DESIGN OF HEATING
SURFACES
The first step in the design
of heating surfaces is determining the heat duties of different components of
the boiler heating surfaces. Atypical boiler would use the following four types
of heating surfaces:
Economizer
Evaporator
Superheater
Reheater (for reheat boilers)
Heat duty of these elements
depends on the designed steam parameter of the boiler. It is best illustrated
by an example (Figure 1), which shows how the relative heat duty of different
boiler elements changes with steam pressure. As the steam pressure increases,
the heat duty of the evaporator decreases and that ofsuperheater increases.
Figure 1
At low pressure the
evaporator duty is so high that a water wall or wing wall alone cannot absorb
the required amount of heat. So, a separate heating surface, called bank tubes,
is needed. After the heat duties of individual elements (economizer,
evaporator,superheater, and reheater)
are determined by the steam table, their disposition can be determined. From
the viewpoint of heat absorption, a CFB boiler may be divided into two regions,
the CFB loop and back-pass
1.1. Primary Loop
The CFB loop includes the
furnace, cyclone/impact separator, loop-seal, and external heat exchanger.
2.2. Secondary Loop or
Back-pass
The back-pass is the section
of gas pass between the exit of the cyclone/impact separator and the exit of
the air heater. The furnace usually accommodates:
Evaporator tubes
Parts of the superheater
Parts of or the entire reheater
The economizer is normally
located in the back-pass between thesuperheater and
the air heater. Evaporator tubes may form the walls of the furnace and those of
the back-pass. Parts of it may also be located in the external heat exchanger.
Sometimes, thesuperheater tubes also form parts
of the back-pass enclosure. The disposition of the reheater and superheater tubes in the furnace, back-pass, and
external heat exchanger is the designer’s choice. This choice is, however,
influenced by the type of fuel, as shown below. Some designs also use a
steam-cooled cyclone