HK1102935B

HK1102935B - A vapor compression system and a method of sizing an accumulator for vapor compression system

Info

Publication number: HK1102935B
Application number: HK07107405.7A
Authority: HK
Inventors: Tobias H. Sienel; Yu Chen
Original assignee: Carrier Corporation
Priority date: 2003-12-19
Filing date: 2004-12-20
Publication date: 2009-11-13

Description

Vapor compression system and method for sizing accumulator of vapor compression system

Technical Field

The present invention relates generally to a vapor compression system including an accumulator sized to protect the system from over-pressurization when the system is not operating.

Background

Chlorine-containing refrigerants have been eliminated in most countries of the world due to their potential ozone depletion potential. "natural" refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Carbon dioxide has a low critical point, which results in most air conditioning systems utilizing carbon dioxide as a refrigerant operating transcritically, or partially above the critical point, under most conditions, including when not operating. Under supercritical operating conditions, the pressure within the system becomes a function of temperature and density.

Vapor compression systems often operate over a wide range of operating conditions. When not operating, external air conditions, including temperature, can affect the pressure of the system. The system components (compressor, condenser/air cooler, expansion device, evaporator, and refrigerant lines) are designed to withstand the maximum pressure, and exposure to higher pressures can result in damage to the components. For most systems, when not in operation, the pressure within the system is a direct function of the system temperature. However, when the temperature is near or above the critical point of the refrigerant, additional factors must be considered. For supercritical fluids, the pressure in the system is a function of the fluid temperature and density. This is not a particular concern for most refrigerants because their critical point is near or above the normal storage temperature. However for carbon dioxide (CO)₂) System of which thisIt becomes a problem because the critical point is very low (88F.).

Pressure relief valves are specifically added to the system to protect the system and components from over pressurization. If the pressure in the system approaches the over-pressurization point, the pressure relief valve will automatically open to vent refrigerant from the system and reduce the pressure to a safe range to protect components from damage.

Vapor compression systems are typically designed to store at a certain maximum temperature, and the system components are designed to withstand the maximum pressure associated with this temperature. Higher storage temperatures generally require higher design pressures. The bulk density of the refrigerant is important in determining the system pressure and therefore the design pressure when the storage temperature is near or above the critical temperature of the refrigerant. This is schematically illustrated in fig. 1, where fig. 1 depicts how the carbon dioxide system pressure varies above the critical point as a function of temperature and bulk density.

Previous vapor compression systems include an accumulator between the evaporator and the compressor for storing excess refrigerant. The accumulator is sized only to provide sufficient capacity to store excess refrigerant during operation to prevent excess refrigerant from entering the compressor. The reservoir may also be used to control the high pressure and therefore the coefficient of performance of the system during supercritical operation. However, when the system is not operating or in storage, the reservoir is not sized to determine the maximum pressure.

Accordingly, there is a need in the art for a vapor compression system and a method that includes a reservoir sized to prevent over-pressurization of the system when not in operation; the method is used to set the size of the reservoir.

Disclosure of Invention

The present invention provides a vapor compression system including an accumulator that acts as a buffer to prevent over-pressurization of the system when the system is not operating.

Pressure is a function of temperature and density as the fluid approaches or goes above its critical point. By knowing the maximum storage temperature and the maximum storage pressure, the refrigerant density of the entire system can be calculated and used to determine the desired volume of the system.

In particular, a method of sizing an accumulator for a vapor compression system is provided, comprising the steps of:

a) determining a maximum storage temperature of a system refrigerant;

b) determining a maximum storage pressure of a system refrigerant; and

c) the maximum storage temperature and the maximum storage pressure are utilized to prevent over-pressurization of the system when the refrigerant is at the maximum refrigerant temperature and the maximum refrigerant pressure to determine an optimal storage volume of the accumulator.

The bulk density in a system is the mass of refrigerant in the system divided by the volume of the system. Thus, by dividing the mass of refrigerant by the desired maximum storage density, the desired volume of the overall system can be determined. The total volume of the system without a reservoir is subtracted from the desired volume of the entire system to calculate the optimal reservoir volume. The optimum accumulator volume is used to size the accumulator so that it can prevent over-pressurization of the system when the refrigerant in the system is stored near or above the critical temperature of the refrigerant.

These and other features of the present invention can be best understood from the following specification and drawings.

Drawings

The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:

FIG. 1 schematically depicts a graph of how the pressure of carbon dioxide varies above a critical point as a function of temperature and bulk density; and

figure 2 schematically illustrates a schematic of a vapor compression system using an accumulator in accordance with the present invention.

Detailed Description

Fig. 2 depicts an example vapor compression system 20 that includes a compressor 22, a heat rejecting heat exchanger 24 (a gas cooler in a supercritical cycle), an expansion device 26, and a heat accepting heat exchanger 28 (an evaporator). Refrigerant circulates through the closed circuit system 20 by refrigerant lines.

In one example, carbon dioxide is used as the refrigerant. Because carbon dioxide has a low critical point, systems using carbon dioxide as a refrigerant typically operate transcritically. Although carbon dioxide is described herein, other refrigerants may be used.

The refrigerant exits the compressor 22 at a high pressure and enthalpy. The refrigerant then flows through the heat rejecting heat exchanger 24 at high pressure. A fluid medium 30, such as water or air, flows through a heat absorbing member 32 of the heat rejecting heat exchanger 24 and exchanges heat with the refrigerant flowing through the heat rejecting heat exchanger 24. In the gas cooler 24, the refrigerant rejects heat to the fluid medium 30, and the refrigerant exits the gas cooler 24 at a low enthalpy and a high pressure. Since the critical temperature of carbon dioxide is 87.8 ° f, the removal of heat will occur in the supercritical region, and the fluid temperature of the removed heat is typically above this temperature. When the vapor compression system 20 is operating transcritically, the refrigerant in the high pressure portion of the system is in the supercritical region where pressure is a function of temperature and density.

A pump or blower 34 pumps the heat source fluid medium 30 through the heat sink 32. The cooled fluid medium 30 enters the heat sink 32 from the heat sink inlet or return 36 and flows in a direction opposite to the direction of refrigerant flow. After exchanging heat with the refrigerant, the heated fluid 38 exits the heat sink 32 from a heat sink outlet or supply 40.

The refrigerant then passes through an expansion valve 26, which expands the refrigerant and reduces its pressure. After expansion, the refrigerant flows through the channels 42 of the evaporator 28 and exits at a high enthalpy and a low pressure. Within evaporator 28, the refrigerant absorbs heat from heat source fluid 44, heating the refrigerant. The heat source fluid 44 flows through a heat sink 46 and exchanges heat with the refrigerant flowing through the evaporator 28 in a known manner. The heat source fluid 44 enters the heat sink 46 through a heat sink inlet or return 48. After exchanging heat with the refrigerant, the cooled heat source fluid 50 exits the heat sink 46 through a heat sink outlet or supply 52. As the refrigerant flows through the evaporator 28, the temperature differential between the heat source fluid 44 and the refrigerant in the evaporator 28 drives the transfer of thermal energy from the heat source fluid 44 to the refrigerant. A fan or pump 54 moves the heat source fluid 44 through the evaporator 28, maintaining the temperature differential and evaporating the refrigerant. The refrigerant then reenters the compressor 22, completing the cycle. The system 20 transfers heat from the low temperature accumulator to the high temperature energy absorption device.

The system 20 also includes an accumulator 56 located between the evaporator 28 and the compressor 22. The accumulator 56 may store excess refrigerant within the system 20 and also control the high pressure of the system 20 and, thus, the coefficient of performance of the system 20 during supercritical operation. During operation of the system 20, the accumulator 56 prevents excess refrigerant from entering the compressor 22.

When the vapor compression system 20 is stored or transported in a high temperature climate, such as a desert climate, the temperature of the refrigerant may increase due to the high temperature of the environment. The elevated temperature increases the pressure within the system 20 and can result in excessive pressurization, which can result in the activation of a pressure relief valve or the bursting of a refrigerant line or system 20 component.

Bulk density is defined as the mass of refrigerant in the system divided by the system volume. Since the temperature and density of the refrigerant affects the pressure of the system when the system is stored at or above the critical point of the refrigerant, the system volume of the vapor compression system 20 also affects the pressure within the system when the system is stored at or above the critical point of the refrigerant. As the system volume increases at a given temperature at or above the critical point of the refrigerant, the system pressure decreases.

When the system 20 is not operating, the accumulator 56 may act as a buffer to reduce the build-up of excess pressure and prevent over-pressurization of the system 20. The size of the accumulator 56 affects the overall volume of the system 20 and, therefore, the maximum storage pressure of the system 20. By increasing the volume of the accumulator 56, the bulk density of the refrigerant within the system 20 may be reduced, and thus the pressure of the refrigerant within the system 20 may be reduced. By reducing the volume of the accumulator 56, the pressure of the refrigerant within the system 20 increases. Fig. 1 illustrates this effect on the system using carbon dioxide as the refrigerant. In the present invention, the accumulator 56 is preferably sized to prevent over-pressurization of the system 20 when not in operation or being transported. That is, the reservoir 56 is sized large enough to prevent over-pressurization, but not so large as to be too expensive.

The volume of the accumulator 56 is determined according to the maximum design storage temperature and the maximum storage pressure of the refrigerant. As the storage temperature increases, the temperature of the refrigerant within the system 20 increases. The increase in refrigerant temperature increases the refrigerant pressure within the system 20. The decrease in refrigerant temperature reduces the refrigerant pressure within the system 20. The maximum storage temperature of the refrigerant in the system 20 depends on the climate. In high temperature climates, the maximum storage temperature increases due to an increase in air temperature. In colder climates, the maximum storage temperature is lower due to the reduction in air temperature. The highest storage temperature will typically be selected due to the global manufacturing requirements of the system.

For a system 20 having a relatively high critical temperature of the refrigerant, the temperature is not near the maximum storage temperature of the system, and therefore the maximum storage temperature determines the maximum storage pressure solely through the saturation characteristics of the refrigerant. This can be seen in figure 1 where the temperature is below about 60 f. For a system 20 that uses a refrigerant with a relatively low critical temperature (e.g., carbon dioxide), the maximum storage temperature and the bulk density of the system together determine the maximum storage pressure of the system 20. This can be seen in fig. 1 where the temperature is above about 60 f. That is, by knowing the maximum storage temperature that the refrigerant will reach when not in operation, and the maximum design storage pressure, the optimum bulk density can be calculated and used to set the size of the accumulator in the system.

The maximum design storage pressure of the system is generally limited by the low pressure side of the system. In operation, the low pressure side of the system is typically at a lower pressure than when not operating or stored. For refrigerants having a relatively high critical point, the selection of the maximum design pressure generally requires reference only to the maximum design temperature. For refrigerants having a relatively low critical point, additional factors, such as the manufacturing cost required for thicker walled components, are required to be taken into account. Typically, systems using carbon dioxide as the refrigerant have a maximum storage pressure between 1000 and 2500 psi.

When outside the saturation region, density is a function of temperature and pressure. Thus, if the maximum storage temperature and the maximum storage pressure are known, the maximum storage bulk density can be determined. The volume can be calculated by dividing the mass by the density. Dividing the mass of refrigerant by the maximum storage density determines the optimum volume of the overall system. The following calculations can be used to obtain the desired overall system volume:

the components in the system 20, except for the accumulator 56, have known component volumes. These components include a compressor 22, a heat rejecting heat exchanger 24, an expansion device 26, an evaporator 28, and refrigerant lines connecting the components. The accumulator 56 is the only component of the system 20 of unknown volume. By subtracting the volume of all components from the total system volume, the optimal accumulator volume can be determined. It is understood that the total component volume includes the total volume of all components in the system 20 except the accumulator 56. By the above equation, the optimal reservoir volume can be calculated:

the above equation determines the optimum volume of the accumulator based on the maximum storage pressure of the refrigerant, the maximum storage temperature of the refrigerant, the refrigerant mass, and the volume of the system components. Preferably, the volume of the accumulator 56 may be selected between 80% -120% of the calculated optimal size to achieve a desired accumulator 56 size that protects the system 20 from over-pressurization during non-operation or transportation.

It is to be understood that the described example of a single stage system using carbon dioxide is merely one example. Optimal accumulator size may also be determined for multi-stage compression systems, systems using internal heat exchangers, and systems using other additional system components such as oil separators and filter dryers. An optimal accumulator size may also be determined in a system having a multi-stage heat rejecting heat exchanger 24, an expansion device 26, and a heat accepting heat exchanger 28. In addition, the accumulator described in this example is disposed between the evaporator and the compressor. However, it is understood that the reservoir may be located in other locations. The present invention is equally applicable to systems that use a liquid storage assembly located elsewhere in the system, such as at the inlet of the evaporator or between the condenser (or gas cooler) and the evaporator. In addition, the reservoir may be divided into two or more reservoir assemblies located in different parts of the system, with the optimal reservoir size being used as the sum of the volumes of each reservoir assembly.

The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, and thus, those skilled in the art will recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.

Claims

1. A method of sizing a vapor compression system accumulator comprising the steps of:

a) determining a maximum storage temperature of a system refrigerant;

b) determining a maximum storage pressure of a system refrigerant; and

2. The method of claim 1, further comprising the steps of calculating a desired system volume using the maximum storage temperature and the maximum storage pressure, calculating the volume of system components prior to the step of adding the optimal accumulator volume, and calculating the optimal accumulator volume by subtracting the volume of the components from the desired system volume.

3. The method of claim 2, wherein the optimal accumulator volume is selected to be in the range of 80% -120% of the calculated accumulator volume.

4. The method of claim 2, wherein the step of calculating the optimal accumulator volume comprises determining a density at a maximum storage temperature and a maximum storage pressure of the refrigerant and dividing the mass of the refrigerant by the density of the refrigerant.

5. The method of claim 2 wherein calculating the volume of the component comprises summing the volume of all compressors of the at least one compressor, the volume of all heat rejecting heat exchangers of the at least one heat rejecting heat exchanger, the volume of all expansion devices of the at least one expansion device, the volume of all heat accepting heat exchangers of the at least one heat accepting heat exchanger, and the volume of all refrigerant lines of the refrigerant lines.

6. The method of claim 5, wherein the step of calculating the volume of the assembly further comprises adding the volume of all internal heat exchangers of the at least one internal heat exchanger, adding the volume of all oil separators of the at least one oil separator, and adding the volume of all filter dryers of the at least one filter dryer.

7. The method of claim 6, wherein the step of calculating the volume of the component further comprises adding the volume of all of the additional components of any additional component.

8. The method of claim 1, wherein the refrigerant is carbon dioxide.

9. The method of claim 1 wherein the maximum storage pressure is between 1000 and 2500 psi.

10. A vapor compression system comprising:

at least one compression device for compressing refrigerant to a high pressure;

at least one heat rejecting heat exchanger for cooling said refrigerant;

at least one expansion device for reducing the refrigerant to a low pressure;

at least one heat accepting heat exchanger for evaporating said refrigerant; and

an accumulator of optimal size and sized to prevent over-pressurization of the system when the refrigerant is at a maximum refrigerant temperature and a maximum refrigerant pressure.

11. The vapor compression system as recited in claim 10 wherein the maximum refrigerant temperature and the maximum refrigerant pressure are utilized to determine a desired system volume, and wherein the optimal size of the accumulator is defined as follows:

volume of_{Component part}Is the total component volume of the components in the system prior to the addition of the reservoir.

12. The vapor compression system as recited in claim 10 wherein the refrigerant is carbon dioxide.

13. The vapor compression system as recited in claim 10, wherein the size of the accumulator is between 80% -120% of the optimal size.

14. The vapor compression system as recited in claim 10, wherein the maximum storage pressure is between 1000-.

15. The vapor compression system as recited in claim 10, wherein the optimal size of the accumulator is determined by utilizing a maximum storage temperature, a maximum storage pressure, a mass of refrigerant, and a total component volume of the system.

16. The vapor compression system of claim 15, wherein the total assembly volumes of the system comprise a total compressor volume of the at least one compressor, a total heat rejecting heat exchanger volume of the at least one heat rejecting heat exchanger, a total expansion device volume of the at least one expansion device, a total heat accepting heat exchanger volume of the at least one heat accepting heat exchanger, and a total refrigerant line volume of the refrigerant line.

17. The vapor compression system of claim 16, further comprising at least one internal heat exchanger, an oil separator, and a filter-dryer, and wherein the overall assembly volume further comprises an overall internal heat exchanger volume of the internal heat exchanger, an at least one oil separator volume of the oil separator, and an overall filter-dryer volume of the filter-dryer.

18. The vapor compression system of claim 17, wherein the module volume further comprises a total additional module volume of any additional module.

19. The vapor compression system as recited in claim 11 wherein the optimal accumulator volume is a total volume of all of the liquid storage components in the system.