Cooling Tower Blowdown and Bleed-Off: The Setting Everyone Gets Wrong
Somewhere on your cooling tower there is a valve, a controller, or a timer that decides how much water gets thrown away. On most sites in Malaysia it is set wrong, it has been set wrong for years, and depending on which way it is wrong it is either costing you water and chemicals or quietly growing scale on your condenser tubes.
Blowdown is the least glamorous setting on the plant and one of the most consequential. Get it right and you cut water bills, chemical bills and energy bills at the same time. Get it wrong in the generous direction and you pour money down the drain. Get it wrong in the stingy direction and you build an insulating layer of scale that raises condenser pressure and drags your chiller efficiency down.
This is how the number actually works, how to find the right one for your water, and the specific ways it goes wrong here.
First, the words, because they cause half the confusion
Blowdown and bleed-off are the same thing. Two names, one action: deliberately draining a portion of the concentrated circulating water so you can replace it with fresh make-up. Some people reserve “bleed-off” for a continuous trickle and “blowdown” for a periodic dump, but the water balance does not care which word you use.
To place it among the other water losses:
Evaporation is the useful loss. It is how the tower rejects heat, and it is nearly pure water leaving as vapour — the dissolved minerals stay behind. A common rule of thumb is roughly 1% of the circulating flow evaporates for every 5.5°C (10°F) of range.
Drift is water carried out as droplets in the air stream. Unlike evaporation, drift takes its dissolved solids with it. Good drift eliminators keep this small.
Blowdown is the loss you control deliberately, to keep dissolved solids from concentrating to the point where they precipitate as scale.
Make-up is everything you add to replace the three losses above:
Make-up = Evaporation + Drift + Blowdown
Evaporation is fixed by your heat load. Drift is fixed by your equipment. Blowdown is the one number you actually set — which is exactly why it is where the money is won or lost.
The concept that runs the whole thing: cycles of concentration
When pure water evaporates, the minerals it was carrying stay in the tower. The remaining water gets saltier. Keep evaporating and never bleeding, and eventually calcium carbonate and silica exceed their solubility limits and crystallise onto the hottest surfaces in the system — your condenser tubes.
Cycles of Concentration (CoC) measures how concentrated the tower water has become relative to the make-up water. Running at 4 cycles means the dissolved solids in the circulating water are four times the level in the make-up. Blowdown is simply how you hold the cycles where you want them: bleed more, cycles fall; bleed less, cycles rise.
The easiest way to measure CoC in practice is with a conductivity meter:
CoC = conductivity of tower water ÷ conductivity of make-up water
That is a two-minute measurement with a handheld meter, and it is the single most useful number about your tower’s water chemistry that almost nobody on site actually checks.
The formula, and the trap hidden inside it
Blowdown relates to evaporation and cycles like this:
Blowdown = Evaporation ÷ (CoC − 1)
That “minus one” is where the intuition breaks, so it is worth seeing the numbers.
Take a 1,000 ton tower. Industry figures put evaporation at roughly 25 to 26.5 gallons per day per ton, so call it about 25,000 gallons a day evaporated.
- At 2 cycles: blowdown = 25,000 ÷ (2−1) = 25,000 gallons/day. You throw away as much as you evaporate.
- At 4 cycles: blowdown = 25,000 ÷ (4−1) = about 8,300 gallons/day.
- At 6 cycles: blowdown = 25,000 ÷ (6−1) = 5,000 gallons/day.
Look at the shape of that. Going from 2 to 4 cycles cuts blowdown by two-thirds. Going from 4 to 6 cuts it by a further 40%. But going from 6 to 8 saves comparatively little — the curve flattens hard.
This is the setting everyone gets wrong, in both directions:
Running at low cycles (2 to 3) is the common default, and it is expensive. Many towers are left bleeding heavily “to be safe.” That safety is paid for in water you buy and immediately discard, and in treatment chemicals that go down the drain with it. Pushing a tower from 3 cycles to 6, where the water chemistry allows, can cut total make-up demand by roughly 20%.
Chasing very high cycles is the opposite mistake. Because the savings curve flattens above about 6, the water you save by going from 6 to 10 cycles is small — but the scaling and corrosion risk you take on is not. You spend more on treatment chemistry to hold off scale than you ever save in water. There is a sweet spot, and it is usually lower than the maximum the chemistry could theoretically survive.
The right answer is: as high as your make-up water chemistry safely allows, and no higher.
What decides the ceiling: your make-up water
The maximum safe cycles is not a universal number. It is set by what is dissolved in the water you feed the tower, because that is what concentrates.
The usual limiting culprits:
Calcium and total hardness. Concentrate calcium far enough and it drops out as calcium carbonate scale — the hard, insulating layer that coats condenser tubes. High-hardness make-up hits its ceiling at lower cycles.
Silica. Often the true limiting factor, because silica scale is stubborn and hard to remove once formed. Many treatment programmes cap total silica in the circulating water, and that cap sets your maximum cycles regardless of everything else.
Alkalinity and pH. Rising alkalinity as water concentrates pushes the balance toward scaling.
Chlorides and sulphates. These drive corrosion rather than scale, and they set a ceiling on galvanised and steel components.
Two towers on the same street can have completely different correct settings if one draws municipal supply and the other uses borewell or recycled water. Which is why the honest version of this article cannot hand you a number — it can only hand you the method.
Continuous bleed on conductivity: how it should be done
The correct way to run blowdown on any tower that matters is automatically, on conductivity, not on a timer and not by a valve someone cracked open years ago.
A conductivity controller continuously measures the circulating water and opens the bleed valve only when conductivity rises above your setpoint, closing it once fresh make-up brings it back down. The tower then self-corrects for changing load: high load means more evaporation, faster concentration, and the controller bleeds more; low load means it barely bleeds at all.
Set against the two bad alternatives:
Timer-based blowdown bleeds the same amount whether the tower is flat out at midday or idling overnight. It is either wasteful at low load or inadequate at high load, and usually both at different hours.
A manually cracked valve is the worst of all. It has no relationship to water quality whatsoever, nobody remembers why it is set where it is, and it continues bleeding at exactly the same rate as your load, weather and water quality all change around it. If your tower has a permanently open bleed valve and no conductivity controller, you have found your first improvement.
The controller pays for itself quickly in most installations, through reduced water, reduced chemicals, and protection of the condenser it serves.
The chain of consequences
The reason this obscure setting deserves attention is that it does not stop at the water bill. It runs straight into your energy cost.
Scale is an excellent thermal insulator. A thin layer of calcium carbonate on the condenser tubes forces the refrigerant to condense at a higher temperature and pressure to reject the same heat. Higher condensing pressure means the chiller compressor works harder for every ton of cooling. The widely used field figure is that each 1°F rise in condensing temperature costs somewhere around 1 to 2% in chiller efficiency.
So a blowdown setting that is too stingy does not just risk the tower. It quietly lifts the electricity bill of the chiller behind it — and under the tariff structure in force since 1 July 2025, where the Peninsular base rate rose from 39.95 to 45.4 sen/kWh, that penalty costs more than it used to.
The failure runs in a chain: wrong blowdown setting → wrong cycles → scale on condenser tubes → higher condensing pressure → higher compressor power → higher bill, plus a shortened tower and chiller life. All from a valve nobody looks at.
The over-generous version of the mistake is cheaper in consequence but more visible in cash: water you paid the utility for, treated with chemicals you paid for, drained to the sewer within minutes of adding it — and, increasingly, a discharge you may need to account for.
Why this bites differently in Malaysia
Water quality varies enormously by source. A tower on treated municipal supply in the Klang Valley has a very different ceiling from one on borewell water in an industrial estate or one running reclaimed water. The correct cycles for each is different, and a setting copied from another site is a guess.
The reclaimed-water push is changing the make-up chemistry. As Malaysia moves data centres and large industrial users toward alternative water sources — SPAN issued guidelines in September 2025 encouraging data centres to use non-potable sources — the make-up water feeding many towers is becoming harder and more variable. That makes conductivity-based control more important, not less, because a fixed setting cannot cope with make-up water that changes.
The Johor water constraint raises the stakes on every litre. With Johor authorities having asked large water-cooled users to defer expansions and tightened approvals for high water-use facilities in late 2025, blowdown is no longer only a cost question. Water you needlessly discard is water you may struggle to be allocated in the first place. Optimising cycles is one of the cheapest ways to cut a facility’s total water draw.
Year-round high load means year-round evaporation. There is no cool season to slow concentration down. Whatever your blowdown setting is doing, right or wrong, it is doing it every day of the year.
Airborne fouling loads the water. Dust, haze and industrial particulates entering an open tower add suspended solids that interact with your scaling and blowdown decisions. Side-stream filtration — continuously filtering the basin water — often lets you run higher cycles safely by removing particulates that would otherwise force you to bleed more.
How to get your setting right
A practical sequence any competent plant team can follow:
- Measure the conductivity of your make-up water and your circulating water. Divide them. That is your current CoC. Most people are surprised by the answer.
- Get your make-up water analysed — hardness, silica, alkalinity, chlorides. Your water treatment supplier will usually do this. This defines your ceiling.
- Have your treatment specialist set a target CoC and corresponding conductivity setpoint for your specific water and treatment programme. This is genuinely a job for someone who knows your chemistry; the wrong target scales your condenser.
- Fit a conductivity controller if you do not have one, or verify the calibration if you do. Controllers drift and probes foul.
- Close any manual bleed valve that is bypassing the controller.
- Consider side-stream filtration if suspended solids are what is holding your cycles down.
- Recheck after any change in make-up water source, and periodically regardless. The right setting is not permanent.
None of this is expensive relative to a scaled condenser bundle or a year of over-bled water. Most of it is a meter, a controller and one good water analysis.
The short version
Blowdown is the one water loss you control, and the setting that governs it decides your water bill, your chemical bill, and — through condenser scaling — a slice of your energy bill. Too low and you scale the condenser and pay for it in compressor power. Too high and you throw away treated water by the thousands of litres. The right number depends entirely on your make-up water, it should be held automatically on conductivity rather than by a timer or a stuck valve, and it usually sits around the point where the savings curve flattens — often near 5 or 6 cycles, but only your water chemistry can say for sure.
Go and measure the conductivity of your tower water and your make-up water this week. The ratio will tell you, in two minutes, whether you have been getting this wrong.
