Ammonia is one of the oldest and thermodynamically most efficient refrigerants in the history of refrigeration. Because its latent heat of vaporisation is very high, little refrigerant circulates for the same capacity; compressor efficiency is good, leaks are noticed instantly by the sharp smell, its effect on the ozone layer and climate is zero, and it is cheap. This is why meat plants, dairies, ice rinks, large cold storage warehouses and blast freezing tunnels run on ammonia.
Its safety class is B2L: toxic and mildly flammable. Its smell is detected in air at 25 ppm, and above 300 ppm it is dangerous. For this reason ammonia systems are installed in a machine room separate from the refrigerated area, and heat is carried to the room side by a glycol or CO₂ secondary circuit or by pumped systems; gas detection, emergency ventilation and trained operating staff are mandatory. It is incompatible with copper; all pipework is steel or stainless steel.
Ammonia is not suitable for small and medium cold rooms; its investment and operating requirements are high. In plants above 500 kW that run continuously and where energy cost is decisive, however, it is usually the most economical solution. In recent years low-charge ammonia chillers and ammonia/CO₂ cascade systems have been reaching medium-sized plants as well.