Cooling Tower Water Consumption Explained

A 1,000 RT chiller plant running hard on a hot Kuala Lumpur afternoon turns roughly six to seven cubic metres of water an hour into vapour, and none of it comes back. That is cooling tower water consumption in one sentence: the tower is not leaking, it is doing its job. In most air-conditioned commercial buildings in Malaysia, the cooling tower is the single biggest water user on the incoming meter, ahead of the toilets, ahead of the kitchen, ahead of landscaping. The uncomfortable part is that a meaningful share of what it draws is genuinely avoidable, and hardly any building can tell you which share, because the tower make-up line is almost never metered on its own.

Why the tower is the biggest water user in the building

A cooling tower rejects heat by evaporating a small part of the water flowing through it. Evaporating one kilogram of water absorbs roughly 2,400 kJ, which is an enormous amount of heat for a very small amount of liquid, and that is the whole trick. If you want the mechanical walkthrough of fill, fans, drift eliminators and the condenser water loop, read how a cooling tower works first.

The arithmetic that follows is simple enough to do on a napkin. A chiller rejects about 1.25 times its cooling load through the condenser, and evaporating that heat costs roughly 1.8 to 2 litres of water per hour for every kW of cooling delivered. Call it 1 m3 per hour for every 500 kW of chiller load. Run a 3,500 kW plant at high load for a ten hour office day and evaporation alone is around 65 to 70 m3, before you have counted a single flush. That number is not a fault. It is physics, and no amount of maintenance will make it go away.

In the tropics it is also relentless. There is no winter shutdown, no shoulder season where the plant idles for three months. Malaysian towers run every working day of the year, in sunlight, in haze, with warm humid air feeding algae and bacteria, which is exactly why the water treatment side of this story matters as much as the water bill.

Where cooling tower water consumption actually goes

Make-up water, the water you pay for, splits into four streams:

- Evaporation. The productive loss. It scales with cooling load and nothing else, and you cannot reduce it without reducing the cooling you deliver or improving chiller and load efficiency upstream.
- Blowdown or bleed. Deliberately dumped water, drained to keep dissolved solids in the loop under control. This is the stream you can actually manage.
- Drift. Fine droplets carried out of the tower by the fan airflow. Modern drift eliminators in good condition hold this to a few thousandths of a percent of circulating flow. Damaged, missing or badly seated eliminators can lose many times that, and the giveaway is a visible mist plume and a film on the cars parked downwind.
- Leaks and overflow. Stuck float valves, cracked basins, an over-filled sump spilling continuously down the overflow, a make-up solenoid that never quite closes. This stream is invisible, runs 24 hours a day, and is the one that turns up in a water bill investigation.

Evaporation is fixed by load. Drift and leaks should be near zero. So the real lever on cooling tower water consumption is blowdown, and blowdown is governed by cycles of concentration.

What are cycles of concentration?

Picture a pot of tap water simmering on a stove. As steam leaves, the water level drops, but every mineral that came in with the tap water stays behind. The remaining water gets saltier and saltier. A cooling tower is exactly that pot, running continuously.

Cycles of concentration is simply how many times more concentrated the tower water is than the make-up water. Three cycles means the water circulating in the loop carries three times the dissolved solids of the supply from Air Selangor or your state operator. In practice it is measured with a conductivity probe: divide the conductivity of the tower water by the conductivity of the make-up.

The useful consequence is a piece of arithmetic every building manager should know. Blowdown equals evaporation divided by (cycles minus one).

At two cycles, blowdown equals evaporation, so total make-up is twice the evaporation. You buy two litres for every one that does useful cooling. At four cycles, blowdown falls to a third of evaporation, and total make-up is about 1.33 times evaporation. Moving a tower from two cycles to four cycles cuts total water consumption by roughly a third for exactly the same cooling output. At six cycles you are down to 1.2 times evaporation, though the returns flatten quickly after four or five.

Can you just keep running the cycles up?

No, and this is where a well-meaning cost-cutting exercise turns into a repair bill. As cycles climb, calcium, alkalinity, silica and chlorides all concentrate. Push past what the make-up chemistry and the treatment programme can hold in solution and you get scale on the condenser tubes, corrosion in the pipework, and warm nutrient-rich water that encourages biofilm and Legionella risk.

The good news locally is that treated supply in much of Peninsular Malaysia is drawn from surface sources and tends to be relatively soft, which usually allows higher cycles than a hard-water region would tolerate. The right ceiling is set by your actual make-up water analysis and your treatment chemistry, so ask your water treatment contractor what cycles the programme is designed for, then ask what cycles the tower is actually holding. In a lot of buildings those two numbers are nowhere near each other.

The common failure modes are mundane. A manual bleed valve cracked open years ago and never adjusted since, dumping water at a fixed rate regardless of load. A conductivity controller whose probe has fouled, reading low, so it bleeds continuously. A controller in permanent manual override because someone was troubleshooting in 2019. None of these announce themselves.

Poor treatment shows up twice: water and kilowatts

Here is the part that turns a housekeeping issue into a real financial one. Scale on condenser tubes is an insulating layer. It raises the condenser approach, which raises condensing temperature, which forces the compressor to work across a bigger lift. As a rule of thumb, each additional 1 degree C of condensing temperature costs somewhere around 2 to 3 percent of chiller efficiency.

So a tower that is scaling because cycles were pushed too high, or because the treatment programme lapsed, charges you twice: once in make-up water, and again, much larger, in chiller kWh every hour the plant runs. On most Malaysian commercial tariffs the electricity penalty dwarfs the water saving that caused it. Water and energy on a chiller plant are the same problem wearing two hats.

How to meter make-up water properly

You cannot manage any of this from a monthly bill that lumps the tower in with every toilet in the building. The starting point is a dedicated meter on the tower make-up line, with a pulse or Modbus output, logged at fifteen minute intervals rather than read by hand once a month. Where the piping allows, meter the blowdown line as well, because make-up divided by blowdown gives you cycles of concentration continuously, without anybody walking up to the roof with a conductivity pen.

Once the data is logging, three checks do most of the work:

- Litres per kWh of cooling. Compare make-up volume against chiller load from the BMS. That ratio should be stable. When it drifts upward, either the bleed has opened up or something is leaking.
- The overnight flat line. In an office building where chillers shut down at night, make-up flow should fall to near zero once the loop settles. Steady flow at 3am is a stuck float valve, a leaking basin or an open bleed, and it is running every night.
- Step changes. Consumption that jumps on a specific date and never returns is a valve position or a controller setting, not a change in weather.

This is the same discipline the rest of the building needs, and it works best when the tower is one metered branch inside a complete building water balance audit rather than a standalone project. CobiNeural does this alongside energy: continuous make-up metering, night-flow leak detection, consumption baselines tied to actual chiller load, and WhatsApp or email alerts when the tower starts drinking more than the cooling it delivers can explain. It runs standalone or over an existing BMS.

One more thing worth checking with your water and sewerage operators: water that evaporates from a cooling tower never reaches the drain, and some tariff arrangements treat separately metered evaporative loss differently from water discharged to sewer. Whether that applies to your premises depends on your operator and account category, so confirm it directly rather than assuming.

If you want to know what your cooling tower is really costing in water and in chiller efficiency, talk to us about water monitoring.