We all know that a long hose off a compressor will result in pressure loss...

Is there a general rule of thumb or a standard for the ratio of length to loss?

Originally posted by dittoz:
We all know that a long hose off a compressor will result in pressure loss...

Is there a general rule of thumb or a standard for the ratio of length to loss? Actually the length of the hose will not affect the pressure. If your tank is set to put out 90lbs once the hose is pressurized, no leaks, then you will have 90lbs at the end of the hose. Doesn't matter if it's a 1 ft hose or a hundred ft hose. What it will do is reduce the volume of air delivered. A smaller diameter hose restricts airflow just as a longer hose restricts airflow. I don't know of any formula to figure that out. It would depend on the hose diameter, length, and pressure. Is there a particular problem you're trying to solve or you trying to set up a tank installation?

No problem (yet). Was having a discussion with a co-worker...

However, it seems to me I've read on here repeatedly that there IS a pressure loss over extended hose. My feeling was exactly what you stated as well, that once pressurized, it shouldn't matter, but now I can't find a link to anything citing this.

If it doesn't make a difference, why do we worry about measuring pressure at the gun instead of at the tank valve?

I KNOW I've read it several times however...!

Originally posted by dittoz:
No problem (yet). Was having a discussion with a co-worker...

However, it seems to me I've read on here repeatedly that there IS a pressure loss over extended hose. My feeling was exactly what you stated as well, that once pressurized, it shouldn't matter, but now I can't find a link to anything citing this.

If it doesn't make a difference, why do we worry about measuring pressure at the gun instead of at the tank valve?

I KNOW I've read it several times however...! Now I see what you're saying. I guess what occurs is a small hose may not be able to supply enough CFM for a particular tool. So the net affect is pressure drops at the tool because you can't move enough air through it fast enough to maintain the pressure. So in that respect yes it looks like the pressure drops because of the long hose but it is really the side affect of not being able to move enough air through the hose. Same thing would occur with a short hose with a very small diameter, it just can't move enough air.

Actually hose length greatly affects air pressure and volume.
A cheap 1/4"ID air hose(typical air hose) that is 50 ft long, 80psi at the compressor will read 50psi at the gun. At 25ft you lose 16psi.

Your high flow air hoses, 3/8"ID will drop 8psi over a 50ft length.

I'll have to do my test over again. I lost the info from the last time :(

I have a test cap as well to PROVE the longer OR smaller hose will greatly effect the AT THE CAP pressure which in turn effects CFM.

Thank you Brian! I figured you, Eric or Bob would be the one with the answers.

So, WHY is there a pressure loss in a relatively sealed system?

-c

Remember, air like everything else isn't a "whole" it is made up of microscopic "pieces".

My understanding (never explained to me, this is just how I see it) is that with the small hose the air "pieces" can't keep up with the ones leaving the hose at the open end.

I see a subway tunnel full of people, the end opens up and all the people rush out. If all those same people were in a little five foot diameter tube that was ten times as long as the subway was when the end opened up, the people sure as heck couldn't get out as fast as with the larger subway tunnel.

Originally posted by MARTINSR:
Remember, air like everything else isn't a "whole" it is made up of microscopic "pieces".

My understanding (never explained to me, this is just how I see it) is that with the small hose the air "pieces" can't keep up with the ones leaving the hose at the open end.
Sounds like a good explanation since unlike liguids, air compresses. Guess I didn't consider that difference in my first response.

I can see this is going to get into a big messy discussion, but let me expand a little on it from my engineering and aeronautical experience.
(Or at least after staying in a name-brand hotel last night...)

What you're referring to Brian is an offshoot of what's known as the Venturi effect. A fixed gas flow (CFM) will increase speed (PSi) when it is forced through a smaller opening. In order for a fixed volume to pass thru a reduced space, it must move faster. This is why a river moves faster when the channel in which it flows narrows. The same volume of water is moving thru a smaller area and thus must move faster to keep up the flow and not back up. It's also one of the principles applied in a jet engine when the tail cone is narrowed to increase power.

Anyway, I suppose rather than widening the route (the hose), if we EXTEND it, that would have the same affect and the pressure would likewise reduce. But, if we extend it's length AND use a larger inside diameter, we're obviously increasing the volume in the passageway (the hose), so shouldn't the pressure be further reduced? According to Bob, it sounds like pressure is increased as a result of a wider hose? From a physics perspective, this simply isn't possible.

Unless of course, we're talking about CFMs and not PSI? Hank's comment of compressability would apply since higher compression equates to higher CFM. But I still don't see that, since the hose is open to the tank and therefore the compression ratio of the air should remain consistent throughout the system.

If it's a wider / longer hose, and assuming the tank pressure remains constant the hose becomes an extension of the tank. Once pressurized, the only thing that would change is that a wider inside diameter hose with pass more air resulting in a higher CFM at the same pressure if the compressor remains running.

If none of this makes sense, imagine a water hose nozzle. If you leave the faucet turned on full that simulates the compressor and the air tank. If you crank the hose nozzle so the jet is sharp and hard, you're still keeping the same pressure at the faucet but reducing the passageway, resulting in lower CFM but higher PSI. If you completely remove the nozzle and have the hose open, you're still maintaining pressure as it enters the hose but decreasing it at the exit point. You're also increasing the volume of water that comes out because it's not restricted by the nozzle, which is why you can fill a bucket faster with the nozzle removed. Equate this to CFM and PSi: The faucet is your tank pressure and the hose nozzle is the gun and it's pressure...

So, whether the hose is long or short, thick or thin, I don't see (in an enclosed pressurized system) why the length of the hose would matter...

Does ANYBODY understand what I just said....?

:confused: :eek: :confused:

Air passing through the eye of a needle won't effect static pressure but it sure will effect flow pressure.
:D

I think a lot of people do get confused about the difference.

WOW! That's very profound.

Sounds like something I'd find in a fortune cookie!

When I took my sherwin williams painting classes they handled out refinishing manuals. Anywho in this manual there is a section about air hose length and diameter and how it affects air pressure.

With a 1/4" inside dia hose with 80psi
5ft hose air loss=5.5psi
15ft air loss=11.5
25ft air loss=16
50ft air loss=25

With a 3/8" hose with 80psi
5ft=2.5psi loss
15ft=3.0
25ft=4.0
50ft=6.0

Thanks Eric - we can now rest on the first part...

I think I got an answer to the 2nd part too...

FRICTION... In this case it's air particle friction. The smaller the diameter of the hose, the more the air molecules tend to bounce off of each other and the surrounding hose, causing them to tumble and bounce in to each other more, causing friction, which slows the air down, resulting in lower pressure...

...I think!

With a 1/4" inside dia hose with 80psi
5ft hose air loss=5.5psi
15ft air loss=11.5
25ft air loss=16
50ft air loss=25

With a 3/8" hose with 80psi
5ft=2.5psi loss
15ft=3.0
25ft=4.0
50ft=6.0 I think this is dependent on what amount of air is passing throught the tool on the end of the hose. Example- If your only pissing air through a pinhole on the end of the hose the pressure difference would be almost none from one end of the hose to the other. If you've got an open end on the hose the pressure drop would be very measurable with the hose restrictions being length, diameter, and tubing texture all playing a role in airflow restriction. Sprayguns all flow different amounts of air so the amount of pressure drop while in use is dependant on how much air the gun uses and how well the air flows through the hose. The same theory can be applied to an engine intake manifold where valve size, port length, port crossectional area, and texture are all related to airflow in terms of volume and velocity. JMO

Yes friction.

This is the same thing as a voltage drop across the starter wire ONLY WHILE CRANKING.

Wouldn't the fact that the pressure and volume of air in the compressor tank isn't constant have something to do with it? The compressor will be kicking on and turning off, and that would vary the volume & pressure in the tank, even though it's regulated to the lines. Just adding to the jumble. :D

BL

I am not an engineer, I certainly don't understand a lot of what I have been told over the years. But one thing I know, EVERY SINGLE paint gun manufacture, EVERY SINGLE paint manufacture has very similar information on this subject.

All will mention "Air hose pressure loss" regarding hose size. In my Devilbiss "Spraymaster" course it reads... "ID has the greatest effect on pressure loss", "As length increases, pressure drop increases"

It then has a chart with hose size and PSI needed to maintain a given CFM over a given length.

"Use the largest ID practical to reduce the pressure drop" and "Use the shortest length possible to reduce pressure drop" is also instructed.

My S-W "Trouble shooting guide" has a chart with recommended pipe size for your compressor relating to compressor size and LENGTH of the piping. With of course the longer pipe for a given compressor needing to be larger.

EVERY SINGLE one of the many gun or paint clinics I have ever been to have the same recommendations.

Like Henry Ford said, "I may not be the smartest man alive, but give me five minutes and I can find someone to answer your questions".

I think what it boils down to is the friction of the air on the inside of the hose as it passes thru the hose. As the air molecules "rub" the hose they slows, in turn slowing down the air molecules that they are touching, and they in turn slow down the ones they're touching...

A larger diameter hose allows more air molecules to remain further away from the hose wall, allowing more unimpeded passage.

The longer the hose, the more opportunity for friction.

I think...

Thanks guys! I can now sleep better! graemlins/thumbsup.gif

Sounds good Curt but I still think it's more because of the compressablity of air.

Closed system, 90 psi at both ends of hose. Open the end, use an air tool, sprayer, etc. The air at the end of the hose sees less pressure on one side than on the other and starts to decompress, the psi drops. Now the air behind that sees less pressure on one said than on the other and starts to decompress. This works it's way back to the compressor which is still feeding 90 psi. It's like a wave effect. Now the more cfm you're pushing or the longer the hose the more chance the air has to decompress between the tank and the tool and you're trying to compress more air. It's not until the end is sealed again that the air has a chance to compress equally end to end.

I think...

Hank, I think you are both right. There is "laminar air" with little tubulance and "turbulent air" with, well, turbulance. Both can occur depending on what it is going thru.

Originally posted by MARTINSR:
Hank, I think you are both right. There is "laminar air" with little tubulance and "turbulent air" with, well, turbulance. Both can occur depending on what it is going thru. Brian, sounds good to me. I'm sure that friction, which will cause turbulance, has some affect. I think the air decompressing over the length of the hose between the compressor and tool also has an affect.
Everybody happy with that? :D

Tomorrow a mech engineer will come along and shoot holes through all of it.

Me thinks you are all bull$hitting us... except Brian or anyone who has ever used a pressure sensitive paint gun or any other tool knows that the length of the hose affects the pressure from the compressor to the end of the line.

You armchair physics majors and guessers are ignoring reality.

Sevt posted some numbers on the approximate pressure drop based on ID hose size and length. Pay attention.

Oh, come on now guys you can't argue with scientific facts. However, to arrive at a conclusion you need all the facts. I think where the engineer may loose it is when there is a tiny fact that is forgotten or not mentioned. For instance, the hose expanding? How about restrictions like hose couplings, filters and the like? I am no engineer but I would think that any one of these or many others I can't think of could skew the conclusion.

Like I said, I will get out into the garage in the next few days and to a test. I have a hundred feet or so of hose and I will check. I don't have a way to read CFM but I do have a pressure cap for my Devilbiss and my Sharpe. The last time I did my test with more hose you had to bump up the inlet pressure to maintain the 10lbs at the cap.

But I will document what I find, it won't be as thorough as I would like as I only have one 1/4" hose and my compressor is set up pretty nice so I don't have a "standard" poor set up to see improvment but at least it will give us something.

Originally posted by vettefella:
Me thinks you are all bull$hitting us... except Brian or anyone who has ever used a pressure sensitive paint gun or any other tool knows that the length of the hose affects the pressure from the compressor to the end of the line.

You armchair physics majors and guessers are ignoring reality.

Sevt posted some numbers on the approximate pressure drop based on ID hose size and length. Pay attention. Pay attention to the replies. I don't think anybody was denying that it happens. Just trying to explain why it happens.

Basic fundmentals of supply and demand here from what I understand, the larger the air consumer the larger the air supplier needs to be. No engineer needed here, besides sometimes engineers get stuck thinking inside the box- often times it's just a dummy that comes up with the really good ideas because he hasn't been taught that it won't work. :rolleyes:

Man, I leave this page for a few days and some of you get all worked up! smile.gif

If the hose expands, that's fine, but it's a finite amount of expansion and when the hose' limit is reached, the tank pressure and the pressure inside the hose will equalize since they are a "closed system" together. Regardless of the size of the hose, the fittings, etc, the pressure will remain consistent as long as there is an open path thru all parts. That much I KNOW. But this is the PSi only in a closed system.

It's when the valve (gun) is open and the airflow begins that the physics part kicks in and all the variables start impacting. I agree with Bob that it's a supply and demand issue, but from the point of creating a model, you must initally assume that the supply is larger than the demand so that the flow will remain stable. This is the CFM part...

As far as being an armchair physics major, I have a 5-yr engineering degree and my initial college education included 2 yrs of aeronautical engineering and flight theory which directly translates to the movement and resulting impact of air movement on a controlled object. We just weren't moving air from a compressed source through a tight system, which is why I posed this question in the first place.

Brian - if you'd like to take a drive up to Sacto, I'd be more than happy to let you test out your pressure cap and such on a really cheap 1/4" setup too! ;)

-c

WOW! That's very profound.

Sounds like something I'd find in a fortune cookie!

dittoz, interesting info. There was quite a bit of input on this smile.gif

Yeah, I think we're going stale at this point Bob.

Once in awhile, I get to thinking and wonder "...why?" and this is the result; Fortune Cookies!

It continually amazes me how the combined knowledge and experience here pretty much know EVERYTHING! What folks would do without a resource like this. Now, I just need someone to come over and show me how our dang'd car goes together!

Thanks for the discussion! graemlins/thumbsup.gif

Now here is some useful information.....
http://www.engineersedge.com/fluid_flow/pressure_drop/pressure_drop.htm

Determin Reynolds Number
Re = w x D / v
Re = Reynold number
w = velocity of Flow
d = diameter of pipe
v = kinematic viscosity

Man I read this whole thread just trying to figure out what size hose to buy when I set up my new compressor and I still don't know for sure. My compressor is a 2 stage 175 max psi. I'll put in about 50 feet of pipe with all the recommended filters and traps etc. Would a 50 ft. 3/8" ID hose in a retractable reel near the ceiling be OK for most anything you normally do with it?

Pressure loss is always a point of disagreement no mater what type of system it is in. If pressure is applied to a closed object then the pressure will be distributed equally to all parts. The key word is closed. Every turn, bend, ruff internal surface etc in a open system does couse pressure drops. This is all taken into account when engineering piping systems etc. With that said when you are spray painting the volume is your major concern. As lond as your volume is sufficent and you do not loss pressure at the regulator then everything else is a constant and even thou there is pressure loss you should not see changes while you spray. (if you did all the math and you have a 8 psi loss on a 25' x 5/16" hose at 18cfm. you will have it through out you work) So the key things are to have a large enough hose, a must if using a HVLP gun, a compressor that can maintain the volume needed, and getting your initial adjustment done. If you have all the above and have good adimization at the begining you should have no problems.

are you confused yet?

Gary

As long as you have a sufficient source, definitely go with the 1/2" hose. It has the greater flow and the lesser pressure loss, again assuming you have enough compressor at the top end...