Discussing gas detection, leak response, and responsibilities around refrigerant releases.

The veteran BBC journalist Alistair Cooke once remarked that people are willing to take advice from experts on every subject under the sun except politics, religion and football. For the refrigeration community, I think it’s fair to add defrost systems to that list.

The most common defrost system in industrial systems is to use “hot” gas. If you disagree with that statement, I refer you to the previous paragraph. But even within the hot gas camp, there are massive differences of opinion that put even the Packers vs. Bears debate in the shade. Some people are so one-sided in this regard that they don’t even realize that an alternative exists. To understand this, it is necessary to start with the basic physics behind hot gas defrosting.

The basic defrost process seals off an air cooler from the suction side of the refrigeration plant and opens it up to the discharge side, allowing higher pressure gas to flow into the tubes of the evaporator. This gas gives up its latent heat and condenses to liquid, adding the heat to the tubes and fins of the evaporator. This warms them up and, provided the gas pressure is high enough, they will get hot enough to melt any frost that has formed, allowing the melt water to flow down the drain.

The gas doesn’t need to be terribly hot. If it condenses at about 50°F (about 10°C), which equates to about 75 psig (about 5 bar G), for ammonia this will be enough to give a quick and efficient defrost. However this is where the arguments start.

The simplest and cheapest way to achieve this effect is to fit a pressure regulator to the outlet of the coil so that when the pressure reaches the set level (see above) the coil is vented to the wet suction line, pushing the condensed liquid back to the receiver. Fundamentalists are quick to point out that a pressure regulator can’t tell the difference between gas and liquid, so there is a risk (they would say a certainty) that a lot of uncondensed gas will also return to the receiver, imposing a big additional load on the compressor. They advocate a float valve instead of a pressure regulator so that only liquid can get past.

These two camps can be further divided into “top feeders” and “bottom feeders” depending on where they prefer the liquid refrigerant to enter the evaporator in normal operation, but both camps agree that liquid has to be drained from the coil before the defrost starts; otherwise, so they say, there is a grave risk of liquid hammer, or “condensate-induced shock,” to give it its Sunday name.

My preference is to ignore all that received wisdom and design the system to fill the coil with liquid at the start of the defrost and then blow gas into the bottom, allowing the bubbles to rise through the tubes, warming the liquid as they go. It’s true that this gives a slower start to the process, but it then provides a much more even heat across the whole coil, especially at the bottom, and generally doesn’t take longer to complete than the more common method. It’s amazing how often I hear “That’ll never work!” even though we’ve been defrosting that way since the 1970s. When it is done properly, it also completely eliminates the possibility of condensate-induced shock.