Centrifugal Fans

In process plants, centrifugal fans are mostly found providing air to boilers and furnace air preheaters. Where inlet air filters are required, these should be elevated to reduce the risk of sucking in airborne dust and generously sized. Since fans operate with relatively low pressure ratio, even apparently modest pressure drop across a filter can result in a large increase in fan power required.

Fans have either straight radial vanes, forward curved vanes or backward curved vanes. Straight vanes are generally used on smaller fans with higher pressure ratio and speed than large industrial fans.

Impeller vanes may be either curved forward or backward as shown in Figure 8.5.

Forward curved vanes are used in low pressure, high flow applications. Backward curved vane impellers can be designed for a wide range of specific speeds but are mostly selected at medium specific speed for medium flow high pressure service.

Figure 8.6 compares the characteristics of the three types-all with the same impeller diameter and speed.

It can be seen that the forward curved unit is capable of higher head and flow than the others and so needs a physically smaller impeller and casing for the same duty provided by a radial or backward curved vane design. This is therefore a low cost option.

Centrifuge Fan
Figure 8.5 Centrifugal fan types.
Fan Performance Curve
Figure 8.6 Comparative fan performance curves.

Figure 8.7 shows the typical performance of an impeller with forward curved vanes. Efficiency peaks around 65% but at less than half maximum flow and about 30%of maximum power. If the process duty is selected for best efficiency operation, it is important that flow is controlled to avoid overloading the driver.

Figure 8.8 shows that the backward curved vane design peaks at over 80% efficiency, and has a non-overloading characteristic. It is therefore better, though initially more expensive due to increased size, to select the backward curve option.

Fan control has traditionally used dampers in the ductwork. There are three options -a multi-flap damper in the suction ducting, the same in the discharge, or variable angle inlet guide vanes (VIGV). From an energy standpoint, a discharge damper is the worst, suction damper better and inlet guide vanes the best. Figure 8.9 shows a typical inlet guide vane assembly.

Figure 8.7 Forward curve vane characteristics.

% Maximum flow

Figure 8.7 Forward curve vane characteristics.

Inlet Guide Vanes Discharge Damper

Figure 8.8 Backward curve vane characteristics.

% Maximum flow

Figure 8.8 Backward curve vane characteristics.

These vanes have adjustable angle and give varying degrees of pre irotation to the air entering the fan. This in turn adjusts the energy imparted by the impeller and so reduces the power required.

The most efficient option is to vary the fan speed to match process demand. For instance, a good example is to use a VFD electric motor driving a forced draft fan for a boiler or furnace where the speed is linked to the stack oxygen reading. In this case, any dampers or guide vanes are set to 100% open giving little or no restriction to flow.

Figure 8.9 Typical inlet guide vanes. Axial Fans

These are generally used in high volume, low head applications. Depending on the service, VFD may be applicable. Variable angle inlet guide vanes also offer good efficiency at part load. For large fans, efficiency can be improved by selecting aerofoil section blades rather then pressed sheet. The possible efficiency improvements detailed in Section 8.2.8 should be checked.

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