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

( 7 X )

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. Prime mover and control Range Usage Typical overall efficiency Va N ‘/2 N V* N N DC motor with AC input through thyristor control Speed and torque Adjustable over range Speed reduction required at full torque 70 86 88 89 AC motor with variable vee rope drive Speed Adjustable Down to % N. Torque Increases with Decreasing Speed Infrequent requirements for speed reduction 70 80 83 85 AC motor with inverter voltage and frequency control in rotor circuit Speed adjustable down to X2 N. Torque reduces substantially with speed 50 60 77 85 AC slip-ring motor with resistance control in rotor circuit Full speed control only possible down to 14 N. Minor speed adjustment or easy starting as control losses substantial 22 45 67 89 AC motor with slip coupling Adjustable over whole range with electronic equipment and tachometer generator Limited speed reduction and easy starting 20 41 62 82 Steam turbine and gearbox with variable supply Adjustable over whole range but requires suitable steam supply Good speed reduction but requires gearbox to match optimum turbine speeds 70 86 88 90

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