Flow measurement in ducts include mainly the measuring of air/gases.Hence these fluids are compressible in nature, it occupies different volumes at different pressure.
So, if the measurement is on volumetric basis, the measurement taken at actual conditions should be converted
either to Standard temperature (0degC) and pressure (760mm Hg) base (STP base) or
Normal temperature (20degC) and pressure (760 mm Hg) base (NTP base).
Differential pressure flowmeters are well understood way to measure flow and this upside of technology is very low cost.
Also, it can be paired up with temperature/pressure sensors to provide mass flow for gases.
The main types of differential pressure flowmeters for circular pipes or ducts are Orifice Plates, Venturi Tubes,
Flow Nozzle, Pitot Tubes, Cone Flow meters, Wedge Flow meters etc.
For square/ rectangular ducts, conventional DP flow meters cannot be used effectively.
Only rectangular venturi tubes and pitot tube can be used, but it has the limitation for long straight run requirement.
Aerofoil flowmeter is specially designed to measure flow through square/rectangular duct.Like conventional DP flow elements, it has upstream and throat pressure sensing points.
In this, a series of ports are used to take tapings and a piezometer tube connects all these ports.
So, an average value of pressure will get as output and also it helps to reduce the error due to the velocity distribution across the duct.
The wing shaped restriction creates an acceleration of fluids towards the throat and the throat pressure also sense by a series of ports.
This gradual and smooth inlet of aerofoil makes a less pressure loss across the element and the aerofoil tail will recover most the pressure drop at the throat.
The main application of Aerofoil for the measurement of following areas.Primary, Secondary & Tertiary air flow in the boilers.
Flue gas/ hot air measurement in large ducts.
Primary & Secondary air measurement have an important role in fuel combustion rate also to maintain the stoichiometric rate.