Ventilation-Booths and spray painting

[smbrm1]

The size of the fan is critical as it needs to overcome the various losses due to the design of the enclosure, face opening size, length and diameter of piping, internal texture of piping, number and angle of bends, and any filter restriction. The benefits of a large fan can easily be lost in the rest of the system components. I recall from somewhere that 100 cfm at the booth face is a good target. It would seem then that researching the system as a whole and not limiting your perception to just the fan motor power might achieve a more efficient result. Cheers

[Makulit]

Open question :

Would a smaller diameter hose 2inch would have a better flow than the avg 4-inch hose?

[CK]

Definitely not. A bigger diameter hose of the same length always offer less air resistance and a better flow.

[Makulit]

Definitely not. A bigger diameter hose of the same length always offer less air resistance and a better flow.

my theory : blowing thru a straw will move faster than a paper towel roll with the same CFM fan ?

[viperdoug]

Just some info, I went from a 4" to a 5" hose and was surprised at the increase in flow.

[sky1911]

Makulit,
regarding your example of straw vs. paper towel roll. Yes, it may travel faster through that thin straw. But also pay attention of how much pressure you have to apply to move the same amount of air in the same amount of time through the smaller opening. Just for arguments sake take square opening straws (as if there is such a thing ) because it is easier to calculate and the number are more obvious. Staw: 1x1 cm (= 1 square cm) , paper towel roll: 2x2 cm (= 4 square cm). So let's assume the towel roll is your "normal" opening. Now reducing the opening to the size of the straw means only 1/4 of whatever fits through the straws opening, but you still arrive with the normal volume at that opening. What is going to happen? Think of the drain holes in your shower tub. Say there are 4 holes normally and the flow of water is constant. Plug all but one hole - what happens is the water will rise. So for the straw / roll situation - in order to get more of whatever through the straw you have to push harder. Guess by how much? - probably 4x (assuming linear behaviour). So you need to exercise 4x the pressure to get the same amount through the smaller opening.
Now assume your fan can only run at one speed setting like the water flowing into your shower tub. It basically says that your fan cannot push harder. In the example of the straws it would now have to push 4x harder to get the same amount through, since it can't... well what is going to happen? Same as with your drain. Flow rate won't change. So the water will rise. If you take that thought with the air further it might get to the point where the fan cannot push anymore whatever forward, since the area behind it is already full with whatever. So you will have some sort of pressure pushing in the opposite direction. And then, just for argument's sake, place a filter right before that straw opening and do the same with the towel roll. The effect will be even worse for the straw.

The explanation might be a bit convoluted but I think it illustrates my point . Larger diameter = better flow as Viper indicates as well.