George Briley discusses how to design a "soft" defrost system.

Figures 1 and 2. With SDS defrost, the system is actuated automatically only when it is necessary. The defrost system can return the defrost liquid to the system’s intermediate pressure vessel or direct the defrost liquid into the wet suction.
In my last two columns, I have talked about defrosting ammonia refrigeration systems. The primary focus of these discussions has been on the use of the demand defrost concept, a method that makes certain a coil is defrosted with hot gas only when it is necessary; that is, when too much frost builds up on the coil, retarding the heat transfer process.

This hot-gas defrost approach generally is accepted as effective on systems with coils that produce up to about 20 tons of refrigeration (TR). However, personally, I prefer to use a soft defrost system (SDS) for coils that produce 15 TR and above. Figures 1 and 2 indicate the piping, valves and controls for an SDS system.

The major differences between the demand-defrost system and the soft-defrost system approaches are as follows:

First, the No. 2 hot-gas defrost solenoid valve (SV-HDG2) is sized at 20 percent to 25 percent of the size of the larger, No. 1 hot-gas defrost solenoid valve (SV-HGD1). The No. 2 hot-gas defrost solenoid valve is opened at the start of defrost procedure after the fans have been turned off and the coil has been pumped down. At about the same time, the No. 3 hot-gas solenoid valve (SV-HG3) is opened. As a result, the coil pressure increases to about 75 to 80 psig over a period of about 3 to 5 min. (The precise timing depends on the size of the coil.) The No. 1 hot-gas defrost solenoid valve then is actuated by the No. 2 pressure switch (PS2) to allow a full flow of hot gas to the coil. The hand expansion valves downstream of the hot-gas solenoid are employed to “fine tune” the hot-gas flow.

After the coil is defrosted -- usually in 10 to 15 min, depending on whether the coil is made of aluminum or galvanized steel -- the No. 1 and No. 2 hot-gas defrost solenoid valves are closed. Next, open the low temperature recirculated suction solenoid valve (SV-LTRS), which is sized at about 20 percent to 25 percent of the suction stop valve A. This allows the gas in the coil, which is at 75 to 80 psig, to expand gradually into the wet return low temperature recirculated suction (LTRS). This is a slow process, and it is that way on purpose. After about 3 to 5 min, hot-gas solenoid valve No. 3 is closed. The coil then can be brought down to temperature.

A thermostat (TS) set at 40oF (4.4oC) is employed to terminate the defrost sequence if the control system fails.

In general, the soft defrost system with a demand actuator can be employed for coils up to 100 TR that are installed in holding freezers. If the coils are used in a spiral freezer, an individually quick frozen (IQF) freezer or a blast freezer, the demand feature should not be used. The defrost sequence should only be initiated manually.

There are several reasons why SDS is the ultimate defrost system:

  • The system uses the least amount of energy possible for defrost.

  • Because the coils are offline for a minimum amount of time, the overall system efficiency increases.

  • The piping system is under less stress because there usually are fewer defrost cycles.

  • The SDS process should eliminate clatter and potential damage to stop valve A.

  • SDS reduces the potential for hydraulic shock, which can be initiated if the hot gas for defrost is not metered into the coil slowly. It saves drain pan damage, too.

  • SDS also minimizes the possibility of vapor-induced hydraulic shock, which can be initiated if the 80 psig vapor from the coil is introduced instantly into the low temperature recirculated suction (LTRS) line.

For all of these reasons, you might define SDS as a “safer defrost system” as well as a soft defrost system. So, if you have ammonia coils that make noise during hot-gas defrost, you might want to consider converting to SDS. If you decide to install this kind of system, be sure that the piping around and to the coil is sized in accordance with the “IIAR Ammonia Refrigeration Piping Handbook.”