mixing chamber to ensure proper mixing of outside air and return
air. Figure 29 contains a sketch of a mixing plenum which is
poorly designed for mixing of outside and return air and one
properly designed to promote mixing of outside and return air.
Freezing Due to Buildup of Condensate in the Coil. See
Figure 30 for coil piping to prevent freezing. There are a
number of oversights in design and installation that can lead to
buildup of condensate in a steam coil. These oversights
primarily fall into the following categories:
a) Inappropriate Coil Selection. Many coil designs
promote steam short circuiting - a phenomenon where steam takes
the path of least resistance through coil tubes that have the
least condensate flow. This increases the coil return header
pressure to the point where the condensate in other coil tubes
cannot drain properly. Coil designs have been developed to help
minimize problems like this. The single row distribution tube
and multi-row series flow coils minimize the potential for
short-circuiting and trapping condensate inside the coil. These
coil designs should be considered when designing a system where
there is substantial risk of coil freezing.
b) Improper Installation. Even though the proper coil
is selected, where the coil is not installed properly, freezing
can still occur. Many coils are installed in such a way that
gravity drainage is not possible. Though initially installed
properly, coils can move into an undesirable position through
building settling or weakening of supports. Closely inspecting
coil installations with these problems in mind can help avoid
problems in this area.
c) Improper Venting, Vacuum Elimination, and Steam
Trapping Practices. If a coil is not designed for proper venting
of noncondensables, vacuum elimination, and condensate removal, a
well selected and installed coil can still experience problems
with condensate buildup. Consider carefully proper steam piping
practices in these areas and consult available Navy and industry
d) Modulating Valves.
Avoid using modulating valves
for control of preheat coils.
e) Traps. Trap each coil separately.
have equal pressure drops.