HVAC Bellows-type thermostat
On modern condensing units, low-pressure control switches are largely superseded by thermostatic-control switches. A thermostatic control consists of three main parts: a bulb, a capillary tube, and a power element or switch. The bulb is attached to the evaporator in a manner that ensures contact with the evaporator. It may contain a volatile liquid,
such as a refrigerant. The bulb is connected to the power element by means of a small capillary tube (see Fig. 14-13).
Operation of the bellows is provided by a change in temperature. Or the operation of the thermostatic-control switch is such that, as the evaporator temperature increases, the bulb temperature also increases. This raises the pressure of the thermostatic-liquid vapor. This, in turn, causes the bellows to expand and actuate an electrical contact. The contact closes the motor circuit, and the motor and compressor start operating. As the evaporator temperature decreases, the bulb becomes colder and the pressure decreases to the point where the bellows contracts sufficiently to open the electrical contacts, thus turning off the motor circuits. In this manner, the condensing unit is entirely automatic. Thus, it is able to produce exactly the amount of refrigeration needed to meet any normal operating condition.
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