more transparency and improved coherence across manufacturers. The end goal is to help prevent fans being sold on the basis of having potentially low capital cost but subsequently consuming excessive amounts of power owing to low practical operating efficiencies. Across an operating range of 50% to
100% of full load, properly selected motors that drive the fans have a reasonably flat efficiency curve. However, the same cannot be said of most fans that may typically have an efficiency curve similar to that shown in Figure 1.
this detailed assessment may be thrown to the wind by not taking sufficient care of the fan’s relationship with the local ductwork system, the so-called ‘system effect’.
The installation of the fan in the system Fan curves are produced by the manufacturer by testing a fan in standardised conditions as prescribed by the relevant standard (such as BS EN ISO 5801). The test rig ensures that the air entering the fan has an even (as opposed to turbulent) flow pattern, otherwise the fan will not produce a repeatable performance (leading to reduced flowrate and pressure), and it will create increased noise and vibration. Internationally there are four standard
Preferred operating range
volume flow
Figure 1: Fan efficiency varies significantly with operating point
This will mean that if the characteristics
that determine the resistance to air flow in the system alter (through dirt, blockages or control mechanisms), the operating point may well shift from the preferred operating range and the fan operating efficiency may be significantly reduced. Such a reduction in fan efficiency is likely to reduce, or even negate, any savings that might be expected when using dampers to reduce flowrate. The ‘air power’, Pu , is given by the product
of volume flowrate and system pressure loss, qv x pt
. However, the electrical power input,
Pe, can be far removed from this, and is expressed in BS EN ISO 5801:20084
as Pe= qv·pf ηr·ηb·ηT·ηm·ηc where
Pe is electrical input power in watts; qv is flow rate, in cubic metres per second; pf is fan pressure in pascals; ηr is fan impeller efficiency; ηb is fan bearing efficiency; ηT is transmission efficiency; ηm is motor efficiency; and ηc is control efficiency. So, there are clearly many contributing
factors requiring consideration in the assessment and selection of the whole fan assembly that will influence the power consumption of the fan. Practically, however,
58 CIBSE Journal February 2012
test configurations, as shown in Figure 2, that attempt to represent the range of basic fan applications. Category D is the one most likely to be closest to the representation of fans in many ventilation and air conditioning systems. Centrifugal fans in air handling units or plenums are likely to be represented by manufacturers as category B, and roof exhaust fans by category A. Where a duct forms part of the test
rig, circular ducts are used with standard transformations converting the rectangular outlet from a centrifugal fan smoothly, and with little energy loss, to a circular duct. The test duct would normally include
a ‘straightener’ (particularly important for axial flow fans) downstream of the fan outlet (such as the low loss honeycomb straightener shown in Figure 3) that will stop the swirl in the air so that measurements of pressure and velocity may be reliably taken. In a real application, the lack of such a device may mean that the air continues to swirl for many duct diameters’ length and, potentially, adversely affect the reliability of flow measuring points and alter the expected pressure loss through the duct, duct fittings and components. In all the test categories where there is
a length of duct shown (see Figure 2), it is sufficiently long so that it simulates a long, straight duct in real life. However, in real life it is very unlikely that there will be a long, straight unobstructed duct at either the inlet or outlet. The manufacturer’s quoted performance is contingent on the application being similar to the method of test. However, it can be difficult for the designer to calculate the effective ‘derating’ of the fan due to the so-called ‘system effect’ of the variations from the idealised test. In many cases the system effect will only truly come to light as the system is commissioned.
Figure 3: AMCA flow straightener (Source: BS EN 5801)
The fan system effect The number of specific possibilities for the installation of fans in ductwork systems is infinite, and so the variations that may lead to the poor performance of fans are equally numerous. The degradation in performance can be principally attributed to uneven or spinning air at the inlet to the fan and obstructions or inappropriate connections at the fan inlet or outlet. Of course, this sounds simple (or even flippant) and easy to overcome through proper design and installation. However, the practicalities of site installations and time constraints inevitably mean that there are often sacrifices made when the fans are installed. There are many publications and websites
that provide details of poor fans and how their installation might be improved (for example,
http://goo.gl/uwqUy). There is some guidance, such as the Air Movement and Control Association’s (AMCA) Publication 201, that provides numerical
www.cibsejournal.com
Category A – Open inlet and outlet (ie, no ducting)
Fan Fan Category B – Open inlet and ducted outlet Fan Fan Category C – Ducted inlet and open outlet Fan Category D – Ducted inlet and ducted outlet Fan
Figure 2: Standard fan testing configurations (Source: FMA Guidance Note 15
)
n e fan total pressure
f
a
fi
f
e
i c
n y c
efficiency
pressure
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