Understanding air flow on compressors

[Scott Shepherd]

For longer than I should admit, air settings on air operated tools and compressors has stumped me and I'm hoping I can start my education today and stop being ignorant of air :)

We have various devices that require air, HVLP guns, Sand Blasting cabinets, etc.

The part I don't understand is the things people require. For instance, our Sand Blaster, we might be blasting at 20 psi (detailed work on glass). The regulator at the sand blaster is set to 20. However, the specs from the manufacturer say that you need no less than 90 psi coming into that fitting. Why do I need 90 psi coming into a regulator that is going to throttle it back to 20 psi? The compressor has a large tank, so what's the need for it? And if memory serves me correctly, if you do turn the compressor down to 30-40 psi, the sand blaster won't work.

On HLVP guns, they say that your gauge at the gun needs to be 25-30, so that it is 12-15 at the actual tip. However, if I put the compressor at 90 psi and the regulator a the gun to 25, then it's WAY too much pressure. If I scale back the compressor to about 40, then it all works fine.

It seems to me like there is a direct relationship between the air pressure on the compressor and what's coming out there other end, even though there are regulators in place.

In my mind, if I have a compressor and it's set to 100 psi, and then I have a device with a regulator on it that's set to 10 psi, if I am discharging 10 psi out there device, and I walk back to the compressor and turn it to 25 or 200, it shouldn't change the air output of the device one bit. However, I don't think that's my experience.

Anyone want to take on the challenge of trying to help me understand that relationship?

[Robert J Thomas]

From what I understand (I'm hoping someone will correct me if I'm not quite right), most air tools are low volume/high pressure. This means that the air coming in needs to be at a very high pressure, high velocity, but not much air is needed. For example, a normal impact gun will use about 5 cfm at 90 psi. HVLP literally means high volume/low pressure, so of course the volume goes up, but the pressure (and therefore the velocity through the nozzle) goes down. Something like 15 cfm and somewhere shy of 40 psi.

Pressure and volume are related by the ideal gas law. P*V=R*T where P and V are pressure and volume, R is the ideal gas constant and T is temperature. We can assume that T is constant because there won't be much heating at this scale. So P and V are inversely proportional for our simple case. As one goes up, the other goes down. IN a perfect world, when the pressure or flow are restricted, we should have v1*A1=v2*A2, where v is the velocity and A is the flow area.

There are also losses in the system, especially through nozzles like the sandblaster tip or the HVLP tip. This is the reason for the differential in pressure between the tool inlet and the outlet. I'm thinking that at high pressure, your compressor may not be able to keep up with the volume requirements of the HVLP (I could be wrong), even with the pressure regulated to 40 psi. There is theoretically up to a 40% loss through a regulator or other flow restricting device (nozzles, orifices, etc).

This is all based on my thermodynamics and physics courses. I'm sure it doesn't exactly answer your question, but hopefully it gives you some things to consider...

[Scott Shepherd]

Thanks Robert, that's a little helpful, but still leaves me with many questions. Mainly, if I have 2 regulators in line, and I set one to 100 and the one at the end where the tool is to 20, then why would the output change after the 20 if I changed the 100 setting? To me, the tank is full and under pressure and it should be regulated at the point where it's set the lowest, in this case 20. I can't wrap my head around why changing the regulator that's on 100 should impact the tool that's after the 20. To me, if I had 1000 psi at the tank and 20 at the final regulator, then it should be 20 coming out the final outlet (assuming it's just coming out a open hose, not being restricted by a device). Also, if I set it on 30 coming out the tank and 20 at the source, again, the pressure at the source should be 20 in my mind. But that's not what appears to be happening, which is the part I'm trying to understand. Of course I could be totally wrong and misunderstanding what I'm seeing.

To me, it's baffling.

[Malcolm McLeod]

Robert is correct, and I think your issues (and perhaps misunderstanding) involve 'flow'. You are correct as well, that if your have a downstream regulator set at 20psi, then it should maintain that downstream pressure - - so long as the upstream pressure is anything greater than 20psi. And this holds true right up to the moment you open a valve, spray tip, etc. - - and introduce flow.

Look at the flow capacity of your regulators and demand of your tools. The tool should demand the lowest flow and pressure of the devices in the system's flow path. If you have a compressor rated for 70SCFM (standard cubic feet per minute) at 120psi, a regulator upstream rated for 60SCFM at 100psi, a tool regulator rated for 20SCFM at 20psi, and a tool that needs 40SCFM at 20psi, you will have problems. Static pressure readings will be fine, and then you pull the trigger. In this case, the dynamic pressure - - pressure while flowing - - at the tool will be too low. The tool regulator will not be flowing enough air.

Or, if you drop the upstream regulator to 60psi (where it may only be rated for 30SCFM??) , it may not have sufficient capacity to deliver the flow rate necessary to keep the pressure high enough at the downstream regulator, and so allow it to deliver its rated capacity.

...Or something like that!:confused:

[Chris Parks]

Most compressor/air systems work at a set pressure which is way to high, say 120 PSI. The use of regulators then limits the pressure needed to air tools, spray guns etc. Why is it so high? Some tools like big impact guns need all the pressure they can get as most systems have big pressure drops at the tool due to losses in the supply system. It would be inconvenient to change the pressure setting on the compressor so we use regulators to do that where needed.

[Tom Stenzel]

Malcolm has it right.

Scott, let's look at your example: lowering a pressure regulator upstream affects the downstream equipment when common sense says the pressure should be enough.

The equipment has a requirement for air typically expressed in pressure and flow requirements. The result is an amount of air in mass. If you lower the upstream pressure regulator (90 psi in your example) down to any lower number, to maintain the mass the line coming out of the regulator has to increase the velocity.

It's sorta like an electric motor having to increase the amount of current to compensate for a loss of voltage to maintain power (watts).

The result is the air line between the regulators has a higher velocity and therefore a higher pressure drop across it. The lower pressure and the increased drop add up and are conspiring* against you. If the line had been made larger you might have gotten away with the lower pressure. But likely not, these regulators aren't anywhere close to perfect, they have internal losses and drops as well.

-Tom

*It's us against the machine!

[Malcolm McLeod]

Scott, guess I should have mentioned that the solution to the hypothetical situation I outlined is to make sure the regulator (or FRL group) is large enough to pass the required volume at the set pressure. This may be simply a matter of replacing a 1/4" FRL with a 3/8" or 1/2" model. It may involve re-sizing your pipe runs to match as well?

And another clarification (clearly, I can't think and type at same time): Maybe obvious to most, but for the sake of the rookies, the compressor doesn't necessarily have to continuously supply the full rated flow of a tool. So long as the tool usage or process is intermittent, the compressor can 'catch up' by filling the tank while I take a break.