Heating Plants. Heating plants normally do not require operating
steam pressures over 150 psig (1035 kPa). Lower pressures have smaller heat
losses from distribution piping. Cast iron valves can be used up to 125 psig
(862 kPa) but steel valves are required between 125 psig (862 kPa) and 150
psig (1035 kPa) unless 250 psig (1725 kPa) class cast iron valves are used.
For this reason, the recommended operating pressure range is 100 psig (690
kPa) to 125 psig (862 kPa) for maximum economy of distribution piping
including valves and fittings. When the maximum plant and distribution loads
are large and the most remote terminal requirements cannot be satisfied with
sufficient quantities of steam, combinations of higher pressures and related
pipe sizes should be analyzed to compare installation and operating costs of
various steam pressures up to 250 psig (1725 kPa) (the next higher pressure
rating of cast iron valves).
Process Loads Requiring Higher Pressures than 100 psig (690 kPa).
When a process load is required in excess of 100 psig (690 kPa) an economic
analysis should determine whether steam should be generated and distributed at
the pressure required at the process, distributed at two pressure levels, or
generated at the higher pressure locally by a separate heating plant as
Equipment Types and Classifications. Watertube and firetube
boilers are the two general types of boilers available. The boilers are also
classified as either high pressure or low pressure and whether they produce
hot water or steam.
A high pressure boiler is a boiler which operates at pressures
above 15 psig (103 kPa). The advantage of the high pressure boiler is reduced
boiler size and distribution piping required to equal the capacity of a low
pressure boiler. Most large capacity boilers are high pressure boilers. A
hot water boiler is a misnomer since the water does not boil. A high
temperature water (HTW) boiler is a hot water boiler which produces water at
temperatures higher than 250 degrees F (121 degrees C). Special
considerations for hot water boilers are discussed in Section 4.
Other classifications for boilers are type of fuel and method of
firing. Burners and stokers are the two general methods of firing with
further categories of these types of firing schemes.
Boiler Type Selection. Once the operating parameters (load, fuel,
etc.) have been determined, the type of boilers which will provide the best
operating characteristics shall be selected. Table 2 lists the advantages and
disadvantages of high temperature hot water boilers versus steam boilers.
Section 4 contains additional design requirements for high temperature hot
water boilers. Table 3 lists acceptable boiler types and criteria to consider
when selecting the type and possible pollutants to control. Pollution control
equipment will be a major cost factor when evaluating the most cost effective