GB2314149A

GB2314149A - Thermosyphon refrigeration apparatus

Info

Publication number: GB2314149A
Application number: GB9712239A
Authority: GB
Inventors: Stephen Forbes Pearson
Original assignee: Star Refrigeration Ltd
Current assignee: Star Refrigeration Ltd
Priority date: 1996-06-14
Filing date: 1997-06-13
Publication date: 1997-12-17
Anticipated expiration: 2017-06-13
Also published as: GB2314149B; GB9712239D0; GB9612445D0

Abstract

A refrigeration apparatus operable either by thermospyhon or vapour compression circulation of refrigerant comprises a condenser 2 for condensing refrigerant, an evaporator 24 for providing cooling effect and delivering a partially evaporated mixture of liquid refrigerant and refrigerant vapour. A receiver 30 is located between the evaporator and the condenser which collects excess refrigerant charge during thermosyphon mode, which might otherwise collect in the evaporator and inhibit operation thereof. During vapour compression operation liquid collecting in the receiver is evaporated away by heat transfer in heat exchanger 12 with liquid refrigerant at high temperature from the condenser and/or by heat transfer in fluid cooling means 48 with a portion of the warm fluid (e.g. water) which is to be cooled in the evaporator. In thermosyphon mode boiling of refrigerant in the evaporator is assisted by heat exchange from warm fluid to be cooled in a fluid heat exchange coil 54 located in the inlet manifold 22 to the evaporator, the heat exchange coil having a surface treatment to promote boiling.

Description

THERMOSYPHON REFRIGERATION APPARATUS The present invention describes a refrigeration apparatus capable of operation in a thermosyphon mode when ambient temperatures are low, and generally has the ability to operate by conventional vapour compression at higher ambient temperatures.

Concerns about ozone depletion and accelerated global warming have led to a desire to return to the use of ammonia as a refrigerant, as it is efficient and has zero ozone depleting potential and negligible global warming effect.

Refrigeration apparatus which can operate in other vapour compression mode or in thermosyphon mode depending on ambient temperatures are known in the art and are described for example in patent specification GB 2233080. When such an apparatus is running in thermosyphon mode, the compressor may be switched off, allowing refrigeration to continue by natural circulation (including evaporation and condensation of the refrigerant) without power input into the compressor. This can result in a substantial cost saving. However, the apparatus can operate in thermosyphon mode only when the substance to be cooled is at a temperature well above ambient. The apparatus described in patent specification GB 2233080 employs a shell and tube evaporator having specially treated tubes to promote boiling at small temperature differences between the refrigerant and the substance (typically water) to be cooled. However, the apparatus described in GB 2233080 is not necessarily suitable for all refrigerants, in particular ammonia.

Previous attempts to use ammonia in dual mode vapour compression/thermosyphon systems employing a plate heat exchanger have been unsuccessful. The ammonia charge required for successful operation in a vapour compression refrigerating system with a plate heat exchanger evaporator is significantly greater than the allowable charge for operation as a thermosyphon system without running the compressor.

In thermosyphon mode the excess ammonia drains to the lowest parts of the refrigeration circuit. This in turn, produces a static head effect at the plate heat exchanger evaporator which prevents the ammonia from evaporating from the lower parts of the evaporator and inhibits refrigerant circulation.

British patent specification GB1502607 discloses the use of a low pressure receiver to control overfeed of refrigerant during vapour compression refrigeration. However, there is no mention of use of such a facility during thermosyphon operation.

It is an object of the present invention to provide a refrigeration apparatus capable of operation in thermosyphon mode, and which is suitable for use with ammonia as a refrigerant.

Generally speaking, the present invention is based on the discovery that undesirable static pressure head effects on the evaporation of the refrigerant may be mitigated by providing a receiver vessel to receive an excess portion of the refrigerant charge during thermosyphon.

The present invention in particular provides a refrigeration apparatus operable under thermosyphon circulation of refrigerant which comprises; - a condenser for condensing refrigerant and having an inlet for receiving vapour and an outlet for liquified refrigerant; - an evaporator having an inlet connected for receiving liquified refrigerant from the condenser and an outlet for delivering a partially evaporated mixture of liquid and vapour refrigerant; - a receiver having an inlet connected to the evaporator outlet for receiving the liquid and vapour refrigerant mixture and an outlet for delivering refrigerant vapour to the condenser inlet, the receiver having a capacity to collect and retain during thermosyphon circulation a volume of liquid refrigerant which is in excess of that which is required to establish thermosyphon circulation of refrigerant around the apparatus, and such that said excess liquid refrigerant does not create a pressure head in the evaporator which inhibits evaporation of refrigerant therein.

Generally, the refrigeration apparatus also comprises a compressor means, expansion valve means and suitable valving to allow it to also operate in vapour compression mode.

The refrigerant may be any suitable refrigerant known in the art. Preferably the refrigerant is ammonia.

Generally, the evaporator is a plate-type heat exchanger. Such heat exchangers are well known and comprise a multiplicity of corrugated plates arranged in a stack and clamped together to ensure a seal. The corrugations define flow channels between adjacent plates, and generally the arrangement is such that the alternate spaces between adjacent plates contain refrigerant (eg. ammonia) and fluid to be cooled (eg.

water) respectively. Suitable gaskets between the plates provide inlet and outlet manifolds within the heat exchanger for refrigerant (and inlet and outlet manifolds for fluid to be cooled) such that the flows through the spaces between the plates are in parallel.

The receiver is of a type sometimes referred to as a "low pressure receiver" and is intended for receiving excess refrigerant during thermosyphon mode so as to prevent the build-up of an excessive static pressure head in the evaporator. Thus, during thermosyphon mode, the receiver contains a portion of the refrigerant charge in liquid form. The liquid is carried over from the evaporator as a mixture of liquid and vapour due to incomplete evaporation of refrigerant in the evaporator. During thermosyphon mode, liquid refrigerant collects within the receiver, such that the overall amount of recirculating refrigerant is reduced to the correct amount.

However, during vapour compression mode, a full refrigerant charge is usually required to be recirculated, so that there is relatively little liquid refrigerant in the receiver. In vapour compression mode, the refrigerant is compressed employing the compressor means and becomes heated to a much higher temperature and pressure than during thermosyphon mode. Heated high pressure liquid refrigerant from the condenser is used to evaporate liquid refrigerant in the receiver, so that any overfeed of refrigerant from the evaporator is vapourised and returned to the compressor. To this end, heated liquid refrigerant from the condenser may be directed through a heat exchanger in thermal contact with the receiver prior to the refrigerant passing to the evaporator. The heat exchanger is generally sized such that during normal vapour compression operation, there is a minimum of liquid refrigerant within the receiver.

A receiver of a type suitable for use in the present invention is described in prior patent specification GB 1502607. However, the arrangement in the present invention is to be contrasted with that disclosed in the prior reference, which is concerned solely with vapour compression refrigeration and does not address the problems of operating under thermosyphon conditions. Moreover, in this reference the receiver (referred to as a "low pressure receiver") is designed to operate with a substantial charge of liquid refrigerant within the receiver at all times.

In another embodiment of the present invention, boiling of liquid refrigerant from within the receiver during vapour compression mode may be achieved by heat exchange with fluid to be cooled (eg. water). For example, a portion of the fluid to be cooled may be passed through a fluid cooling means in thermal contact with the receiver. Thermal contact with the fluid to be cooled is effective when that fluid is at a relatively high temperature i.e. requires to be cooled by a large amount, which corresponds to the conditions where vapour compression refrigeration giving a higher degree of cooling would be required.

Thermosyphon mode would generally be used where lower amounts of cooling (and lower fluid inlet temperatures) are required.

However, in many circumstances, it may be desirable to use both heat exchange with hot compressed refrigerant (employing said heat exchanger) and heat exchange with fluid to be cooled (using the fluid cooling means) in order to evaporate liquid refrigerant from the receiver during vapour compression mode.

As described above, steps may need to be taken in the present invention to enable boiling of liquid refrigerant in the lower parts of the evaporator during thermosyphon mode. In thermosyphon mode, the condenser is generally arranged to be above the level of the evaporator so that liquified refrigerant from the condenser passes by gravity downwards to the evaporator, thereby creating thermosyphon circulation.

During thermosyphon mode, there will generally speaking be a relatively small temperature difference between the liquified refrigerant and the fluid to be cooled which may make it difficult to initiate boiling of the refrigerant. This may be alleviated by taking steps such as outlined above to carefully control the static pressure head of liquid refrigerant within the evaporator. However, in a further preferred embodiment of the invention, further provision is made for initiating boiling of the liquid refrigerant within the evaporator. Thus, liquid refrigerant generally enter the lowest point of the evaporator, i.e. the refrigerant inlet manifold is located in a lower portion of the evaporator below the outlet manifold. Generally, the flows of refrigerant and fluid to be cooled through the evaporator are concurrent, that is to say that the coldest refrigerant first encounters the warmest fluid to be cooled. In order to promote boiling, it is preferred to additionally provide a fluid heat exchanger means within the refrigerant inlet manifold to the evaporator, this fluid heat exchanger being provided with a portion of the fluid to be cooled from the fluid inlet manifold. Preferably, this fluid heat exchanger is in the form of a coil of high flux tubing treated to promote the boiling of refrigerant on the outside of the tubing. High flux tubing is known and comprises a surface treated to provide nucleation cavities to facilitate the initiation of bubbles during boiling. The surface of the high flux tubing may include a thin layer of heat-conductive particles defining therebetween nucleation voids. Typically the heat-conductive particles are metal particles such as aluminium or other high conductivity metals or alloys known in the art. The heat-conductive layer produces a textured surface which consists of a large number of re-entrant cavities. These small cavities facilitate the production of tiny pockets of vapour and thus effectively enhance the boiling heat transfer capacity of the tubing.

A particular preferred method of making the high flux tubing is described in patent specification EP0612858. The high flux tubing so described is produced by concurrently spraying the outer surface of a tube with liquid metal particles and liquid carbon dioxide. This results in the production of a high flux boiling tubing which has a high coefficient of heat transfer and is particularly suitable for use in evaporative refrigeration systems.

The use of high flux tubing in the refrigerant inlet manifold to the evaporator is effective in initiating boiling within the lowest part of the evaporator where the temperature difference with the fluid to be cooled is at its highest, notwithstanding the relatively small temperature differences which may prevail during thermosyphon mode.

Furthermore, it is generally advantageous to split the refrigerating load into two or more components with the fluid to be cooled travelling in series through a number of evaporators. In this way, the maximum temperature difference between refrigerant and fluid to be cooled is encountered at the upstream end of the first evaporator, thereby maximising heat transfer.

Generally, sensors, possibly linked to computer means, will be provided to switch the apparatus from vapour compression mode into thermosyphon mode when high refrigeration capacity is not required e.g. at low ambient temperatures. In vapour compression mode, a compressor is generally provided between the outlet of the receiver and the inlet of the compressor in order to compress the refrigerant vapour. In thermosyphon mode, this compressor is by-passed.

Furthermore, an expansion means (such as an expansion valve) is generally employed in vapour compression mode to allow expansion of the cooled liquified refrigerant prior to the evaporator. This expansion valve is not required in thermosyphon mode and is therefore generally also by-passed. In thermosyphon mode, evaporative cooling occurs due to boiling of the liquid refrigerant in the lower parts of the evaporator.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawing wherein: Figure 1 is a schematic drawing of a refrigeration apparatus according to the present invention.

The refrigeration apparatus comprises a condenser 2 for rejecting heat from the refrigerant and allowing the refrigerant to become liquified, having a level controller 4 on an outlet thereof, which is in turn connected via a duct 6 to a strainer vessel 8. This is connected via duct 10 to a heat exchanger 12 in thermal contact with a liquid refrigerant receiver 30 (to be described in more detail) whereby heat is transferred from the liquified refrigerant to the receiver. A duct 14 connects the heat exchanger 12 with an expansion valve 16. In vapour compression mode, cold liquified refrigerant is expanded through the expansion valve 16 where it becomes partially evaporated and the partially evaporated refrigerant passes through duct 20 to inlet manifold 22 of a plate-type evaporator 24.

In thermosyphon mode, the expansion valve 16 is by-passed by opening ball valve 18.

The evaporator 24 comprises a series of parallel corrugated plates clamped together to form a plate heat exchanger. This comprises a cylindrical inlet manifold 22 extending through the stack of plates at right angles to the plane of each plate, at a lower end of the evaporator. A corresponding outlet manifold 26 extends through the plate heat exchanger at an upper end thereof. Refrigerant passes upwardly through the spaces between each alternate pair of plates from the inlet manifold 22 to the outlet manifold 26. There are thus a multiplicity of parallel refrigerant flow paths through the evaporator.

The plate-type evaporator also comprises an inlet manifold 42 and an outlet manifold 44 for fluid to be cooled (usually water). These manifolds also run at right angles to the plane of each plate and are arranged so that fluid to be cooled passes through the remaining alternate spaces between heat exchanger plates. There are thus a multiplicity of parallel flows of fluid to be cooled through the evaporator.

Such plate-type heat exchangers are well known and allow high heat transfer rates at relatively low pressure drop.

Refrigerant passes from the refrigerant outlet manifold 26 through duct 28 into the liquid refrigerant receiver 30. The receiver is arranged to contain excess unrequired refrigerant in liquid form during thermosyphon mode so as to prevent an excessive static head of liquid refrigerant being applied to the evaporator (which would inhibit vaporisation therein).

Under vapour compression conditions, such refrigerant is evaporated by the action of heat from heat exchanger 12 and by the action of heat transfer from fluid cooling means 48 which contains fluid to be cooled at its highest temperature.

Refrigerant vapour passes from the receiver via duct 32 to compressor 34 wherein it is compressed and recirculated via duct 40 to the condenser again. In thermosyphon mode, a three-way valve 38 in duct 40 is turned so as to by-pass the compressor and allow refrigerant to flow through duct 36.

As mentioned above, fluid to be cooled enters the evaporator via inlet manifold 42 and exits via outlet manifold 44. Warm fluid to be cooled is taken from the inlet manifold via duct 46 and directed into fluid cooling means 48 wherein it is cooled somewhat by thermal contact with liquid within the receiver 30, before returning to outlet manifold 44 via duct 50.

During thermosyphon mode, there may be a problem in initiating boiling of the liquid refrigerant within the refrigerant inlet manifold 22. This problem is tackled by taking a further stream of warm fluid to be cooled from the inlet manifold 42 along duct 52 and into a fluid heat exchange coil 54 extending along the refrigerant inlet manifold 22. This fluid heat exchange coil 54 has an outer surface in contact with the refrigerant which carries a surface treatment intended to promote boiling of liquid refrigerant within the refrigerant inlet manifold 22. The surface treatment generally comprises particles which form reentrant cavities which can provide vapour bubble nucleation centres which assist the initiation of vapour bubbles and thus allow boiling to occur. In this way, boiling is initiated in the lowest part of the evaporator. The presence of vapour bubbles enhances heat transfer from the refrigerant contained between the plates of the evaporator with the fluid to be cooled which passes through alternate inter-plate spaces.

The apparatus is capable of operating in vapour compression mode, particularly when ambient temperatures are relatively high so that the heat rejection capacity of the condenser 2 is reduced, and also when the temperature of water to be cooled in inlet manifold 42 is also relatively high thus requiring higher refrigeration capacity of the apparatus. Such vapour compression refrigeration is carried out in conventional manner employing compressor 34 and expansion valve 16. In this mode, there is a minimum of liquid refrigerant within liquid refrigerant receiver 30, the refrigerant charge being optimised for vapour compression operation.

Thermosyphon operation is indicated when ambient temperatures are relatively low, so that the heat rejection efficiency of condenser 2 is enhanced.

Furthermore, the inlet temperature of water to be cooled may be lower, so that less refrigeration capacity is required. In this situation, energy can be saved by not running the compressor 34 and allowing refrigerant circulation by thermosyphon. In this case, compressor 34 is by-passed by three-way valve 38 and expansion valve 16 is by-passed by ball valve 18. Liquid refrigerant (such as ammonia) becomes liquified by heat rejection in condenser 2 and flows by gravity through ducts 6 and 10 to heat exchanger 12. Little heat is lost from the liquid refrigerant in heat exchanger 12 since the refrigerant is at a relatively low temperature. The liquid refrigerant then passes through line 14 to evaporator inlet manifold 22. Here, there is a relatively low temperature difference between liquid refrigerant in inlet manifold 22 and warm water in fluid inlet manifold 42, which may be insufficient to initiate boiling. However, warm water is passed through fluid heat exchange coil 54 which has a surface treatment which enhances initiation of boiling within the refrigerant inlet manifold, and thus within the whole of the evaporator. A mixture of liquid and vapour refrigerant leaves the evaporator through line 28 and passes to the receiver 30 where liquid refrigerant is collected. The receiver 30 is arranged to receive excess liquid refrigerant not required during thermosyphon mode. Such excess liquid refrigerant would otherwise create an undesirable static head on the liquid refrigerant contained within the refrigerant inlet manifold 22 and thereby inhibit boiling. To avoid this problem, when required, liquid refrigerant is collected in receiver 30. Some warm water passes from the inlet manifold to fluid cooling means 48. This allows some evaporation of overfed liquid from the receiver and thus allows the plate heat exchanger to operate flooded even under themosyphon conditions.

In this way, excess refrigerant can be removed from circulation within receiver 30 during thermosyphon mode, and can be re-evaporated into circulation by the action of heat exchanger 12 and fluid cooling means 48 during vapour compression operation. Furthermore, boiling during thermosyphon mode in the evaporator is enhanced by the use of the fluid heat exchanger 54, some degree of overfeeding of the evaporator 24 being maintained by the action of fluid cooling means 48.

Claims

CLAIM

1. A refrigeration apparatus operable under thermosyphon circulation of refrigerant which comprises; - a condenser for condensing refrigerant and having an inlet for receiving vapour and an outlet for liquified refrigerant; - an evaporator having an inlet connected for receiving liquified refrigerant from the condenser and an outlet for delivering a partially evaporated mixture of liquid and vapour refrigerant; - a receiver having an inlet connected to the evaporator outlet for receiving the liquid and vapour refrigerant mixture and an outlet for delivering refrigerant vapour to the condenser inlet, the receiver having a capacity to collect and retain during thermosyphon circulation a volume of liquid refrigerant which is in excess of that which is required to establish thermosyphon circulation of refrigerant around the apparatus, and such that said excess liquid refrigerant does not create a pressure head in the evaporator which inhibits evaporation of refrigerant therein.

2. Apparatus according to claim 1 which comprises ammonia as refrigerant.

3. Apparatus according to any preceding claim operable also under vapour compression mode which further comprises a compressor means and expansion valve means.

4. Apparatus according to claim 3 which further comprises heating means operative on the receiver such as to evaporate liquid refrigerant from the receiver during operation of said compressor means during vapour compression mode.

5. Apparatus according to claim 4 wherein the heating means operative on the receiver comprises a heat exchanger in thermal contact with the receiver and connected to receive heated liquid refrigerant from the condenser.

6. Apparatus according to claim 4 wherein the heating means operative on the receiver comprises a fluid cooling means in thermal contact with the receiver and connected to receive a portion of the fluid to be cooled in the evaporator.

7. Apparatus according to claim 5 and 6 which comprises both said heat exchanger and said fluid cooling means in thermal contact with the receiver.

8. Apparatus according to any preceding claim wherein the evaporator is in the form of a plate-type heat exchanger and the evaporator inlet is in the form of an inlet manifold, and which further comprises a fluid heat exchanger means within the evaporator inlet manifold, the fluid heat exchanger means being fed with a portion of fluid to be cooled in the evaporator.

9. Apparatus according to claim 8 wherein the fluid heat exchanger means is in the form of high flux tubing having a surface treatment to provide nucleation cavities to facilitate the initiation of boiling of liquid refrigerant in the evaporator inlet manifold.

10. Apparatus according to any of claims 3 to 8 which further comprises means to by-pass the compressor means and means to by-pass the expansion valve means during thermosyphon circulation.

11. A method of refrigeration under thermosyphon circulation of refrigerant which comprises; - condensing refrigerant in a condenser having an inlet for receiving vapour and an outlet for liquified refrigerant; circulating the liquified refrigerant to an evaporator having an inlet connected for receiving the liquified refrigerant from the condenser and an outlet for delivering a partially evaporated mixture of liquid and vapour refrigerant; - a receiver having an inlet connected to the evaporator outlet for receiving the liquid and vapour refrigerant mixture and an outlet for delivering refrigerant vapour to the condenser inlet; the method further comprising collecting and retaining in the receiver during thermosyphon circulation a volume of liquid refrigerant which is in excess of that which is required to establish thermosyphon circulation of refrigerant around the apparatus, such that said excess liquid refrigerant does not create a pressure head in the evaporator which inhibits evaporation of refrigerant therein.