The steady-state condition is rare and, in general, the design capacity of a system will exceed the load because loads change continually. The state maintained in the conditioned space will not stay constant if plant capacity is uncontrolled.
As an example, consider the plenum ventilation system illustrated in Figure 13.1. When the outside temperature rises above its design winter value fabric heat losses will diminish and the heater battery capacity will be too great, with a consequent rise in room temperature. To keep this at a nominally constant value four things must occur. Firstly, it must be measured. Secondly, the change in temperature must be used to send a signal to the heater
Fig. 13.1 Closed loop system.
Battery. Thirdly, the strength of the signal must produce a matching change in battery output. Finally, the time taken for all this to happen must not be so long that further load changes occur in the meantime and the battery output becomes significantly out of phase with the load.
Note that a temperature change must be measured if the battery capacity is to be altered, implying that temperature cannot be kept constant. Such a deviation is inherent in control systems but a sustained deviation (offset) can be prevented, with a lapse of time, in certain refined systems, described later. Several methods are adopted for the signal transmission but its strength must be related to the measured deviation so that the capacity of the regulating device at the battery can gauge its response to this.
The principle of using an observed deviation in the load to give a corrective response in capacity is termed ‘negative feedback’ and a control system using this principle is called a ‘closed loop’. Another form of control system is an ‘open loop’, which does not use negative feedback but, instead, regulates capacity in a pre-arranged manner. For example, since fabric heat loss is proportional to the difference between room and outside temperature, and since heater battery capacity is related to the flow temperature of the hot water, it is possible to reduce the value of the latter as the outside air temperature rises, according to a calculated schedule. Room temperature could then be kept constant without actually having been measured. Such a system (termed ‘compensated control’) works as an open loop. An open loop system usually contains sub-systems which are themselves closed loops, in order to work properly.
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