Cooling Tower Makeup Water, Blowdown & Cycles of Concentration

1. The four water flows in every cooling tower

Water leaves an open cooling tower in three ways, and makeup water replaces all three:

• Evaporation (E): the water that actually does the cooling. Evaporating water absorbs heat from the water that stays behind. This is by far the largest loss, and you cannot reduce it without reducing the heat load.
• Blowdown (B): water drained on purpose. Evaporation leaves dissolved minerals behind, so some concentrated water must be removed and replaced with fresher makeup water to stop scale and corrosion.
• Drift (D): fine droplets carried out by the fan airflow. Drift eliminators keep this to a small fraction of a percent of circulating flow on a well-maintained tower.
• Makeup (M): the fresh water you buy: M = E + B + D.

2. Sizing basics: range, approach and tons

Range is the hot water temperature entering the tower minus the cold water temperature leaving it. Approach is the cold water temperature minus the entering air wet-bulb temperature. A tower can never cool water below the wet-bulb temperature, and a closer approach means a larger, more expensive tower.

The heat a tower rejects is:

The 500 comes from 8.33 lb per gallon × 60 minutes per hour × 1 Btu per lb per °F. One cooling tower ton is 15,000 Btu/h, more than a chiller ton (12,000 Btu/h), because the tower also rejects the heat added by the chiller compressor. The common rating point of 95°F hot water, 85°F cold water and 78°F wet-bulb at 3 gpm per ton gives a 10°F range and a 7°F approach.

3. The water balance formulas

The evaporation formula is a rule of thumb: about 1% of the flow evaporates per 10°F of range. On a hot, dry day nearly all heat leaves by evaporation; on a cool day more leaves as sensible heat, so actual evaporation is lower. That is why metered makeup and blowdown always beat the formula once you have them.

4. Worked example

A tower circulates 1,000 gpm with a 10°F range, runs at 4 cycles of concentration, and has drift eliminators rated at 0.002%.

That is at full design load. Over a year the load varies, so the calculator scales evaporation by your average load. At a 75% average load for 8,000 hours a year, the same tower uses about 4.08 million gallons of makeup water, of which about 1.01 million gallons is blowdown.

5. Why cycles of concentration matter so much

Evaporation is fixed by the heat load. Blowdown is the part you control, and it depends entirely on cycles. Here is the same tower at full load across a range of cycles:

Two lessons stand out:

• The first few cycles are worth the most. Going from 2 to 4 cycles cuts makeup by a third. Going from 8 to 10 saves almost nothing, while scaling risk keeps rising.
• Every gallon of blowdown you avoid is a gallon of makeup you don't buy. In the annual example, moving from 4 to 6 cycles saves about 408,000 gallons of makeup and about 408,000 gallons of sewer discharge each year.

6. Where the operating cost goes

A cooling tower's running cost comes from four places:

• Makeup water: priced on every gallon bought, including the water that evaporates.
• Sewer charges: often billed on water bought, even though evaporated water never reaches the drain. Separate makeup and blowdown meters can let you claim evaporation credits where the utility allows it.
• Fan energy: fan power falls with roughly the cube of fan speed, so a fan on a variable frequency drive at 80% speed draws about half of full power. Replacing single-speed or two-speed fan control with VFDs is one of the most common tower energy upgrades.
• Water treatment: scale inhibitors, corrosion inhibitors and biocides, plus the service contract.

Condenser water pump energy is also part of the system, but it is usually counted with the pumps or the chiller plant rather than the tower.

7. Tower types at a glance

• Open vs closed-circuit: in an open tower the process water itself is sprayed over the fill. A closed-circuit cooler (fluid cooler) keeps the process fluid inside a coil and sprays a separate water loop over the outside. Closed-circuit units keep the process loop clean but cost more for the same capacity.
• Crossflow vs counterflow: in crossflow towers air moves horizontally across falling water, and the gravity-fed hot water basins are easy to inspect. In counterflow towers air moves upward against the falling water, which usually allows a smaller footprint for the same duty.

8. Maintenance and Legionella

Warm, recirculating water can grow Legionella bacteria, and drift can carry it beyond the site. In the United States, ASHRAE Standard 188 calls for a documented water management program for buildings with cooling towers, and the CDC publishes a toolkit for building one. Routine tasks include:

• Inspecting and cleaning fill, basins and drift eliminators
• Checking that the conductivity controller and blowdown valve work and are calibrated
• Monitoring biocide levels and microbial counts on the schedule in your water management plan
• Repairing damaged drift eliminators, which raise both water loss and health risk

Frequently asked questions

Sources and further reading

• U.S. Department of Energy, Federal Energy Management Program (FEMP): cooling tower water-efficiency guidance
• Cooling Technology Institute (CTI): ATC-105 acceptance test code and drift and thermal performance standards
• ASHRAE Standard 188: Legionellosis risk management for building water systems
• CDC: Toolkit for developing a Legionella water management program