One-Pipe Loop-Main. A single distribution pipe is used to
distribute HTW to the connected structures. Return water from the building is
fed back into the loop main. The effect of lower temperatures at buildings
farthest from the central plant must be considered. See Figure 9.
Primary and Secondary Distribution Systems. This is actually not a
separate system but a type of distribution that can be utilized by any of the
previously described systems. HTW carries the water economically over long
sections of pipe. At the connected structure, the HTW is converted through
heat transfer devices into low temperature water. The designer can then use
standard heat transfer devices for the buildings. See Figure 10.
Hot Water Generators
a) Design the heating plant for not less than two generators
totaling 135 to 150 percent of the heating load.
b) Where practicable, the ultimate heating plant shall not require
operation of more than three simultaneously fired generators.
a) HTW generators should be of the controlled forced circulation
water tube type, specifically designed for HTW service.
b) Fire-tube boilers and natural-circulation water-tube boilers
are not recommended.
Design. Generators should be designed and constructed to be
suitable for the intended HTW service and should be certified as such by the
Maximum Pressure Drop through Generator. Hot water generators
should be designed to have a maximum pressure drop through the generator of 15
psi (103.5 kPa). Keeping the pressure drop low, usually eliminates the need
for a separate generator circulation pump, which reduces pump operating and
Steam Space. A HTW generator should be designed to have no steam
space. Inclusion of steam space of any kind within the generator can cause
serious circulation problems in the internal tube circuits and possible tube