A large industrial complex used water-cooled compressors to supply 115-psi compressed air to its processes. The cooling water temperature was quite low throughout the year and provided compressed air at lower-than-normal temperatures. This took moisture load off the air dryers, but these cool temperatures also caused problems.
The plant had been designed with redundant compressor capacity, with only one compressor running, while the second the second unit remained on standby as a spare. But early into the history of the system, there were problems identified. The cooling water circuit feeding the compressors was uncontrolled, so a flow of water continued to pass through the spare unit when it was turned off. Because of the cold temperatures, significant condensation occurred, causing damaging corrosion to the compressor cooling surfaces. A decision was made to keep the spare unit running to prevent this issue — rather than installing an electric control valve to automatically turn off the water when the compressor was down, a cost of about $500.
As a result, the spare compressor runs continuously, consuming 33 kW of continuous power, while producing zero compressed air. At 10 cents per kW, the electrical cost for this operation mode calculates to $28,900 per year. Also, the cooling water has to be processed and treated in the effluent system, adding costs on top of the additional maintenance needed because the compressor is accumulating running hours for no reason.
When shown the numbers by a compressed air auditor, the plant engineer quickly raised a work order to make corrections!