Air regulator/hvlp question


I'm going to experiment a bit with an inexpensive hvlp conversion gun. I've hooked together all the pieces -gun, regulator, in-line water filter. My compressor usually sits at 90 psi for nailers etc. so I figured I could regulate the pressure at the gun regulator. I remember someone telling me that around 30-40 psi was best for these guns.
Here's where I'm confused (and my vast knowledge of air pressure related things shows through :)
The pressure shown on the guage at the gun never changes - same as it is at the compressor - I shut it down at about 60 psi then played with the regulater at the gun. The volume and speed of the air leaving the gun varies with the setting of the regulator, but the pressure guage is always shows the same as at the compressor guage.
Obviously I don't understand this properly. Do I need to go back to the compressor and regulate the air pressure to 40 psi or so there ?
btw - I've been told not to get my hopes to high with the results of this setup - but hey, the stuff is here and all I'm out is a little time and a little fininsh to play with it.
jim
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Throw the regulator next to the gun away and only use the readings of the true regulator on the compressor. The so-called regulator that came with the gun is doing the same thing that you do when you pinch a garden hose. Allow for about a 5-6 lb. drop for every 10' of air hose. Example, if gun is at the end of a 20' hose set the compressor regulator 10 lb higher then you want to end up with. The gun pressure depends on the viscosity of the material that you are shooting. Gun should be 10-12" from subject, and kept perpendicular to subject. Use the spec. that came with gun.
Ken
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NOT SPELLCHECKERED!!!
You want a regulator close to the gun...and run 10-15 feet, or so, of hose to the gun.
It is a bit hard to get one's head around the concept of HVLP. I will try to explain..........
There is static pressure and there is velocity pressure. The movement of the same quantity of air from a to b can happen under high pressure but slowly, or very quickly, but lower pressure. You can't increase the speed at which the air is delivered and keep the pressure the same, without increasing the AMOUNT of air.
Many folks are confused by the LP part of the HVLP. They think it means Low Pressure, in the sense that they can drop the regulator pressure. Not necessarily so. The LP is caused by the HV part of the HVLP.
The only difference is that in a HVLP gun, the air is moving much faster than in an ordinary gun. The Lower Pressure is at the tip....not at the inlet/hose. The inlet pressure has been 'converted', in the gun, to velocity thereby reducing the pressure...again..at the tip.
Because the velocity of the air in a HVLP gun is so much faster than a regular gun, the paint particles slam into the surface to be sprayed with much more speed making it more difficult for the paint particle to change direction (over spray) and waste paint. I run my HVLP at 30 PSIG with a 1.4 tip for laquer...1.8 or 2.0 for heavy primers/fillers.... but still 30 PSIG. My set-up doesn't have to suck the paint out of a cup though..it's pot-fed....WELL worth the money. There are some neat tricks to reclaim all the material from the hose etc...more on that if anybody is interested.
The turbine HVLP guns, start off with high velocity air, hence the big hoses, but they never build up very much pressure. The paint particles fly with the same insane speed and hit the target the same way as a 'converted' HVLP gun. (The 'converter' in a non turbine HVLP gun is just a nozzle before the nozzle.. nothing more than an increase in diameter of the air-path, dropping the pressure and therefore increasing the velocity.)
I hope this helps... and I realize this was a bit 'all-over-the-place'...but hey, it's Saturday night.
Rob
PS..that 'regulator' at the gun, is, like Ken said, nothing but a valve...it doesn't 'regulate' anything. Those things are handy when you're spraying metalics...you step back, throttle the gun at the source, turn off the exhaust fan and 'dust' the entire wet area with floating particles in the hope they'll spread out nice and even. Guys who know how to do that properly, deserve a medal.
(As an aside, I think for the HVLP to work best, the paint should be pressure-fed from a pot to the gun. That way, the gun will work in all positions, upside down etc. Smatter of fact, why use air at all... just pressurize the paint to 3000 PSIG and make it atomize itself. Airless is the way to go.....and I'm not talking Wagner... more like Titan, Graco etc. then when you electrostatically charge your work-piece, you don't even have to move your gun...much.. the paint finds it way to the work piece all by itself. <G>)
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Yeah, but April Fools day was Friday. Some folks are confused by the HV part of the HVLP. Seems that stands for High VOLUME Low Pressure. Because the velocity of the air leaving a HVLP gun is so much SLOWER than a regular gun, the paint particle lands on the surface and sticks instead of bouncing off as overspray. The turbines never develop very much pressure; they need big hoses to move sufficiently high volumes of air to atomize paint at low speed. (The 'converter' in a non turbine HVLP gun is just a nozzle before the nozzle.. nothing more than an increase in diameter of the air-path, dropping the pressure and therefore DECREASING the velocity.)
I am not disputing your methods or results, but I think you have some misconceptions about why HVLP works.
How DO you get the material out of your hoses? I bought a gravity fed gun just because pressure fed looked to be wasteful and a hassle to clean up.
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I repeat:

Once one understands the difference between static and dynamic(velocity) pressure and how they add together relative to a given volume of air, the concept is easier. Read Bernoulli's laws.
One way to demonstrate the concept is to take two 6" chunks of 2x4 on their flat side... and place them parallel on a flat surface about 24" apart. Then span them with a trip of thin laminate or 1/8" ply...something that will bend a bit...mmm about 4 " wide. You should have something that looks like a bridge. If you push on the bridge in the middle, it should deflect downward relatively easy. Now blow air from your compressor with a cleaning nozzle so that the airflow (at high velocity) goes underneath the 'bridge.
I suck at ASCI art:
legend:
] = block ========, = bridge viewed from the top. ^^ = direction of air flow,
Top/plan view:
]============[ ...............^................ ..............air................
keep the nozzle about 3" away from the bridge, otherwise the bridge will fly off.
Observe the pressure below the bridge reducing to the point that the bridge will deflect downward.....unless you have another explanation?
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Thanks as always for all the input.
Well I tried it yesterday. I had built an oak piano bench for my music room - thought that would get me back into the swing of things after being away from WW for several years.
Some things never change - I still hate stain. I had just started experimenting with dyes when I got away from WW a few years ago - I'm reminded why now.
Anyway - for the hvlp per the topic. I didn't remove the in-line 'regulator' (just lazy I guess) but I did open it all the way up with the idea that it would't have an impact then. I set the compressor for 50 lbs. on a 25' hose. I used Enduro water based finishes. I sprayed 1 coat of sanding sealer and 4 coats of clear satin poly, experimenting with different material/air settings, and another setting knob located on the side of the gun - it appears to affect the actual fan width, and also has impact on the amount of air delivered.
While I'm not estatic, I'm also not displeased at all. I can see where this is going to be OK, and would really be great with a turbine instead of a cheap conversion gun. I like the Enduro product. Fast dry, clear, good flowout (which was really good when you're a newbie, experimenting the whole time you're working). The whole project came out pretty well I must say.
I did have a lot more overspray than I expected. Not a bunch - but certainly noticible. A lot more than I saw with the Turbineair that we used in class I took at Woodcraft on HVLP. But of coures there - he was pushing the Turbinair pretty hard because that's what Woodcraft has.
There seems to be a point at which you can adjust the air/material and you begin to get 'specks' of the material instead of a mist - I'm guessing that the mix of air is not enough to atomize the amount of material you're feeding ? I believe I could have fed more material, maybe if I'd brought the air up a little more - but then the overspray would have gotten worse.
All a learning experience I guess - I'll just stay with it until I get more comfortable (and can save for a turbine setup)
jim
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