Handbook of air conditioning and refrigeration / Shan K

8.12 CHAPTER EIGHT
Cast-Iron Sectional Boiler
combustion chamber, where it is initially ignited by an ignition device. The mixture burns, and the
combustion process sustains itself once the heat it generates is greater than the heat it transfers to
the water, i.e., once a high enough temperature in the combustion chamber is attained. The injected
air-fuel mixture is then burned spontaneously.
The direct radiation from the flame and the high temperature in the combustion chamber both
conduct heat through the wall of the chamber—the primary surface adjacent to the water that fills
the shell. The combustion product from the combustion chamber—flue gas—is directed into fire
tubes by headers. As the flue gas at a higher temperature flows through the fire tubes, it transfers
heat to the surrounding water through the pipewall of the tubes—the secondary surface. The flue
gas temperature drops, and its volume contracts. Because the number of fire tubes continuously decreases
in the second, third, and fourth passes of fire tubes, which matches the volume contraction
of flue gas, the gas velocity is maintained in a more uniform manner. Flue gas leaves after the
fourth pass and is vented into the stack.
Return water enters the side of the boiler, sinks to the bottom, is heated, and rises again. Hot
water is finally supplied at the top outlet. Such an arrangement prevents cold return water from
surrounding the combustion chamber and producing thermal shock. It also promotes good water
circulation.
Construction Characteristics. The constructional characteristics for Scotch marine packaged
boilers are as follows:
● Four flue gas passes. Each pass means a horizontal run of the fluid flow passage. A four-pass flow
arrangement for the flue gas, with a gradual reduction in gas flow area as the flue gas becomes
colder, helps maintain a higher gas velocity and, therefore, a clean surface and a higher rate of
heat transfer.
● Sufficient heat-transfer surface area. In a packaged Scotch marine boiler, the combustion chamber,
fire tubes, and tube sheets at both ends are all heating surfaces. The amount of heating surface
area directly affects a boiler’s output. And 3 to 5 ft 2 (0.9 to 1.5 m 2 ) of heating surface area for
each boiler horsepower, or 33,475 Btu/h (9.8 kW), of output, is the key value for providing sufficient
capacity and a long-lasting boiler. A smaller heating surface area for each boiler horsepower
of output often results in a higher heat flux, probably a higher surface temperature, and therefore a
shorter life for the boiler.
● Forced-draft arrangement. In a forced-draft arrangement, the fan is located adjacent to the burner.
The fan forces air into the combustion chamber. It also forces the flue gas to flow through the fire
tubes and to discharge from the vent or chimney. A forced-draft fan supplies a controllable quantity
of combustion air to the combustion chamber. The forced-draft fan is located upstream of the
combustion chamber, so it handles dense, clean boiler room air at a comparatively lower temperature
and lower volume flow rather than the hot, dirty, high-temperature, expanded flue gas downstream.
The temperature fluctuation for the room air is smaller than that for the exhausted flue
gas, so more accurate control of combustion air is possible than with an induced-draft design.
A cast-iron sectional boiler consists of many vertical cast-iron hollow sections in the shape of an
inverted U filled with water. When the sections are linked together by bolts, the center part of the
inverted U forms the furnace or the combustion chamber.
Because of its thick, heavy sections, a cast-iron sectional boiler has a large heat storage capacity
and thus is slow to heat up. This reduces temperature swings when heat demand varies. Thick,
heavy sections are also helpful in extending the boiler’s service life by preventing corrosion.
The heating capacity of a cast-iron sectional boiler depends on the number of sections connected.
Because of its relatively large combustion chamber and lower flow resistance, such a boiler
is able to use atmospheric gas burners with a lower chimney height. Cast-iron sectional boilers are
low-pressure boilers used in residences and in small and medium-size commercial buildings. They
can be field-assembled and fitted in existing buildings.