Heat Exchanger Duty Calculator

Computes heat exchanger duty using the classic Q = U × A × ΔTlm relationship, where the log mean temperature difference (LMTD) captures how the temperature gap between hot and cold streams changes as they move through the exchanger. The calculator handles both counter-current flow (hot and cold streams moving in opposite directions, where ΔT1 is the hot-inlet/cold-outlet gap and ΔT2 is the hot-outlet/cold-inlet gap) and parallel/co-current flow (both streams entering the same end), which changes which temperature pairs define the two end-approach differences. LMTD itself is (ΔT1 − ΔT2)/ln(ΔT1/ΔT2); when the two approach temperatures are nearly equal the formula is numerically unstable (dividing by a near-zero log), so the calculator falls back to using ΔT1 directly, which is the correct limiting value from L'Hôpital's rule.

Duty is reported in kW, W, and BTU/hr for convenience. The overall heat transfer coefficient U is the single biggest source of uncertainty in any real exchanger duty estimate — it depends heavily on fluid properties, fouling, and exchanger geometry, so the helpText ranges given (800-1500 W/m²K for water-water, much lower for gas-gas service) are only starting points, not substitutes for vendor data or a fouled-condition design margin. Note that counter-current arrangements always achieve a higher LMTD than parallel flow for the same four terminal temperatures, which is why counter-current is preferred whenever the process allows it — this calculator will show that difference if you toggle the flow arrangement with the same four temperatures.