RU2039914C1 - Method of control of pressure at high-pressure side of device and cooling or heating device (versions) - Google Patents

Abstract

FIELD: refrigerating engineering. SUBSTANCE: after compression of cooling agent, supercritical pressure is regulated through instantaneous change of its amount at high-pressure side of loop due to change of total amount of cooling agent contained in receiver; increase of pressure is achieved through change of total amount of cooling agent. EFFECT: enhanced efficiency. 8 cl, 4 dwg

Description

 The invention relates to cyclic devices with steam compression in transcritical conditions, such as refrigeration units, air conditioning units and heat pumps that use refrigerant operating in a closed circuit in transcritical conditions, and, in particular, to methods of modulation and performance control of such devices.

 A typical cyclic steam compression device for cooling, air conditioning or a heat pump consists of a compressor, a condenser heat exchanger, a throttle valve and an evaporative heat exchanger. These elements are connected into a closed circulation circuit in which refrigerant circulates. The principle of operation of the cyclic device with vapor compression is as follows. The pressure and temperature of the refrigerant vapor is increased by the compressor before it enters the condenser, where it is cooled and condensed, giving off heat to the secondary coolant. Then, the liquid under high pressure is throttled to the pressure and temperature of the evaporator by means of an expansion valve. In the evaporator, the refrigerant boils and absorbs heat from the surrounding area. The vapor at the outlet of the evaporator is drawn into the compressor, completing the cycle.

Conventional steam compression cyclic devices use refrigerant (e.g. R-12, CF ₂ Cl ₂ ), operating completely at subcritical pressures. A number of different substances can be used as a refrigerant. The choice of refrigerant, among other factors, is determined by the condensation temperature, since the critical temperature of the liquid sets an upper limit at which condensation still occurs. To maintain sufficient efficiency, it is advisable to use a refrigerant with a critical temperature of at least 20-30 K above the condensation temperature. Typically, when creating and operating conventional systems, close to critical temperatures are avoided.

 This technology is described in detail in the literature, such as reference books published by the American Society of Thermal, Refrigeration, and Air-Conditioning Engineering: Fundamentals 1989 and Refrigeration 1986.

 The destructive effect on the ozone layer of the earth of the currently widely used refrigerants (halocarbons) has led to a strong international reaction aimed at reducing or prohibiting the use of these liquids. Therefore, there is an urgent need to develop alternative technologies to the existing one.

 The performance control of a conventional cyclic device with steam compression is mainly controlled by the mass flow rate of the refrigerant passing through the evaporator. This is done, for example, by controlling compressor capacity, throttling or bypass. These methods necessitate the use of more complex circulation circuits and components, additional equipment and devices, lower efficiency at part load and lead to other complications.

 A common type of fluid flow control device is a thermostatic expansion valve, which is controlled by superheated refrigerant at the outlet of the evaporator. Corresponding valve operation under varying operating conditions is achieved by using a significant portion of the evaporator to overheat the refrigerant, resulting in a low heat transfer coefficient.

 In addition, the heat transfer in the condenser of a conventional steam compression cycle takes place mainly at a constant temperature. Therefore, there are thermodynamic losses due to large temperature differences during heat transfer to the secondary coolant with a significant increase in temperature, which occur in heat pumps or when the available secondary coolant flow is negligible.

Previously, the operation of the steam compression cycle in transcritical conditions was practiced to a small extent. Prior to the transition to the widespread use of halocarbons, 40-50 years ago, carbon dioxide (CO ₂ ) was widely used as a refrigerant, especially on ships for cooling food supplies and cargo. Systems have been developed that operate normally at subcritical pressures with evaporation and condensation. Sometimes, when the vessel is floated in the tropics, the temperature of the cooling seawater could be too high for normal condensation and cooling unit has been forced to work under supercritical conditions on the high pressure side of the loop (the critical temperature for CO ₂ of about 31 ° ^C). In this case, it was practiced to increase the charge of refrigerant (the amount of refrigerant introduced into the system) on the high pressure side in a place where the pressure at the compressor outlet increased to 90-100 bar in order to maintain a sufficient refrigerating capacity. Cooling technology using CO ₂ is described, for example, in the books of Ostertag P. Kalteprozesse. Springer 1933 or Mac Intirs HJ Refrigeration Engineering, Wiley 1937.

It was common practice in older CO ₂ systems to add the necessary excess refrigerant from individual storage cylinders. A receiver mounted downstream of the capacitor would normally be incapable of performing the functions contemplated by the invention.

 Another way to increase the productivity and efficiency of a cyclic device with steam compression operating at supercritical pressure on the high pressure side is described in German patent N 278095 (1912). This method includes two-stage compression with intermediate cooling in the supercritical region. Compared to the standard system, this necessitates the installation of an additional compressor or pump and heat exchanger. In the book of Gosney W.B. Principles of Refrigeration. Cambridge Univ. Press, 1982, some features of work at subcritical pressures are indicated. It is contemplated that an increase in refrigerant charge on the high pressure side could be achieved by temporarily closing the expansion valve so that some of the charge is transferred from the evaporator. But this could lead to insufficient liquid in the evaporator, causing a decrease in productivity at the most inopportune moment.

 The aim of the invention is the development of a new improved simple and effective means for modulating and controlling the performance of a cyclic device with steam compression in transcritical conditions, free from the above disadvantages of the known means.

 Another objective of the invention is to develop a steam compression cycle in which refrigerants based on organofluorine compounds are not used and at the same time, several attractive refrigerants are possible from the point of view of safety, environmental friendliness and price.

 Another objective of the invention is the development of a new method of capacity management, which includes operating at an almost constant weight flow rate of the refrigerant and simple modulation of performance by actuating the valve.

 Another objective of the invention is the development of a cycle that gives off heat at a low temperature, reducing losses during heat transfer in applications where the secondary heat carrier flow is negligible or when the secondary heat carrier must be heated to a relatively high temperature.

 The objectives of the invention are achieved by the development of a method of operation usually under transcritical conditions (i.e., supercritical pressure on the high pressure side, subcritical pressure on the low pressure side of the cycle), where the thermodynamic properties in the supercritical state are used to control the cooling capacity and heat capacity of the device.

 The invention includes adjusting the specific enthalpy at the inlet of the evaporator by intentionally changing the pressure before throttling to control capacity. Productivity is controlled by changing the difference in the enthalpy of the refrigerant in the evaporator by changing the specific enthalpy of the refrigerant before throttling. In the supercritical state, this can be accomplished by an independent change in pressure and temperature. In a preferred embodiment of the invention, this modulation of specific enthalpy is accomplished by changing the pressure before throttling. The refrigerant is cooled to the maximum possible extent by means of the available cooling medium and the pressure is regulated to obtain the required enthaly.

 In FIG. 1 schematically illustrates a cyclic apparatus with steam compression under transcritical conditions, a preferred embodiment (this embodiment includes a container as an integral part of an evaporative system containing refrigerant in a liquid state); in FIG. 2 a cyclic action device with vapor compression under transcritical conditions in accordance with a second embodiment of the invention (this embodiment includes a receiver connected directly to the circulation circuit between the two valves); in FIG. 3 a cyclic device with steam compression under transcritical conditions in accordance with a third embodiment of the invention (this embodiment includes a special receiver for containing refrigerant in a liquid state or in a supercritical state); in FIG. 4 is a graph illustrating the dependence of pressure on the enthalpy of a cyclic device with vapor compression under transcritical conditions, shown in FIG. 1, 2 or 3, under various operating conditions.

The cyclic device with steam compression under transcritical conditions contains a refrigerant, the critical temperature of which is between the inlet temperature and the average temperature of the cooled object and a closed circuit of the working fluid in which the refrigerant circulates. Suitable working fluids may be, for example, ethylene (C ₂ H ₄ ), diborane (B ₂ H ₆ ), carbon dioxide (CO ₂ ), ethane (C ₂ H ₆ ) and nitric oxide (N ₂ O). The closed circuit of the working fluid consists of a circulation loop of the refrigerant with an integrated storage unit.

 In FIG. 1 storage unit is an integral part of the evaporation system. The circulation circuit includes a compressor 1 connected in series with the heat exchanger, a counterflow heat exchanger 2 and a throttling valve 3. The throttling valve can be replaced by some other expansion device. The evaporative heat exchanger 4, the separator-receiver 5 for separating the liquid and the counterflow heat exchanger 2 located on the low pressure side are connected to each other in the circulation direction between the throttle valve 3 and the inlet 6 of the compressor 1. The liquid receiver 5 is connected to the output 7 of the evaporator, the gas-phase output of the receiver is connected to counterflow heat exchanger 2.

 A counterflow heat exchanger 2 is not necessary for the operation of the device, but it increases its efficiency, in particular, the reaction rate to the requirements of increased productivity. To this end, the pipeline for the liquid phase from the receiver 5 (shown in dashed line in FIG. 1) is connected to the suction pipe either in front of the countercurrent heat exchanger 2 in place 8, or after it in place 9, or somewhere else between these points. Fluid flow i.e. refrigerant and oil controlled by a suitable fluid flow limiting device (not shown). Introducing a certain excess amount of liquid refrigerant into the steam line, excess liquid is obtained at the outlet of the evaporator.

 In FIG. 2, the accumulating fluid circuit block includes a receiver 10 integrated in the circulation circuit between the valve 11 and the throttling valve 3. The other elements of the circulation circuit are identical to the elements of the previous embodiment, although the heat exchanger can be removed without any serious consequences. The pressure in the receiver 10 is maintained intermediate between the pressure on the high pressure side and the pressure on the low pressure side of the circulation circuit.

 In FIG. 3, the accumulating block of the working fluid circuit includes a special receiver 12, where an intermediate pressure is maintained between the pressure on the high pressure side and the pressure on the low pressure side of the circulation circuit. In addition, the storage unit includes valves 13 and 14, which are connected to the high and low pressure parts of the circulation circuit, respectively.

 In operation, the refrigerant is compressed to a suitable supercritical pressure in compressor 1. FIG. 4, compressor output 15 is shown as state a. The refrigerant circulates through the heat exchanger 16, where it is cooled to state b, transferring heat to a suitable cooling medium, such as cooling air or water. If desired, the refrigerant can be further cooled to state c in the counterflow heat exchanger 2 before throttling to state d. By reducing the pressure in the throttling valve 3, a two-phase gas-liquid mixture is formed, shown in FIG. 2 as state d. The refrigerant absorbs heat in the evaporator 4 by evaporation of the liquid phase. From state e at the outlet of the evaporator, the refrigerant vapor may overheat in the counterflow heat exchanger 2 to state f before it enters compressor input 6, completing the cycle. As shown in FIG. 1, in a preferred embodiment of the invention, the state e at the outlet of the evaporator should be in the two-phase region due to excess liquid at the outlet of the evaporator.

 The modulation of the performance of a cyclic device operating under transcritical conditions is performed by changing the state of the refrigerant at the inlet of the evaporator, i.e. point d in FIG. 4. The refrigerating capacity per unit mass flow rate of the refrigerant corresponds to the enthalpy difference between state d and state e. This enthalpy difference corresponds to the horizontal section on the pressure dependence of enthalpy in FIG. 4.

 Throttling is a constant enthalpy process, so the enthalpy at point d is equal to the enthalpy at point c. As a result, the cooling capacity (in kilowatts) at a constant flow rate of refrigerant can be controlled by changing the enthalpy at point c.

 In a transcritical cycle, single-phase refrigerant vapor does not condense under high pressure, but decreases its temperature in the heat exchanger 16. The temperature of the refrigerant at the outlet of the heat exchanger (point b) should be several degrees higher than the temperature of the incoming cooling air or water, if countercurrent is used. Then the high pressure steam can be cooled several degrees to point c in the counterflow heat exchanger 2. However, the result is that at a constant inlet temperature of cooling air or water, the temperature at point c should be almost constant, independent of the pressure level on the high pressure side .

 Therefore, the modulation of the performance of the device is carried out by changing the pressure on the high pressure side, while the temperature at point c is almost constant. Bending of isotherms near a critical point leads to a change in enthalpy with a change in pressure, as shown in FIG. 4. In FIG. 4 shows a reference cycle (abcdef), a cycle with reduced productivity due to reduced pressure on the high pressure side (a'-b'-c'-d'-ef) and a cycle with increased productivity due to higher pressure on the high pressure side (a ' '-b' '- c' '- d' '- ef), it is assumed that the pressure in the evaporator is constant.

 The pressure on the high pressure side is temperature independent, as it is filled with a single-phase liquid. To change the pressure, it is necessary to change the mass of the refrigerant on the high pressure side, i.e. add or remove a little refrigerant from the current refrigerant charge on the high pressure side. These changes must be carried out by a buffer device to prevent overflow of liquid or drying of the evaporator.

 In the preferred embodiment of the invention shown in FIG. 1, the mass of the refrigerant on the high pressure side can be increased by temporarily decreasing the orifice of the butterfly valve 3. Due to the accidentally reduced refrigerant to the evaporator, the excess liquid fraction at the outlet of the evaporator should decrease. However, the flow of liquid refrigerant from the receiver 5 into the suction pipe is constant. Therefore, the equilibrium between the liquid flow entering and leaving the receiver 5 is violated, which leads to a decrease in the liquid content of the receiver and the corresponding accumulation of refrigerant on the high pressure side of the circulation circuit.

 An increase in refrigerant charge on the high pressure side is accompanied by an increase in pressure and thereby increased cooling capacity. This transfer of the mass of refrigerant from the low pressure side to the high pressure side of the circulation circuit continues until an equilibrium is established between refrigerating capacity and load.

 Opening the throttle valve 3 increases the excess liquid fraction at the outlet 7 of the evaporator, since the evaporated amount of refrigerant is almost constant. The difference between this fluid flow entering the receiver and the fluid flow from the receiver to the suction pipe increases. As a result, delivery of the refrigerant charge in its pure form from the high pressure side to the low pressure side of the circulation circuit is carried out with a decrease in the amount of refrigerant on the high pressure side accumulated in the liquid state in the receiver. By reducing the refrigerant charge on the high pressure side and thereby the pressure, the productivity of the device is reduced until an equilibrium is established.

 The delivery of a certain amount of liquid from the receiver to the suction pipe is also necessary to prevent the accumulation of lubricant in the liquid phase of the receiver.

 In the embodiment of the invention shown in FIG. 2, the mass of refrigerant on the high pressure side can be increased by simultaneously closing valve 11 and modulating the throttle valve 3 to provide the evaporator with a sufficient liquid flow. This reduces the flow of refrigerant from the high pressure side to the receiver through the valve 11, while the mass of refrigerant is transferred from the low pressure side to the high pressure side by the compressor.

 The decrease in loading on the high pressure side is obtained by opening the valve 11 while maintaining a substantially constant flow through the throttle valve 3. This transfers the mass from the high pressure side of the circulation circuit to the receiver 10.

 In the embodiment of the invention shown in FIG. 3, the mass of refrigerant on the high pressure side can be increased by opening valve 14 and simultaneously reducing the flow through the throttling valve 3. Thus, on the high pressure side, the mass of refrigerant is accumulated due to the reduced flow through the throttle valve 3. Adequate liquid flow to the evaporator is ensured by opening valve 14.

 Reducing the load on the high pressure side can be achieved by opening the valve 13 to transfer some of the load from the high pressure side to the receiver. Thus, the performance control of the device is carried out by modulating the valves 13 and 14 and simultaneously actuating the throttle valve 3.

 As shown in FIG. 1, a preferred embodiment of the invention has the advantage of simplicity with performing performance control by actuating only one valve. In addition, the cyclic steam compression device under transcritical conditions according to this embodiment has a certain ability to self-regulate by adapting to changes in the load through changes in the liquid contents of the receiver 5, including changes in the charge of the refrigerant on the high pressure side and, thus, the cooling capacity. In addition, working with excess liquid at the outlet of the evaporator provides favorable heat transfer characteristics.

 As shown in FIG. 2, this embodiment of the invention has the advantage of simplified valve operation. The valve 11 controls only the pressure on the high pressure side of the device, and the throttle valve 3 only ensures the normal operation of the evaporator. Thus, a conventional thermostatic expansion valve can be used for throttling. Oil is returned to the compressor easily by allowing refrigerant to flow through the receiver. However, in this embodiment, a high pressure side performance control function below a critical pressure is not proposed. The volume of the receiver 10 should be relatively large, since it only works under conditions of intermediate pressure between the outlet pressure and the pressure in the pipeline with the liquid.

 The embodiment of the invention shown in FIG. 3, has the advantage of operating as a conventional cyclic steam compression device when it is operating in stable conditions. Valves 13 and 14, through which the receiver 12 is connected to the circulation circuit, are actuated only during the performance control. This embodiment of the invention necessitates the use of three different valves during periods of performance change.

 Recent embodiments of the invention have the disadvantage of increased pressure in the receiver compared to the preferred embodiment of the invention. However, the differences between the individual systems regarding design and operating parameters are not very significant.

 Steam compression cyclic devices under transcritical conditions according to the described embodiments of the invention can be used in several fields. This technology is very suitable for small-sized and medium-sized stationary and mobile air conditioning units, small-sized and medium-sized refrigerators-freezers and small installations with heat pumps. One of the most promising applications is an automobile air conditioning unit, where there is an urgent need to use new, lightweight, non-organofluorine compounds and more efficient alternative systems to a system using R-12 refrigerant.

 The above embodiments of the invention serve only as an example and do not limit the scope of the invention. Obviously, it is also possible to control the performance of a cyclic device operating under transcritical conditions by maintaining an almost constant pressure on the high pressure side and regulating the temperature of the refrigerant before throttling (state c) by changing the speed of circulation of cooling air or water. By reducing the flow of coolant, i.e. air or water, the temperature before throttling increases and performance drops. The increased flow of coolant reduces the temperature before throttling and thereby increases the performance of the device. Combinations of pressure and temperature control are also possible.

The practical application of the invention for cooling or use in heat pumps is illustrated by examples, which show the test results of a cyclic device with steam compression under transcritical conditions according to the embodiment of the invention shown in FIG. 1 using CO ₂ as a refrigerant.

The laboratory experimental device used water as a heat source, i.e. water was cooled by heat exchange with boiling carbon dioxide in the evaporator 4. Water was also used as a cooling medium heated by carbon dioxide in the heat exchanger 16. The experimental device included a piston compressor with a cylinder volume of 61 cm ³ and a receiver 5 with a total volume of 4 l. The system also included a counterflow heat exchanger 2 and a fluid conduit from the receiver to point 8, as shown in FIG. 1. Throttle valve 3 was manually actuated.

 PRI me R 1 shows how the cooling capacity is controlled by changing the position of the throttle valve 3, thereby changing the pressure on the high pressure side of the circulation circuit. By changing the pressure on the high pressure side, the specific enthalpy of the refrigerant at the inlet of the evaporator is controlled, leading to modulation of the refrigerating capacity at a constant weight flow.

Water temperature at the entrance of the evaporator 4 is maintained constant at 20 ^{° C,} the water temperature at the inlet of the heat exchanger 16 was kept constant at 35 ^C. Water circulation was carried out continuously in the evaporator 4 and a heat exchanger 16. The compressor operates at a constant speed. The adjustment of the position of the throttle valve 3 was carried out by a single manipulation. Cooling capacity was easily controlled by actuating the throttle valve 3.

Under stable conditions, the circulating mass flow of CO ₂ is practically constant and does not depend on cooling capacity. The temperature of CO ₂ at the outlet of the heat exchanger 16 is also almost constant. The change in device performance was the result of a change in pressure only on the high pressure side. The increased pressure on the high pressure side led to a decrease in the liquid level in the receiver due to the transfer of part of the CO ₂ charge to the high pressure side of the circulation circuit. The transition period during the increase in productivity was not accompanied by any significant overheating at the outlet of the evaporator, i.e. there were only minor changes in the temperature of CO ₂ at the outlet of the evaporator.

Example 2. At a higher temperature of the water at the inlet of the heat exchanger 16 (for example, a higher ambient temperature), in order to maintain a constant cooling capacity, it is necessary to increase the pressure on the high pressure side. The water temperature at the inlet of the evaporator was kept constant at 20 ^° C, and the compressor was operating at a constant speed.

 Cooling capacity can be maintained almost constant with increasing ambient temperature by increasing pressure on the high pressure side. The mass flow rate of the refrigerant is almost constant. The increase in pressure on the high pressure side was accompanied by a decrease in the liquid content in the receiver, which is noted by the readings of the liquid level.

Claims (7)

 1. A method of controlling pressure on the high pressure side of a device operating along the steam compression cycle, comprising sequentially compressing the refrigerant to supercritical pressure, cooling, throttling and evaporating, characterized in that the supercritical pressure after compression is controlled by changing the instantaneous amount of refrigerant on the high side the pressure of the circuit by changing the total amount of refrigerant in the receiver of the circuit, and the increase in pressure is achieved and Menenius total amount of refrigerant.  2. The method according to claim 1, characterized in that the pressure control on the supercritical pressure side is carried out to provide modulation of the cooling capacity of the circuit.  3. The method according to claims 1 and 2, characterized in that the refrigerant is carbon dioxide.  4. The method according to claim 1, characterized in that a vapor-liquid mixture is obtained from the outlet of the evaporator by providing an excess of liquid prior to heat exchange with the refrigerant on the supercritical pressure side after compression.  5. A refrigeration or heating device operating along the contour of the steam compression cycle, comprising a series-installed compressor, cooler, throttle device and evaporator, characterized in that it further comprises a receiver and a counter-flow heat exchanger, the receiver being installed between the evaporator and the compressor, one cavity of the counter-current heat exchanger is included between the receiver and the compressor, another cavity between the cooler and the throttle device.  6. A refrigeration or heating device operating along the contour of the steam compression cycle, including a series-installed compressor, cooler, throttle device and evaporator, characterized in that it further comprises a receiver with an inlet valve located between the cooler and the choke, and a two-cavity counterflow heat exchanger, one cavity which is connected between the evaporator and the compressor, and the other between the cooler and the receiver valve.  7. A refrigeration or heating device operating along the steam compression cycle, including a compressor, cooler, throttle device and evaporator installed in series, characterized in that it further comprises a receiver with inlet and outlet valves placed parallel to the throttle device between the cooler and evaporator, and a two-cavity counterflow heat exchanger, one cavity of which is connected between the evaporator and the compressor, and the other cavity is connected to the cooler by its inlet, and Exit from the throttle valve device and the input of the receiver.

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