Of all the bits of kit on a cooling tower, the bleed valve gets the least love. It's a small solenoid valve and a probe, tucked away in the pipework, quietly letting water go to drain. But get it wrong and you'll either be pouring water and chemicals down the drain, or slowly scaling up the system and making life easier for bacteria.
February is a good time to look at it. Loads are low, the tower's ticking over, and it's a sensible moment to check your setpoints before the summer heat arrives. Here's how it all works.
Why towers need bleeding at all
A cooling tower loses water by evaporation. But only the water evaporates. The dissolved minerals and salts that came in with the mains water stay behind. Top the tower up with fresh water, more minerals come in, more water evaporates, and the concentration keeps rising.
Left alone, the water would eventually get so concentrated that minerals start coming out of solution as scale. Bleed-off (sometimes called blowdown) deliberately drains some of the concentrated water away so it can be replaced with fresher make-up water. That keeps the concentration where you want it.
What "cycles of concentration" means
Cycles of concentration is just a way of saying how concentrated the tower water is compared with the make-up water. It's usually worked out from conductivity, which goes up as dissolved solids go up.
To keep the maths simple: if your make-up water reads 500 µS/cm and the tower water reads 2,000 µS/cm, the tower water is four times as concentrated, so you're running at four cycles. That's an illustration, not a target. The right number for your tower depends on your water and your treatment programme.
How conductivity control works
Most towers use a conductivity controller. A probe in the circulating water measures conductivity continuously. When it rises above a setpoint, the controller opens the bleed valve and concentrated water goes to drain. Make-up water comes in through the ball valve or level control to replace it, conductivity drops, and the bleed valve closes again.
On many systems the inhibitor is dosed in proportion to make-up water, using a pulsing water meter, so as fresh water comes in, the right amount of inhibitor goes in with it. Biocide dosing is often linked to the bleed too, with the bleed locked out for a period after a shot dose so the biocide isn't drained away before it's had time to work. Those timings come from your treatment programme.
What happens when the setpoint is wrong
Too high (not enough bleed)
- Scale forms on heat exchangers, fill and pipework, cutting efficiency and raising energy use
- Deposits give bacteria somewhere to settle and make biocides less effective
- Corrosion can increase in some water chemistries
- Suspended solids build up, adding to sludge in the pond
Too low (too much bleed)
- Water use and water bills go up
- Treatment chemicals go down the drain before they've done their job, so chemical costs go up too
- Inhibitor levels can drop if dosing doesn't keep pace
The setpoint is a balance, and it should be set by whoever designs your treatment programme based on your make-up water quality (things like hardness and alkalinity) and the inhibitor you're using. Hard water areas, which include much of London and the South East, need particular care. Some sites soften the make-up water to allow more cycles.
Picture a tower where the conductivity controller shows a steady, healthy reading week after week. It all looks fine, but the water bill is creeping up and there's white scale appearing on the fill. A handheld meter tells a different story: the probe is coated and reading low, the actual conductivity is well above setpoint, and meanwhile the bleed valve has been opening at odd times. Cleaning and recalibrating the probe, and fixing the valve, gets things back where they should be.
Simple checks you can build in
- Compare readings. Check the controller reading against a calibrated handheld meter regularly. If they disagree, believe the handheld and get the probe looked at.
- Watch the bleed operate. Does the valve open when conductivity reaches the setpoint and close again afterwards? Is water running to drain when it shouldn't be?
- Read the water meters. If you have meters on make-up and bleed, record them. Sudden changes in water use are an early sign of a problem.
- Keep an eye on chemistry. Conductivity, pH, hardness and inhibitor levels, as your written scheme sets out. Our monitoring page covers the usual frequencies.
- Look for scale. White or crusty deposits on the fill, pond walls or around the distribution are a visual clue that concentration has been too high.
Record it all in the log book. As we said in our post on what goes in a log book, trends are where the value is.
Don't forget the low-load months
In winter, many towers run at low load. There's less evaporation, so less make-up, less bleed and less inhibitor going in if dosing is proportional to make-up. Biocide still needs to reach the whole system, and low flow can leave parts of it stagnant. It's worth checking with your treatment contractor that the programme still works at low load, not just in summer.
In short
Bleed control keeps the water from getting too concentrated, and the setpoint is a balance between scale and waste. Keep the probe clean and calibrated, check the valve works, and watch the trends. A stuck bleed valve is one of the classic causes of trouble, alongside the others in our post on dosing failures. Our dosing and control service includes calibration and checks on every visit, and if you'd like your setpoints reviewed before summer, just ask.
Want your bleed and setpoints checked?
We'll check the controller, probe and valve, compare the chemistry with your make-up water and suggest sensible setpoints.