Book - School of Science and Technology

Systems controls 639where, in consequence, a simpler type of controller could not produce a stable conditionwithout the proportional band being wide beyond acceptable limits. In addition, thismode is used more generally for applications where close control is required.Proportional plus integral plus derivativeAbbreviated to PID, this mode of control would be used where there are sudden andsignificant load changes and where zero off-set from the desired set-point is required.There is seldom a case for such control in heating and air-conditioning work.Systems controlsCommon to all systems is the requirement, often by legislation, that they be started andstopped by some form of time switch. There are two basic types; the simple on/off switchand optimum start (and stop) control.On/off time switchSimple devices of this type are suitable for small systems, those where the number of daysof use is limited, such as church heating, or those where the thermal inertia of the buildingis so high that little practical purpose would be served by varying the system start-up timewith respect to external temperature. Time switches for other than the simplest domesticscale buildings would normally be of the 7-day type, enabling each day of the week to beprogrammed separately. Microprocessor based systems can provide for a complete year,including Bank Holidays, Daylight saving change over, etc., to be entered to memory atone time. To reduce the heat-up period, an additional feature to time switch control,known as fixed time boosted start, is the ability to run the system at maximum output for aperiod until either the desired internal space conditions are achieved or the end of a fixedpreheat period is reached, at which time the controls change to normal mode underthermostatic control.Optimum-start controlThis system serves to delay the start time of a heating or air-conditioning system until thelatest possible time to give the shortest preheat period, normally at full output, which willachieve the desired conditions at the start of the occupancy period. The system of controlmonitors the inside and outside temperatures, see Figure 22.23, and takes account of thethermal response of the building and the system. Modern equipment is self-adapting, inthat it can monitor its own performance (in achieving correct conditions at the start of theworking period) and take corrective action to improve these from one day to the next.Optimum-stop control is also available to switch plant off as soon as possible, consistentwith maintaining acceptable temperatures at the end of the occupied period. This is quitesatisfactory for heating systems, but there is concern over its application to all air airconditioningsystems where stopping the plant in advance of occupants leaving thebuilding also stops the supply of outside air to the space.The characteristics of the optimum-start principle are shown in Figure 22.24 fromwhich it can be seen that the switch `on' time is related to the fall in internal temperature.It also shows that the outside temperature affects the rate of fall of inside temperature,