Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
329
BOILER SYSTEM CONTROL
INTRODUCTION
A boiler is a closed vessel intended to heat water and produce
hot water or steam through combustion of a fuel or through the
action of electrodes or electric resistance elements. Steam and
hot water boilers are available in standard sizes from very small
boilers for apartments and residences to very large boilers for
commercial and industrial uses.
BOILER TYPES
Boilers are classified by water temperature or steam pressure.
They are further classified by type of metal used in construction
(cast iron, steel, or copper), by type of fuel (oil, gas, or
electricity), or by relationship of fire or water to the tubes
(firetube or watertube).
– Low-pressure boilers are those designed to produce
steam up to 15 psig or hot water up to 250F with
pressures up to 160 psig.
– Medium and high pressure boilers produce steam above
15 psig or hot water above 160 psig or 250F or both.
CAST IRON AND STEEL BOILERS
Boilers are typically constructed of cast iron or welded steel.
Cast iron boilers (Fig. 37) are made of individually cast sections
and are joined using screws or nuts and tie rods or screwed
nipples to join sections. The number of sections can be varied
to provide a range of capacities.
Photo Courtesy of Cleaver Brooks
Fig. 37. Typical Cast Iron Boiler (Watertube).
JACKET
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Steel boilers come in a large variety of configurations. They
are factory assembled and welded and shipped as a unit. Figure
38 illustrates a firetube boiler. The fire and flue gases are
substantially surrounded by water. The products of combustion
pass through tubes to the back then to the front and once more
to the back before finally exiting at the front. This makes it a
four-pass boiler. Firetube boilers are manufactured in many
other configurations such as:
– External firebox, firebox not surrounded by water.
– Dry back, firetubes directly available from clean-out
doors at back of boiler.
– Scotch-Marine, low water volume and fast response.
Photo Courtesy of Cleaver Brooks
Fig. 38. Typical Firetube Boiler.
Watertube boilers are steel body boilers used for high capacity
requirements of more than two million Btuh. Watertube boilers
use a water-cooled firebox which prolongs the life of furnace
walls and refractories.
MODULAR BOILERS
Modular boilers are small hot water boilers rated at 200,000
Btuh to 900,000 Btuh input. These boilers are available with 85
percent or higher gross efficiency. Figure 39 shows features of a
typical modular boiler. These boilers are often used in tandem to
provide hot water for space heating and/or domestic hot water.
For example, if the design heating load were 2,000,000 Btuh,
four 600,000 Btuh (input) modular boilers might be used. If the
load were 25 percent or less on a particular day, only one boiler
would fire and cycle on and off to supply the load. The other
three boilers would remain off with no water flow. This reduces
flue and jacket (covering of the boiler) heat losses.
M11416
FIRE TUBES
JACKET