The ways a controller can decide how much scale and corrosion inhibitor to feed a cooling tower, what each gets right and wrong, and how to size the feed.
Inhibitor is lost the same way minerals are — out with the blowdown and drift. To hold a steady level in the system, feed has to replace what leaves. Every method below is a way of estimating that loss; they differ in how closely they track it.
| Method | How it decides | Tracks the load? | On the InteliHive |
|---|---|---|---|
| Meter-proportional | A dose for every set volume of make-up water | Yes — make-up is the load | Volume Dosing |
| Percent timer | On for a fixed share of every cycle, whatever the tower is doing | No | Percent Timer |
| Bleed-and-feed | The feed pump runs whenever the bleed valve is open, with a limit on each run | Roughly | Not offered as a mode; use Volume Dosing |
| Bleed-then-feed | After each bleed closes, feed for a percentage of how long it was open | Roughly | Not offered as a mode; use Volume Dosing |
| Scheduled | Fixed times on a calendar | No | Calendar — for biocides, not inhibitor |
A water meter on the make-up line counts every gallon that comes in, and the controller feeds a set dose per volume. Feed rises and falls with the tower’s load automatically, and it does not depend on the bleed at all. This is the method to use whenever a make-up meter is available.
A fixed feed rate, set once. It is right only at the load it was set for: on a cool day or a shut-down weekend it overfeeds, and on the hottest days it underfeeds. Use it where there is no meter, and protect it with a flow switch interlock so it cannot feed into a stagnant system.
Older controllers often ran the inhibitor pump alongside the bleed valve. It is simple, and feed does follow load in a rough way — a heavily loaded tower bleeds more. But it feeds only while bleeding, which is uneven, and if the bleed line clogs the pump runs on with nowhere for the water to go. Controllers that offer it pair it with a limit timer that stops the pump after a set time in any one bleed.
A refinement: the controller times each bleed, then runs the pump for a percentage of that time after the bleed closes, so fresh inhibitor is not immediately bled out. It still depends on the bleed, and still needs a maximum feed time per cycle.
The dose is set to give the target product level in the system, which is the make-up level multiplied by the cycles.
Product per make-up gallon = target in the system ÷ cycles
Example. The program calls for 100 ppm of inhibitor in the system, and the tower runs at 5 cycles. Each gallon of make-up needs 100 ÷ 5 = 20 ppm.
Check the answer by daily usage. At 12.5 gpm of make-up (see Cycles of Concentration), the tower takes 18,000 gal/day: 180 doses × 7.6 mL ≈ 1.4 L (0.36 gal) of product a day. Compare that with drum draw-down after a week, and adjust.