10 January 2002
8-1. GENERAL. Although it is impractical to cover all special considerations, which arise in heating and
cooling distribution designs, this chapter presents typical design problems and solutions associated with
steam, high temperature hot water, low temperature hot water and chilled water systems.
8-2. STEAM SYSTEMS.
a. Trap Selection. Steam traps are used to separate the condensate and non-condensable gases
from the steam. Many types of traps are used on drip legs for steam distribution systems. Those trap
types include float and thermostatic (F&T), inverted bucket, thermostatic and thermodynamic (disc).
However, for buried heat distribution drip leg applications, inverted bucket or thermostatic (bimetallic type)
should be the trap types selected. For drip leg applications where freezing is a consideration,
thermodynamic type (installed vertically) or bimetallic thermostatic type should be selected.
b. Trap Sizing and Location. Trap sizing is important for obtaining an efficient steam distribution
system. Condensation in the steam line is caused by heat loss from the steam line. Trap life will be
shortened, function affected and excessive energy will be wasted if traps are oversized to handle the higher
initial startup condensate flows. Therefore, the traps should be sized for the condensate load seen during
the distribution system normal operation. Because the traps are not sized for startup loadings, the bypass
must be opened at startup to allow condensate to pass until the steam line has reached normal operating
temperatures (see figure 8-1). The designer will calculate heat loss and condensate flow for that particular
design using the methods presented in appendix D for determining condensate loads during normal
operation. It is critical that the designer calculate trap capacity using the Appendix D method for each trap
station in the design to ensure proper steam system operation. In addition to trap capacity, steam trap
type, differential pressures, and inlet pressure must always be provided on the contract documents. Do not
locate steam drip legs, with associated traps, more than 500 feet apart.
c. Drip Leg Sizing. Drip legs, installed vertically down from the steam pipe, are used to collect
condensate. Design all steam lines to slope at 1 inch in 20 feet minimum toward these drip legs. It is
preferable to slope the steam lines in the direction of steam flow whenever possible. The steam trap line
and bypass line are connected to the drip leg as indicated in figure 8-1. The drip leg will be the same
nominal pipe size as the main line (up to a 12-inch line) and will provide a storage capacity equal to 50% of
the startup condensate load (no safety factor, one-half of an hour duration) for line sizes 4 inches in diameter
and larger and 25% of the startup condensate load (no safety factor, one-half of an hour
duration) for line
sizes less than 4 inches. In no case will the drip leg be less than 18 inches in length or larger than 12
inches in diameter for all steam line sizes. The designer will calculate startup loads for drip leg sizing using
the methods presented in appendix D.
d. Trap Station Layout. All trap stations will be piped as indicated in figure 8-1, as a minimum. Valve
and strainer sizes will match the line sizes on which they are installed. Pipe lines to and from the steam trap
will be sized based on calculated trap capacity but will be no less than 3/4-inch nominal size (line size "A" on
figure 8-1). If reducing fittings are needed at the trap inlet and outlet, eccentric reducers must be used. The
bypass line will be sized to accommodate warm-up condensate loads. For steam systems with an operating
pressure of 150 psig or less and pipe sizes 12 inches or less, provide a -inch bypass line. If the condensate
return main is a low pressure or gravity flow type, the trap discharge line will be routed through an
accumulator, as indicated in figure 8-2. The accumulator will lower the trap discharge temperature and
minimize flashing when the condensate is introduced into sloped condensate lines which are routed to
receiver/pump sets located in valve manholes. The pumps push the condensate back to the central plant in a
separate pressurized condensate line. See figure 8-3 for a duplex condensate pump set connection detail.