GB2566671A

GB2566671A - Heat pump apparatus

Info

Publication number: GB2566671A
Application number: GB1711428.1A
Authority: GB
Inventors: Andrew Simon
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-07-17
Filing date: 2017-07-17
Publication date: 2019-03-27
Anticipated expiration: 2037-07-17
Also published as: GB201711428D0; GB2566671B

Abstract

Method of defrosting an evaporator 14 of an air source heat pump 10 comprises isolating one or more evaporator(s) to be defrosted by a defrosting arrangement 22 from a plurality of evaporators in a refrigerant circuit 12, and continue to operate the remaining evaporator(s) in the circuit whilst the isolated one or more evaporators is defrosted. The circuit comprises at least one compressor 16, a condenser 18, an expansion valve 20 and the plurality of evaporators. The defrost arrangement may comprise a primary line 22 having a primary valve 28 and branches off the circuit downstream of the compressor and upstream of the condenser, and respective secondary lines 26 each having a secondary valve 30 extend from the primary line to each evaporator. First 32 and second 34 circuit valves along with the secondary line valves may be used to isolate the evaporator(s) for defrosting by causing an increase in pressure of compressed refrigerant in the isolated evaporator. A bleed line (38, fig 3) having a bleed valve (40) may release the compressed refrigerant back into the circuit. Between 2 and 10 evaporators may be used and their condition monitored with the use of a wireless controller and sensor array.

Description

Heat Pump apparatus

TECHNOLOGICAL FIELD

Examples of the disclosure relate to a heat pump apparatus, and particularly an air source heat pump apparatus comprising a defrost arrangement.

BACKGROUND

Air source heat pump apparatus are known and can provide useable heat from ambient air. Such air source heat pump apparatus comprise a refrigerant circuit for circulating refrigerant. The refrigerant circuit typically comprises an evaporator, a compressor, a condenser, and an expansion valve.

In use, ambient air from outside is drawn over the evaporator and exchanges heat with a refrigerant, such as ammonia, which is at a low temperature (0°C) and pressure and exists as a mixture of liquid and vapour. This causes the refrigerant to change state to a low temperature (0°C) saturated vapour.

The saturated vapour is compressed by the compressor and exits the compressor as a superheated vapour.

The superheated vapour is condensed by the condenser to a saturated liquid which removes the superheat by cooling the vapour. The superheat is transferred to a heating system, for instance, by heating water directly or indirectly. The condensation process occurs at essentially constant pressure.

The saturated liquid passes through the expansion valve undergoing an abrupt decrease in pressure resulting in an adiabatic flash evaporation and autorefrigeration returning the refrigerant to its original state to repeat the cycle.

In use, and particularly when outside temperatures are below 5°C, ice can form on the heat exchanging surface of the evaporator leading to a reduced efficiency. In such circumstances known air source heat pump apparatus are temporarily shut down either in response to a low outside temperature to avoid ice formation, or alternatively to defrost the heat exchanging surface of the evaporator once ice has formed. Known air source heat pumps cannot therefore be relied upon for continuous heating, and particularly on cold days when heating is most needed.

There is a requirement therefore to provide improved air source heat pump apparatus which can continuously operate to produce heat, even on cold days.

BRIEF SUMMARY

According to various, but not necessarily all, examples of the disclosure there is provided an air source heat pump apparatus, the apparatus comprising a refrigerant circuit for circulating refrigerant, the refrigerant circuit comprising: a plurality of evaporators, at least one compressor, a condenser, and an expansion valve, the apparatus further comprising a defrost arrangement, wherein the apparatus is configured such that in use the defrost arrangement is operable to defrost a selection of the plurality of evaporators in isolation from the non-selected evaporators such that the apparatus can continue to operate with the non-selected evaporators.

The apparatus may be configured such that in use the defrost arrangement is operable to selectively defrost a one of the plurality of evaporators in isolation from the non-selected evaporators such that the apparatus can continue to operate with the non-selected evaporators.

The defrost arrangement may comprise a primary line extending from the refrigerant circuit downstream of the at least one compressor, and respective secondary lines extending from the primary line to each respective evaporator such that in use compressed refrigerant can be directed through the primary and respective secondary lines to any of the plurality of evaporators.

The respective secondary lines may connect to the refrigerant circuit upstream of each respective evaporator.

A primary line valve may be provided in the primary line operable in use to control the flow of compressed refrigerant through the primary line by opening, closing or partially closing the primary line.

A secondary line valve may be provided in each of the respective secondary lines operable in use to control the flow of compressed refrigerant through the respective secondary lines by opening, closing or partially closing the respective secondary lines.

First and second refrigerant circuit valves may be provided in the refrigerant circuit for each respective evaporator, wherein for each respective evaporator the first refrigerant circuit valve is provided upstream of the secondary line, and the second refrigerant circuit valve is provided downstream of the evaporator.

The apparatus may be configured such that in use the defrost arrangement is operable to defrost a selection of the plurality of evaporators in isolation from the non-selected evaporators by opening the primary line valve and selected secondary line valves to allow flow of compressed refrigerant through the primary line and the selected secondary lines, closing the first refrigerant circuit valve to prevent flow of refrigerant to the selected evaporators through the refrigerant circuit, and closing the second refrigerant circuit valve to prevent flow of compressed refrigerant from the selected evaporators through the refrigerant circuit thereby causing an increase in pressure of compressed refrigerant around the selected evaporators to defrost the selected evaporators.

The second refrigerant circuit valve may be configured to act as an evaporator pressure regulator such that in use a pre-set pressure of compressed refrigerant around the selected evaporators can be maintained.

The apparatus may also comprise a bleed line connected to the refrigerant circuit either side of the second refrigerant circuit valve, the bleed line comprising a bleed valve, the bleed valve being operable to release the compressed refrigerant from around the selected evaporators back into the refrigerant circuit to reduce pressure once the selected evaporators have been defrosted and the selected secondary line valves have been closed.

The apparatus may be configured such that once a required reduced pressure has been obtained around the selected evaporators the bleed valve is operable to close, and the first and second refrigerant circuit valves are operable to open to permit operation of the selected evaporators.

The defrost arrangement may be configured to be manually operable, or may be configured to be automatically operable.

The defrost arrangement may be operable by a controller, and may be wirelessly operable. The apparatus may comprise a sensor array configured to monitor the condition of the plurality of evaporators, the controller being configured to select evaporators for defrost in response to input from the sensor array.

There apparatus may comprise 2 to 10 evaporators, and may comprise 3 to 6 evaporators, and may comprise 5 evaporators.

According to various, but not necessarily all, examples of the disclosure there is provided a method comprising providing an air source heat pump apparatus, the apparatus comprising a refrigerant circuit for circulating refrigerant, the refrigerant circuit comprising: a plurality of evaporators, at least one compressor, a condenser, and an expansion valve, the apparatus further comprising a defrost arrangement, wherein the apparatus is configured such that in use the defrost arrangement is operable to defrost a selection of the plurality of evaporators in isolation from the non-selected evaporators such that the apparatus can continue to operate with the non-selected evaporators.

The apparatus may comprise any of the features described in any of the preceding statements or following description.

The methods may comprise any of the features described in any of the preceding statements or following description.

According to various, but not necessarily all, examples of the disclosure there may be provided examples as claimed in the appended claims.

BRIEF DESCRIPTION

For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only to the accompanying drawings in which:

Fig. 1 illustrates an apparatus;

Fig. 2 illustrates another apparatus;

Fig. 3 illustrates a section of an apparatus; and

Fig. 4 illustrates a system comprising the apparatus.

DETAILED DESCRIPTION

Figs. 1 to 3 illustrate an air source heat pump apparatus 10 and method according to examples of the present disclosure. Fig. 4 illustrates a system comprising an air source heat pump apparatus 10.

The apparatus 10 comprises a refrigerant circuit 12 for circulating refrigerant. The refrigerant may be ammonia. The refrigerant circuit 12 may be a pressurised closed circuit cooling coil loop.

The refrigerant circuit 12 comprises: a plurality of evaporators 14, at least one compressor 16, a condenser 18, and an expansion valve 20.

In the example shown in Fig. 1, the apparatus 10 comprises two evaporators 14, whereas in the example shown in Fig. 2, the apparatus 10 comprises five evaporators 14. In other examples, the apparatus 10 may comprise a different number of evaporators 14.

In the examples shown in Figs. 1 and 2 the apparatus 10 comprises a single compressor 16. In other examples, the apparatus 10 may comprise a plurality of compressors 16, for instance, two compressors 16, which may be in series.

In examples of the disclosure comprising two compressors 16, a water heat exchanger (not illustrated), known as a de-superheater, may be provided between the compressors 16. In such examples, the first compressor is a low stage electric motor variable speed refrigerant compressor, and the second compressor is a high stage electric motor variable speed refrigerant compressor.

In the examples shown in Figs. 1 and 2 the apparatus 10 comprises a single expansion valve 20. In other examples, the apparatus 10 may comprise a plurality of expansion valves 20, and may comprise an expansion valve 20 for each of the plurality of evaporators 14.

The apparatus 10 further comprises a defrost arrangement 22. The apparatus 10 is configured such that in use the defrost arrangement 22 is operable to defrost a selection of the plurality of evaporators 14 in isolation from the non-selected evaporators 14 such that the apparatus 10 can continue to operate with the nonselected evaporators 14.

The air source heat pump apparatus 10 according to examples of the disclosure can therefore operate continuously to produce usable heat, even on cold days, because only the selected evaporators 14 are shut down to allow defrosting. The evaporators 14 which are not selected for defrost are not shut down, and the apparatus 10 can continue to operate with the non-selected evaporators 14.

In some examples, the defrost arrangement 22 is operable to selectively defrost a one of the plurality of evaporators 14 in isolation from the non-selected evaporators 14 such that the apparatus 10 can continue to operate with the nonselected evaporators 14. For instance, the defrost arrangement 22 may be operable to selectively defrost one of the five evaporators 14 of the apparatus of Fig. 2 in isolation from the four non-selected evaporators 14 such that the apparatus 10 can continue to operate with the four non-selected evaporators 14.

In other examples, the defrost arrangement 22 is operable to selectively defrost more that one of the plurality of evaporators 14 in isolation from the nonselected evaporators 14 such that the apparatus 10 can continue to operate with the non-selected evaporators 14. For instance, the defrost arrangement 22 may be operable to selectively defrost two of the five evaporators 14 of the apparatus of Fig. 2 in isolation from the three non-selected evaporators 14 such that the apparatus 10 can continue to operate with the three non-selected evaporators 14.

The apparatus 10 comprises a number of evaporators 14 sufficient to provide the required heat output from the apparatus 10 when at least one of the evaporators is being defrosted and is not therefore operational. Each respective evaporator may have an output of about 100 KW. Other evaporators 14 may have a different output, and therefore a different number of evaporators 14 may be required to provide the required heat output.

In the illustrated examples, the defrost arrangement 22 comprises a primary line 24 extending from the refrigerant circuit 22 downstream of the at least one compressor 16, and respective secondary lines 26 extending from the primary line 24 to each respective evaporator 14. In use, compressed refrigerant can be directed through the primary line 24 and respective secondary lines 26 to any of the plurality of evaporators 14. The compressed refrigerant is typically at a temperature of about 68°C, and a pressure of about 30.39 BarG.

The respective secondary lines 26 connect to the refrigerant circuit 12 upstream of each respective evaporator 14.

A primary line valve 28 is provided in the primary line 24 operable in use to control the flow of compressed refrigerant through the primary line 24 by opening, closing or partially closing the primary line 24. A secondary line valve 30 is provided in each of the respective secondary lines 26 operable in use to control the flow of compressed refrigerant through the respective secondary lines 26 by opening, closing or partially closing the respective secondary lines 26.

First and second refrigerant circuit valves 32, 34 are provided in the refrigerant circuit 12 for each respective evaporator 14. For each respective evaporator 14, the first refrigerant circuit valve 32 is provided upstream of the secondary line 26. Accordingly, the first circuit valve 32 is located in the refrigerant circuit 12 upstream of the point where the secondary line 26 connects to the refrigerant circuit 12. For each respective evaporator 14, the second refrigerant circuit valve 34 is provided downstream of the evaporator 14.

Accordingly, first and second refrigerant circuit valves 32, 34 are provided for each respective evaporator 14.

In the illustrated examples, the apparatus is configured such that in use the defrost arrangement 22 is operable to defrost a selection of the plurality of evaporators 14 in isolation from the non-selected evaporators 14 by opening the primary line valve 28 and selected secondary line valves 30 to allow flow of compressed refrigerant through the primary line 24 and the selected secondary lines 26, closing the first refrigerant circuit valve 32 to prevent flow of refrigerant to the selected evaporators 14 through the refrigerant circuit 12, and closing the second refrigerant circuit valve 34 to prevent flow of compressed refrigerant from the selected evaporators 14 through the refrigerant circuit 12 thereby causing an increase in pressure of compressed refrigerant around the selected evaporators 14 to defrost the selected evaporators 14.

In some examples, the second refrigerant circuit valve 34 is configured to act as an evaporator pressure regulator such that in use a pre-set pressure of compressed refrigerant around the selected evaporators 14 can be maintained.

As illustrated in Fig. 3, in some examples the apparatus 10 comprises a bleed line 38 connected to the refrigerant circuit 12 either side of the second refrigerant circuit valve 34. The bleed line comprises a bleed valve 40. The bleed valve 40 is operable to release the compressed refrigerant from around the selected evaporators 14 back into the refrigerant circuit 12 to reduce pressure once the selected evaporators 14 have been defrosted and the selected secondary line valves 30 have been closed. In some instances, the primary line valve 28 may also be closed, and may be closed simultaneously with the selected secondary line valves 30.

In some examples, the apparatus 10 is configured such that once a required reduced pressure has been obtained around the selected evaporators 14 the bleed valve 40 is operable to close, and the first and second refrigerant circuit valves 32, 34 are operable to open to permit operation of the selected evaporators 14.

The defrost arrangement 22 may be configured to be manually operable such that a user could manually select one or more of the plurality of evaporators 14 to be defrosted. Alternatively the defrost arrangement 22 may be configured to be automatically operable such that one or more of the plurality of evaporators 14 are defrosted according to a pre-set schedule, for instance, according to the time elapsed since the last defrost.

As illustrated in the system of Fig. 4, the defrost arrangement 22 may also be operable by a controller 42, and may be wirelessly operable. The apparatus 20 may comprise a sensor array 44 configured to monitor the condition of the plurality of evaporators 14, the controller 42 being configured to select evaporators 14 for defrost in response to input from the sensor array 44. In some examples, if ice is detected on one or more of the evaporators 14 by the senor array 44, the controller 42 automatically instructs the defrost arrangement 22 to defrost the one or more evaporators 14 covered with ice.

In use, ambient air from outside is drawn into the plurality of evaporators 14 by variable speed controlled fans (not illustrated) and over refrigerant evaporator cooler tubes (not illustrated) whereby the air exchanges heat with the refrigerant in the refrigerant circuit 12. The refrigerant may be, for instance, ammonia. Before heat exchange, the refrigerant is at a low temperature (0°C) and pressure, and exists as a mixture of liquid and vapour. Heat exchange causes the refrigerant to change state to a low temperature (0°C) saturated vapour.

The saturated vapour is subsequently compressed by the compressor 16, or several compressors 16 in series, exiting the compressor 16 as a superheated vapour with a temperature of about 68°C and a pressure of about 30.39 barG.

In examples of the disclosure comprising two compressors 16 in series, a water heat exchanger (not illustrated), known as a de-superheater, may be provided between the compressors 16. In such examples, heat from the refrigerant compressed by the first compressor 16 is exchanged with relatively cold water in the heat exchanger, before the refrigerant is re-compressed to a higher pressure by the second compressor 16.

The superheated vapour is subsequently condensed by the condenser 18 which removes the superheat by cooling the vapour to a saturated liquid with a temperature of about 65°C and a pressure about 30.39 barG. The condensation process occurs at essentially constant pressure. The superheat is transferred to a heating system, for instance, by heating mains water directly or indirectly. The heated water could be used, for instance, to heat a building, such as a house or factory.

The saturated liquid passes through the expansion valve 20 undergoing an abrupt decrease in pressure resulting in an adiabatic flash evaporation and autorefrigeration returning the refrigerant to its original state to repeat the cycle. Each respective evaporator 14 may comprise an expansion valve 20.

In some instances, the refrigerant circuit 12 may comprise a liquid sub-cooler (not illustrated) between the condenser 18 and the expansion valve 20, and the saturated liquid refrigerant passes from the condenser 18 through the liquid subcooler to the expansion valve 20.

To defrost, for instance, a one of the plurality of evaporators 14 in isolation from the non-selected evaporators 14, the first refrigerant circuit valve 32 is electrically de-energised to close to prevent flow of refrigerant to the selected evaporator 14 through the refrigerant circuit 12.

After a period of time, for instance three minutes, the second refrigerant circuit valve 34 is electrically energised to close.

The cooler fans (not illustrated) of the selected evaporator 14 are then stopped from operating.

The secondary line valve 30 is then electrically energised to open to allow flow of compressed refrigerant through the selected secondary line 26. The primary line valve 28 may be simultaneously opened with the secondary line valve 30, or may already be open. As the second refrigerant circuit valve 34 is closed the pressure and thus temperature of compressed refrigerant increases around the selected evaporator 14 causing ice to melt and therefore defrosting the selected evaporator 14. The water produced from melting ice is collected in a drain tray located in the evaporator. The collected water may be reused.

After a pre-determined amount of time, for instance 30 minutes, the secondary line valve 30 is electrically de-energised to close, and the bleed valve 40 (see Fig. 3) is electrically energised to open to release the compressed refrigerant from around the selected evaporator 14 back into the refrigerant circuit 12 to reduce pressure. Once a required reduced pressure has been obtained around the selected evaporator 14, which may take around five minutes, the bleed valve 40 is electrically de-energised to close.

The second refrigerant circuit valve 34 is then electrically de-energised to open. After a short time, which may be around one minute, the first refrigerant circuit valve 32 is electrically de-energised to open. Subsequently, after about one minute, the cooler fans (not illustrated) of the selected evaporator 14 re-start, such that the selected evaporator is operational.

During the above defrost process, the non-selected evaporators 14 continue to operate such that the apparatus 10 continues to produce useable heat.

The terms upstream and downstream are used in the context that the direction of flow of the refrigerant in the refrigerant circuit is from the evaporators 14, to the compressor 16, to the condenser 18, to the expansion valve 20 and back to the evaporators 14.

The figures not only illustrate an apparatus 10, but also a method of forming the apparatus 10, and the operation of the apparatus 10.

There is thus described an apparatus and methods with a number of advantages as described above.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

The term “comprise” is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use “comprise” with an exclusive meaning then it will be made clear in the context by referring to “comprising only one...” or by using “consisting”.

In this brief description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term “example” or “for example” or “may” in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus “example”, “for example” or “may” refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a subclass of the class that comprise some but not all of the instances in the class. It is therefore implicitly disclosed that a features described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

CLAIMS ο co

1. An air source heat pump apparatus, the apparatus comprising a refrigerant circuit for circulating refrigerant, the refrigerant circuit comprising: a plurality of evaporators, at least one compressor, a condenser, and an expansion valve, the apparatus further comprising a defrost arrangement, wherein the apparatus is configured such that in use the defrost arrangement is operable to defrost a selection of the plurality of evaporators in isolation from the non-selected evaporators such that the apparatus can continue to operate with the non-selected evaporators.

2. An apparatus according to claim 1, wherein the apparatus is configured such that in use the defrost arrangement is operable to selectively defrost a one of the plurality of evaporators in isolation from the non-selected evaporators such that the apparatus can continue to operate with the non-selected evaporators.

3. An apparatus according to claims 1 or 2, wherein the defrost arrangement comprises a primary line extending from the refrigerant circuit downstream of the at least one compressor, and respective secondary lines extending from the primary line to each respective evaporator such that in use compressed refrigerant can be directed through the primary and respective secondary lines to any of the plurality of evaporators.

4. An apparatus according to claim 3, wherein the respective secondary lines connect to the refrigerant circuit upstream of each respective evaporator.

5. An apparatus according to claims 3 or 4, wherein a primary line valve is provided in the primary line operable in use to control the flow of compressed refrigerant through the primary line by opening, closing or partially closing the primary line.

30

6. An apparatus according to claims 3 to 5, wherein a secondary line valve is provided in each of the respective secondary lines operable in use to control the flow of compressed refrigerant through the respective secondary lines by opening, closing or partially closing the respective secondary lines.

7. An apparatus according to claims 3 to 6, wherein first and second refrigerant circuit valves are provided in the refrigerant circuit for each respective evaporator, wherein for each respective evaporator the first refrigerant circuit valve is provided upstream of the secondary line, and the second refrigerant circuit valve is provided

5 downstream of the evaporator.

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8. An apparatus according to claims 3 to 6, wherein the apparatus is configured such that in use the defrost arrangement is operable to defrost a selection of the plurality of evaporators in isolation from the non-selected evaporators by opening the primary line valve and selected secondary line valves to allow flow of compressed refrigerant through the primary line and the selected secondary lines, closing the first refrigerant circuit valve to prevent flow of refrigerant to the selected evaporators through the refrigerant circuit, and closing the second refrigerant circuit valve to prevent flow of compressed refrigerant from the selected evaporators through the refrigerant circuit thereby causing an increase in pressure of compressed refrigerant around the selected evaporators to defrost the selected evaporators.

9. An apparatus according to claim 8, wherein the second refrigerant circuit valve is configured to act as an evaporator pressure regulator such that in use a pre-set pressure of compressed refrigerant around the selected evaporators can be maintained.

10. An apparatus according to claims 8 or 9, wherein the apparatus comprises a bleed line connected to the refrigerant circuit either side of the second refrigerant

25 circuit valve, the bleed line comprising a bleed valve, the bleed valve being operable to release the compressed refrigerant from around the selected evaporators back into the refrigerant circuit to reduce pressure once the selected evaporators have been defrosted and the selected secondary line valves have been closed.

30

11. An apparatus according to claim 10, wherein the apparatus is configured such that once a required reduced pressure has been obtained around the selected evaporators the bleed valve is operable to close, and the first and second refrigerant circuit valves are operable to open to permit operation of the selected evaporators.

12. An apparatus according to any of the preceding claims, wherein the apparatus comprises 2 to 10 evaporators.

13. An apparatus according to any of the preceding claims, wherein the apparatus 5 comprises 3 to 6 evaporators.

14. An apparatus according to any of the preceding claims, wherein the apparatus comprises 5 evaporators.

10

15. An apparatus according to any of the preceding claims, wherein the defrost arrangement is operable by a controller.

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16. An apparatus according to any of the preceding claims, wherein the defrost arrangement is wirelessly operable by a controller.

17. An apparatus according to claims 15 or 16, wherein the apparatus comprises a sensor array configured to monitor the condition of the plurality of evaporators, the controller being configured to automatically select evaporators for defrost in response to input from the sensor array.

18. An apparatus according to any of the preceding claims, wherein the defrost arrangement is configured to be manually operable.

19. A method comprising providing an air source heat pump apparatus, the 25 apparatus comprising a refrigerant circuit for circulating refrigerant, the refrigerant circuit comprising: a plurality of evaporators, at least one compressor, a condenser, and an expansion valve, the apparatus further comprising a defrost arrangement, wherein the apparatus is configured such that in use the defrost arrangement is operable to defrost a selection of the plurality of evaporators in isolation from the

30 non-selected evaporators such that the apparatus can continue to operate with the non-selected evaporators.

20. An air source heat pump apparatus substantially as hereinbefore described and/or with reference to any of the accompanying drawings.

21. A method substantially as hereinbefore described and/or with reference to any of the accompanying drawings.