Header flow maldistribution check

How this calculator works

When a process fluid splits from a single header into several parallel branches — heat exchangers in parallel, air cooler bays, reactor tubes or distribution manifolds — the flow through each branch depends on the pressure drop through the branch relative to the pressure variation along the header. The key parameter is the ΔP ratio = branch ΔP / header ΔP.

When the branch ΔP is much larger than the header ΔP (ratio above 10), the header behaves almost as a constant-pressure plenum, and every branch sees nearly the same driving force: good distribution, with less than about 5 % maldistribution. When the ratio falls below 5, the branches nearest the header inlet get significantly more flow than those at the far end: poor distribution.

This calculator estimates the header ΔP from friction along the header plus one velocity head (a conservative treatment of momentum effects), and the branch ΔP from friction through the branch pipe and its equivalent length of fittings. Both use the Colebrook friction factor.

Example: 4 branches of 100 mm pipe (10 m long + 5 m equivalent fittings), fed from a 300 mm header 3 m long, total 200 m³/h of water at 30 °C: branch ΔP 0.04 bar, header ΔP 0.003 bar, ratio 14 — good distribution with about 7 % maldistribution.

Limits: the estimate is a screening tool; real maldistribution depends on the branch take-off arrangement (dividing or combining tee), the equipment ΔP in each branch (heat exchangers, orifice plates), two-phase flow and detailed header geometry. For critical applications use computational fluid dynamics (CFD) or a detailed manifold model. Adding orifice plates or balancing valves in each branch is the usual fix when the ΔP ratio is too low.