Book - School of Science and Technology

624 Automatic controls and building management systemsto the item being controlled, in order to reduce wiring (power and/or controls) costs. Thisallows the information for the operators to be centralised by the data bus.Less popular these days are pneumatic controls. The compressed air for these, if derivedfrom a central supply, is usually taken through a reducing valve at a gauge pressure ofabout 100 kPa. If the supply were provided independently, a small air compressor andstorage cylinder is required, normally duplicated, automatically maintaining a constantgauge pressure of perhaps 400 kPa in order to give a good storage of air for suddendemands and supplying again through a reducing valve.Compressed air for pneumatic controls must be provided from oil-free compressors,preferably duplicated, and air coolers and driers, also in duplicate, must be fitted at thedischarge to a storage vessel and pressure reducing valve. Piping must be run with great carein order to provide adequate drainage of the water which will condense out of the air: theoperation of the control system would be upset by any water or oil carry-over and traps mustbe provided at all low points. In consequence of these necessary precautions, it will beunderstood that it is very unlikely that a supply from an industrial compressed air plant willbe suitable for service to a control system. Where plant and control devices are distributedaround a site, a central compressor plant may be used, providing a high pressure air supplyto an air storage vessel and pressure reducing valve at each plant room location.Air is supplied to each sensing device and by the action of some form of mechanicalmovement from it, variable amounts of controlled air bleed away thereby changing the airpressure in the supply line which is the signal transmitted to the control device. Pneumaticsystems would in modern practice be considered only for industrial plant or when eithertight closure or large actuator forces (say for rapid closing or opening) are required. Thisis due to the inherently safe `stall characteristics' of pneumatic actuators which, unlikeelectrical or electronic ones can safely operate at full power against a load for long periodsof time.Other forms of control system operation such as hydraulic, using water or oil, are notused today in building services work.Sensing devicesSiting of sensing elements is critical to the achievement of good control. In pipework orductwork, sensors must be so arranged that the active part of the device is immersed fullyin the fluid and that the position senses the average conditions. Where necessary, averagingdevices serpentined across the full cross-section of a duct should be used. Sensorsshould be protected from the radiant effects of local heat exchangers, such as heaterbatteries in an air handling unit.In sensing space conditions, the device must not be in direct solar radiation or belocated on a surface not representative of the space conditions such as on a poorlyinsulated outside wall. Local effects from heat sources, radiators or office equipmentfor example, will also give unsatisfactory results. Where necessary aspirated sensorsshould be used to improve sensitivity to air temperature.TemperatureThermal expansion of metal or gas or a change in electrical characteristics due totemperature variation are the common methods of detection. Figure 22.3(a) shows asimple bi-metallic type thermostat having closing point contacts; a two-wire type would,