6.4 Process Water, Cooling Towers, and Equipment

Process Water Is Operational Water Use

Process water is water consumed by building equipment and operations rather than by occupants at plumbing fixtures. It includes cooling towers, commercial dishwashers, clothes washers, ice machines, food steamers, pre-rinse spray valves, and laboratory or manufacturing equipment. Because process loads are not in the indoor fixture calculation, LEED addresses them through separate appliance requirements and the Cooling Tower Water Use credit. The exam specifically expects you to treat a cooling tower as process water, never as a restroom fixture.

Cooling Towers and Cycles of Concentration

A cooling tower rejects building heat by evaporating water; the evaporated water leaves dissolved minerals behind, so the remaining water grows more concentrated. Cycles of concentration is the ratio of dissolved solids in the recirculating water to dissolved solids in the makeup water - effectively, how many times the tower reuses its water before that water must be discharged as blowdown (bleed-off). Higher cycles mean less makeup water and less blowdown, so more cycles equals more efficiency.

The LEED Cooling Tower Water Use credit awards points for maximizing cycles of concentration without exceeding limits for six key water-chemistry parameters (such as conductivity, chlorides, calcium hardness, and silica). The structure rewards higher cycles:

Makeup water is the fresh water added to replace evaporation and blowdown. Substituting nonpotable makeup (captured rainwater, condensate, reclaimed water) reduces potable demand, but the source must be treated so it does not foul the tower or violate chemistry limits.

Appliance and Equipment Efficiency

For non-tower process loads, LEED v4 requires named appliances and equipment to meet efficiency standards - for example, ENERGY STAR (or comparable) commercial dishwashers, clothes washers, ice machines, and steam cookers, and pre-rinse spray valves rated at or below a low gpm threshold. These requirements live in the Indoor Water Use Reduction credit's appliance and process section, separate from the FTE fixture math.

Worked Scenario

A data center cooling tower runs at 3 cycles of concentration and discharges large volumes of blowdown. An engineer adds conductivity controllers and side-stream filtration so the water can safely reach 6+ cycles within chemistry limits. Doubling the cycles roughly halves the makeup and blowdown volumes - a direct process-water saving that an efficient restroom fixture could never deliver, illustrating why category discipline matters.

How a Cooling Tower Loses Water

A cooling tower loses water four ways, and understanding them clarifies why cycles of concentration matter. Evaporation is the intended heat-rejection loss and is unavoidable. Drift is fine water droplets carried out in the air stream; high-efficiency drift eliminators capture most of it. Blowdown (also called bleed-off) is the deliberate discharge of concentrated water to keep dissolved solids in check. Leaks and overflow are unintended losses.

Only blowdown is directly controllable by raising cycles of concentration: the more times you safely reuse the water, the less you must bleed off and the less fresh makeup you must add.

Worked Numbers

If a tower runs at 3 cycles of concentration, the blowdown rate is roughly half the evaporation rate. Pushing to 6 cycles cuts blowdown to about one-fifth of evaporation, and reaching the 10 cycles that LEED rewards for the top credit point shrinks blowdown dramatically while slashing makeup demand. The constraint is water chemistry: as solids concentrate, scaling, corrosion, and biological fouling threaten the equipment, so LEED caps the achievable cycles by limits on six parameters - conductivity, chlorides, calcium hardness, silica, alkalinity, and the Langelier or comparable scaling index.

Common Traps

• Selecting efficient toilets does not solve a cooling tower problem; the correct lever is cycles of concentration or nonpotable makeup.
• A cooling tower involves both energy and water, but the WE credit scores water; energy is handled in Energy and Atmosphere.
• Pushing cycles too high without chemistry control causes scaling and corrosion, so LEED caps cycles by the parameter limits, not infinity.
• Drift is not the same as evaporation; drift eliminators reduce droplet carryover, while only blowdown is controlled by cycles of concentration.
• Process water often pairs with submetering so operators can confirm equipment performance over time.
• A pre-rinse spray valve is process water, not a lavatory fixture, even though it looks like a faucet - classify by function, not appearance.

When a stem names any equipment - tower, washer, dishwasher, steamer, ice machine, lab apparatus - default to the process-water category and reach for cycles of concentration, appliance efficiency standards, or nonpotable makeup as your levers, never the occupant-fixture FTE calculation.

Where Process Water Hides

Process water appears wherever a building does work with water rather than serving an occupant directly: heat rejection (cooling towers and chillers), food service (dishwashers, pre-rinse spray valves, ice machines, steamers), laundry (commercial clothes washers), and specialized loads in labs, data centers, and medical facilities. The exam tests recognition - if you can identify that a use is process rather than fixture-based, you will reach for the right lever (cycles of concentration, appliance efficiency, or nonpotable makeup) instead of a restroom-fixture answer.

A reliable tell is the presence of equipment in the stem; equipment plus water almost always means the process-water category, governed by separate appliance standards rather than the FTE occupant calculation.