Parallel Compressors For Sandblasting Without Crashing Your Grid

[Hannah] is restoring a 1962 Volkswagen Bug. The goal is to get the car on the road in time for her driver’s test. This is no easy task, as the lower 3 inches of all the body work is rusted out, and the engine is…. well, missing. Basically, the car needs a frame off restoration. This means that [Hannah] will have a lot of metal bodywork to clean up. One of the easiest ways to do that is sandblasting.

Large scale sandblasting is a bit different from most air-powered operations. Sandblasting needs only a modest air pressure, but a high air flow. [Hannah] need 25 Sustained Cubic Feet Per Minute (SCFM) at 80 PSI for sandblasting. Most compressors can easily supply that pressure, but 25 SCFM is asking quite a lot. She could go with an expensive 3 phase unit, or rent a diesel screw compressor. However, [Hannah] decided to connect 4 compressors in parallel to give her the flow she needed.

Connecting the air outputs in parallel is easy. The problem is the electricity. Each compressor is rated for 9 amps while running. They draw quite a bit more while starting up. The compressors have to be wired to individual 15 amp circuits to avoid blowing fuses. They also need to be started in sequence so they don’t pull down the AC for the entire house while starting.

Hannah could have used any sort of delay for this, but she chose an Arduino. The Arduino’s wall wart is wired up to the master compressor. Turning on the master powers up the Arduino which immediately starts a 2 second delay. When the delay times out, the Arduino fires up the second compressor. After several delay loops, all 4 compressors are running together.

hannah-schThe Arduino’s GPIO pins can’t handle 9 amp AC loads, so [Hannah] wired them to TIP120 transistors. The TIP120s drive low power relays, which in turn drive high current air conditioning relays. The system works quite well, as can be seen in the video below the break.

If you’re interested in air compressor projects, check out this setup made from an old refrigerator compressor. For more background on the TIP120, check out this article about these useful transistors.

77 thoughts on “Parallel Compressors For Sandblasting Without Crashing Your Grid

    1. Looks like she spent about $500 on compressors alone – not including the extra hoses, electrical circuits, control stuff. A belt-driven single phase 208-220 compressor would’ve been quieter, lower surge current, simpler…but that’s still a lot less than what a 7.5-10HP compressor would cost, which is at least $1800 new. Used you could probably do a lot better.

      On the flip side, you do get a lot larger tank when you buy that much compressor.

      Harbor Freight compressors almost certainly draw more power than they should, and probably don’t even deliver their rated CFM/pressure.

      1. Harbor Freight compressors are absolute junk and should not even remotely be classified as a compressor or even be considered for purchase by anybody. I get that a proper compressor costs money but this is just a creative way of wasting it.

      2. Even more cost effective would be to farm out the blasting to a dustless blasting company or an acid dip. On top of it, it’s an air cooled VW, parts are everywhere for it and cheap. It’s hardly cost effective to media blast it to restore rotted metal when it can be replaced quickly, cheaply, and easily.

  1. so when the first compressor cuts out at full pressure the others won’t be at full pressure?
    I guess if its balanced enough the compressors will just run constantly? isn’t that bad?
    no I don’t know anything about air compressors

    1. It’s to solve the problem of the massive current draw of starting each motor. All four running concurrently is fine, but starting all four is just too much amperage.
      I would think that they would all switch off almost simultaneously when max pressure is reached. Within a few seconds maybe.

      Never seen this done before, good hack.

  2. Sand blasting is terrible for sheet metal. She’ll quickly find out if she hasn’t already that it doesn’t take much blasting before warping horribly. There are better ways to take car of a rusted out old bug, and mainly its cutting out said rust and (the whole lower portion of the body, usually) and replacing it with aftermarket metal. The money spent on compressors could have bought a better body more than likely.
    I haven’t seen a bug that didn’t have the floor pans completely gone or replaced. And its not as though bugs are rare.

      1. I’ve seen a lot of different media used, it will still warp. I’ve personally warped 1/4″ aluminum plate with volcanic grit so fine it looks like baby powder, and I was being careful!

    1. It all depends on how big of a pressure you use to blow the metal with. I have a compressor, that’s the size of 2 or 3 of those and i blasted my cars roof. It has holes in it too (that’s the most delicate part i’ve done so far). No warping. The rest of the body i had blasted by a professional, but i said not to blast the outer sheet metal to avoid warping (roof and rear fenders). I bet with his equipment they would’ve warped. Granted, my equipment is selfmade, propably wastes some pressure, but crank the pressure lower, if it warps.

      It’s just that it takes a loooooot of time with small equipment like that. That’s why i didn’t want to do it myself for the whole body. I would’ve been doing it for weeks and i have a lot of those things to do. And the weather does not support that either. I don’t have a barn to do that in either.

      Besides, even if you cut the rusted part out, how many times have you seen a body with good paint and only the bottom rusted out? If the paint is very thick already then it’s should be removed too. It can be done with blasting or chemicals, but if you have rust here and there you are probably going to have to blast the whole body anyway.

      You can also use chemicals to remove rust. That’d be the best method imho, but dipping a the whole body requires quite big of a pool and some chemicals are hard to get rid of (don’t be an ass and pour dangerous chemicals on the ground or sewer). For fenders and such it’s a worthy way.

      1. I’m here watching Fast’n loud and sure enough, it’s the episode they blast the truck hood and warp it. It’s just funny that experienced guys like that let the sheet metal to be warped. But sure, you have to be careful. Don’t just blast and not pay attention.

      2. Chemical stripping of exposed surfaces could work but I’ve heard chemical dipping can result in more rust issues down the road since there are cavities in a VW pan that will get stripped and never be able to be recoated for protection.

  3. Does she have to power them up often? I’m all for a bit of automation, but what’s wrong with flicking a switch, counting to two, flicking the next, etc.? Six seconds of her life she’ll never get back?

    1. She’d have to turn off each compressor when it turned itself off in order to ensure that it didn’t turn on again at the wrong time.

      The simpler solution is to adjust the pressure settings for each compressor so they turn on/off at different times. From the start of the project I would’ve suggested she just buy a compressor unit and a gas engine to power it. The combination of those two pieces would have been $250 from the same supplier compared to the $140 she paid for each of the four compressors.

          1. Yes, you’re correct, but you don’t need a big tank; when you’re sandblasting you just need the constant volume so a small tank (with a pressure relief!) would be sufficient. Start the engine when you’re ready to go and blast away. The only problem here is you’d need to depressurize the system or add a centrifugal clutch to make it possible to pull start the engine.

   — 15.2 CFM at 90 PSI for $169.99.

  4. 8.09 she call a transistor “relay” and she is quiet sure about it saing “the tip120 will turn on the next relay” , I don’t know, when I see young people with impressive skills I always think about the the father who “help” his son with the science project to show of at school…

    1. Of course her parents are helping her – she has 4 air compressors and is making a control box for them to help her restore a 50 year old car. Kids with parents that push them this was are truly lucky, and it is also not like they are doing this in a vacuum.

      Compare her skills to the celebrated clock builder and it is no contest. She can probably even hold a soldering iron correctly.

        1. There is reason to think all winning science fair entries have major help from parents, if not professional scientists, or professional scientist parents… It is serious business…

          And I agree.. I had to mostly make my own projects, but without my dads help I probably wouldn’t have managed to finish science fair entries myself.

          I see stories like ’15 year old girl invents diagnostic test for cancer, wins science fair’, and can only wonder what their parents do all day.. Likely biomedical research, right?

          1. Speak for yourself. I did not get any help from my parents for my science fair projects and won medals at the national level. Neither of my parents have written a line of code in their life.
            Although there were a few projects at the nationals that got there because they had duped their local judges, the majority were genuinely brilliant kids.

          2. I was speaking for myself.. Lol.. Congrats on your good showing.. I am old enough that resources like the internet were not existant… The early and mid 90s when i was in middle and high school was a time of disdain and ignorance for electronics hobby. All we had was nuts n volts magazine and $50+ basic stamps…

            I still think home life has a lot to do with a kids success in science fairs, though I know there are very impressive exceptions.

            “15 year old girl invents cancer diagnostic test”.. I am super skeptical of these ones. I would be delighted to be wrong though.

          3. Disdain for WHO? Maybe if you were a piece of trash student who didn’t care to learn, I grew up in schools that welcomed interest in electronics and discovery. I’d say most if not all of the science projects in the science fairs I was part seemed to be solely the work of the student.

          4. disdain for the hobby.. it wasnt like the 70s and 80s with the many many hobby resources and electronics stores back then. it also was very far from what we now have. it was w brief dark ages for electronics hobbyists.

            i have seen the projects in science fairs also. many of them had huge input from parents. huge.

            the disdain was societies disdain for electronics as a hobby. access was non existent. $50 bucks for a 13 year old, back then especially, was a big expense. there were very few stores to get individual components in. it was just 200 in one kits, all the way down.

            you had to actually order electronic parts from paper printed catalogs, which suppliers often refused to even send to a young student.

            apparently you were either old enough to not face these challenges, or you were not involved.

          5. A lot of people don’t appreciate how valuable it is to simply have some guidance. Kids like her are able to do what they do not necessarily because their parents are doing it for them. For some kids it’s enough to just give them tips, like where they can find information, how NOT to do certain things, what is necessary for something etc.
            Other kids who weren’t so lucky to have family members, teachers etc. who guided them, even if talented, struggle because they spend a lot of time straying and failing. Sure, if they are determined they’ll eventually achieve success, but they won’t become child prodigies that way.
            You can grow up to become an equally skilled person either by failing or achieving successes along your way, making you a very different person, but both ways have their merits.

        1. I diagnosed faults and fixed them with the wiring in our new house when I was 8, which had supposedly been wired by a competent adult.
          Don’t assume younger people are not competent or indeed more competent than adults.
          It’s depending on experience, intelligence, natural aptitude and someone willing to give them a chance.

          1. For sure, even if you were to be very strict about it 16 years of age is the minimum to start an Electrical Apprenticeship, so 15 for a person of above average intelligence is perfectly reasonable, so long as the work is supervised by somebody who is qualified.

    2. And if she had said ‘solid state relay’ it would have been fine, right?

      I have to simplify words when I explain components to certain family members.
      LED is called a light, big capacitors get compared to mini-batteries.
      BLDC, stepper, and voice coil all get explained as being a motor. ;)

    3. Eh, she’s still learning a lot. My dad bought my first soldering iron when I was 7 years old. It’s not like he said “that’s yours” but really it was his, no, he gave me a soldering iron and after a day of “fixing” some wires he cut for me, it was mine to burn the house down if I wished!! (I didn’t, thankfully). I just hope she keeps interested in this stuff, for some kids it’s just a phase. But I’m sure she knows more about electronics than most kids her age…

  5. Nice idea. I have similar issues with getting a large enough compressor for blasting at a reasonable price. I generally use soda blasting. It is easier on sheet metal and soft metals like aluminum. It is much safer than sand, although more expensive. . I think I will try something similar with cheap compressors. Thanks.

  6. This is super cool. Rehabbing the car is a long-term project with so much draw for this age group. It has the potential to go off on a million tangents (like this compressor project) which she will learn so much from. I also like it that she’s taking time to document with posts and videos as that really helps shed light on the gaps in her knowledge and will end up giving her a full understanding.

    For those poking holes in her plan… running into problems along the way is the best way to learn.

  7. Great effort. Sand blasting is the best prep for coatings. To truly get the rust, check out Evapo-Rust, an amazing chelating complex chemical combination that is safe to handle and to even discard in the garden. Best for immersion and though not specifically recommended, a little pump to bathe bigger areas works. Great stuff if you can’t get any Ubik (safe when used as directed).

    On the other hand, an old bug? – Whhhhyyyyy! An FJ40? Definitely. A BMW? Fantastic. But a bug? Less horsepower than a 1930 Model A Ford and not as safe? The car Bob Pease was killed in by driving off the road after Jim Williams memorial? Farhfegnugen: The feeling you get when stuck behind a VW going over a mountain pass :-(

    1. Bugs are nice looking things, simple too (unlesss you need to replace big parts of the curved surfaces), lot’s of spare parts, cheap unlike BMWs, which are ugly as hell (expect the most expessive models, but even those are not anywhere near top 100 of beautiful cars).

      You can always soup up the engine.

      1. A bug is a terrible car for a teenager. It has about zero safety features like anti-lock brakes, traction control, air bags, passenger compartment integrity and so on. Even the seat belts are not safe by today’s standards.
        Yes people lived driving them back in the day but then again people survived trench warfare but that is no reason to throw a mustard gas party.

        1. teenagers need to learn to drive without all those things, so they don’t rely on them to to make up for poor driving. Learn to drive in a standard tranny pickup with no power drum brakes, and you won’t wait to brake till you get right up to the stop sign. Helps to be in a really rural area too,

  8. Why do something easily when you can completely overkill it and learn at the same time? Offsetting the pressure regulators and firing the compressors up manually would have been the smart thing to do, but wouldn’t have taught her anything about electronics, electrical safety or arduinos.

    I wonder how long those TIP120 will last given there is no diode protection against the relay coils back EMF…

  9. This is dumb. Typically, you don’t need to sand-blast paint off of car parts unless the paint is lacquer (or similar). Rotary tools will work fine (rent them, consummables are cheap). If the paint is lacquer, the problem is rotary tools like conformal rotary sanders melt the paint, which only makes removal more difficult. An alternative for lacquer is caustic paint removal, faster but be careful.

    Generally, avoid sand-blasting if you can, it mechanically compromises parts, especially thin parts like fenders and quarter-panels. The problem is even worse if you aggressively sand-blast a uni-body shell. Mechanical integrity will be compromised.

    As for the compressor problem: If she is obsessed with sand-blasting, she could have done a simple rental of a self-contained (including built-in engine/compressor/air-reservoir) sand-blaster from her local Rent-it-All (or whatever it’s called in her town). The rental will optionally come with consummables, including proper respirators and peen stock. The rental company will deliver the device on an easy movable wheeled frame ready to go – and picked it up when she was done. The daily rental cost would be relatively small, especially considering the time and money wasted taking her “Maker” approach.

    1. Rotary tools will strip paint, but are really beastly to cover large areas with. On the other hand, sandblasting is extremely uncomfortable to do without a proper sealed suit. You get sand in your ears, between your toes and EVERYWHERE in between. In the distant past, I stripped an old van complete with rust. Some was done with sandblasting (out in the open) and most with a 9″ right angle grinder and a knotted cup wire wheel. 8 hours a day for for 5 days left me sore for a long time. The biggest thing that i learned from that project was not to do that again!

    2. Plus side is she now has a heck of a shop air supply now. Also don’t forget the fun in building something like this and all the fun had learning/problem solving.

      I sometimes do stuff like this in software purely to see 1) if it can be done and 2) entertainment.

  10. There are cheap vents that lets the compressor start before closing, so that the motor doesn´t have to start against the load of the remaining pressure.

    I got one because my compressor in my service truck (i used to race cars) was too hefty for the 240V generator i had.

    1. I thought every compressor has this. The pressure regulator turns off the motor and vents the discharge line, which has a check valve to the tank. Mostly because asynchronous motors, especially single phase have poor start up torque. I am pretty sure to see this on this units too.

  11. i will never call anyone a little girl again, as apparently these days they kick ass. respect, very nice presentation and technical log. Hope she inspires others in the same age group.

    1. Not sure it will help her peers much, by the time you are that age your personality is well established, but it may inspire parents to do a better job of empowering all of their children, when they see what the right environment and opportunities can inspire.

      I do agree about not assuming anything about a person based on their age (young or old), it is actual experience that counts and many people waste most of their life on things that do not benefit them, so a dedicated “kid” can acquire significant skills rather quickly if they are focused and have the resources.

  12. Nice work, even included the requisite Arduino, and time travel. Hannah’s memory and presentation skills are impressive too.

    Thanks for the post Adam, my kids will really enjoy following Hannah’s project.

  13. Hey – I am just glad to read a story about a kid who is excited about learning about cars. Back in the 70s me and all of my friends were car crazy. We bought old junkers we could afford and we fixed them up, souped them up and basically got into all kinds of trouble. Now kids drive mom’s minivan or grandma’s old buick. They don’t care to learn how to work on them or fix them up. They think that neon wipers and fat farty mufflers are cool.
    Hats off to this kid! Go get em!

  14. This could have been done without any of the control electronics by using delay relays that have been in use for almost fifty years in power applications. They are available in fixed or variable delays and control for all compressors could be done for under $20 with much better longevity.

    1. You know this, how? You know them personally and were there observing this?

      Or more likely, you’re a misogynistic virginal neckbeard living in his mother’s basement at 35 years old, who can’t fathom the concept of a female teenager using her brain and her hands to accomplish something to be proud of, something you’re insanely jealous of so you come here and dribble your tired inanity all over it.

      Keep tipping that fedora and toking on your vaporizer wishing it was something more phallic; meanwhile people with more than two brain cells to rub together realize that this kid is going places.

  15. Wouldn’t it have been easier to connect at least three of the compressors to separate phases and start them simultaneously like a 3ph motor? This should in no way place a heavier load on the mains grid as the currents in the neutral balance out. But I am not that sure over the US power grid.

    The fourth one of course would create a load imbalance anyway.

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