Q&A | Ammonia Leaks and Coil Defrost

Question: Is it good practice to put an air-cooling evaporator into a defrost cycle during an ammonia leak in the evaporator coil?

Answer: In order to answer this question, we need to have a basic understanding of the four types of defrost available: 1. Water defrost; 2. Hot gas defrost; 3. Electric defrost; 4. Air defrost.

Water Defrost
This type of defrost uses water as the medium for removing the frost from the coil surface. If a coil is put into water defrost during an ammonia leak in the coil, the water would help adsorb the ammonia, thus potentially reducing the smell of ammonia. However, the water would also introduce heat which would raise the pressure of the coil, thus increasing the instantaneous rate of the leak. I would not recommend this option.

Hot Gas Defrost
When using hot gas defrost, the liquid supplied to the coil is stopped and hot gas from the compressors is introduced to the coil, thus melting frost on the coil surface. In the event of a liquid ammonia leak in a coil, initiating hot gas defrost might help flush the liquid out of the coil and change the leak into a gas leak, which might be a better scenario, but still not a great scenario.

Electric Defrost
During electric defrost, an electric resistance heater is turned on to melt ice from the coil. Like in water defrost, this will increase the coil pressure, thus increasing the instantaneous rate of release, so I wouldn’t recommend this option either.

Air Defrost
In air defrost, the coil is basically shut down and allowed to thaw. I would recommend this procedure in the event of an ammonia leak although I think it is more helpful to say that “in the event of an ammonia leak in an air-cooling evaporator, it is best practice to shut the evaporator off”.

To summarize…during an ammonia leak in an evaporator coil it is important to shut off the supply of ammonia while not substantially increasing the coil pressure. Defrosting a coil during an ammonia leak has no inherent value, but shutting off the liquid supply to coil does.