Refrigerant NH3 (ammonia) has been used for more than a century in industrial and larger refrigeration plants. It has no ozone depletion potential and no direct global warming potential. The efficiency is at least as good as R-22. In some areas, the efficiency is even more favorable. Its contribution to indirect global warming is therefore small.
There are some negative aspects that restrict the wider use in commercial areas or require costly and sometimes new technical developments. NH3 has a high isentropic exponent (NH3 = 1.31, while R-22 = 1.18) which results in a discharge temperature that is higher than that of R-22. Single-stage compression is therefore already subject to certain restrictions below an evaporating temperature of around -10°C (16°F).
There is also a question of suitable lubricants. The oils used previously were not soluble with complicated technology and seriously limit the use of direct expansion evaporators due to the deterioration in heat transfer. Special demands are made on the thermal stability of the lubricants due to the high discharge gas temperatures. This is especially valid when automatic operation is considered, where the oil should remain for years in the circuit without losing any of its stability.