Optimizing Defrost Systems, Part 2
Failure to defrost an ammonia refrigeration system will reduce the system's efficiency. At the same time, defrosting should not be performed too frequently because that also will reduce the system's efficiency. That means the duration of the defrost process, as well as the frequency, should be kept to a minimum. Defrosting only should take place as often as is necessary to keep the coils clean.
In part one, I talked about the value of demand defrost, which is actuated by a pressure device that measures the air pressure drop across the coil, a good way to minimize daily defrost time. Using this method, the coil is defrosted automatically only when it is necessary. This initiation system, plus the addition of a float drainer to direct the liquid formed during the defrost to an intermediate vessel, comprises what many consider to be the most efficient system available. Figures 1 and 2 give you details of this kind of system.
So, what's the best way to design a hot gas defrost system? There are several different approaches. Figures 1 and 2 show a typical demand defrost system for both upfeed and downfeed coils. This design returns the defrost liquid to the system's intermediate pressure. An alternative is to direct the defrost liquid into the wet suction. It is much better to employ a float drainer or thermostatic trap instead of the relief regulator and install a hot gas regulator at the hot gas inlet to the coil. This system uses a photo-helix pressure switch for measuring the air pressure drop across the coil and in turn actuating the defrost. It uses a thermostat to terminate the defrost cycle. And, just in case, a timer is put into the system as a backup to make sure the defrost terminates.
Another thing I should talk about is the design of hot-gas piping. Basically, there are two approaches to supplying the hot gas to the evaporators:
- Install a pressure regulator in the equipment room at the receiver, set at approximately 100 psig, and size the piping accordingly.
- Install a pressure regulator at each evaporator and size the piping for minimum design defrost pressure, which should be 75 to 85 psig.
In both cases, be sure to refer to the IIAR Ammonia Refrigeration Piping Hand-book for guidance on sizing hot-gas piping to coils. While both options will adequately do the job, I think the first option is the better of the two because it offers more convenience for adjusting and fine-tuning the defrost system.
For example, consider a defrost cycle actuated by a pressure sensor at a predetermined setting of approximately 1" w.c. The steps in the defrost cycle include:
- De-energize the recirculated liquid line solenoid valve (SV-LTRL) and stop the fans.
- Delay energizing the hot-gas defrost solenoid valve (SV-HGD), closing the gas-powered automatic shutoff valve (A). The coil will defrost.
- The coil defrost period is determined by the thermostat (TS), which is set at 40oF (4oC). The override timer should be set at 30 min to shut down the hot-gas solenoid valve (SV-HGD) if the thermostat (TS) malfunctions.
- Turn off the hot-gas solenoid valve using the thermostat that terminates the hot-gas defrost cycle (TS) by opening the gas-powered automatic shutoff valve (A).
- Power up the recirculated liquid line solenoid valve (SV-LTRL) and wait 2 to 4 min.
- Start up the fans.
- Return the liquid to the intercooler or intermediate temperature recirculator to save energy; alternatively, return the liquid to a wet suction line downstream of a stop valve.
This concept is based on the assumption there is a hot-gas outlet-pressure regulator in the equipment room. If high pressure gas is used, an outlet pressure regulator with an electric shutoff should be substituted for the hot-gas defrost solenoid valve and set at 75 to 90 psig.