General. Low pressure steam in HVAC systems is defined
as 15 psi or less. High pressure steam in HVAC systems is
defined as being 16 psi to 150 psi. There is a trend away from
using direct steam in HVAC systems. If one gets heat from
district steam or a steam boiler, the designer will often convert
the steam to hot water through a heat exchanger to supply the
HVAC system. The reasons for using hot water HVAC systems
include the following:
Eliminates maintenance of many steam traps.
More flexible when planning temperature controls.
Low Pressure Steam Systems
Pipe Sizing. Pipe sizing for low pressure steam and
return mains is found in ASHRAE Handbook, Fundamentals. See
Tables 10 through 13. Note the following:
a) Slope steam mains in the direction of the flow
( inch in 10 feet is recommended).
Slope return mains similarly.
Avoid lifts on low pressure returns, or make them
Pumped returns are preferred to gravity returns.
Provide a step-up and drip at the base of steam
f) Drip the low point if steam flow is against the
condensate flow and upsize piping to the next larger size to
allow condensate to run back in the bottom of the steam pipe.
High Pressure Steam Systems
Refer to par. 220.127.116.11.
See Figures 18 through 21 for steam pipe sizing.