a) Base selection of the distribution system and route on the
results of life-cycle economic analyses of alternatives. Consider esthetics
within the limits of the Station Master Plan.
b) Operation and maintenance costs depend on the type of system
design and past experience with various systems.
Steam Versus High Temperature Water Distribution. For criteria on
steam versus high temperature water distribution, refer to DM-3.6, Central
Heating Plants. Some advantages and disadvantages of each system type are
summarized on Table 2.
High-Pressure (above 50 psig) (344.5 kPa) Steam Versus Low-Pressure
(0 to 15 psig) (0 to 103.4 kPa) Steam Distribution. Compare costs of higher
pressure pipe, valve, and fitting standards against lower pressure standards
plus costs of pressure reducing stations in selecting the most economical
system. Low pressure steam may not require full-time boiler operator
attendance. If operationally adequate, consider medium-pressure steam
systems, 15 to 50 psig (103.4 to 344.5 kPa). End-use temperature requirements
of terminal equipment must be met by the system selected.
Insulation for Shallow Trench Systems Which Exceed 500 Feet. If
the estimated distribution line length exceeds 500 feet (152.5 m), determine
the required insulation thickness as follows and edit NFGS-02693 as required.
Heat Loss Formula. For concrete shallow trench systems, perform
the economic analysis with heat losses calculated using the following
Equation: Q = 6.28(Tf - T)/1n(rp/r)/Kp + 1n(ri/rp)/Ki + 0.5/ri
inside radius of pipe, feet (one half of id)
outside radius of pipe, feet (one half of od)p
radius of insulation, feet (one half of od)
temperature of fluid, degrees Ff
temperature of ambient, degrees F (use 75)
pipe heat loss, Btu/hr (ft of pipe)