High Side Pressure Regulation For Transcritical Vapor Compression System

Abstract

An expensive expansion device may be eliminated in favor of a less expensive pressure regulator in a CO2 vapor compression system such as is used in a bottle cooler or small-capacity air conditioner, refrigerator, or other system.

Description

BACKGROUND OF THE INVENTION

The invention relates to refrigeration. More particularly, the invention relates to beverage coolers.

As a natural and environmentally benign refrigerant, CO₂ (R-744) is attracting significant attention. In most air-conditioning operating ranges, CO₂ systems operate in transcritical mode. 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. This pressure control strategy involves a fairly high cost valve, a sophisticated controller 50, and a sensor 52 for measuring the high side pressure. This equipment adds a significant amount of cost to the CO₂ vapor compression system, causing the CO₂ vapor compression system to be less attractive compared to an HFC system.

It is possible to use a fixed expansion device in a transcritical vapor compression system, but this approach has limitations which may cause a loss of performance or functionality. During steady state operation, a fixed expansion device (e.g., a fixed orifice or capillary tube) can work well to regulate the system high side pressure to a near optimum pressure. During pulldown, when the system is started and the evaporation temperature and pressure can be very high, the flowrate through a fixed speed and displacement compressor can become relatively high. This high flowrate can cause the high side pressure to exceed a safe limit.

SUMMARY OF THE INVENTION

An expensive expansion device may be eliminated in favor of a less expensive pressure regulator in a CO₂ vapor compression system such as is used in a bottle cooler or small-capacity air conditioner, refrigerator, or other system. The potential for overpressurization may be reduced by using an inexpensive, multi-step fixed expansion device based on one or more solenoid valves.

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 vapor compression system.

FIG. 2 is a schematic of a first inventive CO₂ vapor compression system.

FIG. 3 is a schematic of a second inventive CO₂ vapor compression system.

FIG. 4 is a schematic of a third inventive CO₂ vapor compression system.

FIG. 5 is a schematic of a fourth inventive CO₂ vapor compression system.

FIG. 6 is a schematic of a fifth inventive CO₂ vapor compression system.

FIG. 7 is a schematic of a sixth inventive CO₂ vapor compression system.

FIG. 8 is a schematic of a seventh inventive CO₂ vapor compression system.

FIG. 9 is a side schematic view of a display case including a refrigeration and air management cassette.

FIG. 10 is a view of a refrigeration and air management cassette.

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

DETAILED DESCRIPTION

The current invention relates to high-side pressure optimization for a CO₂ vapor compression system. For HVAC & R products which do not have broad operating envelopes, the optimal high side pressures for all operating conditions do not vary much. Therefore, a fixed expansion device (e.g., an orifice or capillary tube) can be used to regulate the high side pressure to a preset constant value for all steady state operating conditions of the CO₂ vapor compression system. The preset value should be determined such that the CO₂ vapor compression system can achieve the best overall Coefficient of Performance (COP) for the entire operating envelope. Using a fixed expansion device can significantly reduce the cost of the pressure control components in a CO₂ vapor compression system.

For pulldown conditions, the compressor flowrate will be significantly higher than during steady state conditions. The high-side pressure should be optimized such that the pulldown cooling capacity of the CO₂ vapor compression system can be maximized, but the flow through the pressure regulator does not exceed the flow through the compressor (so that the system pressure becomes too great). This optimal high-side pressure for maximizing capacity is usually higher than the optimal high-side pressure for maximizing the overall COP. However, because the compressor flowrate is much higher during pulldown conditions than during steady state conditions, the expansion device may be configured to have a larger flow capacity during pulldown conditions. A simple multi-position expansion device may provide this. There are a number of ways through which this can be achieved through the use of solenoid valves to enable a two or more position pressure control system.

The following examples reflect modifications of the basic system of FIG. 1. Accordingly, the same reference numerals are used to identify the compressor 22, gas cooler 24, and evaporator 28. In any reengineering or remanufacturing situation, these components may be identical to those of the baseline system or may be further modified. FIG. 2 shows a system 60 in which the refrigerant flow path 62 is split into two parallel branches/segments 64 and 66 between the gas cooler 24 outlet and evaporator 28 inlet. The first branch 64 has a first fixed expansion device 68. The second branch 66 includes, in series, a solenoid valve 70 and a second fixed expansion device 72. Although the solenoid valve 70 is shown upstream of the second fixed expansion device 72, this order may be reversed. The exemplary solenoid valve 70 has two settings/conditions. One setting/condition is a fully closed condition in which no flow may pass along the second branch 66. The second setting/condition is a fully open condition allowing flow to pass through the second branch 66 with a minimal pressure loss across the solenoid valve 70.

During steady state operating conditions, when the compressor flowrate is relatively low, the solenoid valve 70 is kept fully closed. During pulldown conditions, the compressor flowrate is relatively high. In order to avoid overpressurization during pulldown, the solenoid valve 70 is opened, allowing flow through the second fixed expansion device 72. The combination of both expansion devices 68 and 72 regulates the high-side pressure to avoid overpressurization while still delivering good system performance.

In operation, a pulldown condition may be detected by means of one or more temperature sensors 75 and pressure sensor 74 coupled to a controller 76 coupled to control the solenoid valve 70. The controller 76 may also be coupled to the compressor and/or fan(s) to control their respective operation. For ease of illustration, the sensor and controller are not illustrated in the following examples although they may be present.

FIG. 3 shows a system 80 wherein the refrigerant flow path 82 has two segments/branches 84 and 86 in parallel upstream of a first fixed expansion device 88. The first branch 84 includes a solenoid valve 90. The second branch 86 includes a second fixed expansion device 92. During steady state operating conditions, the solenoid valve 90 is closed to prevent flow along the first branch 84. The second branch 86 acts as a bypass with restricted flow passing through the second fixed expansion device 92 before then passing through the first fixed expansion device 88. During pulldown conditions, the solenoid valve 90 is open, allowing an essentially unrestricted flow along the first branch 84. A small additional flow may flow along the second branch 86, with the combined flow then passing through the first expansion device 88. In alternative embodiments, the first expansion device 88 may be upstream of the branching rather than downstream. Control methods and components (not shown) of this system and those discussed below may be similar to those of the system 60.

FIG. 4 shows another system 100 wherein the flow path 102 has first and second segments/branches 104 and 106 between the gas cooler and evaporator. A fixed expansion device 108 is located in the first branch 104. A solenoid valve 110 is located in the second branch 106. The solenoid valve 110 combines aspects of a solenoid valve and a fixed expansion device. Specifically, the open condition may still be relatively restricted compared with the open condition of the solenoid valve 90. Therefore, the pulldown pressure drop through the solenoid valve 110 is significant and the high-side pressure of the system is controlled to the preset constant optimal value by the combination of the solenoid valve 110 and the fixed expansion device. For steady state operation, the solenoid valve 110 is fully closed and all flow passes through the expansion device 108.

FIG. 5 shows a branch-less system 120 in which, along the flow path 122, a solenoid valve 124 and fixed expansion device 126 are located in series. The solenoid valve 124 combines aspects of the solenoid valve and a fixed expansion device differently from the valve 110 of FIG. 4. Specifically, the valve element (e.g., the solenoid plunger) of the solenoid valve 124 may have a small orifice so that its closed condition is only a partially closed condition. The open condition, however, is an essentially fully open condition with low pressure drop. Accordingly, during steady state operating conditions, the solenoid valve 124 is in its closed condition passing a relatively low flow and creating a substantial pressure drop (individually and combined with the expansion device 126). In the steady state condition, the solenoid valve is open, permitting the flow rate to be dictated essentially solely by the expansion device 126. As with the other systems, the series order may be reversed.

FIG. 6 shows a system 140 combining aspects of the systems 80 and 120. Specifically, the flow path 142 has two segments/branches 144 and 146 in parallel upstream of a first fixed expansion device 148. The first branch 144 includes a solenoid valve 150. The second branch 146 includes a fixed expansion device 152. The exemplary solenoid valve 150 may, similar to the solenoid valve 124, have a closed condition that is only partially closed. During pulldown conditions, the solenoid valve 150 is open. During steady state conditions, the valve 150 is closed. In the steady state condition, there is a relatively small flow along each of the branches. During pulldown conditions, a larger flow may pass along the first branch 144, with a residual flow along the second branch 146.

FIG. 7 shows another system 160 wherein the flow path 162 includes a solenoid valve 164 that combines solenoid valve and orifice functions. Specifically, the element of the solenoid valve 144 includes an orifice so that the closed condition is only partially closed. During steady state conditions, the valve 144 is in its closed condition with the orifice passing the relatively small flow. During pulldown conditions, the valve is open so that a larger flow is passed.

FIG. 8 shows a system 180 wherein the flow path 182 includes segments/branches 184 and 186 between the gas cooler and the evaporator. A solenoid valve 188 and 190 is located in each of the branches. The elements of these solenoid valves may include orifices. Independent control over the valves may provide more than two alternative effective flow restrictions. For example, with different size orifices, the two valves provide up to four different effective restrictions. A minimal restriction may be present with both valves open. A maximal restriction may be present with both valves closed. A pair of intermediate restrictions may be achieved with one of the valves closed and the other open. To provide a more than trivial difference amongst the three least restrictive conditions, the conduit of the branches may be sized or the valve sized or additional restriction may be present so that with only one valve open there is not essentially free flow. An alternative embodiment could feature such valves in series rather than parallel.

A variety of sensor and/or user inputs may be used to control the solenoid valve(s). Direct measurement of the high-side pressure may be made by the sensor 74. When this pressure exceeds one or more associated thresholds, the controller 76 may cause the valve(s) to assume an associated relatively free-flow condition. Alternatively or in addition to high-side pressure measurement would be sensor 74, input may be received from an air temperature sensor. The exemplary sensor 75 may be positioned to be exposed to air in or from the cooler interior (e.g., to the flow 36 upstream of the evaporator 28). The sensor 75 may form part of a control thermostat. Accordingly, use of such a sensor alone may permit cost savings through the elimination of the pressure sensor 52 or 74.

For fixed speed and displacement compressor, the flow through the system is a direct function of the density of the refrigerant entering the compressor and, to a lesser extent, the pressure ratio of the compressor. The inlet density is a direct function of the saturation temperature and superheat of the refrigerant. These, in turn, are direct functions of the air temperature, system size, and charge. For a simple system, these parameters may be determined in the design stage as a function of air temperature flowing through the evaporator. A correlation can be produced which matches the evaporator air temperature to the refrigerant inlet density. In operation, the solenoid valve(s) would remain in the open position until the output of the evaporator temperature sensor 75 drops below a predetermined value. When this happens, the solenoid valve or one of the solenoid valves is closed. This can be repeated for systems having multiple solenoid valves further reducing the effective expansion orifice area as the temperature drops so as to maintain a mere optimal pressure in the high pressure portion of the system.

If a high-side pressure is directly measured (e.g., by the sensor 74) a different correlation may be used. The optimal high-side pressure may be known as a function of evaporator temperature and, optionally, the ambient temperature. The solenoid valve or valves may be actuated to maintain the pressure within certain limits.

FIG. 9 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. 10 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 i28 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 refrigeration system (60; 80; 100; 120; 140; 160; 180) comprising: a compressor (22) for driving a refrigerant along a flow path (62; 82; 102; 122; 142; 162; 182) 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;a second heat exchanger (28) along the flow path upstream of the compressor in the first mode; anda pressure regulator (68; 72; 88; 92; 108, 110; 124, 126; 148, 152; 164; 188; 190) in the flow path downstream of the first heat exchanger (24) and upstream of the second heat exchanger (28) in the first mode.

2. The system of claim 1 wherein the pressure regulator comprises a non-valve fixed orifice expansion device (68; 72; 88; 92; 108; 126; 148; 152).

3. The system of claim 1 wherein the pressure regulator comprises a non-valve fixed orifice expansion device (126; 148) in series with a solenoid valve (124; 150) with a valve element having an orifice.

4. The system of claim 1 wherein the pressure regulator comprises a non-valve fixed orifice expansion device (88; 148) in series with a parallel combination of a solenoid valve (90; 150) and bypass conduit (86; 146).

5. The system of claim 1 wherein there are first and second such pressure regulators in parallel.

6. 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.

7. The system of claim 1 wherein: the first and second heat exchangers and compressor are removable from a housing of the system as a unit without need to previously empty contents of the system.

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

9. The system of claim 1 being a cooler containing: a plurality of beverage containers in a 0.3-4.0 liter size range.

10. The system of claim 9 wherein the cooler is selected from the group consisting of: a cash-operated vending machine;a transparent door front, closed back, display case; anda top access cooler chest.

11. A refrigeration system comprising: a compressor driving a CO2-based refrigerant along a flow path in at least a first mode of system operation;a first heat exchanger along the flow path downstream of the compressora second heat exchanger along the flow path upstream of the compressor; andmeans in the flow path downstream of the first heat exchanger and upstream of the second heat exchanger for expanding the refrigerant in the absence of an electronic expansion device.