This calculator provides an
estimate of static pressure as air flows through ducts in a grain aeration or
closedloop fumigation system.
Calculations are based on air at 60 F and standard atmospheric pressure through
constant crosssection ducts.
If more accurate estimates are needed consult a professional engineer.
This calculator is provided as is and OSU assumes no responsibility for results
obtained. The parameters needed to estimate stastic pressure are discussed
below in the order that they appear on the calculator web page.
Enter the dimensions of the duct in
inches. First select the crosssection shape of the duct, and then fill
in the dimensions in the boxes below the selected duct shape in inches.
Enter the Length of Duct in feet. The total length of duct should be
entered here. If the complete duct
system consists of different sizes or shapes of ducts connected together, the pressure
drop through the complete duct system can be calculated by calculating the
pressure loss for each section of duct and then adding them together.
Select duct material. Pressure drop due to friction is
partially dependant on the roughness of the inside of the duct. As the surface roughness increases so
does pressure drop. Roughness values
for each material are:
Duct material

roughness (ft)

PVC or Plastic

0.00001

Steel

0.0002

Concrete or wood

0.005

Corrugated plastic drain pipe

0.02

Enter the number of each type of
fitting in the boxes below.
Bends and discontinuities in the ducts also increase the resistance to airflow,
and thus the static pressure, in a duct system.
There is also a static pressure due to air entering or exiting a duct
system. The minor losses from all
fittings are added and used in the final calculation of static pressure for the
duct system.
Fitting type

Minor loss per fitting

90 degree elbow

0.7

30 or 45 degree elbow

0.35

Coupling

0.1

Ball Valve (fully open)

0.1

Duct entrance

0.5

Duct exit

1

Enter the flow rate in cubic feet per
minute through the duct. As airflow rate increases so does the
static pressure per unit length of duct.
CALCULATE button uses
the information entered above to calculate the following estimates using the
indicated equations/methods:
Velocity in duct: CFM/Duct crosssectional area plus appropriate unit
conversions.
Reynolds number: This dimensionless number is used to calculate the
friction factor (f) and also determine if the flow is laminar (Re<2100) or
turbulent (Re>2100).
Re=Density*diameter*velocity/viscosity;
Density=2.38X10^{3}Slugs/ft^{3}, Viscosity=3.74x10^{7}lbs/ft^{2}. For noncircular ducts, “diameter” is
replaced by hydraulic diameter D_{h}= 4*area/perimeter.
Reynolds friction factor (f): If Re<2100 then f=Re/64, otherwise
f=0.25/[Log(roughness/3.7D + 5.74/Re^{0.9})]^{2}
Static pressure in duct is calculated using the following equation:
Static pressure = density*V^{2}/(2) [f*L/D* + sum of minor losses]