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

Related Vendors

Thus we have eight variables and four equations. If we specify the inlet and outlet temperatures of the water, the inlet state of the air and the air flowrate (four variables in total), then for any particular coil we have four equations and four remaining variables. As a result, the capacity of the coil on the air side (and hence also on the fluid side), the outlet state of the air and the water flowrate are all uniquely determined.

Coils from Standard Series Heat Exchanger

For economic and logistical reasons most AHU manufacturers today use a preselected set of standard water coils, typically 3–12 sizes of heating coils and 3–12 sizes of cooling coils. Faced with the design specification for the example above, imagine that we do not have the coil needed to meet the duty exactly (a single-row coil with a fin pitch of 1.81 mm and 20 coil circuits). Instead we must choose between two standard models: a single-row coil with a fin pitch of 2.12 mm and 20 coil circuits (Figure 2), or a two-row coil with a fin pitch of 2.54 mm and 20 coil circuits (Figure 3). Each simulation screenshot shows three scenarios:

Fix water temperatures at 90 °C inlet and 70 °C outlet; or

Set water flowrate at 4.4 m³/h as in the original specification to meet the heat duty; or

Set the heat duty at 100 kW.

Of the two options, we will probably select the two-row coil (Figure 3), but we must bear in mind that in this case if we run the system so that the heat duty is 100 kW as specified, the water outlet temperature will be only 37° C.

From this we can conclude that if a ven-dor using a limited range of coils claims to be able to match both the design heat load and the specified water outlet temperature, this is probably wishful thinking. It is understandable, however, because the vendor knows that to meet the specified water outlet temperature, the air outlet temperature will have to differ from the specified value. Design engineers often put too much emphasis on the water side, either because they are being pedantic or simply because they do not understand the compromises involved.

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