Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question on proper airflow

  1. Mar 23, 2004 #1
    hi all, I'm going to be building a screen printing table and ran into a question of airflow. Not being much of a science minded person I came across this site and thought I might find some help. Basically think of a box (4x25x35 roughly) it's airtight. except for a bunch of 1/16" holes drilled through the top. The holes are to hold down the paper. Now I need something to propel the air out of the box so that it sucks in through the holes, sucking down the paper. I had thought computer fans would be nice as they would fit nicely and they are light and can be transported easily. my question is how much cfm would the fans have to be able to push to pull the paper flat (the paper is relatively thick BTW). Is there a formula of somekind that could help me here, or does anyone know roughly through experience what it would take? I looked around and most fans seem to move between 35-119 that are the right size. Another question is if this could be done with say two or three, et cetera, would it push as much air if the fan were lined side by side as if they were sandwiched together producing a tunnel of sorts. It almost seems logical to me that a tunnel might move more air, any insight is appreciated.
    Patrick
     
  2. jcsd
  3. Mar 23, 2004 #2
    a small-vac will do the trick. A 2 gallon vac will produce enough vaccum to firmly hold paper down. You might get away with a gallon vac even. I'd buy the smaller one, give it a try. If it doesn't work, take it back, and get a slightly larger one.
     
  4. Mar 24, 2004 #3

    Cliff_J

    User Avatar
    Science Advisor

    Suction power is far more important than flow rate. Faust9's suggestion is the most inexpensive way to create a vacum table.

    BTW, plugging unused holes on the table surface around the border of the piece (assuming it doesn't cover them all) will reduce the flow rate needed by the fan also, so you might get by with the little vac, or if you need the flexibility to leave some holes open you might need a bigger one.

    Cliff
     
  5. Mar 24, 2004 #4
    I appreciate the feedback, I was trying to get around using a vacuum though as the unit needs to be easy to transport. I'm now thinking a bathroom exhaust fan might be the ticket. Another question though, how is cfm measured, is it the amount of air, or the speed of the air, the reason I ask is many ceiling fans are the same as computer fans which seemed odd as I thought it was an amount.
    Patrick
     
  6. Mar 25, 2004 #5

    Cliff_J

    User Avatar
    Science Advisor

    CFM is an amount, like a gallon is an amount of liquid. Speed would be MPH or ft/s.

    A regular computer fan doesn't generate much positive/negative pressure. A bathroom fan might be a little better, but most will just barely hold up a paper towel (something to try in your bathroom).

    A vacum is typically optimized for higher pressure (negative, or vacum) and lower flow. A one gallon shop vac like Wal-Mart carries is pretty tiny. Maybe even a dust-buster sized unit.

    Cliff
     
  7. Mar 25, 2004 #6
    thanks for the info cliff. the cfm was hanging me up. I did try a bathroom fan with the thick paper i will be using and it was able to suck the paper flat and stay by itself even with the weight of the paper working against it. the table will have the weight of the paper working with it so I think it will work. basically I'm trying to make this as cheap as possible and a ceiling fan can be had for 15 bucks where as a vacuum is usually quite a bit higher.
    Patrick
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?