Transcritical Refrigeration With Pressure Addition Relief Valve

Abstract

A refrigeration system (20) includes a pressure addition relief valve (62) in parallel with an expansion device (63).

Description

BACKGROUND OF THE INVENTION

The invention relates to refrigeration. More particularly, the invention relates to transcritical refrigeration systems such as CO₂ beverage coolers.

Transcritical vapor compression systems have an extra degree of control freedom when compared to subcritical vapor compression systems. In subcritical systems, pressure in the high and low pressure components of the system are largely controlled by the heat exchanger fluid temperatures. If the system is an air-to-air system, the evaporator pressure is a strong function of the air temperature entering the evaporator, and the condenser pressure is a strong function of the air temperature entering the condenser. This is because these temperatures are closely correlated with the saturation pressures in the heat exchangers. In a transcritical system, the high pressure side of the system does not have any saturation properties, and thus pressure is independent from temperature. It is well known that the choice of the high side pressure has a very strong effect on the performance of the system, and that there is an optimal pressure which provides maximum energy efficiency. This optimal pressure will change as the operating conditions of the unit change. Control of the high side pressure can be achieved in many different ways, but for systems which have fixed speed and volume compressors, the strongest influence is through the expansion device.

FIG. 1 schematically shows transcritical vapor compression system 20 utilizing CO₂ as working fluid. The system comprises a compressor 22, a gas cooler 24, an expansion device 26, and an evaporator 28. The exemplary gas cooler and evaporator may each take the form of a refrigerant-to-air heat exchanger. Airflows across one or both of these heat exchangers may be forced. For example, one or more fans 30 and 32 may drive respective airflows 34 and 36 across the two heat exchangers. A refrigerant flow path 40 includes a suction line extending from an outlet of the evaporator 28 to an inlet 42 of the compressor 22. A discharge line extends from an outlet 44 of the compressor to an inlet of the gas cooler. Additional lines connect the gas cooler outlet to expansion device inlet and expansion device outlet to evaporator inlet.

The major difference between transcritical and conventional operation is that heat rejection in the gas cooler is in the supercritical region because the critical temperature for CO₂ is 87.8° F. Consequently, pressure is not solely dependent on temperature and this opens additional control and optimization issues for system operation.

For a fixed gas cooler discharge temperature, as the high side pressure is increased, the exit enthalpy of the refrigerant decreases, yielding a higher differential enthalpy through the gas cooler. The capacity of the gas cooler is a function of the mass flowrate of refrigerant and the enthalpy difference across the gas cooler. For a beverage cooler, the evaporator may be essentially at the cooler interior temperature. It is typically desired to maintain this temperature in a very narrow range regardless of external condition. For example, it may be desired to maintain the interior very close to 37° F. This temperature essentially fixes the steady state compressor suction pressure.

For a fixed compressor suction pressure, as the high side pressure increases, the amount of energy used by the compressor increases, and the volumetric efficiency of the compressor decreases. When the volumetric efficiency of the compressor decreases, the flowrate through the system decreases. The balance of these two counteracting effects is typically an increase in gas cooler capacity as the high side pressure is increased. However, above a certain pressure the amount of capacity increase becomes very small. Because the expansion device is usually isenthalpic, the evaporator capacity will also typically increase as the high side pressure increases.

The energy efficiency of a vapor compression system, the Coefficient of Performance (COP), is usually expressed as a ratio of the system capacity to the energy consumed. Because an increase in pressure typically produces both a higher capacity and a higher energy consumption, the balance between the two will dictate the overall COP. Therefore, there is typically an optimal pressure which yields the highest possible performance.

An electronic expansion valve is usually used as the device 26 to control the high side pressure to optimize the COP of the CO₂ vapor compression system. An electronic expansion valve typically comprises a stepper motor attached to a needle valve to vary the effective valve opening or flow capacity to a large number of possible positions (typically over one hundred). This provides good control of the high side pressure over a large range of operating conditions. The opening of the valve is electronically controlled by a controller 50 to match the actual high side pressure to the desired set point. The controller 50 is coupled to a sensor 52 for measuring the high side pressure.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a prior art CO₂ bottle cooler.

FIG. 2 is a schematic of a modified CO₂ bottle cooler.

FIG. 3 is a sectional view of a pressure addition relief valve of the cooler of FIG. 2 in a closed condition.

FIG. 4 is a sectional view of a pressure addition relief valve of the cooler of FIG. 2 in an open condition.

FIG. 5 is a graph of discharge pressure against ambient temperature for three different expansion methods.

FIG. 6 is a graph of coefficient of performance against ambient temperature for said three different expansion methods.

FIG. 7 is a graph of capacity against ambient temperature for said three different expansion methods.

FIG. 8 is a graph of discharge pressure against evaporating temperature during pulldown for said three different expansion methods and an inventive method.

FIG. 9 is a graph of coefficient of performance against evaporating temperature during pulldown for said three different expansion methods and said inventive method.

FIG. 10 is a graph of capacity against evaporating temperature during pulldown for said three different expansion methods and said inventive method.

FIG. 11 is a side schematic view of a display case bottle cooler including a refrigeration and air management cassette.

FIG. 12 is a view of a refrigeration and air management cassette.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

A pressure addition relief valve (PARV) may be used in combination with a primary expansion device. FIG. 2 shows a system 60 formed as a modification of the prior art system 20. In the example, the PARV 62 and expansion device 63 are coupled in parallel between a high pressure (upstream) portion 64 of the refrigerant flow path from the gas cooler and a low pressure (downstream) portion 66 to the evaporator. A combination of the pressures at the opposite sides of the PARV 62 acts to open the PARV to permit flow therethrough. The PARV and expansion device may be combined in a combination valve 68.

The exemplary valve 68 (FIG. 3) has a body 70 with an inlet port 72 receiving the conduit portion 64 and an outlet 74 receiving the conduit portion 66. The inlet port 72 and outlet port 74 respectively communicate with a high pressure volume 76 and a low pressure volume 78, both within the body. The exemplary body 70 includes a main portion 80 and a cover 82 secured thereto. The exemplary cover seals and secures the periphery of a membrane 84 to the main portion 80. The exemplary membrane is a disk of sheet spring steel.

The membrane has a front face/surface 86 normally engaged/sealed to a seat surface 88 of the body main portion 80. The volumes 76 and 78 have respective ports 90 and 92 in the surface 88. The ports 90 and 92 are normally blocked by engagement with membrane front face 86. The engagement may be assisted by a biasing spring 94 if the particular membrane is not sufficiently self sprung (e.g., a film rather than a metal sheet spring). An exemplary biasing spring 94 is a coil compression spring having a first end 96 engaging the backside/face 98 and a second end 100 engaging an underside 102 of the cover 82. A membrane backside volume (backspace) 104 is formed containing the spring. A port 106 in the cover may expose the backside volume 104 to a reference pressure. The reference pressure may be ambient air pressure, may be a vacuum or other sealed fixed pressure (in which case, the port 106 might be omitted), or a pressure dependent upon a system condition (e.g., connected via a conduit 108 to a TXV-type bulb 110 located elsewhere in the system to provide a variable pressure force). This backside pressure serves to maintain the membrane in its closed condition.

The pressures in the high and low pressure volumes 76 and 78 act on the membrane with forces based upon the relative areas of their ports 90 and 92 and in view of mechanical advantage factors such as port positioning. These pressures act counter to the pressure of the backside volume 104. If the relative balance of the

If the effect of combined high pressure and low pressure forces exceed the effect of backside pressure and spring force on the backside of the membrane, then the membrane will flex outward to an open condition (FIG. 4), allowing a flow 112 from the port 90 and back into the port 92 (and thus through the valve 68). If the combined effects of the high pressure and low pressure forces do not exceed those of the forces on the backside of the membrane, then the membrane will remain closed, allowing little to no flow. In this way, the membrane and associated components act as a PARV to regulate the additive pressure force to a controlled level.

The PARV is used in combination with a primary expansion device to provide a better mechanism for controlling the high pressure. The primary expansion device can be a simple orifice as discussed further below, or can be another type of expansion device, such as a capillary tube, TXV, EXV, or other valve. For example, a TXV type valve can be used with the bulb sensing the temperature of the exit of the gas cooler or condenser in one embodiment. In another, a dual bulb TXV can be used to sense the air temperature and gas cooler or condenser discharge difference.

In the FIGS. 3&4 example, an orifice 120 passing a flow 122 provides the principal function of the fixed expansion device portion of the combined valve. For purposes of discussion, the term PARV may be used to identify both the pure PARV and the combined valve.

An exemplary system design may reflect specific design external (ambient) and internal temperatures. An exemplary design ambient temperature is 90° F. (32° C.). An exemplary design pulldown temperature is 16° F. (−9° C.).

A theoretical optimal control is that which yields the highest possible COP.

FIG. 5 shows a plot 400 of discharge pressure against ambient temperature for the optimal control strategy. A plot 402 represents a fixed orifice dimensioned to provide the same pressure at the design ambient pressure. For lower ambient temperatures, the fixed orifice will produce higher than optimum discharge pressure. For higher ambient temperatures, the fixed orifice will produce lower than optimum discharge pressure. A plot 404 represents a fixed pressure situation which involves even greater departures from optimum.

FIG. 6 shows a plot 410 of coefficient of performance against ambient temperature for the optimal control strategy. A plot 412 represents the fixed orifice and a plot 414 represents the constant pressure situation. FIG. 7 shows a plot 420 of capacity against ambient temperature for the optimal control strategy. A plot 422 represents the fixed orifice and a plot 424 represents the constant pressure situation. From FIGS. 5-7 it is seen that that the fixed orifice provides a small difference in pressure relative to the optimal control. This difference causes a relatively modest reduction in efficiency (COP) and an even smaller reduction in capacity. The low cost of the fixed orifice device may outweigh these modest performance reductions. However, there are other considerations.

FIG. 8 shows a plot 430 of discharge pressure against evaporating temperature during pulldown for the optimal control strategy. A plot 432 represents the fixed orifice and a plot 434 represents the constant pressure situation. From the plot 432, it can be seen that pulldown conditions cause the fixed orifice to produce much higher discharge pressures than the optimal control. At higher evaporator temperatures, the resulting high pressures might damage the system. Thus, the problem with using only a simple fixed orifice is that the high (discharge) pressure will exceed a practical design pressure for the system hardware when the low pressure is much higher, such as during a pulldown condition (e.g., when the system is turned on with a high temperature in the volume to be refrigerated and a high ambient temperature). The PARV may function to avoid such high discharge pressures. A plot 436 represents the orifice and PARV combination and is shown departing from the plot 432 at/above a temperature of an exemplary 7.5° C. to cap the discharge pressure at an exemplary 12000 kPa selected based upon hardware strength. The PARV will not allow the pressure to exceed a certain value during pulldown, thus preventing damage to the system, and allowing the use of a simple pressure control device such as the fixed orifice device. The effect of the parv strategy on the regulation of the high pressure is a mechanism which acts very close to the optimal pressure control, but which does not over pressurize during periods of excessive refrigerant flow.

FIG. 9 shows a plot 440 of coefficient of performance against evaporating temperature during pulldown for the optimal control strategy. A plot 442 represents the fixed orifice, a plot 444 represents the constant pressure situation, and a plot 446 represents the orifice plus PARV combination. FIG. 10 shows a plot 450 of capacity against ambient temperature for the optimal control strategy. A plot 452 represents the fixed orifice, a plot 454 represents the constant pressure situation, and a plot 456 represents the orifice plus PARV combination. Plots 446 and 456 show that efficiency (COP) is not dramatically affected, while capacity is actually gained by the use of the PARV relative to a controlled expansion device regulating to the optimum pressure. The effect of this is that the pulldown time will be reduced and therefore the overall energy consumption of the device will be reduced as well.

A particular area for implementation of the PARV is in bottle coolers, including those which may be positioned outdoors or must have the capability to be outdoors (presenting large variations in ambient temperature). FIG. 11 shows an exemplary cooler 200 having a removable cassette 202 containing the refrigerant and air handling systems. The exemplary cassette 202 is mounted in a compartment of a base 204 of a housing. The housing has an interior volume 206 between left and right side walls, a rear wall/duct 216, a top wall/duct 218, a front door 220, and the base compartment. The interior contains a vertical array of shelves 222 holding beverage containers 224.

The exemplary cassette 202 draws the air flow 34 through a front grille in the base 224 and discharges the air flow 34 from a rear of the base. The cassette may be extractable through the base front by removing or opening the grille. The exemplary cassette drives the air flow 36 on a recirculating flow path through the interior 206 via the rear duct 210 and top duct 218.

FIG. 12 shows further details of an exemplary cassette 202. The heat exchanger 28 is positioned in a well 240 defined by an insulated wall 242. The heat exchanger 28 is shown positioned mostly in an upper rear quadrant of the cassette and oriented to pass the air flow 36 generally rearwardly, with an upturn after exiting the heat exchanger so as to discharge from a rear portion o the cassette upper end. a drain 250 may extend through a bottom of the wall 242 to pass water condensed from the flow 36 to a drain pan 252. A water accumulation 254 is shown in the pan 252. The pan 252 is along an air duct 256 passing the flow 34 downstream of the heat exchanger 24. Exposure of the accumulation 254 to the heated air in the flow 34 may encourage evaporation.

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a remanufacturing of an existing system or reengineering of an existing system configuration, details of the existing configuration may influence details of the implementation. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. A cooler system comprising: a compressor (22) for driving a refrigerant along a flow path in at least a first mode of system operation;a first heat exchanger (24) along the flow path downstream of the compressor in the first mode so as to act as a gas cooler;a second heat exchanger (28) along the flow path upstream of the compressor in the first mode so as to act as an evaporator to cool contents of an interior volume of the system;an expansion device (63) in the flow path downstream of the first heat exchanger (24) and upstream of the second heat exchanger (28); anda pressure addition relief valve (62) in parallel with the expansion device.

2. The system of claim 1 wherein: the pressure addition relief valve (62) is a purely mechanical valve.

3. The system of claim 1 wherein: the expansion device (63) is a purely mechanical device.

4. The system of claim 1 wherein: the pressure addition relief valve (62) is normally closed and configured to open responsive to a combined force produced by pressures essentially respectively immediately downstream of the first heat exchanger and upstream of the second heat exchanger.

5. The system of claim 4 wherein: a bias force acts opposite the combined force, the bias force comprising at least one of: a supplemental spring (94) bias force;a bias force provided by a system condition sensor (110); anda bias force exerted by ambient air pressure.

6. The system of claim 1 wherein: the pressure addition relief valve (62) is integral with the expansion device (63).

7. The system of claim 1 wherein: the expansion device (63) comprises a fixed orifice (120) in a common body (70) of the pressure addition relief valve.

8. The system of claim 1 wherein: the expansion device (63) comprises non-EEV device.

9. The system of claim 1 wherein: the pressure addition relief valve comprises a sheet metal spring membrane (84) and no other spring.

10. The system of claim 1 wherein: the pressure addition relief valve comprises a membrane (84) and a coil biasing spring (94).

11. The system of claim 1 wherein: flowpath portions upstream (76) and downstream (78) of the expansion device have effective counterbias areas of the pressure addition relief valve, the lesser being no less than 10% of the greater.

12. The system of claim 1 being a self-contained externally electrically powered beverage cooler positioned outdoors.

13. The system of claim 1 wherein: the refrigerant comprises, in major mass part, CO2; andthe first and second heat exchangers are refrigerant-air heat exchangers.

14. The system of claim 1 wherein: the refrigerant consists essentially of CO2; andthe first (24) and second (28) heat exchangers are refrigerant-air heat exchangers each having an associated fan (30; 32), an air flow across the first heat exchanger being an external to external flow and an airflow across the second heat exchanger being a recirculating internal flow.

15. The system of claim 1 in combination with said contents which include: a plurality of beverage containers in a 0.3-4.0 liter size range.

16. The system of claim 15 being selected from the group consisting of: a cash-operated vending machine;a transparent door front, closed back, display case; anda top access cooler chest.

17. A transcritical CO2 refrigeration system comprising: a compressor (22) for driving a refrigerant along a flow path in at least a first mode of system operation;a first heat exchanger (24) along the flow path downstream of the compressor in the first mode so as to act as a gas cooler;a second heat exchanger (28) along the flow path upstream of the compressor in the first mode so as to act as an evaporator;an expansion device (63) in the flow path downstream of the first heat exchanger (24) and upstream of the second heat exchanger (28); anda pressure addition relief valve (62) in parallel with the expansion device.

18. A method for operating a transcritical CO2 refrigeration system comprising: compressing and driving a refrigerant along a flow path in at least a first mode of system operation;cooling the compressed refrigerant along the flow path downstream of the compressing;expanding the cooled refrigerant; andheating the expanded refrigerant;

19. A method for remanufacturing a transcritical CO2 refrigeration system or reengineering a configuration thereof wherein a baseline configuration comprises: a compressor (22) for driving a refrigerant along a flow path in at least a first mode of system operation;a first heat exchanger (24) along the flow path downstream of the compressor in the first mode so as to act as a gas cooler;a second heat exchanger (28) along the flow path upstream of the compressor in the first mode so as to act as an evaporator; andan expansion device (26) in the flow path downstream of the first heat exchanger (24) and upstream of the second heat exchanger (28),