Plain intake with mesh
Length
Volume flow (m3 Velocity (m/s) Velocity
Pressure (Pa) ζ
Static pressure loss (Pa)
/s) -
0.7 -
Duct A-B
3
0.7 5.7
19.5 1.47 28.7 - 3
Fan - 0.7
Duct C-D
1.5 0.7 5.7
- - - Figure 2: Ductwork sizing using constant pressure drop of 1 Pa/m
There are three principal methods commonly used to size ducting: l ‘Constant pressure drop’ – commonly used, easily applied method for low pressure (+/- 500Pa) comfort ventilation systems – particularly for final distribution ductwork and constant volume systems;
l ‘Constant velocity’ – particularly useful where there is a need to maintain a minimum velocity (for example, to convey particulate matter along ducts); and
l ‘Static regain’ – a more complicated iterative method (although readily computerised) that can provide better stability, particularly for VAV systems and main distribution ductwork. Although any one of them could be
used, in this example the more frequently employed ‘constant pressure drop’ method will be applied for a small single office ventilation system. The actual value for the static pressure drop (per metre) used as the constant value will be determined by the application – larger values will not only consume more power but are also likely to cause more noise with the smaller duct sizes and the resulting higher velocities. For the purposes of this example, the time-honoured value of 1Pa/m will be used. This is a value that many years of experience has apparently proved to be a good compromise between the demands to keep ducts small, so as to limit space needs and keep capital costs low, and the higher noise and operating costs that arise from smaller ducts. (Conveniently, it also has a nearly equivalent value in the IP system of 0.1 inches wg per 100 feet ductwork.) Guide B3i
provides extensive guidance on
the appropriate limits for the associated air velocities to maintain appropriate noise levels. The data used to determine the
pressures throughout the duct system are shown in Figure 2. Clearly, the pressure drop in the straight lengths of ductwork is just a small part of the total pressure
64 CIBSE Journal October 2011 Point Velocity pressure (Pa)
Total pressure (Pa) (Gauge)
O A B C D E F G H R 0 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 0 Static pressure (Pa) (Gauge) 0 -48.2 -51.2 69.4 67.9 42.9 40.9 36.0 35.0 0 0 -28.7 -31.7 88.9 87.4 62.4 60.4 55.5 54.5 0 Figure 3: Pressures throughout duct system
drop in the system. The pressures in the system may be plotted as in Figure 4. The pressure at the start and at the end of the ‘system’ (in the outdoor air and in the room) are shown as being the same, although frequently the pressure may be increased or reduced in the room by supplying slightly more or less air than is being extracted, as a means of controlling infiltration and exfiltration of air from adjacent spaces or outdoors. From the data shown in Figure 3 and
Figure 4, the Fan Total Pressure may be obtained. This is the difference in the total pressure across the fan, and in this case is 88.9 – (-31.7) = 120.6Pa. This compares with the sum of the static pressure drops of 101.1Pa – the difference being a velocity pressure. For ducted air systems, the requirement
is to select a fan that meets the required total flowrate and fan total pressure, so in this case a fan would be needed to provide a volume flow, qv, 0.7m3
/s against a total
pressure, pt, of 120.6 Pa. Assuming that a fan could be perfectly matched to this,
then the electrical power, Pef , that would be needed to provide the required airflow
would be: Pef = (pt · qv)/η0
, where η0 is the overall
efficiency of the fan and the motor assembly. So if, say, the combined fan and motor
efficiency is 65% then, in this case, the electrical input would be (0.6m3
/s x
120.6Pa)/0.65 = 111 watts and, if the duration of operation is known, this can be simply changed to the energy consumption per time period. So, for example, if the system was to operate for eight hours per day, five days per week,
52 weeks per year, the annual electrical energy consumed would be: 111 watts x 3,600 seconds x 8 hours x 5 days x 52 weeks = 831 MegaJoules or, alternatively, (111 watts/1000) x 8 hours x 5 days x 52 weeks = 231 kWh. This can be readily related to energy
cost – assuming electricity is a nominal 0.15 pounds per kWh, this particular system would consume 231 x 0.15 = £34.65 per year. Similarly, the carbon impact of operation, based on Carbon Trustv
figures,
may be calculated as 231kWh x 0.54 kg CO2
/kWh = 125 kg CO2 per annum. The Building Regulationsvi relates the
effectiveness of ventilation systems to ‘Specific Fan Power’ (SFP). In terms of the regulations, this is defined as the sum of the power used in both the supply and extract ventilation systems (and their associated control systems) divided by the ventilation flowrate in litres/second. However, SFP is frequently applied to single system (as opposed to the sum of the supply and extract), so in this case the SFP would be 111/600 = 0.185 J/l. (As a comparison, the Non-Domestic Building Services Compliance Guidevi
that
supports the England and Wales Building regulations would allow a maximum value of 0.6 J/l for such a system – but note that this example contains no filters, extract system or control power!) Even this apparently simple system
could usefully have changes made that would reduce the system pressure drop and so reduce the operational energy use. So, for example: l Depending on space availability, the ductwork could be made circular – this will have a lower pressure drop. The
www.cibsejournal.com 19.5 - 1.5
Heater -
0.7 -
- - 25
Duct E-F
2
0.7 5.7
19.5 - 2
Bend -
0.7 5.7
19.5 0.25 4.9
Duct G-H
1
0.7 5.7
19.5 - 1
Diffuser 1
0.7 -
- - 35
Total 101.1
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