Heat Exchangers

A recreation complex's domestic hot water was lukewarm despite the boiler system running. The piping arrangement allowed boiler water to flow backwards through the shell-and-tube heat exchanger, feeding it return water instead of supply. The facility manager's parting question — 'How could they balance it when it's flowing backwards?' — remains unanswered.

Logarithmic mean temperature difference (LMTD) is the driving force behind heat transfer, and counter-flow arrangements produce a higher LMTD than parallel flow. For installers, the rule is simple: hot-to-hot, cold-to-cold — or for coils, air in where water goes out. Follow the labels.

A radiant floor system isolated by a flat plate heat exchanger wasn't working because the PHE had become plugged with pebbles and debris. Two strainers were installed downstream of the exchanger instead of upstream — protecting nothing. A reminder to always put strainers where they'll actually do some good.