Boiler System Control; Introduction; Boiler Types; Cast Iron And Steel Boilers - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

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 200 kPa or hot water up to 120 C with
pressures up to 1200 kPa.
– Medium and high pressure boilers produce steam above
200 kPa or hot water above 1200 kPa or 120 C 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).
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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 590 kW. 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 60 kW to
260 kW 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 600 kW, four 180 kW (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.
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