GB2474259A

GB2474259A - Vapour compression refrigeration circuit

Info

Publication number: GB2474259A
Application number: GB0917595A
Authority: GB
Inventors: John Malcolm Elliott
Original assignee: Ebac Ltd
Current assignee: Ebac Ltd
Priority date: 2009-10-08
Filing date: 2009-10-08
Publication date: 2011-04-13
Also published as: GB0917595D0

Abstract

In a refrigeration circuit 1 a refrigerant is compressed adiabatically by a main compressor 2 before passing through a condenser 3 to release heat, the liquid output from the condenser is used to drive an expansion device 4. The cooled refrigerant then passes through an evaporator 5 to absorb heat. The circuit can include a pre-compressor 6 arranged to assist the main compressor 2, or can include means to generate power from the expansion device. The expansion device may drive an electrical generator for producing electrical power output, or the expansion device may be a turbine which produces mechanical power output. The power output from the expansion device 4 may be used to assist the main compressor 2 by driving the pre-compressor 6 to assist the compressor in circulating refrigerant vapour around the circuit, or the output from the expansion device may be used to directly assist the compressor by electrical power or mechanical power means. The expansion device thus recovers a proportion of the energy that would normally be lost, and this is used to reduce the power consumption of the system.

Description

VAPOUR COMPRESSION REFRIGERATION CIRCUITS

TECHNICAL FIELD OF THE INVENTION

This invention relates to vapour compression refrigeration circuits. In this specification the term "refrigeration circuit" will be understood to encompass all kinds of refrigeration systems, including (without limitation) those used in refrigerators for cooling food etc., water coolers, dehumidifiers, air conditioning, geothermal heat pumps etc.

BACKGROUND

Refrigeration circuits of the vapour compression type utilise adiabatic compression and expansion of a recirculating refrigerant medium to move heat from one place to another.

Pressurised refrigerant vapour gives up heat through condensation in a first heat exchanger known as a condenser and absorbs heat through evaporation in a second heat exchanger known as an evaporator. Pressurisation of the refrigerant normally uses electrical energy which is used to drive a mechanical compressor. After passing through the condenser the refrigerant is subjected to expansion to reduce its temperature and pressure before entering the evaporator.

In recent years environmental concerns have produced a strong public demand for energy wastage to be minimised and make energy-consuming systems as energy efficient as possible. To date, manufacturers have paid little attention to improving the energy efficiency of refrigeration circuits themselves, concentrating instead on reducing losses in the heat source (e.g. a refrigerator Compartment) or the heating target (e.g. a building).

US 6 185 944 Bi attempts to improve the efficiency of a refrigeration system by incorporating a liquid pump between the condenser and the expansion device to increase the pressure of the liquid entering the expansion device. The pump may be built into the compressor and driven by the same motor.

However, it is believed that such a system would be relatively inefficient compared with a conventional refrigeration system and would not produce any significant benefit.

Instead of using a mechanical compressor DE 4 415 199 Al uses a thermal compressor comprising an refrigerant absorber and desorber, but when the volumetric capacity is high the thermal compressor IS augmented by a mechanical compressor.

In general, absorption refrigeration systems have low efficiency, although they can be useful under certain circumstances, e.g. when solar power is available to provide free energy.

The present invention seeks to provide a new and inventive form of vapour compression refrigeration circuit which has improved operating efficiency relative to existing refrigeration circuits.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention proposes a vapour compression refrigeration circuit in which a compressor circulates a refrigerant through a condenser, an expansion device and an evaporator, wherein a mechanical pre-compressor is incorporated between the evaporator and the compressor to increase the temperature and pressure of the refrigerant vapour leaving the evaporator before entering the compressor.

Under certain conditions it can be been shown that raising the input pressure to a mechanical compressor gives a bigger increase in performance than the energy used to raise the pressure. This occurs when the compressor would otherwise run at lower input pressure than it's design optimum.

According to another aspect, the invention provides a vapour compression refrigeration circuit in which a compressor circulates a refrigerant through a condenser and an evaporator, wherein refrigerant leaving the condenser passes through an expansion device which generates power which is used to assist the compressor in circulating refrigerant around the circuit.

Refrigeration circuits in accordance with this aspect of the invention are capable of providing even greater performance for a given amount of power consumed.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice.

In the drawings: Figure 1 is a schematic diagram of a vapour compression refrigeration circuit in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1. shows a vapour compression circuit 1 of the kind which is often referred to as a refrigeration system or heat pump, depending on the intended use. A mechanical compressor 2 circulates refrigerant vapour under pressure through a condenser 3 and an evaporator 5. The compressor raises the pressure of the refrigerant causing an increase in temperature and pressure without increasing the heat content. The condenser 3 is essentially a heat exchanger which is capable of removing heat from the superheated refrigerant, e.g. by transferring heat to ambient air or another cooling medium.

Upon entering the condenser the vapour gives up heat and condenses causing the liquid refrigerant to become cool. The refrigerant then passes through a pressure reducer 4 which reduces the pressure and therefore the temperature of the liquid. When the cooled refrigerant enters the evaporator 5 it absorbs heat, e.g. from ambient air or another heat exchange medium, causing the liquid to evaporate. Refrigerant vapour from the evaporator is re-circulated by the compressor 2 in a continuous cycle of condensation and eyaporation, absorbing heat into the evaporator and expelling it from the condenser, assuming a favourable temperature gradient at each.

The main function of the compressor is to raise the temperature and pressure of the refrigerant adiabatically so that the superheated refrigerant vapour can give up heat in the condenser. This process must then be reversed so that the refrigerant can absorb heat again in the evaporator. This is the function of the pressure reducer 4. In known vapour compression circuits the pressure reducer 4 is some kind of expansion device such as a valve, a small bore tube, or another form of restriction. Whereas the compressor requires energy input in order to function, usually in the form of electrical energy, a significant proportion of this energy is simply lost in the expansion device.

In the present invention the expansion device 4 (pressure reducer) is used to recover a proportion of the energy used in compression of the refrigerant (typically about 10%) which may, for example, be used to pre-compress the refrigerant vapour leaving the evaporator or otherwise assist the compressor. In one example the expansion device could take the form of an electrical generator which is driven by pressurised liquid, i.e. the refrigerant. The electrical power can then be used to drive a pre-compression pump 6. This increases the pressure of refrigerant entering the compressor 2 so that the compressor is required to do less work to achieve the same temperature and pressure of gas entering the condenser 3. A pre-compressor 6 also enables the temperature and pressure of refrigerant to be reduced in the evaporator 5, producing a further increase in efficiency.

Electrical generators are not very efficient, so that if the generator is only say 50% efficient this would only recover about 5% of the power input, but this would still produce a useful increase in overall efficiency. However, it is possible to use the energy stored in the pressurised liquid more directly.

The greatest advantage could be obtained by using part of the energy used in expansion of the refrigerant to directly assist the compressor. For example, the pressurised liquid leaving the condenser could drive a turbine incorporated into the compressor to assist rotation of the compressor shaft.

It is important to note that the energy used to assist the compressor is being recovered from the compressed liquid, and -.7-is energy that would otherwise be lost in expansion of the refrigerant. This energy is instead being recovered and used to increase the overall efficiency of the system.

It can be been shown that in certain circumstances, raising the input pressure into a standard compressor gives a bigger increase in performance than the energy used to raise the pressure. This occurs when the compressor would otherwise run at lower input pressure than it's design optimum. The return pressure in some refrigeration systems varies depending on the ambient temperature, e.g. if the ambient is say 10 °C the evaporating temperature could be at -5 °C while if the ambient fell to say 5 °C the evaporating temperature would also fall, which means the pressure is lower and causes the compressor to pump less refrigerant. This is not important when the ambient temperature is consistent and thereby the optimum design can be used e.g. in air conditioning which usually operates at a design condition of 22 °C or refrigeration which operates in a fixed ambient of say -18 °C. However, a heat pump will operate under different ambient Conditions at the evaporator. In cases where the evaporation is fixed the condensing will vary and similar benefits may occur if there is a pre-compression pump on the condenser side.

Whilst the above description places emphasis on the areas which are believed to be new and addresses specific problems which have been identified, it is intended that the features disclosed herein may be used in any combination which is capable of providing a new and useful advance in the art.

Claims

CLAIMS1. A vapour compression refrigeration circuit in which a compressor circulates a refrigerant through a condenser, an expansion device and an evaporator, wherein a mechanical pre-compressor is incorporated between the evaporator and the compressor to increase the temperature and pressure of the refrigerant vapour leaving the evaporator before entering the compressor.
2. A vapour compression refrigeration circuit according to Claim 1 in which liquefied refrigerant passing through the expansion device produces power which is used to drive the pre-compressor.
3. A vapour compression refrigeration circuit according to Claim 2 in which the expansion device generates electrical power which drives the pre-compressor.
4. A vapour compression refrigeration circuit according to Claim 2 in which the expansion device generates torque which drives the pre-compressor.

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5. A vapour compression refrigeration circuit In which a compressor circulates a refrigerant through a condenser and an evaporator, wherein refrigerant leaving the condenser passes through an expansion device which generates power which is used to assist the compressor in circulating refrigerant around the circuit.
6. A vapour compression refrigeration circuit according to Claim 5 in which the expansion device generates electrical power.
7. A vapour compression refrigeration circuit according to Claim 6 in which the electrical power is used to drive a pre-compressor which increases the temperature and pressure of refrigerant vapour leaving the evaporator before entering the compressor.
8. A vapour compression refrigeration circuit according to Claim 5 in which the expansion device generates torque which is used to assist the compressor.
9. A vapour compression refrigeration circuit according to Claim 8 in which the expansion device is a turbine.
10. A vapour compression refrigeration circuit according to * Claim 9 in which the turbine and the compressor share a common shaft. -11 -
11. A vapour compression refrigeration circuit substantially as described with reference to the drawings.