Determining dimension of exhaust pipe from an air duct

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SUMMARY

The discussion focuses on determining the diameter of an exhaust pipe connected to a 14" air duct with a supply air flow of 5000 scfm at 2.5 psi and 120°F. The exhaust pipe must accommodate the airflow while maintaining a constant process flow by applying the continuity equation. Participants confirm that incompressible flow can be assumed if the Mach number remains below 0.30, although compressibility effects may need consideration due to pressure variations. The ideal gas law is recommended for calculating flow rates at various points in the system.

PREREQUISITES
  • Understanding of fluid dynamics principles, specifically the continuity equation.
  • Familiarity with the ideal gas law and its application in airflow calculations.
  • Knowledge of compressible vs. incompressible flow characteristics.
  • Experience with HVAC systems and airflow management.
NEXT STEPS
  • Study the application of the continuity equation in HVAC systems.
  • Learn about the ideal gas law and its implications for airflow calculations.
  • Research compressibility effects in high-pressure airflow scenarios.
  • Explore modeling techniques for airflow in duct systems using software tools.
USEFUL FOR

Engineers, HVAC professionals, and anyone involved in designing or optimizing air duct systems and exhaust configurations.

achan88
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Hello. I need to determine the diameter of an exhaust pipe coming out of a 14" air duct. The exhaust would be perpendicular to the duct and connect to a vertical stack which exhausts to atmosphere. Supply air is 5000scfm at 2.5psi and 120F going into a gas heater which adds 400scfm air to the system. This process air is recirculated back through the blower. In order to maintain a constant 5000scfm process flow, I need to exhaust part of the input air prior to the heater.

What would be the best way to model the air flow for this problem? Considering 14" duct with 5000scfm, velocity = 4677.206 ft/min. Can I assume incompressible flow and apply continuity equation here with Qin = Qto heater+Qexhaust?
 
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achan88 said:
Hello. I need to determine the diameter of an exhaust pipe coming out of a 14" air duct. The exhaust would be perpendicular to the duct and connect to a vertical stack which exhausts to atmosphere. Supply air is 5000scfm at 2.5psi and 120F going into a gas heater which adds 400scfm air to the system. This process air is recirculated back through the blower. In order to maintain a constant 5000scfm process flow, I need to exhaust part of the input air prior to the heater.

What would be the best way to model the air flow for this problem? Considering 14" duct with 5000scfm, velocity = 4677.206 ft/min. Can I assume incompressible flow and apply continuity equation here with Qin = Qto heater+Qexhaust?
It's not clear why you think the continuity equation wouldn't apply.

As long as the Mach number of the air flow is kept below about 0.30, you can assume incompressible flow without loss of accuracy in your calculations.
 
Well, the pressure is pretty high so even at low velocity, you still have to consider compressibility (if pressure varies)...not to mention temperature. But I don't think that should be very hard. Just sketch it out and calculate the flow rate at every relevant point based on the pressure and temperature changes and ideal gas law.
 

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