GB2202966A - Control of heating or cooling - Google Patents

Abstract

A method of controlling a compressor driven vapour compression heat movement system, e.g. a refrigeration system, air conditioning system or heat pump, in which a common compressor system heats or cools a plurality of load units and is operated in cycles each of which include a compressor system higher capacity period and a compressor system lower capacity period, is characterised in that the lower capacity period is made or controlled to be sufficiently long that when the compressor system is switched to the higher capacity a majority of the load units are demanding heating or cooling and that the higher capacity period is made sufficiently long that when the compressor system is switched to lower capacity one or more of the load units have had their heating or cooling demand satisfied. The length of the lower capacity period may be controlled in response to the length of the preceding higher capacity period. The higher capacity period may be terminated when the load on the compressor system falls below a set point which may be adjustable. The load may be inferred from the compressor inlet pressure or from current consumption, power consumption or power factor.

Description

Refrigeration Systems This invention relates to controlling compressor driven vapour compression heat movement systems as used for example in refrigeration and air conditioning for cooling and in heat pump arrangements for heating.

Reference will be made primarily to refrigeration systems for explaining the invention in detail.

Figure 1, which will be described in more detail below, shows a typical refrigeration system in which there are three refrigerated food cabinets. The two graphs in Figure 2 illustrate two alternative known modes of controlling such a system. As will also be explained in more detail below, the mode illustrated in the lefthand graph permits the compressor inlet pressure to fall to extremely low values on occasions, and on these occasions the system is operating very inefficiently and removing moisture excessively from food stored in the cabinets. The mode shown in the right-hand graph avoids these problems but on occasions may result in the compressor being switched on and off unacceptably often.

It is of course desirable that the system should run as efficiently as possible so as to minimise its power consumption, and due to limitations in the physical design of compressors, they are given by the manufacturers a rating as to the maximum frequency with which they can be turned on without unacceptably shortening their requirements for servicing and repair. Typical ratings would lie between about six and ten starts per hour.

The invention aims to provide an improved method of controlling a compressor driven heat movement system which enables the system to be run efficiently, without exceeding the starts per hour rating of the compressor.

A further aim of the present invention is to provide a method of controlling a compressor driven vapour compression heat movement system having a plurality of load units (e.g. refrigerated cabinets, air-conditioning units or heat pump output units) which is more efficient than the type of system at the left hand side of Figure 2.

More particularly, the invention provides a method of controlling a compressor driven vapour compression heat movement system in which a common compressor system heats or cools a plurality of load units and is operated in cycles each of which include a compressor system higher capacity period and a compressor system lower capacity period, characterised in that the lower capacity period is made sufficiently long that when the compressor system is switched to the higher capacity a majority of the load units are demanding heating or cooling and that the higher capacity period is made sufficiently long that when the compressor system is switched to lower capacity one or more of the load units have had their heating or cooling demand satisfied.

The compressor system involved can be a single compressor as in the embodiment which will be described in detail below. In that event its higher capacity mode will be when the compressor is running and its lower capacity mode will be when it is not running during which period of course its capacity is actually zero. However, the invention can also be applied where the compressor system includes a plurality of compressors. Then, in the higher capacity mode some of the compressors are running and in the lower capacity mode a lesser fixed number, which may be zero, are running. For example, it may be desirable to have one compressor, which may be relatively small, which always runs so as to prevent liquid refrigerant accumulating in the inlet route to a main compressor, which is switched on and off.

In one embodiment, the length of the lower capacity period is controlled in dependence upon the length of the preceding higher capacity period.

In relation to a single compressor, if the on period becomes less than a predetermined value (e.g. two minutes), then the compressor is switched off for longer during the next cycle, so that it will also stay on for longer before the pressure comes down to the predetermined pressure value, thus lengthening the cycle until the on-time again exceeds two minutes. By setting a minimum (say four minutes) to the length of off period employed, the frequency is limited, though not to an exactly calculated frequency value, by the fact that the cycles will always be long enough that the on period of the cycle will only very occasionally fall short of two minutes and hence the entire cycle will only occasionally fall short of six minutes.It may be said that the inlet pressure excursions from cycle to cycle are forced, either by making the compressor on periods or off periods long enough, to be sufficiently great that, in the prevailing load conditions, the cycle time would be long enough for the starts per hour rating of the compressor not to be, or only rarely to be, exceeded.

The low capacity period may be controlled so as to be sufficiently long that when the compressor system is switched to higher capacity a majority of the load units are demanding heating or cooling or, where the operating conditions are sufficiently predictable, the low capacity period may be fixed at a sufficiently long value to ensure that the same condition is met. In the case where the low capacity period is controlled, the control may be exercised in response to various different sensed characteristics and these will be referred to in more detail below.

So far as concerns making the higher capacity period sufficiently long that when the compressor is switched to low capacity at least some of the load units have had their demand satisfied, preferably this will comprise sensing a variable which represents the load on the compressor system and terminating the higher capacity period when the sensed variable indicates that the load on the compressor system is falling.

Generally, in operating a control method in accordance with the invention for a cooling system, because a majority of the load units are demanding cooling when the compressor is brought on, there will be a substantial period of time during which the majority of the load units will continue to require cooling and therefore there will be a substantial flow of refrigerant through the system and the inlet pressure of the compressor system, and hence the temperature in the evaporators of the load units, will remain at a reasonably constant level so long as this situation prevails. This may conveniently be referred to as a plateau in the load on the compressor system and also in the ' inlet pressure against time curve.Furthermore, the method of the invention involves turning the compressor system off when one or more of the load units have had their cooling demand satisfied i.e. not very long after the pressure has started to fall from the plateau following one or more of the load units ceasing to take refrigerant flow because it is no longer demanding cooling.

Consequently, when a method in accordance with the invention is used, most of the heat transfer from the load units occurs in the vicinity of the plateau level of inlet pressure and evaporator temperature and it will become apparent that this means the system is operating primarily above the band Poff-Pon of the prior art system of the left hand side of Figure 2, and hence is operating more efficiently in terms of energy consumed per unit of refrigeration or cooling achieved.

Various variables may be sensed to indicate the load on the compressor system and these will be described in more detail below. The higher capacity period may be terminated when the sensed variable reaches a set point value.

It has previously been mentioned that when operating a control method in accordance with the invention the compressor inlet (or in the case of a heat pump system, the outlet) pressure variation will exhibit a plateau during the higher capacity period. Preferably, the invention further comprises sensing the occurrence of a substantially constant level (or "plateau") of load on the compressor system during its higher capacity period and automatically adjusting the set point value to a value which would represent a load level below said substantially constant level.

For example, in a cooling system the set point value may be automatically adjusted during each cycle to lie at 80% of the plateau level measured in terms of absolute value of the compressor inlet pressure, though values between 60% and 90g may be employed depending on the circumstances. A "substantially constant level" may be defined for the above purpose as the pressure varying by less than 10% over a significant period (e.g. between 30 and 60 seconds) of time. Also, if a variable other than pressure is sensed, its set point value may need to be set at a different percentage of the plateau value in order to achieve the desired percentage for the pressure level itself.

This preferred feature prevents the occurrence of problems which may otherwise arise as a consequence of the fact that the level of the plateau will not necessarily be the same during each higher capacity period, but may vary from cycle to cycle or drift over a substantial period of time due to various types of change in operating conditions. For example, in a single compressor system where the load units are refrigerated cabinets, the plateau will occur at a lower value when a number of cabinets are taken out of service, as sometimes happens in practice. If the set point were fixed, this could result in the compressor being switched off before the plateau level is reached in which case adequate cooling of the remaining cabinets would not be achieved.

Inaccurate manual setting of the set point pressure could have the same effect. Drift may also occur in the characteristics of pressure transducers and this could result in undesirable shift of the effective set point in a system where the set point is ostensibly fixed. Also, the different operating conditions encountered in winter as compared with summer, the natural plateau level being higher in summer than it would be in winter, means that for uniform operation throughout the seasons a lower set point level be used in winter than in summer. The preferred feature of automatic adjustment of set point level in response to plateau level mitigates the problems just referred to.

In order that the invention may be more clearly understood, embodiments thereof will now be described with reference to the accompanying diagrammatic drawings in which: Figure 1 is a simplified illustration of a typical compressor driven refrigeration system; Figure 2 shows in a simplified form the relationship between compressor inlet pressure and time for two different known methods of controlling such a system; Figure 3 shows the same relationship but in relation to a method of control in accordance with the invention; Figure 4 shows the components for carrying out the method of Figure 3; Figure 5 is a flow chart showing the steps of a control method according to Figure 3; Figure 6 shows in more detail the likely practical form of one cycle in the method of Figure 3 plus other information useful in understanding the invention; and Figure 7 shows the form of a compressor outlet pressure cycle in a heat pump heating system.

The refrigeration system of Figure 1 is typical of systems that might be found in, for example, supermarkets, where a number of refrigeration cabinets need to be kept cold and have their temperatures controlled.

There may be any number of cabinets, but the Figure 1 system would involve three. A compressor 2 feeds compressed gaseous refrigerant to condenser 4 where it is condensed to liquid which flows to a receiver or reservoir 6. From the receiver it flows on three parallel paths through three evaporators (one per cabinet) indicated at 8 and from the evaporators back to the compressor 2 in gaseous form, the liquid refrigerant having evaporated within the evaporators to produce the cooling effect. In standard manner, expansion valves 10 precede the evaporators 8 and are automatically controlled in known manner so as to maintain correct conditions within the evaporators. Each cabinet is provided with a temperature sensor 12 which exercises thermostatic control over an on/off valve 14 for that particular cabinet.Thus, each evaporator only takes liquid refrigerant from the receiver when the temperature of its associated cabinet has risen to such a level that it requires further cooling.

The left-hand graph in Figure 2 illustrates the use of a control method in which the compressor inlet pressure (which is related to the evaporator temperature when refrigerant is boiling in the evaporator) is measured and compared with a set point value Poff which is set so low that the pressure will fall below it only when all three evaporators have been turned off by their own thermostatic temperature control systems. Consequently, so long as any one of the evaporators is working, the compressor will be running, but with its inlet pressure varying according to how many evaporators are on.The compressor inlet pressure would be approximately at either level 3, 2 or 1 illustrated in Figure 2 according to whether three, two or one compressors are operating, and would fall to the level 0 only when all the evaporators were turned off, so that it is only in this condition that the compressor itself would be turned off. Pressure then rises until P is reached, when the on compressor is re-started. With such a system, at times when the demand for refrigeration is low, then the compressor inlet pressure is permitted to become very low, and the evaporator temperatures will be correspondingly low, and in these conditions the efficiency of the system is very poor in terms of heat removal per unit of energy input.A further and substantial disadvantage of this method of control is that the very low evaporator temperature causes excessive icing with the attendant inconvenience and cost of having to defrost the cabinets more often whilst, undesirably, the product in them warms up.

Another is that certain food products will have moisture removed from them excessively.

These problems are avoided by using a control system which as illustrated in the right-hand graph of Figure 2 has two set point values Pon and P off which are set relatively close together and are both quite high. This necessarily prevents the compressor inlet pressure from ever falling to low values but at the same time, when the demand for refrigeration is low, so that the compressor inlet pressure falls very rapidly when the compressor is on, the operating cycles of the compressor become very short and the starts per hour rating of the compressor will be exceeded so that the compressor will be subject to excesssive wear and its servicing and repair costs, and the frequent inconvenience of servicing, will be unacceptable.

Figure 3 illustrates a method of controlling the compressor in accordance with the invention. In Figure 3, the compressor is run for a period Ton which is terminated when the compressor inlet pressure reaches the set point Pset The compressor is then turned off for a calculated period of time Toff which may be equal for example to four minutes. After the off period, the compressor is turned on again until the inlet pressure has again fallen to Pset The off period of the compressor is derived with reference to the time that it takes for compressor inlet pressure to fall to PSet after the compressor has been turned on, this time being taken as a characteristic indicative of the load on the compressor system.If the on period becomes undesirably short, indicating low load, for example less than two minutes, then the subsequent period Toff is extended so as to increase the length of the subsequent period Ton and hence increase the length of the next cycle. The limiting or set value of Ton which causes Toff to be increased for subsequent cycles is selected, bearing in mind the other parameters of the system, such that the total cycle time will be long enough for the number of starts per hour of the compressor rarely if ever to exceed its rated value.Using this method of control, there are the additional benefits that the cycle time is permitted to become quite long when the demand for refrigeration is such as to permit that, and that the system never operates at a compressor inlet pressure lower than set' so that its efficiency is with certainty maintained at a relatively high level.

Figure 4 shows hardware required to operate the control method of Figure 3, including a compressor inlet pressure sensor 16, an inlet pressure set point device 18, and a source of time pulses 20 all of which feed their outputs to a controller 22. The controller may be a digital controller which operates in accordance with the flow chart shown in Figure 5 and provides an output signal on line 24 which opens and closes a contactor 26 to switch the compressor 2 off and on. The time factor used in controlling the compressor cycles is derived with reference to the time pulses produced by the generator 20.

Figure 5 is a flow chart showing the operation of the controller 22 in order to perform the control method of Figure 3. Initially Toff/set is set to four minutes, the compressor is then started, compressor inlet pressure is compared with P set until they are equal at which point the compressor is stopped, Ton is recorded and the compressor off period Toff/set starts to run.

If the read value of Ton is greater than two minutes and less than fifteen minutes, the compressor is switched on again when it has been off for four minutes.

If Ton is less than the minimum desired value of two minutes, then Toff/set is increased in inverse proportion to the read value of Ton, but subject to a maximum of twelve minutes, and during the next operating cycle the compressor is held off for the new increased period of Toff/set If Ton should become greater than fifteen minutes, then Toff/set is reduced in inverse proportion to Ton, but subject to a minimum of four minutes, for the next cycle.

Thus, the lower limit value, in this instance four minutes, for T off/set sets a lower limit on the frequency with which the compressor can be started and hence protects it against being started at rates beyond its starts per hour rating, whilst the upper limit of fifteen minutes on Toff/set avoids the temperatures in the cabinets becoming too high.

Reference will now be made to Figure 6 to explain in more detail what is occurring in the course of one of the cycles shown in simplified form in Figure 3. When the compressor has been off for some while, any liquid refrigerant in the evaporators of the cabinets will have evaporated to gas and the gas will have warmed up to a greater or lesser degree. In this condition, the com pressor inlet pressure bears no particular relationship to the gas temperature. When the compressor is switched on, the pressure initially falls very rapidly as the gas is pumped out of the evaporators. This is indicated by the steeply falling part 28 of the curve in Figure 6. At the point indicated at 30, the expansion valves of the cabinets open and liquid refrigerant starts to flow into the evaporators and to evaporate in them.When the expansion valves have been open for a little while, the rate of evaporation of refrigerant liquid in the evaporators reaches a relatively high level such that the pressure stops falling or falls only- gradually, in part 32 of the curve. As individual cabinets reach the temperatures set on their thermostats, their thermostatic valves close off, thus limiting the total refrigerant flow in the system, and the pressure starts to fall again as indicated at part 34 of the curve. When the pressure reaches Poet, the compressor is turned off and the compressor inlet pressure initially rises very rapidly because the expansion valves are open, one or more of the thermostatic valves is open, and so liquid is entering the evaporators and boiling in them.This is shown at part 36 of the curve and during this phase open thermostatic valves may or may not close. When the pressure reaches a certain value, to which the expansion valve controls have been set, the expansion valves close and the remaining refrigerant in the evaporators boils off during part 38 of the curve. When all the liquid has boiled off, the pressure at the compressor inlet rises only slowly and at a decreasing rate as the gas at that point becomes gradually warmer, this happening along part 40 of the curve.

For control purposes, the pressure at the inlet of the compressor only has significance when it is fairly close to the pressure at which refrigerant is boiling in one or more of the evaporators of the system. For this reason, any pressure measurements made at the peak of part 28 of the curve would not be significant. They only become significant at a point shortly after (perhaps five seconds after) the compressor has been switched on, as inidicated at 42 in Figure 6, at which time it will be certain that liquid has entered one or more of the evaporators, and that therefore the compressor inlet is approximately at the pressure under which that liquid is boiling therein. Thus, the pressures at which the compressor is shown being switched on and off in Figure 3 actually represent the points 42 and 44 in Figure 6.

From the above, it can be seen that in accordance with the invention insofar as a fixed set point value is used, only a single set point pressure value has to be set when the system is being installed, and consequently only a single value has to be adjusted in order to adjust the operation of the system.

A control method in accordance with the invention can be applied to air conditioning, where the principles involved are the same as those in refrigeration.

Furthermore, it can be applied to heat pumps. Heat pump systems are equivalent to refrigeration systems except that the purpose is to deliver heat in the condenser rather than remove it in the evaporator. Consequently, in applying the invention to heat pumps, it would be the compressor outlet pressure that is measured rather than its inlet pressure, this being an indication of the vari able which it is intended to control, namely the temperature at which refrigerant is condensing in the condenser.

Other attempts have been made to achieve the benefits of the present invention. For example, a rectifier/ inverter has been used to convert the mains frequency to a variable frequency so as to run the compressor at a variable speed to match the refrigeration demand, but this is complex and expensive.

Other arrangements for seeking to match capacity to demand include mechanical arrangements for shutting off a number of the cylinders of a compressor to reduce its capacity, over re-expansion compressors in which the compression ratio can be changed to alter the capacity, and multiple compressors which can be switched on in varying numbers. In some applications, a control method of the present invention applied to a simple single compressor may achieve similar results to these more complex systems. However, the invention can also be applied to the control of such systems to improve their efficiency.

It has been mentioned above that control systems which cause the compressor system to operate as described with reference to the left-hand part of Figure 2 are relatively inefficient.

Figure 6 shows a downward continuation of part 34 of the pressure curve to illustrate in more detail how a system operating in accordance with the left-hand side of Figure 2 performs. Typically, Poff is set at a relatively low value and the pressure continues to fall to that value along the broken-line part 50 of the curve.

Poff is set so low that it will not be reached until the thermostatic valves of all the cabinets have closed i.e.

the demand for cooling has become zero. The compressor system is then switched off and the pressure rises along part 52 of the curve until a pre-set value Pon is reached at which time the pressure starts to fall again as the compressor system comes into operation, this being along the broken line 54. The result is that the system cycles in the band between Poff and on which lies in a relatively low pressure range and so the system is operating at a correspondingly low average efficiency.

The invention achieves greater efficiency. In using the invention, the values of Toff/set employed in the Figure 5 algorithm are, for any given installation of refrigerated cabinets and associated compressor system, made long enough that at the end of the off period a majority of the cabinets (preferably at least 75%, for example 5 out of 6) will have their thermostatically controlled valves 14 open i.e. they are demanding cooling.As a result, when the compressor system is brought on, and residual gas has been cleared from the compressor inlet- side of the system, there will be a total flow of refrigerant through the individual cabinet cooling systems which approaches the maximum amount of flow possible, and consequently a plateau 32 in the compressor inlet pressure will occur at a relatively high level e.g., with refrigerant R502, and with chill (produce temperature +4 C) cabinets as a load, at about 2.8 bar gauge pressure. A major part of the refrigeration of the cabinets will occur during this phase of the cycle, and therefore at relatively high efficiency. In contrast, a pump-down control system as illustrated on the left in Fig. 2 would typically operate at an average pressure of about 1.8 bar gauge under the same conditions.Referring to the same Figure, the plateau when using the invention would occur at the three-cabinet running level, i.e. at a higher pressure (above say 2.1 bar) than is ever reached by the pump-down system when the latter is running between its Pon and Poff values.

As the cooling demand of the cabinets begins to be satisfied, their thermostatic valves 14 will start to close individually and so the pressure drops from the plateau on part 34 of the curve but Poet is set at such a level that it will not drop very far before the compressor system is switched off at point 1414.

The off period is then controlled so as to be sufficiently long that at the end of it a majority of the cabinets will once again be demanding cooling. This control may be achieved in various ways. When the Figure 5 algorithm is employed, then the programming of the controller 22 (Figure 4) will be arranged, empirically if necessary, such that the relationship between the current demand for cooling as indicated by the length of the on period in each cycle, and the length of the off period as calculated by the algorithm, will result in the majority of the cabinets demanding cooling at the end of the off period.

Alternatively, instead of the length of the off period being determined by the programming of the controller, the off period may be terminated in response to a sensed characteristic of the load units. When each of the load units includes a thermostat system, the sensed characteristic may be the condition of the thermostat systems and, for example, the off period may then be terminated when a majority of the load units are demanding cooling as indicated by the conditions of their thermostat systems. Figure 4 shows in chain-dotted lines 56.connections from the three thermostat switches 12 of Figure 1 by means of which the controller is informed of the conditions of the thermostatic switches and hence can be programmed to detect the closure of a majority of them and in response switch on the compressor system via line 24.

In a system which is not subject to major variations in load, it is possible for the off period actually to be fixed at a length which can be relied on to allow the majority of the load units to be demanding cooling when the compressor system is switched on, though provision may be made for manual adjustment of the length of the off period in the event that monitoring of the system indicates that the desired pattern of operation is not in practice being achieved.

So far as the on period is concerned, it is desired that this period be terminated when one or more of the load units have ceased to demand cooling i.e. not very long after the compressor system inlet pressure has dropped from the plateau 32. This of course may be achieved by the Figure 5 algorithm in accordance with which the on period is terminated as soon as the pressure falls to the value set However, other variables may be sensed in order to detect the falling load on the compressor system which is indicative of the plateau region having been passed.

When the compressor system is electrically powered, the current consumption, power consumption and power factor of the motor or motors will all be reduced as the load on the system falls from the plateau and hence, as illustrated in Figure 14, a sensing unit 58 may be associated with the power supply to the compressor motor to sense current consumption, power consumption, or power factor.

Suitable sensing units are readily available and therefore need not be further described. The output from sensing unit 58 is sent by line 60 to the controller 22, where it will be compared with the output of the set point device 18 to detect when the load, falling from its plateau level, reaches the set point value. Of course, the set point device 18 will be arranged to deliver a set point signal indicative not of a set point pressure, but of a set point current consumption, power consumption or power factor value.

It will be evident that the lengths of the on period and the off period may be determined independently of each other by separate systems so long as those systems are compatible with each other.

It has been mentioned that it is advantageous to vary set' or whatever alternative set point value may be employed, automatically in response to the level at which the plateau 32 occurs, because although the plateau 32 will necessarily occur at a relatively high level owing to the high load existing at the beginning of the on period, its exact level will vary according to operating conditions at the time.To achieve this, the controller 22 may be programmed so as to monitor a variable indicative of load, for example compressor inlet pressure from pressure sensor 16, the number of thermostatic values open as indicated on lines 56, or one of the electrical parameters of the power supply as indicated by unit 58, to recognise when that measured variable does not change by more than 10%, or preferably 5%, during a predetermined period of time, and to treat the detection of that occurrence as an indication that the plateau level is then occurring. It can further be programmed to then provide on line 62 a signal effective to adjust the set point device 18 so that it will give a set point output value to the controller equal to, for example, 80% of the measured value in absolute units of the plateau level.

It should be appreciated that a method according to the invention can be applied to a multiple-capacity compressor system which runs at more than one different level of capacity during a compressor on period, but not at all during the off period. In that event, steps may occur in the plateau level when an additional capacity stage is switched in but nevertheless it is possible to detect the fall in load from the end of the plateau by any of the techniques referred to above.

It has previously been mentioned that the method of the invention may be applied to a compressor system which includes one, preferably relatively low capacity, compressor which runs all the time, and a main compressor which is operated in cycles, the purpose of the small compressor being to ensure that liquid refrigerant does not accumulate on the outlet sides of the evaporators which will be capable of damaging the main compressor when it is switched on. In such a system, the continuous running of the small compressor would depress the level of the maximum pressure reached at the inlet of the main compressor as indicated by the broken line curve parts 28' and 40' shown in Figure 6. The pressure reduction may be even greater than is illustrated.

In applying a method in accordance with the invention to compressor driven heat pump systems, the variable which is measured to give an indication of load will be the compressor outlet pressure. Figure 7 shows how this outlet pressure varies throughout a cycle in a manner opposite to that of the inlet pressure. The outlet pressure rises whilst the compressor is on and, provided that a majority of the heat pump output units are demanding heating at the time when the compressor is switched on, there is a plateau as shown at 32' at a relatively low pressure level, which represents efficient operation of the heat pump system. Following the plateau, the compressor outlet pressure-starts rising again as the heating demand from the output units falls, and the on period is then terminated in a similar manner to that used in the refrigeration system described in more detail. For example, the level of the plateau may be sensed, and a set point value may be set in response to that sensing at a level which is a desired percentage higher than the plateau level so as to ensure that the compressor is switched off relatively soon after the demand from the output units starts to fall. The compressor is then held off, for example in the ways already described in relation to a refrigeration system, for a sufficient period that when it comes on again at least a majority of the output units are again demanding heating.

Claims (22)

CLAIMS:

1. A method of controlling a compressor driven vapour compression heat movement system in which a common compressor system heats or cools a plurality of load units and is operated in cycles each of which include a compressor system higher capacity period and a compressor system lower capacity period, characterised in that the lower capacity period is made sufficiently long that when the compressor system is switched to the higher capacity a majority of the load units are demanding heating or cooling and that the higher capacity period is made sufficiently long that when the compressor system is switched to lower capacity one or more of the load units have had their heating or cooling demand satisfied.

2. A method as claimed in claim 1, wherein the lower capacity period is controlled so as to be sufficiently long.

3. A method as claimed in claim 2, wherein the length of the lower capacity period is controlled in dependence upon the length of the preceding higher capacity period.

14. A method as claimed in claim 2, wherein the lower capacity period is terminated in response to a sensed characteristic of the load units.

5. A method as claimed in claim 14, wherein the load units each include a thermostat system, and the sensed characteristic is the condition of the thermostat systems.

6. A method as claimed in claim 1, wherein the length of the lower capacity period is fixed at a sufficiently long value.

7. A method as claimed in any preceding claim, wherein said majority is at least 75%.

8. A method as claimed in any preceding claim, comprising sensing a variable which represents the load on the compressor system and terminating the higher capacity period when the sensed variable indicates that the load on the compressor system is falling.

9. A method as claimed in claim 8, comprising terminating the higher capacity period when the sensed variable indicates that the load on the compressor system has fallen from a substantially constant load level.

10. A method as claimed in claim 8 or claim 9, wherein the system is a cooling system and the sensed variable is the compressor system inlet pressure.

11. A method as claimed in claim 8 or claim 9, wherein the system is a heating system and the sensed variable is the compressor system outlet pressure.

12. A method as claimed in claim 8 or claim 9, wherein the compressor system is electrically powered and the sensed variable is its current consumption.

13. A method as claimed in claim 8 or claim 9, wherein the compressor system is electrically powered and the sensed variable is its power consumption.

14. A method as claimed in claim 8 or claim 9, wherein the compressor system is electrically powered and the sensed variable is its power factor.

15. A method as claimed in claim 8 or claim 9, wherein the load units each include a thermostat system and the sensed variable is the number of thermostat systems demanding heating or cooling.

16. A method as claimed in any one of claims 8 to 15, comprising terminating the higher capacity period when the sensed variable reaches a set point value.

17. A method as claimed in claim 15, comprising sensing the occurrence of a substantially constant level of load on the compressor system during its higher capacity period and automatically adjusting the set point value to a value which would represent a load level lower than said substantially constant value.

18. A method as claimed in claim 17, wherein the system is a cooling system and the set point value is set such that when the sensed variable reaches the set point value the compressor inlet pressure is between 60% and 90% of the absolute value which it has when the load is at said substantially constant level.

19. A method as claimed in any preceding claim, wherein the compressor system consists of a compressor and the higher capacity mode is when the compressor is on and the lower capacity mode is when the compressor is off.

20. A method as claimed in any one of claims 1 to 18, wherein the compressor system includes a plurality of compressors, and in the higher capacity mode at least some of the compressors are running and in the lower capacity mode a lesser, fixed number which may be zero, are running.

21. A method of controlling a compressor driven vapour compression heat movement system in which a common compressor system heats or cools a plurality of load units and is operated in cycles each of which include a compressor system higher capacity period and a compressor system lower capacity period, characterised in that the lower capacity period. is made sufficiently long that when the compressor system is switched to the higher capacity a majority of the load units are demanding heating or cooling and that the duration of the higher capacity period is controlled by monitoring a variable which represents the load on the compressor system and terminating the higher capacity period when the sensed variable indicates that the load on the compressor system has fallen from a substantially constant level.

22. A method as claimed in claim 21 including the further features specified in any of claims 2 to 7 or 10 to 20.