Heat Transfer Basics
Understand the three modes of heat transfer, how they apply to industrial equipment, and the fundamental equations that govern thermal engineering.
Three Modes of Heat Transfer
- Conduction: Heat transfers through a solid material from the hot side to the cold side. The rate depends on the material's thermal conductivity and the temperature difference. Metals conduct well; insulators conduct poorly.
- Convection: Heat transfers between a solid surface and a moving fluid. The fluid carries heat away from (or toward) the surface. The rate depends on fluid properties, flow velocity, and the temperature difference.
- Radiation: Heat transfers through electromagnetic waves (infrared). Does not require a medium (can travel through vacuum). Significant at high temperatures (above 500 degrees C). Usually negligible in typical process heat exchangers.
Key Equations
The overall heat transfer coefficient (U) accounts for all resistances to heat transfer: convection on the hot side, conduction through the wall, convection on the cold side, and any fouling layers. Designers must select a U value appropriate for the fluids, flow arrangement, and expected fouling.
Heat transfer rate
Q = U x A x LMTD. Where Q is heat transfer rate (W), U is overall heat transfer coefficient (W per square metre per degree C), A is heat transfer area (square metres), and LMTD is Log Mean Temperature Difference (degrees C). This is the fundamental equation for all heat exchanger calculations.
LMTD Explained
The Log Mean Temperature Difference (LMTD) is the effective temperature difference that drives heat transfer in a heat exchanger. It accounts for the fact that the temperature difference changes along the length of the exchanger.
For counter-flow (fluids flow in opposite directions), LMTD is highest, giving the most efficient heat transfer. For parallel flow (same direction), LMTD is lower. Counter-flow can achieve closer approach temperatures than parallel flow.
Typical U Values
| Fluid Combination | U (W/m2/C) | Notes |
|---|---|---|
| Water to water | 1,000 to 2,500 | Very efficient. Common in heating/cooling systems |
| Water to oil | 100 to 350 | Lower due to oil's lower thermal conductivity |
| Steam to water | 1,000 to 3,000 | Condensing steam has very high heat transfer coefficient |
| Steam to air | 30 to 300 | Air side is the bottleneck. Use fins on the air side |
| Water to air | 30 to 100 | Common in cooling coils. Air side limits performance |
| Oil to oil | 100 to 300 | Lower thermal conductivity on both sides |
Tip: When a heat exchanger's performance drops, it is usually because fouling has added a thermal resistance layer. The overall U value drops, reducing heat transfer. Monitoring the calculated U value over time is the best way to detect fouling. Clean when U drops 20 to 30% below the clean baseline.