Medium pressure duct desing

[ssn61]

What is a simple procedure to calculate the duct sizes when a main duct splits into two different directions? For example, if we have a duct that carries 4000 cfm and splits into 1500 cfm and 2500 cfm, can the area of the smaller and larger ducts be proportional to the area of main duct, using their cfm's? ex. Smaller duct area would be 1500/4000 or 0.375 of area of the main duct? I am mainly concerned about dividing up the airflow. I have attached a detail I have seen regarding this issue and wonder what do the equations indicate.

 

[Xera]

Tys90 answered your question in your original post for this question. If you are only concerned in matching velocities then yes simple ratios. But any other answer will require more details from you on this system.

 

[ssn61]

Xera, thanks for the reply. I really appreciate it. What do you make of the equestions in the attached detail (drawing)?

 

[lma1]

Since 1995, SMACNA has discontinued the use of the term “low pressure” & “high pressure” in ductwork design. “Duct pressure class” is now used.

In general, there is the industry accepted general practice of using equal friction, static regain, velocity reduction or total pressure to size a HVAC ductwork system. Equal friction is a very common method used in consulting firms.

The asymmetrical diverging (or converging) “wye” fitting should be made as symmetrical and smooth as possible, the elbows should be full-radius, the change should be gradual so that the loss is minimized. It should in general conform to any of those as depicted in the SMACNA or ASHRAE tables, charts & duct fitting loss database or those as shown in Carrier or Trane handbook so that the loss can be easily estimated using the tabulated loss coefficients.

 

[Xera]

ssn61, the equations do not make sense to me. A little more details please. Why wouldn't D=B and E=C?