The need for flowrate control

Every fan is selected and installed for a given flowrate and sys­tem pressure, but there will be many occasions when the de­mand will not be at this design maximum. Boiler induced draught units will have to cope with gas flows varying according to the amount of fuel being burned and therefore the boiler out­put. Afan on a grain drying installation will have to blow through more crop as the harvest progresses. On a ventilation plant there may be differences between winter and summer duties whilst on VAV (variable air volume) systems the fan capacity and system requirements must be continuously balanced.

For these and many other examples, the fan manufacturer needs to provide or advise on capacity control systems. Before considering specific cases, it is necessary to determine how the system demand may vary as on this will depend not only the best method of control to use, but also which type of fan is most suitable. To operate the fan at a higher rate than necessary is wasteful in energy. Whilst increasing the initial cost, fan control systems will usually more than pay for themselves over the life of the fan.

The manner in which fan demand may vary can be categorised categorized as follows although combinations of these are also possible.

Constant orifice systems

In these the plant remains unchanged, but the air flowrate through it may need to vary. When there are fixed elements such as straight ducting, bends, takeoffs etc., and the flow is fully turbulent, then we may apply normal systems resistance “laws”.

Thus system pressure ps oc (flowrate Q)2. If the capacity control is to maintain its efficiency constant then as fan power

P cc Q x ps Poe QxQ2

Or P cc Q3 Equ 6.1

As fan capacity Q oc N it will be seen that speed variation is the optimum solution provided that power source efficiency can also remain constant over the range required. With AC electric motors, good efficiencies are maintained down to about 50% power (i. e. 80% fan flowrate).

It should again be noted (see Chapter 5) that whilst a system may be fully turbulent for the design flowrate and just below this figure, this will not be the case for high turn down ratios. Inevita­bly, flow will become laminar as zero is approached. Then

P oc Q x p P oc Q x Qn

Or PocQn + 1 Equ 6.2

Where n varies continuously from just less than 2 at the design flow down to 1 at zero flow.

Fan speed and efficiency will also vary.

Parallel path systems

Here the airflow may vary, but pressure required remains virtu­ally constant. Examples which come to mind are mechanical draught systems where one fan may cater for more than one boiler. As boilers are shut off or started up according to de­mand, so the gas flowrate will vary. Provided the common ductwork is short, however, the pressure drop through each boiler and therefore through the system will remain unchanged. If the capacity control is to maintain a constant efficiency then as

Power P oc flowrate Q x fan pressure ps

I. e. P oc Q Equ 6.3

Similar situations can arise in central extract systems where dampers in parallel ducting legs may be shut according to whether a machine is or is not operating and therefore emitting dust or fumes. With an on-floor grain drying plant, the floor area to be ventilated will increase as the harvest progresses but at constant grain depth and drying rate the pressure demand would remain unchanged.

Series path systems

The airflow needs to remain fairly constant, but pressure re­quired will vary. For a fan ventilating a tunnel during construc­tion, the air requirements at the working face will remain con­stant, depending only on the number of men working and air required to cool or supply the machinery. The length of ducting taken to a fresh airsource will, however, increase as the work progresses. If the fan control is to maintain a constant effi­ciency then as

Power P oc flowrate Q x fan pressure ps i. e. P CC Ps

Similar situations can arise in drying plants with bottom venti­lated bins where pressure will increase with the depth of bed.

Variable air volume (VAV) systems

In a VAV system, as applied to the air conditioning of a building environment, the airflow rate to each separate room or occu­pied space is varied both individually and continuously. Thus the instantaneous cooling demands of a room may be satisfied. Such a system is shown in Figure 6.1 and consists of a central unit (1), ducting (2), flow variators (3) and supply air terminals (4). Each flow variator is controlled by a room thermostat (5) and demands a constant pressure in the ducting. This is main­tained by the pressure transducer (6) which controls the fan flowrate by altering fan speed, inlet guide vane angle, disc throt­tle position, impeller pitch angle or such other method of flow variation as installed.

1 Central unit 4 Supply air terminals 2 Ducting 5 Room thermostat 3 Flow variators 6 Pressure transducer

Figure 6.1 Variable air volume (VAV) system

The system pressure required may be divided into three main parts:

Pa: Pressure loss in the air handling unit, which varies gen­erally as something less than the square of the fan air flow (any filters Pf may be oc Q) Pa ^ Q2

Pb: Frictional pressure loss in the ducts, which varies as something less than the square of the air flow. pb oc Q2

Pc: Constant pressure loss across the flow variator. This can amount to between 10% and 50% of the total pres­sure loss in the system. pc = c

Reference to Figure 6.2 shows that the resulting system curve of “orifice” is far from the usual square law relationship where Ps oc Q2. When assessing the suitability of the fan we must, therefore, consider that the resultant

Ps=(Pa+Pb + Pc)ccQ2+c Equ 6.4

Even this is not the complete truth. For the reasons given in Chapter 5 and Section 6.2.1

Ps=(Pa+Pb+Pc)0CQfl+1+c Equ 6.5

It must be emphasised that no type of fan flowrate control is ap­plicable to all installations. The type selected will depend on the

Air flow Q Figure 6.2 System pressure in a VAV system

Turndown ratio required, how the system resistance varies and the presence of contaminants or high temperatures. Where the system has high values of fixed resistance elements, variable speed solutions will not operate to best advantage. With reduc­tion in fan speed, the fan may develop insufficient pressure to satisfy system requirements. Some of the features and advan­tages/disadvantages of the various designs are detailed in the following Sections.

Posted in Fans Ventilation A Practical Guide