BACKGROUND ART The water has traditionally been heated in two ways. A first water heating method uses a water cooled desuperheater heat exchanger where only the discharge superheat is collected, with no latent heat being taken. In this case the leaving water temperature can approach the inlet superheated refrigerant gas temperature. This method can give moderately high water temperatures in standard temperature refrigeration systems, typically 60°-70°C, but due to the low superheat fraction in the rejected heat only low volumes are produced.

The second method of water heating utilises a water cooled condenser where both superheat and latent heat together are collected. In this case the leaving water temperature can normally only approach the refrigerant condensing temperature. With this method, where all the rejected heat is collected, but lower water temperatures are achieved (typically only up to 45°C), the heated water is produced in significantly higher volumes.

The limited amount of high temperature water achieved in desuperheaters has prevented the wide application of this heating method to smaller commercial refrigeration systems of between 10-50kw.

The requirement for large volumes of high temperature water at 60-70°C or even higher was only previously satisfied by using a water cooled condenser and elevating the condensing temperature to the desired leaving water temperature. With the technical limitations on compressor operation, an upper limit on condensing temperature is imposed for each refrigerant, with the highest being for R134a at 75°C. So a requirement for large volumes of hot water in excess of 75°C could not be met without exceeding the compressor manufacturers limitation on condensing temperature, and water at 85°C in commercial volumes was impossibility.

It is this requirement for commercial volumes of high temperature hot water that the technology in NZ Patent PCT/NZ00/00186 (published as WO 01/22011) relates. It discloses the capability of obtaining water heated to temperatures well in excess of the condensing

temperature, while still collecting all the rejected heat, both superheat and latent heat, and giving the combined advantages of the above two water heating methods without any of their individual disadvantages.

The method in NZ Patent PCT/NZ00/00186 to Thompson provides a refrigeration system extracting heat from a low temperature source, namely cold water at 4°C. The refrigeration system then operates as a heat pump elevating the low grade heat extracted from the water to a high temperature in a single vapour compression stage and then rejecting the heat to a flow of water via a series condenser and desuperheater.

This involves using the same refrigerant to provide the cooling of the water to collect source heat for the heat pump and also provide the water heating via the series condenser and desuperheater. The refrigerant R134a is chosen to allow for a condensing temperature as high as practical for the heat pump while keeping within the compressor application envelope.

However the use of R134a as the refrigerant for the collection of the source heat from the water at low evaporating temperatures in a single compression stage imposes inefficiency in the compression cycle when the compression ratio is 11: 1.

It is an object of the present invention to provide for water heating which allows for the provision of worthwhile quantities of hot water at relatively high temperatures reliant upon heat extracted from a refrigerated zone by a refrigeration system.

It is another object of the present invention to provide a system that allows optimising of a refrigeration system yet still allows for the extraction of worthwhile heat therefrom for the purpose of heating worthwhile quantities of water to a relatively high temperature.

DISCLOSURE OF INVENTION In a first aspect the present invention consists in a method of heating a liquid to a temperature in the range of from 60 to 90°C which comprises or includes passing that liquid in a non mingling heat exchange with a working fluid such that a heat exchange occurs initially which allows uptake by the liquid of (preferably at least some of the sensible heat and preferably all of) the latent heat of the working fluid and thereafter another heat exchange which allows the uptake of all of the superheat from the working fluid, the heat exchangers being part of a heat pump system deriving its input heat by a first heat exchange.

Reference herein to a source of energy for the first heat exchange preferably refers to heat at a temperature below that required for the resultant hot liquid (e. g. water) whether derived by the first heat exchanger from a fluid or not. Whilst as hereinafter described in more detail a preferred source is the working fluid of a refrigeration system, other sources are contemplated.

Examples include gases or liquids or mixtures thereof moving in a system irrespective of whether or not subject to a compression/expansion cycle as typified in a refrigeration system.

The source of heat preferably can be considered as"lower grade"heat but in sufficient quantity whereby it can be used to provide the heat output liquid (e. g. water) having as enthalpy therein a"higher grade"heat.

Preferably the latent heat and superheat heat exchange is by spaced heat exchangers, one heat exchanger being a desuperheater (preferably not operating to condense the working fluid even though it may leave as a fully saturated vapour) and the other heat exchanger by condensing the vapour received from the desuperheater extracting at least (preferably most or all) latent heat and preferably some sensible heat.

Preferably the heating COP is above 4 (preferably about 4.5).

Preferably the working fluid is cooled overall from its desuperheated but fully saturated vapour temperature (s) by from 30°C to 60°C (more preferably from 40°C to 50°C) (most preferably about 45°C).

Preferably the desuperheating heat exchanger takes the temperature of the liquid to greater than 70°C (preferably above 80°C).

Accordingly, in a further aspect the present invention consists in a water or other liquid ("water") heating system comprising or including a first heat exchanger adapted to extract heat from a fluid or other source of energy, a heat pump system having a circulating second working fluid adapted to extract heat via the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and"third heat exchanger"respectively), the third heat exchanger feeding the second working fluid back to the second heat exchanger, and

a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage.

In still a further aspect the present invention consists in a water or other liquid ("water") heating system comprising or including a first heat exchanger adapted to extract heat from a fluid or other source of energy, a heat pump system having a circulating second working fluid adapted to extract heat via the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and"third heat exchanger"respectively), the third heat exchanger feeding the second working fluid back to the second heat exchanger, and a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage, wherein the third heat exchanger heats the water with superheat of the vapour of the second working fluid, and wherein the second heat exchanger floods downwardly with the condensed /condensing vapour of the second working fluid thereby heating the water with sensible and/or latent heat.

In still a further aspect the present invention consists in a water heating system comprising or including a first heat exchanger adapted to extract heat from a fluid or other source of energy, a heat pump system having a second circulating working fluid adapted to extract heat via the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and"third heat exchanger"respectively), the third heat exchanger feeding the second working fluid back to the second heat exchanger, and a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage wherein the third heat exchanger heats the water with superheat of the vapour of the second working fluid,

and wherein the second heat exchanger heats the water with sensible and/or latent heat.

In yet a further aspect the present invention consists in a water heating system comprising or including a first heat exchanger adapted to extract heat from a fluid ("first fluid"), a heat pump system having a circulating working fluid ("second fluid") adapted to extract heat via the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and "third heat exchanger"respectively), the third heat exchanger feeding the second fluid (i. e. a working fluid) back to the second heat exchanger, and a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage wherein the third heat exchanger heats the water with superheat of the vapour of the second fluid, and wherein the second heat exchanger heats the water with sensible and/or latent heat, and wherein the first and second fluids differ.

In still a further aspect the present invention consists in a method of heating a liquid (such as water) to a temperature above 50°C which involves the operative use of apparatus in accordance with the present invention as previously defined.

Accordingly, in another aspect the present invention consist in water or other liquid ("water") heating system comprising or including a refrigeration system having a circulating working fluid ("first working fluid"), the system including at least a first heat exchanger adapted to extract heat from the compressed first working fluid of the refrigeration system, a heat pump system having a circulating working fluid ("second working fluid") adapted to extract heat from the first working fluid reliant upon the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and"third heat exchanger"respectively), the third heat exchanger feeding the second working fluid back to the second heat exchanger, and

a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage.

In a further aspect the present invention consists in water or other liquid ("water") heating system comprising or including a refrigeration system having a circulating working fluid ("first working fluid"), the system including at least a first heat exchanger adapted to extract heat from the compressed first working fluid of the refrigeration system, a heat pump system having a circulating working fluid ("second working fluid") adapted to extract heat from the first working fluid reliant upon the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and"third heat exchanger"respectively), the third heat exchanger feeding the second working fluid back to the second heat exchanger, and a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage wherein the third heat exchanger heats the water with superheat of the vapour of the second working fluid, and wherein the second heat exchanger floods downwardly with the condensed /condensing vapour of the second working fluid thereby heating the water with sensible and/or latent heat.

In still a further aspect the present invention consists in a water heating system comprising or including a refrigeration system having a circulating working fluid ("first working fluid"), the system including at least a first heat exchanger adapted to extract heat from the compressed first working fluid of the refrigeration system, a heat pump system having a circulating working fluid ("second working fluid") adapted to extract heat from the first working fluid reliant upon the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and"third heat exchanger"respectively), the third heat exchanger feeding the second working fluid back to the second heat exchanger, and

a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage wherein the third heat exchanger heats the water with superheat of the vapour of the second working fluid, and wherein the second heat exchanger heats the water with sensible and/or latent heat.

In yet a further aspect the present invention consists in a water heating system comprising or including a refrigeration system having a circulating working fluid ("first working fluid"), the system including at least a first heat exchanger adapted to extract heat from the compressed first working fluid of the refrigeration system, a heat pump system having a circulating working fluid ("second working fluid") adapted to extract heat from the first working fluid reliant upon the first heat exchanger, the heat pump system including downstream of the heat pump system compressor two heat exchangers in series ("second heat exchanger"and"third heat exchanger"respectively), the third heat exchanger feeding the second working fluid back to the second heat exchanger, and a water system adapted to feed water to be heated first through the second heat exchanger and then through the third heat exchanger and from thence, as heated water, to an off take or to storage wherein the second heat exchanger heats the water with superheat of the vapour of the third working fluid, and wherein the second heat exchanger heats the water with sensible and/or latent heat, and wherein the first and second working fluids differ.

Preferments for any such forms of the present invention are as follows: Preferably the water heating system is one having a refrigeration system having a refrigerated zone, e. g.; a milk vat or the like.

Preferably the refrigeration system in addition to the first heat exchanger includes between the first heat exchanger and the refrigerated zone an air cooled condenser.

Preferably between the air cooled condenser and the refrigerated zone is a liquid receiver to collect the first working fluid as a liquid.

Preferably an expansion valve is provided between the liquid receiver and the refrigerated zone.

Preferably a refrigerated pad or the equivalent for heat removal is provided at the refrigerated zone.

Preferably the first heat exchanger operates with the working fluid as a vapour compressed by a compression ration below 11 : 1.

Preferably the compression ratio is below 5: 1 and with a saturated evaporation condition for the first working fluid at 0°C.

Preferably the first working fluid entering the first heat exchanger is a vapour at a temperature sufficient to evaporate the second working fluid at a temperature above 15°C (e. g. preferably about 25°C).

Preferably the second working fluid has an evaporation temperature above that of the first working fluid.

Preferably the difference in evaporation temperatures of the two working fluids is greater than 10°C (preferably greater than 15°C) (and preferably about 25°C).

Preferably the second working fluid is able to condense under the action of the second heat exchanger at a temperature above 50°C (preferably about 75°C).

Preferably an expansion valve is provided in the flow line between the second heat exchanger and the first heat exchanger, the second working fluid being primarily liquid when passing through the expansion valve.

Preferably said third heat exchanger acts as a desuperheater heat exchanger and preferably the second heat exchanger acts as a condenser-subcooler heat exchanger.

Optionally, there is means to vary the critical refrigerant charge (e. g. a surge receiver) responsive to water or liquid inlet thereby to control outlet temperature (irrespective of whether or not there is any direct monitoring of inlet temperature).

Preferably the water feed is from, for example, a collection vessel or a header tank or the like and preferably is to a storage vessel for hot water from which the water can be expressed with the expressed hot water being replaced, on demand, by an infeed through the second and subsequently third heat exchangers to the storage vessel.

Whilst reference is made hereto to water heating system persons skilled in the art will appreciate that other liquids can be substituted for water and thus the term"water heating system"should be interpreted broadly as including any other liquid that would benefit from heating.

In still a further aspect the present invention consists in a method of heating a liquid (such as water) to a temperature above 50°C which involves the operative use of apparatus in accordance with the present invention as previously defined.

In still a further aspect the present invention consists in a method of heating a liquid to a temperature in the range of from 60 to 90°C which comprises or includes passing that liquid in a non mingling heat exchange with a working fluid such that a heat exchange occurs initially which allows uptake by the liquid of at least some of the sensible heat and all of the latent of the working fluid and thereafter another heat exchange which allows the uptake of all of the superheat from the working fluid, the heat exchangers being part of a heat pump deriving its input heat by heat exchange from a refrigeration system (preferably having a working fluid with a lower evaporation temperature than that of the working fluid in direct heat exchange with the liquid to be heated).

Preferably said method is performed using apparatus in accordance with the present invention.

Preferably the liquid to be heated is water although not necessarily so.

In still a further aspect the present invention consists in a method of heating a liquid reliant on a liquid and two other fluid systems when performed substantially as herein described with or without reference to the accompanying drawing.

In still a further aspect the present invention consists in apparatus for performing a method as aforesaid, said apparatus being substantially as hereinafter described with reference to the accompanying drawing.

In still a further aspect the present invention consists in a refrigeration system for milk which includes a water heating system which, when considered in conjunction with the milk refrigerating system, is a water heating system in accordance with the present invention.

As used herein the term"and/or"means"and"and"or".

As used herein the term" (s)" following a noun means the singular or plural or both forms of that noun.

As used herein the term"system"includes the apparatus of the system.

BRIEF DESCRIPTION OF DRAWINGS A preferred form of the present invention will now be described with reference to the accompanying drawings in which Figure 1 shows one embodiment, and Figure 2 shows another embodiment as a flow diagram in order to indicate energy and temperature levels, the arrangement of Figure 2 showing to the right of the broken line'A'a standard dairy vat cooling unit and to the left of that broken line a cascade water heater which includes (in the broken line surround'B') an optional control system for varying water inlet.

BEST MODE FOR CARRYING OUT THE INVENTION To overcome the shortcomings of the high compression ratio of the single stage compression using R134a as applied in NZ Patent PCT/NZ00/00186 to Thompson, and to allow for a more effective refrigerant to collect the source heat, we propose a cascade heat pump system to operate almost as a separate entity from the refrigeration system (e. g. save for working fluid to working fluid heat exchange) and both can be at optimum efficiency with a most effective refrigerant.

The refrigeration system with a saturated evaporating condition at 0°C is most efficient using R404a with a compression ratio of 3.5 : 1.

The rejected heat from this system is used for the heat source of a cascade heat pump with a saturated condensing condition of 75°C using R1 34a with a compression ratio of 4.0 : 1.

The cascade heat pump uses a method whereby the refrigerant discharge vapour of a the refrigeration system is passed to the primary side of a brazed plate heat exchanger which then acts as the condenser for the refrigeration system. The rejected heat of condensation in the refrigerant discharge vapour is transferred to the secondary side of the brazed plate heat exchanger which acts as a refrigerant evaporator providing the heat source of the cascade heat pump.

After temperature and pressure elevation by the vapour compression cycle through the cascade heat pump compressor, the superheated refrigerant discharge vapour is passed to the

primary side of a brazed plate desuperheater and brazed plate condenser/subcooler coupled in series which both use the same flow of water to collect the rejected heat from the refrigerant discharge vapour causing it to be cooled while simultaneously causing the water to be heated.

The water to be heated enters at a low temperature 15°C and is first passed through the secondary side of the brazed plate condenser/subcooler where it is heated by the transfer of sensible and latent from the refrigerant vapour to leave the condenser/subcooler at a close approach to the condensing temperature. The heated water is then passed on through the secondary side of a brazed plate desuperheater where it is further heated by the transfer of superheat from the refrigerant vapour to leave at a close approach to the discharge vapour temperature thereby leaving at a temperature in excess of the condensing temperature.

The refrigerant discharge vapour to be cooled flows counter to the water, and enters at a high temperature and through the primary side of the brazed plate desuperheater where it is cooled by the transfer of superheat to the water to leave the desuperheater exactly on the point of condensation. The desuperheated refrigerant vapour is then passed on through the primary side of the brazed plate condenser where it gives up all its latent heat and a fraction of sensible heat to the water thereby leaving as a subcooled liquid refrigerant at a temperature less than the condensing temperature.

This water heating by series condenser and desuperheater is embodied in NZ Patent PCT/NZ00/00186 to Thompson. Obtaining a water off temperature in excess of the condensing temperature in a single water cooled desuperheater/condenser heat exchanger is possible in certain long path countercurrent heat exchanger designs particularly the tube in tube type.

By physically separating the desuperheater from the condenser the NZ Patent PCT/NZ00/00186 to Thompson ensures that the water leaving the desuperheater is above the condensing temperature.

It has been our determination, for efficiency, that the two heat transfer mechanisms, desuperheating and condensing, be thermodynamically separated by at least the second and third heat exchangers being properly separated.

The system finds a balance point for saturated condensing temperature which is determined by the water inlet temperature. In normal operation the two refrigerant phases (i. e. superheated vapour and condensing vapour) do not co-exist in the same heat exchanger.

The refrigerant leaves the desuperheater as a fully saturated vapour and usually at no other

state as refrigerant will not normally condense in the desuperheater even though the water entering is below the saturated condensing temperature. This ensures the water leaving the desuperheater is above the condensation temperature.

The use of the cascade heat pump method to collect the source heat from the discharge vapour of a refrigeration system and the use of a brazed plate condenser as applied in NZ Patent PCT/NZ00/00186 to Thompson and as optimised by us as both a condenser and refrigerant subcooler provides for a greater amount of heat to be transferred to the water and increases the overall efficiency of the heat pump cycle raising the heating COP to 4.5.

The NZ Patent PCT/NZ00/00186 to Thompson provides for latent heat only to be transferred in the condenser.

We have found it is possible to extract an amount of sensible heat from the refrigerant after condensation by configuring the condenser to be vertically downward counterflow for the condensing refrigerant and providing an extra transfer area at the bottom of the condenser to subcool the refrigerant after condensation. The amount of subcooling achievable for water entering at 20°C is 45°C below the saturated condensing temperature of 75°C thereby allowing liquid refrigerant at 30°C to pass to the expansion valve. This greatly improves the efficiency of the evaporation of the R134a by reducing the flash gas fraction after the expansion valve. The sensible heat in the refrigerant in cooling from 75°C to 30°C passed to the incoming water thus increasing the amount of heat taken up by the water.

The simultaneous requirement for hot water at elevated temperatures and refrigeration occurs in a milking shed where refrigeration is required to cool the milk in a vat prior to collection and significant volumes of sterilizing water are needed for washdown of the vat and milking equipment.

The refrigeration has traditionally been provided by a standard air cooled condensing unit using the main HCFC refrigerant R404a, while the required volume of water has been heated by electrical resistance heaters in a conventional storage cylinder.

By allowing the refrigeration of the milk vat to occur as normal via a standard air cooled condensing unit operating on R404a evaporating at 0°C provides the milk refrigeration at optimum efficiency. The low grade rejected heat from the R404a milk refrigeration system is then used as a heat source for a cascade heat pump using R134a. The rejected heat is of sufficient temperature to allow the cascade heat pump to evaporate at 25°C and condense at 75°C a far more efficient system than a single compression step from 0°C.

The use of the water cooled condenser and desuperheater as outlined in NZ Patent PCT/NZ00/00186 to Thompson together with the application of the brazed plate condenser as a refrigerant subcooler in a cascade heat pump allows the efficient generation of a suitable quantity of hot water at 85°C.

By having the counterflow directions of water and refrigerant in the condenser configured so that the condensed refrigerant is draining downwards it is possible to flood the condenser with condensed refrigerant and then allow this to be subcooled by the incoming counter flow water. This then utilises a portion of the condenser internal volume to act as a subcooler and the exiting subcooled refrigerant approaches to within 10°C of the entering water. The water is typically entering at 20°C so the refrigerant is subcooled to 30°C. This extra sensible heat fraction from 75°C down to 30°C is then collected by the counterflowing water and increases the total rejected heat by up to 30%. This extra heat collected elevates the water temperature to 30°C as it starts to condense the refrigerant and the approach of temperature of the exiting water to the condensing temperature is reduced when compared to a system with no subcooling, thus degrading the condenser performance slightly.

The loss in condenser performance due to the effect of the subcooling load on the counterflowing water is far outweighed by the extra heat rejection gained from the subcooling effect, and the increased thermodynamic efficiency derived from reduced flash gas that is generated during the expansion process. The overall performance is still improved by over 25% by subcooling the refrigerant directly in the condenser using the incoming water. In order to flood the condenser and subcool in the same heat exchanger the system has a critical charge of refrigerant and no liquid receiver is incorporated in the circuit.

Since the R134a cascade heat pump is not intimately linked to the R404a milk refrigeration system, milk refrigeration can occur with security even if hot water generation is not required. Control and operation of the two systems is totally separate with the cascade heat pump being sized exactly to the hot water generation requirements while the milk cooling condensing unit is sized exactly to the milk cooling load, enabling the combined system to be in total balance.

The diagram in Figure 1 shows the schematic interconnection of the milk vat refrigeration system and the cascade heat pump. The R404a refrigeration system for the milk vat is totally standard in operation and consists of a compressor (1), air cooled condenser (2) liquid receiver (3), expansion valve (4) and milk vat (5) with refrigerated pad for heat removal.

The one modification in the milk vat refrigeration system is the discharge line from the compressor (1) enters the primary circuit of the brazed plate heat exchanger (19) at the inlet (20) where the discharge gas is totally superheated at 65°C.

Heat is rejected in the brazed plate heat exchanger (19) as the R404a entering at (20) desuperheats and partially condenses in the primary of exchanger with the partly condensed wet mixture leaving via the outlet (21) at a saturated temperature of 35°C. The balance of the condensation of the R404a takes place in the aircooled condenser (2) of the milk vat system at a saturated temperature of 35°C and continues through the circuit.

The secondary side of the brazed plate heat exchanger (19) acts as the evaporator for the cascade heat pump system and the R134a refrigerant is fed into the inlet (22) of the exchanger via the expansion valve (25). The refrigerant evaporates in the secondary side of the brazed plate heat exchanger (19) at a saturated temperature of 25°C and is then superheated by 25K by the R404a in the primary side to leave at the outlet (23) of the brazed plate heat exchanger as a superheated vapour at 50°C.

The superheated R134a vapour is then piped to the compressor (7) and enters the suction inlet (24) and is compressed and leaves at the discharge outlet (13) as a vapour at a saturated condensing temperature of 75°C and superheated to 105°C.

The superheated refrigerant vapour is piped to the primary side of the brazed plate desuperheater (8) and enters at the inlet (11). The refrigerant vapour gives up all of the superheat to the counterflowing water in the secondary side, leaving at the outlet (12) as a totally saturated vapour at a temperature of 75°C.

The saturated refrigerant vapour is then piped to the primary side of the brazed plate condenser/subcooler (14) and enters at the inlet (17). The saturated refrigerant vapour gives up all of it's latent heat and sensible heat to the counterflowing water in the secondary side, leaving at the outlet (18) as a subcooled liquid at a temperature of 30°C. The subcooled liquid is then fed to the expansion valve (25) valve and then enters the inlet of brazed plate heat exchanger at (22).

The farm fresh water supply at a temperature of 20°C leaves the bulk storage tank (26) and is piped to the secondary of the brazed plate condenser/subcooler (14) and enters at the inlet (15). The water absorbs the sensible and latent heat from the counterflowing refrigerant in the primary side, leaving at the outlet (16) at a temperature of 70°C.

The water is then piped to the secondary side of the brazed plate desuperheater (8) and enters at the inlet (9). The water absorbs the all the superheat from the counterflowing refrigerant in the primary side, leaving at the outlet (10) at a temperature of 85°C.

The flow rate of the water is controlled to maintain the leaving water temperature at 85°C and the hot water enters the bulk hot water cylinder (27) for storage, and is supplied to the dairy shed via the supply line (28).

The efficiency of the cascade heat pump is considerably enhanced by allowing the incoming water entering the brazed plate condenser/subcooler (14) inlet at (15) to subcool the liquid refrigerant leaving at the outlet (18). The amount of rejected heat that is available from the R134a vapour during the condensation process is the sum of the superheat fraction in the vapour and the latent heat of condensation in the liquifaction process and this would provide condensed liquid at 75°C leaving the condenser at the outlet (18).

By having the counterflow directions of the water and refrigerant in the brazed plate condenser/subcooler (14) configured so that the condensed refrigerant in the primary side drains vertically downwards, it is possible to flood the bottom portion of the primary side of the brazed plate condenser/subcooler (14) with condensed refrigerant. The water entering the secondary side of the brazed plate condenser/subcooler (14) at the inlet (15) can subcool the liquid refrigerant held in the primary side allowing it to leave at the outlet (18) subcooled to 30°C.

This extra sensible heat fraction in the liquid refrigerant that is subcooled from 75°C to 30°C is then transferred to the counterflowing water and increases the total rejected heat by 30%. This extra heat collected elevates the water temperature to 30°C as it starts to condense the refrigerant and the approach of temperature of the exiting water to the condensing temperature is reduced when compared to a system with no subcooling, thus degrading the condenser performance slightly.

The loss in condenser performance due to the effect of the subcooling load on the counterflowing water is far outweighed by the extra heat rejection gained from the subcooling effect, and the increased thermodynamic efficiency derived from reduced flash gas that is generated during the expansion process. The overall performance is still improved by over 25% by subcooling the refrigerant directly in the condenser using the incoming water. In order to flood the condenser and subcool in the same heat exchanger the system has a critical charge of refrigerant and no liquid receiver is incorporated in the circuit.

CONTROLS REQUIRED FOR OPERATION In normal operation of the system such as exemplified by Figure 1 or 2 several parameters contribute to the production of water at 85°C. These parameters are, primary refrigerant superheated inlet vapour temperature to the cascade heat exchanger, condensing temperature of the primary refrigerant in the cascade heat exchanger, evaporating temperature of the secondary refrigerant in the cascade heat exchanger, superheated outlet vapour temperature of the secondary refrigerant from the cascade heat exchanger, superheated discharge vapour temperature of the secondary refrigerant after compression, saturated condensing temperature of the secondary refrigerant vapour after compression and amount of subcooling of the condensed second refrigerant on leaving the condenser.

The condition of the primary refrigerant inlet vapour and its subsequent condensing temperature is determined by a simple fan control on the condensing unit. This control of the condenser fan operation by a pressure switch maintains the condensing temperature at 35°C at all times. The primary superheated vapour is thereby maintained at 65°C by the suction conditions of the condensing unit.

The heat pump system parameter that is easiest to control with a relatively simple device is the saturated condensing temperature. This is achieved with a pressure operated water regulating valve whereby the required saturated condensing temperature of the heat pump is maintained at 75°C by controlling the water volume into the condenser.

The fixing of this parameter enables the other system parameters to be independently set and therefore ensure effective operation of the heat pump.

The saturated evaporating temperature in the secondary side of the cascade heat exchanger is controlled at 20°C by the expansion valve and this also provides superheated vapour at 50°C which enters the compressor.

The combination of these suction conditions together with the fixed saturated discharge temperature of 75°C provide the required superheated discharge vapour temperature of 115°C.

The liquid subcooling is provided by flooding refrigerant into the bottom of the condenser. This requires a critical charge which provides sufficient condenser flooding to provide liquid subcooling and the required refrigerant temperature of 35°C.

When all the conditions are in balance water is heated to 85°C from 20°C at maximum cycle efficiency.

CONTROL SYSTEM FOR VARYING WATER INLET TEMPERATURE The operating of the system as described above with fixed critical charge provides 85°C outlet water with 20°C water inlet. The system provides the most efficient heat pump operation with subcooled liquid leaving the condenser at 35°C. This subcooling occurs when the condenser is partially flooded with refrigerant and hence with a critical charge the operating conditions for 85°C outlet water are very specific. A variation in water inlet temperature changes the performance of the condenser. If the inlet water temperature increases then the condenser capacity is reduced. By maintaining a fixed condensing temperature of 75°C by means of the water regulating valve then an increase in inlet water temperature requires an increase in the volume of water entering the system. The increase in water volume for the same heat of rejection therefore reduces the outlet temperature unless some other parameters are changed.

If the inlet water temperature decreases then the condenser capacity is increased. By maintaining a fixed condensing temperature of 75°C by means of the water regulating valve then a decrease in inlet water temperature requires a decrease in the volume of water entering the system. The decrease in water volume for the same heat or rejection therefore increases the outlet temperature unless some other parameters are changed.

With a critical charge in the system to maintain the desired subcooling effect, the variation in outlet temperature with a variation in inlet temperature does not cause difficulty within a small range. A+/-3°C variation in water inlet temperature will provide a +/-1°C variation in outlet temperature.

If the heat pump water heater is applied in a situation where the inlet can widely vary outside this narrow band and a subsequent wide variation in water outlet temperature is not acceptable then a control system is required to adjust the system parameters to maintain the water outlet at 85°C.

The most effective means of altering the temperature of the outlet water under any operating conditions is by altering condensing temperature via the pressure regulating valve.

However the temperature cannot be raised due to the upper limit of 75°C on the condensing temperature. An alternative means of altering the water outlet temperature is to alter the

critical refrigerant charge which in turn changes the subcooling effect and the amount of effective condenser surface area.

A means of modulating the refrigerant charge while still maintaining the fixed saturated condensing temperature at 75°C will provide the required temperature control, i. e. raising the water off temperature if the water on temperature rises, and reducing the water off temperature if the water on temperature falls.

The method involves the fitting of a vertical surge type receiver in the liquid line to the expansion valve.

The receiver then fills from the bottom from the liquid line and in doing so removes liquid from the condenser, thereby reducing the subcooling effect and increasing the water off temperature. The water off temperature can then be reduced by pushing refrigerant out of the surge receiver which then passes through the system to flood the condenser and increase the subcooling effect which reduces the water off temperature.

The means of pushing the refrigerant out of the receiver is to inject discharge vapour from the compressor into the top of the receiver. The discharge vapour pressure is slightly higher than the receiver pressure and therefore this pressure difference moves the refrigerant out into the liquid line.

The receiver used is or can be standard refrigerant drier which has a shell that is rated well above the system operating pressure of 22.3 Bar. The discharge vapour from the compressor is fed to top of the receiver via a' inch solenoid valve.

Water temperature is sensed at the outlet of the condenser and the control thermostat switches the solenoid on at water outlet 86°C and off at water outlet 85°C thus providing a stable control mechanism.

This method of liquid volume modulation provides a steady temperature control for the outlet water temperature from water inlet temperature between 5°C to 30°C.

This invention includes the use of a cascade heat pump fluid heating system for producing hot fluid at temperatures at least equal to the condensing temperature in a heat pump system. In one embodiment relates to a cascade heat pump fluid heating system by cascade operation with a dairy vat refrigeration unit for the production of hot water at 85°C suitable for use in sterilizing dairy farm milking equipment after use. The use of cascade heat collection from a refrigeration system discharge at high temperature levels provides an

operational efficiency for the heat pump far in excess of that previously achieved with non cascade heat pumps collecting low grade heat from air or low temperature fluids.

In respect of Figure 1 a cascade heat pump system (6) for raising the temperature of a fluid comprises a compressor (7) for compressing a working fluid; a desuperheater heat exchanger (8) provided with an inlet (9) and outlet (10) for a fluid to be heated and an inlet (11) and outlet (12) for the working fluid to be desuperheated, the working fluid inlet (11) being communicated with an outlet (13) from the compressor (7); a condenser-subcooler heat exchanger (14) provided with an inlet (15) and outlet (16) for the fluid to be heated and an inlet (17) and outlet (18) for the working fluid to be condensed and subcooled, the condenser- subcooler heat exchanger fluid outlet (16) being communicated directly with the desuperheater heat exchanger fluid inlet (9), and the condenser-subcooler heat exchanger working fluid inlet (17) being communicated directly with the desuperheater heat exchanger working fluid outlet (12); and a condenser-evaporator heat exchanger (19) with an inlet (20) and outlet (21) for a fluid to be condensed and an inlet (22) and outlet (23) for the working fluid to be evaporated, the working fluid inlet (22) communicated with the condenser- subcooler heat exchanger working fluid outlet (18) and the outlet (23) communicated with the inlet (24) to the compressor (7).

In the embodiment of Figure 2 there is shown to the right of the broken line'A'a stand dairy vat cooling unit. In this unit there is provided a milk vat 29, a milk receiver 30, a drier 31, a solenoid valve 32, a sights glass 33, a TX valve 34, a compressor (e. g. R404a) 35, a dual pressure switch (high pressure/low pressure) 36 and an air cooled condenser 37.

The optional control system for varying water inlet as described herein includes a surge receiver 51 and a' inch solenoid valve 52.

The cascade system employs as a water source a 2500 litre bulk water silo 38, a general farm pump 39 to provide farm water reticulation 40 but also feeding through the condenser 42, the system also includes a TX valve 43, the PHE heat exchanger 44, the compressor (R134a) 45, the dual pressure switch 46 (similar to that of 36), the desuperheater 47, a 900 litre bulk hot water cylinder preferably with an emergency electric heater 53 so as to provide 85°C water via a dairy hot water pump 49 for the purpose of washing milk machinery at 50.