This variation trend
can be understood from two aspects. First, at fixed shell inner
diameter, the helix pitch B and the cross-flow area S decrease with
the decrease of helix angle (Eqs. (9) and (10)), and at same mass
flow rate the shell side velocity increases with the decrease of
cross-flow area (Eq. (8)). Thus the convective heat transfer is enhanced
with the decrease of the helical angle because of the increased
velocity. Second, with the decrease of the helical angle,
the flow pattern of shell-side fluid flow gradually approaches the
external flow cross a tube bank, which has better heat transfer
intensity than the flow parallel to a tube bank at the same fluid
velocity [15]. Again, it is to be noticed that the heat transfer