Sand blaster troubleshooting
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  1. #1
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    Default Sand blaster troubleshooting

    Greetings fellow nerds.
    I am troubleshooting an issue with my sandblaster and was hoping for an advice here.

    Currently I have a very underpowered compressor, 2.2SCFM at 90 PSI. i know...
    Anyhow, i was looking at reducing the flow rate by using small opening on the blaster 5/32nds vs 6mm that came stock. The problem that i am having now is that i have air blowing out of the pick-up tube because my 1/4 inch supply line basically provides more air than can escape through the nozzle opening. While getting a bigger compressor is the long term solution, i am trying to understand what variables i can change to get a small spray pattern at 90 psi and low flow rate? Do I have to reduce the hose size from the regulator to reduce the supply volume to match the smaller nozzle? I'd hate to do that to avoid psi drop in the hose and additional power loss. Do i need to put flow rate choke, basically a 1/8th " tube right next to the nozzle air inlet?

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    There is no blasting a damn thing with 2.2 cfm of air.

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    Quote Originally Posted by pcm81 View Post
    Greetings fellow nerds.
    I was going to type up a good article to help you......

    Instead, Try TPtools.com

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    Quote Originally Posted by digger doug View Post
    I was going to type up a good article to help you......

    Instead, Try TPtools.com
    Awww... I am a nerd and I watch NurdRage videos on youtube...

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    You can sandblast at low cfm.

    Dentists used to have available a sandblaster to do teeth. I've seen them.

    For doing 2x2x2 inch parts with a bead blasted finish prior to anodizing, I have the set up for that now. Get away from the big nozzle and get a little set up. like going from a 4 1/2 inch grinder to a Dremel.

    If I want to sand blast a car body, I use a set up like you likely have with ceramic nozzle, pickup tube suck in media. To keep good flow in the media, it has to be dry and particle size has to fit good in nozzle. The compressor sucks moisture out of the air and adds it into the media stream and you have to stop once in a while and clear lines. To get my cfm into higher numbers I connect 2 or 3 air compressors, my pancake, two of those $90 on sale at HFT #68740. And I have 1 or 2 air tanks. I connect them all together and I can blast sand all day. You can get seriously hurt or killed doing it that way though, if you have compressed air and break a line, the air is stored energy, it can whip a hose with a connector attached and whip around until the air is gone.

    If you can blast using short air use time the compressor can keep up but it will take an hour to do 10 minutes work

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    Quote Originally Posted by Pathogen View Post
    You can sandblast at low cfm.

    Dentists used to have available a sandblaster to do teeth. I've seen them.

    For doing 2x2x2 inch parts with a bead blasted finish prior to anodizing, I have the set up for that now. Get away from the big nozzle and get a little set up. like going from a 4 1/2 inch grinder to a Dremel.

    If I want to sand blast a car body, I use a set up like you likely have with ceramic nozzle, pickup tube suck in media. To keep good flow in the media, it has to be dry and particle size has to fit good in nozzle. The compressor sucks moisture out of the air and adds it into the media stream and you have to stop once in a while and clear lines. To get my cfm into higher numbers I connect 2 or 3 air compressors, my pancake, two of those $90 on sale at HFT #68740. And I have 1 or 2 air tanks. I connect them all together and I can blast sand all day. You can get seriously hurt or killed doing it that way though, if you have compressed air and break a line, the air is stored energy, it can whip a hose with a connector attached and whip around until the air is gone.

    If you can blast using short air use time the compressor can keep up but it will take an hour to do 10 minutes work
    This is exactly what i am after, your 2x2x2 inch pats example. I am trying to use small nozzle at 90+psi for small rust removal. I have in-line air dryer to avoid clogging issues. The problem i am facing is that with a small nozzle my compressed air is blowing out of the pick-up tube instead of sucking media in. With a large nozzle psi in tank drops too fast. I am trying to go from aggressive blast for 10 seconds, wait 1 minute for tank refill to less aggressive blasting non-stop. Need to keep around 90 psi though for rust removal.
    I am running 1/4 inch hose from pressure regulator on my tank to a desiccant dryer. The drier is full open, so its regulator just puts out the same psi as tank puts in. That goes to a pedal valve and then to Eastwood blasting gun in glove box. If I use 6mm nozzle it works, if i switch to 3mm nozzle the air is blowing out of the pick up tube. I'd like to keep air leaving my small nozzle at around 90-100 psi; much lower than that and my 100-170 grit glass beads to not take rust off.

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    29 gal. 2 HP 150 PSI Cast Iron Vertical Air Compressor

    Get yourself a 20% off coupon and at least have something that can sort of maybe do what you are trying to do...

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    IMHO the answers a pressure pot.

    You need to understand you have 2 nozzles in your gun to make a venturi, first funnels the air into the second that then accelerates the grit. The suction between the 2 is what lifts your grit.

    Swap the output for a smaller nozzle and yeah, you get no suction but back pressure, you have to reduce both the air nozzel and the ceramic grit nozzle to still generate the suction to lift your grit.

    On a pressure pot setup you just have the one nozzel. My HV02 hydrovane pretty much holds 90 psi constantly on a 3mm nozzel, that gives about a effective 1" ish blast spot dia. a 2" by 2" square of aluminum would take low single digit seconds. Underneath of a car bonnet to strip pain takes me 20 minutes or so, so its still painfully slow for industrial use, but for what i need its great!

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    Quote Originally Posted by kustomizingkid View Post
    29 gal. 2 HP 150 PSI Cast Iron Vertical Air Compressor

    Get yourself a 20% off coupon and at least have something that can sort of maybe do what you are trying to do...
    MORE POWA FOR DA WIN!!! YEAH!!!!
    And when i move into a larger house with neighbours far far away i am getting a 20 scfm eaton polar air or similar class compressor may be Bendpak TS-5. Until then its all about 120 VAC and low dB rating.

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    Adama hit the nail on the head. A typical sand blaster cabinet uses the venturi effect to siphon the media. As adama mentions, match the air and media reducer to your small output nozzle.

    I would second TPTools.com. I've bought from them in the past and have been satisfied.

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    Quote Originally Posted by adama View Post
    IMHO the answers a pressure pot.

    You need to understand you have 2 nozzles in your gun to make a venturi, first funnels the air into the second that then accelerates the grit. The suction between the 2 is what lifts your grit.

    Swap the output for a smaller nozzle and yeah, you get no suction but back pressure, you have to reduce both the air nozzel and the ceramic grit nozzle to still generate the suction to lift your grit.

    On a pressure pot setup you just have the one nozzel. My HV02 hydrovane pretty much holds 90 psi constantly on a 3mm nozzel, that gives about a effective 1" ish blast spot dia. a 2" by 2" square of aluminum would take low single digit seconds. Underneath of a car bonnet to strip pain takes me 20 minutes or so, so its still painfully slow for industrial use, but for what i need its great!
    I wonder if i can reduce the incoming flow to match 90 psi through 3mm nozzle by adjusting pressure regulator on my drier? Basically have regulator on tank set at 90, the regulator on drier set to say 45psi, but due to low flow through the 3mm nozzle actually end-up with 90 psi at the nozzle? May be just have a section of 1/4" hose with some blockage in it to have equivalent diameter to 3mm instead of 1/4"?

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    Quote Originally Posted by pcm81 View Post
    I wonder if i can reduce the incoming flow to match 90 psi through 3mm nozzle by adjusting pressure regulator on my drier?
    Nope, and this isn't going to fix your suction problem. Imagine a smooth pipe with a plate welded across the inside and a hole/orifice drilled in the plate. With a solid supply, the upstream side is going to be at your supply pressure, while the downstream/output side is going to be at a lower pressure and a particular volume of flow determined by the size of the orifice and the supply pressure. The smaller the orifice, the greater the pressure drop.

    For the blaster suction to work, there are two orifices, one in the upstream flow and one from your blast nozzle and they have to be be properly sized relative to one another. When you reduce the nozzle size, you need to also reduce the upstream orifice size, and you probably should reduce the diameter of the media suction hose, too, to keep the suction air speed up so media will stay in suspension. Just reducing the supply pressure won't restore the balance.

    See if you can find or make a smaller upstream orifice. I'd start by trying a diameter reduction proportional to the nozzle diameter reduction.

    Pressure pots work on a different principle. They don't rely on orifice pressure drop, and push rather than pull the media from the bulk.

    You can buy a teeny tiny media blaster that runs on negligible amounts of air from hobbiest and jewelers tool outlets. See here, here, here or here, for examples. (I looked for something at Rio Grande, but all they are currently listing is spare parts, not a full blaster.) You can probably adapt the gun off on of those units to run in your full-size cabinet without too much trouble. Just make sure you use media that's small enough.

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    Quote Originally Posted by sfriedberg View Post
    Nope, and this isn't going to fix your suction problem. Imagine a smooth pipe with a plate welded across the inside and a hole/orifice drilled in the plate. With a solid supply, the upstream side is going to be at your supply pressure, while the downstream/output side is going to be at a lower pressure and a particular volume of flow determined by the size of the orifice and the supply pressure. The smaller the orifice, the greater the pressure drop.

    For the blaster suction to work, there are two orifices, one in the upstream flow and one from your blast nozzle and they have to be be properly sized relative to one another. When you reduce the nozzle size, you need to also reduce the upstream orifice size, and you probably should reduce the diameter of the media suction hose, too, to keep the suction air speed up so media will stay in suspension. Just reducing the supply pressure won't restore the balance.

    See if you can find or make a smaller upstream orifice. I'd start by trying a diameter reduction proportional to the nozzle diameter reduction.

    Pressure pots work on a different principle. They don't rely on orifice pressure drop, and push rather than pull the media from the bulk.

    You can buy a teeny tiny media blaster that runs on negligible amounts of air from hobbiest and jewelers tool outlets. See here, here, here or here, for examples. (I looked for something at Rio Grande, but all they are currently listing is spare parts, not a full blaster.) You can probably adapt the gun off on of those units to run in your full-size cabinet without too much trouble. Just make sure you use media that's small enough.
    Thanks for advice. I'll look at the options tomorrow. Past my bed time now.

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    sfriedberg is right on track, although I'd suggest a proprotional reduction in nozzle area rather than diameter. Hose diameter is a little trickier.

    There are two fluid dynamic processes going on here, and they both depend on momentum transfer. Momentum is proportional to the mass of the fluid and the square of the velocity (mv^2). For the best performance they need to be matched. The gun passes air through a small nozzle which creates a high velocity stream of air. This stream transfers momentum to the air in the body of the gun and drives it out through the blasting nozzle. ( Venturi effect - Wikipedia )

    With a compressible fluid like air, under normal conditions the highest velocity through the nozzle is the speed of sound at that pressure ( Choked flow - Wikipedia ). The momentum is proportional to the square of that velocity. Reducing the flow with upstream restrictions reduces the velocity through the air nozzle so the suction decreases dramatically. With too low a flow through the nozzle, there's insufficient momentum to overcome the pressure drop at the blasting nozzle and some of the air comes out the suction tube. If you assume that a nozzle has 8 units of momentum at sonic flow, cutting the volume in half by upstream pressure reduction reduces the momentum to 1 unit of momentum. If you cut the volume in half by decreasing the size of the nozzle, you cut the momentum in half to 4 units of momentum.

    The second process is the transfer of momentum from the air to the blasting medium. With the reduced momentum from the improperly sized nozzle, the energy transferred to the blasting medium is reduced, resulting in poor blasting performance even if the medium is drawn into the blasting stream.

    It's really a lot more complicated when you fully account for the compressibility of air, temperature effects from the expansion of the air, and the actual geometry of the gun, but this is what's going on at a high level. The manufacturers have worked this all out, so using a gun with their recommended specifications will be a lot more successful than trying to hone in on it yourself.

    PS. The equation for flow through the air nozzle inside the gun:
    choked-flow.jpg

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    Again fundamentally what your trying to do just will not work, suction guns use a significant amount of energy just suctioning the grit, you just don't have enough input energy to accelerate a meaning full qty of grit to a usable velocity, due to how this all works it does not scale linearly, your losses remain pretty dang constant on a small cabinet blaster setup, hence smaller nozzles won't really work at all well, the difference between your current compressor and one 4 times the size just does not scale down into your gun even if you get the orifices to match up size wise and spacing.

    Don't get me wrong, blasting is great, but for the energy consumed its a bloody inefficient process, the losses are horrific energy wise to use full work done. Your so far down the scale energy wise, all your getting are losses right now.

    Industrial blasters work on the notion of 100PSi at the output of the nozzle, to achieve that you need a lot more than 90psi at a regulator on a bit of 1/4" hose. At 20-30 psi my pressure pot will barely mat aluminum, can comfortably hold your hand in the stream, at 60 its a nice stream, theres enough energy in every particle any exposed skin and ricochets sting! up at 90+ its very very effective and you will soon not have any non covered up skin.

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    Give the guys at Tacoma sandblasting a call. Sandblasting is their business, they will be able to help you with any questions you have.

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    Thank you all for very good input. I'll be honest, I have not thought about the suction aspect of the set-up at all. And as was mentioned here the pressure losses in a small set-up are far more parasitic than in a larger unit. I guess I’ll just have to play with it until I get the best "possible" combination for my low power set-up. Looks like trying a pressure pot may be another thing to experiment with. Its a table-top glove box, so having a small pressure pot inside is too tight. I guess I’ll have to see if the 1/4" air feed hose can be re-purposed for media feed from pressure pot blaster. Might be too flow restricting with air and media though...

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    Pressure pot cabinets, don't have the pot in the cabinet, they normally have it under and the bottom of the cabinet just dumps back into the pot once you let off the pressure.

    fine grit and a 1/4" air hose with grit should work, your limitation is still nozzel size, 2mm nozzels or smaller like you will have to run are very prone to clogging, your going to be restricted to fine medias only.

    Again my pressure pot, 3hp constant duty and i can only just hold 90-100 psi with a 3mm nozzle. But i do mean constant as for 20+ minutes at a time no issue. 3.5mm nozzel and the pressure drops to nearer 70 psi, still effective but noticeably more delicate and slower!

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    Pressure pot cabinets, don't have the pot in the cabinet, they normally have it under and the bottom of the cabinet just dumps back into the pot once you let off the pressure.

    fine grit and a 1/4" air hose with grit should work, your limitation is still nozzle size, 2mm nozzles or smaller like you will have to run are very prone to clogging, your going to be restricted to fine medias only. odd big bits will block things up so you have to sift your media pretty well.

    Again my pressure pot, 3hp constant duty and i can only just hold 90-100 psi with a 3mm nozzle. But i do mean constant as for 20+ minutes at a time no issue. 3.5mm nozzel and the pressure drops to nearer 70 psi, still effective but noticeably more delicate and slower!


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