Page 31 - Boiler plant and distribution system optimization manual

16 Boiler Plant and Distribution System Optimization Manual

RATING BOILER CAPACITY TIME—TEMPERATURE—TURBULENCE

Sometimes one can become confused by The type of fuel burned determines the size
the different ways boiler ratings are described. of the combustion chamber and ultimately the
For example firetube boilers are rated by horse- overall size of the boiler. If gas or lighter grades
power, commercial boilers are rated in Btus per of distilled fuel oil are used, the burn out is faster
hour and industrial boilers are rated in thousands than for heavier residual grade oils and coal. The
of pounds of steam per hour. This complicated key is the carbon content of the fuel. The higher
system has a historical background. Originally the carbon content, especially fixed carbon an op-
firetube boilers were used in applications where posed to carbon gasses, the longer the fuel burn-
horsepower was a term commonly where fire out takes requiring larger and larger furnaces.
tube boilers were first used in mining· operations The key to good combustion relies on the three Ts:
and ship propulsion. In buildings; heating and air Time, Temperature and Turbulence. See Figure 1.19.
conditioning requirements are calculated in Btus
and this is commonly used for their boiler speci-
fications. Many industries use pounds or tons of TURNDOWN RATIO
steam per hour as a way of expressing energy re-
quirements. Figures 1.17 and 1.18 provide addi- A measure of the quality of a burner and
tional information. control system lies in its capability to modulate
through a firing range, this is known as turndown.

Figure 1.17—The systems for rating boilers has developed with usage over the
years. Firetube boilers were first used to power ships and heavy duty equip-
ment where a horsepower convention was in use. Industrial facilities adopted
the use of thousands of pounds of steam per hour and building architects and
engineers calculate heating needs in terms of Btus and called for boilers rated
in terms of Btus. The EPA has issued rules using millions of Btus per hour.