a) Steam boiler, separate expansion vessel, direct-contact heat
exchange of steam and water in expansion vessel.
b) Steam boiler, integral expansion vessel (boiler drum), water
drawn from below water line of drum.
System Water Velocities
Central heating distribution systems should be zoned.
b) Irrespective of flow rates and velocities, each zone should be
designed so that the total pressure drops in the zones do not vary more than
15 percent from each other.
c) A minimum velocity of 2 ft/sec (0.61 m/s) in the distribution
system should be maintained to prevent stratification.
d) A reasonable average system water velocity is 5 ft/sec
(1.52 m/s). Velocities should be based on the median temperature of the
system, provided that operating temperatures are in excess of 350 degrees F
(177 degrees C) and that the temperature drop through the heat exchange
equipment is no less than 90 degrees F (50 degrees C) nor more than 130
degrees F (54.4 degrees C).
Types of Distribution Circuits. There are many types of
distribution circuits in general use. The selection of the best system for
the particular terrain and situation is essential for satisfactory operation
of a HTW plant. Extreme care must be exercised not to design a distribution
system that is difficult to operate, balance, and control.
Direct-Supply, Single-Circuit. HTW is fed from the central plant
directly to the buildings to be served, passes through the heat exchange
equipment, and returns to the generators through the return main. This is a
simple system to design and is the most prevalent in use. It must be
understood that the pressure at the entrance to each connected structure is
different and the sizing of control valves to give balanced flow must be
carefully analyzed. In a series circuit the decreasing supply-water
temperature from structure to structure must be accounted for in heat
exchanger design. See Figure 6.
Direct-Supply, Radial. This system utilizes a number of individual
distribution circuits. In such a design the length of runs tends to be
shorter, and differences in pressure at the entrances of buildings served are
less. Control-valve sizing is not as difficult as with the direct-supply
single-circuit. See Figure 7.
Direct-Supply, Reverse-Return. As depicted in Figure 8, the return
lines are reversed. HTW from the central plant is fed to the connected
structures. The return mains are reversed so that the farthest buildings,
which have the longest supplies, have the shortest returns. In this manner,
the system is more easily balanced and pressure differentials at all connected
structures are nearly equal.