Because of the high incidence of damage in recent years to industrial refrigeration plants attributable to hot-gas defrosts, the recommended procedures are now more tightly prescribed. The critical times are at the initiation and at the termination of the defrost. The piping, valves and controls should be provided to achieve the following sequence.
Keep defrost gas mains free of liquid. Even before any defrost steps associated with the coil begin, the condition of the refrigerant in the defrost gas main from the machine room to the various coils should be addressed. These
lines may run through low-temperature spaces or even above the roof and may be cold at times of the year. The moderately high-pressure defrost gas is likely to continuously condense, and the resulting liquid can be troublesome when the defrost begins. These mains should be equipped with liquid traps (like the float drainer of Fig. 6.56) which continuously pass liquid to a lowpressure pipe or vessel.
Pump out the coil. Close the solenoid valve in the liquid refrigerant supply line to the coil. Keep the suction valve open and continue fan operation, which will boil liquid refrigerant out of the coil.
Start defrost. Stop the fan, close the suction valve, and open the valve in the defrost line, preferably in stages. This is one of the crucial moments, because any retained liquid in the defrost mains that has not been purged and liquid remaining the coil after the pumpout can be propelled around the coil, resulting in possible damage. Two parallel valves, one small and the other capable of handling the total defrost gas flow, may be controlled so that the small one allows a low flow to build up the pressure in the coil before opening the main valve.
Complete the defrost. Once the defrost is in progress, allow it to continue until all the defrost water has melted and as much as possible drained. Failure to allow the pan to drain adequately will cause a progressive problem with the undrained water freezing in the pan when refrigeration resumes. On the next defrost the thickness of ice increases until it surrounds the lower tubes and possibly crushes them. Close the defrost gas valve.
Slowly relieve the pressure in the coil. The bypass valve around the main suction valve, as shown in Fig. 6.48, is first opened to slowly bleed down the pressure so that high-velocity vapor does not rush into the liquid/vapor return line. A high flow rate of vapor could propel the liquid in the line against end caps, elbows or valves. Furthermore, high-temperature vapor contacting cold liquid could cause condensation shock which reverberates throughout the piping. The combination of the bypass and main valves is available in one body with the valve first opening to 10% of its capacity, then when the pressure differential has dropped to 150 kPa (22 psi) the main valve opens.
Open the solenoid in the liquid refrigerant supply line. When this valve is opened, the coil is once again refrigerating, but the fan is not yet started. The reason for the delay is to permit the coil to become cold and freeze droplets of water that cling to the surfaces during defrost. Were the fan started at this time, this water would be blown off the coil and into the space or on product where it would quickly freeze.
Restore fan operation. This final step completes the defrost sequence.