Process Cooling
U-Tube Heat Exchangers

Process Cooling U-Tube Heat Exchangers

Tower, Chilled and Glycol Water Duty

Cooling water arrives from a tower, a chiller or a glycol loop, and each brings a different supply temperature, a different cleanliness and a very different cost per unit of heat removed. That choice, more than anything in the exchanger, sets what the duty will cost to run.

Tower water is the workhorse and the dirtiest. Its supply temperature follows wet bulb, so the hot humid day is the design case and winter is the control problem. It also carries whatever the tower scrubs out of the air, concentrates dissolved solids as it evaporates, and supports biological growth. Everything about a tower-water cooler is a fouling decision.

Which stream goes in the tubes. On a U-tube exchanger the tube interiors cannot be mechanically cleaned, so the cleaner stream belongs there. Where tower water is the dirtier of the two, it goes on the shell side despite the widespread instinct to put cooling water in the tubes. Getting this backwards produces an exchanger that cannot be maintained.

Fouling factor is a design decision, not a default. An over-generous allowance produces excess surface, and excess surface means lower velocity when the unit is clean. Low velocity encourages the very deposition the allowance was meant to cover, so the exchanger fouls its way toward the condition it was designed for. An inadequate allowance produces a unit that meets duty when clean and fails within months. Your own water quality and cleaning history beats any published table.

Seasonal swing is larger than most designs acknowledge. A tower supplying 85 degrees in August may supply far cooler water in February. The exchanger has to be sized for the summer case and controlled through the winter one, and where the range is very wide a control valve on the coolant side or a bypass arrangement is usually needed for stable operation.

Clean on condition, not on the calendar. Trend the approach temperature or the overall heat transfer coefficient over time. When it degrades past a threshold you have set, clean. Plants that adopt this almost always find they clean less often and have better availability than plants cleaning to a fixed schedule.

Related pages: gas cooling, hot water and glycol loops, titanium and copper-nickel and replacement bundles.