Ideally, the reverse-return system provides nearly
equal total lengths for all terminal circuits (Fig.
31). The reverse return concept is basically to
use the extra piping to balance pressure drops between
nearby and faraway coils. The idea is that the closest
coil has the shortest supply piping and the longest
return piping while the farthest coil has the opposite.
So overall, each coil has approximately the same
pressure drop. In this case the total piping length
of the coil 1 equals AB + BG + GF + FE + EH. The
coil 2 total piping length adds the section BC but
rests GF, so the total piping length remains constant.
However, reversed return piping circuit requires
longer piping length which would drive the cost
higher.
Past experience with chilled-water system design
has been to favor reversed return piping over direct
return. This comes from the fact that if all terminal
sub-circuits have the same head loss, the piping
circuit will be balanced. When terminal sub-circuit
head losses are different, simple adjustment of
the balance valves will provide for required full
load flow balance. While this basic design rule
is widely accepted, it is true that a direct return
piping system design with knowledge can be superior
to a reversed return piping design used only because
of the paradigm that the reversed return must provide
flow balance. The reversed return solution to off-balance
problems in bigger chilled water loops as campus-like
facilities sometimes becomes impractical and costly.
To install reverse return piping for so many coils
is so extensive that the cost would be all but prohibitive.
Direct return piping has been successfully applied
where the designer has guarded against major flow
unbalanced by:
