Air Compressor Modification?

[onedash]

Can an air compressor that delivers X amount of SCFMS be upgraded to produce more SCFM's? Maybe by replacing the regulators????

[BobSch]

Nope. The regulator controls the pressure, not the flow. The SCFM is determined by the rating of the compressor itself. If it's a belt-drive unit you could possibly try replacing the pullies to get the compressor to run faster (With the risk of over-heating the motor and the compressor itself). With a direct-drive compressor you're pretty much stuck with what you've got.

[onedash]

That doesn't seem to make sense. If I have two tanks that are both the same size how can one provide more CFM's than the other?
If you have your compressor turned off it will deliver that CFM rating until pressure is to low then it turns on and puts more air back in the tank.
Right?
I have a craftsman dual stage compressor 1.6hp or something dual stage oil free and it has a max PSI of 175 but the SCFM is rated lower than my paint gun needs.
My paint gun does work but I am guessing it would work better if my compressor was rated at what the gun needed.
___OR____ Does that CFM Rating mean it can run continously and the motor provides enough air to go non stop even if the tank gets low????
My HVLP gun will spray paint but I have to thin it first. I am under the impression it would work better with a bigger compressor. Is that true?

[Tom Miller]

The CFM ratings are what the motor and compressing piston can produce continuously, irrespective of tank size. It's a simple calculation of the piston displacement multiplied by the number of strokes per minute. As mentioned, you can tweak the strokes per minute if you change pulleys. On a direct system, let's just say it's a little more involved.

The flow rate that you can achieve out of a tank is just a function of the pressure drop of the plumbing, and the pressure in the tank. Keep in mind that the pressure in the tank will be falling at some rate if the compressor motor can't keep up. And, if you can start off with a higher tank pressure, the longer you can have a higher flow.

Regards,
Tom

[newbie2wood]

You are confusing pressure with CFM. The regulator control the pressure in the tank. It shuts off the compressor at preset level. The CFM is a "measure" of the air produced by your compressor. Imagine your unit without a tank and you turned on the compressor, CFM is the measure of output.
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[LCHIEN]

Continuous SCFMs are limited by the piston size of the compressor and RPM of the motor.

The simplified explanation is that each stroke of the compressor pumps a pistonful of air into the system. Since the piston is a fixed volume (stroke length times Pi x R-squared), the maximum CFMs is limited.

The tank is a storage device, to smooth out the pressure AND it also delivers more CFM for a short period of time (until the tank pressure drops too low). You can get more peak CFMs for longer by adding tank size or an auxiliary tank. But ultimately the average continuous CFMs is limited by the motor speed and compressor piston size.

[onedash]

ok. My compressor has 5.6 SCFM @ 40 PSI and 5.1 @ 90PSI. Its a 25 Gallon tank. My HVLP Paint gun says it requires 7 SCFM @ 40PSI.
It sprays paint if I thin it. Would I still have to thin the paint if I was getting more than 7 SCFM? Its the only one I ever used so I can't compare it to anything. I obviously can't paint non stop. I paint a little but let it fill up the tank then move on. Im sure thinning it makes excessive overspray and probably defeats the purpose of HVLP????
I never thought I would get a paint gun when I bought the compressor.

[Stick]

Loring, as usual, in his engineering manner, has hit the nail right on the head as to a totally perfect explanation. I'm not an engineer, I'm a millwright and power station operator, but I sure think like an engineer. LOL! I have to make an effort to meet Loring some day!

Bottom line is..............you just don't have enough CFM to paint continuously. Not usually a huge problem. Just like using a sandblaster or a die grinder....you have to pause to let the tank charge up. Same with a welder, they have a duty cycle, where you can only weld for a set time at a set current, then you have to allow a cooldown period.

You can add an auxilliary tank, or wait for yours, or get a bigger compressor.

Messing with the drive ratio is IMHO, a very bad idea. On a big compressor, you can go the other way, as in have a compressor head turned slower by a smaller motor, but its bearings and valves and pistons are only designed for a certain maximum RPM. I would not want to exceed that.

My big old compressor at the farm is run by a 10hp 3 phase motor. It originally had a 20hp motor but ran at 600psig. I "detuned" it to run at 175psig. Works fine like that and will likely outlast my grandchildren. It's a 3 stage, and I have more volume (CFM) than I can use. It originally charged 4 tanks that were 8' around and 20' high. Today it runs on an old tank about 3 feet by 8 feet, maybe 500 gallon?

[BobSch]

The viscosity that a particular gun can spray is dependent on the design of the gun itself. Normally you have to thin most liquids to match the gun's specs.

[mpc]

Onedash - as others have already said, it's the piston part of the compressor assembly that determines long-term airflow capacity. The tank gives you a short-term "surge" capacity.

Air compressors pump the tanks to some "cut off" or maximum pressure; then when the tank pressure falls a little bit (generally 10 to 20 psi drop) to the "cut in" pressure the motor starts up to recharge the tank. If the motor cycles on and off while your air tool or your paint sprayer is actually running... then the compressor's CFMs are more than enough for the tool - the motor/pistons are able to out-run the air tool and top-off the tank. If the motor starts and runs continuously... and the tank pressure keeps dropping, then the CFMs of the compressor are too low for the tool; the motor/pistons can't keep up let alone top off the tank. Such use can wear out the compressor quickly too; most small home style compressors use "compressor duty" electric motors which are not designed to run at full power continuously; they need cool-down time.

About your paint sprayer and thinning: you can determine for yourself if more CFMs will reduce the need to thin... let your compressor run until it shuts itself off - i.e. so you're running on the "tank" air. You can get LOTS of CFMs for a minute or two this way - well above the 5.6CFM @ 40psi rating. If your sprayer won't spray un-thinned paint this way then going to a larger compressor won't help. Most paint sprayers have very small orifices for the paint; raw (thick) paint just plugs these holes. Air pressure does not shove the paint through the holes either; in most hand-held guns the paint is drawn out via suction from the air wizzing past... the Bernoulli effect. (to see what that is: fill a glass with soda pop, stick a straw half-way into the soda, and blow ACROSS the top of the straw. You'll see the sode climb up the straw a little bit. The air zipping above the straw is actually creating a small vacuum which sucks the soda upwards.) Some air sprayers do pressurize the paint bucket so they can work with thicker paints... but very few sprayers work with "raw" unthinned paint.

One thing I've found though on a lot of "home" rated compressors: the stock pressure regulators can't flow much CFMs - often they can't flow anywhere near the same CFMs as the pistons can generate. When this happens you'll see the air pressure guage on the regulator (or a guage right at the tool) drop a lot when your air tool is turned on. This will affect your paint sprayer if it's happening.

mpc

[onedash]

Ok. just went and checked. With the blower nozzel on and the tank full and pressure to the hose at 150PSI it drops at least 20 PSI when I pull the trigger. It will run 30-45 seconds before the pump kicks in.
Let it fill up dropped the pressure to 40 and hooked up my paint gun and had at least 10-15 psi drop when I pulled the trigger.
Would a new regulator fix this? or putting a regulator at the other end and keeping enough pressure in the hose to maintain 40 PSI at the gun? Or do you think the pressure would drop after that regulator too?
I also have a water filter right after the regulator. It turns off at 175 PSI and turns on at 150PSI. I can't get more than 150 PSI past the 2nd regulator.

[Tom Slick]

have you tried painting with your current setup? My home setup is similar to yours and it works just fine for me.

the pressure drop you are seeing is from your regulator, filter, air fittings, and hose. you can deal with a number of ways.
the easiest/cheapest is to adjust to the pressure you need while spraying the gun, that accounts for the pressure drop. you have already done that, and that is the most reasonable fix and it will work fine.
the next easiest is to shorten your hose, that 3/8" 50' hose in your picture has alot of restriction at 50' long, a 3/8" 20' or 1/2" 50' hose would have less pressure drop.
standard 1/4" industrial air fittings are very restrictive. you have a 3/8" ID hose and if you look at your fitting there is a 3/16" hole in it cutting way back on flow. look for low restriction fittings at the local automotive paint supply or buy some "european style" quick connects from www.mcmaster.com . they have a much larger ID.
the most expensive, but best way to have teh least restriction is all the above and replace your regulator and filter with units for larger 3/8" pipe and use a larger hose. you could also plumb 3/4" pipe over to your work area then use a shorter 3/8" hose to your gun.
In my finishing dept. at work we have 3/4" pipe running into the booth at 90 psi, then a regulator, then 1/4" or 3/8" hose running to the gun with a regulator at the gun. even the 1/4" hose supplies plenty of air for our HVLP guns.

even if you did all that you will run out of air at some point because the pump on the compressor can only put out so much air. you'll have to paint in shorter sessions to make up for that. you'll probably find out that your current setup works fine for doing occasional projects.

[onedash]

Well this is the only gun I have ever had or used. As long as I thin out the paint enough and mess around long enough tweaking it out it seems to be pretty good. Maybe I was expecting it to do better. I thought that HVLP was supposed to have much less overspray and waste and maybe this is much better than a high pressure one but sure seems like there is way to much. Last time I did it outside because everything in the garage tends to take on a light tint of whatever color I use if I do it in the garage.

[Tom Slick]

HVLP has less overspray becuase there is less "bounce" from the surface. they do put out alot more material so you do have to work faster. have you tried less pressure?

[onedash]

last time I used it was with some rusolium and I used it in the garage. I did wear goggles and a mask but after a little while my legs felt funny. I itched my leg and it turned black. All the hair on my legs was covered in paint. And everything I moved was much lighter underneath. So I finished it up outside. I might have overthinned it that time.
I dont use it enough to figure out the best consistancy. It seems to work awsome during clean up (water or mineral spirits) But it still turns out better than with a brush or roller.