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  • Spray Paint Area Exhaust Fan Size

    I have a rather small area where I do my spray painting with rattle cans. It is in a storage shed in my back yard and I use one shelf that is about 2' deep, 2' high and about 4' wide. That's 16 cubic feet. Not a lot of space. The building is about 8' x 15' and 8' high with no ceiling or overhead insulation. The roof is the hip type and not too tall as there is virtually no snow here. Perhaps 1100 cubic feet or so.

    I want to install an exhaust fan to remove the paint odors and to bring in some outside air with a vent located across the room or a crack in the window. The exhaust fan will be located in the wall behind that shelf that I use for spray painting so it will be fairly close to where I am painting. I want to change out the air, but I do not want a strong air current to interfere with the painting.

    I am also experimenting with using heat lamps to help the drying process and I don't want to negate the heating effect of the lamps or bring in so much fresh air that humidity becomes more of a problem.

    My question is just how do I calculate the amount of air volume that I need to move. I probably want to get a fan that has a bit more (1.5X - 2X) than the calculated value as I will be adding a filter to keep the fan's internals as paint free as possible. I plan to change the filters often. But how do I do the basic calculation?

    Fans seem to be rated in cubic feet per minute. So this fan will be bringing that amount of air in through the front opening and perhaps half of the side opening of my shelf. The top, bottom, back, and other side are all closed with walls or shelves. Front = 8 sq feet and Side = 4 sq feet. So perhaps 10 sq feet of air intake to the area, in all. To use a round number lets start with an air flow of 100 cubic feet per minute. It should be easy to scale up or down from there. It would seem that the air would flow in at 10 feet per minute. That sounds rather slow. Of course, the velocity will increase as it nears the fan's intake. Should it/could it be faster without disturbing the painting process? Or should it be less? What criteria should I use?

    I am fairly free in where I locate the fan. Center, left, right, side wall, up or down, etc. I am thinking the corner or side wall may be a good choice. And that is closer to the available power.

    Are there any vent/exhaust fans that have a speed control? That may be the perfect solution.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  • #2
    How about a shutter system on the fan? Open or close until it is just right.

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    • #3
      Get yourself a fan assembly from a furnace. They typically have 4-6 speeds (hidden from the user) and are used to adjust the exit temperature.

      You really only need small fan to remove smell. Think toilet fan - 50-150 cfm

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      • #4
        Do you just want something for convenience?

        Or do you want some semblance of what it is supposed to be?

        In an actual commercial spray booth, there is a spec for the velocity of air across the open face of the spray booth. More airflow than you might suspect is required for a commercial booth. About 50 feet per minute minimum thru the face of the paint booth, as I recall, which creates quite a wind in the booth. Given that, you might want more fan that you suggested.

        Or, if you are just wanting not to have so much in the way of fumes, it might be considerably less and be OK for you. After all you have nothing particular right now, and you are doing OK. For fumes, figure your total volume of the room,because it will fill up with fumes, and then see what it takes to change that air volume in say 5 or 10 minutes. When you combine that air exchange rate with the airflow, you should be fine.

        If you do put in a fan, I might suggest putting a frame in front of it (on the inside) to hold two furnace filters. That will pick up the paint overspray that the fan might pull through, and keep the fan from getting covered so badly.

        Furnace fans of the centrifugal type, with a belt drive, are good, because they keep the motor out of the airflow. If you spray solvent-based paint, that cuts the fire hazard.
        Last edited by J Tiers; 07-30-2017, 11:05 AM.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #5
          Oh, the numbers I used in the first post were not any kind of suggestion. I simply used 100 CFM as a figure that could be easily scaled. 200 CFM should give twice the air velocity, 50 CFM would give half the calculated velocity, etc. As I said, it is easy to scale up or down from that point. I could, just as easily have used 1 CFM or 10,000 CFM but those would be a bit out of the practical range.

          I am looking for the numbers to plug into this ratio to give me a good result.

          The 50 feet/minute that J suggested would translate to a 500 CFM fan in this case. That sounds a bit high, but if I could have a variable speed fan with that as a top end, perhaps it would work.

          Perhaps I need two speeds: low for while I am spraying and high for drying time.
          Paul A.

          Make it fit.
          You can't win and there is a penalty for trying!

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          • #6
            filter the incoming air. keeps the dust and particles from attaching to the painted project.

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            • #7
              The size/volume of the fan you use will be determined primarily by the opening in the barrier separating the spray area from the rest of the room. And to a lesser secondary extent by the force that the spray cans generate when the blast hits the object and tries to bounce back out the opening. You need enough velocity across the face of the opening to prevent that from occurring. And to get away with the least fan volume and avoid a shop cyclone area you really do want a paint hood style enclosure along the lines suggested by JTiers above. Using an enclosure and access opening of limited size really does reduce the required air volume by a rather massive degree compared to the whole room being the "spray booth".

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              • #8
                When I had my down draft paint booth that was about 30' x 16' I had a 3' x 20' pit in the floor where the exhaust inlet was. Grated floor over the pit. The ceiling had a double row of 2' sq. filters. That was the intake. It was a DeVilbiss exhaust fan and stack 24" in dia. 4 blade fan and don't remember the HP of the motor, 1 HP maybe.
                It worked pretty good for the most part, no dead areas where the over spray was stagnant.

                Biggest issue was it sucked all the heat out of the shop during the winter.

                JL..................
                Last edited by JoeLee; 07-30-2017, 09:53 PM.

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                • #9
                  Dust and particles are not much of a problem here. I do need to keep the mosquitoes and other bugs out and will screen the intake.



                  Originally posted by 1-800miner View Post
                  filter the incoming air. keeps the dust and particles from attaching to the painted project.
                  Paul A.

                  Make it fit.
                  You can't win and there is a penalty for trying!

                  Comment


                  • #10
                    JoeLee, that sounds great, but I think we are talking mustard seed vs. Yankee Stadium here.



                    Originally posted by JoeLee View Post
                    When I had my down draft paint booth that was about 30' x 16' I had a 3' x 20' pit in the floor where the exhaust inlet was. Grated floor over the pit. The ceiling had a double row of 2' sq. filters. That was the intake. It was a DeVilbiss exhaust fan and stack 24" in dia. 4 blade fan and don't remember the HP of the motor, 1 HP maybe.
                    It worked pretty good for the most part, no dead areas where the over spray was stagnant.

                    Biggest issue was it sucked all the heat out of the shop during the winter.

                    JL..................
                    Paul A.

                    Make it fit.
                    You can't win and there is a penalty for trying!

                    Comment


                    • #11
                      Old small fan heater backwards, disconnect the heater coils, usually free
                      Mark

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                      • #12
                        Not intended as direct competition to an open front Binks style booth......googlefoo the discontinued Grizz G0532-533,2hp dry spray booth.

                        I can think of three or four reasons it got discontinued,all leading down a corporate lawyer sink hole.But,that's for the end user to decide?

                        It's a weekend build,with a used HF dust collector.Buy filter materials off epay.

                        In the "perfect" world,it would be a plug/play system that you'd still be able to use as a DC.But,those HF 2hp units used are so cheap,hardley worth the effort(make it a dedicated unit,sell or use the leftover parts in the shop).You're looking for air speeds of about a slow walking pace,1 -1 1/2 mph.
                        Last edited by BWS; 07-31-2017, 07:14 AM.

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                        • #13
                          If the fan motor is going to be in the exhaust air stream and unless you use exclusively water passed paints don't forget that the motor needs to be explosion proof.

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                          • #14
                            I can spray water based paints in the shop. This area is for solvent based ones.

                            Wouldn't any brushless motor be "explosion proof"? No sparks: no ignition. I don't plan to have one with any kind of integral switch or thermostat. As I said above, the controls will be elsewhere. What I think I need is a brushless motor that can be speed controlled. VFD?

                            OTOH, a vent fan that belched fire might be interesting and fun at parties.



                            Originally posted by Frank K View Post
                            If the fan motor is going to be in the exhaust air stream and unless you use exclusively water passed paints don't forget that the motor needs to be explosion proof.
                            Paul A.

                            Make it fit.
                            You can't win and there is a penalty for trying!

                            Comment


                            • #15
                              I would hook up a car radiator fan to a 12v battery. Cheap (free usually), portable and shifts a lot of air.
                              Peter - novice home machinist, modern motorcycle enthusiast.

                              Denford Viceroy 280 Synchro (11 x 24)
                              Herbert 0V adapted to R8 by 'Sir John'.
                              Monarch 10EE 1942

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