The second-most-popular method of defrosting industrial refrigeration coils is by spraying water on the coil, although the method is a distant second to the popularity of hot-gas defrost. Water is sprayed over the coil and the mixture of water and melted frost is collected by the drain pan and passed outside the refrigerated space. There are some situations where water defrost is advantageous, and some where its use is not recommended. Spraying water on the coil when the objective is to remove water from the surface may seem incongruous, but the real goal is to attack the frost which is on the outside surface of the coil, and water defrost meets this objective.
Several advantages of water defrost in comparison to hot-gas defrost are: (1) inexpensive source of defrost medium, (2) short defrost time, and (3) provides a cleaning action on the coil. Water defrost may be appropriate when the refrigeration system is dedicated to only the coil(s) in question and the source of refrigerant vapor is limited for hot-gas defrost. Water defrost finds application in a variety of installations, including large low-temperature spaces, but one of the ideal applications is in a spiral freezer chamber operating in a production mode. In this application, the defrost must be rapid in order to return to production quickly. Furthermore, during the defrost and cleanup interval the temperature in the chamber usually rises above freezing temperatures, so the the presence of excessive water is not a problem. In a refrigerated warehouse, on the other hand, an increase of temperature in the refrigerated space is to be avoided.
Some design specifications and precautions surrounding water defrost are:
– the rate of defrost water should be between 1 to 1.36 L/s per m2 of coil face area (1–1/2 to 2 gpm per ft2).
– a water temperature of about 16°C (61°F) is an acceptable compromise. The higher the water temperature the more rapid will be the defrost, but a high water temperature also results in excessive fogging in the neighborhood of the coil. The rate of vaporization of water into fog is controlled by the water vapor pressure which in turn is a function of the water temperature. Furthermore the water vapor pressure is 3–1/2 times higher at 30°C (86°F) than it is at 10°C (50°F).
– the rate of water to be discharged is that of the melted frost plus the defrost water, so the quantity that the drain pan and drain lines must handle will be considerable more than with a coil defrosted by hot gas.
– the solenoid valve controlling the defrost water should be in a warm environment so that the water line will not freeze. Also, from the position of this valve, the piping should be sloped so that negligible water is retained in the line between the valve and the sprayheads at the coil.
– the pumpout phase to evacuate the coil of refrigerant first specified for hot-gas defrost is equally important for water defrost.