Compressed Air Flow Rate
How to calculate the compressed air flow rate by thermal mass flowmeter and vortex meter technilogies?
When calculating the compressed air flow rate, we need to take temperature and pressure factors into account, as air is compressible. The actual volume of a constant number of moles of gas depends on the measurements of temperature and pressure. 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 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 .
