How an open recirculating cooling tower loses and gains water, why its dissolved solids climb, and what the controller does to keep them in range.
A cooling tower cools water by evaporating a small part of it. Evaporation removes pure water and leaves the dissolved minerals behind, so the water left in the system gets steadily more concentrated.
| Flow | What it is | Carries minerals out? |
|---|---|---|
| Evaporation | The water that does the cooling | No — minerals stay behind |
| Drift | Fine droplets carried out of the tower by the air | Yes, but very little on a modern tower |
| Blowdown (bleed) | System water deliberately drained to control concentration | Yes — this is the control lever |
| Make-up | Fresh water added to replace all of the above | Brings new minerals in |
Make-up = evaporation + blowdown + drift and leaks. The only one of these the controller governs is blowdown.
A useful rule of thumb: evaporation is about 1 % of the recirculation rate for every 10 °F (5.5 °C) of range, where range is the temperature drop across the tower. A tower recirculating 1000 gpm with a 10 °F range evaporates about 10 gpm. Expressed per ton of cooling, it is roughly 0.03 gpm (1.8 gal/h) per ton.
| Job | Why | How, on the InteliHive |
|---|---|---|
| Bleed on conductivity | Keeps the concentration of dissolved solids at the target cycles, below the point where scale forms | Setpoint Control, Rising, on system conductivity — see Cycles of Concentration |
| Feed inhibitor | Scale and corrosion inhibitor must track the water added, since that is where the load comes from | Volume Dosing on a make-up meter, or Percent Timer — see Chemical Feed Methods |
| Control pH | Acid feed to hold pH where the inhibitor program needs it | Setpoint Control, Rising, on pH |
| Feed oxidizing biocide | Holds a residual of chlorine or bromine | Setpoint Control, Falling, on ORP |
| Slug non-oxidizing biocide | Periodic kill of what the oxidizer misses, with bleed held off so it is not drained away | Calendar — see Biocide Programs |
| Stop everything with no flow | Probes read stagnant water and pumps feed into a dead line | A flow switch interlock on every mode |
| Page | Covers |
|---|---|
| Cycles of Concentration | What cycles are, how blowdown sets them, choosing a conductivity setpoint, worked examples |
| Scale and Corrosion | LSI and Ryznar with a worked example, other scales, corrosion rates, and what the controller does about each |
| Chemical Feed Methods | Bleed-and-feed, bleed-then-feed, percent, and meter-proportional feed — and when to use each |
| Biocide Programs | Oxidizing and non-oxidizing biocides, pre-bleed, bleed lockout, and scheduling |
| Sample Line and Bleed Plumbing | Where the probes go, flow through the sample line, and plumbing the bleed valve and water meter |