CPD PROGRAMME
Figure 3 - Full set of fan characteristic curves Figure 4 - System and fan operating point
Figure 5 – The effect of varying air density of fan and system operation
cooking is taking place). So aside from altering the mechanisms or the physical size (or possibly geometry) of the fan, the increasingly adopted option is to alter the fan rotational speed. This would have traditionally been undertaken through mechanical devices such as hydraulic clutches, fluid couplings, and adjustable belts and pulley. (Altering pulley sizes on indirectly driven fans is still routinely used as an efficient way of setting a fan’s installed speed.) Electrical devices have also been used, such as eddy current clutches (where the torque passed from the motor drive to the fan is varied) and wound rotor motor controllers (where motor voltage or resistance is altered). More recently variable frequency drives, VFDs (often known as ‘inverters’), have become a cheaper alternative. VFDs vary the effective supply
frequency to the AC fan motor giving speed reductions down to 20%. The
www.cibsejournal.com
Figure 6 – Variable speed fan providing operating point at required flowrate
power conversion efficiency of such drives is typically above 96%, although the imperfect output waveform created by the electronics (ie not perfectly sinusoidal) may reduce peak motor efficiency by 1 or 2%. Motor efficiency may itself reduce significantly at speeds below 75%ii When the fan speed is reduced, the
.
curves for fan performance and power both move downwards and to the left. Since the fan efficiency curve shifts to the left as the speed is reduced, the fan can still maintain high efficiencies (as in Figure 4) even at lower output. Since the principal element of
noise from a fan outlet is due to its aerodynamics, reducing the fan speed will mean that the noise emitted by the fan is also likely to reduce. This will then lessen the adverse effects of the ventilation system on occupants and may also reduce the need for sound attenuation. (Note that when comparing two different fans, a slower fan
will not necessarily be quieter – the octave band sound power levels of the individual fans at the design flow rate and pressure must be used to compare the noise).
The fans There are two principal types of fans used in building services engineering for ducted systems – centrifugal and axial. A future CPD article will consider the details, variants and application of these fans and reflect on how installation will affect performance. © Tim Dwyer 2011 For further reading in this area the CIBSE Fan application guide (CIBSE TM42: 2006) provides extensive detail on fan types and their application. A book that is currently out of print but particularly useful is Woods Practical Guide to Fan Engineering. It is still widely available as a second-hand purchase. ASHRAE HVAC Handbook 2008 Chapter 20 also provides core material.
November 2011 CIBSE Journal 63
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76