dissolved oxygen by bringing the feedwater to the boiling point. This
also tempers the water to reduce the potential for damaging the boiler
with a shot of cold water.
Feedwater makeup to boilers is accomplished with feedwater
pumps. If feedwater is heated to near the boiling point, the pumps
must have a low net positive suction head (NPSH) to avoid cavitation.
Small plants often have a dedicated pump for each boiler with a level
control on the boiler drum which cycles the pump on a call for more
water. Larger plants usually have a continuously running pump for
several boilers with modulating valves and automatic level controls to
maintain a constant level in the boiler steam drum.
7.3.1 Steam plant controls
In a very small steam system, a space thermostat may cycle the boiler
on and off, and the steam drum-level control will activate the feed-water pump. In a more complex system, the boiler(s) will maintain a
constant steam pressure in the main header, and a pressure control
will modulate the fuel input to match the load. For multiple-boiler
operation, there may be a plant master control which will apportion
the load to the several boilers on a proportional or a programmed
basis.
7.3.2 Flue gas economizers
Flue gas economizers are often used on steam boilers to pick up an
additional 3 to 7 percent of combustion efficiency. Reclaimed heat from
the economizers may be used for combustion air preheating or feed-
water preheating. In either case, care must be taken to keep the ex-
iting flue gas above the water vapor condensation temperature, and
for feedwater heating, there must be adequate flow to avoid steaming
in the economizer.
7.3.3 Boiler testing
It is often desirable or necessary to test steam boiler performance. To
this end, a valve to open for discharge to atmosphere is included in
the plant design. The test valve discharge line should include a sound
silencer to minimize the noise.
7.4 Central Hot Water Plants
Some general concepts of heating water distribution were discussed
in Chap. 6. Chapter 10 discusses boilers and some other pieces of heat-
ing plant equipment.
Low-temperature water (LTW) heating systems (150 to 250
F) are
simple in design. They include boiler(s), pump(s), and secondary com-
ponents such as water treatment, air eliminators, and expansion
tanks. See Fig. 7.2. The simplicity of these systems is compelling. They
become so automatic and reliable that even in larger sizes they are
often taken for granted.
Most hot water plants serve loads of varying magnitude. If constant-
flow systems were common in the past, variable-flow systems are be-
coming more common because of the reduced pumping energy which
can be obtained at lower loads.
Water heating plants are usually designed for a heating differential
of 20 to 40
F through the boilers. Return water temperatures below
140
F to the boiler should be avoided in most cases out of concern for
flue gas vapor condensation and for ‘‘cold shock’’ of the boiler itself.
Multiple hot water boilers are almost always piped in parallel. See
Figs. 7.3 and 7.4. Where two boilers are selected, it is common to sizeeach for 60 percent of the peak load, to allow one boiler to keep the
system ‘‘alive’’ if the other boiler fails. For a three- or four-boiler or
more system, boilers are usually sized so that the entire load can be
carried even if the largest boiler fails. There is usually a smaller boiler
sized to the summer load. Care must be taken not to underestimate
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