Cooling tower selection: range, approach, wet-bulb and certified ratings

Three temperatures decide most of a cooling tower enquiry. Get them right before comparing cells and fans.

A cooling tower rejects heat by evaporating part of the water it circulates. How much heat it can reject depends less on its box size than on three temperatures: the water entering, the water leaving and the wet-bulb temperature of the air. Define those three before asking a supplier for a model, and most of the selection follows.

Range, approach and wet-bulb

Range is the hot water temperature entering the tower minus the cold water temperature leaving it. It is set by the heat load and the water flow, not by the tower's quality.

Approach is the cold water leaving the tower minus the wet-bulb temperature of the entering air. It measures how hard the tower is working. Water cannot be cooled below the entering wet-bulb, so approach is never zero; a smaller approach needs a much larger tower.

Design wet-bulb is a site climate value, not a catalogue value. Use the figure your engineer has chosen for the project location and state it in the enquiry. A tower rated at one wet-bulb does not deliver the same leaving-water temperature in a more humid climate.

Brentwood's explanation adds a useful diagnostic: if range rises while approach stays steady, the load or flow has changed; if approach rises at steady range, the tower itself is performing worse, for example through fouled fill or reduced airflow.

Counterflow and crossflow

In a counterflow tower air moves upward against falling water. In a crossflow tower air moves horizontally across the falling water. Each has consequences for footprint, pump head, access to the distribution system and icing behaviour, and suppliers often build both.

On this site, Henan Jinchuang publishes both layouts. The JCN counterflow towers are described for hot-water duties with modular cells, and the JCR crossflow towers are presented with condition-specific selection. Read the two pages together to see how one manufacturer documents each layout.

Closed circuits and dry alternatives

An open tower exposes the process water to air. Where the condenser water must stay clean, a closed-circuit tower or a dry cooler keeps the fluid in a coil, and some plants keep an open tower but separate the circuits with a plate heat exchanger. Closed-loop process packages such as the Grand LSS closed pure-water cooling systems keep the water quality under control in a sealed circuit. Dry coolers use no evaporation and so cannot approach the wet-bulb; their duty depends on dry-bulb temperature. The Envicool dry coolers and the AIRSYS ADIACOOL adiabatic dry coolers show the trade-off: adiabatic pre-cooling adds water use only in hot hours.

Integrated equipment can also include the heat rejection. The Takamura TS-C-UNI evaporatively cooled chiller combines the chiller and its evaporative condenser, which changes what you compare.

Certified ratings

The Cooling Technology Institute runs a thermal performance certification under its STD-201 standard. CTI certifies that every model in a manufacturer's certified line performs in accordance with its published ratings, retests products periodically and excludes custom-built units. Eurovent Certification cooperates with CTI so that product lines certified through Eurovent can be tested to STD-201. Energy codes such as ASHRAE 90.1 refer to certified ratings for heat-rejection equipment.

A Chinese supplier may not hold CTI or Eurovent certification for a line. That is not unusual, but it changes the evidence you need: ask for a performance test method and, for large projects, a site acceptance test against agreed conditions.

What else to put in the enquiry

Water quality decides the fill and materials. State whether the tower serves a chiller condenser, a process or a heat pump, and give make-up water quality and any treatment programme.

Fans drive power and sound. Ask for fan type, motor power, speed control and the sound data basis; our note on equipment sound data explains sound power and sound pressure. Ask whether the axial fan and motor are made by the tower supplier or bought in.

Structure and basin: state the installation site, wind and seismic requirements, basin material and whether an access platform, ladders and basin heaters are included.

Questions to ask

What range, approach and design wet-bulb is the quoted model selected for, and at what water flow? Is the line certified under CTI STD-201 or Eurovent, and does the certificate cover the quoted model? What is the fan motor power at design and at part load, and how is part-load control arranged? What is the drift rate and how is it measured? What materials are used for fill, casing, basin and structure?

For the chiller connected to the tower, read our chiller quotation guide so the condenser water temperatures in both offers match.

Common questions

Can a bigger tower always deliver colder water? Only down toward the wet-bulb. Approaching it closely costs a disproportionately larger tower.

Is a dry cooler a direct substitute? Not where the process needs water below ambient dry-bulb. Compare the required leaving temperature with the site's hottest hours.

Does CTI certification guarantee site performance? It verifies published thermal ratings for the certified line under test conditions; site layout, recirculation and maintenance still matter.

By HVACDeck Editorial · Updated 2026-11-04