Heat transfer coefficient — W/(m²·K) and BTU/(h·ft²·°F)
h joins fluid, geometry and flow. Wrong unit means the exchanger is 5× too big or 5× too small.
Heat transfer coefficient h captures the combined effect of convection, boundary layer thickness and fluid properties. It slots directly into Q = hAΔT.
The exact factor
1 W/(m²·K) = 0.17611 BTU/(h·ft²·°F) > 1 BTU/(h·ft²·°F) = 5.678 W/(m²·K)
Typical values
Natural convection, air: 5–25. Forced convection, air: 25–250. Natural convection, water: 100–1000. Forced water: 500–10 000. Boiling water: 3000–100 000. Condensing steam: 5000–100 000.
Worked example — shell-and-tube exchanger
Water/oil exchanger: h_water = 3000, h_oil = 400. Tube wall thermal resistance ~0.0002 m²·K/W. U = 1 / (1/3000 + 0.0002 + 1/400) = 337 W/(m²·K). Q = UAΔT. For 50 kW and 30 °C LMTD, A = 50 000/(337 × 30) = 4.95 m². Convert h_oil incorrectly and area doubles.
Recap
h in W/(m²·K). Take the reciprocal sum for U through composite walls.
Try the converters mentioned in this article
FAQ
What is a good h for design?+
Depends on fluid and flow. Handbook correlations (Dittus-Boelter, Nusselt) generate h from Re, Pr and geometry.
Is fouling accounted for separately?+
Yes — fouling factors (h·ft²·°F/BTU or m²·K/W) add to 1/U and are conservatively assumed for design life.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.