Heat Exchanger Sizing Calculator

Required area is A = Q/(U×LMTD), so it's inversely proportional to the overall heat transfer coefficient (U). Water-to-oil systems have a much lower U — roughly 20-60 BTU/hr·ft²·°F versus 150-300 for water-to-water — because oil's poor thermal conductivity and higher viscosity resist heat transfer, so for the same duty and temperature driving force, a water-to-oil exchanger needs proportionally more surface area.

The mass flow rate conversion from GPM is hardcoded to water's density (8.33 lb/gal), so the calculator always assumes a water-like fluid for that step, but heat duty (Q = ṁ·Cp·ΔT) still depends on the fluid's actual specific heat. If you're sizing for glycol or oil, enter that fluid's specific heat (glycol ≈ 0.81, oil ≈ 0.45-0.55) even though the density behind the flow conversion isn't adjusted.

Effectiveness is the actual heat transferred divided by the theoretical maximum possible with infinite area, assuming the hot stream is the side with the smaller thermal capacity rate (mass flow × specific heat). If your cold side actually has the smaller capacity rate instead, that assumption breaks and the number won't reflect the true thermodynamic limit — you'd need to identify the real minimum-capacity side yourself.

The standard log-mean formula divides by the natural log of the ratio of the two end-temperature differences, which is undefined when those differences are equal (the ratio is 1, and ln(1) = 0). The calculator detects that near-equal case and falls back to the simple arithmetic difference, since in the limit the log-mean and arithmetic mean converge to the same value anyway.