Both Sides Matter, and the Pressure Drop Is the Input People Omit

A liquid-to-liquid selection needs six numbers before it can be made properly: flow rate, inlet temperature and required outlet temperature on each side. Two more make it reliable: the allowable pressure drop on each side. That last pair is the input most often left off an enquiry and the one that most often changes the answer. An exchanger can meet the thermal duty perfectly and still be the wrong unit, because the pump in the system cannot push the design flow through it. On oil duty this happens routinely, because oil-side pressure drop rises steeply with both flow and viscosity.

The second thing worth stating is which side fouls. On a U-tube bundle the cleanable side is the shell side, because nothing rigid passes a 180 degree bend. That makes the standing rule straightforward: clean fluid in the tubes, fouling fluid in the shell. It is worth checking on an existing installation, because a change of duty over twenty years occasionally leaves the fouling stream on the side that cannot be reached, and no amount of chemical cleaning fixes a geometry problem.

Pass count is how tube-side velocity is bought. Two-pass is standard across the WU range; four and six pass arrangements raise velocity where the flow rate alone would leave the tubes laminar and the coefficient poor. Each additional pass costs pressure drop, and the increase is steeper than the thermal gain, so the optimum is frequently at two or four rather than higher. The compact CHX and CHXS ranges include single-pass arrangements precisely because on a small shell there is not much flow to distribute and forcing it through four passes buys velocity a small pump cannot pay for.