Cooling Tower Control for Plating and Etch Lines

A cooling tower removes heat from the water circuits that serve rectifiers, heat exchangers, chillers and etch lines, and it does so by evaporating a small part of the water it circulates. That evaporation is the whole mechanism, and it also explains why everything else about the tower is a water chemistry problem. Water leaves as vapour while the dissolved solids stay behind.

In a plating shop the consequence is direct. A tower that cannot reject the heat will raise the temperature of a plating or etch bath, and a bath that is too warm changes its own chemistry. Cooling capacity problems rarely announce themselves as cooling problems; they appear first as a plating defect or as an etch rate that will not hold. A tower that is short of capacity is a process variable, not a utility detail.

What the Cooling Tower Has to Remove

The heat load is the sum of every duty served by the tower, and in a plating shop that includes rectifier cooling, bath heat exchangers and any compressed air or chiller condenser on the same loop. Each duty has a design figure, and the tower has to reject the total at the worst combination of ambient conditions. A hot, humid day is the design case, not an average day.

Because the load is shared, a fault anywhere on the loop shows up as a general loss of cooling capacity. A partly blocked heat exchanger on one tank will raise the return water temperature and reduce the margin available to every other duty. The heat exchanger itself is described in the plating heat exchanger control notes.

Heat Load and the Plating Schedule

The load follows production, and production is not steady. A shop that runs a heavy plating schedule on two shifts and a light one on the third will see the tower load swing with the rota. Capacity planning has to follow the peak rather than the average, which is one reason cooling systems are usually sized with margin. Instrumentation on the return line is what makes that total visible.

Where the tower is running close to its limit, the schedule becomes a control. Moving a heavy current job away from the hottest part of the afternoon is a crude but effective measure, and it costs nothing except planning. A shop that tracks tower load against production can see the peak coming instead of reacting to it.

Water Chemistry and Cycles of Concentration

Evaporation concentrates everything dissolved in the make-up water, and the ratio between the concentration in the circulating water and the concentration in the make-up is the number of cycles. Each cycle of concentration reduces the water that has to be bought and treated, so running more cycles is cheaper on water. It also raises the risk of scale and corrosion. Every cycle saved is water that does not have to be bought or discharged.

The practical limit is set by the chemistry of the make-up water and by the treatment programme. Hard water reaches its scale limit sooner than soft water, which is why the same tower can run at very different cycles in two locations. Water treatment for the site is described in the PCB process water treatment notes.

Blowdown, Make-Up and Conductivity

Blowdown is the deliberate removal of circulating water to keep the concentration in check, and it is normally controlled from conductivity. The conductivity of the circulating water is compared with the make-up water, and the difference drives the blowdown valve. This makes conductivity the single most useful control on the tower.

The blowdown rate should be set so that the concentration stays below the limit set by the treatment programme rather than at a figure someone once chose. A tower that is blown down too little will scale; one blown down too much wastes water and treatment chemical. Both faults are visible in the same reading.

Fouling, Scale and Biological Growth

Scale forms on the hot surfaces first, and a thin layer of scale reduces heat transfer far more than its thickness suggests. Deposits also create places for biological growth to establish, and the growth then blocks the fill and reduces the tower performance further. The three problems feed each other. A tower that looks clean from the ground can still be fouled inside the fill.

Fouling of the fill and the distribution deck is usually visible as dry patches on the tower or as water channeling instead of even flow. Cleaning should follow inspection rather than a calendar, because a tower on dirty make-up water will foul faster than the schedule assumes. Treatment chemical dosing should be checked at the same time.

Drift, Aerosol and Safety

Drift is the small fraction of water carried out of the tower as droplets, and it carries dissolved material and any biological growth with it. Drift eliminators reduce it, and their condition is therefore a safety matter as well as a water loss matter. A damaged eliminator should be repaired promptly.

Cooling towers are a recognised source of aerosol borne bacteria, so the treatment programme has to include a biocide regime and the tower has to be cleaned on a schedule. Records of treatment and cleaning are usually required by local regulation as well as by good practice. Anyone working on the tower should follow the site procedure for cleaning and disinfection.

Winter, Shutdowns and Freeze Protection

A tower in a cold climate needs freeze protection, and the usual answers are basin heaters, a remote sump or a controlled fan cycle. A tower that freezes will crack the fill and the basin, and the repair takes longer than the protection would have cost. The control logic has to be tested before the first cold night rather than after it. The control logic is worth testing before the weather turns cold.

Shutdowns bring their own problems, because water that stands in a warm tower is a good environment for growth. A tower that is to be idle should be cleaned, drained where possible and returned to service with a full treatment dose. Bringing it back without a check is how a biological problem is introduced to a loop that was clean.

Records and Verification

The record should carry conductivity, cycles, blowdown rate, treatment dose, make-up volume and the dates of cleaning and inspection. Those figures show whether the tower is running economically and whether the treatment programme is holding. They also show when the tower is being asked for more than it can deliver. Blowdown disposal is covered in the PCB waste treatment and recycling notes. A log that is kept for a year shows whether the tower is drifting.

Acceptance of the finished work follows the published IPC documents, and a bath that holds its temperature is part of what makes that acceptance repeatable. A cooling problem that is traced quickly is a plating problem that never happens.

FAQ

Why does plating quality change on hot days? Because the cooling tower has less margin, so bath temperatures rise and the chemistry behaves differently.

What is the single best cooling tower control? Conductivity, because it sets the cycles of concentration and therefore drives the blowdown.

Can a cooling tower be shut down without treatment? No. Standing warm water encourages growth, so the tower should be cleaned and dosed before it is returned to service.