Heat Transfer Coefficient
The heat transfer coefficient (h) is a proportionality constant that quantifies the rate of heat transfer per unit area per unit temperature difference between a surface and a fluid in contact with it. It combines the effects of conduction through the fluid boundary layer and convection driven by fluid motion, making it central to the design of heat exchangers, reactors, and process equipment. Higher values indicate more efficient heat transfer, and the coefficient depends strongly on fluid properties, flow velocity, geometry, and surface roughness.
Key Formula
q = h × A × ΔT
LaTeX: q = h \cdot A \cdot \Delta T
| Symbol | Meaning | Unit |
|---|---|---|
| q | Rate of heat transfer | W |
| h | Heat transfer coefficient | W/(m²·K) |
| A | Heat transfer area | m² |
| ΔT | Temperature difference between surface and fluid | K or °C |
Worked Example
Problem
Steam condenses on the outside of a pipe with h = 8,000 W/(m²·K). The pipe has outer diameter 0.05 m, length 2 m, and the steam temperature is 120 °C while the pipe wall is at 100 °C. Calculate the heat transfer rate.
Solution
Step 1: Calculate heat transfer area: A = π × d × L = π × 0.05 × 2 = 0.3142 m². Step 2: Temperature difference: ΔT = 120 − 100 = 20 K. Step 3: Apply Newton's Law of Cooling: q = h × A × ΔT = 8,000 × 0.3142 × 20. Step 4: q = 8,000 × 6.284 = 50,272 W ≈ 50.3 kW.
Answer
Heat transfer rate q ≈ 50.3 kW
Typical Heat Transfer Coefficient Values for Common Fluids and Conditions
| Fluid / Condition | h (W/m²·K) | Flow Type | Typical Application | Notes |
|---|---|---|---|---|
| Free convection, air | 5–25 | Natural | Finned radiators | Low h |
| Forced convection, air | 25–250 | Turbulent | Air coolers | Moderate h |
| Forced convection, water | 500–10,000 | Turbulent | Shell & tube HX | High h |
| Boiling water | 2,000–45,000 | Two-phase | Boilers, evaporators | Very high h |
| Condensing steam | 5,000–100,000 | Two-phase | Condensers | Highest h |
Interactive Tools
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Related Terms
Dimensionless Group
A dimensionless group is a combination of physical variables and constants that yields a pure number with no units, enabling the comparison and scaling of physical phenomena independent of the system of measurement. In chemical engineering, dimensionless groups like the Reynolds, Nusselt, and Prandtl numbers are used to correlate experimental data, predict transport phenomena, and scale laboratory results to industrial equipment. They arise from dimensional analysis (Buckingham Pi theorem) and are fundamental to similarity theory and the development of engineering correlations.
Pressure Drop
Pressure drop (ΔP) is the reduction in fluid pressure between two points in a flow system due to frictional resistance from pipe walls, fittings, valves, packed beds, or other flow restrictions. It determines the pumping or compression power required to maintain flow and is a critical factor in the economic design of pipelines, heat exchangers, distillation columns, and catalytic reactors. For incompressible flow in pipes, the Darcy-Weisbach equation relates pressure drop to fluid velocity, pipe geometry, and friction factor.
Industrial Evaporation
Industrial evaporation is the unit operation in which a solvent (usually water) is vaporised from a dilute solution or liquid feed to concentrate a dissolved solute, using heat supplied by steam or other heating media. Unlike drying, the product remains a concentrated liquid rather than a solid, and the process is continuous in industrial settings. It is central to the sugar industry, dairy processing (condensed milk), caustic soda production, and fruit juice concentration, and is typically performed in multiple-effect evaporator trains to maximise steam economy.
The concept formalised by Newton in his Law of Cooling (1701), later attributed to Josef Stefan and Ludwig Boltzmann. "Coefficient" comes from Latin "co-" (together) + "efficiens" (accomplishing). The symbol h was standardised in heat transfer literature by McAdams (1933) and subsequent ASME standards.