Single Pass vs Multi-Pass

Single Pass and Multi-Pass Arrangements

Buying Velocity With Pressure Drop

A pass is one traverse of the bundle. In a single-pass arrangement the tube-side fluid crosses once through every tube; in a four-pass arrangement it crosses four times through a quarter of them each time, so velocity is roughly four times higher at the same flow.

Why velocity matters. Below turbulent flow the tube-side coefficient collapses, and low velocity also lets particles settle and deposits build. Most liquid duty targets a velocity band that keeps flow turbulent without eroding the tube wall.

The pressure drop penalty is steep. Drop rises roughly with the square of velocity and with the length of the flow path, so doubling the pass count multiplies pressure drop considerably. The fourth pass frequently costs more in pumping than it returns in surface reduction.

Availability varies by series. The SU, WU, DSU, DWU, HTWU and the compact ranges offer multiple arrangements; the tank heaters are 2 pass throughout; the aftercoolers and GC are single pass because air and gas pressure drop is too expensive to spend.

U-tube pass counts are always even. The fluid enters one leg and returns through the other. Where a single pass is genuinely required, that points at straight-tube construction or a different range.