High pressure fans

Casings of high pressure fans need to be of sufficient thickness and strength to withstand the internal bursting pressure. This is normally calculated by determining the hoop stress in the scroll and the bending stress in the volute sides. Another consider­ation is the thrust load on the fan bearings. In a closed circuit fan, this can be considerable. There is also the attendant leak­age at the shaft entry hole. The features detailed in Sections

8.6.1 and 8.6.2 reduce both the thrust and any outward leakage.

Scavenger blades

These are narrow (usually radial) blades attached to the rear of the impeller backplate and running in the space between the volute side and the impeller (see Figure 8.22). Air is induced at the shaft entry hole and an axial thrust developed in the oppo­site direction to that of the main impeller. The resultant axial load at the fan bearing can thereby be reduced to a very low fig­ure albeit with an increase in absorbed power.

High pressure fans

Figure 8.22 Cross-section of fan with scavenger blades

Pressure equalizing holes

These are small holes in the impeller backplate which allow a minimum quantity of air to pass through, thereby reducing the pressure difference between the space behind the impeller and the suction zone at its inlet, see Figure 8.23. Again the resultant axial load on the fan bearing is reduced, with a slight reduction in fan efficiency. Stresses in the fan backplate will increase and the holes may act as a stress raiser.

High pressure fans

Figure 8.23 Cross-section of fan with equalising holes

Duplex bearings

An alternative solution, without reducing loads, is to fit a duplex bearing housing. A ball thrust race is contained within the same bearing housing or plummer block as the radial load bearing.Construction features for axial and mixed flow fans

Features applicable

Many of the features described for centrifugal fans in Sections

8.1 to 8.6 inclusive, are also applicable to axial and mixed flow fans. Examples which readily come to mind are inspection doors (with the provisos detailed) and drain points. The latter may be used where the fan is at the lowest point of the system. Cooling discs may be used with bifurcated fans (see Section

8.9.4) where the air is above 75 °C and heat transmitted along the shaft could otherwise damage the motor.

Scavenger blades and pressure relief holes are not of course applicable but the reduced pressure development of these fans make them unnecessary.

Tube axial fans may be provided with so-called “short” or “long” casings.

Short casings are normally used on fans at the entry (Installa­tion Category B) or at the exit (Installation Category C) of the ducting system. They can also be used in non-ducted situations (Installation Category A). Access to the motor and impeller in all these cases is then easy. See Figure 8.24.

High pressure fans

Figure 8.24 Short cased axial flow fan

The terminal box can be on the motor carcase in its normal po­sition, noting however that there is some blockage to the airflow where this is along the motor body length. A terminal box on the motor endshield may be preferable for this reason.

Long casings are normally used on fans contained within a ducting system which has elements on both the fan inlet and outlet (Installation Category D). The fan casing will be suffi­ciently long to encompass the impeller and motor length, nor­mally terminating in flanges, see Figure 8.25. An external termi­nal box is fitted, so that electrical wiring can be carried out without access to the motor, the fan manufacturer providing the wiring between this box and the motor terminals. This wiring is normally contained within rigid piping or a flexible conduit. Vane axial fans with downstream guide vanes and mixed flow fans are invariably provided with long casings.

High pressure fans

Figure 8.25 Long cased axial flow fan

Increased access casings for maintenance

There are a number of variants on this theme which are particu­larly popular for marine use and for kitchen extraction.

A) A short cased fan is manufactured with an external termi­nal box. The motor mounting arms are bolted on the inside of the fan casing, enabling the motor with impellerto be re­moved for overhaul while the casing remains in situ and any attached ducting does not need to be disturbed. An extension duct bolted to one of the fan flanges with a door

High pressure fans

Figure 8.27 Marine fan with swing-out “Maxcess” casing

Figure 8.28 True bifurcated axial flow fan

подпись: figure 8.28 true bifurcated axial flow fan

Figure 8.29 Bifurcated axial flow fan with one-sided motor compartment

подпись: 
figure 8.29 bifurcated axial flow fan with one-sided motor compartment

Figure 8.26 Marine fan with downstream duct section having large inspection doors

подпись: 
figure 8.26 marine fan with downstream duct section having large inspection doors

V

подпись: v

I

подпись: iOr doors gives an opening of 180 ° for this removal (see Figure 8.26).

B) Instead of the extension duct detailed above, a more sim­ple “inspection” duct can be substituted. This is fitted with an access door of ample size for inspection, lubrication or cleaning. On larger sizes the door may be carried on hinges instead of being bolted on.

C) For the most arduous duties, the so-called “Maxcess” cas­ing is preferred. Here the motor and impeller are mounted on a very large hinged door which can be swung out for ac­cess and maintenance, without disturbing any associated ducting. (See Figure 8.27.)

Bifurcated casings

Directly driven axial flow fans have their motors in the airstream, which can be both an advantage and disadvantage. Whilst the moving air cools the motor, if there is high tempera­ture or corrosive elements present, then it is desirable for the motor to be outside. A bifurcated, or “split” casing is a solution. This is shown in Figure 8.28. The airstream is diverted either side of the motor compartment and then rejoins again down­stream. Thus the motor is open to the cooler or cleaner ambient atmosphere. True bifurcated fans can be installed vertically at high level in chimneys where the wind can blow through the motor compartment to give excellent cooling.

A variant on the true bifurcated fan is for the motor compartment to be only open to atmosphere on one side, see Figure 8.29. The blockage effect is less but requires a diversion plate to be fitted to encourage a cooling air path if a TEFV motor, as dis­cussed in Chapter 13, is fitted.

8.

Posted in Fans Ventilation A Practical Guide


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