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SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
www.swegon.comCalculations aids
Graphs and formulae
Pressure drop graph for circular section ducts
Pressure drop graph for circular section bends
Pressure drop graph for circular section bends
Pressure drop graph, dimensions changes in circular ducts
Area increase
A = Number of dimension changes.
(Example from 016 - 020 = 1 dim.changes.)
Area reduction
A = Number of dimension changes.
(Example from 020 - 016 = 1 dim.changes.)
Pressure drop graphs for circular T and Y jones
Pressure drop includes dimensionen reduction where S < 3 x D, as shown in the diagram.
Pressure drop graph for rectangular section ducts
Ducts with the same equivalent diameter De may have different cross section areas. The velocity curves plotted above are therefore approximate with a maximum error of <5%.
Pressure drop graph for rectangular section bends
Mollier graph
A = Wet thermometer
B = Iced-over thermometer.
C = Water vapour saturation pressure, kPa
Symbols: | |
h | = enthalpy for1 kg dry air, kJ/kg |
x | = water content of 1 kg dry air, kg/kg |
j | = relative humidity |
J | = dry thermometer temperature, °C |
v | = wet thermometer temperature, °C |
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The graph is correct at atmospheric pressure = 101.3 kPa = 1013 mbar. | |
Formulae overview
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Air flow, q m3/s |
q = A · v |
A = cross section area, m2 |
v = air velocity, m/s |
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Dynamic pressure, pd Pa |
Pd = P v2/2 |
r = air density kg/m3 |
v = air velocity, m/s |
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Hydraulic diameter, dh m |
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dh = 4 · A/O |
A = cross section area, m2 |
O = circumference of duct, m |
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dh for rectangular ducts |
dn = 2 · a · b / a + b |
a and b are the duct sides |
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dh for circular ducts |
dh = d = duct diameter |
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Total pressure drop - supply air, pt Pa |
pt = ps + pd |
ps = static pressure drop Pa |
pd = dynamic pressure drop, Pa |
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Total pressure drop - exhaust air pt Pa |
pt = (-ps) + pd |
ps = negative static pressure drop, Pa |
pd = dynamic pressure drop, Pa |
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Cross section area, circular duct, A m2 |
A = p · d2 / 4 |
d = duct diameter, m |
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Circumference, circular duct, O m |
O = p · d |
d = duct diameter, m |
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Air density, r kg/m3 |
rt = 1.293 · B / 1013 · 273 / 273 + t |
B = barometer reading, mbar |
t = air temperature, oC |
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Cooling/heating effect, P kW |
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q = air flow, m3/s |
r = air density, kg/m3 |
Cp = specific heat capacity of air, kJ/kg,K
(» 1.0 at 20 °C) |
Dt = temperature difference, oC, between supply and exhaust air |
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Throw at different terminal velocities, Lx m |
Lx = l0.2 · 0.2/Vx |
l0.2 = throw to terminal velocity 0.2 m/s in accordance with catalogue data, m |
Vx = selected alternative, m/s |
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