CPD PROGRAMME
factors in an attempt to quantify the system effect that could be taken as increasing the system resistance to take account of typical fan system effect. The impact will be dependent on the specifics of the application. When the Fan Manufacturers’ Association (FMA) published the Fan and Ductwork Installation Guide in 1993, it compared some of the AMCA 201 values with work undertaken at the UK government’s National Engineering Laboratory (NEL) (as foretold by A.N. Bolton6
in 1990 in one of the rare UK
publications that considers this area). As an example, considering inlet bends (some similar to those shown in Figure 4a), the NEL experiments determined values of ‘pressure loss’ for tube axial, vane axial and backward curved centrifugal fans and compared them with AMCA’s data. There was reasonable agreement with
the tube axial fans, but the losses for the vane axial and backward curved centrifugal were significantly different. That is not to say that either was ‘right’ or ‘wrong’, but the important message is that the determination of fan system effect is highly dependent on the particular situation – that is, the details of the duct and fitting fabrication, the manufacture of the fan, and the dynamics of the air. CIBSE TM42 – Fan Application Guide
(created in conjunction with the FMA) has collected together examples of fan installations, and just one of the many examples is shown in Figure 4. The fan in Figure 4 (a) has a ducted input (and probably a ducted output), so is likely to be referred to as a category D application. However, it is clear that the inlet is being compromised by the bend immediately before the fan inlet, with turbulent flow – indicated by the red areas – affecting the pattern of air entering the fan. Ideally, there would be a straight length of duct at the entry to the fan (AMCA 201 recommends that bends near the fan’s inlet be located at least three duct diameters upstream of the fan, and notes that they can affect the fan up to five diameters upstream). Inevitably, there will often be a need
for a bend and, if that is the case, the arrangement in Figure 4 (b) would be preferable – a small change producing a significant benefit in fan performance that is likely to halve the effective loss in available fan output. (Downstream of the fan, AMCA’s data for building services applications suggests a straight duct of at
www.cibsejournal.com Figure 5: Improving a straight entry to a centrifugal fan (Source: CIBSE TM42, 2006) Flexible connections (a)
Cascade turning vanes fitted
(b)
Figure 4: Examples of reducing system effect (Source: CIBSE TM42, 2006) (a)
Smooth taper improves the air flow
(b)
WRONG Flexible connectors should not be slack, as this will cause ‘necking’, which will starve the impeller blade tips of air, reduce fan performance and increase noise (starves the blades).
Figure 6: Example of guidance from manufacturer7
least 2.5 duct diameters from centrifugal fans prior to a bend, and half this distance for vane axials.) Even straight entries, such as that in
Figure 5 (a), can be significantly improved by adding a 15° taper, as in Figure 5 (b). Fan manufacturers supply some excellent guidance based on the experience of applications, such as that in Figure 6, and will be able to provide specific data that relates to their fan geometry. If the system effect cannot be avoided,
AMCA 201 (or similar) provides a means of estimating its impact on the performance. And later this year, a new ISO standard on Fan System Effects is scheduled for publication (being developed as ISO/NP TR 16219). But, pragmatically, the best solution is to understand the possible consequences of swirl, bends and obstructions on fan performance, and to use good practice to design them out and ensure that they are installed with proper consideration of the system effect. © Tim Dwyer 2012
Further reading: The Carbon Trust has freely downloadable guides that give useful guidance in this area – Energy savings in fans and fan systems (GPG383) and Energy-efficient mechanical ventilation systems (GPG257) – see
www.carbontrust.co.uk The CIBSE Knowledge Portal provides instant access to CIBSE TM42 and CIBSE TM30 – see
www.cibseknowledgeportal.co.uk
References 1. Non-Domestic Building Services Compliance Guide: 2010 Edition, DCLG, April 2010.
2. Specific Fan Power – a tool for better performance of air handling systems, Jorma Railio and Pekka Mäkinen, Clima, 2007.
3.
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri =OJ:L:2011:090:0008:0021:EN:PDF Accessed 1 November 2011.
4. BS EN ISO 5801:2008, BS 848-1:2007 Industrial fans. Performance testing using standardized airways.
5. Fan Manufacturers’ Association Guidance Note 1 – Fan Installation Effects – A Guide to Installed Fan Performance, FETA, 2001.
6. Installation Effects in Fan Systems, A.N. Bolton, Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy August 1990, vol. 204 no. 3 201-215.
7. JM Aerofoil Fan Installation Factors, FläktWoods, 2001. February 2012 CIBSE Journal 59
RECOMMENDED Flexible connectors should be just long enough for mechanical isolation and should be taut.
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