What are the specific constraints of refrigerating compressors using ammonia as a refrigerant?

 specificity of a lubricant for refrigerating a compressor is that it permanently contains a small quantity of refrigerant. The lubrication must thus be good in the presence of this small quantity of refrigerant. In the case of ammonia, there is a total immiscibility between the lubricant and the refrigerant; it is thus enough for the lubricant to have a pour point lower than the temperature of the evaporator to make sure of its proper return to the compressor. 

Moreover, the lubricant is pulled in small quantities by the refrigerant and goes through the refrigerating installation. Therefore, it must first endure a long exposure to the pressure and temperature and so must have a good thermal stability in the presence of ammonia (it is necessary to avoid the formation of black sludge).