Cooling Tower Selection for a Factory: Capacity, Water Quality and Cost Drivers

A cooling tower is sized from four numbers: the heat to reject, the water range (hot water temperature minus cold), the design wet-bulb temperature of the site, and the approach, which is how close the cold water gets to that wet-bulb. Water quality, noise, space and the tower's material then decide the price. For a water-cooled chiller the heat rejected is the cooling load plus the compressor power.

Which numbers fix the size of the tower?

NBC 2016 Part 8 defines the two terms that matter. The range is the temperature difference between the entering and leaving water. The approach is the difference between the leaving water temperature and the ambient air wet-bulb temperature. A tower cannot cool water below the wet-bulb, so approach is the margin you buy above it.

The heat to reject comes from the chiller. For a vapour-compression chiller it is the evaporator duty plus the electrical power the compressor puts into the refrigerant. The flow then follows from a simple heat balance: flow in cubic metres per hour equals the heat in kilowatts divided by 1.163 times the range in degrees Celsius. The Bureau of Energy Efficiency's chiller testing code gives a worked case that shows the scale: a 1,180 TR package with condenser water of 1,427 cubic metres an hour entering at 27 °C and leaving at 32 °C, a range of 5 °C. Multiply 1,427 by 1.163 by 5 and you get about 8,300 kW of heat to reject.

Start your own numbers in the chiller capacity calculator, then give the tower supplier the heat, flow, entering and leaving temperatures and the design wet-bulb for your site. If the supplier is asked only for "a 300 TR tower", the answer is a catalogue size that may not match your duty. Tower makers also rate in different units from refrigeration tons, so ask for the heat rejection in kilowatts on the datasheet.

Why do wet-bulb and approach set the price?

Because the tower gets larger, quickly, as the approach shrinks. For a fixed range and flow, a tower that delivers cold water 3 °C above wet-bulb needs more fill, more airflow or more cells than one that delivers it 5 °C above. Cold condenser water lowers chiller power, which is why NBC 2016 asks the designer to endeavour to reduce the approach to maximise energy conservation, and the BEE code lists holding as low an approach as possible at the rated temperature among its tower measures. The trade-off is capital against running cost, and it has no universal answer.

Two practical points follow. First, the design wet-bulb must be a figure for your location that you can defend, not the hottest day on record and not the average. Ask your designer which dataset was used. Second, ask each bidder to rate the same duty at two approaches. The price gap per degree shows what colder condenser water really costs, and you can set that against the chiller's power saving.

BIS published IS 18758:2024 on the thermo-hydraulic design of induced-draught counterflow towers. Ask each bidder whether the rating follows it, and on what fill, cell size and fan basis the duty was rated, so that two quotations for the same duty are comparable.

What does water quality do to the tower and the water bill?

A cooling tower cools by evaporating a small part of the circulating water, which leaves dissolved salts behind. NBC 2016 gives planning figures for the losses: evaporation is usually about 1 percent of the circulation rate, drift stays below 0.1 percent and blow-down below 0.8 percent. Cycles of concentration, the ratio of dissolved solids in the circulating water to those in the make-up, follow from the mass balance as one plus evaporation divided by blow-down, ignoring drift. With the NBC figures that is about 2.25 cycles or more.

The BEE circuit in the example above circulates 1,427 cubic metres an hour. One percent of that is about 14 cubic metres an hour of evaporation, a few hundred cubic metres a day on a full-load day. Make-up water is therefore a continuing cost, and a poor make-up supply forces more blow-down, higher chemical dosing and shorter fill life.

NBC adds three requirements you can write into the contract: provide a make-up water tank for the tower that is separate from the tank serving drinking water; treat make-up water that has contaminants or hardness that could harm the plant; and treat water with a pH below 5. It recommends treated water with hardness of 50 ppm as calcium carbonate or less for air-conditioning applications, and suggests make-up from treated sewage, waste-water treatment or rainwater where it can be had. Test your supply before you choose materials and the treatment package.

What moves the cost of a cooling tower?

We do not quote a tower price here, because the number depends on the points below and on the supplier's rating. The table lists the drivers, why each moves the price and what to pin down in the quotation.

Space is easy to underestimate. NBC 2016 recommends a floor area of 0.07 to 0.10 square metres per TR for mechanical-draft towers and 0.15 to 0.20 for natural-draft ones. Add clearance for air intake and discharge: the BEE code advises following the manufacturer's clearances and relocating structures that interfere with air intake or exhaust.

How do you check what you were sold?

NBC 2016 requires each major piece of plant to be tested on completion. For a cooling tower, capacity is calculated from the temperature difference, the approach to ambient wet-bulb and the condenser water flow, power consumption is checked from motor current, and every calculated and measured value must match the data in the technical submittal. The manufacturer or an authorised representative must certify the tower ready to start, and the instruments used must be calibrated with the certificate on record. BIS also lists IS 11561:2018, a code of practice for testing water cooling towers, which the contract can name as the test method.

Write the test into the contract before the order: who measures, at what load and wet-bulb, with which instruments, and what happens if the tower falls short because the day's wet-bulb was lower than design. Our guide to testing and commissioning covers the wider handover record. A tower that is never tested is a tower whose shortfall shows up as a hot chiller in May.

How Secured Engineers helps

Secured Engineers designs the chilled water and condenser water system in-house, with the electrical supply, fire protection and plumbing for the same plant, under one MEPF contract if you want it. Fire NOC, CEIG and DISCOM net-metering files are prepared in-house. We work from offices in Noida (Delhi NCR) and Ludhiana, across 535+ projects in 18+ states, and 89.77% of our clients return.

If you have a chiller plant budget or a tower quotation, send it for a review: we check the duty, the approach, the water analysis and the scope lines against NBC 2016 and the BEE code. The chiller plant page describes the service. You can reach us on WhatsApp or at +91 70099 85817, or ask for a free proposal.