Split again - air compressor psi/cfm

[chris99z71]

(this thread split from here)

From what I can see Titan=Vaper (Titan = model, Vaper = brand).
Check here Titan 19200 HVLP 1.4 and 1.7 and 1.0 mini Spray Gun

Currently on sale with free shipping for $95
(but for some reason, comes to $105 when I put it in the cart????)​

There are some specs there:

[FONT=Verdana, Arial, Helvetica, sans-serif]• ALL Guns Features:[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• Aluminum paint cups[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• Die cast Aluminum body with black Nickel finish[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• 2-step trigger control[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• Non-bleeder type, gravity feed external mix[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• Stainless Steel nozzles[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• Adjustable spray width, air inlet and fluid outlet[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]Includes 3 Spray Guns:[/FONT]​

[FONT=Verdana, Arial, Helvetica, sans-serif]• 1.0mm nozzle set for lighter material and touch-up work[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• 1.4mm nozzle set for lighter materials[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]• 1.7mm nozzle set for heavier materials[/FONT]​

[FONT=Verdana, Arial, Helvetica, sans-serif]PLUS - Locking pressure regulator with gauge[/FONT]
• 1000ml aluminum paint cups
• Spray pattern control knob on side of guns
• 1/4" air inlet
• 15 - 45 PSI operating pressure
• 7.0-8.0 CFM air consumption​

 

[Gofor]

Re: Split: Northern Tools HVLP guns

Looking at the specs on Chris's post, I see 7.0 - 8.0 CFM @ 45 psi. That's equates to probably about a 6 hp compressor, but realize that will be what is needed for the large gun and /or heavy coatings. (Thats the required cfm for a Binks Mach 1 industrial grade gun with the largest tip) The detail gun will use less air, and the thinner (less viscosity) coatings will need less air to properly atomize the spray. For an HVLP to properly work (ie, less overspray) you do not want to exceed the max psi, but that does not mean you need that high of pressure.
To maximize your compressor's ability in cfm, run no smaller than 3/8' ID air hose to within 10 feet of the gun (closer if possible). 25 feet of 1/4" air hose cannot deliver 8 cfm continuously even at 100 psi, regardless of the compressor's ability.
Another thing that will help is running higher air pressure from the compressor and stepping it down at or close to the gun with a pressure regulator. You can tell the in-line pressure regulator (available at the BORG) from a volume regulator by the fact that the pressure regulator will have a gauge on it and will be a bit more expensive. Altho both may say pressure regulator on the package, the one with just a small knob on it is nothing more than a needle valve that restricts the air flow, which cuts the volume. A true pressure regulator has a diaphragm/spring assembly in it that will allow full volume of air but not let it exceed the set pressure.
Also available, but not sure if the Borg has them, are 3/8 ID air fittings. (both male and female quick-disconnect). The standard ones are 1/4" id. Using the larger ones will remove additional volume restrictions in the air supply. (Some are made with 1/4" npt so are an easy replacement for existing fittings)
If you are running a 3 cfm @ 45 psi air compressor, you will probably have to upgrade or get a BIG collector tank. If you are running 5 or 6 cfm, you will probably be okay with the thinner coatings, but may have to thin poly or varnish a little more than normal. The additional thinning with solvent coatings kind of defeats the intent of the HVLP which was designed to lower VOC (volatile organic chemical) emissions, tho. Water-based coatings may pose a problem because water does not atomize easily. If DNA can be used as a thinner, it will atomize better, and is an EPA "exempt" solvent. It will increase the flammability and toxicity of the fumes.

My .002
Go

 

[chris99z71]

Re: Split: Northern Tools HVLP guns

If you're at 4.2 cfm at 90 psi, that same compressor should be able to put out double the volume at half the pressure (8.4 cfm at 45 psi). Am I missing something here? I believe it's just the ideal gas law PV=nRT. nRT is constant so P1*V1=P2*V2. 4.4*90/45=8.4
Does this not work?

Wayne, good advice on the line sizing. The friction loss in 1/4" hose will be much more than in 3/8". 3/8" will flow a lot more air. I think that these guns are designed to be run with the tank pressure high and to have the included regulator placed at the gun to step down the pressure.

 

[Grgramps]

Re: Split again - air compressure psi/cfm

Chris, Thanks for splitting this. I was ready to start a new thread, but obviously you are more computer savvy. Be patient, some of this is repetitious.

I have a smaller than ideal compressor, so I have been reluctant to attempt spraying with it. It's the Makita MAC2400 which has been great for me, but the output of 4.2CFM @ 90PSI doesn't meet the requirements of most of the conversion guns on the market. If I ever finally decide to get into spraying, I might be just as well off buying the HVLP unit.

This prompted the following from Chris:

"If you're at 4.2 cfm at 90 psi, that same compressor should be able to put out double the volume at half the pressure (8.4 cfm at 45 psi). Am I missing something here? I believe it's just the ideal gas law PV=nRT. nRT is constant so P1*V1=P2*V2. 4.4*90/45=8.4
Does this not work?"
​

Now that sounds perfectly logical to me, in fact using what I thought was obvious, I posed the question to a professional who makes his living selling spray equipment (many of you know him). His reply was that no such assumption could be made.

So where am I now. I'm not, nor have I ever been an engineer, yet this sounds like it would hold true. Or is there some law of physics that comes into play at lower pressures that would affect this. I understand the compressor would probably run a lot, if not all the time that I was spraying.

Gofor, Your thoughts have been noted. Thanks
Roy

 

[chris99z71]

Re: Split again - air compressure psi/cfm

Chris, Thanks for splitting this. I was ready to start a new thread, but obviously you are more computer savvy.

Nope, I just have moderator privileges

Roy, I'm going to take a step back on my last post after thinking about it. I don't think that calculation works. The cfm calculated is based on "standard cubic feet per minute" (scfm), which means that the cubic feet of air is corrected to standard atmospheric pressure and temp. That means that 5 cfm at 90psi is the same as 5 cfm at 10psi, becuase it's corrected to standard cubic feet.
So, basically, a compressor has a higher rating at a lower pressure, that's saying that it can pack more scfm of air into the tank when working at a lower tank pressure. It doesn't have to work as hard pushing against the pressure that's already in the tank. (kinda like: it's easier to push your car up a gradual slope than a steep slope)
This is why a two stage compressor is more efficient. One piston pushes the air from say 0 psig to 50 psig (50 psi increase) and the second piston pushes the air from 50 psig to 100 psig (50 psi increase). This requires less energy than pressing the air from 0 psig to 100 psig in one step.

So back to HVLP. When using a smaller compressor, your compressor will be able to move more air if the tank pressure is lower. But, (the big BUT) you will have slower flow rate through the hose to the gun when using a lower pressure. Will your compressor be able to keep up if you run it at higher pressure? Will you gun get enough air if you run at a lower pressure?
Solution: Get a compressor big enough to supply enough cfm at the higher pressure OR get an air tank big enough that your compressor can fill the tank and then not have to kick on again until your paint job is done.

Am I right?

 

[DavidF]

Re: Split again - air compressure psi/cfm

Nice answer Chris. FYI my compressor is 3HP 230V and can provide 10.3CFM at 90psi so a 6hp motor as proposed by gofor should not be required. I would say that tank size is the crucial element in a spray setup with a smaller motor/pump. I have a 20Gall tank and I hope this will be sufficient for HVLP. Most compressor adverts seem to state the divide/2 rule when quoting cfm/pressure. I may well get some 3/8" hose and connectors as well. BTW, is there a recommended way of cleaning the hose if it may have been contaminated? cleaning the gun is easy enough, but the hose?

 

[chris99z71]

Are you talking about the air hose or are you using a pressure-pot sprayer that has hoses?

 

[DavidF]

Are you talking about the air hose or are you using a pressure-pot sprayer that has hoses?

The air hose. Directly from the compresser filter to the gun.

 

[chris99z71]

Hoses are pretty cheap, is it worth it to even try to clean it? Even the heavy 3/8" ones can be had for under $20.
What about having a separate hose setup with oil separator/moisture trap just for spraying and another hose for air tools?

 

[DavidF]

Hoses are pretty cheap, is it worth it to even try to clean it? Even the heavy 3/8" ones can be had for under $20.
What about having a separate hose setup with oil separator/moisture trap just for spraying and another hose for air tools?

That's probably what I'll do. It is just that my existing hoses were "aquired" but have been used on other compressors in the passed. I guess a new hose is the way to go.

 

[Gofor]

I stand corrected David F. I have a 5 hp (not 6) 15 amp @110v, 7.5 amp@220 compressor with a 20 gal tank. It is rated at "avg SCFM 6.8 @ 90 psi". It will push an HVLP at 45 psi okay. I think Roy's at 4.2 cfm @ 90 is marginal, altho it may do a detail gun. My guess is he will get a around 6 at 45.

Both at work, at home and I found the same true for many body shop guys, the 'clean air' hoses are kept separate (marked) from the "dirty air" hoses. I have never heard of anyone successfully cleaning the fine oil out of a hose. In my case at home, I bought yellow hoses from Sam's club, which are my "clean air" hoses, and orange ones from the Borg which are my "dirty air" hoses. Different color zip ties work well also. It pays to shop around on hoses. Altho the prices don't significantly vary, flexibility does. If you aren't continually dragging them over rough concrete, etc, the softer, more flexible will give a lot less headaches.

Most of the trades people I worked with usually made a small stand to hold a good (1/2" inlet/outlet) oil/water separator with two outlets. One side usually had a second smaller separator for the spray guns, and the other an in-line oiler for the rotary tools.
That way a large hose could be run from the compressor to the stand, which was close to the work, and then a smaller flexible hose run from the appropriate side to the tool. One that large shouldn't be needed for a home shop, but if the inlet is only 1/4", it will restrict air flow. If it has a drain valve on the bottom, that is a plus when it needs draining.
I wouldn't invest in an in-line oiler unless you extensively use rotary tools. If you do use rotary air tools preparing for the finish (i.e. DA sanders, etc) the in-line oiler can provide just enough oil for lubrication without overloading it (oil spraying out the air exhaust) that is common right after you put some in the tool by manually oiling it. Setting the oil rate too high will have the opposite effect, tho.

Another half-penny's worth
Go

 

[chris99z71]

I have a 5 hp (not 6) 15 amp @110v, 7.5 amp@220 compressor with a 20 gal tank. It is rated at "avg SCFM 6.8 @ 90 psi".

That brings up another subject altogether-manufacturers lying about horsepower.
This is a formula for calculating horsepower (I stole it from here):

HP = V x I x Eff
746​

HP = horsepower
V = voltage
I = curent (amps)
Eff. = efficiency

If you're drawing 15A at 115V, even if your motor is 100% efficient (Eff.=1), it's physically impossible to generate over 2.3 horsepower. I always cringe when I see something with a 110V (15A) plug labeled over 2hp.
Not trying to call you out, Wayne, just a buyer-beware issue with anything regarding electric motors. I don't think you can even use their quoted hp to compare one manufacturer's motor to another, because they all lie to a different degree. I have no idea where they come up with their numbers.​

 

[Grgramps]

Air compressor psi/cfm

Chris, You said, ".....they all lie to a different degree. I have no idea where they come up with their numbers." While I have no proof, I suspect the compressor manufacturers hired the same ad agency that does the commercials for the diet pills and cold remedies. ;-) Unfortunately, many of us were duped by these ads from companies that we assumed were honest.

Here's a link to info on the class action suit filed against major compressor manufacturers at: THE AIR COMPRESSOR LITIGATION WEBSITE. I hope that now we can have a bit more faith in the labels affixed to the compressors that we buy.

There's a rather interesting essay on the subject of HP and CFM at: Evaluating True Horsepower and CFM Ratings of Air Compressors

As I said earlier, I'm not an engineer, but there seems to be logic in the essay. Later I plan to attempt some of his ideas in the shop to get a better idea of what I have. The labels on the MAC 2400 show: 2.5 HP, 12.3 Amps, with output of 4.2 CFM @ 90 PSI. While they didn't claim this to be a 6 HP motor, the 2.5 sounds a bit optimistic.

I'm not sure whether we got into this earlier, but mine is rated for 50% run time while the professional grades are often rated for 100%.
Roy

 

[Gofor]

No problem Chris. Thanks for the hp info. I quit paying attention to "horsepower ratings" years ago when they started using "developed hp" and all of a sudden the same item that used to be 1 hp became "1 1/2.". That is why I included the amps info from the motor, as that is the comparison I use when buying electric tools. I am glad that it was pertinent info for developing this discussion which hopefully will help Roy make a difficult decision.
I have a side question concerning "power" ratings. My compressor has the previously described motor, but runs on a pulley/belt (the one on the compressor head significantly larger than the one on the motor). Am I right that this increases the torque, allowing it to drive the large piston, but the hp is the same?
Regardless of the calculations, this compressor (Campbell Hausfeld Extreme Duty) does push enough air to power a Binks Mach 1 HVLP, living up to its rated cfm. It does run continuously when spraying a large surface (side of a house), and gradually loses pressure by the time about 1 gal sprayed from a pressure pot. Based on my experience with it sanding a 26' boat hull for repainting with a 6" DA sander, spraying the boat and spraying the house I previously lived in, I think it is about the minimum needed for HVLP spraying of a large item (i.e boat or car) and for using air tools to prep paint surfaces (the continuous use rotary tools like sanders generally use more cfm than the spray gun). For small items such as furniture, a smaller compressor may suffice, but a larger collector tank would definitely help.
My advice for those starting out would be to try what you have before buying a new compressor, or maybe go with the self-contained turbine type if you know what you have is lacking and have no desire to upgrade the compressor. I have no experience with the turbine type so cannot recommend nor defame them.
Realize that the above info is based on my spray technique. Having used a spray gun for many years, I spray right and left handed and rarely release the trigger far enough to shut off the air flow except when changing hands or moving location if I can't continue spraying while doing so. This puts maximum load on the air source as there is little recovery time. Someone with a different technique may use less air.

HTH's and adds to the discussion.

Go

PS. My moniker is "Go". Wayne is the county I live in. Its the one with no sawyers, lumber sources or good tool stores (we have one Lowes and a couple builder's supply places for construction stuff only).

 

[chris99z71]

My moniker is "Go". Wayne is the county I live in.

Whoops, my mistake. Sorry, man. It would help if I could read! :slap:

You're definately right on using the amps as a means for comparison. That's the only thing that won't lie.
As for the pulley sizes- hp can't be created using pulleys. I believe that configuration is common for a few reasons:

Speed change - Most compressor pumps are designed to run at a speed considerably less than the 3450rpm that is common to most motors.

Torque - You're right on with this one. A pulley system can multiply torque by sacrificing speed with the same horsepower. The end result is just like downshifting on a ten speed bike-you pedal faster to maintain the same speed, but the torque required to move your pedals is less.

Momentum - The additional mass of the large spinning flywheel on the compressor pump makes everything run smoother. If there is more resistance at one point, such as during the compresson vs intake portions of the piston cycle, the additional momentum of the large flywheel helps to carry the piston through the cycle and even out the load on the motor.

Again, I'm not an engineer and if someone who IS an engineer can tell me that I'm off on any of this - please do!