What Really Happens on the Water Side of Finned Pack Heat Exchangers?

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Figure 1 shows a simulation of the problem. The first row represents the design conditions: to heat 10,700 Nm³/h of air from –8 to 20° C against water which is cooled from 90 to 70° C. A precisely selected coil transfers the required 100 kW. Row 3 shows what happens if with the same coil we increase the heat duty to 120 kW by reducing the air inlet temperature to –13.6 °C, leaving the air outlet temperature unchanged at 20° C. The result is disappointing: increasing the water flow by 20 percent to 5.3 m³/h increases the heat duty by only 10 percent, to 110 kW.

A Look at the Theory of Heat Transfer

Four equations apply to the coil:

Conservation of energy: capacity on the air side (variable 1) = capacity on the water side (variable 2).

Air balance: capacity on the air side corresponds to the air flowrate (variable 3) and the inlet (variable 4) and outlet (variable 5) states of the air.

Fluid balance: capacity on the fluid side corresponds to the fluid flowrate (variable 6) and the inlet (variable 7) and outlet (variable 8) temperatures of the water.

Heat transfer: the heat duty is specified by the logarithmic mean temperature difference between air and water, the heat transfer surface area, the coil design, the velocity of the water inside the tubes, and the velocity of the air flowing over the coil.

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