Controls for Air-Conditioning



Control systems for air-conditioning installations are complex and varied.

A lot can depend on the control system sold by the control equipment manufacturer and also on how sophisticated the engineer wants the controls to be.

Dew point control is one basic method for air conditioning systems and an understanding of dew point is necessary.



Constant Dew Point Control


Constant dew point control for air-conditioning systems means that the supply air is maintained at a constant dew point. 

The supply air condition will therefore be at the same moisture content for both summer and winter conditions as shown on the psychrometric chart  below.





A typical air-conditioning system is shown below.





























Control Sequence


1.     In summer it is best to admit minimum outside air.

        Damper motors M1, M2 and M3 are adjusted by temperature sensor T1 which measures outside air temperature.

Minimum fresh air is usually about 20% of the total supply air quantity.

The actual minimum fresh air quantity can be calculated for high occupancy buildings based on statutory requirements and recommended levels.


2.     In summer dew point is controlled by thermostat T2 operating the cooling coil control valve V2 which will alter the amount of chilled water through the coil. Thermostat T2 has a set point say of 10oC and if there is a rise in outdoor temperature or return air temperature then T2 will sense this temperature rise and open the cooling coil valve V2 to admit more chilled water through the coil and maintain the thermostat set point of 10oC in this example.

If the outside air cools a little in summer, then thermostat T2 will eventually sense an air temperature lower than its set point and will reduce the amount of cooling by diverting more chilled water away from the cooling coil.


3.     In mid-season, when the outdoor air temperature is cool   enough ‘free cooling’ may be used.

This is probably when outside air temperature is lower than space temperature and is measured by sensor T1. 

If the outdoor temperature falls to say 21oC then sensor T1 will operate the damper motors to increase the amount of fresh air admitted to the system. 

The proportion of fresh air can be increased if the outdoor temperature falls to some outdoor temperature where 100% fresh air is an advantage for ‘free cooling’.


4.     In mid-season dew point will continue to be controlled by thermostat T2  operating the control valve V2  as for summer.


5.            In winter, with outside conditions well below that required in the conditioned space, valve V4 will maintain dew point control by thermostat T2.   

Valve V4 operates the heater battery.

The anti-frost preheater in the outside air supply will have thermostat T3 operating valve V3 to maintain a temperature of say 4oC.  

If the anti-frost preheater is electric then switches are used to control electric elements.


6.     If the amount of latent heat changes within the conditioned space then humidistat H1 can be arranged to reset thermostat T2 and to control valve V5 to introduce humidity to the air stream.

        The change in latent heat is a variation in dew point.

        The humidistat H1 can be placed either in the space or the return air duct.



7.     The final dry-bulb temperature leaving the plant will be controlled by thermostat T4 operating valve  V4  to admit heat to the heater battery.


8.     To provide for variation in sensible load in the conditioned space, and thus for adjustment of dry-bulb temperature leaving the plant, thermostat T5  placed either in the space or the return air duct may be arranged to reset thermostat T4 .





Controls for Refrigeration Systems



Air-Cooled Condenser Control


Air-cooled condensers are increasing in popularity because of the absence of water piping, also water does not come in contact with air as in cooling towers and their simple operation.

Their use in air-conditioning is usually limited to about 70kW total plant output.



The demand for cooling varies widely throughout the year and even varies widely from morning to afternoon to evening in warm summer days.

This variation in demand can be met by good control of each item of refrigeration and air-conditioning plant.


It is relatively easy to control fan speeds or more often switch on and off fans in systems with several roof-mounted air-cooled condensers in accordance with the cooling requirement.


It is always desirable to keep a stable condensing pressure with reciprocating compressors, otherwise in mild periods when the refrigeration plant is on, the condensing pressure will be too low to meet a reduced demand for air-conditioning.





Controls for Lighting Systems


Lighting systems may be automatically controlled resulting in annual electrical energy savings.

In modern offices for example the lighting level may be about 25 Watts per metre square floor area.

If this can be reduced even by a small percentage significant savings can still result.


One typical method of control is to switch off the row of lights near windows on the external walls.

This is achieved with the use of a photo-cell so that when the required lighting level can be maintained by daylight ( about 400 - 700 lux) a row of light fittings may be automatically switched off. 


One of the problems associated with switching off lights is that even light distribution is no longer achieved and although the average lighting level may be adequate in a room there may be a noticeable difference in lighting levels in some areas.


Another system of lighting control measures room occupancy and lights are switches on only when the room is occupied. 

Movement sensors or a door switch can be used to indicate if the room is occupied or not. 

A typical installation would be a toilet with a door switch so that when the door opens the lights are automatically switched on (and if necessary the ventilation also).

A time delay built into the system will avoid frequent switching.

PIR’s (Passive Infra-red Sensors) act as movement and occupancy sensors and are useful in offices, classrooms where occupancy is variable.