GB2469616A

GB2469616A - Transcritical Refrigeration System

Info

Publication number: GB2469616A
Application number: GB0902192A
Authority: GB
Inventors: Andrew Brash Pearson
Original assignee: Star Refrigeration Ltd
Current assignee: Star Refrigeration Ltd
Priority date: 2009-02-11
Filing date: 2009-02-11
Publication date: 2010-10-27
Anticipated expiration: 2029-02-11
Also published as: GB2469616B; US20100199707A1; GB0902192D0

Abstract

A refrigerating system has at least one compressor 1A, 1B, 1C, a heat rejection device such as a cooler, at least one expansion device 15, at least one evaporator 5A, 5B, 5C associated with an automatic control device 14 an and a receiver 4. The system is capable of operating with the compressor discharge pressure higher than the critical pressure of refrigerant, which may be carbon dioxide operating at transcritical pressure. Within the system, the 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, and the refrigerant flow to the at least one evaporator is regulated at the inlet to the evaporator by the automatic control device which permits intermittent flow of liquid refrigerant to the receiver during normal operation of the system. The receiver acts to trap any liquid received from the evaporator, and ensures a dry gas flow to the compressor suction.

Description

A REFRIGERATION SYSTEM

Technical Field

The present invention relates to a refrigeration system, especially utilising carbon dioxide as refrigerant.

Background

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 mid 19th century. Carbon dioxide has no effect on the ozone layer, and is present in the atmosphere at a level of 38Oppm. This bulk contributes to global warming, but the gas used in industrial systems, including refrigeration plant, is recovered from combustion processes and is therefore beneficially used in sealed equipment.

Since 1990 several inventors have proposed novel ways of using carbon dioxide for refrigeration, including Pearson in GB2258298 (1991), Lorentzen et al in US5245836 (1993) and Komatsubara et al in EP1790712 (2007). A comprehensive overview of ideas for systems is given in Rieberer, R, GassIer, M and Halozan, H, Control of C02 heat pumps" Proc hR Conf -4th Gustav Lorentzen, Purdue (2000).

The present invention extends the application of these previous ideas and provides for additional reliability in operation.

In a conventional refrigeration system heat is rejected by condensation of refrigerant at a pressure below the critical point of the fluid. When a thermostatically controlled expansion valve (or electronic equivalent) is used to control the flow of refrigerant the volume of liquid in the low pressure side of the system, primarily the evaporator, is widely variab'e, depending on the system load. The variation in liquid volume cannot be stored in the low pressure side of such a system, and so a high pressure liquid receiver is required at the outlet of the condenser to accumulate the variation in distribution of refrigerant between the high and the low side.

Transcritical operation of carbon dioxide refrigeration systems has been widely reported, including Bergdoll in US1860447 (1932), and many variant systems have been developed. When operating in this transcritical mode all of the refrigerant on the high pressure side of the system is in the form of gas. 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. Previous systems have used intermediate pressure receivers to provide some liquid storage volume for example Gernemanri and Heinbokel in W02006015741 (2006), or have allowed the pressure in the high pressure side of the system to vary, thereby accommodating variation in the refrigerant mass by changing the gas density. Designs of the latter type are suitable for very small refrigeration systems, typically characterised by having a single evaporator, a single compressor and a simple on/off control strategy.

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.

Summary of the Invention

The present invention comprises a refrigeration circuit comprising one or more refrigerant compressors capable of raising the refrigerant pressure from the evaporating condition up to a pressure suitable for rejection of heat to the atmosphere or a cooling fluid, coupled with one or more heat rejection heat exchangers, a flow control device and one or more heat extraction heat exchangers.

To achieve the novel mode of operation unique to this invention the system also includes a receiver between the heat extraction heat exchangers and the suction of the compressor(s), preferably a gas separator, a gas vent valve from the separator to the low pressure portion of the receiver and a set of flow control devices in the inlet to each heat extraction heat exchanger. The refrigerant is preferably capable of operating under transcritical conditions, and carbon dioxide is particularly preferred.

The evaporating pressure can be anywhere from the triple point of carbon dioxide (5.18 bar absolute) up to 60 bar absolute, representing an evaporating temperature of 22 °C. When the ambient temperature or cooling fluid temperature exceeds approximately 25 °C this requires the compressor to discharge gas at a pressure above the critical point of carbon dioxide (73.8 bar absolute). The preferred range for operation of the compressor discharge under these conditions is between 80 bar absolute and 120 bar absolute. When the ambient temperature or cooling fluid temperature is lower than approximately 25 °C the compressor can run at lower discharge pressures enabling the refrigerant to condense in the heat rejection device.

Unlike previous systems, the pressure in the refrigerant pipe which leads from the heat rejection device to the evaporator is permitted under these low ambient operating conditions to rise to the condensing pressure of the refrigerant. A preferred feature of the present invention is that the pressure regulator in the outlet of the heat rejection device will modulate when the system operates in the transcritical mode and will be fully open in the subcritical mode, whereas the pressure regulating valve in the vent line which leads from the gas separator to the receiver will modulate when the system operates in the transcritical mode and will be fully closed in the subcritical mode, albeit capable of operation in the event that the liquid line pressure rises above the set pressure level of the vent valve.

The pressure between the pressure control valve and the expansion device is permitted to rise to approximate the heat rejection device operating pressure by driving the pressure control valve fully open.

Generally, the receiver is located downstream of the pressure control valve and operates at a pressure intermediate the compressor discharge pressure and the evaporating pressure. It acts to receive liquid and gas refrigerant. Usually, an internal heat exchanger within the receiver contains a flow of refrigerant from the heat rejection device and to the evaporator(s). Low pressure refrigerant from the evaporator(s), possibly containing liquid and gaseous refrigerant is received into the low pressure portion of the receiver and any liquid separated out. Dry refrigerant is directed to the compressor inlet. The receiver is generally sized to be able to accommodate the full charge of refrigerant in the system.

A gas separator may be provided between the pressure control valve and the receiver. This separates the gas refrigerant from the liquid and passes the gas refrigerant into the low pressure portion of the receiver regulated by the gas vent valve, whilst the liquid passes through the internal heat exchanger within the receiver.

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.

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.

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.

A further object of the current invention is that, in the event of total power loss to the system, the rise in pressure which occurs due to heat infiltration through the pipe and vessel walls shall not result in a significant loss of refrigerant through the pressure relief devices, and shall not inhibit normal function of the refrigerating system automatically when power is restored. This feature of the design of the current invention is generally achieved through the sizing and disposition of the main system components. In the event that the system was correctly charged prior to loss of power then the pressure rise due to heat gain may be limited by the volume of liquid within the system in relation to the total volume of the system, and should not reach or exceed the set pressure of the safety relief device fitted to the receiver in the compressor suction line. In the event that the system had been overcharged and contained an excess of liquid, then it is possible that the pressure would reach or exceed the set pressure of the safety relief device fitted to the receiver in the compressor suction line for a limited time. Such excess pressure may be vented by the pressure regulating relief device or pressure relief device fitted to the receiver, but only to the extent of the allowable pressure of the system. In this manner, when power is restored and the system restarts automatically there shall be sufficient refrigerant contained within the system to resume normal operation without any manual intervention.

The invention also relates to a method of conducting refrigeration.

This invention can be applied to a wide range of refrigerating systems including, but not limited to supermarket refrigeration systems, air conditioning systems, iT cooling systems, refrigerated warehouse systems and refrigerated food processing systems.

A preferred embodiment of the invention will now be described by way of

example only.

Description of preferred embodiments

One embodiment of the invention is illustrated schematically in Figure 1.

In this system the refrigerant carbon dioxide is used for cooling of chiJled food display cases in a supermarket. A set of one or more compressors (1A, lB etc) pump refrigerant gas to a high pressure condition at which it can reject heat to atmosphere or some other cooling medium. The gas is conducted from the compressor(s) through pipes (6B, 6C) to a heat rejection device (2). From the heat rejection device the cooled refrigerant passes through pipes (6D, 6F) to a heat exchanger located in the receiver (4) and from there through pipes (6G) to a set of one or more system evaporators (5A, 5B etc).

When the temperature of the heat sink (the zone to which heat is rejected from heat rejection device (2)) is higher than approximately 25 °C then the compressors will be required to raise the carbon dioxide gas to a supercritical pressure, typically in the range 80 to 120 bar absolute. This can be described as "transcritical mode of operation." Under these conditions the pressure regulating valve (7) shall restrict the flow of refrigerant under control of an upstream pressure measurement device. This control may be to a fixed pressure condition, or it may use an optimising algorithm to adjust the control point. The pressure downstream of the regulating valve will be at an intermediate pressure level somewhere between the compressor discharge pressure and the evaporating pressure. The expansion process in regulating valve (7) will reduce the pressure from above the critical point to a level below the critical point, and some of the gas will turn to liquid as it passes through the pressure reduction valve. In order to ensure correct operation of the expansion devices (15) at the evaporators (5A, 5B etc) during this transcritical mode of operation it is necessary to vent the gas remaining after the first expansion. This is done in a pipe fitment (3) used as a gas separator. Unlike previous inventions this pipe fitment is not a pressure vessel, but merely a particular arrangement of the pipework configured to ensure that liquid flows from the bottom of the fitment and gas, with perhaps a small proportion of liquid, flows from the upper portion of the fitment. The gas vented from the gas separator is passed through pipes (6E) and a second pressure regulator (8) set to maintain a pressure on the upstream side equivalent to the maximum normal operating pressure for the liquid lines. In the preferred embodiment the allowable pressure (design pressure) in the liquid line (6F, 6G) and the suction line (6H) is 75 bar absolute and the operating pressure of the vent regulator (8) is 65 bar absolute.

It may be appropriate to include a refrigerant filter and a refrigerant drier in the refrigeration circuit. Item 11 in Figure 1 illustrates one such embodiment of this feature whereby the functions of drying and filtering the refrigerant are combined into a single device, but it is recognised that there are many possible alternative embodiments which achieve the same desired function.

The flowrate of refrigerant which passes from the internal heat exchanger within the receiver (4) to the expansion devices (15) is regulated according to the evaporating pressure and the evaporator outlet temperature to provide some liquid overfeed from the evaporator to the receiver. This regulation is achieved by sensors (Si, S2 etc) and a control element (14) connected to each of the expansion devices.

In the preferred embodiment, the control setpoint for evaporator outlet superheat is 2K or less, and normal system fluctuations result in liquid flowing out of the evaporator intermittently. This liquid travels with the evaporated refrigerant to the receiver vessel (4). The receiver acts as a trap for the liquid, ensuring that the gas flow to the compressor suction is dry.

When the temperature of the heat sink is lower than approximately 25 °C then the control of the pressure regulating valve is over-ridden and the valve is opened to its fullest extent. This can be described as "subcritical mode of operation.' Under these conditions the pressure in the liquid refrigerant pipe from the heat rejection device (2) to the evaporators, including the liquid in the gas separator, the internal heat exchanger of the receiver (4) and the pipe feeding to the evaporators shall be at the condensing pressure of the refrigerant. The vent regulator valve (8) shall

B

typically be closed under these conditions because the condensing pressure shall be less than 65 bar absolute.

The expansion valves at the evaporators (5A, SB etc) shall operate in the same manner as previously described, providing an intermittent flow of liquid mixed in with the evaporator outlet gas from the evaporator to the receiver.

Where the compressors use lubricating oil, it may be appropriate to use an oil separator (12) in the discharge from the compressors. In this case oil will return from the separator to the compressors through a series of tubes (13). Whether or not an oil separator is included in the system, the preferred embodiment of the invention will include a method for returning oil from the receiver (4) to the compressors (1A, lB etc). This method will preferably be fully automatic using a distillation system.

Protection against excess pressure for the system is provided by pressure relief valve (9) connected to the receiver. In addition a pressure regulating relief valve (10) is fitted to the receiver to permit small quantities of refrigerant to be released in a controlled manner if the pressure rises close to the sethng of the relief valve. It is a feature of the present invention that the dimensions of the receiver vessel are carefully selected in relation to the overall refrigerant content of the system to ensure that this pressure rise is self-limiting. With this consideration included in the design of the system the loss of refrigerant in the event of a major power cut will be minimised.

The heat rejection device may take the form of a finned heat exchanger in contact with atmospheric air, but in an alternative embodiment of the invention will be a heat exchanger in contact with a cooling fluid such as cooling tower water, process water requiring to be heated or some other medium.

Claims

Claims 1) A refrigerating system comprising a compressor, a heat rejection device, an expansion device, an evaporator and a receiver, capable of operating with the compressor discharge higher than the critical pressure of the refrigerant; where -the flow outlet from the heat rejection device is regulated by a pressure control valve, -the pressure downstream of the pressure control valve is regulated by a gas vent valve, -the refrigerant flow to the evaporator is further regulated by an automatic control device at the inlet to the evaporator, and -the automatic control device being set to permit intermittent flow of liquid refrigerant to the receiver during normal operation of the system.
2) A refrigerating system in accordance with claim 1 where the refrigerant is carbon dioxide.
3) A refrigerating system in accordance with claim 1 where the pressure between the pressure control valve and the expansion device is permitted to rise to approximate the heat rejection device operating pressure by driving the pressure control valve fully open.
4) A refrigerating system in accordance with any preceding claim where lubricant, which has accumulated in the receiver, can be automatically returned to the compressors through a sampling device.
5) A refrigerating system in accordance with any preceding claim where pressure rise due to heat gain when the compressors are non-operational is self-limiting by virtue of the size and disposition of the refrigeration system components.
6) A refrigerating system in accordance with any preceding claim where the total refrigerant mass contained in the system can be accumulated in the receiver.
7) A refrigeration system in accordance with any preceding claim where the heat rejection device rejects heat to a cooling fluid requiring to be heated.