Technical note

Side-Stream Flow: Three Constraints

Side-stream flow is not a guessed percentage: size from scaling rate, solids load and equipment hydraulics, then take the largest.

“How much side-stream flow do I need?” It is the first question when adding electrochemical treatment or side-stream filtration. The answer is not a percentage; it is the largest of three constraints: scaling rate, suspended-solids load, and the equipment's own flow window.

1. Size it from the scaling rate first

The amount of scale formed depends on the calcium, magnesium and alkalinity entering with make-up water, and on the cycles of concentration. Convert make-up calcium hardness and total alkalinity to calcium carbonate equivalent, multiply by make-up flow and the system's retention, and you get the mass of scale-forming material entering per day. The side-stream treatment capacity must exceed that mass, otherwise scale still accumulates on heat-transfer surfaces. The first step is therefore not choosing a flow but calculating water quality and quantity.

2. Size it from suspended solids and biofilm load

A cooling tower is a natural scrubber: dust, micro-organisms and algae enter with air. Side-stream filtration and electrochemical treatment together determine how clean the circulating water can stay. Turbidity, suspended solids and heterotrophic plate count are three measurable indicators. If make-up turbidity is high or the tower sits in a dusty area, the side-stream ratio must be higher; removing scale but not mud still leaves exchangers fouled.

3. Check it against equipment capacity and hydraulics

Electrochemical equipment has rated flow and residence-time requirements. Too little flow means low efficiency; too much means insufficient treatment per pass. Selection must consider side-stream pump head and piping resistance, the location of take-off and return points (to avoid short-circuiting), and the equipment's allowable inlet temperature and pressure. Many site failures are not equipment failures: the take-off sits in a dead zone and treated water returns next to the suction.

4. How engineering practice brackets it — and how to verify

Side-stream flow is usually expressed as a percentage of circulating flow, but the number varies with water quality and duty. We deliberately do not quote a universal percentage here, because it would mislead. The correct method: calculate the minimum treatment flow from the scaling rate, add a margin for suspended solids, then verify piping and pump capacity hydraulically — and take the largest of the three. If the three differ widely, the system needs another fix first (additional filtration or a make-up change).

5. Send us the data

We need the four make-up water numbers, circulating flow, cycles of concentration, tower type and surroundings, and details of any existing side-stream equipment. We will work through the sequence above and give the recommended side-stream flow, take-off and return locations, and the assumptions behind the check — not a single number claimed to fit everything.