Book - School of Science and Technology

Controller modes of operation 635FacedamperCoilBypassdamperFigure 22.17 Face and bypass controlabsorbed by the damper is high, due to the operating conditions of the plant, an additionalresistance, such as a perforated plate, may be incorporated in the bypass to increasethe authority of the damper.Another common application of control dampers is to modulate the rate of flowthrough a cooling coil to provide what is termed face and bypass control. This is mostcommonly used where high amounts of dehumidification are required of a cooling coilrelative to the sensible temperature decrease required across it. The principle is illustratedin Figure 22.17. In sizing the dampers, the same logic as above may be applied, to provideconstant pressure drop across the combination throughout the range of operation andhence not affect the flow rate in the system. As for outside/recirculating air dampers, thecharacteristics of the face and bypass dampers must complement one another. It wouldnormally be the case that the face damper would be the same size as the coil, but thebypass damper will usually be smaller than the associated duct.Controller modes of operationThere are various ways in which a controller can cause a control device to operate inresponse to a signal from a sensing device. The most common modes that may apply aredescribed here.Two-position controlA typical application is on/off switching, in which the sensing device, perhaps a simpleswitching thermostat, provides two signals, for example opening contacts below a setpointand closing them above a set-point. The device may be arranged to operate in theopposite way and this is then called reverse acting.The interval between the switching actions, an inherent characteristic of the device, isnormally referred to as the differential gap. Where a differential gap is too wide, anaccelerator (in the case of a thermostat, a heating element energised when the thermostatcalls for heat thereby anticipating the response) may be used to reduce the range, or swing,of the controlled condition. The swing between the controlled space temperature may begreater than the differential gap in the sensing device due to slow response of the controlsand the thermal inertia inherent in the building. Figure 22.18 illustrates the performanceof such a method of control.