MIL-HDBK-1003/3
7.2.1.4
Pipe Sizing. Refer to ASHRAE Handbook, Fundamentals or
Figures 11, 12, and 13 for pipe sizing. Guidance parameters for
sizing are as follows:
a) Charts are based on use of 60 degree F water. They
are conservative (e.g., overestimate the pressure drop) for 200
degree F water; and are not conservative for chilled water.
b) A mean pressure drop selection might be 2.5
feet/100 feet. Pressure drops of over 4 feet/100 feet can be
used for pipes over 2 inches in diameter.
c) Keep the minimum velocity above 2 feet per second,
and increase this velocity in down-feed return mains. Lower
velocities cause separation of entrained air in the mains.
7.2.1.5
Valve and Fitting Pressure Drops. Add the equivalent
valve and fitting piping length to the system when performing
hydronic computations.
7.2.1.6
Return Arrangements. The two-pipe reverse return
system has two mains, one supplying and one returning water from
heating units. The return piping is longer than that of a direct
return system and is arranged so that the equivalent lengths of
supply and return piping to any heating unit are about equal.
Use reverse return piping arrangements on closed systems, if all
units or parallel circuits have nearly equal resistance. Do not
use reverse return piping on open or once-through systems.
Provide a reverse return in an up-feed system, and provide a
reverse supply in a down-feed system. Isolation and drain valves
are required in piping systems.
7.2.1.7
Air Vents. Water flow, especially through heat
transfer equipment, shall be directed to permit natural air
venting. Install air vents at system high points.
7.2.2
Hot Water Heating Systems
7.2.2.1
Hot Water Piping.
Refer to pars. 7.1 and 7.2.1 for
piping and pipe sizing.
7.2.2.2
Hot Water Coils. Hot water coils are used in ducts of
an HVAC system to heat the air. Their extended surface is
provided by fins that are bonded to the coil tube. The most
common design is attaching aluminum fins to copper tubing with a
mechanical bond. Headers are usually provided between the tubes,
with supply water entering the coil low and return high so the
coil will be self-venting of any entrained air that enters the
coil.
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