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.
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.
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.
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.
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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
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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