Fan Selection

Fan Selection

Package steam generators generating up to 120 tons/h of steam today use a single fan. The furnaces of oil- and gas-fired boilers are pressurized. Estimating the flow or head inaccurately can lead to operation of the fan in an unstable region or result in the horsepower being too high and the operation inefficient. The inlet density of air and the flow volume and temperature should be estimated accurately considering flue gas recirculation (FGR) if any. If flue gas at, say, 150°C is with ambient air and sent to the fan inlet, the density will be lower as the mixture temperature is higher than ambi- ent temperature. The flow volume will also be higher considering the amount of flue gas recirculated. In addition, the elevation factor should be kept in mind. The fan is selected for the lowest density case as the fan always delivers the same volume, and hence, the mass flow is lower at lower density. Ensuring the required mass flow of air for combustion at the lowest density case will result in higher mass flow of air at higher density cases (say, lower ambient temperatures). The fan also delivers a lower head at lower density, while the steam generator may require a constant head due to

a given mass flow of flue gas at a particular load. Hence, provision should be made to turn down the fan under such circumstances. The effect of air density on fan perfor- mance is shown in Figure 6.16.

One of the mistakes made by consultants or plant engineers is the use of large margins on flow and head. This leads to oversizing of the fan (considering that a single fan is often used) and operation close to the unstable region as shown in Figure 6.16 at part loads. Fan also will have a large motor, which is unnecessary. Typically, 10% margin on flow and a maximum of 15%–20% on head should be adequate for package boilers. Else, inlet vane control and variable speed drives may be required for the turndown making the fan system expensive. In a large utility boiler, two or even three fans are used, and when the turndown is low, say, 50%, a fan may be cut off. With a single fan system, this results in operation at low flows, and hence, the excess air control becomes difficult. Underestimating the fan head can also cause the fan to operate in the unstable region as shown in Figure 6.16a. The fan operating point should always

be in the negatively sloping portion of the head versus flow curve; else, the fan could operate in the unstable region causing surges and vibration. The fan inlet duct and downstream ductwork to the burners must have proper flow distribution. Pulsations and duct vibrations are likely if the inlet air flow to the fan blades is not smooth.

354 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Not recommended Standard air ad

3 He

ad Lower density

He Not recommended

(a) Volume

(b)

Volume V

System resistance

ad He

100 % Vane opening

(a) Fan performance and range of operation. (b) Effect of system resistance on fan horsepower. Note: Curve 1 is the actual operating curve while curve 2 is the estimated. Operating at point 1 is not recommended. Also, a fan delivers a lower head at lower density. (c) Effect of vane position on flow reduction in fans.