Determining dimension of exhaust pipe from an air duct

In summary, the conversation discusses the need to determine the diameter of an exhaust pipe from a 14" air duct, which will connect to a vertical stack and exhaust to atmosphere. The supply air is at 5000scfm, 2.5psi, and 120F, with an additional 400scfm added by a gas heater. To maintain a constant process flow, part of the input air needs to be exhausted prior to the heater. The best way to model the air flow is through the continuity equation, assuming incompressible flow as long as the Mach number is kept below 0.30. Compressibility and temperature must also be considered in the calculations.
  • #1
achan88
4
1
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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  • #2
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.
 
  • #3
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.
 

Related to Determining dimension of exhaust pipe from an air duct

1. What is the purpose of determining the dimension of an exhaust pipe from an air duct?

The purpose of determining the dimension of an exhaust pipe from an air duct is to ensure proper ventilation and airflow in a system. The dimension of the exhaust pipe will determine the amount of air that can be removed from the system, which is important for maintaining air quality and preventing overheating or buildup of harmful gases.

2. How do you determine the dimension of an exhaust pipe from an air duct?

The dimension of an exhaust pipe can be determined by measuring the diameter of the air duct and matching it to the appropriate exhaust pipe size. It is also important to consider the length and shape of the air duct, as well as the airflow rate and temperature of the system, when determining the dimension of the exhaust pipe.

3. What factors should be considered when determining the dimension of an exhaust pipe?

Several factors should be considered when determining the dimension of an exhaust pipe from an air duct. These include the size and shape of the air duct, the airflow rate and temperature of the system, the type of exhaust being produced, and any regulations or standards that must be met.

4. Can the dimension of an exhaust pipe be changed once it is installed?

In most cases, the dimension of an exhaust pipe cannot be easily changed once it is installed. Therefore, it is important to carefully determine the correct dimension before installation to avoid any potential issues or complications with the ventilation system.

5. What are the consequences of using an incorrectly sized exhaust pipe in an air duct?

Using an incorrectly sized exhaust pipe in an air duct can lead to a decrease in ventilation efficiency, causing the system to work harder and potentially resulting in higher energy costs. It can also lead to poor air quality and potential health hazards if harmful gases are not properly removed from the system.

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