. The process as claimed in claim 1, in which the compression of
the second heat-transfer fluid is carried out by one or more
compressors and the adjusting of the temperature of the second
heat-transfer fluid at the evaporation is carried out by regulating
said compressors.
12. The process as claimed in claim 11, in which the regulating of
said compressors comprises an adjustment of the speed of rotation
of the compressors or is carried out by successively starting up
and shutting down the compressors.
13. The process as claimed in claim 1, which is a process for
cooling a compartment comprising foods, which are deep-frozen or
frozen.
14. An installation for cooling a fluid or a body, comprising at
least: a first vapor compression circuit comprising a first heat
transfer fluid; a second vapor compression circuit comprising a
second heat-transfer fluid; a cascade heat exchanger, configured
for exchanging heat between the first heat-transfer fluid and the
second heat transfer fluid; the first vapor compression circuit
comprising: a first evaporator configured for exchanging heat
between the first heat-transfer fluid and said fluid or body; one
or more first compressors; a first expansion device; the second
vapor compression circuit comprising: one or more second
compressors; a second condenser configured for exchanging heat
between the second heat-transfer fluid and an external medium; a
second expansion device; the installation also comprising: a device
for measuring the temperature of the external medium; and means for
adjusting the evaporation temperature in the cascade heat
exchanger, as a function of the measurement of the temperature of
the external medium.
15. The installation as claimed in claim 14, additionally
comprising a module for calculating an optimum evaporation
temperature as a function of the measurement of the temperature of
the external medium.
16. The installation as claimed in claim 15, in which the means for
adjusting the evaporation temperature in the cascade heat exchanger
are configured for adjusting the evaporation temperature in the
cascade heat exchanger to the optimum evaporation temperature.
17. The installation as claimed in claim 15, in which the optimum
evaporation temperature corresponds to the evaporation temperature
for which the overall coefficient of performance of the first vapor
compression circuit and of the second vapor compression circuit is
at a maximum.
18. The installation as claimed in claim 15, in which the optimum
evaporation temperature is defined by the formula
T.sub.opt=A.times.T.sub.ext+B, in which T.sub.ext is the
temperature of the external medium in degrees Celsius, A is a
dimensionless constant and B is a constant in degrees Celsius.
19. The installation as claimed in claim 18, in which the constant
A has a value from 0.3 to 0.6 and the constant B has a value from
-50.degree. C. to 0.degree. C.
20. The installation as claimed in claim 14, configured for cooling
the body or the fluid to a temperature of -50 to -15.degree. C.
21. The installation as claimed in claim 14, in which: the first
heat-transfer fluid is chosen from carbon dioxide, hydrocarbons,
hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and
mixtures thereof; and/or the second heat-transfer fluid is chosen
from ammonia, hydrocarbons, hydrofluorocarbons, ethers,
hydrofluoroethers, fluoroolefins and mixtures thereof.
22. The installation as claimed in claim 14, in which the means for
adjusting the evaporation temperature in the cascade heat exchanger
comprise means for regulating the second compressors.
23. The installation as claimed in claim 22, in which the means for
regulating the second compressors are configured for adjusting the
speed of rotation of the second compressors or are configured for
successively 30 starting up and shutting down the second
compressors.
24. The installation as claimed in claim 14, comprising a
compartment configured for receiving foods, which are deep-frozen
or frozen.
25. The process as claimed in claim 7, in which the constant A has
a value from 0.4 to 0.45 and the constant B has a value from
-30.degree. C. to -20.degree. C.
26. The process as claimed in claim 1, in which the fluid or body
is cooled to a temperature of -40 to -25.degree. C.
27. The process as claimed in claim 1, in which: the first
heat-transfer fluid is carbon dioxide; and/or the second
heat-transfer fluid is a tetrafluoropropene.
28. The process as claimed in claim 27, in which: the
tetrafluoropropene is 2,3,3,3-tetrafluoropropene or
1,3,3,3-tetrafluoropropene.
29. The installation as claimed in claim 18, in which the constant
A has a value from 0.4 to 0.45 and the constant B has a value from
-30.degree. C. to -20.degree. C.
30. The installation as claimed in claim 14, in which the fluid or
body is cooled to a temperature of -40 to -25.degree. C.
31. The installation as claimed in claim 14, in which: the first
heat-transfer fluid is carbon dioxide; and/or the second
heat-transfer fluid is a tetrafluoropropene.
32. The installation as claimed in claim 32, in which: the
tetrafluoropropene is 2,3,3,3-tetrafluoropropene or
1,3,3,3-tetrafluoropropene.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a U.S. national stage application
of International Application No. PCT/FR2013/050034, filed on Jan.
8, 2013, which claims the benefit of French Application No.
12.50746, filed on Jan. 26, 2012. The entire contents of each of
International Application No. PCT/FR2013/050034 and French
Application No. 12.50746 are hereby incorporated herein by
reference in their entirety.
TECHNICAL FIELD
[0002] The disclosure relates to a cascade refrigeration system
designed to operate optimally and to a refrigeration process
carried out in this system.
TECHNICAL BACKGROUND
[0003] Refrigeration systems are generally based on a thermodynamic
cycle comprising the vaporization of a fluid at low pressure (in
which the fluid absorbs heat); the compression of the vaporized
fluid up to a high pressure; the condensation of the vaporized
fluid to give a liquid at high pressure (in which the fluid
discharges heat); and the reduction in pressure of the fluid in
order to complete the cycle. The choice of a heat-transfer fluid
(which can be a pure compound or a mixture of compounds) is
dictated, on the one hand, by the thermodynamic properties of the
fluid and, on the other hand, by additional constraints. Thus, an
important criterion is that of the impact of the fluid under
consideration on the environment. In particular, chlorinated
compounds (chlorofluorocarbons and hydrochlorofluorocarbons)
exhibit the disadvantage of damaging the ozone layer. Thus,
nonchlorinated compounds, such as hydrofluorocarbons, fluoroethers
and fluoroolefins, are from now on generally preferred to them.
[0004] Another environmental constraint is that of the global
warming potential (GWP). It is thus essential to develop
heat-transfer compositions exhibiting a GWP which is as low as
possible and good energy performances.
[0005] Some specific refrigeration systems are based on the use of
several refrigeration circuits and in particular on two circuits
coupled together, namely a high-temperature circuit and a
low-temperature circuit: these systems are said to be "cascade"
systems. The two circuits generally comprise different
heat-transfer fluids.
[0006] A cascade system exhibits a number of advantages in terms of
safety. In particular, it is possible to use, for reasons of cost
or performance, a certain heat-transfer fluid in the
high-temperature circuit and to use another heat-transfer fluid,
which is less flammable or less toxic, in the low-temperature
circuit. Thus, the total charge of the most flammable or most toxic
heat-transfer fluid is minimized and this most flammable or most
toxic heat-transfer fluid is restricted to an unconfined region
and/or to a region without risk of contact with the public or
personnel in the event of escape.
[0007] For example, carbon dioxide is a highly advantageous
heat-transfer fluid due to its nonflammability, as well as from the
environmental viewpoint. However, due to its low critical point, it
is generally less effective than a conventional heat-transfer fluid
(hydrocarbon, hydrofluorocarbon, and the like). An optimal solution
may consist in using a cascade system comprising carbon dioxide in
the low-temperature circuit and a conventional heat-transfer fluid
in the high-temperature circuit.
[0008] The documents WO 2008/150289 and WO 2011/056824 provide
examples of cascade refrigeration systems.
[0009] The paper Theoretical analysis of a CO.sub.2-NH.sub.3
cascade refrigeration system for cooling applications at low
temperatures, by Dopazo et al. in Applied Thermal Engineering, 29,
1577-1583 (2009), and also the paper Experimental investigation on
the performances of NH.sub.3/CO.sub.2 cascade refrigeration system
with twin-screw compressor, by Bingming et al. in International
Journal of Refrigeration, 32, 1358-1365 (2009), describe the
performance of a cascade system using carbon dioxide in the
low-temperature circuit and ammonia in the high-temperature
circuit.
[0010] However, a need still exists to improve the efficiency and
the performance of cascade refrigeration systems and in particular
a need still exists to minimize the overall energy consumption of
these systems and also the associated environmental impact.
SUMMARY OF THE INVENTION
[0011] The disclosure relates first to a process for cooling a
fluid or a body by means of at least one first vapor compression
circuit comprising a first heat-transfer fluid and of at least one
second vapor compression circuit comprising a second heat-transfer
fluid, the process comprising: [0012] in the first vapor
compression circuit: [0013] the at least partial evaporation of the
first heat-transfer fluid by exchange of heat with said fluid or
body; [0014] the compression of the first heat-transfer fluid;
[0015] the at least partial condensation of the first heat-transfer
fluid by exchange of heat with the second heat-transfer fluid;
[0016] the reduction in pressure of the first heat-transfer fluid;
[0017] in the second vapor compression circuit: [0018] the at least
partial evaporation of the second heat-transfer fluid by exchange
of heat with the first heat-transfer fluid; [0019] the compression
of the second heat-transfer fluid; [0020] the at least partial
condensation of the second heat-transfer fluid by exchange of heat
with an external medium; [0021] the reduction in pressure of the
second heat-transfer fluid;
[0022] the process additionally comprising: [0023] the measurement
of the temperature of the external medium; and [0024] the
adjustment of the temperature of the second heat-transfer fluid at
the evaporation, as a function of the temperature of the external
medium.
[0025] According to an embodiment, the adjustment of the
temperature of the second heat-transfer fluid at the evaporation is
carried out continuously or is carried out at least once per
hour.
[0026] According to one embodiment, the process comprises the
detection of variations in the temperature of the external medium
and the adjustment of the temperature of the second heat-transfer
fluid at the evaporation comprises an increase in the temperature
of the second heat-transfer fluid at the evaporation if an increase
in the temperature of the external medium is detected and a
decrease in the temperature of the second heat-transfer fluid at
the evaporation if a decrease in the temperature of the external
medium is detected.
[0027] According to one embodiment, the process comprises the
calculation of an optimum evaporation temperature as a function of
the measurement of the temperature of the external medium.
[0028] According to one embodiment, the temperature of the second
heat-transfer fluid at the evaporation is adjusted to the optimum
evaporation temperature.
[0029] According to one embodiment, the optimum evaporation
temperature corresponds to the evaporation temperature for which
the overall coefficient of performance of the first vapor
compression circuit and of the second vapor compression circuit is
at a maximum.
[0030] According to one embodiment, the optimum evaporation
temperature is defined by the formula
T.sub.opt=A.times.T.sub.ext+B, in which T.sub.ext is the
temperature of the external medium in degrees Celsius, A is a
dimensionless constant and B is a constant in degrees Celsius.
[0031] According to one embodiment, the constant A has a value from
0.3 to 0.6, preferably from 0.4 to 0.45, and the constant B has a
value from -50.degree. C. to 0.degree. C., preferably from
-30.degree. C. to -20.degree. C.
[0032] According to one embodiment, the fluid or body is cooled to
a temperature of -50 to -15.degree. C., preferably of -40 to
-25.degree. C.
[0033] According to one embodiment: [0034] the first heat-transfer
fluid is chosen from carbon dioxide, hydrocarbons,
hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and
the mixtures of these, and is preferably carbon dioxide; and/or
[0035] the second heat-transfer fluid is chosen from ammonia,
hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers,
fluoroolefins and the mixtures of these, is preferably
tetrafluoropropene and more particularly preferably is
2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene.
[0036] According to one embodiment, the compression of the second
heat-transfer fluid is carried out by one or more compressors and
the adjusting of the temperature of the second heat-transfer fluid
at the evaporation is carried out by regulating said
compressors.
[0037] According to one embodiment, the regulating of said
compressors comprises an adjustment of the speed of rotation of the
compressors or is carried out by successively starting up and
shutting down the compressors.
[0038] According to one embodiment, the process is a process for
cooling a compartment comprising products, preferably foods, which
are deep-frozen or frozen.
[0039] The disclosure furthermore relates to an installation for
cooling a fluid or a body, comprising at least: [0040] a first
vapor compression circuit comprising a first heat-transfer fluid;
[0041] a second vapor compression circuit comprising a second
heat-transfer fluid; [0042] a cascade heat exchanger, appropriate
for exchanging heat between the first heat-transfer fluid and the
second heat-transfer fluid;
[0043] the first vapor compression circuit comprising: [0044] a
first evaporator appropriate for exchanging heat between the first
heat-transfer fluid and said fluid or body; [0045] one or more
first compressors; [0046] a first expansion device;
[0047] the second vapor compression circuit comprising: [0048] one
or more second compressors; [0049] a second condenser appropriate
for exchanging heat between the second heat-transfer fluid and an
external medium; [0050] a second expansion device;
[0051] the installation also comprising: [0052] a device for
measuring the temperature of the external medium; and [0053] means
for adjusting the evaporation temperature in the cascade heat
exchanger, as a function of the measurement of the temperature of
the external medium.
[0054] According to one embodiment, the installation additionally
comprises a module for calculating an optimum evaporation
temperature as a function of the measurement of the temperature of
the external medium.
[0055] According to one embodiment, the means for adjusting the
evaporation temperature in the cascade heat exchanger are
appropriate for adjusting the evaporation temperature in the
cascade heat exchanger to the optimum evaporation temperature.
[0056] According to one embodiment, the optimum evaporation
temperature corresponds to the evaporation temperature for which
the overall coefficient of performance of the first vapor
compression circuit and of the second vapor compression circuit is
at a maximum.
[0057] According to one embodiment, the optimum evaporation
temperature is defined by the formula
T.sub.opt=A.times.T.sub.ext+B, in which T.sub.ext is the
temperature of the external medium in degrees Celsius, A is a
dimensionless constant and B is a constant in degrees Celsius.
[0058] According to one embodiment, the constant A has a value from
0.3 to 0.6, preferably from 0.4 to 0.45, and the constant B has a
value from -50.degree. C. to 0.degree. C., preferably from
-30.degree. C. to -20.degree. C.
[0059] According to one embodiment, the installation is appropriate
for cooling the body or the fluid to a temperature of -50 to
-15.degree. C., preferably of -40 to -25.degree. C.
[0060] According to one embodiment: [0061] the first heat-transfer
fluid is chosen from carbon dioxide, hydrocarbons,
hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and
the mixtures of these, and is preferably carbon dioxide; and/or
[0062] the second heat-transfer fluid is chosen from ammonia,
hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers,
fluoroolefins and the mixtures of these, is preferably
tetrafluoropropene and more particularly preferably is
2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene.
[0063] According to one embodiment, the means for adjusting the
evaporation temperature in the cascade heat exchanger comprise
means for regulating the second compressors.
[0064] According to one embodiment, the means for regulating the
second compressors are appropriate for adjusting the speed of
rotation of the second compressors or are appropriate for
successively starting up and shutting down the second
compressors.
[0065] According to one embodiment, the installation comprises a
compartment appropriate for receiving products, preferably foods,
which are deep-frozen or frozen.
[0066] The disclosure makes it possible to meet the needs felt in
the state of the art. More particularly, it provides refrigeration
processes and corresponding installations in which the overall
energy consumption and the environmental impact are minimized.
[0067] This is accomplished by adjusting the evaporation
temperature of the heat-transfer fluid of the high-temperature
circuit as a function of the external temperature (ambient
temperature). It has been discovered that such an adjustment makes
it possible to optimize the overall performance of the system.
BRIEF DESCRIPTION OF THE FIGURES
[0068] FIG. 1 is a diagram of an installation according to an
embodiment of the disclosure.
[0069] FIG. 2 is a graph representing: (1) the change in ambient
temperature during a typical day taken as an example (white
circles, left-hand axis of the ordinates, values in degrees
Celsius); and (2) an example of conventional change in the
refrigerating capacity necessary for the preservation of
deep-frozen foods during this typical day (black squares,
right-hand axis of the ordinates, values in kW); this being as a
function of the hours of the day (axis of the abscissae).
[0070] FIG. 3 is a graph illustrating the optimum evaporation
temperature (in degrees Celsius, axis of the ordinates) as a
function of ambient temperature (in degrees Celsius, axis of the
abscissae) for a cascade refrigeration system in which the
heat-transfer fluid of the high-temperature circuit is: (1)
HFO-1234yf (white squares); or (2) HFO-1234ze (black circles).
[0071] FIG. 4 is a graph illustrating the total energy consumption
of a refrigeration system during a typical day, in kWh, according
to whether the refrigeration system is according to an embodiment
of the disclosure (gray bars, evaporation temperature of the
high-temperature heat-transfer fluid adjusted as a function of
ambient temperature) or is a conventional system (black bars,
evaporation temperature of the high-temperature heat-transfer fluid
set at -10.degree. C.). The two series of data correspond to the
case where (1) the heat-transfer fluid of the high-temperature
circuit is HFO-1234yf, and (2) the heat-transfer fluid of the
high-temperature circuit is HFO-1234ze.
[0072] FIG. 5 is a graph illustrating the TEWI index of a cascade
refrigeration system over a typical day in various scenarios:
conventional refrigeration system and HFO-1234yf in the
high-temperature circuit (R1234yf bar); refrigeration system
according to an embodiment of the disclosure and HFO-1234yf in the
high-temperature circuit (opti R1234yf bar); conventional
refrigeration system and HFO-1234ze in the high-temperature circuit
(R1234ze bar); refrigeration system according to an embodiment of
the disclosure and HFO-1234ze in the high-temperature circuit (opti
R1234ze bar). The values correspond to the percentage of TEWI index
with respect to the reference situation (conventional refrigeration
system and HFO-1234yf in the high-temperature circuit). The
conventional system is a system in which the evaporation
temperature of the high-temperature heat-transfer fluid is set at
-10.degree. C., and the system according to an embodiment of the
disclosure is a system in which the evaporation temperature of the
high-temperature heat-transfer fluid is adjusted as a function of
ambient temperature.
[0073] FIG. 6 is a graph equivalent to that of FIG. 4, but with a
conventional system where the evaporation temperature of the
high-temperature heat-transfer fluid is set at -18.degree. C.
[0074] FIG. 7 is a graph equivalent to that of FIG. 5, but with a
conventional system where the evaporation temperature of the
high-temperature heat-transfer fluid is set at -18.degree. C.
DESCRIPTION OF EMBODIMENTS
[0075] The disclosure is now described in more detail and without
implied limitation in the description which follows.
[0076] The term "heat-transfer compound" or "heat-transfer fluid"
(or refrigerant) respectively is understood to mean a compound or a
fluid respectively capable of absorbing heat by evaporating at low
temperature and low pressure and of discharging heat by condensing
at high temperature and high pressure, in a vapor compression
circuit. Generally, a heat-transfer fluid can comprise just one,
two, three or more than three heat-transfer compounds.
[0077] The term "heat-transfer composition" is understood to mean a
composition comprising a heat-transfer fluid and optionally one or
more additives which are not heat-transfer compounds for the
application envisaged.
[0078] The disclosure is targeted at installations for cooling of
fluid or a body, and at associated cooling processes. These
installations can be stationary or mobile air conditioning
installations or, preferably, stationary or mobile refrigeration
and/or freezing and/or cryogenic installations.
[0079] With reference to FIG. 1, according to one embodiment, the
installation according to an embodiment of the disclosure comprises
a first vapor compression circuit 10 (or low-temperature circuit),
which comprises a first heat-transfer fluid, and a second vapor
compression circuit 20 (or high-temperature circuit), which
comprises a second heat-transfer fluid. A cascade heat exchanger 30
(or evaporator-condenser or refrigerant-to-refrigerant heat
exchanger) provides the thermal coupling between the two vapor
compression circuits.
[0080] The first vapor compression circuit 10 comprises at least
one first evaporator 11, at least one first compressor 12 and at
least one first expansion device 14. Between the first compressor
12 and the first expansion device 14, the circuit passes through
the cascade heat exchanger 30, which acts as condenser for this
first circuit (first condenser).
[0081] Fluid transportation lines are provided between all the
components of the circuit.
[0082] The vapor compression circuit 10 operates according to a
conventional vapor compression cycle. The cycle comprises a change
in state of the first heat-transfer fluid from a liquid phase (or
liquid/vapor two-phase system) to a vapor phase at a relatively low
pressure (in the first evaporator 11), then the compression of the
fluid in the vapor phase up to a relatively high pressure (in the
first compressor 12), the change in state (condensation) of the
heat-transfer fluid from the vapor phase to the liquid phase at a
relatively high pressure (in the cascade heat exchanger 30), and
the reduction in the pressure in order to recommence the cycle (in
the first expansion device 14).
[0083] The second vapor compression circuit 20 comprises at least
one second compressor 22a, 22b, 22c, at least one second condenser
23 and at least one second expansion device 24.
[0084] Between the second expansion device 24 and the second
compressor 22a, 22b, 22c, the circuit passes through the cascade
heat exchanger 30, which acts as evaporator for this second circuit
(second evaporator).
[0085] Fluid transportation lines are provided between all the
components of the circuit.
[0086] The second vapor compression system 20 operates analogously
to the first.
[0087] It is possible to provide an accumulator 27 in the circuit
in order to form a reserve of fluid in the liquid state. The level
of the liquid in the accumulator varies according to the
requirement of the installation as a function of the conditions of
use.
[0088] The first heat-transfer fluid receives heat from the part of
the fluid or body to be cooled in the first evaporator 11. For
example, when the body to be cooled consists of one or more frozen
or deep-frozen products (in particular foodstuffs), this body can
be placed in a compartment, at least a portion of the walls of
which is in direct contact with the first evaporator 11 (or at
least a part of the walls of which belongs to the first evaporator
11).
[0089] Alternatively, the exchange of heat between the fluid or
body to be cooled and the first heat-transfer fluid can be carried
out via an auxiliary circuit comprising a heat-exchange fluid, such
as air or else a glycol compound, for example (with or without
change in state).
[0090] The first heat-transfer fluid gives up, in turn, heat to the
second heat-transfer fluid, in the cascade heat exchanger 30 which
provides the coupling between the two circuits. The transfer of
heat from the first heat-transfer fluid to the second heat-transfer
fluid brings about, on the one hand, the condensation of the first
heat-transfer fluid and, on the other hand, the evaporation of the
second heat-transfer fluid.
[0091] Finally, the second condenser 23 allows the second
heat-transfer fluid to give up heat to the external medium. The
external medium is preferably the surrounding air.
[0092] The exchange of heat between the second heat-transfer fluid
and the external medium can be carried out either directly or via
an auxiliary circuit of heat-exchange fluid (with or without change
in state).
[0093] Use may in particular be made, as compressors, in the
abovementioned circuits, of single-stage or multistage centrifugal
compressors or of centrifugal minicompressors. Rotary, piston or
screw compressors can also be used. The compressors can be driven
by an electric motor or by a gas turbine (for example fed by the
exhaust gases from a vehicle, for mobile applications) or by
gears.
[0094] Use may be made, as heat exchangers for the implementation
of the disclosure, of cocurrentwise heat exchangers or, preferably,
countercurrentwise heat exchangers. It is also possible to use
microchannel exchangers.
[0095] Each item of equipment (condenser, expansion device,
evaporator, compressor) can consist of one unit or of several units
arranged in series and/or in parallel. When several units in
parallel are used, as is the case for the second compressors 22a,
22b, 22c in FIG. 1, a distributor 25 and a collector 26 are
provided, if necessary, in order to distribute the fluid into the
various units and to collect the fluid resulting from the various
units.
[0096] It is also possible to provide several first
vapor-compression (low temperature) circuits coupled to a single
second vapor compression (high temperature) circuit or also several
second vapor compression (high temperature) circuits coupled to a
single first vapor compression (low temperature) circuit.
[0097] The first heat-transfer fluid is preferably chosen from
carbon dioxide, hydrocarbons, hydrofluorocarbons, ethers,
hydrofluoroethers, fluoroolefins and the mixtures of these. It can
in particular be carbon dioxide.
[0098] The second heat-transfer fluid is preferably chosen from
ammonia, hydrocarbons, hydrofluorocarbons, ethers,
hydrofluoroethers, fluoroolefins and the mixtures of these. It can
in particular be tetrafluoropropene and more particularly
preferably 2,3,3,3-tetrafluoropropene (HFO-1234yf) or
1,3,3,3-tetrafluoropropene (HFO-1234ze), in the cis or trans form
or in the form of a mixture of cis and trans forms.
[0099] According to one embodiment, the first heat-transfer fluid
is carbon dioxide and the second heat-transfer fluid is
HFO-1234yf.
[0100] According to another embodiment, the first heat-transfer
fluid is carbon dioxide and the second heat-transfer fluid is
HFO-1234ze.
[0101] Other possible examples for the second heat-transfer fluid
are: [0102] A mixture of HFO-1234yf and HFC-134a
(1,1,1,2-tetrafluoroethane), which is preferably a binary mixture
and which preferably comprises from 50% to 65% of HFO-1234yf and
ideally approximately 56% of HFO-1234yf. [0103] A mixture of
HFO-1234ze and HFC-134a, which is preferably a binary mixture and
which preferably comprises from 50% to 65% of HFO-1234ze and
ideally approximately 58% of HFO-1234ze. [0104] A mixture of
HFO-1234yf and HFO-1234ze, which is preferably a binary mixture and
which preferably comprises from 35% to 65% of HFO-1234yf and
ideally approximately 50% of HFO-1234yf. [0105] A mixture of
HFO-1234yf, HFO-1234ze and HFC-134a, which is preferably a ternary
mixture and which preferably comprises from 40% to 45% of HFC-134a,
from 35% to 50% of HFO-1234ze and from 5% to 25% of HFO-1234yf.
[0106] A mixture of HFO-1234yf and ammonia, which is preferably a
binary mixture and which preferably comprises from 15% to 30% of
ammonia. [0107] A mixture of HFO-1234yf, HFC-152a
(1,1-difluoroethane) and HFC-134a, which is preferably a ternary
mixture and which preferably comprises from 2% to 15% of HFC-134a,
from 2% to 20% of HFC-152a and from 65% to 96% of HFO-1234yf.
[0108] A mixture of HFO-1234ze, HFC-134a and HFO-1336mzz
(1,1,1,4,4,4-hexa-fluorobut-2-ene), which is preferably a ternary
mixture.
[0109] Within the above ranges, the proportions of the different
compounds are proportions by weight.
[0110] Various additives can be added to the heat-transfer fluids
in the context of the disclosure in the vapor compression circuits.
They can in particular be lubricants, stabilizing agents,
surfactants, tracers, fluorescent agents, odorous agents and
solubilizing agents.
[0111] The stabilizing agent or agents, when they are present,
preferably represent at most 5% by weight in the heat-transfer
composition. Mention may in particular be made, among the
stabilizing agents, of nitromethane, ascorbic acid, terephthalic
acid, azoles, such as tolutriazole or benzotriazole, phenolic
compounds, such as tocopherol, hydroquinone, t-butylhydroquinone or
2,6-di-(tert-butyl)-4-methylphenol, epoxides (alkyl, optionally
fluorinated or perfluorinated, or alkenyl or aromatic), such as
n-butyl glycidyl ether, hexanediol diglycidyl ether, allyl glycidyl
ether or butylphenyl glycidyl ether, phosphites, phosphonates,
thiols and lactones.
[0112] Mention may be made, as tracers (agents capable of being
detected), of deuterated or nondeuterated hydrofluorocarbons,
deuterated hydrocarbons, perfluorocarbons, fluoroethers, brominated
compounds, iodinated compounds, alcohols, aldehydes, ketones,
nitrous oxide and the combinations of these. The tracer is
different from the heat-transfer compound or compounds making up
the heat-transfer fluid.
[0113] Mention may be made, as solubilizing agents, of
hydrocarbons, dimethyl ether, polyoxyalkylene ethers, amides,
ketones, nitriles, chlorocarbons, esters, lactones, aryl ethers,
fluoroethers and 1,1,1-trifluoroalkanes. The solubilizing agent is
different from the heat-transfer compound or compounds making up
the heat-transfer fluid.
[0114] Mention may be made, as fluorescent agents, of
naphthalimides, perylenes, coumarins, anthracenes, phenanthracenes,
xanthenes, thioxanthenes, naphthoxanthenes, fluoresceins and the
derivatives and combinations of these.
[0115] Mention may be made, as odorous agents, of alkyl acrylates,
allyl acrylates, acrylic acids, acryl esters, alkyl ethers, alkyl
esters, alkynes, aldehydes, thiols, thioethers, disulfides, allyl
isothiocyanates, alkanoic acids, amines, norbornenes, norbornene
derivatives, cyclohexene, aromatic heterocyclic compounds,
ascaridole, o-methoxy(methyl)phenol and the combinations of
these.
[0116] The choice may in particular be made, as lubricants or
lubricating oils, of compounds chosen from oils of mineral origin,
silicone oils, paraffins of natural origin, naphthenes, synthetic
paraffins, alkylbenzenes, poly(.alpha.-olefin)s, polyol esters,
polyalkylene glycols and/or polyvinyl ethers. Polyol esters and
polyvinyl ethers are preferred. Polyalkylene glycols are very
particularly preferred.
[0117] Embodiment of the disclosure are very particularly
appropriate for fluids or bodies to be cooled to a temperature of
-50 to -15.degree. C., preferably of -40 to -25.degree. C. The
temperature of the external medium typically varies from -10 to
50.degree. C., in particular from 0 to 40.degree. C. and very
particularly from 10 to 35.degree. C.
[0118] The temperature of the evaporation of the first
heat-transfer fluid (temperature in the first evaporator 11) is
preferably from -60 to -20.degree. C., more particularly from -50
to -25.degree. C.
[0119] The temperature at the condensation of the second
heat-transfer fluid (temperature in the second condenser 23)
depends on the external temperature and it is typically from 20 to
60.degree. C., more particularly from 20 to 45.degree. C. It can,
for example, be +10.degree. C. with respect to the external
temperature.
[0120] The condensation temperature of the first heat-transfer
fluid in the cascade heat exchanger 30 depends on the evaporation
temperature of the second heat-transfer fluid in this same
exchanger. It can, for example, be +5.degree. C. with respect to
said evaporation temperature.
[0121] In addition, an embodiment of the disclosure provides a
device for measuring the temperature of the external medium 41 and
also means for adjusting the evaporation temperature 42 in the
cascade heat exchanger 30, as a function of the temperature of the
external medium which is measured.
[0122] It has been found by the inventors that the overall
performance of the installation is at an optimum (that is to say,
that the energy consumption is at a minimum, for a given cooling
temperature of the fluid or body to be cooled) when the temperature
of the second heat-transfer fluid in the cascade heat exchanger 30
is adjusted as a function of the external temperature. The higher
the external temperature, the higher the temperature of the second
heat-transfer fluid in the cascade heat exchanger 30 has to be, for
better effectiveness, and vice versa.
[0123] According to a preferred embodiment, the evaporation
temperature in the cascade heat exchanger 30 is adjusted to an
optimal evaporation temperature, which is determined by a
calculation module, as a function of the temperature of the
external medium which is measured.
[0124] The optimum evaporation temperature is preferred defined as
being the evaporation temperature in the cascade heat exchanger 30
for which the overall coefficient of performance of the
installation is at a maximum and for which the overall energy
consumption of the installation is at a minimum (for a given
refrigerating capacity and/or for a given cooling temperature of
the cooled fluid or body).
[0125] For a given installation, the optimum evaporation
temperature can be determined either by directly using the data
supplied in example 1 below in connection with FIG. 3; or by
carrying out a calculation analogous to that presented in example 1
below, for the installation in question; or also experimentally or
empirically, by measuring the energy consumption of the
installation for different evaporation temperatures of the high
temperature circuit, and by establishing the correlation with
respect to the external temperature.
[0126] Means for determining the optimum evaporation temperature
can be included in the installation. Alternatively and preferably,
the function connecting the optimum evaporation temperature to the
external temperature is determined beforehand and then only this
function is incorporated in the abovementioned calculation
module.
[0127] The evaporation temperature in the cascade heat exchanger 30
can also be adjusted to a different temperature from the optimum
evaporation temperature, in order to take into account other
constraints. For example, it may be appropriate to limit the
possible variations in the evaporation temperature in the cascade
heat exchanger 30 to a certain temperature range T.sub.1-T.sub.2.
In this case, the evaporation temperature in the cascade heat
exchanger 30 is adjusted to the optimum evaporation temperature, if
the latter belongs to the range T.sub.1-T.sub.2, or else it is
adjusted to the temperature T.sub.1, if the optimum evaporation
temperature is less than T.sub.1, and, finally, it is adjusted to
the temperature T.sub.2, if the optimum evaporation temperature is
greater than T.sub.2.
[0128] Many other variations are possible. It is possible in
particular to provide a delayed adjustment or a hysteretic
adjustment of the evaporation temperature in the cascade heat
exchanger 30 as a function of the temperature of the external
medium, in order to prevent excessively frequent or excessively
sudden adjustments.
[0129] Generally, the optimum evaporation temperature is an
increasing function of the temperature of the external medium.
Consequently, it is desirable, when an increase in the temperature
of the external medium is detected, for the evaporation temperature
in the cascade heat exchanger 30 to be increased and, when a
decrease in the temperature of the external medium is detected, for
the evaporation temperature in the cascade heat exchanger 30 to be
reduced. Or, according to another embodiment, the adjustment is
such that, for all given temperatures T.sub.1 and T.sub.2 of the
external medium with T.sub.2>T.sub.1, the evaporation
temperature in the cascade heat exchanger 30 is respectively
adjusted to temperatures T.sub.1' and T.sub.2' with T.sub.2'
greater than or equal to T.sub.1'.
[0130] The adjusting of the evaporation temperature in the cascade
heat exchanger 30 can be obtained by regulating the second
compressors 22a, 22b, 22c. For example, the means for adjusting the
evaporation temperature 42 in the cascade heat exchanger 30 can
comprise means for adjusting the speed of rotation of the second
compressors 22a, 22b, 22c, or also means for successively starting
up and shutting down the second compressors 22a, 22b, 22c.
[0131] The adjusting of the evaporation temperature in the cascade
heat exchanger 30 can be carried out either continuously or at
separate moments and, for example, at regular time intervals (every
minute, every 15, 30, 45 or 60 minutes, and the like). The
adjusting of the temperature can also be carried out by taking, for
reference, a mean of the temperature of the external medium
measured over a certain period, for example over 10 minutes, 30
minutes or 1 hour.
EXAMPLES
[0132] The following examples illustrate embodiments of the
disclosure without limiting it.
Example 1
Demonstration of an Optimum Evaporation Temperature
[0133] FIG. 2 provides a typical example of the variation in the
temperature of the external medium (ambient temperature) over a
day, and also a typical example of the refrigerating capacity
requirements over this day, in order to refrigerate compartments
comprising frozen or deep-frozen products in a store of the
supermarket type.
[0134] The refrigeration installation is of the type represented
diagrammatically in FIG. 1. The low-temperature circuit comprises
carbon dioxide and the high-temperature circuit comprises
HFO-1234yf or HFO-1234ze.
[0135] For the low-temperature circuit, the evaporation temperature
is -40.degree. C., the overheating is 25.degree. C. and the
undercooling is 5.degree. C. The compressor is a screw compressor
with an isentropic efficiency according to the following equation:
.eta..sub.iso=0.00476 .tau..sup.2-0.09238 .tau.+0.8981, .tau. being
the ratio of pressures (see Thermodynamic analysis of optimal
condensing temperature of cascade-condenser in CO.sub.2/NH.sub.3
cascade refrigeration systems by Tzong-Shring et al. in
International Journal of Refrigeration, vol. 29, No. 7, 2006, pp.
1100-1108).
[0136] The condensation temperature is 5.degree. C. greater than
the evaporation temperature in the high-temperature circuit.
[0137] As regards the high-temperature circuit, the evaporation
temperature is either fixed at a constant value (-10.degree. C. or
-18.degree. C.) or is variable as a function of the external
temperature. The overheating is 25.degree. C. and the undercooling
is 5.degree. C. The compressor is a screw compressor with an
isentropic efficiency according to the following equation:
.eta..sub.iso=0.00060079 .tau..sup.2-0.03002352 .tau.+0.90880781
(reference: ASHRAE 2008 Handbook, HVAC system and equipments,
Chapter 37, p. 22, Twin screw compressor, FIG. 34). The
condensation temperature is 10.degree. C. greater than the external
temperature.
[0138] With the evaporation temperature as parameter (evaporation
temperature in the high-temperature stage), the coefficient of
performance (COP) is optimized as a function of the ambient
temperature. The value of the COP for the entire installation
corresponds to the following formula:
COP Cascade = COP 1 COP 2 1 + COP 1 + COP 2 ##EQU00001##
(in which COP.sub.1 and COP.sub.2 are the coefficients of
performance of the low-temperature and high-temperature
circuits).
[0139] The correlation between the ambient temperature (T.sub.ext)
and the optimum evaporation temperature in the high-temperature
circuit (T.sub.opt) is visible in FIG. 3.
[0140] The tendency equations are very similar for the two
refrigerants tested:
For HFO-1234yf, T.sub.opt=0.4411.times.T.sub.ext-26.549 (in degrees
Celsius).
For HFO-1234ze, T.sub.opt=0.4208.times.T.sub.ext-26.107 (in degrees
Celsius).
Example 2
Gains Offered by an Embodiment of the Disclosure
[0141] In this example, the optimum evaporation temperature
demonstrated in example 1 is used to achieve energy savings.
[0142] Thus, the graph of FIG. 4 illustrates the comparison
between: (1) the overall energy consumption of the installation
operating in accordance with an embodiment of the disclosure (in
gray), that is to say with an adjustment to the evaporation
temperature of the high-temperature circuit to its optimum value as
a function of the ambient temperature (as determined in FIG. 3),
hour by hour, it being assumed that the temperature changes over
the day according to the curve of FIG. 2; and (2) the overall
energy consumption of the same installation operating
conventionally (in black), with a constant evaporation temperature
in the high-temperature circuit equal to -10.degree. C. (this is
the value most normally selected).
[0143] The graph of FIG. 5 illustrates a comparison between the
same two situations but with respect to the TEWI (Total Equivalent
Warming Impact) index as defined in Annex B to the standard EN
378-1:2008+A1:2010. In the graph, the indices are with respect to a
reference of 100 for the installation operating conventionally with
HFO-1234yf in the high-temperature circuit.
[0144] The graphs of FIGS. 6 and 7 are analogous to those of FIGS.
4 and 5, except that the installation operating conventionally
operates with a constant evaporation temperature in the
high-temperature circuit equal to -18.degree. C. instead of
-10.degree. C.
[0145] It has also been confirmed that an embodiment of the
disclosure makes it possible to correctly anticipate (in particular
on the basis of the graph of FIG. 3) the daily energy consumption
in the case of ambient temperatures which are either warmer or
colder than those of the typical day of FIG. 2.
* * * * *