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R717 Ammonia as Marine Refrigerant

The ammonia used for refrigeration systems based on the use of a compressor, condenser, expansion valve and an evaporator is dry (anhydrous) in that there is no water in solution with it. It has the chemical formula NH3 but as a refrigerant, it is coded with the number R717.

The good qualities of ammonia as a refrigerant have been offset by its toxicity, flammability and pungent odour, so that carbon dioxide and then CFCs (which replaced CO2) were used at sea in preference. Now that R12 is to be phased out in the short term and R22 at a later date, ammonia is being considered as a replacement, because despite its local harmful effects and disadvantages, it is ozone friendly.

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The upper and lower explosive limits for pure ammonia in air, are 27% and 16% by volume, respectively. With oil contamination, the latter may reduce to 4%. Long-term exposure to ammonia should be restricted to the current threshold limit value (TLV).

Exposure to higher concentrations of 1500 ppm will result in damage to body tissue and death may result at 2500 ppm. However, ammonia leaks are instantly detected at less than 10 ppm concentration by the pungent odour and this is a safety feature. Very few people can endure ammonia when its concentration exceeds the TLV. In the liquid form, ammonia causes chemical and frost burns. Corrosion of brass, bronze and similar alloys , occurs in ammonia systems if there is any water present. These materials are being avoided with steels being used instead.

Ammonia is highly soluble in water with which it forms ammonium hydroxide, a weak base. About 1300 volumes of ammonia can be dissolved in I volume of water at low temperature. However it is easily expelled by boiling. This action makes the vapour absorption refrigerator possible. Refrigerators, of this type do not require a compressor for operation, only a heat source.



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