• Lowering the gas velocities over the tubes can reduce gas pressure drop in the boiler. But this reduces the heat transfer rates and makes the boiler larger and

More expensive. A balance between the heat transfer rate and the pressure drop is necessary, which is largely taken care of by following the recommended optimum gas velocities and tube spacing cited in Chapter 6.

• Lowering the excess air quantity is necessary to lower the volumetric capacity of the fans; this concept helps in choosing fans of correct size and reducing the fan power.

• Choosing the highest available fan efficiency, even by paying a bit more, is prudent as the fan power may reduce drastically. Refer to Chapter 8 for more details on fan selection.

• For small fans up to —150 kW, belt drives are popular as they provide the optimal revolutions per minute (rpm) for the duty that consumes the least power.

• Variable-speed hydraulic couplings, installed between the constant-speed motor and the fan, reduce the speed of the fans at lower loads, and help to reduce the power consumption more effectively than inlet guide vanes.

• Variable-frequency drives, which vary the motor speeds by varying the fre­quency of the power to the motor, are better but more expensive.

• When the fans are large, as in utility applications, axial flow design is consid­ered, at least for clean application, as axial flow is inherently more efficient. They are, no doubt, more expensive, are delicate, and demand more careful operation, but become more economical for larger sizes based on life cycle costing.