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sures up to 15 lb/ in2
gauge for steam and 30 lb/ in2
gauge for water.
They are either atmospheric or power burner gas-fired, and they come
in inpidual heat transfer sections which are modularized to obtain
a range of capacities. The small sections allow for installation in
spaces which are inaccessible to package boilers. They are easy to
maintain and have the longest physical life of any type of boiler. Care
must be taken to avoid the problem of thermal (cold) shock of these
boilers for they fail if the castings crack.
10.4.2 Fire tube boilers
In fire tube boilers, the products of combustion are confined within a
series of tubes surrounded by water. The most popular type is the
Scotch marine boiler in which the combustion furnace is in the shape
of a cylinder surrounded by water. Other types have steel firebox fur-
naces, brick-set firebox furnaces, and in some cases a combination of
both. Capacities go up to about 1000 boiler horsepower (bhp) (1 bhp
33,480 Btu/h). Their popularity is due to their low first cost. Their
useful life is less than that of either cast-iron or water tube boilers,
and some fire-tube designs are susceptible to thermal shock under
wide temperature differentials and sudden load shifts. The maximum
operating pressure is usually 250 lb/ in2
gauge or less. Many older
boilers had atmospheric-type burners. Current practice favors forced-
draft burners. The Scotch marine boilers are all forced-draft design.
10.4.3 Water tube boilers
In water tube boilers, the water is inside the boiler tubes and the
products of combustion surround the tubes. There is a wide variety of
configurations, including slant-tube (Fig. 10.1), bent-tube (D-type),O-type, C-type, and express type. They range in size from small resi-
dential units to large utility boilers. They have an extended service
life if proper water treatment and maintenance are provided. Water
tube boilers may be factory-assembled and tested, package type, or
field-assembled; the field-assembled boiler is more common in sizes
above 200,000 lb/h capacity. Operating pressures of 150 to 900 lb/ in2
gauge or greater are used where process requirements are severe or
where power generation is a consideration.
10.4.4 Thermal liquid boilers
Thermal liquid boilers are of the water tube type, but instead of water,
a special thermal liquid is used. This liquid permits the generation of
high temperatures—600 to 800F—at low pressures. These units are
often found in manufacturing facilities, with the thermal liquid used
in processes. Steam is generated through a heat exchanger for use in
space heating and other purposes. These boilers are prevalent in Eu-
rope but have seen limited application in the United States. Thermal
liquids are often elusive in containment. Special consideration must
be given to joint systems and device seals.10.4.5 Steam quality
Heating and domestic hot water applications utilize saturated steam.
Saturated steam is at a temperature and pressure that correspond to
the saturation conditions discussed in Sec. 6.2 and is said to have 100
percent quality. Steam with some free moisture present has less than
100 percent quality (down to zero quality for condensed water). ‘‘Su-
perheated’’ means that additional heat is applied to the steam to drive
its temperature above the saturation temperature at the existing pres-
sure. In the boiler, this is accomplished in a special tube bank called
a superheater. Superheat is required for many turbine and process
applications, including cogeneration, but this steam must be ‘‘desu-
perheated’’ for use in normal heating and domestic water applications.