What a cooling tower has to do

A water-cooled chiller removes heat from the building and adds the heat of compression from its own compressor. The cooling tower must reject both to the outdoor air, mainly by evaporating a small part of the condenser water. How well it can do that depends on the outdoor wet bulb temperature, not the dry bulb, which is why tower sizing starts with the site’s design wet bulb.

Step 1: Calculate heat rejection

Heat rejection is the cooling load plus the compressor work:

Heat rejection = Cooling load × (1 + 1 / COP)

For typical electric chillers this works out at about 1.25 times the cooling load, which is where the industry’s “cooling tower ton” comes from: 1 tower ton = 15,000 Btu/h of heat rejection, compared with 12,000 Btu/h for a refrigeration ton. Absorption chillers reject far more heat, often around 2.5 times the cooling load, so always use the chiller’s actual condenser heat rejection when you have it.

Step 2: Calculate condenser water flow

Flow (GPM) = Heat rejection (Btu/h) / (500 × Range (°F))

Flow (L/s) = Heat rejection (kW) / (4.19 × Range (°C))

The standard rating point is 3 GPM per refrigeration ton (0.054 L/s per kW) with a 10°F (5.6 K) range. Many modern plants use 2 to 2.5 GPM per ton with a larger range to cut pump energy, but the chiller must be selected for the same conditions. You can check flows with the Chiller Tonnage Calculator.

Step 3: Set range and approach

Two temperature differences define the tower’s duty:

• Range = hot water entering the tower − cold water leaving the tower. It is set by heat rejection and flow.
• Approach = cold water leaving the tower − design wet bulb. It is a design choice, and the biggest driver of tower size.

An approach of 5 to 10°F (3 to 5.5 K) is the usual design range, with 7°F (3.9 K) a common choice. Going from a 7°F to a 5°F approach can increase tower size significantly for a small gain in cold water temperature, so tighter approaches only make sense where the chiller efficiency gain pays for it.

Step 4: Use the right design wet bulb

Select the tower at the site’s design wet bulb from climate data, typically the ASHRAE 0.4% or 1% annual value. Using a dry-bulb-based value or a generic figure is the most common sizing mistake. Hot, humid coastal sites can have design wet bulbs above 85°F (29.4°C), while hot, dry inland sites may be well below 75°F (23.9°C) even with very high dry bulb temperatures. You can convert between dry bulb, humidity and wet bulb with the Psychrometric Calculator; for the theory, see Psychrometric Processes in HVAC.

Cold water temperature then follows directly: Cold water = Design wet bulb + Approach. Check that the chiller can accept this entering condenser water temperature at full load.

Step 5: Calculate make-up water

Water leaves the tower in three ways, and make-up water must replace all of them:

Evaporation (GPM) ≈ 0.00085 × Flow (GPM) × Range (°F), roughly 1% of the flow for every 10°F (5.6 K) of range.

Drift is water carried out as droplets. Modern drift eliminators keep it to 0.001 to 0.005% of the circulating flow.

Blowdown (GPM) = Evaporation / (Cycles of concentration − 1). Cycles of concentration (COC) is set by water treatment, typically 3 to 6.

Make-up = Evaporation + Drift + Blowdown

Cooling tower sizing: worked example for a 500 TR chiller plant

A 500 TR (1,758 kW) electric chiller plant has a design wet bulb of 80°F (26.7°C). The design uses 3 GPM per ton, a 7°F approach and 4 cycles of concentration.

The tower is specified as 1,500 GPM from 97°F to 87°F at 80°F wet bulb, and the manufacturer selects the cell size, fan and fill to meet it. The make-up water supply and storage must cover about 3.9 m³ (1,025 gal) for every hour of full-load operation.

Run your own numbers in the Cooling Tower Calculator. Condenser water is an open loop, so pump head must include static lift and nozzle pressure; see the open loop section of Chilled Water Pump Head Calculation and check suction conditions with the Water Pump NPSH Calculator.

Common mistakes

• Sizing the tower on refrigeration tons instead of heat rejection.
• Using a design dry bulb or a generic wet bulb instead of the site’s design wet bulb.
• Picking a very small approach without checking the cost against the chiller efficiency gain.
• Forgetting blowdown when sizing make-up water supply and storage.
• Placing towers where the discharge air recirculates into the intake, which raises the effective wet bulb.

Frequently asked questions

What is a cooling tower ton?

15,000 Btu/h of heat rejection. It covers the 12,000 Btu/h refrigeration ton plus about 3,000 Btu/h of compressor heat.

What is a good cooling tower approach?

5 to 10°F (3 to 5.5 K). A 7°F (3.9 K) approach is a common balance between tower size and chiller efficiency.

How much water does a cooling tower use?

Roughly 1% of the circulating flow per 10°F of range for evaporation, plus blowdown and drift. For a 500 TR plant this is about 17 GPM at 4 cycles.

Why does wet bulb matter more than dry bulb?

A cooling tower cools water mainly by evaporation, and the lowest temperature evaporation can reach is the wet bulb temperature.