Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Title of the invention
Traps-critical vapour compression cycle device.
Field of the invention
This invention relates to vapour compression cycle devices
such as refrigerators, air-conditioning units and heat
pumps, using a refrigerant operating in a closed circuit
under traps-critical conditions, and more particularly, to
methods for modulating and controlling the capacity of such
devices.
Background of the invention
A conventional vapour compression cycle device for refri-
geration, air-conditioning or heat pump purposes is shown in
principle in Fig. 1. The device consists of a compressor
(1), a condensing heat exchanger (2), a throttling valve (3)
and a evaporating heat exchanger (4). These components are
connected in a closed flow circuit, in which a refrigerant
is circulated. The operating principle of a vapour compres-
sion cycle device is as follows: The pressure and tempera-
ture of the refrigerant vapour is increased by the compres-
sor (1), before it enters the condenser (2) where it is
cooled and. condensed, giving off heat to a secondary cool-
ant. The high-pressure liquid is then throttled to the eva-
,porator pressure and temperature by means of the expansion
valve (3). In the evaporator (4), the refrigerant boils and
absorbs heat from its surroundings. The vapour at the eva-
porator outlet is drawn into the compressor, completing the
cycle.
Conventional vapour compression cycle devices use refrige-
rants (as for instance R-12, CFzCl2) operating entirely at
sub-critzoal pressures. A number of different substances or
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mixtures of substances may be used as a refrigerant. The
choice of refrigerant is among others influenced by the
condensation temperature, as the critical temperature of the
fluid sets the upper limit for the condensation to occur. In
order to maintain a reasonable efficiency, it is normally
desirable to use a refrigerant with critical temperature at
least 20-30K above the condensation temperature. Near-
critical temperatures are normally avoided in design and
operation of conventional systems.
The present technology is treated in full detail in the
literature, e.g. the Handbooks of American Society of Heat-
ing, Refrigerating and Air Conditioning Engineers Inc.,
Fundamentals 1989 and Refrigeration 1986.
The ozone-depleting effect of todays common refrigerants
(halocarbons) has resulted in strong international action to
reduce or prohibit the use of these fluids. Consequently
there is a urgent need for finding alternatives to the pre-
sent technology.
Capacity contxol of the conventional vapour compression
cycle device is achieved mainly by regulating the mass flow
of refrigerant passing through the evaporator. This is done
e.g. by controlling the compressor capacity, throttling or
bypassing. These methods involve more complicated flow cir-
cuit and components, need for additional equipment and ac-
cesories, reduced part-load efficiency and other compli-
cations.
A common type of liquid regulation device is a thermostatic
expansion valve, which is controlled by the superheat at the
evaporator outlet. Proper valve operation under varying
operating conditions is achieved by using a considerable
part of the evaporato r to superheat the refrigerant, resul-
ting in a low heat transfer coefficient.
Furthermore, heat rejection in the condenser of the conven-
tional vapour compression cycle takes place mainly at con-
stant temperature. Therefore, thermodynamic losses occur due
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to large temperature differences when giving off heat to a
secondary coolant with large temperature increase, as in
heat pump applications or when the available secondary cool-
ant flow is small.
The operation of a vapour compression cycle under trans-
critical conditions has been formerly practiced to some
extent. Up to the time when the halocarbons took over - 40-
50 years ago - COi was commonly used as a refrigerant,
notably in ships refrigeration for provisions and cargo. The
systems were designed to operate normally at sub-critical
pressures, with evaporation and condensation. Occasionally,
typically when a ship was passing tropical areas the cooling
sea water temperature could be too high to effect normal
condensation, and the plant would operate with supercritical
conditions on the high-side. (critical temperature for COz
-31°C). In this situation it was practiced to increase the
refrigerant chacge on the high-side to, a point where the
pressure at the compressor discharge was raised to 90-100
bar, in order to maintain the cooling capacity at a reason-
able level. COZ refrigeration technology is described in
older literature, e.g. P. Ostertag "Kalteprozesse", Springer
1933 or H.d. MacIntire °'Refrigeration Engineering", ~iley
1937.
The usual practice in older COz-systems was to add the ne-
cessary extra charge from separate storage cylinders. A
receiver installed after the condenser in the normal way
will not be able to provide the functions intended by the
present invention:
Another possibility to increase the capacity and efficiency
of a given vapour compression cycle device operating with
supercritical high-side pressure is known from German patent
278095 (1912). This method involves two-stage compression
with intercooling in the supercritical region. Compared to
the standard system, this involves installation of an ad-
ditional compressor or pump, and a heat exchanger.
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The textbook "Principles of Refigeration" of W.P Gosney
(Cambridge Univ. Press 1982) points at some of the pe-
culiarities of near-critical pressure operation. It is
suggested that increasing the refrigerant charge in the
high-pressure side could be accomplished by temporarily
shutting the expansion valve, so as to transfer some charge
from the evaporator. But it is emphasized that this would
leave the evaporator short of liquid, causing reduced capa-
city at the time when it is most wanted.
Objects of the invention
It is therefore an object of the present invention to pro-
vide a new, improved, simple and effective means for modu-
lating and controlling the capacity of a trans-critical
vapour compression cycle device, avoiding the above short-
comings and disadvantages of the prior art.
Another object of the present invention is to provide a
vapour compression cycle avoiding use of CFC refrigerants,
and at. the same time offering possibility to apply several
attractive refrigerants with resgect to safety, environ-
mental hazards and price.
Further object of the present invention is to provide a new
method of capacity control, which involves operation at
mainly constant refr~.gerant mass flow rate and simple capa-
city modulation by valve operation.
Still another object of the present invention is to provide
a cycle rejecting heat at gliding temperature, reducing
heat -exchange losses in applications where secondary coolant
flow is small or when the secondary coolant is to be heated
t.o a.relatively high temperature.
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Summary of the invention
The above and other objects of the present invention are
achieved by providing a method operating normally at trans-
critical conditions (i.e. super-critical high-side pressure,
sub-critical low-side pressure) where the thermodynamic
properties in the super-critical state are utilized to con-
trol the refrigerating and heating capacity of the device.
The present invention involves the regulation of specific
enthalpy at evaporator inlet by deliberate use of the pres-
sure and/or temperature before throttling for capacity con-
trol. Capacity is controlled by varying the refrigerant
enthalpy difference in the evaporator, by changing the spe-
cific enthalpy of the refrigerant before throttling. In the
super-critical state this can be done by varying the pres-
sure and temperature independently. In a preferred embodi-
ment this modulation of specific enthalpy is done by varying
the pressure before throttling. The refrigerant is cooled
down as far as it is feasible by means of the available
cooling medium, and the pressure regulated to give the re-
quired enthalpy. Another embodiment involves modulation of
enthalpy by variation of the refrigerant temperature before
throttling. This is done by controlling the heat rejection
from the device.
Brief description of the drawings
The invention will now be described in more detail, in the
following ref erring to attatched drawings, Fig. 1, 2, 3, 4,
5, 6, 7 and 8, where:
Fig. 1 is a scheanatic representation of a conventional (sub-
critical) vapour compression cycle device.
Fig: 2 is a schematic representation of a traps-critica l
vapour compression cycle device constructed in accordance
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with a preferred embodiment of the invention. This embodi-
ment includes a volume as an integral part of the evaporator
system, holding refrigerant in the liquid state.
Fig. 3 is a schematic representation of a trans-critical
vapour compression cycle device canstructed in accordance
with a second embodiment of the invention. This embodiment
includes an intermediate pressure receiver connected direct-
ly into the flow circuit between two valves.
Fig. 4 is a schematic representation of a trans-critical
vapour compression cycle device constructed in accordance
with a third embodiment of the invention. This embodiment
includes a special receiver to hold refrigerant as liquid or
in the super-critical state.
Fig. 5 is a graph illustrating the relationship of pressure
versus enthalpy of the trans-critical vapour compression
cycle device of Fig. 2, 3 or 4, at different operating con-
ditions.
Fig. 6 is a collection of graphs illustrating the control of
refrigerating capacity by the method of pressure control in
accordance with the present invention. The results shown are
measured in a laboratory demonstration system built accord-
ing to a preferred embodiment of the invention.
Fig. 7 is a~collection of graphs illustrating the control of
refrigerating capacity by control of the heat rejection, in
accordance with the present invention. The results shown are
measured in a laboratory demonstration system build accord-
ing to a preferred embodiment of the present invention.
Fig. 8 is test results showing the relationship of tempera-
ture versus entropy of the trans-critical vapour compression
cycle device of Fig. 2, operating at different high-side
pressures, employing carbon dioxide as refrigerant
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Detailed description of the invention
A trans-critical vapour compression cycle device according
to the present invention includes a refrigerant, of which
critical temperature is between the temperature of the heat
inlet and the mean temperature of heat submittal, and a
closed working fluid circuit where the refrigerant is circu-
lated.
Suitable working fluids may be by the way of examples: ethy-
len (C2H4), diborane (BZH6), carbon dioxide (COZ), ethane
(CZH6) and nitrogen oxide (N20).
The closed working Fluid circuit consists of a refrigerant
flow loop with an integrated storage segment. Fig. 2 shows a
preferred embodiment of the invention where the storage
segment is an integral part of the evaporator system. The
flaw circuit includes a compressor 10 connected in series to
a heat exchanger 11, a counterflow heat exchanger 12 and a
throttling valve 13. The throttling valve can be replaced by
an optional expansion device. An evaporating heat exchanger
14, a liquid separator/receiver 16 and the low-pressure side
of the counterflow heat exchanger 12 are connected in flow
communication intermediate the throttling valve 13 and the
inlet 19 of the compressor 10. The liquid receiver 16 is
connected to the evaporator outlet 15, and the gas phase
outlet of the receiver 16 is connected to the counterflow
heat exchanger l2.
The aounterflow heat exchanger 12 is not absolutely neces-
sary for the functioning of the device but improves its
efficiency, in particular its rate of response to a capacity
increase requirement. It also serves to return oil to the
compressor. For this purpose a liquid phase line from the
receiver (16) (shown with broken line in Fig. 2) is connect-
ed to the suction line either before the counterflow heat
exchanger (12) at l7 or after it at 18, or anywhere between
these points. The liquid flow, i.e, refrigerant and oil, is
controlled by a suitable conventional liquid flaw restrict-
ing device (not shown in the figure). By allowing some ex-
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cess liquid refrigerant to enter the vapour line, a liquid
surplus at the evaporator outlet is obtained.
In a second embodiment of the invention indicated in Fig. 3,
the storage segment of the working fluid circuit includes a
receiver 22 integrated in the flow circuit between a valve
21 and the throttling valve 13. The other components 10-lA
of the flow circuit is identical to the components of the
previous embodiment, although the heat exchanger 12 can be
omitted without any great consequence. The pressure in the
receiver 22 is kept intermediate the high-side and low-side
pressures of the flow circuit.
In a third embodiment of the invention indicated in Fig. 4,
the storage segment of the working fluid circuit includes a
special receiver 25, where the pressure is kept between the
high-side pressure and the low-side pressure of the flow
circuit. The storage segment further consists of the valves
23 and 29 which are connected to the high pressure and low
pressure part of the flow circuit respectively.
In operation, the refrigerant is compressed to a suitable
supercritical pressure in the compressor 10, the compressor
outlet 20 is shown as state "a" in Fig. 5.. The refrigerant
is circulated through the heat exchanger 11 where it is
cooled to state "b", giving' off heat to a suitable cooling
agent, e.g. cooling air or water. If desired, the refrig-
erant can be further cooled to state "c" in the counterflow
heat exchanger 12, before throttling to state "d". By the
pressure reduction in the throttling valve 13, a two-phase
gas/liquid mixture is formed, shown as state "d" in Fig. 3.
The refrigerant absorbs heat in the evaporator 19 by eva-
poration of the liquid phase. From state "e" at the evapora-
for outlet, the refrigerant vapour can be superheated in the
counterflow heat exchanger 12 to state "f" before it enters
the,compressor inlet 19, making the cycle complete. In the
. preferred embodiment of the invention, as shown in Fig. 2,
the evaporator outlet condition °'e" will be in the two-phase
region due to the liquid surplus at the evaporator outlet. ''
g
Modulation of the traps-critical cycle device capacity is
accomplished by varying the refrigerant state at the eva-
porator inlet, i.e. point "d" in Fig. 5. The refrigerating
capacity per unit of refrigerant mass flow corresponds to
the enthalpy difference between state "d" and state "e".
This enthalpy difference is found as a horizontal distance
in the enthalpy-pressure diagram, Fig. 5.
Throttling is a constant enthalpy process, thus the enthalpy
in point "d" is equal to the enthalpy in point "c". In con-
sequence, the refrigerating capacity (in kW) at constant
refrigera:~t mass flow can be controlled by varying the en-
thalpy at point "c".
It should be noted that in the traps-critical cycle the
high-pressure single-phase refrigerant vapour is not conden-
sed but seduced in temperature in the heat exchanger 11. The
terminal temperature of the refrigerant in the heat ex-
changer (point "b") will be some degrees above the entering
cooling air or water temperature, if counterflow is used.
The high-pressure vapour can then be cooled a few degrees
lower, to point "c", in the counterflow heat exchanger 12.
The result is, however, that at constant cooling air or
water inlet temperature, the temperature at point "c" will
be mainly constant, independent of the pressure level in the
high side.
Therefore, modulation of device capacity is accomplished by
varying the pressure in the highside, while the temperature '
in point "c" is mainly constant. The curvature of the iso-
therms near the critical point result in a variation of
enthalpy with pressure, as.shown in Fig. 5. The figure shows
a reference cycle (a-b-c-~d~e-f), a cycle with reduced capa-
city due to reduced high side pressure (a'-b°-c°-d'-e-f) and
a cycle with increased capacity due to higher pressure in
the high side,(a"-b"-c"-d"-e-f). The evaporator pressure is
assumed to be constant.
The pressure in the high-pressure side is independent of
temperature, because it is ~ilied with a single phase fluid.
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To vary the pressure it is necessary to vary the mass of
refrigerant in the high side, i.e. to add or remove same of
the instant refrigerant charge in the high side. These vari-
ations must be taken up by a buffer, to avoid liquid over-
flow or drying up. of the evaporator.
In the preferred embodiment of the invention indicated in
Fig. 2, the refrigerant mass in the high side can be in-
creased by temporarily reducing the opening of the throt-
tling valve 13. Due to the incidentally reduced refrigerant
flow to the evaporator, the excess liquid fraction at the
evaporator outlet (15) will be reduced. The liquid refri-
gerant flow from the receiver 16 into the suction line is
however constant. Consequently, the balance between the
liquid flow entering and leaving the receiver 16 is shifted,
resulting in a net reduction in receiver liquid content and
a correspotzding accumulation of refrigerant in the high
pressure side of the flow circuit.
The increase in high side charge involves increasing pres-
sure and thereby higher refrigerating capacity. This mass
transfer from the low-pressure to the high-pressure side of
the circuit will continue until balance between refrigerat-
ing capacity and load is found.
Opening of the throttling valve 13 will increase the excess
liquid fraction at the evaporator outlet 15, because the
evaporated-.amount of refrigerant is mainly constan t. The
difference between this liquid flow entering the receiver.
and the liquid flow from the receiver into the suction line,
will accumulate: The result is a net transport of refri-
gerant charge from the high side to the low side of the flow
circuit, with the reduction in the high side charge stored
in liquid state in the receiver. By reducing the high-side
charge and thereby pressure, the capacity of the device is
reduced, until balance is found.
Some liquid transport from the receiver into the compressor
suction line is,also needed tb avoid lubricant accumulation
in the liquid phase of the receiver. ~ .
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In the second embodiment of the invention indicated in Fig.
3, the refrigerant mass in the high side can be increased by
simultaneously shutting the valve 21 and modulating the
throttling valve 13 to provide the evaporator with suf-
ficient liquid flow. This will reduce the refrigerant flow
from the high side into the receiver through valve 21, while
refrigerant mass is transferred from the low side to the
high side by the compressor.
Reduction of high-side charge is obtained by opening the
valve 21 while keeping the flow through the throttling valve
13 mainly constant. This will transfer mass from the high-
side of the flow circuit to the receiver 22.
In a third embodiment of the invention indicated in Fig. 4,
the refrigerant mass in the high side can be increased by
opening the valve 24 and simultaneously reducing the flow
through the throttling valve 13. By this, refrigerant charge
is accumulated in the high-pressure side due to reduced flow
through the throttling valve 13. Sufficient liquid flow to
the evaporator is obtained by opening the valve 24.
A reduction in the high side charge can be accomplished by
opening the valve 23 to transfer some refrigerant charge
from the high side to the receiver. Capacity control of the
device is thus accomplished by modulation of the valves 23
and 24, and simultaneously operating the throttling valve
13.
The preferred embodiment of the invention, as indicatzd in
fig: 2 has the advantage of simplicity, with capacity con-
trol by operation of one valve only. Furthermore, the trans-
critical vapour compression cycle device built according to
this embodiment has a certain self-regulating capability by
adapting to~ changes in cooling load through changes in.
liquid content in the receiver 16, involving changes in
highside charge and thus cooling capacity. In addition, the
operation with liquid surplus at evaporator outlet gives
favourable heat transfer characteristics.
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The second embodiment, as indicated in Fig. 3, has the ad-
vantage of simplified valve operation. Valve 21 only regu-
lates the pressure in the high side of the device, and the
throttling valve 13 only assures that the evaporator is fed
sufficiently. A conventional thermostatic valve can thus be
used for throttling. Oil return to the compressor is easily
achieved by allowing the refrigerant to flow through the
receiver. This embodiment however does not offer the capa-
city control function at high-side pressures below the cri-
tical pressure. The volume of the receiver 22 must be rela-
tively large since it is only operating between the dis-
charge pressure and the liquid line pressure.
Still another embodiment as indicated in Fig. 4, has the
advantage of operating as a conventional vapour compression
cycle device, when it is running at stable conditions. The
valves 23 and 24, connecting the receiver 25 to the flow
circuit, are activated only during capacity control. This
embodiment requires use of three different valves during
periods of capacity change.
The latter embodiments has the disadvantage of higher pres-
sure in the receiver, as compared to the preferred embodi-
ment. The differences between the individual systems regard-
ing design and operational characteristics are however not
very significant.
Trans-critical vapour compression cycle devices built ac-
cording to the described embodiments can be applied in seve-
ral areas. The technology is well suitable in small and
medium-sized stationary and mobile air-conditioning units;
small and medium=sized refrigerators/freezers and in smaller
heat pump units. One of the most promising applications is
in automotive air-conditioning, where the present need for a
new; non-CFC, lightweight and efficient alternative to R12-
systems'is urgent.
The abbve described embodiments of this invention are in-
tended to be exeraplative only and not limiting. It will be
appreciated that it is also possible to control the capacity
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of the traps-critical cycle device by keeping the high-side
pressure mainly constant, and regulate the refrigerant tem-
perature before throttling (state "c") by varying the circu-
lation rate of cooling air or water. By reducing the flow of
cooling fluid, i.e. air or water, the temperature before
throttling will increase and the capacity will drop, in-
creased cooling fluid flow will reduce the temperature be-
fore throttling, arid thereby increase the capacity of the
device. Combinations of pressure and temperature control are
also possible.
Examples
The practical use of the present invention for refrigeration
or heat pump purposes is illustrated by the following ex-
amples, giving test results ~rom a traps-critical vapour
compression cycle device, built according to the embodiment
of the invention shown in Fig. 2, employing carbon dioxide
(COZ) as refrigerant.
The laboratory test device uses water as heat source, i.e.
the water is refrigerated by heat exchange with boiling COz
in the evaporator 19. water is also used as cooling agent,
being heated by C02 in the heat exchanger 11. The test de-
vice includes a 61 ccm reciprocating compressor (10) and a
receiver (16) with total volume of 4 liters. The system also
includes a counterflow heat exchanger (12) and liquid line
connection from the receiver to point 17, as indicated in
Fig. 2. The throttling valve 13 is operated manually.
Example 1
This example shows how control of refrigerating capacity is
obtained by varying the position of the throttling valve 13,
thereby varying the pressure in the high-side of the flow
czrcuit. By variation of high-side pressure, the specific
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refrigerant enthalpy at the evaporator inlet is controlled,
resulting in modulation of refrigerating capacity at
constant mass flow.
The water inlet temperature to the evaporator 19 is kept
constant at 20°C, and the water inlet temperature to the
heat exchanger 11 is kept constant at 35°C. Water circu-
lation is constant both in the evaporator 14 and the heat
exchanger 11. The compressor is running at constant speed.
Fig. 6 shows the variation of refrigerating capacity (Q),
compressor shaft work (W), highside pressure (pH), COZ mass
flow (m), COZ temperature at evaporator outlet (tA), C02
temperature at the outlet of heat exchanger 11 (tb) and
liquid level in the receiver (h) when the throttling valve
13 is operated as indicated at the top of the figure. The
adjustment of throttling valve position is the only mani-
pulation.
As shown in the figure, capacity (Q) is easily controlled by
operating the throttling valve (13). It is further clear
from the figure that at stable conditions, the circulating
mass flow of COZ (m) is mainly constant and independent of
the cooling capacity. The COZ temperature at the outlet of
heat exchanger 11 (tb) is also mainly constant. The graphs
show that the variation of capacity is a result of varying
high side pressure (pH) only.
zt can also be seen from the diagram that increased highside
pressure involves a reduction in the receiver liquid level
(h), due to the COz charge transfer to the highpressure side
of the circuit.
Finally, it can be noted that the transient period during
capacity increase is not involving any significant
superheating at the evaporator outlet, i.e. only small
fluctuations in tB.
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Example 2
With higher water inlet temperature to heat exchanger 11
(e.g. higher ambient temperature), it is necessary to in-
crease the high side pressure to maintain a constant refri-
gerating capacity. Table 1 shows results from tests run at
different water inlet temperature to heat exchanger 11 (tw).
The water inlet temperature to the evaporator is kept
constant at 20°C, and the compressor is running at constant
speed.
As the table shows, the cooling capacity can be kept mainly
constant when the ambient temperature is rising, by in-
creasing the high side pressure. The refrigerant mass flow
is mainly constant, as shown. Increased high-side pressures
involve a reduction in receiver liquid content, as indicated
by the liquid level readings.
Table 1
Inlet temperature (tw) 35.1 45.9 57.3 C
Refrigerating capacity 2.4 2.2 2.2 kW
(Q)
High side pressure (pH) 84.9 99.3 114.1 bar
Mass flow (m) 0.026 0.024 0.020 kg/s
Liquid level (h) 171 166 115 mm
Example 3
This example illustrates the possibility to modulate and
control the capacity of the device by adjustment of the flow
of: coolant (e. g. air or water) circulating through heat
exchanger ll, keeping the high-side pressure constant.
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Fig. 7 shows the variation of refrigerating capacity
when the circulation rate of cooling water (mW) is regulated
as shown at the top of the figure. The mass flow of COz (m),
the high-side pressure (pH) and the water inlet temperature
to heat exchanger 11 (ti) are kept constant. The compressor
is running at constant speed and both the temperature and
flow rate of water entering the evaporator are kept
constant.
The refrigerating capacity is easily controlled by variation
of the water flow, as shown in the figure. Mass flow of C02
is mainly constant.
Example 4
Fig. 8 is a graphic representation of traps critical cycles
in the entropy/temperature diagram. The cycles shown in the
diagram axe based on measurements on the laboratory test
device, during operation at five different high-side
pressures. The evaporator pressure is kept constant.
refrigerant is C02.
The diagram gives a good impression of the capacity control
principle, indicating the changes in specific enthalpy (h)
at evaporator inlet caused by variation of the high-side
pressure (p).
