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High side pressure regulation for transcritical vapor compression system

High side pressure regulation for transcritical vapor compression system description/claims

Benefit is claimed of U.S. patent application Ser. No. 60/663,960, filed Mar. 18, 2005, and entitled “High Side Pressure Regulation for Transcritical Vapor Compression System”, the disclosure of which is incorporated by reference herein as if set forth at length.

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.

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.

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.

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