The most widely used methods of defrosting industrial refrigeration coils are using: air, electric, water, and hot gas.
Air defrost may take several different forms. In refrigerated spaces operating at temperatures above about 2°C (36°F), F), it is possible to shut off the supply of refrigerant to the coil and allow heat from the space to melt the frost. An extremely long period is required unless the fan continues to operate, forcing air through the coil. Even so, the process may be slow, so the designer must insure that the capacity of operating coils is adequate to meet the refrigeration load. Another form of air defrost that was used in the past isolates the coil and the chamber, which is then supplied with warm air from an external source that blows over the coil to defrost it.
For defrosting electrically, an electric resistance heater is mounted in good thermal contact to the coil. One design that achieves penetration of the heat into the inner sections of the coil is through the insertion during coil assembly of cylindrical heating elements that become dummy tubes. The first cost of electric defrost is probably the lowest of any method, but the operating costs may be the highest. The reason for the high cost is the expense of the electricity in comparison, for example, with hot-gas defrost where the defrost gas is a byproduct of the system operation.
Electric defrost is more common in smaller commercial coils than in large industrial coils, but electric defrost is often adaptable to flooded coils built of aluminum. The sequence of defrost is to first close a valve located between the surge drum and liquid leg, and also close the solenoid valve in the liquid supply line to the surge drum. The fan continues to operate, which supplies heat to the evaporator and evaporates or pumps down the coil. When most of the liquid ammonia is removed, the suction valve is closed, which isolates the evaporator. The heater is then turned on to defrost the coil. When the defrost has been completed, the valve in the liquid leg and the suction valve are opened. The coil then becomes cold and freezes any drops of water that have not drained off. Thus, water is not blown off the coil when the fan resumes operation. Complete refrigeration operation resumes when the fans start and the liquid line solenoid valve opens.
Water defrost and hot-gas defrost are the most important methods, and each will be discussed later in their own sections.
Whatever defrost method is used, the drain line carrying defrost water from the coil to outside the space should be trapped, as in Fig. 6.45. Were no trap installed, cold air from the space could be blown to the outside (in the case of a blow-through coil), or warm, moist air from the ambient could be drawn into the space in the case of a draw-through coil. Each defrost water drain should be trapped individually.