Ammonia is liquefied in a steady-state process by first compressing the gas and then cooling until all the ammonia condenses. The cooling takes place in a heat exchanger using water at 90 °F. The ammonia enters the compressor at 80 psia and 80 °F. The efficiency of the compressor is 80%.

a) What is the minimum pressure that we must have at the exit of the compressor in order to achieve liquefaction? 

b) Calculate the amount of work in the compressor. 

c) Calculate the amount of heat that must be removed by the cooling water.