Technical note

Three signals before your heat exchanger scales

Approach temperature and vacuum move first, not water temperature. Three numbers to log weekly, and when to act.

Scaling does not happen overnight

Most plants notice scaling when a heat exchanger is already blocked and output drops. In practice the system had been sending signals for weeks — nobody was recording them. This note lists the signals in the order they appear and the three numbers worth logging every week.

1. The approach temperature moves first

Heat exchanger approach temperature (the difference between the hot-side outlet and the cold-side outlet) is the most sensitive indicator of fouling: the deposit is a thermal resistance, so the same heat duty needs a larger temperature difference once it forms. If the approach creeps up while circulating water temperature, flow and load stay flat, something is growing on the heat transfer surface.

Steam condensers give a second, blunter signal: vacuum. Falling vacuum and a unit that will not reach rated load usually show up earlier than a temperature alarm. Many plants only watch water temperature — approach and vacuum are never logged, so the first warning arrives at the point where the unit must be taken off line for cleaning.

2. Then the cleaning interval shortens

When the gap between cleanings gets shorter, the fouling rate has increased. It is a lagging indicator, but an honest one: it tells you the current treatment can no longer keep up with the water. Writing the cleaning date on every exchanger is the cheapest monitoring you will ever install.

3. Three numbers on the water side

Equipment readings are not enough. Measure calcium hardness, total alkalinity and chloride at least weekly. The first two drive the scaling tendency used in LSI/RSI; chloride sets the corrosion risk and the material boundary. Measure make-up water and circulating water together and calculate cycles of concentration, so you know how far the water has been concentrated.

GB/T 50050-2017 sets explicit limits: cycles of concentration should not be below 3.0, and treatment measures are required when calcium hardness plus total alkalinity exceeds 1100 mg/L as CaCO₃, or when RSI is below 3.3. These are design boundaries, not suggestions.

4. Log three numbers, review every two weeks

The log sheet only needs three columns: approach temperature, cycles of concentration, days since last cleaning. If the approach rises for two consecutive weeks while cycles stay flat, do two things: recalculate LSI/RSI, and check the actual run rate of the side-stream treatment — plenty of systems have equipment installed that has been running at low load for months, which is the same as not having it.

5. When to change the treatment approach

Any one of these means "more chemical and more frequent cleaning" has run out of road: cleaning intervals have halved against design; the approach no longer returns to baseline after cleaning; blowdown is capped by discharge limits or water cost. Adding more chemical at that point only moves the problem downstream, while chemical cost, discharge cost and downtime all rise together.

If your plant is stuck at that point, send us one month of approach readings plus calcium hardness, alkalinity and chloride for both make-up and circulating water. We will recalculate the water balance and give you a calculation sheet showing how long the current approach can hold and where the boundary sits with side-stream electrochemical treatment — assumptions and data gaps stated.