Refrigeration system

Abstract

A refrigerating system comprises a compressor (1A, 1B, 1C), a heat rejection device (2), an expansion device (15), an evaporator (5A, 5B, 5C) and a receiver (4), capable of operating with the compressor discharge higher than the critical pressure of the refrigerant; wherethe flow outlet from the heat rejection device is regulated by a pressure control valve (7),the pressure downstream of the pressure control valve is regulated by a gas vent valve (8),the refrigerant flow to the evaporator is further regulated by an automatic control device (41, 52, 14) at the inlet to the evaporator, andthe automatic control device being set to permit intermittent flow of liquid refrigerant to the receiver during normal operation of the system.The refrigerant may be carbon dioxide, which may operate under transcritical pressures e.g. 80 to 120 bar absolute. The receiver acts as a trap for any liquid from the evaporator and ensures that the gas flow to the compressor suction is dry.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application claims priority of United Kingdom Patent Application 0902192.4 filed Feb. 11, 2009.TECHNICAL FIELD[0002]The present invention relates to a refrigeration system, especially utilising carbon dioxide as refrigerant.BACKGROUND[0003]The prohibition of chlorofluorocarbons as refrigerants, which results from their deleterious effect on the ozone layer, has led to a number of alternative strategies to provide cooling without adverse environmental impact. The majority of systems have used hydrofluorocarbons, which have no effect on stratospheric ozone, but which in many cases have a high global warming potential and are therefore known to contribute to climate change through global warming. Carbon dioxide is a suitable fluid for use as a refrigerant, and was one of the earliest fluids used when mechanical refrigeration was developed in the raid 19th century. Carbon dioxide has no effect on the ozone layer, and is present in the atm...

AI Technical Summary

Benefits of technology

[0018]The evaporators are preferably equipped with individual expansion devices to regulate the flow of the refrigerant into each evaporator. It is a preferred feature of this invention that these expansion devices can be similar in type to the traditional valves used in conventional refrigerating systems, provided they can withstand the higher pressure encountered when operating with sub-critical carbon dioxide. The control of the expansion valves is based upon refrigerant temperature and pressure, but set to a relatively low value such that some liquid will, from time to time during normal modulation of the valve, pass from the evaporator to the receiver. In this respect, although the design and construction of these valves is similar to the traditional valves used in conventional systems, the control of the valves is novel. According to another preferred aspect of the invention, this novel control provides significant advantage in the event of a malfunction of one of the individual expansion devices on the evaporators because the excess liquid passed through the evaporator will be trapped by the receiver and will not cause damage to any of the compressors in the system.

[0019]A further preferred feature of this invention enables lubricant, which has passed through the oil separator and circulated through the refrigerant pipes to the evaporators and hence to the receiver, to be returned to the compressor(s). Failure to return this lubricant would result in a gradual accumulation in the receiver, causing a loss of heat transfer performance of the heat exchanger located in the receiver, and possibly also causing a lack of lubricant at the compressors. The lubricant may be returned by taking a sample of the liquid in the receiver, which may contain a proportion of lubricant, and mixing this sample into the suction gas to the compressor. By providing such lubricant return means to each compressor it can be ensured that lubricant is only returned to a machine which is operation at the time.

[0020]A preferred feature of the current invention is that the components are configured in the system such that functional failure of any of the control elements, comprising the flow control valve in the outlet of the heat rejection device, the gas vent pressure control valve or, as previously mentioned, the individual expansion devices at the evaporators will not cause damage to the rotating machinery incorporated into the circuit. Additionally, a preferred feature of the current invention is that the receiver shall be sufficiently large to contain the full mass of refrigerant, in liquid form, required for correct normal operation of the system.

Problems solved by technology

There is therefore limited possibility for accumulation of refrigerant on the high pressure side, as is required with the use of a conventional expansion valve.

They are not well suited to larger distributed systems where the refrigerating capacity is variable, where there are multiple evaporators and where the evaporators are large enough to be subject to liquid maldistribution.

In such larger systems application of these simple designs is liable to lead to unreliability, lack of refrigerating capacity under off-design conditions and impaired efficiency.

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