Book - School of Science and Technology

622 Automatic controls and building management systemsControllerSensing device(temperature sensor)Control device(motorised valve)Primary flowand returnSecondary flowHeat exchangerSecondary returnFigure 22.1 Simple heat exchanger controlscontrol action to limit deviation from the set-point. An open loop system has no feedbackfrom the controlled variable; an example of this is given later in connection with heatingsystem controls.System typesThere are many classes of control system, but they may be grouped conveniently underheadings:. direct-acting. electric/electronic. pneumatic.The simplest form of controller is direct-acting, comprising a sensing element, say in aroom or in the water flow and which, by liquid expansion or vapour pressure through acapillary, transmits power to a bellows or diaphragm operating a valve spindle. The mostcommon example is the thermostatic radiator valve (TRV) which, when installed in thesupply pipe to a radiator, convector or other heat emitter (as in Figure 22.2), serves thepurpose of an individual room controller.Direct-acting thermostats have little power and their control band can be somewhatwide, although they have been considerably improved in recent years. Direct-actingthermostatic equipment gives gradual movement of the controlling device and thus maybe said to modulate. In order to reduce the extent to which the TRV has to close to controlthe output of the emitter under part load conditions, it is normal to reduce the temperatureof the heating water supplied to it under part load conditions.The most common control system is the electric/electronic, which may be found indomestic applications (thermostat and motorised valve) through to large commercial andindustrial installations. The basic functions in an electric system are switching andresistance variation. Switching may be achieved by closing metallic contacts, using atilting mercury switch, or a relay (electro-magnetic switch) which is an electrical holdingdevice that uses the magnetic effect produced by an energised/de-energised coil. Toproduce a variable signal, a resistance in a sensing device such as a temperature sensor,changes with the temperature sensed. This signal is processed by the controller which thenprovides the signal and power, for example, to provide movement of a modulating controlvalve or damper. The simplest control device is the solenoid valve consisting of a coilwithin which an iron `plunger' slides to give linear movement.Electronic controls operate at 24 volt or less and use smaller strength signals fromthe sensing elements; typically thermocouples or thermistors which have no movingparts. The circuitry incorporates amplifiers to magnify the signal or software to adapt