Cooling tower selection: heat load, approach and wet-bulb temperature

Choosing a cooling tower starts with four values: the heat load the tower has to reject, the water flow rate circulating through it, the water temperatures entering and leaving the tower, and the summer design wet-bulb temperature of the site. These four values set the size of the tower and its fan power. Then the type, open or closed circuit, is chosen by water quality; noise, drift and placement by the site; and finally the casing and fill materials. Picking a tower from a catalogue by asking "how many tons" is misleading: the same tower rejects very different amounts of heat at different temperatures.

Heat load and water flow rate

The tower's job is to pass to the air the heat that a machine or a chiller has given to the water. The link between heat load, flow rate and temperature difference is Q = m · c · ΔT; for water, c is about 4.19 kJ/kg·K.

Example: assume a production line rejects 500 kW of heat and you want water entering the tower at 35 °C and leaving at 30 °C. For a 5 °C difference the flow required is 500 / (4.19 × 5) ≈ 23.9 kg/s, or about 86 m³/h. Rejecting the same heat with a 10 °C difference halves the flow; pumps and pipes get smaller, but you need to check whether the machine or the condenser accepts the warmer return water.

If the tower is to serve a water-cooled chiller, the load is larger than the chiller's cooling capacity: the electricity used by the compressor also turns into heat and is rejected at the condenser. Example: in a unit producing 1,000 kW of cooling, with a COP we assume to be 5, the compressor draws about 200 kW, so the tower has to reject about 1,200 kW.

Size the capacity for the plant's most heavily loaded hour and the most humid days of the year, not for an average day. If a new line or an additional chiller is planned in the near future, a modular arrangement to which cells can be added later is often more flexible than oversizing the tower today.

When comparing quotations, watch one detail: the "ton" in cooling tower catalogues is often not the refrigeration ton (3.52 kW) but the cooling tower ton, which includes the compressor heat (15,000 BTU/h, about 4.4 kW). Ask which ton is meant and at which temperatures the figure is given.

Range and approach: the two differences that size a tower

• Range: The difference between the hot water entering the tower and the cold water leaving it. For water entering at 35 °C and leaving at 30 °C, the range is 5 °C. The range is set not by the tower but by the heat load and the flow rate.
• Approach: The difference between the cold water leaving the tower and the wet-bulb temperature of the air. A tower that delivers 30 °C water when the wet-bulb is 25 °C has an approach of 5 °C.

It is the approach that really sets the size of the tower. Bringing the water to within 3 °C of the wet-bulb instead of 5 °C looks like a small difference on paper, but for the same load the tower becomes noticeably larger; approaches below 2–3 °C are usually not economical. Designs mostly use a value between 3 and 5 °C. If you need water colder than a tower can deliver, the answer is not a bigger tower but a chiller; for sizing the chiller side, see our article on chiller selection.

The wet-bulb temperature of the site

Because a tower cools by evaporation, and evaporation is limited by the humidity of the air, the air value to look at in selection is not the dry-bulb but the wet-bulb temperature. The same 35 °C air corresponds to a wet-bulb temperature of about 21 °C in a dry inland region at 25% relative humidity, and about 27.5 °C on a humid coast at 55% humidity. A tower working with a 4 °C approach delivers 25 °C water at the first site and 31.5 °C water at the second. On the coast the same tower rejects less heat, or a larger tower is needed for the same job.

The design value is taken from the site's meteorological data as the wet-bulb temperature exceeded during only a very small part of the year, for example 0.4% or 1% of the hours. The highest value on record oversizes the tower needlessly, while the summer average leaves it short on hot, humid days. The installation spot itself also changes this value: a tower squeezed between walls, sitting in a roof well or taking in the warm exhaust air of another unit draws back its own humid discharge and in effect works with a higher wet-bulb.

Open or closed circuit?

The type is decided by how sensitive the machine is to water quality.

• Open-circuit tower: Gives the smallest approach for the lowest initial investment. Because the water is in contact with air, it picks up dust and oxygen; sensitive heat exchangers, moulds or hydraulic circuits are separated from this water with a plate heat exchanger. The heat exchanger usually adds a temperature difference of 1–2 °C, and this must be taken into account when selecting the tower.
• Closed-circuit tower: The process water stays inside the coil and never touches the air or dirt; glycol can be used in the circuit. Suitable for induction furnaces, compressors and outdoor lines at risk of freezing. For the same load it is larger and heavier; the spray water that wets the outside of the coil also needs treatment.

The choice between counterflow and crossflow is mostly made on space and pumping: where space is tight a counterflow tower comes first, and where the pump head is to be kept low a crossflow tower does. We explained how the types work in our article what is a cooling tower.

Water quality and make-up water

An open tower uses water continuously. The water that has to be added from the mains is the sum of three losses:

• Evaporation: Most of the heat is rejected by evaporation. In the 500 kW example above this comes to at most about 0.75 m³/h (500 kW ÷ 2,400 kJ/kg). Because part of the heat passes directly to the air as sensible heat, the real figure is a little lower.
• Blowdown: Evaporated water is pure; salts and minerals stay behind in the basin. To keep their concentration within limits, part of the water is drained continuously. Blowdown is found by dividing evaporation by the number of cycles of concentration minus one: at 4 cycles, 0.75 ÷ 3 = 0.25 m³/h.
• Drift: Droplets carried away by the exhaust air. With a high-efficiency drift eliminator it stays below 0.005% of the circulating water; in this example it can be neglected.

In this example the tower needs about 1 m³ of make-up water per hour, or around 24 m³ a day in a plant running 24 hours. The hardness and conductivity of the source water determine how many cycles you can run at, and therefore the amount of blowdown. This is why a water analysis should be done before selecting the tower: in hard or dirty water, the fill type, the casing material and the water treatment system are chosen accordingly. If water is scarce or expensive, a closed-circuit tower, a dry cooler or a hybrid cooler that runs wet only on hot days should also be considered.

Drift, noise and placement

The droplets leaving a tower are not just lost water. Warm tower water is an environment in which Legionella bacteria can grow, and it is these fine airborne droplets that carry the bacteria into the surroundings. Asking for a high-efficiency drift eliminator, placing the tower away from fresh air intakes, opening windows and footpaths, and keeping the basin out of direct sunlight are the selection-stage side of this risk.

A tower has two sources of noise: the fan and the water falling into the basin. For towers near housing or hospitals, low-speed large-diameter fans, slower night operation with a frequency inverter, attenuators on the intake and discharge sides and sufficient distance are considered together. Ask for the sound figure as a sound pressure level measured at a stated distance and direction; two figures given under different conditions cannot be compared. In winter, the white plume rising from a tower in cold air also affects placement: if the wind carries it onto a road or a neighbouring facade, it can cause problems.

If the tower is going on a roof, give the structural engineer its operating weight full of water, not its empty weight; vibration isolators and the base are also part of the selection. Leave enough free space around the tower for air to enter freely, and do not let the intake faces point at a wall or at the discharge of another tower.

Materials, fill and capacity control

• Casing: A glass-fibre reinforced polyester (GRP) casing is light and resists corrosion. Galvanised steel is more economical but needs an extra coating in aggressive water; stainless steel can be preferred for the basin and fixings. Very large field-erected towers use concrete.
• Fill: PVC film fill is the most efficient type, but it usually cannot take inlet water above 50–55 °C and tends to clog in dirty water. PP fill is chosen for hot water and splash fill for dirty water.
• Capacity control: Fans with frequency inverters cut energy use at part load and in cool weather. In a multi-cell tower, cells can be switched off when the load falls, and when one cell stops the others keep running.
• Winter operation: In regions at risk of freezing, a basin heater, a bypass line and fan control should be written into the specification from the start; in a closed-circuit tower the glycol concentration is set by the lowest outdoor temperature.

Selecting together with a chiller

If the tower is to be connected to a chiller's condenser, the two should be selected together, not separately: the temperature of the water coming from the tower sets the chiller's efficiency, and the chiller's condenser flow sets the tower's range. Our industrial chillers are available with a water-cooled condenser connected to a tower as well as with an air-cooled condenser, and when selecting such a unit, the temperature and flow of the condenser water are among the inputs. You can read how a tower and a water-cooled chiller are operated together in the water-cooled chiller article of our sister company Aktek Elektromekanik.

Information to prepare for a quotation

• The heat load to be rejected (kW), or the capacity of the chiller to be connected
• Water temperatures entering and leaving the tower, and the water flow rate
• Installation site: city, altitude, roof or ground, surrounding buildings
• Water source and, if available, a water analysis
• Noise limit and the distance to the nearest home or window
• How sensitive the machine is to dirt and freezing

If the project also includes a chiller, send us this information through our contact page; we will assess the conditions on the tower side together with the chiller selection. You can see which chiller class suits your needs on our chillers page.