HVAC Direct-expansion systems

Figure 10-7 shows a high temperature system [above 32°F (0°C)] with no drip-pan defrost. During the normal cooling cycle, controlled by a thermostat, the room temperature may rise above the high setting of the thermostat. This indicates a need for refrigeration. The liquid solenoid (valve A), pilot solenoid (valve B), and the dual-pressure regulator (valve D) open, allowing refrigerant to flow. When solenoid (valve D) is energized. The low-pressure adjusting bonnet controls the regulator. The regulator maintains the predetermined suction pressure in the evaporator.

When the room temperature reaches the low setting on the thermostat, there is no longer need for refrigeration. At this time, solenoid valve A and solenoid valve D close and remain closed until further refrigeration is required.

The hot-gas solenoid (valve C) remains closed during the normal refrigeration cycle. When the three-position selector switch is turned to “Defrost,” liquid solenoid valve A and valve D, with a built-in pilot solenoid, close. This allows valve D to operate as a defrost pressure regulator on the high setting. The hot gas solenoid (valve C) opens to allow hot gas to enter the evaporator. When the defrost is complete, the system is switched back to the normal cooling cycle.

The system may be made completely automatic by replacing the manual switch with an electric time clock. Table 10-1 shows the valve sizes needed for this system.

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