NH3 (Ammonia) as alternative refrigerant
The refrigerant NH3 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 that of R22, in some areas even more favourable; the contribution to the indirect global warming effect is therefore small. In addition, its price is exceptionally low. Is it therefore an ideal refrigerant and an optimum substitute for R22 or an alternative for HFCs?
NH3 has indeed very positive features, which can be exploited quite well in large refrigeration systems and heat pumps.
Unfortunately there are also negative aspects, which restrict the wider use in the commercial area or require costly and sometimes new technical developments.
A disadvantage with NH3 is the high isentropic exponent (NH3 = 1.31 / R22 = 1.19 / R134a = 1.1), which results in a discharge temperature even significantly higher than that of R22. Single stage compression is therefore already subject to certain restrictions below an evaporating temperature of around -10°C.
The question of suitable lubricants is also not satisfactorily solved for smaller plants in some kinds of applications. The most commonly used mineral oils and poly-alpha-olefins are not soluble with the refrigerant. They must be separated with complex technology and seriously limit the use of "direct expansion evaporators" due to the deterioration in the 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 is supposed to remain in the circuit for years without losing any of its stability.
NH3 has an extraordinarily high enthalpy difference and thus a very small circulating mass flow (approx. 13 to 15% compared to R22). This feature, which is favourable for large systems, makes the control of the refrigerant injection more difficult with small capacities.
Further to be considered is the corrosive action on copper containing materials; pipe lines must therefore be made of steel. This also hinders the development of motor windings resistant to NH3 as basis for semi-hermetic constructions. Another difficulty arises from the electrical conductivity of the refrigerant in case of higher moisture content.
Additional characteristics include toxicity and flammability, which require special safety measures for the construction and operation of such systems.
