Cooling Towers for Chiller Plants

In a water-cooled chiller plant, the cooling tower is the last step in the heat rejection chain. Every BTU the chillers pull out of the building, plus the energy their compressors add, leaves through the tower. This page is written for the engineers who design and maintain those plants. It covers heat rejection loads, condenser water design conditions, open versus closed loop arrangements, and which CTS towers fit common chiller sizes.

Where the tower fits in the chiller plant

Building or process load → chilled water loop → chiller evaporator → compressor → chiller condenser → condenser water loop → cooling tower → atmosphere.

The chiller moves heat from the chilled water loop to the condenser water loop. The tower then rejects that heat to the air through evaporation. Tower performance sets the condenser water temperature, which sets the chiller's lift, which drives most of the plant's energy use.

Calculating heat rejection load

The tower must reject more heat than the chiller's cooling capacity, because the compressor's energy also ends up in the condenser water.

For electric centrifugal and screw chillers, a common planning factor is 1.25. That is why one nominal cooling tower ton equals 15,000 BTU/hr (12,000 × 1.25). In practice, a 300-ton electric chiller needs about 300 nominal tons of tower.

• High-efficiency chillers reject a little less, and older or part-loaded machines can reject more. Use the manufacturer's rated condenser heat rejection when you have it.
• Absorption chillers reject far more heat, roughly twice their cooling load or more for single-effect machines. Size the tower from the absorber/condenser heat rejection on the chiller data sheet.
• Add other loads on the condenser loop, such as heat exchangers, compressors, or process equipment.

Condenser water flow

The 500 comes from 8.33 lb per gallon × 60 minutes. At a 10°F range, 15,000 BTU/hr per ton works out to 3 GPM per ton.

Condenser water design conditions

Design tradeoffs

• Lower condenser water temperature saves chiller energy. Chiller efficiency typically improves by roughly 1–2% per °F of lower entering condenser water, depending on the machine. A larger tower or a tighter approach costs more up front but pays back on the chiller.
• Low-flow design saves pump energy. At 2.0 GPM/ton, condenser pumps and piping shrink. The tower sees hotter entering water, which it handles efficiently, but the chiller runs at a higher condensing temperature. Evaluate the whole plant, not one component.
• Minimum condenser water temperature. Many chillers have a minimum entering condenser water temperature, especially at startup. Use fan control and a tower bypass line to stay above it. See our piping and bypass line guide.
• Fan control. VFDs on tower fans hold the condenser water setpoint with much less energy than on/off cycling, since fan power varies roughly with the cube of speed.

Open loop vs closed loop

CTS supplies plate and shell-and-tube heat exchangers and the LCC closed-circuit cooling tower for closed-loop designs. For an example, see our T-2400 with a plate heat exchanger in Key Biscayne, Florida.

Freeze protection and winter operation

Plants with waterside economizers run their towers in cold weather. Plan for basin heaters, drain-down or indoor sumps, and fan control to prevent icing. See cooling tower freeze protection and basin immersion heaters.

Multiple chillers and redundancy

• Dedicated towers (one per chiller) keep controls simple and make each chiller-tower pair independent.
• Common header designs let any tower serve any chiller. Add isolation valves and basin equalizer lines so water levels stay balanced.
• N+1 towers let one unit come offline for service. Three towers sized so any two carry the load give N+1 with only 50% extra capacity.
• Run all towers at low fan speed rather than fewer towers at full speed when water flow allows. It usually saves fan energy and uses all the fill.

For mission-critical plants, see cooling towers for data centers and cooling towers for hospitals.

CTS towers for common chiller sizes

CTS nominal tons already include the 1.25 heat-of-rejection factor, so for electric chillers at standard conditions, tower tons roughly equal chiller tons. Confirm the selection at your site's design wet bulb.

See the full Model T-2 line. For rectangular crossflow designs, see the LRC-N and the CTI-certified RLC. To compare designs, read counterflow vs crossflow cooling towers.

Worked example: 400-ton chiller plant

A building has two 200-ton electric centrifugal chillers, designed at 3.0 GPM/ton and a 10°F range.

1. Heat rejection: 400 tons × 15,000 = 6,000,000 BTU/hr.
2. Condenser flow: 6,000,000 ÷ (500 × 10) = 1,200 GPM (600 GPM per chiller).
3. Design conditions: 95°F to 85°F. If the site's design wet bulb is 75°F, the approach is 10°F, which matches CTS ratings.
4. Options:
   • One T-2400 (6,000,000 BTU/hr) on a common header. Lowest first cost, no redundancy.
   • Two T-2200s (3,000,000 BTU/hr each), one per chiller. Simple controls; losing a tower limits one chiller.
   • Three T-2200s on a common header. Any two carry the full load, giving N+1.
5. Check the wet bulb: if the design wet bulb is higher than 75°F, the approach tightens and a larger tower may be needed. Send us your conditions and we will confirm the selection.

Why FRP for chiller plant towers

Condenser water is treated, warm, and aerated around the clock, which is hard on galvanized steel. CTS casings and basins are FRP, which does not rust, and carry a 20-year warranty for U.S. orders. Factory-assembled towers also install in hours or days, not weeks.

Related reading: FRP vs galvanized steel, 20-year total cost of ownership, factory-assembled vs field-erected, how to size a CTS cooling tower, and emergency cooling tower replacement.

Frequently asked questions

How do I size a cooling tower for a water-cooled chiller?

Start with the heat rejection load: the chiller cooling load plus compressor energy. For electric chillers this is about 15,000 BTU/hr per ton of cooling, which is one nominal cooling tower ton. Then set the condenser water flow (commonly 3 GPM per ton), the range, and the design wet bulb for your site, and select a tower that meets that duty.

What are standard condenser water design conditions?

AHRI 550/590 rates water-cooled chillers at 85°F entering condenser water and 3.0 GPM per ton. In practice, that means about 95°F water leaving the chiller and 85°F water returning from the tower, a 10°F range. CTS ratings use 95°F / 85°F / 75°F wet bulb.

Should a chiller plant use an open or closed cooling tower?

Most chiller plants use an open tower piped directly to the chiller condenser, because it is the lowest-cost option. A plate heat exchanger or a closed-circuit fluid cooler keeps tower water out of the chiller, protects condenser tubes from fouling, and allows waterside economizing or glycol in the closed loop.

How much does lower condenser water temperature save?

Chiller efficiency typically improves by roughly 1–2% for every 1°F drop in entering condenser water temperature, depending on the chiller. Stay above the chiller manufacturer’s minimum entering condenser water temperature, using a tower bypass if needed.

Can one cooling tower serve several chillers?

Yes. Towers can be dedicated to each chiller or piped to a common header. With a common header, provide isolation valves and basin equalizer lines so water levels stay balanced. Several smaller towers also make N+1 redundancy easier.