Compressor requirements for sand blasting

[lastcowboy32]

Lets say that I wanted to run a small sand blasting cabinet for small parts (recycled farm implement chain for aesthetic projects)

and to be able to use a sand blaster to blast some small areas (like inside corners) in items that I've repaired and welded before painting them (just with something out of a can to protect the weld...I'm not looking to do automotive grade painting)

What would be my air compressor requirements?

I looked up a table of air requirements online, and it said that, to run a 1/8" nozzle at 100CFM (which I read was about the minimum to use...or my projects would take a long time), it would take something like 20CFM of air.

So just for kicks, I went to Lowe's website and looked through their selection of compressors.
30 Gallon Kobalt?...nope...something like 5 CFM
60 Gallon Kobalt?...nope...
80 Gallon Kobalt?... nope

By now, I'm up to 1000 dollars and thinking that wire brush wheels and flappers on my angle grinder are looking pretty good.

From Lowe's lineup, the only compressor that came close to the 20CFM rating was the 1,800 dollar Quincy two stage, cast iron compressor body, 27 Amps at 240V compressor.

Even that didn't put out 20CFM. It was something like 17SCFM (I haven't converted the SCFM (standard CFM to CFM, yet; but I don't think I'm going to gain that much)

Is this the case in peoples' experience here?

Do you need a couple thousand dollars of compressor to run even a very small nozzle?

Thanks in advance for your experience.

Chris

 

[David G]

My 5HP is too small for blast cabinet.

 

[Steve@Advance]

Yes, you're numbers are right, blasting takes a lot of air volume.

You could get by with a smaller compressor with a big tank IF you are willing to blast for a couple minutes, then let it recover.

But the harder you push the air supply, the more likely it will be to start spitting water. Water in a blast cabinet will clump the media, stop the flow.

 

[unc]

I have an industrial true 5HP compressor that was about 18 CFM when it was new. It would keep up with a blast cabinet- barely. The compressor would recover while the operator was loading and unloading the cabinet. I still have that compressor and cabinet, but for occasional use, I have an inexpensive hand held gun that fills from the top. Doesn't hold much media, and it doesn't blast long, but it's handy for cleaning a small area like a weld. I'd get bored if I had to do a big job this way, but it's handy for occasional use. Key here is LOTS of air. A too-small air source will drive you batty on any sandblaster. unc

 

[lastcowboy32]

(quoted from post at 17:48:38 05/06/19) I have an industrial true 5HP compressor that was about 18 CFM when it was new. It would keep up with a blast cabinet- barely. The compressor would recover while the operator was loading and unloading the cabinet. I still have that compressor and cabinet, but for occasional use, I have an inexpensive hand held gun that fills from the top. Doesn't hold much media, and it doesn't blast long, but it's handy for cleaning a small area like a weld. I'd get bored if I had to do a big job this way, but it's handy for occasional use. Key here is LOTS of air. A too-small air source will drive you batty on any sandblaster. unc

Do you have a link or any information on the small media blaster that you have. That sounds like what I'm talking about. I don't need to sandblast graffiti off of hundreds of square feet of brick wall.

I just need to clean up some crevices that I've welded.

For instance, I just welded the fender on my Ford 3000. It snapped off right at the "L" where the base meets the wall of the fender.

That's a spot that is ripe for corrosion as it is, and the process and byproducts of welding can only make it worse. I would really like to be able to just quickly blast some sand in there to clean it up and then passivate with zinc phosphate... or something like that, and then spray some paint on it. Just to try to give it a chance against the elements.

 

[rustyfarmall]

My compressor is a 60 gallon, 6 horsepower (advertised). Rated at 11 or 12 scfm. It WILL keep up with my cabinet blaster, but it runs continously.

 

[sms]

I found an old Schramm trailer compressor
100 CFM gas powered for $500. Works well
for sandblasting. But that is more then
you need.

 

[DR. EVIL]

I always heard a full 5 hp 2-stage compressor with 80 gallon tank was needed 100% duty Cycle to blast
with a 1/8th inch diameter nozzle. 15 cfm minimum required to run the 1/8" nozzle.
I have a Harbor Freight 40# pressure blasting pot, no cabinet yet to recycle my sand. With a brand new
ceramic nozzle 1/8" or smaller my full 7-1/2 hp 80 gallon 24 cfm @ 150 psi 2-stage compressor will keep
up, even get up to pressure and shut off. Problem is the crushed quartz sand I blast with wears the
nozzle larger and my compressor quickly will not keep up. I've used nozzles that wore to about 5/16"
before.

Costs money to sand blast. If your going to pinch Penny's better keep using wire wheel brushes.

 

[unc]

Here's the small blaster I like. It was IIRC about $20 on sale. Try Harbour Freight or a similar store. I got this one at Princess Auto in Canada, and they stock extra nozzles. It's not a production tool, but it sure beats dragging out the pressure pot to blast a few square inches. unc

 

[Stephen Newell]

It comes down to volume. If you are going to sandblast a few parts now and then a pancake compressor would do the job. You might have to wait now and then for it to catch up but would do the job. If you intend to keep one going pretty continuous all day then you probably need a 5hp model. A smaller portable unit would also work but they get very hot if they never shut off and that would wear them out prematurely.

 

[fixerupper]

At least 15 CFM, 20 is better for a small sand blaster with a 1/8" or 3/16" nozzle. Forget the horsepower, advertised horsepower is so deceiving. Look at CFM. Single stage max pressure runs in the area of 110 psi. Two stage runs a max press of around 180 give or take a few. The bigger the tank the more reserve pressure you have to run on before you have to let the compressor catch up. It also takes longer for the compressor to refill a larger tank.

 

[lastcowboy32]

(quoted from post at 16:45:23 05/07/19) It comes down to volume. If you are going to sandblast a few parts now and then a pancake compressor would do the job. You might have to wait now and then for it to catch up but would do the job. If you intend to keep one going pretty continuous all day then you probably need a 5hp model. A smaller portable unit would also work but they get very hot if they never shut off and that would wear them out prematurely.

I understand that.

From an engineering standpoint, that would probably have to do with a cumulative number, such as the average CFM per hour or "duty cycle", storage and maximum stored PSI.

If I bought a reasonably priced compressor, for say, 500 bucks that could do anything else in the shop (tires, air cleaning, impact wrench, nail gun, etc) AND, if it had, say, 60 to 80 gallons of storage... AND if it had a maximum stored PSI of, say, 175 PSI... AND I installed a regulator for the output adjusted to 100 PSI...

AND I just occasionally used a little sandblaster or a small cabinet very sparingly... then I would probably be OK, because the excess volume of air (the 60 or 80 gallons of volume stored at 75 PSI above my working PSI) would float me for short bursts of operation.

Just thinking out loud.

I would like to get some "shop air" capacity. I'm just wondering how much to spend vs what tools I would use eventually.

 

[Destroked 450]

For spot and small parts blasting were you'll only be blasting for a few seconds to a minute or so one of the 17 cfm compressors will do what you need.
One could purchase 2 of the smaller 11 cfm compressors and tie them together to get over 20 cfm but that's a bit of work in the extra wiring needed.
Lowes offers a 7.5 hp Campbell Hausfeld 25 cfm compressor for a little under $2000.

I have a Quincy QR25 series, model V325, it'll run non stop when using a 50 lb blast pot non stop, but it doesn't run out of air.
It also cost a lot more than $2000.

 

[lastcowboy32]

A monster Quincy with a horizontal 325 gallon tank was just posted on Craigslist locally here. For something like 800 bucks.

Only drawback is that it's 3 phase.

That's just an example. I'm hunting a little bit now. It seems that the used compressor market may be a good source, if one has the cash and doesn't need to depend on in-store financing and such.

 

[Stephen Newell]

That size compressor you could probably sandblast all day. The only benefit from a large tank is if you sandblasted sporadically. For continuous use it's the size of the pump that matters. I see compressors for sale all the time that have a huge tank with a little bitty pump on them. That wouldn't be any better than a portable compressor. The larger the tank the longer it would take to run the pressure down but if you were using more air than the compressor was delivering the big tank would be just for show.

 

[Destroked 450]

(quoted from post at 10:32:22 05/08/19)
A monster Quincy with a horizontal 325 gallon tank was just posted on Craigslist locally here. For something like 800 bucks.

Only drawback is that it's 3 phase.

That's just an example. I'm hunting a little bit now. It seems that the used compressor market may be a good source, if one has the cash and doesn't need to depend on in-store financing and such.

I'd jump all over that deal, you can swap out the 3 phase motor for a good compressor duty single phase motor and have around $1200 in a compressor that will last you for ever.
Quincy 325 pumps can be rebuilt, block bored over size, crank reground, disc valves in the heads are replaceable, it's the last air compressor you'll ever need to buy.
Be prepared if you get it, the complete compressor weighs around 700 lbs, the pump along is nearly 300 lbs, their big and heavy and last forever.
325 pump maintains 17.5 cfm at 175 psi, at lower pressures at will pump up to 22 cfm

 

[lastcowboy32]

I understand the CFM issue vs tank size. The serial tag on this one said model 390-22.

It had a 10hp three phase motor on it.

I tried to get CFM specifications off the web, but that's not forthcoming...yet. A similar model in that family runs from a 25hp motor and has a CFM rating of 70.

I can't assume that CFM relates linearly with horsepower, so that doesn't help me much.

And, like I said, it's a curiosity; because I don't have 3 phase.

I'm not getting this, but I used it as an example of what is out there.

 

[Destroked 450]

Sorry I miss read the 325 for the pump size.
A 390 is huge, 10 hp is min making 34 cfm, will make 64 cfm with a 20 hp motor but to big for a home shop.

 

[Stephen Newell]

Three phase isn't that much of a hindrance. A phase converter can be installed to run three phase on single phase however it wouldn't surprise me that this compressor is beyond the capabilities of a phase converter.

 

[warbaby]

I have a 5 HP Champion 2 stage compressor 230V single phase with a 60 gallon tank. I use a TIP sandblast cabinet that measures about 4 ft. x 3 ft. and I use a
pressure pot type sand blaster in my "mobile" blasting operation (a 6 x 9 ft tip trailer with plywood sides with a tarp draped over the walls and floor- you just
blast some parts, then tip the trailer and shake the tarp down into a screen door over another tarp on the ground to recycle the sand).
The compressor handles either blaster just fine.

Whatever set-up you eventually settle on MAKE SURE YOU HAVE A GOOD VENTILATION OR DUST CONTROL SYSTEM or you wont be blasting -or breathing- for long.

In fact, no matter what type of air cleaner you may get, I'd still wear a good particle mask as well. Sand dust will mess your lungs up big-time!

 

[redforlife]

I think the key here is how long you plan on sand blasting continuously, more so than how much air pressure and
volume you are going to be using. If you plan on blasting, say in 15 minute intervals, I don't think it's going to
matter much. With compressor full at the start, it's not going to have to run long untill your done. And a small
compressor with a medium sized tank would be just fine.

Now if you plan on sand blasting used chain all day, then that's a different story. Your compressor running
continuously is what you want to avoid. It needs to kick off and have a break once in awhile. If that's what your
planning on doing, than that's when you need to get a large enough one to keep up with your air usage. But that
can be simplified by taking a break yourself. I am kind of douting you will be blasting continuously and needing a
large compressor.

 

[lastcowboy32]

I get what you're saying.

I tried to do some off the cuff thought experiment based on a couple of facts:

For an ideal gas, PV=nRT

n= PV/RT

n is the number of moles of the gas
R is a constant
T is the temperature, that we can assume is constant
V is the volume

lets assume that the volume is 80 gallons. Lets assume that the compressor stops at 150PSI and kicks back on at 110PSI.

In the middle of that is 130PSI...which I'm going to use as an average for estimating.

so... n is related to the actual amount of air molecules stored in the tank.

If you're sand blasting at 100PSI and 20CFM... 20CFM is about 0.33 CFS (cubic feet per second)... which is about 2.5 gallons per second...at 100PSI.

If I use the ideal gas law and some estimating... that 2.5 gallons per second at 100PSI is about like 1.9 gallons per second at 130 PSI

Really rough numbering here... but... the compressor will kick back on when the tank has lost about 4/15 of the "moles" of air stored... or somewhere around 4/15 times 80... or 21.3 gallons at 130PSI.

All of this for what? I'm estimating that this system would sandblast for about eleven seconds, before the compressor kicks on.

If you kept sand blasting, it will fill the tank at 10 CFM... which is 0.16CFS... which is 1.2 gallons per second... while trying to sandblast at 2.5 gallons per second... the pressure will keep going down and you might get another five or ten seconds before the pressure gets down to 90 PSI.

At that point, the compressor would have been running for five or ten seconds, and if you stop, it would take... oh, I don't know, using my numbers above, somewhere around 30-40 seconds to charge back up.

Seems like, if I had a small amount of work, where I could tolerate this type of blast for 15 or 20 seconds... stop... recover for 30 or 40 seconds... continue... then I could use a compressor with 80 gallons of storage and 10 - 15 CFM.

If I tried to run it continuously, it would just kill it.

 

[lastcowboy32]

But, working off of UNC's comment, about a really small media blaster for spot blasting...

A compressor like this (60-80 gallons of storage, 10-15 CFM) would certainly work for a little spot blasting with a small media blaster.

I found something like what he had up in Canada available here in the US from Lematec.

They're 26 bucks, give or take, on ebay. And for the little bit of work that I'm thinking about, this might be my best option.

https://www.ebay.com/i/292991508971?chn=ps

 

[Stephen Newell]

I believe you are over thinking it. The size compressor you are proposing getting won't run you short of air. Maybe if you had two or three people sandblasting at the same time but certainly not working by yourself. That size compressor will shut off and sit for perhaps as much as five minutes with you sandblasting pretty continuously before it starts back up again.

 

[lastcowboy32]

I can appreciate that. "Overthinking" is my middle name. As it is, I took my wife with me and we walked around Harbor Freight, Lowes and Tractor Supply; while I discussed the numbers with her.

We looked at wire wheels for my angle grinder, compressors and various sand blasting equipment.

What we decided on was probably an odd decision, but one that I'm happy with.

I just so happens that good knotted wire cups and wheels for my angle grinder are very inexpensive. I picked up a few of those.

Knowing that bulk surfaces can be cleaned of rust and paint with those tools lowered my "requirements" for blasting.

For real small nooks and crannys, I found a little "air eraser" at Harbor Freight. It only requires 4CFM @65 pounds. It's essentially an air brush that uses sand.

With that and the wire wheels in hand, the best compressor value (I thought), was a Porter Cable unit from Tractor Supply for 240 dollars.

4CFM@90 PSI
24 Gallons of storage
120VAC, vertical, oil lubricated and portable.

I also ordered the 5lb portable sand blasting gun (as recommended by "unc" in his reply). Now, I know that this requires 7CFM@90 PSI.

But, there is no portable solution that gives 7CFM. The best that you can get are some DeWalt 30Gallon units that have 5.3CFM, and they cost $500.

From there, you're up to stationary units that need 240VAC wiring.

I figure that I'm going to try this 5lb portable gun with this compressor. If it's a real pain in the rear end, I can go buy another one for another 240 dollars. Plug it into another 120VAC breaker in the garage and gang them together for blasting. It would be 48 gallons of storage and 8CFM. Actually more than enough for the 5lb portable gun.

Plus... if I need to do this in the barn... or somewhere else... I can run two of these compressors off of my generator. I might have to do a little finagling to make sure that they don't switch on/off at the same time...but there is more than enough power.

But, either way, ALL of that is worst case analysis. If I use wire wheels exclusively for the bulk of surface cleaning. I might be able to run the 5lb blasting gun with the compressor that I have for a little bit of spot work.

Gun is ordered. Will be in tomorrow.

Then...we test...