Automatic Controls for Heating and Air Conditioning. by K. M. Letherman, N. S. Billington

By K. M. Letherman, N. S. Billington

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The solenoid operates on a sliding armature which is linked directly with the valve plug. These actuators are suitable only for the smaller sizes of valve and with limited pressure drop. They tend to be noisy and hence are not suitable for domestic applications. 28 29 Applications Motor-operated valve actuators are relatively slow in operation, taking up to 2 minutes to drive over their full range. This is not a practical disadvantage for most applications. 2. Pneumatic actuators, or pneumatic motors as they are also called, consist usually of a circular diaphragm on one side of which the control pressure is applied either directly from a controller or via a positioner (see Fig.

The calibration varies with time as atmospheric dust collects on the hair. The hair element should be cleaned and the hygrometer recalibrated at regular intervals. The change in length with relative humidity is not uniform, being more marked at low values of relative humidity, as shown in Fig. 18. In indicating instruments, non-uniform scale markings are used. In recording instruments such as thermo-hygrographs a non-linear mechanism is used so that uniform scale graduations can be employed. It has been found that if the hairs are rolled between steel rollers so that the cross-section is flattened, the total extension can be increased to about 4% of the original length.

Main components in the compressed-air-supply system. reservoir at a pressure of about 5 bar. The compressor is controlled by an on/off pressure switch with a wide hysteresis (about 2 bar) so that the compressor is not switched on too frequently. The air is fed to a second filter and drier unit with a moisture trap which extracts any liquid water which has condensed in the air and 45 46 Automatic Controls for Heating and Air Conditioning would be carried along with it. 2 bar by a pressure-reducing valve and fed to a pipe manifold from where it can be distributed by narrow-bore copper or plastic pipework to the various controllers and sensors.

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