Heat Exchanger Efficiency: How to Calculate & Improve

Key Performance Metrics

• Overall heat transfer coefficient (U-value): Measures how effectively the exchanger transfers heat. Expressed in W/m²·K. Compare the actual U to the design U — when actual drops to 70% of design, cleaning is needed.
• LMTD (Log Mean Temperature Difference): The effective temperature difference driving heat transfer, accounting for the changing temperature along the exchanger. Used in the basic design equation: Q = U × A × LMTD.
• Effectiveness (ε): The ratio of actual heat transfer to the maximum possible heat transfer. Ranges from 0 to 1. An effectiveness of 0.8 means the exchanger achieves 80% of the theoretical maximum.
• Approach temperature: The temperature difference between the hot outlet and the cold inlet (or vice versa). A smaller approach means better heat recovery. In cooling applications, the cold outlet should approach the hot inlet.

Calculating the U-Value

The overall heat transfer coefficient is calculated from operating data:

Where: Q = heat duty (W), A = heat transfer area (m²), LMTD = log mean temperature difference (K)

Step 1: Calculate Q from the cold side: Q = m × Cp × (T_cold_out − T_cold_in)

Step 2: Calculate LMTD:

Where ΔT₁ = hot inlet − cold outlet, ΔT₂ = hot outlet − cold inlet (for counter-flow)

Step 3: Calculate U = Q / (A × LMTD)

Typical U-Values

Why Efficiency Drops

• Fouling — the most common cause. Scale, biological growth, or particulate buildup insulates the heat transfer surfaces.
• Reduced flow rate — lower velocity reduces the heat transfer coefficient (turbulent flow transfers heat better than laminar flow).
• Air in the system — air pockets on the hot side reduce effective heat transfer area.
• Bypass flow — internal baffle damage allows fluid to bypass the heat transfer surfaces.
• Tube blockage — blocked tubes reduce the effective heat transfer area.

Improving Efficiency

• Clean regularly — the most impactful action. Schedule cleaning when U drops to 70% of design.
• Maintain design flow rates — don't operate below 70% of design flow on either side.
• Install antifoulant chemical treatment for process-side fouling
• Use higher-efficiency plate heat exchangers instead of shell-and-tube where pressure ratings allow
• Install an economiser to preheat feedwater with waste heat from the flue gas
• Insulate the exchanger and all piping to minimise heat loss to ambient
• Consider adding surface area (more plates or longer tubes) if the exchanger is chronically undersized