How to Size a Cooling Tower: The Four Numbers That Decide It

Ask three suppliers to quote a cooling tower and you will often get three incomparable offers, because each has assumed something different about your duty. The way to prevent that is to fix the four inputs yourself before you ask anybody for a price.

Everything about a tower selection follows from these:

If the tower serves a chiller, the heat load is not the chiller tonnage. A chiller rejects its cooling duty plus the work its compressor puts in, so the condenser load is roughly 1.25 times the evaporator tonnage for a typical electric chiller. A 300 TR chiller therefore needs a tower sized for around 375 TR.

If the tower serves a process, the heat load is whatever the process sheet says it rejects. Where nothing is documented, it can be measured: heat load in kilocalories per hour equals flow rate in litres per hour multiplied by the temperature drop in degrees Celsius, because the specific heat of water is very close to one.

One TR is 3024 kilocalories per hour. Dividing your kilocalorie figure by 3024 converts it, which is useful because Indian tower catalogues are almost always written in TR.

Range and flow rate are linked. For a fixed heat load, a wider range means less water and a narrower range means more. The industry convention in India is a 5 degree Celsius range, which works out at roughly 0.6 cubic metres per hour of circulation for every ton of refrigeration.

That convention is worth questioning rather than accepting. A wider range moves the same heat with less water, so pumps, pipework and valves all get smaller and the pumping energy falls for the life of the plant. A narrower range needs more water but allows a smaller temperature lift across the process.

If your process can tolerate a 6 or 7 degree range, say so at enquiry stage. The saving lands in the pump and pipe budget, not the tower.

Approach is the gap between the cold water leaving the tower and the ambient wet bulb temperature. It is the single most expensive parameter in a selection, and the one clients most often specify carelessly.

A tower cannot cool water below the wet bulb, and the closer it gets, the larger it must be. Going from a 5 degree approach to a 3 degree approach on the same duty can increase tower size by half again. Going below about 2.8 degrees is generally uneconomic for a standard induced draught tower.

So ask what the process actually needs rather than what would be nice. If your chiller is happy with 32 degree condenser water and the design wet bulb is 28, you need a 4 degree approach and should not pay for a 3.

Everything above depends on the design wet bulb temperature at your site, which is a humidity-adjusted figure, not the temperature on a thermometer. It is the reason a tower that performs perfectly in one city underperforms in another.

Design wet bulb is normally taken as the value exceeded for only a small percentage of hours in the year. Coastal and humid locations run high, which is why the same duty needs a larger tower in Chennai or Mumbai than in Delhi or Jaipur.

Specify the wet bulb you want the tower guaranteed at. If a supplier quotes against a lower wet bulb than your site really sees, their tower looks cheaper on paper and will disappoint every humid afternoon.

Take a 300 TR electric chiller in New Delhi. The condenser load is around 375 TR. At the conventional 5 degree range, circulation is roughly 0.6 times 375, which is 225 cubic metres per hour.

Design wet bulb for Delhi is commonly taken at 28 degrees Celsius. If the chiller needs 32 degree condenser water, the approach is 4 degrees and the hot water returning is 37 degrees.

The complete duty statement is then: 375 TR, 225 cubic metres per hour, 37 degrees in, 32 degrees out, 28 degree wet bulb. Any competent supplier can size against that, and any two quotations you receive will be comparable.

Send the four numbers, your location, and anything about the site that constrains the answer. The constraints matter as much as the duty.