These are also referred to as constant-pressure or pressure-reducer valves (see fig. 108).

Because of their versatility, these valves are produced in a variety of types and sizes, and can be installed in different parts of the sealed system, then adjusted to the pressures desired. They can be used as high- to low-side bypass regulators, evaporator-pressure regulators, compressor overload protection, evaporator freeze-up protection, and also as an expansion valve for one-fourth- to three-ton-capacity room air conditioners, ice cream units, ice makers, water coolers, dehumidifiers, and central air conditioners.

These valves contain a diaphragm, control spring (FS1), seat, and the basic valve needle or ball. The control spring above the diaphragm moves the diaphragm down, this opens the valve. The opposing force is provided by low-side evaporator pressure (FE) and a constant body spring force (FS2), this moves the valve to close.

During the off cycle, evaporator pressure builds up and overcomes the spring pressure. This keeps the valve closed until the next cycle. Then the compressor quickly reduces evaporator pressure. When this pressure equals the control-spring pressure, the valve begins to open. The valve opens when evaporator pressure falls below the control-spring setting. This is the point, or setting, at which the valve opens. (See fig. 116.)

When it is used to control the evaporator temperature, it is mounted on the liquid line at the inlet of the evaporator to control the amount of refrigerant entering the evaporator. By operating only on the low-side pressure, it is activated when the evaporator pressure drops. At that time, the needle valve automatically opens and sprays refrigerant into the evaporator until the evaporator pressure rises to a predetermined point. This can only occur when the compressor is running and creating low pressure in the evaporator. When the compressor is stopped, and no more suction is applied to the low side, the pressure built up in the evaporator prevents
the needle valve from opening. This prevents the evaporator from being flooded (filled with refrigerant) during the off cycle.

In this system, the thermostat sensing bulb is attached to the outlet of the evaporator. When sufficient drop in temperature is sensed by the bulb, the contacts within the thermostat mechanism open, causing power to the compressor to be disrupted, and the cooling cycle stops (see fig. 117).

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