DK167985B1 - PROCEDURE FOR REGULATING A COMPRESSION COOLING SYSTEM AND HEATING / COOLING DEVICE FOR EXERCISING THE PROCEDURE - Google Patents

Description

DK 167985 Bl i

The invention relates to a method of controlling compression cooling systems such as refrigerators, air conditioners and heat pumps, and of the kind set out in the preamble of claim 1.

5

A conventional compression cooling circuit for cooling, air conditioning or heat pump purposes is shown in principle in FIG. 1. The apparatus consists of a compressor 1, a condensation heat exchanger 2, a rumbling valve 3 and a vaporizing heat exchanger 4. These components are connected in a closed circuit in which a refrigerant is circulated. The operation of the compression refrigeration system is as follows: the pressure and temperature of the refrigerant vapor are increased by the compressor 1 before the vapor enters the condenser 2, 15 where it is cooled and condensed, giving off heat to a secondary refrigerant. The high pressure coolant liquid is then swirled to evaporative pressure and temperature by means of the expansion valve 3. In the evaporator 4, the refrigerant boils and absorbs heat from the surroundings.

20 The refrigerant vapor at the evaporator output is drawn into the compressor, completing the circuit.

Conventional compression refrigeration systems, such as uses the refrigerant R-12, CF2C12, works completely at sub-critical 25 pressures. Several different substances or mixtures of substances can be used as refrigerant. The choice of refrigerant is determined, among other things. of the condensation temperature, the critical temperature of the fluid setting an upper limit of condensation. In order to maintain a reasonable efficiency 30, it is usually desirable to use a refrigerant having a critical temperature of at least 20-30 ° C above the condensation temperature. Post-critical temperatures are usually avoided in the design and operation of conventional systems.

35 The present technology is discussed in detail in the literature, e.g. in "Handbooks" from the American Society of Heating, Refrigerating and Air Conditioning Engineers DK 167985 B1 2

Inc. ("Fundamentals" 1989 and "Refrigeration" 1986).

The ozone depleting effect of the now common refrigerants (chlorofluorocarbons CFC) has resulted in extensive international agreements for reducing or bidding against the use of these fluids. Consequently, there is an urgent need to find alternatives to current technology.

10 Capacity control of conventional compression refrigeration systems is mainly achieved by regulating the mass flow of refrigerant passing through the evaporator. This is done e.g. by controlling the compressor capacity, dribbling or shunting. These approaches involve more complicated systems, reduced efficiency and other complications.

A common type of throttle means is a thermostatic expansion valve which is controlled by the superheater in the evaporator outlet. Proper valve operation under varying operating conditions is achieved by using a significant portion of the evaporator to superheat the refrigerant, resulting in a low heat transfer coefficient.

25 Heat transfer in the condenser in conventional compression refrigeration systems takes place mainly at constant temperature. Therefore, thermodynamic losses occur. due to large temperature differences when heat is supplied to a secondary refrigerant with high temperature increase, as is the case with heat pumps, or when the available flow of secondary refrigerant is small.

Operation of a compression cooling system under transcritical conditions has been practiced to some extent in the past. Up to 35 when CFCs took over - 40 to 50 years ago - CO 2 was commonly used as a refrigerant, especially in ship refrigeration systems for supplies and cargo. Application was designed to operate normally at sub-critical pressures with evaporation and condensation and with seawater for condenser cooling. Occasionally, typically when a ship is passing through tropical areas, the temperature of the cooling seawater could become too high to effect normal condensation on the high-pressure side. (The critical temperature for CO 2 is 31 ° C). In this situation, it was practiced to increase the refrigerant quantity on the high pressure side to a point where the pressure at the outlet of the compressor was raised 10 to 90-100 bar to maintain the cooling capacity at a reasonable level. The CO 2 cooling technology is described in older literature, e.g. P. Ostertag "Cold Processing", Springer 1933 or H.J. Maclntire "Refrigeration Engineering", Wiley 1937.

15

The usual practice in older C02 systems was to supply the necessary extra volume from separate storage containers. A liquid collector installed after the capacitor in the usual manner will not be able to provide the functions contemplated by the present invention.

Another possibility of increasing the capacity and efficiency of a given compression refrigeration system operating on the supercritical high pressure side is known from the specification of German Patent No. 278 095 (1912). This method involves 2-step compression with intermediate cooling in the supercritical range. Compared to the standard system, this results in the installation of an extra compressor or pump and an extra heat exchanger.

The book "Principles of Refrigeration" by W.B. Gosney (Cambridge University Press 1982) points out some of the peculiarities of near-critical pressure operation. It is suggested that increasing the amount of refrigerant on the high pressure side could be accomplished by briefly closing the throttle valve to transfer a certain amount from the evaporator. However, it is emphasized that this would leave the evaporator with liquid deficit, which would cause reduced capacity at the time when capacity is most needed.

5

It is therefore an object of the invention to provide a method of the present invention for controlling a transcritical compression refrigeration system in a simple and effective manner and to avoid the above-mentioned drawbacks and disadvantages of the prior art.

Another object of the invention is to provide a cooling process which avoids the use of CFC refrigerants and at the same time allows the use of several with regard to safety, environmental damage and price attractive refrigerants.

It is a further object of the invention to provide a new method of capacity control which involves operating with substantially constant refrigerant mass flow and simple capacity control by means of a valve.

Another object of the invention is to provide a compression cooling process with heat release at sliding temperature so that the heat exchanger losses can be reduced in systems where the secondary refrigerant flow is low or when the secondary refrigerant is to be heated to a relatively high temperature.

30

The above and other objects of the invention are achieved by performing the method as set forth in the characterizing part of claim 1.

The invention involves the regulation of specific enthalpy at the evaporator inlet by the intentional application of pressure before drowning, e.g. by capacity management. The capacity is controlled by variation of the refrigerant enthalpy difference across the evaporator by changing the specific enthalpy of the refrigerant before drowning. In the supercritical state, this can be done by varying pressure and temperature independently of one another. In a preferred embodiment of this control, the specific enthalpy is controlled by variation of the pressure before drowning. The refrigerant is cooled as far as possible by the available secondary refrigerant and the pressure is adjusted to provide the desired enthalpy.

The invention also relates to a heating / cooling device for carrying out the method and of the nature specified in the preamble of claim 6, and the characteristic of the heating / cooling device according to the invention is given in the characteristic part of claim 6.

The invention will now be explained in more detail with reference to the drawing, in which 1 schematically shows a conventional (subcritical) compression cooling system; FIG. 2 schematically shows a preferred embodiment of a transcritical compression cooling system according to the invention; In this embodiment, as an integral component of the evaporator system, a volume containing liquid refrigerant is included; FIG. 3 schematically shows another embodiment of a transcritical compression cooling system; this embodiment contains an intermediate pressure vessel inserted directly into the circuit between two valves; 4 schematically shows a third embodiment of a transcritical compression cooling system according to the invention; this embodiment contains a special container for storing refrigerant as liquid or in supercritical condition, fig. 5 is a graph illustrating the relationship between pressure 5 and enthalpy in the embodiment of FIG. 2, 3 or 4, under different working conditions, shown in FIG. 6 is a collection of graphs illustrating the control of cooling capacity by the method of the invention; the results shown are measured in a laboratory test system constructed in accordance with a preferred embodiment of the invention, and FIG. 7 is a graph showing, based on experimental results, the relationship between temperature and entropy in the embodiment of FIG. 2, when working with different pressures in the high pressure side and using CO2 as a refrigerant.

20

A transcritical compression refrigeration system according to the invention contains a refrigerant whose critical temperature is. between the heat supply temperature and the average heat release temperature, and a closed fluid circuit in which the refrigerant is circulated.

Suitable refrigerants are, for example: ethylene C2Hdi-borane B2Hg, carbon dioxide CO2, ethane C2Hg and nitric oxide.

The closed refrigerant circuit consists of a refrigerant flow loop with an integrated storage element. FIG. 2 shows a preferred embodiment of the invention, wherein the storage element is an integral component of the evaporator system. The current circuit contains a compressor 10 connected in series with a cooler 11, a countercurrent heat exchanger 12 and a throttle valve 13. The throttle valve can be replaced with an expansion apparatus as desired. An evaporator heat exchanger 14, a liquid container 16 and the low pressure side of the countercurrent heat exchanger 12 are connected in the flow direction between the throttle valve 13 and the inlet 19 of the compressor 10. The container 16 is connected to the evaporator output 15 and the gas output of the container 16 is connected to the heat exchanger 12.

The countercurrent heat exchanger 12 is not absolutely necessary for the operation of the system, but improves its efficiency, in particular its reaction rate against increased capacity requirements. It also serves to return oil to the compressor. For this purpose, the liquid conduit from the container 16 (shown in dotted line in Fig. 2) is connected 15 to the suction conduit either before the countercurrent heat exchanger 12 at 17 or thereafter at 18 or any location between these points. The fluid flow, e.g. refrigerant and oil, are controlled by an appropriate conventional throttle member (not shown in the drawing). By allowing a 20 excess amount of liquid to enter the suction line, a corresponding excess of liquid is obtained at the evaporator outlet.

In another embodiment of the invention shown in FIG. 3 25, the storage element of the refrigerant circuit is constituted by a container 22 which is integrated in the current circuit between a valve 21. and the throttle valve 13. The other components 10-14 of the circuit are identical to the components of the previous embodiment, although the heat exchanger 12 can be omitted, 30 without having any major consequences. The pressure in the container 22 is maintained between the high pressure side and the low pressure side pressure.

In a third embodiment of the invention shown in FIG.

4, the storage element of the circuit is a special container 25 where the pressure is maintained between the high pressure side and the low pressure side pressure. The storage element also consists of valves 23 8 DK 167985 B1 and 24, which are connected to the high-pressure side and the low-pressure side of the circuit respectively.

During operation, the refrigerant is compressed to a suitable supercritical pressure in the compressor 10; the state of compressor output 20 is shown as state "a" in FIG. 5. The refrigerant is circulated through the cooler 11 where it cools to state "b", delivering heat to a suitable secondary medium, e.g. air or water. If desired, the refrigerant may be further cooled to state "c" in the countercurrent heat exchanger 12 before being quenched to state "d".

In the pressure reduction in the throttle valve 13, a two-phase gas / liquid mixture is shown as state "d" in FIG. 5. The refrigerant absorbs heat in evaporator 14 by evaporation of the liquid phase. From state "e" at the evaporator output, the refrigerant vapor can be superheated in countercurrent heat exchanger 12 to state "f" and the circuit is completed. In the embodiment shown in FIG. 2 shows the preferred embodiment of the invention. the evaporator output state "e" will be in the two-phase range due to the excess liquid at the evaporator output.

The cooling capacity of the transcritical system is adjusted by varying the refrigerant state at the evaporator input, see point "d" in fig. 5. The cooling capacity per

unit coolant 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 of FIG. 5th

30 • Throttling occurs at constant enthalpy, so that the enthalpy at point "d" is equal to the enthalpy at "c". Accordingly, the cooling capacity (in kW) at constant refrigerant mass flow can be controlled by variation of the enthalpy at point "c".

It should be noted that in a transcritical process, high pressure single phase refrigerant is not condensed, but only cooled in the cooler 11. The outlet temperature from the cooler (point "b") will be some degrees above the inlet temperature of cooling air or water if countercurrent is used. The high pressure steam can then be cooled a few more degrees down to point 5 "c" in the countercurrent heat exchanger 12. The result is that the temperature in point "c" at the constant inlet temperature of cooling air or water will be substantially constant, independent of the pressure level on the high pressure side. Therefore, the cooling capacity of the system is regulated by varying the pressure on the high pressure side, while the temperature in point "c" is substantially constant. The curvature of the isotherms near the critical point implies a variation of the enthalpy with the pressure as shown in FIG. 5. The figure shows a reference process abcdef, a process with reduced capacity 15 due to reduced pressure on the high pressure side a'-b'-c'-d'-ef and a process with increased capacity due to higher pressure on the high pressure side a "- b "c" d "-ef. The evaporator pressure is assumed to be constant.

20 The pressure on the high pressure side is independent of the temperature because this side is filled with single-phase fluid. For variation of pressure, it is necessary to vary the mass of refrigerant on the high pressure side, ie. to add or remove some of the instant refrigerant volume on the high pressure 25 page. These variations must be absorbed by a buffer to avoid overfilling or drying of the evaporator.

In the embodiment shown in FIG. 2, the refrigerant mass on the high pressure side may be increased by temporarily reducing the opening of the throttle valve 13. Due to the short-term reduced coolant flow to the evaporator, the excess liquid in the evaporator outlet 15 will be reduced. The liquid refrigerant stream from. however, the container 16 into the suction line is constant.

Accordingly, the balance between the fluid flow entering and leaving the vessel 16 changes, resulting in a net reduction of the vessel's liquid content and a corresponding accumulation of refrigerant in the high-pressure side of the circuit.

The increase in volume on the high-pressure side results in increased pressure 5 and thus higher cooling capacity. This mass transfer from the low-pressure side of the circuit to its high-pressure side will continue until a balance is achieved between the cooling capacity and the heat load.

Opening the throttle valve 13 will increase the excess liquid volume at the evaporator outlet 15, because the vaporized amount of refrigerant is substantially constant. The difference between this fluid flow entering the vessel and the fluid flow from the vessel into the suction line will accumulate. The result is a net transport of refrigerant quantity from the high-pressure side of the circuit to its low-pressure side with the reduction of the high-pressure side volume stored in liquid form in the container. By reducing the high pressure side volume and thus the pressure, the capacity of the plant, 20 until there is a balance, is reduced between capacity and load.

Some fluid transport from the container into the suction line of the compressor is also required to avoid lubricant accumulation during the liquid phase of the container.

In the second embodiment of FIG. 3, the refrigerant mass on the high pressure side can be increased by simultaneously closing the valve 21 and opening the throttle valve 13 to provide a sufficient liquid flow to the evaporator. This will reduce the refrigerant flow from the high pressure side into the container through the valve 21 while transferring refrigerant from the low pressure side to the high pressure side of the compressor.

Reduction of the high-pressure side volume is achieved by opening the valve 21, while the flow through the throttle valve 13 is kept constant. This will transfer refrigerant from the high pressure side of the circuit to the container 22.

In the third of FIG. 4, the refrigerant mass on the high pressure side can be increased by opening the valve 24 and at the same time reducing the flow through the throttle valve 13. Thereby accumulating refrigerant on the high pressure side due to reduced flow through the throttle valve 13. Sufficient liquid supply to the steam generator is obtained. at the opening of the valve 24.

Reduction of the amount on the high pressure side can be accomplished by opening the valve 23 to transfer a certain amount of refrigerant from the high pressure side to the container. Capacity control of the system can then be performed by controlling the valves 23 and 24 and simultaneously controlling the throttle valve 13.

The FIG. 2, the preferred embodiment of the invention has the advantage of being simple with capacity control by operating only one valve. Furthermore, the compression cooling system constructed in this embodiment has some self-regulating ability in that load changes cause changes in the liquid content of the container 16, which implies changes in the high pressure side volume and thus the cooling capacity. In addition, operation with excess liquid at the evaporator output provides favorable heat transfer properties.

30 The second one in FIG. 3 shows the advantage of simplified valve control. The valve 21 controls only the pressure on the high pressure side of the system, and the throttle valve 13 only ensures that the evaporator is sufficiently fed. There can then-. For example, a conventional thermostatic expansion valve for throttling is used. The return of oil to the compressor is easily achieved by allowing the refrigerant to flow through the container. However, this embodiment does not allow for the capacity control function when the pressure in the high pressure side falls below the critical pressure. The volume of the container 22 must be relatively large, since the container only operates between the compressor discharge pressure and the fluid conduction pressure.

Yet another embodiment shown in FIG. 4 has the advantage of operating as a conventional compression cooling system when operating under stable conditions. The valves 23 and 24 connecting the container 25 to the power circuit are activated only under capacity control. This embodiment requires the use of three different valves during periods of capacity regulation.

The latter embodiments have the disadvantage that there is higher pressure in the container than in the preferred embodiment. However, the difference between the different systems in terms of construction and work characteristics is of no greater importance.

20

Transcritical compression refrigeration systems constructed in accordance with the described embodiments can be used in several areas. The technology is suitable in small and medium sized, stationary and mobile air conditioners, 25 small and medium refrigerators / freezers and in smaller heat pump systems. One of the most promising uses is in passenger car air conditioners, where there is a strong need for an alternative that does not use CFCs, has low weight and good efficiency.

30

EXAMPLES

. The practical application of the invention for cooling or heat pumping purposes is illustrated by the following Examples, which contain experimental results from a transcritical compression refrigeration system constructed according to the one shown in FIG. 2 which utilizes DK 167985 B1 13 carbon dioxide CO 2 as a refrigerant.

This laboratory test plant uses water as a heat source, ie. the water is cooled by heat exchange with boiling 5 CO 2 in the evaporator 14. Water is also used as a secondary refrigerant heated by C0 2 in the heat exchanger 11. The test plant contains a 61 cm piston compressor 10 and a container 16 with a total volume of 4 liters. The plant also contains a countercurrent heat exchanger 12 and liquid conduit connection from the container to point 17 as shown in FIG. 2. The throttle valve 13 is operated manually.

EXAMPLE 1 15 This example shows how control of cooling capacity is achieved by varying the position of the throttle valve 13, thereby varying the pressure on the high pressure side of the circuit. By varying the pressure, the specific refrigerant enthalpy is controlled at the evaporator input, which results in the regulation of the cooling capacity at constant mass flow.

The water inlet temperature of the evaporator 14 is kept constant at 20 ° C and the water inlet temperature of the heat exchanger 11 is kept constant at 35 ° C. The water circulation 25 is constant both in the evaporator 14 and in the heat exchanger 11. The compressor runs at a constant speed.

FIG. 6 shows the variation of the cooling capacity Q, the compressor shaft efficiency W, the pressure in the high pressure side p

H

The CO 2 mass flow m, the CO 2 temperature at the evaporator output t, the CO 2 temperature at the output of the heat exchanger. 11 and the liquid level in the receiver h when the throttle valve 11 is operated as indicated at the top of the figure. Adjusting the throttle valve position is the only manipulation.

DK 167985 B1 14

As can be seen in the figure, the cooling capacity Q is easily controlled by operation of the throttle valve 13. It is further clear from the figure that the circulating mass flow of CO2 ro under stable conditions is substantially constant and independent of the cooling capacity. The CC CC temperature at the output of the heat exchanger 11 t ^ is also substantially constant. The graphs show that the capacity variation is the result only of varying pressure in the high pressure side pH · 10 It is also seen from the diagram that increased pressure in the high pressure side results in a reduction of the liquid level h of the container due to the transfer of CO 2 to the high pressure side of the circuit.

15 Finally, it will be noted that the transitional period during capacity increase does not involve any significant superheating at the evaporator output, ie. that only • small fluctuations occur in t.

EXAMPLE 2

With higher water inlet temperature to the heat exchanger 11 (e.g. higher ambient temperature), it is necessary to increase the pressure in the high pressure side to maintain 25 constant cooling capacity. Table 1 shows results of experiments conducted with different water inlet temperatures for the heat exchanger 11 t.

w

The water inlet temperature of the evaporator is kept constant at 30 ° C and the compressor runs at a constant speed.

As the table shows, the cooling capacity can be kept substantially constant as the ambient temperature rises, by increasing the pressure in the high pressure side. The mass flow of 35 refrigerant is substantially constant as shown. Increased pressure implies a reduction in the liquid content of the container, as shown in the liquid level counters.

15 DK 167985 B1

Table 1

Inlet temperature t 35.1 45.9 57.3 eC

5

Cooling capacity Q 2.4 2.2 2.2 kW

Pressure in the high pressure side pH 84.9 94.3 114.1 bar

Mass flow m 0.026 0.024 0.020 kg / s Fluid level h 171 166 115 mm 10 EXAMPLE 3

FIG. 7 is a graphical representation of transcritical 15 circuit processes in the entropy / temperature diagram. The process curves shown in the diagram are based on measurements at the laboratory experimental plant during operation at five different pressures in the high pressure side. The evaporator pressure is kept constant. The refrigerant is CO 2 · 20

The diagram gives a good impression of the capacity control principle, showing changes in specific enthalpy h at the evaporator input by variation of the pressure p.

25 30 35

Claims (7)

A method of controlling a compression cooling system comprising a compressor (10), a cooler (11), a throttle member (13) and an evaporator (14) connected in series in a closed circuit with supercritical high pressure side , characterized in that the pressure in the high pressure side is regulated by variation of the refrigerant charge in the high pressure side of the system by varying the refrigerant content in a container which forms an integral part of the system and where the specific performance of the system is affected by the pressure regulation. Process according to claim 1, characterized in that the control is carried out by varying the refrigerant content in a container (16) located on the low pressure side. between the evaporator (14) and the compressor (10) alone using the throttle means (13). 20 Process according to claim 1, characterized in that variation of the refrigerant filling in the high pressure side is achieved by using a valve (21) and the throttle (13) to vary the content of refrigerant at supercritical pressure in a container (22) connected in the system between valve (21) and throttle member (13). Method according to claim 1, characterized in that variation of the refrigerant filling in the high pressure side 30 of the system is obtained by continuously controlling the supply or removal of refrigerant to or from a container (25) coupled to the high and low pressure side of the system by means of valves with valves. (23, 24) while maintaining the pressure in the container (25) between the high pressure 35 and the low pressure. 17 DK 167985 B1 Process according to claim 1, characterized in that carbon dioxide is used as refrigerant in the system. A heating / cooling device for carrying out the method 5 according to claim 1 comprising a compressor (10), a cooler (11), a throttle member (13) and an evaporator (14) connected in series in a closed circuit operating at supercritical pressure. on the high pressure side, characterized by,. the device further comprising a container (16) disposed between the evaporator (14) and the compressor (10), and wherein the throttle member (13) is arranged in the system between the cooler (11) and the evaporator (14) and is used to regulate the high pressure. in the system by varying the refrigerant content of the container (16). 15 Device according to claim 6, characterized in that an additional heat exchanger (12) is added to the system in that its low-pressure inlet (17) is coupled to the container (16) and its high-pressure inlet is coupled to the outlet of the cooler (11). ), and that the heat exchanger (12) is arranged in the system between the container (16) and the compressor (10). Device according to claim 6, characterized in that carbon dioxide is used as refrigerant in the system. 25 30 35