Mechanical Matters: Seeking Water Savings

  January 15, 2015

While most of the attention in water-conservation programs for institutional and commercial facilities goes to reductions in the amount of domestic water a facility uses, mechanical systems often offer the highest rate of return for retrofit programs. In a typical building, mechanical systems account for about 25 percent of total water use when they operate properly. But equipment failures and improper setup can dramatically increase this number.

To determine the potential savings of a retrofit program, managers need to identify each system that regularly uses water, including boilers, cooling towers, and once-through refrigeration-cooling systems. For each system, managers need to measure the water used by the system.

For cooling towers, technicians need to bleed off water at a certain rate to prevent the level of suspended solids from exceeding the manufacturer's recommended level. They also must add water to the system to replace water lost from drift from the tower.

Ideally, the system monitors losses and solids and bleeds off the correct amount of water. But in practice, that rarely happens. Bleed rates are typically set too high, or the makeup valve malfunctions, and too much water is added to the tower basin. The result is higher water use by the system.

Boilers also require bleeding off water to prevent the accumulation of solids. A bleed rate that is too high results in wasted water.

For both systems, technicians need to inspect the operation of the makeup and bleed systems, looking for obvious equipment failures. Test the water in both systems, and measure the rate at which water is added. If water rates can be reduced — particularly by adding automated monitoring systems — technicians can determine the amount of water the changes can save in a season.

For refrigeration systems with once-through cooling systems, technicians can identify the equipment the systems serve and the capacity of the systems installed. For each, they can measure the flow rate through the cooling system and determine the amount of time the flow occurs to estimate potential water savings. Even when these systems operate properly, they use large amounts of water. Malfunctions, such as valves that fail to close when water is not needed, make matters even worse.

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