WO1990015289A2

WO1990015289A2 - Air conditioning system

Info

Publication number: WO1990015289A2
Application number: PCT/GB1990/000897
Authority: WO
Inventors: Munther Al-Khafaji
Original assignee: London Electricity Plc
Priority date: 1989-06-08
Filing date: 1990-06-08
Publication date: 1990-12-13
Also published as: GB8913211D0; GB9012800D0; AU5741090A; WO1990015289A3; GB9012799D0; GB2235551A; GB2235524A

Abstract

The present invention relates to an air conditioning system for use in a building arranged in a plurality of zones in each of which the air temperature can be separately controlled. The system comprises two air supply ducts (1, 2) from each of which either heating or cooling air can be supplied to each zone and an individual terminal unit (3) in each zone to which ducts (1, 2) are connected and controllable to determine the air flow rate into the zone at any instant. There is a temperature sensor (6) in each zone, a central control means (9) common to all zones which controls the temperature of the air supplied and the air flow rate in each duct in dependence upon signals sent to the central control means from each zone. There is also a communication network (7) linking the central control means to each zone over which signals are transmitted between the central control means and the zones to control operation of the system.

Description

AIR CONDITIONING SYSTEM

This invention relates to an air conditioning system, and particularly to an air conditioning system for use in a building arranged in a plurality of zones in each of which the air temperature can be separately controlled.

Air conditioning systems are now common in buildings, but a disadvantage of known systems is that they are generally not quickly responsive to short as well as long term changes in the variables, for example temperature, humidity and air throughput, both inside and outside the building, that affect the internal environment of the building. For best results a system should be able to maintain a substantially uniform deviation from the required values of the various controlled parameters of the environment in the building, in response to both short and long term changes in the influencing variables inside and outside the building, and should also ensure an efficient distribution of heating or cooling air throughout the building. This latter requirement can be a particular problem in buildings arranged in a plurality of zones in each of which separate control of the environment is provided.

Known air conditioning systems are described in GB 2215867, GB 2194651 and GB 2183018 which operate on the variable refrigerant volume principle rather than an air distribution system. These systems are also decentralised since the individual terminal units contain fans, heat exchangers, etc., rather than being controlled from a central station.

Another known air conditioning system is described in US 3568760 which is a modification on a basic dual duct system. The operation and control of the system consists of the apportioning of hot and cold air streams by linked dampers to effect desired temperature control. There is, however, no control over the volume of air in the system, i.e the terminals have constant volume of air flow.

According to this invention there is provided an air conditioning system for use in a building arranged in a plurality of zones in each of which the air temperature can be separately controlled, comprising two air supply ducts from each of which either heating or cooling air can be supplied to each zone; an individual terminal unit in each zone to which the ducts are connected and controllable to determine the air flow rate into the associated zone at any instant; an individual temperature sensor in each zone and operative to provide control signals for the associated terminal unit; a central control means common to all of the zones and operative to control the temperature of the air supplied by each duct and the air flow rate in each duct in dependence upon signals sent to the central control means from each zone; and a communication network linking the central control means to each zone and over which signals are transmitted between the central control means and the zones to control operation of the system.

In the system of the invention the temperature and flow rate of the air in each duct is variable and is determined by the central control means in dependence upon the requirements of the zones signalled to the central control means over the communications network. Thus, the central control means will ensure that sufficient air at the required temperature is available at each zone for the heating or cooling required there. Generally one duct will serve fo supplying heating air and the other duct will serve for supplying cooling air to any zone, and the central control means will operate to decide which duct supplies air to a particular zone at any instant in dependence upon the signals received from that zone over the communications network. Under certain circumstances, for example when a relatively high degree of heating or cooling is required, then the central control means will supply either heating or cooling air to both ducts. Thus, the central control means will include means for heating or cooling the air supplied to each duct, which air can be either new air from outside the building, air being recirculated within the building, or a mixture of the two.

The individual terminal unit in each zone is connected to the ducts and operates to determine the rate of air flow from a duct, or ducts, into the associated zone in dependence upon the signals from the associated temperature sensor in the zone, thereby to maintain the required temperature in the zone. Clearly there will be a maximum air flow rate capability of the system and thus preferably the maximum air flow rate available from a duct or ducts at any zone is determined by control signals transmitted to the terminal unit at that zone by the central control means over the communications network in dependence upon the maximum air flow rate capability of the system.

Each terminal unit preferably comprises two modulating dampers which are controlled to determine the air flow rate into the associated zone from each of the ducts respectively.

Preferably each terminal unit has an air discharge diffuser having a variable air discharge area controlled in dependence upon the air flow rate from the duct or ducts supplying air to the terminal unit at any instant, whereby the terminal unit supplies air to the associated zone at a constant air discharge velocity regardless of the air flow rate supplied to the terminal unit from the duct or ducts.

Preferably, the area of the exit aperture from the terminal unit is variable.

Preferably, the exit aperture has located within it a restrictor means which is moveable in dependence on the air flow rate.

Preferably each terminal unit has means to control the direction of air discharge therefrom into the associated zone.

Control signals for the modulating dampers, air discharge diffuser, and air discharge direction control means of any terminal unit can be derived from the central control means or a local control means at the associated zone and possibly in communication with similar local control means at other zones and the central control means over the communications network. The necessary inputs to the local control means can be from appropriate transducers in the associated zone, in a similar manner to the temperature input obtained from the temperature sensor in the zone.

As discussed above, the system of the invention functions by controlling the temperature and flow rate of the air supplied to each zone. However, it will be appreciated that other control parameters, for example humidity, can be incorporated into the system.

Another difficulty which arises with known air conditioning systems is that due to the variable air discharge rates into various zones it is generally necessary for parts of the system to be at an air pressure higher than otherwise essential, in order to ensure that there is sufficient air pressure in the system to satisfy the required air flow rate at every discharge location of the system. In some known systems an attempt is made to meet this difficulty by providing a minimum number of pressure sensors in the system and controlling the air supply means, for example a fan, in dependence upon signals derived from these pressure sensors.

Also according to this invention there is provided an air conditioning system for use in a building arranged in a plurality of zones in each of which there is separate control of the environment, comprising individual flow control means to control the pressure and volume of air supplied to each zone, which flow control means are controlled by signals derived from pressure sensors in the associated zones to maintain the minimum necessary pressure in the air supply to the zone, and a common central control means to which the signals from the pressure sensors in each zone are transmitted, the central control means operating to control the air supply means of the system to maintain the air pressure in the system to that required to satisfy the needs of each zone.

With such a system each flow control means can ask the central control means for increased or decreased air flow as necessary to satisfy its needs, the central control means also taking into account the needs of the other zones in the system.

Preferably a similar flow control means is provided for the extraction of air from each zone.

Such a system ensures efficient air distribution throughout the various zones in a building by balancing the air supply and extraction flow rates at each zone.

Preferred embodiments of the present invention will now be described in detail by way of example with reference to accompanying drawings, in which:-

Figure 1 diagrammatically illustrates a first air conditioning system according to the invention; and

Figure 2 diagrammatically illustrates a second air conditioning system according to the invention; Figure 3 is a first schematic arrangement for measuring the pressure head of the exit air in the exit chamber of a terminal unit;

Figure 4 is an alternative arrangement to that in Figure 3;

Figure 5 is yet a further alternative arrangement to that in Figure 3;

Figure 6 is a second schematic arrangement for measuring the pressure head of the exit air in the exit chamber of a terminal unit;

Figure 7 is a variation on the sprung blades in Figure 6; Figure 8 is a further variation on the sprung blades in Figure 6;

Figure 9 is a perspective view of a terminal unit according to the present invention.

Figure 10 is a schematic view of the proposed air conditioning system;

Figure 11 is a perspective schematic view of the proposed air conditioning system.

The system shown in Figure 1 is an air conditioning system for use in a building arranged in a plurality of zones, for example rooms, in each of which the environment, and in particular the temperature, can be separately controlled. The figure shows the arrangement at each of two zones A and B of the building, the arrangements at the two zones and at the other zones of the system being the sam .

The system includes a first duct 1 for the supply of heating air to each zone, and a second duct 2 for the supply of cooling air to each zone, the apparatus at each zone being connected to each of the two ducts 1 and 2. The apparatus at each zone comprises an air discharge diffuser 3 having a variable air discharge area, shown in two parts, by way of which air from the ducts 1 and 2 is discharged into the zone. Air is supplied to the _^diffuser 3 from each duct 1 or 2 by way of an individual modulating damper 4 which is controlled by signals from a local contol means 5, for example a microprocessor, which receives input signals from a temperature sensor 6 in the zone and from a central control means (not shown) , for example a microprocessor, common to all of the zones of the system. The signals to and from each zone and the central control means are transmitted over a common communications network 7 in any convenient manner. An air discharge pressure controller 8 is connected to the communications network 7 either directly or as shown via the local control means 5, and the diffuser 3 at each zone.

As previously described, under the control of signals from the local control means 5 and/or the central control means, the dampers 4 and the diffuser 3 are controlled to determine the rate, velocity and direction of supply of such air, which control means also operate to determine which of the ducts 1 and 2 supplies air _^to the zone at any instant. The temperature of the air is determined by the central control means in dependence upon signals received from all of the zones over the communications network 7, with the air flow rate in each duct 1 and 2 being similarly determined, whereby sufficient air at the required temperature is available at each zone for the needs of that zone indicated by the temperature sensor 6 (and any other sensors) at that zone.

When the ducts 1 and 2 are carrying air at different thermal levels from a zone, i.e one duct 2 has a cooling capacity and one duct 1 has a heating capacity in relation to the zone conditions, then only one of the two ducts 1 and 2 will be available to the apparatus (terminal unit) in each zone. The supply which will be available is the one to which a terminal unit has indicated a need. For example, a terminal unit which needs heating will have the supply with heating potential available to it but not the one with cooling potential available to it. At no time will the two supplies (ducts 1 and 2) , for the purpose of controlling the room temperature, flow and mix at a terminal unit unless they have the same thermal potential.

The variation in heating and cooling introduced into a terminal unit is achieved by varying the air flow rate.

To ensure a zero or uniform deviation from set levels in temperature, humidity, etc. , throughout the building, the central control means (or master controller) will dictate to the local control means 5 the maximum flow rates they can provide. The terminal units, as explained earlier, each have two modulating dampers 4 whose function is to regulate air flow rate from the ducts 1 and 2. The air diffusers 3 have a variable discharge area which enables a constant air discharge velocity from the air diffusers 3 regardless of the air flow rate to the air diffusers 3. The air diffusers 3 are also provided with flow direction control means which enable air to be delivered into a given zone at the desired flow rate, the desired velocity and in the desired direction. The air diffusers 3 can be any shape, e.g linear, circular and some examples are described later.

Referring now to Figure 2, this shows another air conditioning system for use in a building arranged in a plurality of zones in each of which the environment can be separately controlled. The figure shows the arrangement at one zone, the arrangements at the other zones being the same.

The system includes a first duct 1 for the supply of heating air to each zone, a second duct 2 for the supply of cooling air to each zone, and an extract duct 11 for the extraction of air from each zone. An individual local control means 5 at each of the zones is connected by a common communications network 7 to a common central control means 9, as in the system of Figure 1. Air is supplied from each of the ducts 1 and 2 to the zone by way of an individual flow control means 10 operative to control the pressure and volume of air supplied to the zone from the associated duct, a similar flow control means being included in the connection between the zone and the air extraction duct 11. The three flow control means 10 at each zone are controlled by signals from the associated local control means 5 which receives inputs from pressure sensors 12 which serve to sense the pressure of air supplied from each of the two supply ducts 1 and 2, the signals from the sensors 12 also being sent to the central control means 9 over the communications network 7. The flow control means 10 are controlled to maintain a required minimum pressure in the associated zone, while the central control means controls the air supply means, for example a fan, of the system to maintain the air pressure in the ducts 1 and 2 to that required to satisfy the needs of each zone.

Thus, the air supply and extraction at each zone are interlocked thus ensuring the required balance between the air supply and extraction low rates.

It is always the case that where the air flow rate is varying within a system due to changing discharge rates from the terminal units, that the terminal at the lowest static pressure is difficult to identify. Also because of the lack of sufficient pressure and flow regulating devices, that large sections of duct network are overpressurised, i.e at a static pressure above that required to operate and satisfy the terminal unit. The supply fan 9 is therefore, controlled to meet the set point of one or two pressure sensors located in the main risers of the system. The new balancing system will prevent overpressurisation by maintaining the static pressure at the minimum required level. To this end it utilises the use of pressure and volume regulating boxes 10. These boxes can be installed at various points in the system (e.g floor branches) . These boxes 10 will be under the direction of a local controller 5. These local controllers 5 have their own pressure sensors for feedback and control the boxes 10 to maintain the minimum pressure and communicate with the fan controller 9. This communication with the fan controller 9 will enable each local controller 5 to ask for increased or decreased fan output according to its status and the status of other local controllers 5. The boxes 10 can be fitted with flow rate measuring devices in order to measure the air being introduced into the space. With a similar box 10 fitted to the extract air branch 11 from that space the two quantities, i.e the rate of supply and the rate of extract can be interlinked to ensure the desired rates. This will ensure efficient air distribution throughout the building. The system will utilise two boxes 10 for its two supply ducts 1 and 2 and one for the extract duct 11 for each floor branch. The requirement for a constant exit air velocity demands that the area of the aperture should change with air flow rate. The problem can be solved by giving consideration to the possibliity of using the pressure head of the exit air to directly control the exit pressure. Figure 3 depicts a schematic arrangement for measuring the pressure head of the exit air in the exit chamber in such a way.

The air^-flow into the terminal from the supply ducts 1 and 2 is determined by dampers 20. These are controlled Sa actuators 21.

The air from the dampers 20 is guided and mixed by sound absorbing baffles 24.

A problem with conventional variable flow terminals is that at low flows, the attachment of the discharge flow to the surface in which the unit is mounted, normally the ceiling of the controlled room, is lost and the discharge air does not mix correctly with the air in the room and results in less effective air conditioning of the room and possible unpleasant conditions for the occupants of the room. The present invention is designed to overcome this problem by adjusting the exit aperture to maintain the exit velocity of the air as the air flow is reduced.

A requirement of the terminal is that the exit velocity of the air from the terminal aperture 26 should, essentially, be of constant velocity, or with some defined relationship between air flow rate and exit velocity. By directing the discharging air in a suitable direction, the controlled exit velocity will ensure that the discharge flow will maintain attachment to the surface into which the terminal unit is mounted.

The exit air velocity is related to the total pressure head by well established engineering principles. Thus, the desired exit velocity can be achieved by controlling the. pressure difference across the exit aperture.

Figure 3 illustrates a principle by which this might be achieved when air is flowing out of the exit aperture 26. A diaphragm 27 is mounted on compliant supports 23, such that when the pressure difference across the diaphragm reaches the desired value, the diaphragm is displaced so that it increases the exit aperture. This allows more air to flow out so that the pressure differential decreases to the point that pressure differential balances the excess supporting force on the diaphragm. The resulting displacement of the diaphragm can be sensed by a displacement sensor 22 this information combined with the pressure difference, determined from the diaphragm support characteristics is used to determine the air flow from the terminal.

The terminal can also incorporate a temperature sensor 25 to determine the discharge temperature of the air and combining this measurement with the deduced flow allows the energy dissipation can be determined.

The arrangement in Figure 3 results in small pressure differentials. An improved version of the air control system is shown in Figure 4. In this the displacement of the exit aperture controller 63 is improved by use of a pressure amplifier diaphragm 60 mounted on a compliant support 61. The flow of air past the amplifier diaphragm 60 is determined by the pressure differential across it and the clearance around the edge of the diaphragm. This clearance is determined by displacement of the diaphragm 60 and the profile 64 of the housing into which it fits. The motion of the diaphragm 60 and the exit aperture controller 63 are linked together by a linkage or mechanism 65. The ratio of the pressure differential across the exit aperture controller 63 to the pressure differential across the exit aperture is arranged to be a substantial value, typically 10 to 1, so that the control forces across the diaphragm 60 are substantially increased. The ratio of pressures is determined by the ratio of the two aperture areas 66 and 67. The area ratio is determined by the profiles of the housing around these two apertures and the link displacement of the mechanism. Noise baffles 62 are also provided.

The relationship between the displacement of the air flow control mechanism and the aperture sizes can be achieved by a variety of profile slots or apertures in which the air flow is dependent on the displacement of the mechanism. By way of an example, Figure 5 shows an alternative method by which this might be achieved. In this the amplifier diaphragm 66 is perforated by holes 65 through which are protruding fixed cones 64. These cones are shaped such that the air flow is related in a desired way to the displacement of the linkage 67 between the diaphragm 66 and the exit aperture control 68. Such a design allows the control or the air flow to be more precisely controlled with conventional production methods and tolerances. Figure 6 is a cross-sectional view through a terminal unit which is also able to measure exit velocity by measuring the pressure head of the exit air. Ducts 1 and 2 are shown and the air flow is controlled by control valves 30 (or dampers) which are under the control of the local control means 5 in each zone. The two vertical blades 31 are spring loaded by a spring 32, for example, so that as the pressure behind them increases they are pushed together thus opening the exit apertures 33 to allow air to flow out at the desired pressure and therefore, exit velocity. In effect, the arrangement is a pressure relief valve.

The blades 31 need to be supported along their length and pivot with the minimum of friction. In addition, some degree of blade counterbalancing may also be desirable. Figures 7 and 8 illustrate possible low cost designs which achieve the pivoting and support requirements.

Figure 7 is a detailed, enlarged view of blades 31 which are self-balancing with out-of-balance stiffness. The blades 31 are pivoted and are each provided with a localised mass increase at 34a.

Figure 8 is a detailed, enlarged view of blades 31 which provide compliance in bending. The blades 31 are mounted at point 34b and each have a thinned section 37 to achieve the required compliance.

A method of achieving a two stage pressure drop has already been described in connection with Figure 4. In this the large upper plate or amplifier diaphragm 60 is linked to the exit aperture controller 63 so that the movement of the two is identical. The profile of the housing either side of the upper plate 60 and the lower exit aperture 67 is arranged such as to retain the ratio of aperture areas as the combination moves. The pressure drop thus occurs across the large plate 60, which is some distance from the exit 67 and hence any flow noise created can more easily be absorbed by noise baffles 62 before discharge.

A difficulty with this concept is that the width of the. perture either side of the upper plate would be quite narrow, typically 3mm when fully open. This would result in the need for stringent tolerance control and a consequent increase in production costs. To overcome this, the flow control function may be achieved by arranging the pressure drop aperture as a cone 64 in a hole 65 as shown in Figure 5. , his would allow the required flow control to be achieved with a simple, low tolerance and robust mechanism. The cone profile can be chosen to give any desired flow/velocity relationship, such as increasing velocity with reducing flow to help maintain room penetration at low air flows.

Counterforce on the pressure plate may be achieved by springs, or counterweights, or a combination of the two. In practice, spring loading is probably preferable, as a positive spring rate will contribute to dynamic stability more than would the zero rate provided by a counterweight. It is noted in passing that this concept relies on the terminal being installed in a substantially horizontal orientation.

The construction of the terminal unit itself is a substantially conventional sheet steel fabrication. This provides an optimum mix of low cost, robustness, compatibility and low risk design.

Figure 9 is a perspective view of such a terminal unit 40. The unit 40 is attached to a false ceiling panel 41 and comprises a box 42 connected to supply ducts 1 and 2 (not shown) . The box 42 can have end spigots so that it is able to replace a dual length of ducting thus reducing cost and casing installation. The terminal unit 40 is connected to the ducts 1 and 2 by a duct connectors 51 (only one of which is shown in Figure 9) . From each supply duct 1 and 2 air is bled via a row of holes 43 which discharge upwards into a plenum chamber 44 and whose flow rate is controlled by a sliding aperture plate 45 with a corresponding row of holes 49. From each plenum chamber 44 the two airflows mix in passing through baffle plates 46 which serve to reduce noise. The mixed air then passes downward into the air diffuser 3 through holes 47 and past the diffuser blades 31.

The flow rate is controlled by moving the aperture plates 45 by means of small DC motor-gearbox units 48. The corresponding pairs of flow control holes 49 in the aperture plates 45 should be non-circular in order to linearise the position flow rate relationship. The position of the aperture plates 45 is sensed by a low cost transducer 50 such as an array of electrical contacts whose closures correspond to suitable fractions of plate travel. An alternative arrangement would be to use a stepper motor and an end of travel limit switch but this would prove more costly.

The discharge flow rate is inferred from the exit nozzle deflection having previously calibrated its relationship with flow rate. This deflection may be measured in various ways, such as capacitively, or using a simple coil and moving core device.

Schematic diagrams of the proposed system are shown in Figures 10 and 11.

Figure 10 shows the system at a local area (e.g floor) level. Three features may be highlighted.

First, some parameters such as flow rate and pressure are best measured at the floor supply entry point and their data shared around the network. This makes best use of relatively expensive sensors measuring parameters which will be essentially constant within the local area.

Second, each floor duct is controlled by a damper 70, under the control of the Local Area Controller (LAC) 71. If necessary, local heaters or chillers 72 could also be fitted at these points if the building use requires it.

Third, the local terminal network which links all the terminal controllers and the LAC 71, also includes separate units acting as room sensors 73 or room controllers 74. It is envisaged that these would consist of terminal controller units configured to act in the required mode. They would not be required in all applications, since the terminal controllers themselves would normally act as room sensors. A typical room sensor 73 will sense temperature, smoke, humidity at velocity and occupancy.

Figure 11 indicates that each floor level system is duplicated as required. Each LAC 71 would be linked to the Building anagement Controller (BMC) 75, by a proprietary communication system.

The local terminal network 75 and system network 76 are both indicated in Figures 12 and 13.

Some modes of operation of the air conditioning system will now be described.

WINTER

Both ducts can be utilised for preheating the building. The volume and temperature of the supply air will be decided upon by the requirements of energy efficiency. Once the space is up to temperature and occupied, the hot duct will supply the ventilation air and meet any heating requirements. The cold duct will come into operation only when there is a cooling demand.

SUMMER

As heating demand diminishes with the advent of the spring and finally becomes a cooling load even at perimeter areas, the hot duct will stop to supply any air to the space and only cooling via cold duct will be provided. When cold duct reached its maximum capacity but still cannot meet the cooling demand the (hot) duct starts to operate as cold duct supplying cold air to the space. When the cooling demand is at its maximum both ducts will be carrying cold air at maximum volume and lowest temperature.

INDIVIDUAL SPACES In an individual space where there is one or more terminal units the temperature will be maintained by providing cold air when cooling is required and hot air when heating is required. Each terminal unit has two modulating dampers so the relevant damper will be opened and the other will be closed. The temperature is maintained by varying the volume of supply air into the space. The temperature is set centrally according to demand.

If the cooling or heating demand of an area exceeds the capacity of the terminal units serving it, a message will be transmitted to report such a state. The units can then be replaced with larger units.

PARTIAL OPERATION

When only part of the building is occupied the system can be instructed to isolate air supply and extract from unoccupied areas totally or partially. To this end the branch dampers will be used.

PRIORITIES

Some areas in a building can be given priority over all else. This means set points will be maintained in priority areas and any deviation is spread uniformly over the rest of the building.

Claims

CLAIMS :

1. An air conditioning system for use in a building arranged in a plurality of zones in each of which the air temperature can be separately controlled, comprising two air supply ducts from each of which either heating or cooling air can be supplied to each zone; an individual terminal unit in each zone to which the ducts are connected and controllable to determine the air flow rate into the associated zone at any instant; an individual temperature sensor in each zone and operative to provide control signals for the associated terminal unit; a central control means common to all of the zones and operative to control the temperature of the air supplied by each duct and the air flow rate in each duct in dependence upon signals sent to the central control means from each zone; and a communication network linking the central control means to each zone and over which signals are transmitted between the central control means and the zones to control operation of the system.

2. A system as claimed in Claim 1, in which the maximum air flow rate available from a duct or ducts at any zone is determined by control signals transmitted to the terminal unit at that zone by the central control means over the communications network in dependence upon the maximum air flow rate capability of the system.

3. A system as claimed in Claim 1 or Claim 2, in which each terminal unit comprises two modulating dampers which are controlled to determine the air flow rate into the associated zone from each of the ducts respectively.

4. A system as claimed in any preceding claim, in which each terminal unit has an air discharge diffuser having a variable air discharge area controlled in dependence upon the air flow rate from the duct or ducts supplying air to the terminal unit at any instant.

5. A system as claimed in Claim 4 wherein the area of the exit aperture from the terminal unit is variable.

6. A system as claimed in Claim 5 wherein the exit aperture has located within it a restrictor means which is oveable in dependence upon the air flow rate,

7. A system as claimed in Claim 6 wherein biased restrictor blades are located within the exit aperture.

8. A system as claimed in any preceding claim, in which each terminal unit has means to control the direction of air discharge therefrom into the associated zone.

9-. A system as claimed in any one of Claims 3 to 8, in which control signals for the modulating dampers and/or the air discharge diffuser and/or the air discharge direction control means at any terminal unit are derived from a local control means at the associated zone and receiving inputs from appropriate transducers in the associated zone.

10. A system as claimed in Claim 9, in which each local control means is in communication with the local control means at other zones and the central control means over the communications network.

11. A system as claimed in any one of Claims 1 to 8, in which all the necessary control signals are derived from the central control means.

12. An air conditioning system for use in a building arranged in a plurality of zones in each of which there is separate control of the environment, comprising individual flow control means to control the pressure and volume of air supplied to each zone, which flow control means are controlled by signals derived from pressure sensors in the associated zones to maintain the minimum necessary pressure in the air supply to the zone, and a common central control means to which the signals from the pressure sensors in each zone are transmitted, the central control means operating to control the air supply means of the system to maintain the air pressure in the system to that required to satisfy the needs of each zone.

13. A system as claimed in Claim 12, in which a similar flow control means is provided for the extraction of air from each zone, the signals transmitted to the central control means being dependent upon the rate of supply of air to the zone and the rate of extraction of air from the zone.

14. An air conditioning system as claimed in Claim 1 and as claimed in Claim 12.

15. An air conditioning system as claimed in any of Claims 1 to 14, further comprising a third air supply duct carrying air at a mean temperature.

16. An air conditioning system substantially as hereinbefore described with reference to the accompanying drawings.

17. A terminal unit for use in an air conditioning system for discharging air supplied to the unit by two ducts, including two modulating dampers operative to determine the air discharge rate from each of the ducts respectively.

18. A terminal unit for use in an air conditioning system for discharging air supplied to the unit by a duct or ducts, including an air discharge diffuser having a variable air discharge area controlled in dependence upon the air flow rate from the duct or ducts supplying air to the terminal unit at any instant.

19. A terminal unit as claimed in Claim 17 or Claim 18 wherein the area of the exit aperture from the unit is variable.

20. A terminal unit as claimed in Claim 19 wherein the exit aperture has located within it a restrictor means which is moveable in dependence upon the air flow rate.

21. A terminal unit as claimed in any of Claims 17 to 20, including means to control the direction of air discharge therefrom.

22. A terminal unit as claimed in any of Claims 17 to 21, in which the modulating dampers and/or the air discharge diffuser and/or the air discharge direction control means are controllable by control signals derived from an associated control means receiving inputs from transducers in the zone to which the air is being discharged.

23. A terminal unit for an air conditioning system, substantially as hereinbefore described with reference to the accompanying drawings.