Packaged steam generators of today use a single fan for up to 250,000 lb/h of steam. The furnaces of oil — and gas-fired boilers are pressurized, hence the fan parameters should be selected with care. Estimating the flow or head inaccurately can force the fan to operate in an unstable region or result in the horsepower being too high and the operation inefficient. The density of air should be accurately estimated, so elevation and ambient temperature conditions should be considered. In some cold locations, a steam-air preheat coil is used to preheat the air before it enters the fan, and this adds to the pressure drop. When flue gas recirculation is required, usually the flue gases from the boiler exit are sucked in by the fan, which handles the resistance of the entire system. The density of the mixed air is lower, owing to the higher temperature of the air mixed with the flue gases. The fan should be selected for the lowest density case, as explained in Q9.06, because the mass flow of air is important for combustion and not the volumetric flow. The effect of gas density on fan performance is shown in Fig. 3.16a.

Large margins on flow and head should not be specified, because this leads to oversizing of the fan and can force the fan operating point to the extreme right of the curve in Fig. 3.16b, where the horsepower can be extremely high; a lot of energy is also wasted. Inlet vane control is typically used for controlling the flow

Also. a fan delivers a lower head at lower density

(b)

Figure 3.16 (a) Fan performance and range of operation, (b) Effect of system resistance on fan horsepower. (c) Effect of vane position on flow reduction in fans.

Of air; this system typically operates stably between 20% and 100% vane opening, which does not translate into a large flow difference, as can be seen from Fig. 3.16c. Hence a small margin on flow and head is preferred—about 15% margin on flow and 20-25% on head is adequate; otherwise one may have to use a variable-speed drive or frequency modulation for control, which is expensive. Underestimating the fan head can also cause the fan to operate in the unstable

Region as shown iN Fig. 3.16a . Curve 2 in Fig. 3.16b is the estimated curve, and the actual curve 1 is to the left, close to the unstable region with positive slope. It also delivers less flow than required. The fan operating point must preferably be in the negatively sloping portion of the head versus flow curve; otherwise the fan could operate in the unstable region, causing surges and vibration. The flue gas recirculation lines must be properly sized; typical air and flue gas velocity in ducts is about 40 ft/s.

The flue gas recirculation line is usually connected to the fan inlet in gas and distillate oil-fired boilers. This increases the size of the forced draft fan. The higher gas pressure drop in the boiler due to the increased mass flow should also be considered when selecting the fan. A separate recirculation fan is used occasionally when heavy fuel oils containing sulfur are fired and the flue gases are admitted into the burner wind-box. If the flue gases were allowed to mix with the cold air at the fan inlet, the mixture temperature could fall below the acid dew point, possibly leading to corrosion.

The fan inlet duct and downstream ductwork must have proper flow distribution. Pulsations and duct vibrations are likely if the inlet airflow to the fan blades is not smooth and the maldistribution in velocity is large. Similarly, the ductwork between the fan and wind-box should be designed to minimize flow maldistribution to ensure proper airflow to the burner.