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The fans will draw air in through the inlet cone (in the same way as a housed fan) but then discharge the air radially around the whole 360° outer circumference of the impeller. They can provide a great flexibility of outlet connections (from the plenum), meaning that there may be less need for adjacent bends or sharp transitions in the ductwork that would themselves add to the system pressure drop (and, hence, additional fan power). Overall system efficiency may be improved by using bell mouth entries to the ducts leaving the plenum. One of the benefits of the plug fan is its improved acoustic performance, largely resulting from the sound absorption within the plenum and the lack of ‘direct sight’ paths from the impeller into the mouth of the ductwork. The efficiency will be very dependent on the fan’s location within the plenum and the relationship of the fan to its outlet – the plenum being used to convert the kinetic energy in the air and so increase the static pressure. Substantially different performance and different stabilities of operation will depend on the impeller type – mixed flow impellers (providing a combination of radial and axial flow) have been used to overcome flow problems resulting from the strong radial air flow pattern created using simple centrifugal impellers3
. For smaller units, their compact design
is often complemented through the use of readily controllable EC motors.
Axial fans In axial flow fans, the air passes through the fan in line with the axis of rotation (as shown in the simple tube axial fan of Figure 6) – the pressurisation being produced by aerodynamic lift (similar to an aircraft wing). These can be comparatively compact, low cost and lightweight, particularly suited to moving air against relatively low pressures, so are frequently used in extract systems where the pressure drops are lower than supply systems – the supply normally including the pressure drop of all the air conditioning components in the air handling unit. When the air leaves a simple axial fan, it will be swirling due to the rotation imparted on the air as it passes through the impeller – the performance of the fan may be improved significantly by downstream guide vanes to recover the swirl, as in the vane axial fan shown in Figure 7. The efficiency of an axial fan is affected by the shape of the blade, the distance between the tip of the blade
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channels and then passed axially through the straightening guide vanes. The combined action can produce pressure far higher than is possible with other axial flow fans. Efficiencies and noise levels can be similar to those of a backward curve centrifugal fan.
Figure 5: Example of direct driven plug fan with backward curved blades (Source: Fläkt Woods)
and the surrounding case, and the swirl recovery. The pitch of the blade can be altered to efficiently vary the fan’s output. By reversing the rotation of axial fans, the airflow can also be reversed – although the fan will be designed to work in the principal direction. The characteristic curve for axial fans
has a stall region that can make them unsuitable for systems with a widely varying range of operating conditions, although they have the benefit of a non- overloading power characteristic. Vane axial fans can be as efficient as backward curved centrifugal fans, and are able to produce high flows at reasonable pressures (typically around 2kPa), although they are likely to create more noise. The mixed flow fan is a development
of the axial fan and, as shown in Figure 8, has a conical shaped impeller where air is drawn radially through the expanding
Stalling the air flow The stall in a fan can be thought of as a condition where the flow patterns in the impellers are such that air will not be effectively and consistently moved through the fan. At this point, the flow becomes unstable and can manifest itself in a number of ways including vibrating fans and ductwork, rumbling noise in ductwork, and possibly some resonance that makes the ductwork physically pulse. Continued operation in stall can damage bearings and damage the fan itself. Stall conditions can be averted through appropriate matching of fan to system – in many fans this means keeping the fan operating point so that it is to the right (that is, greater volume flow) of the point of peak efficiency. However, the type of stall and the solutions will vary depending on fan type – the Eurovent document 1/11 Fans and System Stall: Problems and Solutions provides excellent coverage of this area.
The installation of the fan The efforts to provide an effective fan solution may be severely undermined by the relationship between the fan and the local ducted pathways for the air – this will be discussed in a future CPD article. © Tim Dwyer 2011
Figure 6: A tube axial flow fan (Source: GPG383) Figure 7: A vane axial flow fan (Source: GPG383)
For further reading in this area see: CIBSE Fan application guide (CIBSE TM42: 2006) – available for free access for CIBSE members through the CIBSE Knowledge Portal. ASHRAE HVAC Handbook 2008, Chapter 20. GPG383 Good Practice Guide: Energy savings in fans and fan systems – freely downloadable from
www.carbontrust.co.uk For an excellent discussion of ‘stall’ see EUROVENT 1/11 Fans and System Stall: Problems and Solutions – free download
www.eurovent-association.eu
References 1. Murphy, J. ‘Selecting Efficient Fans’, ASHRAE Journal, April 2010.
2. Carbon Trust CTV016 ‘Motor Drive Technology Overview’, Carbon Trust 2007.
Figure 8: Mixed flow inline fan (Source: GPG383)
3. Coward, C. ‘Unhoused (Plug/Plenum) Fans: Is Their Performance Predictable?’, ASHRAE Journal, October 1997.
December 2011 CIBSE Journal 53
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