How to calculate Air flow rate....?

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Discussion Overview

The discussion centers on calculating the airflow rate of compressed air through a pipe of 12 mm diameter at a pressure of 7 kg/cm². Participants explore methodologies for determining flow rates, considering factors such as pipe length, fittings, and flow conditions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant requests a method to calculate the flow rate of air through a 12 mm pipe at a specified pressure.
  • Another participant suggests using Bernoulli's equation for an approximation, provided that pressure drop and velocity profile are not critical.
  • A different participant emphasizes the need for additional information, such as pipe length and fittings, to account for minor losses in the system.
  • Concerns are raised about the potential for choked flow at the given pressure, which would complicate the use of Bernoulli's equation due to compressibility effects.
  • Viscous and turbulent friction losses in the pipe are highlighted as important factors to consider in the calculations.
  • One participant seeks a basic methodology for calculating airflow capacity for pipe sizing recommendations, assuming no choking of flow.
  • A reference to a specific example in a textbook is provided as a resource for understanding flow rate calculations for incompressible fluids, with implications for compressible fluids as well.

Areas of Agreement / Disagreement

Participants express varying views on the necessary conditions and factors to consider for calculating airflow rate, indicating that multiple competing approaches and uncertainties remain in the discussion.

Contextual Notes

Participants note the importance of distinguishing between gauge and absolute pressure, as well as the implications of flow conditions (choked vs. non-choked) on the applicability of certain equations.

Who May Find This Useful

This discussion may be of interest to individuals involved in fluid dynamics, engineering applications related to compressed air systems, and those seeking to understand the complexities of airflow calculations in piping systems.

MANPREET SINGH
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Sir, Kindly let me know how to calculate flow rate (cfm or Litre/m) of air flowing through a pipe of 12 mm dia from a reservoir at pressure 7 kg/cm2 to atmosphere.

i.e ;- how much volume of compressed air could pass within a minute through a "X"mm dia pipe from a constant pressure (7 kg/cm2)

Thanks
 
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What level of accuracy are you going for?

If you don't feel the need to calculate the pressure drop in the pipe, or the velocity profile, you can get an approximation with Bernoulli's. Find the kinetic energy of the air as it leaves, solve for the velocity, multiply by the crossectional area and you've got a volume flow rate.
 
MANPREET SINGH said:
Sir, Kindly let me know how to calculate flow rate (cfm or Litre/m) of air flowing through a pipe of 12 mm dia from a reservoir at pressure 7 kg/cm2 to atmosphere.

i.e ;- how much volume of compressed air could pass within a minute through a "X"mm dia pipe from a constant pressure (7 kg/cm2)

Thanks
You'll also need to know the length of pipe through which the air travels from the reservoir to atmosphere, along with any fittings, elbows, valves, etc. to calculate the minor losses in the system.

Since the reservoir is maintained at a pressure of 7 kg/cm2(is this gauge or absolute pressure?), which is approximately 7 atmospheres, choking of the flow is a distinct possibility, which would render use of the Bernoulli equation invalid, since in the choked condition, the flow has become supersonic and one can no longer ignore the effects of compressibility on the flow of the air inside the pipe.
 
You also need to consider viscous/turbulent friction loss in the pipe.

Chet
 
Chestermiller said:
You also need to consider viscous/turbulent friction loss in the pipe.

Chet
Those were my reasons for asking for more information on the exit piping.
 
thanks for your Suggestions,
@jackwirl - i need to know basic Methodology for calculating air flow capacity of a System at a specified pressure, For Recommendation of Pipe Sizing for a required Air Flow.

@SteamKing - Sir 7 kg/cm2 is gauge pressure & assume with no choking of flow in pipe line and the pipe is open to atmosphere.

i need to know(calculate) what would be the max Flow a 12mm pipe (10m length) could provide from a Air Receiver at 7kg/cm2.

thanks.
 
See Example 6.2-2 "Flow Rate for a Given Pressure Drop" in Bird, R.B., Stewart, W.E., and Lightfoot, E.N., Transport Phenomena, John Wiley, 2002, p. 183.

This example shows how to solve for the case of an incompressible fluid. Based on this, it is easy to figure out how to handle a compressible fluid.

Chet
 

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