Variable speed control
Where high efficiency fans, such as centrifugal units fitted with backward inclined, backward curved or aerofoil impellers or premium efficiency downstream guidevane axial flow fans are installed on constant orifice or series path systems, then reduction in flowrate by varying the speed is preferred. In this wayfull advantage can be taken of the fan characteristic without sacrificing the inherent low energy demand. It should be noted that speed variation is not usually suitable for parallel path systems
Single blade Two blade parallel
HR PR Mil I II I I I I
Four blade parallel Four blade opposed
Figure 6.3 Approximate effect of damper blade opening on flowrate (constant system)
Due to the reduction in pressure developed (Ps °c Q2 cc N2) with decreasing flow.
Whether speed variation can be used on VAV systems will depend on the fixed element of system resistance due to the flow variator. Where this is 10% of the total fan pressure at maximum duty it is acceptable, but at 50% the variation in flowrate possible will probably be unacceptable.
Suitable prime movers for variable speed include:
• AC electric induction motors with inverter drive
• Slip ring and commutator type AC electric motors
• DC electric motors
• Variable vee belt drives with AC electric motors
• Steam turbine and reciprocating motors
Multi-speed dual wound or pole changing electric motors can be used when the operating requirements are clearly defined. For example there may only be specific winter and summer, or continuous and overload, duties to be met. In conjunction with damper control, a wider duty variation is possible, and this combination is often a very simple solution to the problem.
Where continuously variable control down to about 50% design flowrate is required, the economy achieved by slip couplings of
the eddy current, scoop control fluid, or powdertype may be indicated. There is the additional advantage of improved starting by gradually “letting in” the fan inertia. In all such cases close consultation between system designer, fan manufacturer, and coupling manufacturer, is necessary to achieve the best results in energy saving.
Figure 6.4 Instability with speed control of wide backward bladed centrifugal fan
Air flowrate Q
A steam turbine drive with a gearbox to give optimum matching of fan and turbine speeds is usually the most efficient. It is only considered for industrial applications, however, where a suitable steam supply is available.
Table 6.1 below shows typical overall drive efficiencies for a 15 kW input at %, !4, % and full speed.
Table 6.1 Typical overall drive efficiencies
It is not always realised that centrifugal impellers of backward bladed design, whilst shown on performance data as having a smooth continuous characteristic of pressure against flow, often have a small order discontinuity close to their peak pressure point. This discontinuity usually increases with impeller width and is the result of a rotating stall “cell” between adjacent blades. Manufacturers try to obtain the maximum airflow from a given casing size by incorporating wide impellers. This results in the performance being obtained with the smallest space envelope.
For a given resultant pressure rise there is a relationship between the blade inlet and outlet radii. The inlet cone throat diameter is dictated by the blade inlet diameter. Thus there is an optimum width of impeller for the correct inlet throat area/impeller inlet blade area ratio. An increase in this value will result in the impeller being susceptible to inlet disturbance and the resultant discharge airflow may contain disturbing pulsations. These can be difficult to deal with, and the downstream ducting may become “live” to low frequency vibrations.
The area of instability is shown in Figure 6.4 which also indicates a typical VAV system curve. If speed control is used as a means of modulation then entry into this unstable area is inevitable. This is often overlooked with the availability of low cost (but lower efficiency) prime movers.
Posted in Fans Ventilation A Practical Guide