Technical note

Filter installed, heat exchanger still scaling?

Filtration removes particles; electrochemical treatment removes dissolved ions. Different jobs, different installation order.

They solve different problems

A question we hear on site: "We already have a side-stream filter — why is the heat exchanger still scaling?" The answer is simple. A filter removes what has already come out of solution — suspended solids, silt, algae, broken-off scale flakes. Scaling is calcium and magnesium still dissolved in the water coming out of solution on the heat transfer surface. A filter cannot touch dissolved ions, so it is not a substitute for scale control; equally, electrochemical treatment is not a substitute for filtration.

1. What filtration does — and does not do

It does: lower turbidity, capture silt, algae debris and detached scale, and reduce deposition in low-velocity zones and the under-deposit corrosion that follows. It does not: lower calcium hardness, lower alkalinity, or change LSI. Those two are the drivers of scaling tendency. So "with a filter we don't need to worry about scale" is false for any water chemistry.

2. What electrochemical treatment does

A side stream is drawn through an electrolytic cell where calcium and magnesium deposit on the cathode and leave the system with blowdown. It changes how much of those ions remain in the water — it removes the scale-forming components. That is mechanically different from both chemical treatment (keeps the ions in but stops them crystallising) and filtration (removes particles).

3. Why filtered plants still scale

Because make-up calcium hardness and alkalinity have not changed. Once cycles of concentration rise, both rise with them, LSI climbs, and the surface scales anyway. Turbidity can be excellent and the water can look crystal clear — clear does not mean non-scaling. This is why so many plants report "the water is clean but the approach temperature keeps rising".

4. How to combine them

Suggested order: first check make-up turbidity and suspended solids. If they are high, install side-stream filtration first — otherwise particles foul both the equipment and the heat transfer surface. Then check calcium hardness and alkalinity: if they are high and LSI is likely to exceed limits, add side-stream electrochemical treatment to remove the scale-forming ions. Finally tie both to blowdown and cycles of concentration so the system runs inside the standard's limits. Typically each takes its own side stream from the circulating pump discharge and returns to the tower basin, so they do not compete for flow.

5. One-line diagnosis

High make-up turbidity, silt at the basin bottom → filtration first. Clear make-up but high calcium hardness and alkalinity, approach rising → electrochemical first. Both → both, in the order above. If unsure, send us the make-up and circulating water analyses; we will work the water balance and say which to install first, what each saves, and the payback period.