Variable geometry fans

The possibilities for varying the fan geometry are limitless. Many exotic methods have been tried on both centrifugal and axial flow fans. In all systems, the intention is to vary the in­let/outlet velocity flow triangles. At inlet, pre — or contra swirl of varying amounts may be induced by the use of variable angle radial vane inlet controls. They have been used extensively for over forty years with backward inclined or aerofoil bladed cen­trifugal fans where they have proved particularly successful, and also with axial flow fans where the simplicity of a non-rotat­ing control has been desired. The operating range at high effi­ciency with axials is, however, somewhat narrow. Mixed flow fans are becoming more popular and again this method of con­trol is widely used.

For axial flow fans, the alteration of impeller blade pitch angle at rest has been available for many years but over the last two de­cades the means of varying the pitch angle in motion has ex­tended from the high technology mine ventilation and mechani­cal draught installations into the more humble HVAC plant. This is now seen to be an extremely efficient and versatile form of control, rivalling the inverter drive on constant orifice sys­tems. It also gives useful power savings on variable flow/con­stant pressure and constant flow/variable pressure systems.

Other less popular methods of centrifugal fan control have con­sisted of variable angle impeller tips and a rotating plate at­tached to the impeller backplate which can vary its axial posi­tion and, therefore, the impeller blade width. A cylindrical drum moving axially over the impeller periphery to achieve the same result has been more extensively used in North America.

Some of the most popular types are now described in a little more detail.

Radial vane inlet control (RVIC)

The full pressure development of a fan is achieved only when the air enters the impeller eye axially and without swirl. If the air or gas entering the fan is already spinning in the direction of im­peller rotation, the fan will develop less pressure.

Both flowrate and power absorbed will thus be reduced. It is the purpose of this control to induce pre-rotation. In effect, it alters the design pressure/flow characteristic whilst largely maintain­ing the fan’s efficiency. Thus the power consumption can be considerably reduced with lowering fan capacity.

Radial vanes are most effective with backward bladed high flowrate fans where the pressure curve rises considerably above the duty condition, the power is non-overloading, and the impeller inlet velocity vectors are of such a magnitude that they can be materially affected. With other types, especially the for­ward curved, power savings are not nearly so great and often only marginal.

Figure 6.6 RVIC with external operating gear

The relationship between control arm movement and flowrate reduction is intermediate between the two previous types. Such dampers should never be used on direct pneumatic con­veying or high dust burden extract systems as they require many parts within the air/gas stream subject to erosion and/or corrosion.

A typical performance characteristic for a backward aerofoil centrifugal fan is shown in Figure 6.5. Superimposed are the ef­fects of the various types of system and thus the energy sav­ings achieved. It should again be noted that a typical VAV sys­tem will have a system characteristic intermediate between the parallel path and constant orifice systems. When the fixed ele­ment of system resistance is a large proportion of the total, then the power savings will approach those for parallel paths, whilst if it is small, then the power saving will be similar to that for a constant orifice system.

« 5.5 5 45 4 15 3 2.5 3 15 1 4 0

Figure 6.5 Typical performance characteristics of aerofoil bladed centrifugal fan fitted with a RVIC

The mechanical design of the vanes and particularly the mech­anism can cause problems because of the need for continuous maintenance and greasing. This is due to the high friction and corresponding high operating torque required for the operation of the actuating mechanism.

This mechanism usually comprises an external ring and a num­ber of actuating levers, one lever for each vane (see Figure 6.6). The vanes are supported by a larger hollow collar at the centre to allow the fan shaft to pass through. The ring is nor­mally supported by a number of rollers. The actuating levers are connected to the ring via double links to overcome the great differences between the paths of the levers and the external ring.

It is such mechanical problems which have resulted in doubt as to their reliable operation for VAV systems, especially as the fans are often of double inlet design necessitating cross linkage between the two assemblies.

As with speed variation, when considering the use of RVICs as a means of control, then the resultant area of instability may lead to problems. In “wider” impeller designs this area can be large, see Figure 6.7. This has lead some to claim that one should not consider their use if a flowrate of less than 50% of design is required. Below this ratio simple damper control would have to take over, with its resultant inefficiency.

However, by correct impeller/RVIC design selection, the area of instability can be very small with modulation over the entire VAV system curve totally stable. Normally a turndown to 20% can be achieved with a single speed drive motor and this is generally

Air flowrate Q Figure 6.7 Instability with radial vane inlet control of backward bladed centrifugal fans

Sufficient for VAV system use. By using a two speed fan, opera­tion down to 10% of design is feasible.

It should be noted that due to the relatively large clearances necessary at the centre support, zero flow is impossible and even with complete closure there will be a leakage of up to 8%.

As well as inducing pre-swirl, the RVIC imposes an additional and increasing resistance as the vanes approach full closure. This is the explanation for the corresponding reduction in effi­ciency, as this loss of energy is then attributed to the fan/RVIC combination. RVICs are very expensive and the price for two fitted to a double inlet fan can even exceed the price of the bare fan itself.

Controls incorporating an internal mechanism can be less ex­pensive (Figure 6.8) but are usually limited to clean dry air appli­cations.

Figure 6.8 RVIC with internal operating mechanism

Semi-circular inlet regulator

First introduced by Davidson & Co of Belfast, this is a very much simplified device for imparting swirl to the air entering the inlet of a centrifugal fan. It consists of a split circular plate in which the top and bottom halves swing in opposite directions (Figure 6.9) and thereby induce the required circular motion to the incoming gas stream. Extremely simple in concept and therefore

Cheaper to produce, it is only slightly less efficient than the RVIC.

Differential side flow inlet control

Where a centrifugal fan has to be fitted with an inlet box for side air entry, the possibility for incorporating a simplified method of flowrate control is apparent. If the box is fitted with a set of paral­lel bladed dampers then these can impart pre-swirl (Figures 6.10 and 6.11). Thus a power saving almost as good as a RVIC can be achieved, (Figure 6.12).

Figure 6.10 Inlet box incorporating side flow control

Impeller

Rotation

Rotational flow field

Figure 6.11 Flow path of air with differential side flow inlet control

Disc throttle

The unit comprises a profiled circular plate supported co-axially within a centrifugal impeller. It is described in UK Patent 2,119.440B. It is necessary for the inner edges of the blades to be parallel to the impeller axis so that a close clearance can be maintained with the periphery of this disc throughout its move­ment. The plate is carried by an axially extending shaft which projects outwards through the inlet venturi and is moved axially by means of an actuator of any convenient kind. The actuator is

Supported from the fan casing by suitable brackets or rods and where the travel is particularly long, an additional sliding bear­ing may be incorporated to support the shaft. A cross-section of the arrangement is shown in Figure 6.13 and the general layout is shown in Figure 6.14.

Per cent design volume Figure 6.12 Power absorbed by various types of fan control

Movement of the rod alters the position of the disc axially with respect to the impeller’s blades and this effectively controls the flowrate by varying the active width of the blades. The disc does not rotate and it will be seen that there are, therefore, a minimum of moving parts. This produces an inexpensive de­vice, and a high efficiency is maintained for a considerable turndown. A soft rubber ring can be attached to the outer edge of the disc so that when the damper is withdrawn up to the venturi, the inlet flowrate is almost zero.

Conversely, with the plate close to the impeller backplate, the flow is at a maximum and almost the same as that for a fan with­out a disc throttle.

This, therefore, permits the control to be used with very wide im­pellers to achieve the maximum flowrate from a given space en­velope, without the risk of entering the stall range. Its simplicity and effectiveness has been optimised with the development of a special range of impellers having dimensions calculated to make the best possible use of the disc throttle.

Figure 6.13 Cross-sectional arrangement of centrifugal fan with disc throttle for pneumatic actuation

The control offers a substantial energy reduction compared with conventional dampers. There is also an additional power saving compared to radial vane inlet controls. With the damper plate acting on width, operation is unaffected by blade shape and these may, therefore, take many of the forms commonly used in centrifugal fans, such as backward inclined, backward curved, aerofoil, shrouded radial and radial tipped.

Flowrate control is substantially linear over a wide range. Even forward curved bladed fans may be fitted when an additional power saving over normal dampers is made, albeit small, in contradistinction to the radial vane inlet control.

Again, with narrower width high pressure impellers, the power savings become less but the other advantages outlined remain. The disc throttle is a competitive solution to many centrifugal fan flowrate control problems.

As the effective width of the impeller is narrowed, there is still a small stall point at each setting until at about 1/3 effective width this can no longer be detected. The unstable area for disc throt­tle is therefore very unlike the RVIC (see Figure 6.7) and is shown in Figure 6.15.

Figure 6.14 General arrangement of disc throttle Figure 6.15 Instability with disc throttle of wide centrifugal fans

One of the most important parameters in the design of any turbo machine is the angle which the outer edges of the blades make with the tangent of the peripheral motion. As this angle is increased, so the volume flowrate will also increase, and this applies to axial, mixed flow or centrifugal fans. At the same time the pressure, which is a function of the swirl, remains substan­tially constant.

It will, therefore, be seen that if the pitch of the blades of an axial flow fan could be altered in motion, then an effective method of volume control would be available. The technology to do this al­ready existed with the aircraft propeller, albeit where the num­ber of duty hours was considerably less than the humble venti­lating fan. Nevertheless, over the last few years, the systems necessary have been simplified to enable a sufficiently reliable fan to become available for normal HVAC applications.

As previously stated, only variable pitch axial fans can ade­quately meet the needs of constant orifice, constant flowrate or constant pressure systems. The energy savings made have been amongst the highest achieved, and a reasonably good ef­ficiency is maintained over a turndown of 4:1. The aerodynamic performance of such fans is, of course, similar to normal adjust­able pitch-at-rest axials and a typical characteristic is shown in Figure 6.16. The centrifugal force on an individual fan blade can be considerable and is a function of the blade weight and its rotational speed. For a typical application, this force can be as much as 600 times the dead weight. These forces are usually resisted by anti-friction bearings of the ball or roller type. Such bearings have a lower capacity under the virtually static condi­tions prevailing, and in the early days failure was not uncom­mon. With increasing experience, however, the problems have been overcome.

Levers at the base of each blade convert the equal pitch angle adjustment into axial movement of a sliding member within the impeller hub. This may be controlled in a number of ways:

A) By movement of pneumatic bellows against a spring as shown in Figure 6.17. The bellows are expanded by com­pressed air through a rotary air seal onto a shaft exten­sion.

B) By an actuator (either pneumatic or electric) giving axial movement through levers to the stationary race of a ball thrust bearing, the revolving race being coupled to the sliding actuator within the hub.

An alternative pneumatic arrangement is shown in Figures 6.18 and 6.19, with an overall fan assembly shown in Figure 6.20.

Figure 6.17 Cross-sectional arrangement of hub mechanism for VPIM axial flow fan (compressed air operation)

Figure 6.18 Sideways view of alternative form of pneumatically operated VPIM axial flow fan

«s 40

02« * • « G U » » MiMO* 0 — Voka* Hour at* Figure 6.16 Characteristic curves for 710 mm diameter VPIM axial flow fan at 2950 rev/min and handling standard air

Figure 6.19 Cross-section of alternative form of pneumatically operated VPIM axial flow fan

Where:

Figure 6.20 General arrangement of VPIM axial flow fan

In all cases, when the fan is running, a force must be applied to each blade to maintain the required pitch angle or it would ro­tate to a position near zero pitch angle where the centrifugal forces on it were in balance. Weights are sometimes attached to the blade root, at right angles to the blade pitch, to produce a counterbalancing moment and thus reduce the actuating force necessary. In the event of compressed air supply failure, the flowrate will, of course, revert to minimum unless some alterna­tive is available.

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