Nomogram of the specific friction loss R (Pa/m) against the flow L (m³/h). Rising lines are the family of constant diameter «Ø», falling ones — of constant velocity «v». The dot is the result of the calculation at the intersection of its lines. Grid pressure 101.33 kPa, t 20 °C. Hover over the nomogram to magnify it.
Reference
Units
Sizes are in millimetres, flow in m³/h, velocity in m/s, specific loss R in Pa/m, pressure in pascals, temperature in °C, roughness K in millimetres.
What it computes
For a straight duct run: velocity, equivalent diameter, cross-section area, Reynolds number, friction factor λ (Altshul), dynamic pressure and specific friction loss R. Given the run length it also gives the pressure drop ΔP = R·l, and with fittings the local losses Σζ·Pd and the total.
Velocity, noise and the «traffic light»
The recommended velocity depends on the ventilation mode and the network element. Green means the velocity is inside the recommended band (or below it — acoustically safe), yellow means above the band but within the noise limit, red means the noise limit is exceeded.
Duct types
Metal, plastic, flexible ducts and masonry channels differ in roughness K. For a flexible duct the roughness is approximate: the actual loss depends strongly on how far the corrugation is stretched.
Pressure and altitude
Air density and viscosity depend on pressure and temperature. Set the pressure explicitly, derive it from the altitude above sea level, or leave the standard 101 325 Pa. A printed nomogram is drawn for 98 kPa — that is why the result may differ from it.
Section selection
In the «flow + velocity» mode the tool computes the required area and offers the nearest standard duct size, showing the actual velocity and R for it.
Local resistances (fittings)
Add the fittings of the run — the local losses ΔP_loc = Σζ·Pd (at the approach velocity) are added to the friction loss. Tees belong to the network calculation.
Material comparison, saving and export
The same run can be computed for several materials at once — the roughness is taken from the material. The calculation can be saved as a permanent link or exported to Excel.
Laminar and transitional flow
The Altshul formula applies to turbulent flow. Below Re = 2000 (laminar) and between 2000 and 4000 (transitional) the value of λ is indicative — check the input data.
Limits
Velocity 0…30 m/s, equivalent diameter 50…3000 mm, temperature −50…400 °C. The run must be straight: bends and branches are accounted for as local resistances.
How it differs from a printed nomogram
A printed nomogram is drawn for one pressure and one temperature and is read by eye. Here every value is computed for the actual air conditions, so the result is more accurate than the reading of a chart.