Understanding Pressure, Airflow, and Hose Diameter with HVLP Sprayguns

The math gets tricky, but here's some commonsense advice on getting the pressure and flow you need at the spraygun tip. February 23, 2008

I recently purchased a new HVLP spray gun to replace my old HVLP gun and was expecting to see enormous improvements in the finish quality, e.g. the quality of atomization, going from a low end gun into a higher end gun. Both guns are compressor driven and when I hooked up my new gun, I was quite disappointed in the results. In fact my old gun was performing much better than my new higher priced gun. So I started looking for the source of the problem and think it may be in the air supply and not the gun. I thought I understood air pressure and air volume, e.g. cfm, but after reading numerous articles, I have become very confused about this topic.

My confusion boils down to how air pressure effects air volume. Or does it? It seems that everyone is saying you need to have a 3/8 ID hose when using a spray gun in order to supply the right amount of air pressure. Say I have a 25 foot run and I need to have 50psi static pressure at the gun inlet. If I am running 1/4 hose, I would set the pressure at the source to, say, 60psi, assuming a 10psi loss, and for the 3/8 inch ID hose, I would set the pressure at the source to 50psi. Thus I would assume that the end results would be equivalent. Is this true? What about the air volume? Since I need air volume, are these really equivalent? Wouldn’t I have more air at the end of a 3/8 inch hose than I would at the end of a 1/4 inch hose even though the pressure is the same? And what happens when you place a 1/4 inch connector on a 3/8 inch hose? Wouldn’t this effectively give you the same end results as a 1/4 inch hose?

Also in terms of the compressor itself, how does this affect all of this? For example, if I have two compressors, one that is rated at 15cfm @ 40psi and one that is 5cmf @ 40psi. These are the ratings these compressors can maintain, right? So I am assuming that if I hookup a spray gun to each of these compressors using the same hose, etc, the initial few seconds of spraying should be identical and only when the compressor started to cycle would the spray quality start to diminish on the smaller compressor. Is this true or does the larger compressor deliver more air volume from the get go?

Forum Responses
(Finishing Forum)
From contributor A:
Conventional wisdom is that, for runs of 25 ft. - 50 ft., a 3/8" ID hose should be adequate for most HVLP guns. For LVLP guns putting 1/4" fittings on 3/8" hose does not yield the same result as using 1/4" hose. For your system to operate properly, your compressor should be able to supply at least 1 - 1.5x the pressure/CFM requirement of your gun. There may be other issues affecting your new gun's performance, for example - are the needle/nozzle/tip size correct for the material you're spraying?

From contributor M:
I never quite understood the 3/8" vs 1/4" hose either. I have gauges on all of my guns. Whatever I set that gauge to is what my gun gets. What does it matter if it's coming out of a smaller or larger hose? I realize that if you are using the manifold gauge on the wall, the hose diameters would affect the pressure at the gun. What gives?

From the original questioner:

I get the idea of adding a regulator at the gun, which I have always used, but how does this figure into the correct air volume? How do I know if I am getting the correct air flow coming out of the regulator? So how does pressure equal air volume? Say I have 10psi of dynamic air pressure, e.g. with the gun triggered. How do I know if I am getting the needed 12cmf of air volume to atomize correctly? Does 10psi inlet pressure equal 12cmf air volume?

To be more specific, my new gun is an Apollo 7500 setup for air compressor use. This gun is supposed to work on either a turbine or air compressor. The only difference between the turbine and air compressor guns is the inlet connector. One is for a turbine hose quick connect and the other is the standard 1/4 inch male connector. When I hooked this gun to my air hose it appears that I am not getting enough air volume to atomize the finish correctly. I believe I have the correct needle / cap setup. I have tried several different sizes and believe that the 1.5mm with a B cap is correct for the viscosity I am working with, about 18 seconds with a zahn #2. Also I have been spraying this same finish for 3 years in my old Finish Line II with a 1.6mm tip and a #3 air cap with good results. I have also tried using a 1.3mm up to a 2.0mm all with poor results. I believe the issue I am having is with to little air volume. I get droplets, not a fine mist. I also tried air pressure from 40psi which the manufacture recommended up to 80psi. The results improve as the pressure increases but still I have not gotten good results.

To add to the confusion, one of the turbine units that Apollo sells that was recommended for use with this gun states that it delivers 8psi with 135cfm. So if the gun needs 135cfm to atomize the finish, how does it work when connected to a compressor that is pushing maybe 20cfm? I am not looking to buy a turbine, as I have a good air system and don’t do any finishing outside of my shop.

In regard to the connectors, there are many sizes of connectors and quick connects, from 1/8 inch all the way up to 1 inch. Grainger is one vendor that carries hundreds of these. The ones that I was looking at have the same NPT for the 3/8 body as they are for the 1/4 inch body connectors. Both these connectors will fit onto a standard air hose and both will fit onto the male connector on my gun. The difference is in the ID. I am considering the 3/8 body connectors to allow more air volume to the gun. It seems logical that a 3/8 inch hole would allow more air in than a 1/4 inch hole. Is this true? Would using 3/8 connectors yield better air flow at the cap?

Also, has anyone else used the Apollo 7500 with a compressor? How did it perform?

From contributor J:
You don't need to measure or figure out the CFMs going through the gun. Make sure the pressure is there while spraying, and the volume will take care of itself.

The math does get tricky, and I don't have a comprehensive grasp of it by any means. I've mastered enough of it to be able to say that at 12 cfm through 25' of 1/4" hose, you'd have to have a consistent 50 PSI at the compressor end of the hose in order to have 40 PSI at the gun end of the hose. That ~10 PSI drop is specific to a 12 CFM flow rate and only accounts for friction losses in the hose, not turbulence around fittings. With a 3/8 hose, the compressor-end pressure would only have to be about 45 PSI to maintain 40PSI at the gun end, again assuming 12 CFM.

When I was figuring this out, I realized that many of the technical articles are written by engineers who do energy audits for large plants that spend phenomenal amounts of money on energy, and have a great deal to gain by choosing the most efficient equipment. If using larger hoses on a variety of machines enables the plant to lower the tank pressure maintained by its compressors, they stand to save a lot of money. Compressed air is a very expensive way to power anything, compared to electricity, but in a small shop there isn't nearly such a big payoff in terms of energy savings. In other words, the things that matter to the authors of technical articles don't necessarily matter much to you.

One potential problem with a small hose is that if you turn up the pressure enough to ensure sufficient pressure and flow at the gun when spraying, then the pressure at the gun will be higher than it should be at the moment you first squeeze the trigger. A diaphragm regulator at the gun would take care of this.

From contributor E:
You might want to look at the different size water hoses that are sold. If you connect a half inch hose compared to an inch hose you will get more water flowing from the inch size hose. The psi from the inlet valve will be the same but with the larger hose more water will flow out. Same with air. You have to increase the pressure with a smaller hose to get an increase in cfm. This is one reason why they sell HVLP couplings.

From contributor C:
The difference between the 1/4 and 3/8 hose is with the trigger pulled setting the regulator at the gun to match what the air cap requires. The example about the water hose is very good. I don't use the quick connects on my guns. I have 1/2 ball valves to shut off the air.

From contributor W:
Maybe your new gun is a dog. You haven't said which gun it is. Some are just better than others. Price isn't a gauge of how a gun will work.

From contributor T:
What is the issue from the guns that makes you unsatisfied? Is it the atomization of material from the guns? Why riddle the question of cfm, etc.? It could be the air caps that solve all the issues. We also use only 1/4" inlets, chucks, hose, and it boils down to the air caps on both conventional cup guns and our HVLP guns.

From contributor N:
There are two requirements which must be met - both psi and cfm. Your compressor must be able to supply the cfm required by your gun at its recommended inlet pressure.

Please answer these questions:
What is the cfm/psi requirement for your gun?
What is the cfm output of your compressor at 40 and 90 psi (or however the spec. is listed) for your compressor?

Hoses act as restrictors - the longer the hose, the bigger the diameter required to pass the same cfm. The simple answer - for most HVLP applications where runs are around 25' - is to use a 3/8" diameter hose and high flow 1/4" fittings.

From the original questioner:
My questions stem from my recent purchase of an Apollo Atomizer 7500 gun. The results I have gotten from this gun have been terrible. I am reasonably confident that my air supply is not the issue as I am running a 2 stage 80 gal compressor that delivers 18cmf at 90psi. The gun is rated at 20-80psi at 7-12cfm. So my air supply is good. It’s all 3/4 copper trunk line with high flow filters, etc. Not the issue. Besides my old guns, a Finish Line II and an Asturo, both work very well with this air supply.

At this point I am thinking the Apollo gun is just a dog, to say it nicely. I have the compressor model and after spending several days changing tips, playing with the air pressure, adjusting viscosity. Oh yes, I get out that old viscosity cup to make sure everything was just right... Haven’t pulled that thing out in years… never needed to! No matter what I did, the gun just performed poorly. Talked with Apollo support and did exactly what they suggested. Still poor results.

Since the issue I am having with this gun is poor atomization that seems to be caused by lack of air volume, I'm trying to figure out what I might be missing or doing wrong. Since I have been using spray guns for some 15 years, I am somewhat comfortable with my skills and this was a desperate attempt not to see $600 of my money sitting on the shelf in an unusable state. I was hopping that someone could explain how to increase air flow delivered to the gun.

From contributor A:
I believe there are two models for this gun, the 7500T (turbine driven) and 7500C (compressor-driven conversion gun). The requirements for one may not be transferable to the other. I assume you're using the 7500C. The turbine-driven version uses a high volume of air at low pressure (as you noted). HVLP conversion guns are generically broken down into the approximate ratios for the psi required to deliver 10psi maximum at the spray tip (e.g. - a 4:1 gun requires 40 psi at its inlet to deliver a maximum 10 psi at the tip). Conversion guns usually achieve this pressure reduction internally. Some manufacturers manufacture a gun usable on either a turbine or compressor. When used with a compressor, either a special inlet fitting or regulator at the gun's inlet is used to reduce the pressure. Your gun has an unusually large pressure requirement range of 20 - 80 psi. If Apollo recommends 40 psi for most applications, I would expect the gun to deliver good performance at that pressure. Are you are using the Apollo-supplied regulator, and setting the dynamic air pressure (pressure on gun regulator with gun triggered) to 40 psi? The cfm through the gun is set by the gun's design. If you set the psi at 40 (at the gun regulator) with the gun triggered for air only, and the gun regulator continues to read 40 psi over a period of, say, 30 seconds, you likely have sufficient air flow to the gun. If this is what you're doing, and you're getting poor atomization performance, I'd suspect there might be a blockage in the gun (aircap, etc.). Apollo guns generally have very good performance.

From the original questioner:
Thanks for the info. I was a bit surprised as well with the poor performance I received with this gun so I called Apollo and spoke with their technical support, and was told that a 1.5mm tip with the B cap and 40psi static should produce good results with the finish I am using. I double checked he said static pressure. I followed their recommended setup and still received poor results. Prior to talking with Apollo I tried the following setups: 1.3mm and 1.5mm with B cap, 1.8mm and 2.0mm with C cap. For each needle / cap setup I tried starting at 30psi static moving up in 5psi increments up to about 100psi. As the pressure increased the results got better but not good. I tried both with a pressurized cup and without and I also tried with un-thinned and thinned finish matching the viscosity to the needle / cap. What I have observed in all my trials is that the gun just does not seem to have enough air flow to atomize the finish. I could never get a fine mist. The best I seemed to get was small droplets. Between all my testing I broke the gun down and cleaned it and inspected it for any possible clogs, etc but never found anything.

As for a regulator, the supplied Apollo regulator is only for controlling the cup pressure. The cup pressure per Apollo should have a pressure between 2 and 5psi when needed to help push a thicker finish. When I was testing with a pressurized cup I had to set the gun inlet pressure at the compressor regulator and I measured about a 10psi drop to the gun so I compensated for this by setting the regulator by 10psi higher. But when I was testing without a pressurized cup I used my inline regulator at the gun to set the pressure so I charged the line to about 100psi and then set the exact pressure I wanted at the gun. Nothing I tried gives satisfactory results.

The reason I left this out of my post was because I am trying to get a better understanding on how air pressure and air volume work and how different sized hoes and couplings could possibly affect the finish results independent of the gun being used. I am wondering if the 1/4 inch quick connects that I have at the gun could be restricting the air flow even though I have the right pressure.

As you said, there are two different types of guns - a turbine and a conversion gun. But this is where some of the confusion comes in. Per Apollo the Atomizer 7500T and 7500C are the exact same guns. The only difference between the two is the inlet connector. The T model has a turbine quick connect and the C model has a standard 1/4 inch air hose connect. I am told by Apollo that I can take my T model and buy the connector and hook this up to a turbine without any other changes.

So this is where my question about air pressure versus volume comes in and where I am really confused. If I look at the Apollo model 825 turbine this unit is rated to give 5.5psi at 115cfm. So as I understand this, about 115cfm of air would be moving through the cap if I connected my gun to this turbine. Right? Then how could it be that this gun could possibly give the same results when I only have about 12 – 15cfm moving though the air cap when connected to a compressor? This is the heart of my confusion.

From contributor K:
Do you think there might be a missing decimal point in the Apollo literature? That 115 cfm figure may really be 11.5 cfm. Perhaps?

From the original questioner:
Here is the copy that Apollo has listed on their website for two of their turbine units:
Model 825 copy:
# 3 Stage
# 5.5 PSI (0.38 Bar)
# 115 CFM (3.25 cmm)
# Dual Air Filtration
# HVLP Only, Single gun
# 110VAC - 60HZ or 240VAC - 50Hz
# 29 lbs/13.2 Kg
# 15" X 8.5" X 12" (38.1cm X 21.6cm X 30.48cm)
# All EU units shipped
# All 110 volt units are tested and certified by
Model 1025 copy:

# 4 Stage
# 8.0 PSI (.055 bar)
# 130 CFM (3.68 cmm)
# Dual Air Filtration
# HVLP Only - Single Spray Gun
# 110VAC - 60HZ or 240VAC - 50Hz
# 30 lbs/13.6 Kg
# 15" X 8.5" X 12" (38.1cm X 21.6cm X 30.48cm)
# All EU units shipped
# All 110 volt units are tested and certified by

They show 115cmf and 130cfm. Am I reading this information wrong?

From contributor K:
Okay, I just learned something. Went to the Turbinaire website and looked up the specs for their various turbine driven HVLP models. Their CFM ratings ranged from 110 to 130. So I would say you are not reading the information wrong, and furthermore, I stand corrected!

From contributor G:
Call a Kremlin rep and have him demo an LVLP gun. Under $400 and you will be impressed.