Single-pass or once-through cooling systems provide an opportunity for significant water savings. In these systems, water is circulated once through a piece of equipment and is then disposed down the drain. Types of equipment that typically use single-pass cooling include CAT scanners, degreasers, hydraulic equipment, condensers, air compressors, welding machines, vacuum pumps, ice machines, x-ray equipment, and air conditioners.
To remove the same heat load, single-pass systems require 40 times more water than a cooling tower operated at five cycles of concentration. To maximize water savings, single-pass cooling equipment should either be modified to recirculate water or, if possible, should be eliminated altogether.
Operations and Maintenance
To maintain water efficiency in operations and maintenance, federal agencies should:
- Provide proper insulation on piping, chiller, or storage tanks.
- Inventory cooling equipment and identify all single-pass cooling systems.
- Check entering and leaving water temperatures and flow rates to ensure they are within the manufacturer's recommendations. For maximum water savings, water flow rates should be near the minimum allowed by the manufacturer.
- Keep coil loops clean to maximize heat exchange with the refrigerated enclosure.
- Check the operation of the water control valve. Water control valves adjust the flow rate of water based on demand. Regular valve maintenance ensures water is used as efficiently as possible.
The following retrofit options help federal agencies maintain water efficiency across facilities.
- To maximize water savings, eliminate single-pass cooling by modifying equipment to operate on a closed loop that recirculates water instead of discharging it.
- If modification of equipment to a closed-loop system is not feasible, add an automatic control to shut off the entire system during unoccupied night or weekend hours. This option should be considered only when shutdown has no adverse impact on indoor air quality.
- Installation of a chiller and cooling tower or an air-cooled chiller may also be an economical alternative. Excess usable cooling capacity may already exist within the building.
- Find another use for the single-pass effluent in boiler make-up supply or landscape irrigation and implement. Some equipment effluent, such as degreasers and hydraulic equipment, may be contaminated. This effluent must not be used in boilers.
The following replacement options help federal agencies maintain water efficiency across facilities.
- Replace water-cooled equipment with air-cooled equipment or best available energy/water-efficient technology.
- Replace the once-through cooling systems with a multi-pass cooling tower or closed loop system.
- Consult with experts in the field. Your first resource should be local or headquarters engineers, but do not overlook input from experienced contractors or other government agencies.
For more information, read the U.S. Environmental Protection Agency's WaterSense at Work BMP on single-pass cooling.