Pumping cost per day per gpm = $.033 The pumping cost per gpm has increased 65% in addition to the capital costs, not a very efficient modification. Before making the pumping change, it may be possible to reduce frictional energy losses. The existing distribution system should be traced and pressure drop calculations made for sections of the system that appear to have high pressure drops. Maybe a short section of pipe could be replaced with a larger size. Maybe the proposed tie-in point for the cooling water to the new process could be moved closer with a small increase in piping costs, but a significant lowering of frictional energy losses.

Another possible way to cut energy usage is to limit cooling water flow through the exchangers. It is doubtful that the operating procedure covered this aspect. If flow is not throttled, the flow through an exchanger is determined by the AP available from the pumping system and the frictional energy losses in the exchanger and piping. For example, an unthrottled flow showed 8 psi across the exchanger. Design flow was for 800 gpm with a 5 psi drop across the exchanger. Since flow is approximately proportional to the square root of the pressure drops, the flow rate is 800 or 1000 gpm. An inexpensive type butterfly valve with a manual lock positioner could be installed to throttle the flow to 800 gpm, saving 200 gpm of cooling water.

If a cooling water system operated at 6000 gpm and 50 psig before the exchanger flows were throttled and 5000 gpm at 55 psig after the throttling, how much energy was saved? Let us assume there is an improvement in efficiency from 0.50 to 0.52.

Horespower before change

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