I am working on an indirect ammonia chiller that is used to cool the ice in an arena. The system consists of 3 reciprocating compressors, an evaporative condenser with a vfd on the fan, a float throttling valve and a plate heat exchanger acting as the evaporator.
With one compressor operating, the condensing pressure of the ammonia ranges from 140 psi to 180 psi depending on the speed of the fan. When the condensing pressure is higher, the compressor is doing more work. When the fan speeds up, the head pressure lowers which corresponds to a drop in compressor work.
I know that lowering the condensing pressure reduces compressor work and am familiar with the pressure-enthalpy chart that is used to describe the refrigeration cycle. My question is, how does increasing the fan speed cause a drop in compressor work? The fan kicks on and rejects more heat from the gas, but what is the process or mechanism that occurs as you follow the refrigeration cycle that results in the compressor lowering the condensing pressure?