Steel Blower, Venturi setup

[sauna]

I'm new to this forum and trying to learn on the fume hood and venturi setup.




Does anyone have the image posted by 4metals in "Building a chemical fume hood with a lain steel blower"? The writing in the image is not very clear but could make out few details. The duct sizes seems to be 6" or 4" out of the blower, 12" duct to the roof, 500 cfm blower has a duct length which is 6' long before the 45° elbow, 3' up the stack (wonder how far up in the stack it need be from the hood top & does this matter), door opening is 36"x30", hood base is 36"x30" not sure of the overall hood height. I would appreciate if anyone can confirm these numbers and if you'll could share blower details.

Already built the dog house fume hood that is 48"x30" base by 48" high, opening 48"x25" so need about 850 cfm blower. Happy to have found this forum!
Thank you!!
Abhishek

 

[4metals]

The blower used had a 6" discharge and it went into a 12" vertical pipe. The critical thing is the long sweep where the discharge from the blower curves up to go into the vertical pipe. The reason for a long sweep is the fact that you do not want to lose too much velocity with the air bouncing into a 90 degree elbow, a long sweep is best but hard to find or costly. A series of 4, 22 1/2º fittings space at least the pipes diameter apart should work as well. Last resort is 2 45º elbows.
The airflow goes into the 12" pipe approximately 1 foot up the pipe and draws with it air from the hood. Basically it is a low efficiency venturi but it works.

If you use a 500 cfm blower it is likely the smallest blower you can get away with. I would try to use a bigger blower because the air entrained from the hood will not be as great in volume or velocity as the air leaving the blower.

Because of this, the door on the front of the hood (I used plexiglass) left a small 2" gap at the bottom of the opening. This created a nice inward airflow when the door is down.


There are other venturi designs on the forum which are also effective with an air path flowing through a pipe vertically over the hood. Post #20 in this thread shows that venturi. see that thread here

 

[sauna]

The blower used had a 6" discharge and it went into a 12" vertical pipe. The critical thing is the long sweep where the discharge from the blower curves up to go into the vertical pipe. The reason for a long sweep is the fact that you do not want to lose too much velocity with the air bouncing into a 90 degree elbow, a long sweep is best but hard to find or costly. A series of 4, 22 1/2º fittings space at least the pipes diameter apart should work as well. Last resort is 2 45º elbows.
The airflow goes into the 12" pipe approximately 1 foot up the pipe and draws with it air from the hood. Basically it is a low efficiency venturi but it works.

If you use a 500 cfm blower it is likely the smallest blower you can get away with. I would try to use a bigger blower because the air entrained from the hood will not be as great in volume or velocity as the air leaving the blower.

Because of this, the door on the front of the hood (I used plexiglass) left a small 2" gap at the bottom of the opening. This created a nice inward airflow when the door is down.


There are other venturi designs on the forum which are also effective with an air path flowing through a pipe vertically over the hood. Post #20 in this thread shows that venturi. see that thread here

Thank you 4metals for the prompt response, clarification and additional link!

 

[sauna]

Does anyone have the calculation for above venturi setup? What venturi configuration (size of ducts, blower cfm) will provide the necessary cfm or vacuum to the hood? Thank you!