Heat Exchanger Area Calculator (Preliminary)

How is heat exchanger area calculated?

The heat a heat exchanger transfers depends on three things: surface area, the mean temperature difference between the two fluids and how easily heat crosses the surface. The relation is:

Q = U × A × F × ΔT_lm

• Q is the duty (W).
• U is the overall heat transfer coefficient (W/(m²·K)): the combined effect of convection on both sides, wall conduction and fouling.
• ΔT_lm is the log mean temperature difference and F the flow arrangement correction factor (1 for counter and parallel flow).

The area follows as A = Q / (U × F × ΔT_lm).

Where does the duty come from?

The duty usually comes from heating or cooling one of the fluids: Q = ṁ × c_p × ΔT. Heating 1.2 kg/s of water from 20 °C to 80 °C takes 1.2 × 4.18 × 60 = 301 kW.

Example

Suppose that 301 kW is supplied in a counter flow exchanger by geothermal water entering at 160 °C and leaving at 125 °C. The end differences are 80 K and 105 K and the log mean is 92 K. With U = 640 W/(m²·K) the required area is 301 000 / (640 × 92) = 5.11 m².

U and fouling

U is the weakest link in the calculation. Even for the same two fluids, velocity and exchanger type can change U several times over. That is why the tool suggests no value and only gives rough ranges in the help text.

Surfaces collect scale, rust and deposits over time. This is allowed for with a fouling resistance: 1 / U_f = 1 / U + R_f. In exchangers with a high U even a small fouling resistance increases the area noticeably.

Limits

This is a preliminary estimate. In real design U is calculated from the film coefficients on both sides, pressure drops are checked and mechanical design follows the code. If the outlet temperatures are unknown, use the effectiveness and NTU method instead of LMTD.