GB2245967A - Air conditioning system - Google Patents

Abstract

An air conditioning system includes a plurality of independently controlled air-to-water heat exchangers (17) or groups of heat exchangers for heating or cooling air before circulation. Means are provided for supplying hot or cold water to the heat exchangers together with a plurality of variable speed pumps (51a). Each variable speed pump has the function of varying the flow rate of the water supply through one independently controlled heat exchanger or group of heat exchangers according to the required temperature of the air. <IMAGE>

Description

AIR CONDITIONING SYSTEMS The present invention relates to the control of air conditioning systems.

The control of heating or cooling of air in known air conditioning plants is often carried out by varying the flow rate of hot or chilled water to air-to-water heat exchangers. The heat exchangers thereby cause the air flowing past to heat up or cool down as required. The flow rate to the heat exchangers is varied by a control valve which can be continuously modulated between fully opened and fully closed. For good control, the heater or chiller output should be proportional to the valve position. This can only be achieved by careful matching of the control valve and heat exchanger.

This system has many disadvantages as there are a number of factors which make this ideal situation difficult to achieve. The problems include valve sizing and flow balancing.

The pressure drop through the control valve should be high compared to the pressure drop in the rest of the water circuit. Oversizing the valve gives too low a pressure drop and results in little flow modulation as the valve approaches fully open. Undersizing the valve results in excessive pumping power to deliver the required maximum flow rate.

Usually several heat exchangers are fed by parallel water loops from a single central supply. The flows to each of these circuits must be well balanced to ensure that a desired maximum flow rate is achieved in each loop when all of the control valves are fully open. This is achieved by incorporating flow balancing valves in each circuit which are adjusted when the plant is commissioned.

Failure to achieve linearity between the control valve position and the heat exchanger output leads to difficulties in tuning the controller to give good system control, the result being either sluggish response or instability at some part of the control range.

The object of the present invention is to provide an improved system to overcome these disadvantages.

According to the invention there is provided an air-conditioning system comprising means for supplying hot or cold water to a plurality of independently controlled air-to-water heat exchangers or groups of heat exchangers for heating or cooling air before circulation and a plurality of variable speed pumps, each variable speed pump having the function of varying the flow rate of the water supply through one independently controlled heat exchanger or group of heat exchangers according to the required temperature of the air.

The variable speed pump preferably comprises a pump driven by a variable speed motor. The pump may be a positive displacement pump or a centrifugal pump. A control unit may be programmed to control the motor to give a known flow rate for any control signal.

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of an air-conditioning system; Figures 2 and 3 show control diagrams for a known system; and Figures 4 and 5 show control diagrams for embodiments of an improved system according to the invention.

Figure 1 shows one example of an air conditioning system 10 for controlling the heating or cooling of air to a room 11.

Fresh air enters the system at inlet 12 and is mixed in chamber 13 with air returned from the air conditioned room 11 via duct 14. The mixed air passes through filter 15 into a chamber 16 where it is heated or chilled via heat exchangers 17 to the required degree.

This can be controlled by a thermostat (not shown) via a control system shown schematically at 18 which controls the flow of water to the heat exchangers 17. The air is then forced through duct 19 back to the room 11 by a fan 20. The constant supply of air to the room 11 produces a slight excess of pressure in the room which causes the exhaust air to flow back through the return duct 14. Some of the exhaust air may be discharged to the open air via outlet duct 21 or will leak from unsealed windows or doors etc.

A known control system for an air conditioning plant is shown in Figure 2. A primary pump 24 is provided for supplying water from a mains or central supply to a plurality of independently controlled heat exchangers or group of heat exchangers 17. A control circuit 25 for each independently controlled heat exchanger or group of heat exchangers includes a control valve 27 having three ports. A first flow balancing valve 29 is connected to the outlet port of the control valve 27 and determines the magnitude of the flow of water into the control circuit 25. The control valve 27 controls the flow of water through the heat exchanger 17 by allowing part of the flow to pass through the bypass line 31 when the desired heat output from the heat exchanger is below the maximum.The second flow balancing valve 33 is used to set the flow rate through the bypass line 31 when there is no flow through the heat exchanger 17 to have the same value as that through the heat exchanger when there is no flow through the bypass line. Thus the total flow rate through each control circuit remains substantially constant at all times irrespective of the flow rate through the heat exchanger. In this way changing the flow rate through one heat exchanger 17 has no effect on the flow through the other heat exchangers and the pressure drop at the control valve 27 need only be high compared to the pressure drop through the two parallel paths, of the heat exchanger 17 and the bypass line 31, that it serves. It will be appreciated that the flow rate of water through the pump 24 must be constant at all times irrespective of the flow of water through each heat exchanger 17.This is wasteful in electrical energy.

A second embodiment of a known control system for an air conditioning plant is shown in Figure 3. A primary pump 33 supplies water from a mains or central supply to independently controlled heat exchangers or groups of heat exchangers 17. Control of the flow of water through each independently controlled heat exchanger or group of heat exchangers 17 is provided by a control circuit indicated generally at 35. The control circuit 35 includes a two port control valve 37. The output port of the control valve 37 is connected to the heat exchanger 17 to be controlled. The input port of the control valve 37 is connected directly to the distribution mains, and hence to the primary pump 33, and accordingly the pressure drop across the valve 37 must be large compared with that through the distribution mains.A flow balancing valve 39 on the output side of the heat exchanger 17 ensures that a desired maximum flow rate is achieved in each control circuit when all of the control valves 37 are fully opened. The arrangement of Figure 3 allows the flow in the distribution mains to vary as the flow rate through each control valve 37 is changed. However, changing of the flow through one control valve 37 may affect the flow through the other control valve.

Figure 4 shows a control system for use in an air conditioning system according to one embodiment of the present invention. The system includes a plurality of control circuits 41, each control circuit providing independent control for a heat exchanger or group of heat exchangers 17. The control circuits 41 are connected in parallel by distribution mains 43, 45. A primary pump 47 pumps water from a central or mains supply 49 through the mains distribution 43, 45 to and from the control circuits 41. A secondary pump 51, which is a variable speed pump, is provided in each of the control circuits 41 to control the flow of water through the heat exchangers 17.

In the system of Figure 4, the variable speed pumps 51 are centrifugal pumps. A balance pipe 53 ensures that the pump 51 is isolated hydraulically from the rest of the system. Optionally, a non-return valve 55 is provided to prevent flow in either direction through the heat exchanger 17 when the control pump is switched off.

Non-return valves both prevent flow in the backwards direction and require a finite pressure drop thereacross to allow flow in the forward direction. It is this requirement for a finite pressure drop which prevents flow through the heat exchanger in the forward direction, when the pump is switched off, should there be any small pressure fluctuations across the balance pipe 53.

The system of Figure 4 further includes a first balancing valve 57 and a second flow balancing or commissioning valve 59. The first flow balancing valve 57 is set so that the flow rate through it is larger than that required by the rest of the control circuit 41. The precise value of the flow rate allowed through the flow balancing valve 57 is not critical, in contrast to the setting of the equivalent flow balancing valves in the prior art arrangements, so long as the flow setting is greater than a given value. Accordingly, such a setting for the first flow balancing valve 57 may be achieved by design of the pipe work layout and valve selection rather than by actually manually setting the required flow rate.

The bypass line 53 allows any flow through the first flow balancing valve 57 in excess of that required by the remainder of the control circuit 41 to return to the distribution mains 43, 45.

The second flow balancing valve 59 is used to set the correct maximum flow for each variable speed pump 51.

The flow through each second flow balancing valve 59 can be set independently of the flows in the other control circuits. Such flexibility is in contrast to the conventional two port system in which altering the flow to one control valve alters the flows to the others and commissioning is a process of continually setting and resetting the flows until all of the circuits are correctly balanced.

In the control system of Figure 4, the centrifugal pumps 51 may be replaced by positive displacement pumps.

The use of a positive displacement pump would remove the need for the non-return valve 55 and the second flow balancing valve 59.

In particular, the speed of the primary pump 47 in Figure 4 can now be varied to achieve energy savings in pumping power. The only criterion for the speed of the primary pump 47 is that its flow rate as a percentage of its maximum flow rate must always exceed the flow rate as a percentage of the maximum flow rate for any of the secondary pumps 51. For example, if one control circuit is demanding a flow which is 50% of its maximum flow rate and the other control circuits are demanding less than 50% of their maximum flow rates, then the flow in the primary pump 47 can be reduced down to 50% of its maximum flow rate. Such a flow control scheme for the primary pump will be readily achievable with modern building management systems.Thus the system of the present invention will both maintain good control and allow flow reductions in both the mains distribution and the control circuits per se.

An alternative embodiment of a control system for use in the present invention is shown in Figure 5. Like parts to those shown in Figure 4 will be designated by like reference numerals. Each control circuit utilises a positive displacement pump 51a.

Positive displacement pumps deliver a flow rate approximately proportional to the motor speed for a wide range of pressure lifts across the pump. However, such pumps are at present only used for difficult pumping duties such as pumping highly viscose liquids.

In the improved system of Figures 4 and 5 the need for control valves is dispensed with. A given heat output can be achieved for any controlled demand as a known flow rate can be given by the system for any control signal. A controller unit would be incorporated in the system to provide the correct control signal to the secondary pump according to the demand. The unit is programmed with the heat exchanger characteristics.

The improved system has substantial advantages over the existing systems. The design of the system is much simplified as there are no control valve sizing problems.

There are also much simpler piping runs as can be seen by the comparison of the prior art Figures 2 and 3 and the embodiments of Figures 4 and 5.

Existing systems only work by the production of pressure drops into the water flow circuit, both to control the flow and to balance the flows between parallel loops. The proposed systems can be designed for low pressure drop hence reducing the pumping power required.

For any control signal input a known heat output can be produced which enables the controller to be tuned to give good system response across the full range of control inputs.

Modifications to the embodiments described within the scope of the present invention will be apparent to those skilled in the art.

Claims (8)

1. An air conditioning system comprising means for supplying hot or cold water to a plurality of independently controlled air-to-water heat exchangers or groups of heat exchangers for heating or cooling air before circulation and a plurality of variable speed pumps, each variable speed pump having the function of varying the flow rate of the water supply through one independently controlled heat exchanger or group of heat exchangers according to the required temperature of the air.

2. An air conditioning system as claimed in Claim 1 in which the variable speed pump comprises a pump driven by a variable speed motor.

3. An air conditioning system as claimed in Claim 2 further comprising a control unit programmed to control the motor to give a known flow rate for any control signal.

4. An air conditioning system as claimed in any one of the preceding claims in which the pump is a positive displacement pump.

5. An air conditioning system as claimed in any one of Claims 1 to 3 in which the pump is a centrifugal pump.

6. An air conditioning system as claimed in any one of the preceding claims wherein said means for supplying hot or cold water includes a primary pump for supplying water to more than one of said plurality of variable speed pumps.

7. An air conditioning system according to Claim 6 including means for controlling the speed of said primary pump so that the percentage flow rate of water through said primary pump is no less than the percentage flow rate of water through any one of said variable speed pumps.

8. An air conditioning system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.