The air-cooled heat exchanger

The air-cooled heat exchanger, also known as dry cooler, air-cooler or fin-fan cooler, essentially consists of a finned tube heat exchanger bundle arranged above a fan plenum chamber. An induced draft fan respectively draws air across the finned tube bundles. The cooling water flows through the finned tubes, and heat is transferred from the water to the ambient air. With air-cooled heat exchangers, it is not possible to cool the medium below the ambient dry bulb temperature. In this case the difference between the medium outlet temperature and the air inlet temperature is generally referred to as the terminal temperature difference (TTD) or the approach temperature and values in the range of 7-8°C are considered economically feasible [4].

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Fig. 2. Air-cooled heat exchanger

The dry-cooler is an air-cooled compact, finned-tube heat exchanger. The type of surface selected is a finned circular tubes, surface 8.0-3/8T [8], the surface geometric characteristics are:

• Tube outside diameter = 0.0102 m

• Fin pitch = 315 per meter

• Flow passage hydraulic diameter = 0.00363 m

• Fin thickness = 0.00033

• Free-flow area/frontal area, ct = 0.534

• Heat transfer area/total volume, a = 587 m2/m3

• Fin area/total area = 0.913

• Number of tubes = 120

• Number of rows = 6

• Number of passes = 10

The heat transfer from a bank of circular finned tubes in counter flow with both fluids unmixed is considered here. The air and the fluid outlet conditions as well as the fans consumption are calculated with the EES code. The finned-tube rows are staggered in the direction of the fluid velocity.

The heat transfer characteristics of the finned-tube heat exchanger are:

• Heat exchanger effectiveness = 0.68

• NTU (Number of Transfer Units) = 1.893

• Water-side heat transfer coefficient = 12.75 (kW m-2 K-1)

• Air-side heat transfer coefficient = 0.14 (kW m-2 K-1), [9]

The dimensions of the dry-cooler were supplied by the manufacturer, 1.050 m x 2.725 m x 0.15 m for 40 kW nominal cooling capacity. Three fans are installed above the compact heat exchanger and the maximum power of each fan is 0.9 kW.