AIR-COOLED HEAT EXCHANGER DESIGN

THEORY  &   FORMULAE

Sizing of Air-Cooled Heat Exchanger

Heat exchangers are systems that transfer heat between fluid mediums. The fluids or gases in a heat exchanger can be mixed or the energy transference can go through a conductive wall that keeps them separate. Heat exchangers are found in car radiators, furnaces, refrigerators, air conditioning, space heating, refining and chemical processing systems. Air-cooled heat exchangers typically have rectangular bundles containg several rows of tubes. The hot fluid enters at the top of the bundle, while air is blown by fans vertically upwards across the tube bank, i.e. counter current flow.

The calculator here is based on the correlations presented by Smith and Brown, and the series of equations presented by Blackwell to fit the graphs and tables of Smith and Brown. In brief, the method begins with the first equation, hinges on the iterative solution of the second equation below, and ultimately leads to the third equation, as described by Coker:
    
where
     R = number of tube rows
     U = overall heat transfer coefficient
     Q = exchager duty (heat load)
     C_i^'s = correlation constants
     t₁ = air outlet temperature
     t₂ = air inlet temperature
     T₁ = process fluid outlet temperature
     T₂ = process fluid inlet temperature
     A_f = face area of bundle
     V_f = face velocity of air
     W = tube bundle width
     L = tube width