NO171810B - PROCEDURE FOR REGULATING A COMPRESSION DEBT SYSTEM WITH COOLING DEVICE FOR EXECUTING THE PROCEDURE - Google Patents

Description

This invention relates to compression refrigeration devices such as refrigeration systems, air conditioning systems and heat pumps, which use a refrigerant in a closed circuit under trans-critical conditions and more specifically to methods for regulation and control of such devices.

A conventional compression cooling circuit for cooling, air conditioning or heat pump purposes is shown in principle in Fig. 1. The device consists of a compressor 1, a condensation heat exchanger 2, a throttle valve 3, and an evaporation heat exchanger 4. These components are connected in a closed circuit, in which a refrigerant circulates.

The principle of the compression refrigeration system is as follows: The pressure and temperature of the refrigerant vapor is increased in the compressor 1, before it is fed into the condenser 2, where it is cooled, condensed and emits heat to a secondary medium. The high-pressure refrigerant liquid is then throttled to evaporation pressure and temperature using the expansion valve 3. In the evaporator 4, the refrigerant boils and absorbs heat from the surroundings. The refrigerant vapor at the pre-evaporator outlet is sucked into the compressor, and the circuit is completed.

Conventional compression refrigeration systems, which for example use the refrigerant R-12, CF2C12, work exclusively at subcritical pressures. A large number of different substances and mixtures of substances can be used as refrigerants. The choice is, among other things, affected by the condensation temperature, as the critical temperature of the medium sets an upper limit for condensation. In order to maintain a reasonable efficiency, it is usually desirable to use a refrigerant with a critical temperature at least 20-30°K above the condensation temperature. As a rule, one wants to avoid temperatures close to the critical when constructing and operating conventional systems. The current technology is discussed in detail in the literature, e.g. in "Handbooks" of the American Society of Heating, Refrigerating and Air Conditioning Engineers Inc.

(Fundamentals 1989 and Refrigeration 1986).

The ozone-depleting effect of today's common refrigerants (chloro-fluorocarbons, CFCs) has resulted in extensive international agreements to reduce or ban the use of these substances. Consequently, there is an urgent need to find alternatives to the current technology.

Capacity regulation of conventional compression refrigeration systems is mainly achieved by regulating the mass flow of the refrigerant that passes through the evaporator. This can be done, for example, by checking the compressor capacity, by throttling or by bypass circuits. These methods involve more complicated systems, reduced efficiency in part-load operation and other complications.

A common type of throttling device is a thermostatic expansion valve, which is controlled by the superheat in the evaporator outlet. Correct valve function under varying operating conditions is achieved by using a significant portion of the evaporator to superheat the refrigerant, resulting in low heat transfer rates.

Heat release in the condenser in conventional compression refrigeration systems mainly takes place at a constant temperature. When heat is given off to a secondary medium that is to be heated at a sliding temperature, such as in heat pump systems, or when there is a limited supply of secondary medium, thermodynamic losses occur due to large temperature differences.

Operation of compression refrigeration plants under trans-critical conditions has previously been practiced to some extent. Until CFCs took over 40 to 50 years ago, C02 was commonly used as a refrigerant, especially in ships for cooling provisions and cargo. The systems were designed for normal operation at subcritical pressures with evaporation and condensation, and with seawater for condenser cooling. Occasionally, typically in tropical waters, the seawater temperature could become too high to achieve normal condensation on the high-pressure side. (Critical temperature for C02 is 31°C) . In this situation, it was common to increase the refrigerant charge in the high-pressure side so that the pressure at the compressor outlet rose to 90-100 bar in order to maintain the cooling capacity at an acceptable level. C02 cooling technology is described in older literature, e.g. Ostertag "Kalteprozesse", Springer 1933 or H.J. Maclntire "Refrigeration Engineering", Wiley 1937.

The common practice in older C02 systems was to supply the necessary extra charge from separate storage containers. A liquid collector installed after the condenser in the usual way will not be able to cover the intended functions of this invention.

Another possibility for increasing the capacity and efficiency of a compression refrigeration plant operating with a supercritical high pressure side is known from German patent 278095 (1912). This method involves two-stage compression with intermediate cooling in the supercritical area. Compared to the standard system, this entails the installation of an additional compressor or pump, and an additional heat exchanger.

The handbook "Principles of Refrigeration" by W.B. Gosney (Cambridge Univ. Press 1982) points out some of the peculiarities of operation of refrigeration plants near critical pressure. It is suggested that increasing the refrigerant charge on the high pressure side could be achieved by briefly closing the throttle valve, to transfer some charge from the evaporator. However, it is emphasized that this will lead to a lack of liquid in the evaporator and thus reduced cooling capacity at the time it is most needed.

It is therefore the purpose of this invention to provide a new, improved, simple and effective way of regulating and controlling a trans-critical compression refrigeration system, and avoid the above-mentioned shortcomings and disadvantages that characterize current systems. Another aim of the invention is to achieve a cooling process where it is possible to avoid using CFC cooling media, and at the same time open up the possibility of being able to use more favorable substances with regard to safety, environmental friendliness and price.

Another purpose of the invention is to offer a new method for capacity regulation which involves operation mainly with constant mass flow, and simple capacity control by means of a valve.

Another object of the invention is to offer a compression cooling process which emits heat at a sliding temperature, so that the heat exchange losses can be reduced in applications where the flow of secondary medium is small, or when the secondary medium is heated to a relatively high temperature.

The above-mentioned and other purposes of the present invention are achieved by using a compression cooling process which works mainly at trans-critical conditions (i.e. supercritical high pressure side, subcritical low pressure side), where the pressure in the high pressure side is regulated by means of the refrigerant filling in the high pressure side of the system.

The invention involves regulation of specific enthalpy at the evaporator inlet by deliberate use of pressure before throttling, for example by capacity regulation. The capacity is controlled by varying the enthalpy difference of the refrigerant above the evaporator, by regulating the specific enthalpy of the refrigerant before throttling. In supercritical conditions, this can be done by varying pressure and temperature independently of each other. In a preferred embodiment of this regulation, specific enthalpy is controlled by varying the pressure before throttling. The refrigerant is cooled as far as possible using the available secondary medium, and the pressure is regulated to achieve the desired enthalpy.

The invention will be described in detail below, with reference to the attached drawings, Fig. 1, 2, 3, 4, 5, 6, and 7, where: Fig. 1 is a schematic representation of a conventional

(subcritical) compression refrigeration system.

Fig. 2 is a schematic representation of a trans-critical compression shoulder system constructed in accordance with a preferred embodiment of the invention. This embodiment includes a volume as an integral part of the evaporator system, where the refrigerant is kept in a liquid state. Fig. 3 is a schematic representation of a trans-critical compression cooling system in accordance with another embodiment of the invention. This embodiment includes an intermediate pressure vessel connected directly into the system between two valves. Fig. 4 shows a schematic representation of a trans-critical compression cooling system constructed in accordance with a third embodiment of the invention.

This embodiment includes a special container where the refrigerant can be in the liquid phase or in a supercritical state.

Fig. 5 illustrates the relationship between pressure and enthalpy

in a trans-critical compression cooling process such as in the systems in Fig. 2, 3 or 4, at different operating conditions. Fig. 6 is a collection of curves illustrating the regulation of cooling capacity by means of the pressure in the high pressure side, in accordance with the invention. The results shown were measured in a laboratory demonstration plant, built in accordance with a preferred embodiment of the invention. Fig. 7 are measurement results showing the relationship between temperature and entropy in a trans-critical compression cooling process, cf. Fig. 2. The curves show measurement results from experiments with C02 at different pressures in the high-pressure side of the plant.

A trans-critical compression refrigeration system in accordance with the invention includes a refrigerant, with a critical temperature that is higher than the heat absorption temperature and lower than the heat release temperature, and a closed circuit where the refrigerant circulates.

Suitable refrigerants are, for example: ethylene (C2H4), diborane (B2H6), carbon dioxide (C02), ethane (C2H4) and nitrogen oxide (N20).

The closed refrigerant circuit consists of a flow circuit with an integrated storage element. Fig. 2 shows a preferred embodiment of the invention where the storage element is an integral part of the evaporator system. The flow circuit includes a compressor 10 connected in series to a cooler 11, a counterflow heat exchanger 12 and a throttle valve 13. The throttle valve can be replaced by an expansion device. An evaporation heat exchanger 14, a liquid container 16 and the low pressure side of the 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 outlet 15, and the gas outlet of the container 16 is connected to the heat exchanger 12.

Counterflow heat exchanger 12 is not absolutely necessary for the system to work, but improves its efficiency, in particular it reduces the response time when increasing capacity. It also serves to return oil to the compressor. For this purpose, the liquid line from the container 16 (shown by dotted line in Fig. 2) is connected to the suction line either before the counter-flow heat exchanger 12 at 17 or after it at 18, or anywhere between these points. The liquid flow, e.g. refrigerant and oil, is regulated with a suitable conventional throttle device (not shown in the figure). By allowing some excess liquid to enter the suction line, a corresponding excess liquid will be introduced at the evaporator outlet.

In the second embodiment of the invention as shown in Fig. 3, the storage element in the refrigerant circuit constitutes a container 22 integrated in the flow circuit between a valve 21 and the throttle valve 13. The other components 10-14 in the flow circuit are identical to the components of the above embodiment, although the heat exchanger 12 can be avoided without any major consequences. The pressure in the container 22 is kept at a level between high pressure and low pressure in the flow circuit.

In the third embodiment of the invention shown in Fig. 4, the storage element in the refrigerant circuit is a special container 25, where the pressure is kept between the pressure of the high pressure side and the pressure of the low pressure side. The storage element also consists of the valves 23 and 24 which are connected respectively to the high-pressure side and the low-pressure side of the flow circuit.

In operation, the refrigerant is compressed to a suitable supercritical pressure in the compressor 10, the compressor outlet 20 is shown as "a" in

Fig. 5. The refrigerant passes through the cooler 11 and is cooled to state "b" by heat being emitted to a suitable secondary medium, e.g. air or water. If desired, the refrigerant can be further cooled to state "c" in the counter-flow heat exchanger 12, before throttling to state "d". During the pressure reduction in the throttle valve 13, a two-phase gas/liquid mixture is formed, shown as condition "d" in Fig. 3. The refrigerant absorbs heat in the evaporator 14 by evaporation of the liquid phase. From state "e" at the evaporator outlet, the refrigerant vapor can be superheated to state "f" in the counter-flow heat exchanger 12, and the circuit is complete. In the preferred embodiment of the invention, shown in Fig. 2, the state "e" of the evaporator outlet will be in the two-phase region due to the liquid excess in the evaporator outlet. Regulation of the trans-critical system's cooling capacity is achieved by varying the state of the evaporator inlet, see point "d" in Fig. 5. The cooling capacity per unit refrigerant mass flow corresponds to the enthalpy difference between state "d" and state "e". This enthalpy difference can be illustrated as a horizontal length in the enthalpy-pressure diagram, Fig. 5.

Throttling takes place at constant enthalpy, so that enthalpy in state "d" is equal to enthalpy in state "c", Consequently, the cooling capacity (in kW) at constant mass flow can be controlled by varying the enthalpy in point "c".

It should be noted that in a trans-critical process, the high-pressure single-phase refrigerant will not be condensed but only cooled in the cooler 11. The outlet temperature from the cooler (state "b") will be a few degrees higher than the inlet temperature of cooling water or cooling air, if countercurrent heat exchange is used. The high-pressure medium can then be further cooled a few degrees to state "c" in the counter-flow heat exchanger 12. The result is that at a constant inlet temperature of cooling air or cooling water, the temperature at point "c" will be approximately constant, regardless of the pressure level in the high-pressure side. Regulation of the cooling capacity can thus be achieved by varying the pressure in the high-pressure side, while the temperature at point "c" is essentially constant. The course of the isotherms near the critical point means that the enthalpy varies with the pressure, as shown in Fig. 5. The figure shows a reference process (a-b-c-d-e-f), a process with reduced capacity due to reduced pressure in the high-pressure side of the system (a1-b1-c1 -d1-e-f) and a process with

increased capacity due to increased pressure (a"-b"-c"-d"-e-f). The evaporator pressure is assumed to be constant.

The pressure in the high-pressure side of the system is independent of the temperature, because this part of the system is filled with a single-phase medium. In order to vary the pressure, it is necessary to change the refrigerant charge in this part of the system, that is to say that one must add or remove some of the instantaneous refrigerant charge on the high-pressure side. These variations must be taken up by a buffer to avoid overfilling or drying out the evaporator.

In the preferred embodiment of the invention indicated in Fig. 2, the refrigerant filling on the high-pressure side can be increased by temporarily reducing the flow in the throttle valve 13. Due to the short-term reduction in the flow to the evaporator that then occurs, the liquid overflow in the evaporator outlet (15) will be reduced. The liquid flow of refrigerant out of the container (16) into the suction line is, however, constant. Consequently, the balance between liquid flowing into and out of the container will be shifted, resulting in a net reduction in the container's liquid content. A corresponding increase in filling will take place in the high-pressure side of the system.

The increase in filling in the high-pressure side results in increased pressure and thereby higher cooling capacity. This mass transfer from the low-pressure to the high-pressure side of the circuit will continue until a balance between the plant's cooling capacity and the heat load is achieved.

By increasing the flow through the throttle valve 13, the excess liquid in the evaporator outlet 15 will increase, because the evaporated quantity of the refrigerant is essentially constant. The difference between this liquid flow from the evaporator which is fed into the container and the liquid flow out of the container which is fed into the suction line will accumulate. The result is a net transfer of refrigerant from the high pressure side to the low pressure side of the system, with the reduction in high pressure side charge stored in a liquid state in the container. By reducing the high-pressure side filling and thereby the pressure, the cooling capacity will be reduced until a balance is achieved between capacity and load. Some liquid transport into the suction line is also necessary to avoid accumulation of oil in the liquid phase in the container.

In the second embodiment of the container arrangement shown in Fig. 3, the refrigerant charge on the high pressure side can be increased by closing the valve 21 and simultaneously opening the throttle valve 13 to supply the evaporator with sufficient liquid flow. This will reduce the refrigerant flow from the high-pressure side into the container 22 through valve 21, while refrigerant is transported from the low-pressure side to the high-pressure side of the compressor.

Reduction of the filling in the high-pressure side is achieved by opening valve 21 while the flow through the throttle valve is kept more or less constant. Refrigerant will then be transferred from the high-pressure side to the container 22.

In the third embodiment of the invention indicated in Fig. 4, the refrigerant filling in 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. Thus, refrigerant accumulates in the high-pressure side due to the reduced flow through the throttle valve 13. Sufficient liquid supply to the evaporator is achieved by to open valve 24. A reduction in the high-pressure side's filling can be achieved by opening valve 23 to transfer some refrigerant from the high-pressure side to the container. Capacity control of the system will then be achieved by regulating the valves 23 and 24, at the same time as the throttle valve 13 is controlled.

The preferred embodiment of the invention, as shown in Fig. 2, has the advantage of being very simple, with capacity regulation only by operating a valve. Furthermore, a trans-critical compression refrigeration system built in accordance with this design can have a certain self-regulating ability in that load changes cause a change in the liquid filling in the container 16, which causes changes in the filling on the high-pressure side and thus also affects the cooling capacity. In addition, operation with excess liquid in the evaporator outlet will provide favorable heat transfer properties.

The second embodiment shown in Fig. 3 has the advantage of a simple valve control. Valve 21 only regulates the pressure on the high-pressure side of the device, and throttle valve 13 should only ensure that the evaporator receives a sufficient supply of liquid. A conventional thermostatic expansion valve can then be used. Oil return to the compressor is achieved by allowing the refrigerant to flow through the container 22. With this design, however, the capacity control function is not achieved when the pressure in the high pressure side falls below the critical level. The volume of the container 22 becomes relatively large since it only operates between the compressor outlet pressure and the pressure in the liquid line.

An embodiment as indicated in Fig. 4 has the advantage of operating as a conventional compression refrigeration plant when operating under stable conditions. The valves 23 and 24 which connect the container 25 to the flow circuit are only activated during capacity regulation. This design requires the use of three different valves for capacity regulation.

The latter embodiments have the disadvantage of higher pressure in the container than in the preferred embodiment. However, the difference between the various solutions in terms of construction and operating characteristics is not very noticeable.

Trans-critical compression cooling systems built in accordance with the structures described can be used in several areas. The technology is well suited in small and medium-sized stationary and mobile air conditioning systems, small and medium-sized refrigerators/freezers and in smaller heat pump systems. One of the most promising areas of use is in passenger car air-conditioning cooling systems, where there is currently a strong need for an alternative that does not use CFC substances, has low weight and sufficient efficiency.

Examples

The practical use of the invention for cooling and heat pump purposes is illustrated by the following examples, which provide test results from a trans-critical compression refrigeration plant, built in accordance with the embodiment of the invention shown in Fig. 2, and with carbon dioxide (C02) as refrigerant. This laboratory test device uses water as a heat source, i.e. the water is cooled by heat exchange with boiling C02 in the evaporator 14. The water is also used as a cooling secondary medium, i.e. it is heated by C02 in the heat exchanger 11. The test device includes a 61 ccm piston compressor (10) and a container (16) with a total volume of 4 litres. The system also includes a counter-flow heat exchanger (12) and a liquid line from the container to point 17, as indicated in Fig. 2. Throttle valve 13 is of the manual type.

Example 1

This example shows how regulation of the cooling capacity is achieved by varying the position of throttle valve 13, so that the pressure in the high-pressure side of the system is varied. By varying the pressure, the specific enthalpy of the refrigerant in the evaporator inlet changes, and the cooling capacity is thus regulated with an approximately constant mass flow. The water temperature entering the evaporator 14 is kept constant at 20°C, and the water temperature entering the heat exchanger 11 is kept constant at 35°C. The water circulation is constant, both through the evaporator 14 and in the heat exchanger 11. The compressor runs at a constant speed. Fig. 6 shows the variation in the cooling capacity (Q), the compressor shaft power (W), the pressure in the high pressure side (pH), C02 mass flow (m), C02 temperature at the evaporator outlet (te), C02 temperature at the outlet of the heat exchanger 11 ( tb) and liquid level in the container (h) when the throttle valve position is varied as shown at the top of the figure. The adjustment of the throttle valve position is the only influence on the process that takes place.

As shown in the figure, the cooling capacity (Q) can be easily regulated by operating the throttle valve (13). It is further clear from the figure that the mass flow of C02 (m) is approximately constant under stable conditions, and furthermore independent of the cooling capacity. The C02 temperature at the outlet of the heat exchanger 11 (tb) is also approximately constant. The curves show that the variation in cooling capacity is only a result of varying pressure in the high pressure side.

In the diagram you can also see that increased pressure in the high pressure side is accompanied by a reduction in the container's liquid level (h) due to the transfer of C02 to the high pressure side. Finally, it can be noted that the transition period when changing capacity does not cause any noticeable overheating in the evaporator outlet, i.e. only small fluctuations in tea.

Example 2

At a higher inlet temperature of the water circulating through the heat exchanger 11 (ie higher ambient temperature) it is necessary to increase the pressure in the high pressure side to maintain a constant cooling performance. Table 1 shows the results from experiments with different inlet temperatures to the heat exchanger 11 (tM). The inlet temperature of the water circulating through the evaporator is kept constant at 20°C, and the compressor runs at a constant speed. As the table shows, the cooling capacity can be kept approximately constant when the ambient temperature rises, by increasing the pressure pH. The mass flow C02 is approximately constant, as shown. increased pressure implies a reduction in the container's liquid content, as shown in

the table.

Example 3

Fig. 7 is a graphical representation of trans-critical circuit processes in the entropy/temperature diagram. The process curves in the diagram are based on measurement data from the laboratory system. The measurements were carried out at five different pressures in the high-pressure side of the system. The evaporator pressure is kept constant. The refrigerant is C02. The diagram gives a good impression of the capacity regulation principle, and shows the change in specific enthalpy (h) at the evaporator inlet when the pressure (p) varies.

Claims (8)

1. Procedure for regulating a compression shoulder system comprising compressor (10), cooler (11), throttling device (13) and evaporator (14) connected in series in a closed circuit with supercritical pressure on the high pressure side, characterized in that the pressure in the high-pressure side is regulated by varying the refrigerant filling in the high-pressure side of the system by varying the refrigerant content in a container that forms an integral part of the system, and where the specific performance of the system is affected by the pressure regulation. 2. Method according to claim 1, characterized by that the regulation is carried out by varying the refrigerant content in a container (16) on the low pressure side located between the evaporator (14) and the compressor (10) only using the throttle (13). 3. Method according to claim 1, characterized by that variation of the refrigerant filling in the high-pressure side of the system 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 the valve (21) and the throttle (13). 4. Method according to claim 1, characterized by that variation of the refrigerant filling in the high-pressure side of the system is achieved by continuously regulating the supply or removal of refrigerant to or from a container (25) connected to the high- and low-pressure side of the system by means of lines with valves (23,24) at the same time as the pressure in the container ( 25) is kept between the system's high pressure and low pressure. 5. Method according to claim 1, characterized by that carbon dioxide is used as a refrigerant in the system. 6. A heating/cooling device for carrying out the method according to claim 1 comprising a compressor (10), a cooler (11), a throttle device (13) and an evaporator (14) connected in series in a closed circuit which operates with a supercritical pressure of high pressure side, characterized by that the device further comprises a container (16) placed between the evaporator (14) and the compressor (10), and where the throttle (13) placed in the system between the cooler (11) and the evaporator (14) is used to regulate the high pressure in the system by varying the refrigerant content in the container (16). 7. Device according to claim 6, characterized by that a further heat exchanger (12) is added to the system in that its low-pressure inlet (17) is connected to the container (16) and its high-pressure inlet is connected to the outlet of the cooler (11) and that the heat exchanger (12) is placed in the system between the container ( 16) and the compressor (10) . 8. Device according to claim 6, characterized by that carbon dioxide is used as a refrigerant in the system.