Adjusting air compressor for spraying

[Arron]

Hi. I have a 2.5hp, 12cfm belt-driven air compressor. I use it for spraying lacquer, using an inexpensive hvlp gun. It is factory preset for the motor to turn on when the cylinder pressure drops to 6 bar, and turn off when the pressure hits 10 bar. When the tank pressure gets below about 6.5 bar the stream coming out of the gun is noticeably weak and the atomisation suspect. So is it possible to adjust the compressor so that the motor restarts when the pressure drops to 7 bar, but leaving the cut out point at 10 bar ??

A couple of things to note:
- The compressor is a bit underpowered but it has to do- I dont have a 15amp circuit, and I dont want to put one in.
- I dont mind holding off every now and then while the tank refills, I just dont like the way the stream weakens without me noticing.
- As I dont use it long or hard, the motor and head are always cool when I finish using it, so it has room to work quite a lot harder.
- As best as I can see, the restart point of the motor (6 bar) is not affected by how I adjust the outlet regulator or the regulator on the gun. Is that true ?

cheers
Arron

[joez]

Hi Arron,

6 Bar seems high for a HVLP gun I run mine gravity feed guns at about 3.5- 4 Bar max.


I am using a 1.1-1.2mm tip and I usually thin my lacquer by about 15-25%. There should be some improvements you can make to your gun/mix to get more from your compressor.


joez

[Arron]

Hi Arron,

6 Bar seems high for a HVLP gun I run mine gravity feed guns at about 3.5- 4 Bar max.


I am using a 1.1-1.2mm tip and I usually thin my lacquer by about 15-25%. There should be some improvements you can make to your gun/mix to get more from your compressor.


joez

Perhaps I wasnt clear. The regulator on the gun is adjusted to 3 bar, and I usually choke it off quite a bit more with the air screw. Its the tank pressure that varies from 6 to 10 bar, and I'm wondering if its possible to change that to 7 to 10 bar (obviously by making the motor restart at a higher pressure). I dont quite know enough compressor terminology to make things clear.

cheers and thanks for the reply
Arron

[gumred]

Perhaps I wasnt clear. The regulator on the gun is adjusted to 3 bar, and I usually choke it off quite a bit more with the air screw. Its the tank pressure that varies from 6 to 10 bar, and I'm wondering if its possible to change that to 7 to 10 bar (obviously by making the motor restart at a higher pressure). I dont quite know enough compressor terminology to make things clear.

cheers and thanks for the reply
Arron

If the regulator is set at 3 bar, and the compressor cut's in at 6 bar. there should be any differents at the gun .

[joez]

Hi Arron, as long as your tank pressure is higher than your regulated pressure you shouldnt see the pressure drop.

Not all Compressors have adjustable pressure switches, check out this youtube video which will show you what to look for [ame=http://www.youtube.com/watch?v=zh_hfD3rCA0]How To Adjust A Pressure Switch - MASTERTOOLREPAIR.COM - YouTube[/ame]

I had a friend who was having pressure issues, it ended up being a dodgy hose. I would be trying a different gun/hose/regulator before touching my compressor settings.


joez

[Arron]

OK. I think I understand now. The regulator on the tank is set to 6bar, and the regulator on the gun is set to 3 bar, and the air screw on the gun is choking it off even further. Therefore, when the tank pressure gets down to 6 bar its still well above what the gun needs so I shouldnt be noticeing any drop in output. Therefore, there must be something wrong with the hose or one of the fittings. Quite likely any one of these because though the compressor is new all the fittings are fairly old hand-me-downs.

thanks again
Arron

[nearnexus]

The problem with setting the cut in point for the compressor higher is that the electric motor may not/ will probably not have sufficient power to restart the pump against that pressure and you will damage the motor.

The compressors I've used (with PENN switches) only allow you to adjust the cut off pressure.

If the compressor can't keep up supply long enough the cheapest option is to get an extra tank and hook that into the air line to give you more spray time between recharges.

As stated ,if the regulator is working correctly there should be no gun pressure drop while tank pressure is higher. But that's only part of the story - volume is also important and maybe the air line is too small or choked off/icing up somewhere.

Air in lines has friction and once pressure falls far enough in a restricted line, the through put (volume ) will diminish to a point where the spray pattern will be significantly affected.

Sounds a bit like that's what's happening here.

Also some cheap small regulators don't have sufficient through put for high demand use.

If the connectors are the older push button style they can get crap in the seal area (female side) and air flow can drop significantly. Quite OK if in good shape but check them.

The modern Nitto sliding collar type have a bigger bore and are the best bet.

Rob

[Arron]

The compressor didnt come with a hose so I put on one of those thin curly hoses - dont really know where it came from. I never thought about till now, but it could be the problem.
Cheers
Arron

[nearnexus]

I hate those curly coiled air hoses.

[Chris Parks]

I suspect your airline is not big enough. HVLP needs a lot of air so a 12.5mm air supply would be best.

[malb]

I would definitely be considering a larger bore for the airline, I have a long 10mm bore and some of the 6mm bore curly units. My outlet reg is normally set at 90PSI, and running a single curly to a tyre inflator and car tyre, I can drop at least 15PSI (abt 1.1bar) with a moderate flow rate. Trying to run something with a decent flow through the curly would produce significantly greater loss.

Another option worth trying, if you are only having an issue at the very bottom end of a tank charge, is to try and get a direct feed (no reg) off the tank, and into a larger industrial water trap then into the hose to the reg at the gun. Standard issue reg/water traps often drop a few PSI (.3-.5 bar) when the reg input is close to the selected output pressure. This minimum loss is also flow dependant, so more critical at high volumes.

[soundman]

Lets start from the very beginning.

Almost without exception, compressors come with the pressure switces exactly how they came from the manufacturer, the compressor assemblers simply don't bother to adjust them and retailers very rarely will adjust them either.

mostly the pressure switches will be set with a wide window, with the low pressure too low and the high pressure who knows where but usually way below the capacity of the tank and the pump.

SO
the first thing to do when you get a compressor home is adjust the pressure switch....after you fill it with oil of course.

most small compressors we want the pressure switch running arround 90PSI cut in and arround 110PSI cut out, a better quality machine in a higher demand application ya might bump that pressure up to 100 & 120, some workshops higher.

I have mine set at 100 & 120 with my main filter reg set at 90psi & I run a second spraying filter reg set to whatever.

The higher the cut out pressure the higer the current drawn by the motor at the end of run time.

On your pressure switch there will be two adjustments...and this varies.
either there will be an upper and lower limit adjusted by screws or a limit screw and a Histeresis ( window) screw.

these will be under the cover of the pressure switch.

BE CAREFULL, there are almsot certainly live terminals under that cover, if you are not confident in exactly what you are doing, get an electricain or a compressor serviceman to do this for you.

Adjusting consist of letting the compressor pump up and cut out and letting out air till it cuts in.

Moving one adjustment screw at a time will reveal what they do.......yes it is a fiddle.

once you know what the two screws do, you can then fiddle the adjustments....yess fiddle ......they tend to interreact some times, till you have a reliable 90 and 110 PSI or whatever.........90psi = 6 bar, 105psi - 7 bar, 120psi = 8 bar. approximately 15PSI per bar.

Almost all compressors these days including the cheap $100 units have start decompression valves that decompress the pump side of the oneway valve, associated with the pressure switch so there should be no problem with the compressor starting at 90PSI.

If your compressor is already running at these pressures.....look elsewhere for your problem...remember if you are still spraying and the compressor is not keeping up the pressure may be well low.

once you have that you need to look at your main filter regulator.......if you dont have one, you need one.......if this is mounted on the machine...... get it off the machine......a little distance between the tank and the filter reg will help cool the air so the water trap works, and getting the thing off the machine will save it from a premature death from vibration.
Some cheap compressors will have a reg built into the outlet block.....these are generally rubbish, either bypass it, remove it or open it right up, and get a decent proper filter reg.


As others have mentioned, get ya self some decent hose, use a couple of meters of it to seperate you filter reg from the compressor, use the rest for your general use hose.

I have curly hoses, but I dont use them for spraying.

OH..if you are running an on gun regulator ( mean a regulator not just an air control), make sure if your gun does have and air controll that it is wide open.....many HVLP guns are simply standard guns with modifed aircaps and all the facilities of the standard gun remain, including the air controll at the base of the handle.

hope this helps

cheers

[soundman]

I have reread the first post.
Yes I would be adjusting the pressure switch fro a narrower window and 100 to 120 arround 7 to 8 bar would be reasonable.

if you don't have a seperate dedicated spraying air filter/reg set the output reg on your compressor to arround 60 to 70 psi ( 4 bar).

Open the air screw on your gun wide open and set the on gun regulator to arround 30psi (2 bar) and see how you go......do not use the air screw, make all air adjustments using the on gun regulator..that is what it is there for.

cheers

[Arron]

Thanks Soundman,

I have an output regulator on the tank, a water filter on the tank, and a water filter on the gun and a regulator on the gun (in the sense of being a separate unit but attached directly to the gun). There is also an air screw on the gun.

I think I read from what you say that I should set the regulator on the tank wide open (so its doing nothing and not constricting anything), set the air screw wide open, and control the air solely through the reg on the gun.

Also, I should probably remove the water filter on the gun and rely on the one on the tank (ie no sense in having one more constriction unless you know you need it). I left the one on the gun because I thought it was best to filter as close to the gun as possible - I'll test whether I can get by without it.

And of course get some decent hose.

As my motor is cutting in at 90psi and you recommend changing to no more then 100psi then that is probably not enough difference to worry about - especially as I dont really want to touch the compressor if I dont have to.

cheers
Arron

[soundman]

Unless you have other off machine water seperation I would leave the water filter on the gun......lots of water will get past ya first filter reg if it is very close to the main tank.

I would also run the main reg down arround 60 to 70 PSI, so the on gun reg is having an easier time...the less variation it sees the more consistent its output will be.

BUT yeh open the on gun air controll wide open, and use the on gun regulator to controll your air.

cheers