Simple Venturi

[sm625]

I have a 4" diameter pipe through which I need to pass the maximum amount of air possible. I am using an 8" square 100W fan, and several intermediate pieces to adapt the 8" square fan to the 4" round hole. It has worked ok in the past but right now for my current application it is just not enough air. I was wondering if adding a venturi somewhere in the intermediary stage would help, and if so, what is the easiest way to make one? Also, where would I place it for optimal velocity?

Here is a rough representation of the type of intermediary I'm workign with:

 

[Squeetard]

Are you trying to maximize wind speed or CFM?
What is this for, I'm curious?

 

[sm625]

It is for a temperature chamber. I need maximum CFM.

 

[silicon]

I have a 4" diameter pipe through which I need to pass the maximum amount of air possible. I am using an 8" square 100W fan, and several intermediate pieces to adapt the 8" square fan to the 4" round hole. It has worked ok in the past but right now for my current application it is just not enough air. I was wondering if adding a venturi somewhere in the intermediary stage would help, and if so, what is the easiest way to make one? Also, where would I place it for optimal velocity?

Here is a rough representation of the type of intermediary I'm workign with:

you need a fan with more cfm for the job.

 

[Squeetard]

Why is the 4" diameter of the pipe the limiting factor?
Bigger pipe and bigger fans.

 

[rumpleforeskin]

Because of the closed nature of a pipe you will not be able to get more CFM by adding a venturi. The volume of air will be limited by the what the fan can push into the pipe.

Some ideas to improve CFM:
Increase pipe diameter
Increase fan RPM
Improve rate of exhaust from the temperature chamber
Add an aspirator to syphon additional air into the intake