Taking a closer look at reverse-cycle heat pumps in changing climates.

A couple of months ago, I mentioned that defrost methods are like baseball teams: everyone has a favorite and won’t hear a bad word said about it. In that sense, what I am about to say makes me feel like a Mets fan in a Yankees bar.

My preferred defrost method, when it is possible, is to swap the evaporator and condenser of the refrigeration plant and run it as a heat pump for a short time. There are lots of reasons why I like this—I will try to explain some of them here—but first here is an overview of what happens.

The plant is fitted with a four-port ball valve, located in the discharge line between the oil separator and the condenser with the wet return line from the evaporator to the low side receiver passing through it. By rotating the ball in this valve through 90°, the discharge gas can be directed in the reverse direction up the wet return line from the compressor to the evaporator.

The gas gives up its latent heat in the evaporator, turning to liquid which flows in reverse from the evaporator to the condenser, passing through an expansion device on the way and then evaporating in the condenser at low pressure. The evaporated gas passes back through the ball valve and into the low side receiver, and then on to the compressor suction.

I said that I prefer this “when it is possible.” It is best suited to a system with one evaporator and one condenser, although it is possible to serve multiple evaporators provided they all defrost at once. It is best with an evaporative condenser, although we have had very good success in recent years with air-cooled condensers. It can be done with water-cooled condensers, but this is a much trickier proposition and I wouldn’t recommend it.

The sequence of a good reverse cycle defrost is as follows. The compressors stop, and pressure is equalized between the high and low pressure sides of the system. It turns out that reverse cycle is easier through a small vent line, typically settling at about 5°F (about -15°C). This might take five minutes and then the ball valve is turned through 90°. Once the valve position is confirmed, the compressors start in sequence and the refrigeration capacity is gradually increased. The discharge pressure rises quite rapidly until frost on the coil starts melting, which slows down the rate of pressure rise. When the last of the frost has gone, the rate of pressure rise increases again and the defrost is terminated when the discharge pressure reaches a set level, usually about 70°F (about 20°C). At this point the compressors stop and the pressures equalize again, then the valve gets turned back through 90° and the system is ready to start refrigerating again.

The whole process, including pressure equalization and drain down, can be completed in less than 30 minutes. The compressors typically only run for a fraction of this time.

It is necessary to restrict the flow of liquid from the evaporator during defrost, but as the operating conditions are always more or less the same, this doesn’t require any sophisticated control and is usually done with a fixed orifice. It is very easy to set the defrost up to perform reliably, and it eliminates the two biggest problems with traditional hot gas defrost: pipe fractures caused by liquid hammer and leakage from the high to low pressure sides of the system causing gross inefficiency.

When all’s said and done, provided attention is paid to the small details, reverse cycle defrosting is easier than it looks.