A boiler is a plant in which essentially two processes are at work, namely, combustion and heat transfer.

1. Combustion to release heat from fuel in the firing equipment.

2. Transfer of the released heat to water and steam in the various heat traps of the boiler.

Combustion and firing equipment of different types is discussed in detail in Section 6.4. The different heat transfer surfaces are described individually, all of which, together with firing equipment, form a fired boiler.

In a waste heat or unfired boiler, there is no firing equipment, but only heat transfer surfaces, also called heat traps:

1. Evaporator surfaces consisting of, sequentially,

A. Furnace, which is a chamber made of water-cooled surfaces to

I. Give the burning fuel adequate time for completion of combustion.

Ii. Cool the flue gases and transfer the heat to furnace tubes. Furnace tubes in natural circulation boilers transfer heat to boiling water and to subcooled water in supercritical (SC) boilers.

B. Boiler bank (BB) or evaporator bundles to cool the flue gases and transfer heat to the saturated surfaces to add latent heat

2. Superheater (SH) and reheater (RH) tube banks to cool the flue gases and acceler­ate heat to superheat and reheat the steam

3. Back-end equipment consisting of

A. Economizer (ECON) tube banks to further cool the gases to transfer heat to the subcooled water

B. Airheater (AH) to cool the gases in the last stage and transfer heat to the com­bustion air

This chapter provides engineering and sizing inputs for the various heating surfaces (HSs) and hence forms the foundation course for the design of all boilers. The data pro­vided should be very useful for practicing engineers and designers. Like all the design information, there can be some variation with specific data developed by any manufac­turer for a particular geometry and fuel. Thus, the guidelines and limits are generic and meant for essential design insights.