Duct air velocity 5.7m/s (From duct sizing data from CIBSE Guide C4) ζ factors from CIBSE Guide C4
Room R
CPD PROGRAMME
Static pressure Total pressure
Atmospheric pressure
O A Intake B Fan C D E Heater
Velocity pressure F
G Bend
H R Diffuser
The use of electrostatic fi lters will make signifi cant reductions to pressure drops;
● Where sound attenuation is included, the application of longer silencers (or indeed electronic active noise attenuation) will result in lower pressure drops;
Figure 4: Pressures in simplifi ed ductwork system
● Heat recovery systems can impose large pressure drops on the systems, and this should be carefully considered when comparing options. There may be a benefi t in applying bypass systems for periods when the heat recovery is not in use; and
500 400
Cooler Heater Heat recovery Main fi lter Pre-fi lter Pre-heater
300
● Aside from individual elements, the interactions between fans and downstream bends, transitions and components (such a heating and cooling coils) can increase the effective pressure drop, due to high velocity air jets impacting on duct walls and coils.
In this small system, every saving of 10Pa will reduce annual running costs by about £3.26 and will reduce carbon emissions by around 9kg CO2
per year. Of course,
in larger systems, such changes will have magnifi ed benefi ts. Simply using lower pressure drop
200
components and designs can provide remarkable benefi ts. The CIBSE publication Improve life cycle performance of mechanical ventilation systems – CIBSE TM30: 2003viii
covers this area in greater 100 0 Figure 5: Reducing air handling pressure drop through careful selection
actual shape and size of the duct will be infl uenced by a number of factors (see HVCA DW144vii
for detailed guidance);
● The mesh covering the plain intake has a ζ factor related to its free area. This value can vary signifi cantly as the free area is changed. New designs of mesh on the market combine a large free area while maintaining good resistance to rain ingress;
● When selecting components for conditioning air (in this case, just a heater) the focus may be predominantly on the thermal performance of the equipment. However, the pressure drop in the components will affect the performance of the system at all times (whether the component is active or not) and should form part of the selection criteria;
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● The pressure drop in any sections which change shape or direction (such as the bend in this system) can be signifi cantly reduced by adding splitters or turning vanes to keep the air fl owing smoothly – for example, the pressure drop in a rectangular bend can be readily reduced by more than 50% using turning vanes; and
● The design and quality of the fi nal air terminal device (such as the diffuser in this system) will vary between manufacturers. This may not make a large difference to the overall pressure drop but will contribute to continuous savings.
And, for items not in the example system: ● Filters should be selected to meet the requirements of the application. Over- specifi ed fi lter effectiveness is very likely to increase the fi lter’s pressure drop.
detail, and shows that an offi ce using good practices in design and operation can practically halve the fan and pump energy use compared to a traditional (2003) design. For example Figure 5, taken from TM30, shows how the pressure drop of the air handling unit (AHU) may be reduced through thoughtful selection. This publication is highly recommended for anyone involved in ventilation system design, as it provides extensive comparative data and proposes a number of practical techniques to reduce the total life impact of air systems. © Tim Dwyer 2011
i ii
iii iv
v vi vii viii CIBSE Guide B, CIBSE , London 2005
CIBSE/BSRIA Guide to HVAC Building Services Calculations, 2nd Edition, BSRIA 2007
CIBSE Guide C, CIBSE, London, 2007
ASHRAE Fundamentals Handbook, ASHRAE, Atlanta Georgia, 2009
www.carbontrust.co.uk/cut-carbon.../conversion- factors.aspx accessed 13 September 2011
Non-Domestic Building Services Compliance Guide, NBS, London 2010
Specifi cation for sheet metal ductwork HVCA DW/144, Heating and Ventilating Contractors’ Association, London 1998
Improve life cycle performance of mechanical
ventilation systems CIBSE TM30, CIBSE London 2003
October 2011 CIBSE Journal 65
Fan total pressure
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