Pressure drop in a recuperator causes hydraulic energy loss. This loss has to be
subtracted (in primary energy) from the energy (heat) savings, giving the overall savings.
Though common sense is that turbulent flow leads to the highest heat transfer,
the connected pressure drop (quadratic with the flow) causes a lower overall
heat recovery in smaller ducts.
Laminar flow has the advantage of a low pressure drop (linear with the flow)
and a high heat transfer in small ducts (small distance for the conductive heat flow).
The effectiveness as function of the hydraulic diameter of the ducts, both only
thermal and overall, for a recuperator operating at a 10°C temperature difference
is shown in fig.1. The recuperator size is chosen such that, at its overall maximum,
it has the required thermal effectiveness of 93%
Figure 1 overall and thermal effectiveness as function of the
hydraulic diameter of the ducts
The optimum hydraulic diameter is in the order of 1mm.