How to calculating and measuring compressed air flow rate?

Different flow standards

The actual volume of a constant number of moles of gas depends on the measurements of temperature and pressure

For the same mass of gas fluid

Higher the pressure, smaller the volume

Higher the temperature, bigger the volume

Therefore, we can use the reference temperature and pressure conditions to specify the volume of gas measured under those conditions. Once the volume is calculated, we can convert the calculated amount into a number of moles or mass of gas.

Different volume standards

Standard cubic foot Unit: Scf : Reference T: 60 °F (15.6 °C) Reference P: 14.73 psiA

Normal cubic metre Unit: Nm3: Reference T: 20°C Reference P: 101.325 kPaA

Many standards are used worldwide

Nm3 in China :

Ref T= 20 °C,

Ref P= 101.325 kPaA

FAD flow in compressed air:

Ref T= inlet T of the compressor, mostly 20 °C

Ref P= inlet P of the compressor, mostly 101.325 kPaA

Equation of state for ideal gas: Ideal gas are related in accordance with the combined gas law

Pressure (proportional to volume)

Higher the pressure, smaller the volume
Lower the pressure, bigger the volume

Temperature ( inversely proportional to volume)

Higher the temperature, bigger the volume
Lower the temperature, smaller the volume

Therefore we get following formula transferring between actual flow and standard flow

With this equation, we can transfer actual flow in different temperature and pressure to standard temperature and pressure.

Direct standard flow calculation

The flow range of mass flow meter are normally in velocity, Nm/s, or SF/S. Therefore the calculation is between velocity and volume flow

V: velocity of flow meter, unit is m/s
D: pipe inner size, unit is mm;
Volume flow rate unit: m3/hr

Say the inner diameter of a compressed air pipeline is 100mm, flow rate about 2000Nm3/hr, there is a thermal mass flow meter which can measure 0.9~90 Nm/s. Can this meter measure this application

Standard velocity to Standard volume flow:

Therefore the flow range of the meter covers this application.

Standard velocity to Standard volume flow

2000= (V∗π (〖100/2)〗^2∗3600)/1000,000

Therefore the flow range of the meter covers this application.

Standard flow calculation with temperature and pressure compensation

Calculate actual velocity to standard volume flow, and reversely

Calculation between standard and actual flow

The flow range of actual flow meter is normally in actual velocity, m/s, or f/S. Therefore the calculation is between velocity and volume flow, and between standard flow and actual flow
Say the inner diameter of a compressed air pipeline is 100mm, flow rate about 2000Nm3/hr, pressure 7 barG, there is a vortex flow meter which can measure 2~60 Nm/s. Can this meter measure this application.

Actual flow to standard flow :

Vortex flow meter actual velocity range is 2~60m/s
Transfer it to actual volume flow according to the calculation in the last sector: Actual flow= 56.5~1694.9 m3/hr

Now transfer it to Normal flow ref to 15°C, 101.325 kPaA

Standard flow to actual flow:

Transfer 2000Nm3hr to the normal flow

Transfer it to velocity according to the calculation in last secter
Actual velocity=8.1m/s
“Therefore the flow range of the meter covers this application”

Tips:

In the calculation between velocity and volume
To calculate in the previous formula, all unit should comply requirement

In the calculation between actual flow and standard flow
The pressure should be absolute pressure
The temperature should be absolute temperature

The most used flow meter to measuring compressed air flow rate

Based on the nature of compressed air and combines of technology flow meters used, we devided different solutions for compressed air systems.

Thermal mass flow meter the most common flow meters for dry compressed air flow measurement, due to it’s based on thermal diffusion theory so the medim cannot contain water or much moisture.

Vortex flow meter in high accuracy and for wet compressed air, as we know compressed air is compressible, temperature and pressure changes will cause volume of air changes. VFM60MV combines temperature and pressure compensation which will deliver the most accurate readings.

Pitot tube flow meter which can handel wet or dry air .