Timed two-position control
An exact match between load and capacity does not generally exist for two-position control; for example, if the load is 50 per cent then, on average, the plant will be on for half the time and off for the other half. This would be so if the plant were on for 30 minutes in each hour, or if it were on for every alternate minute in each hour. But it is clear that a 30-minute on — off cycle would produce a much wider swing in the controlled variable than would a one — minute cycle. The lower the value of the room air temperature, then the longer the period for which the heater battery, say, must be on, in a small plenum heating installation. Thus, for large loads the ratio of ‘on’ to ‘off’ time would not be to one but, say, 0.8 ‘on’ to 0.2 ‘off’. Simple two-position control can be improved to obtain a timed variation of capacity, which gives a smaller differential gap, by locating a small heater element next to the temperature-sensitive element in the thermostat. If the ambient temperature is low, the heating element loses heat more rapidly and so takes a longer time for its temperature and the temperature of the adjacent sensitive element to reach the upper value of the differential gap which switches off the heater battery. Thus, the heater battery is on for longer periods when the ambient room air temperature is low.
Posted in Engineering Fifth Edition