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 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.

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.

This allows the surface of the element to maintain itself in equilibrium with the surrounding air. When the LiC£ is dry its electrical resistance is high and almost no current is drawn from the supply; hence there is little or no heating effect and the hygroscopic salt is able to absorb moisture from the air. This causes the electrical resistance to fall sharply and a significant current is drawn from the supply. The heating effect of this current tends to drive off moisture from the LiC£ and causes it to dry out so that its electrical resistance rises.

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