A standard regulator is reset by manually turning the adjusting stem, which increases the spring pressure on top of the diaphragm. In an air compensated regulator, a change of pressure on top of the diaphragm is accomplished by introducing air pressure into the airtight bonnet over the diaphragm. As this air pressure is increased, the setting of the regulator will be increased. This will produce like changes of evaporator pressure and refrigerant temperature. The variations in air pressure are produced by the temperature changes of the thermostatic remote bulb placed in the stream of the medium being cooled as it leaves the evaporator.
Temperature changes in the medium being cooled over the remote bulb of the thermostat will cause the thermostat to produce air pressures in the regulator bonnet within a range of 0 to 15 lb. This will cause the regulator to change the evaporator suction pressure in a like amount. A more definite understanding of this operation is obtained by assuming certain working conditions for the purpose of illustration. In cases where a larger range of modulation is required, a three-to-one air relay may be installed. This will permit a 45-lb range of modulation (see Fig. 10-26).