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1/4" vs. 3/8" air hose, does it matter?

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Forum topic by Jim B posted 03-03-2016 11:21 PM 2558 views 1 time favorited 7 replies Add to Favorites Watch
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Jim B

57 posts in 693 days


03-03-2016 11:21 PM

Topic tags/keywords: question trick tip resource spray gun

I think most of us know that 3/8th inch air hose is supposed to be better than ¼ inch when more power is required for air tools. Although most woodworking does not require air tools that need a high volume of airflow, many of us use air tools for other hobbies. I’m in a Gas Dynamics class at school and got to thinking exactly how much better is 3/8th inch than ¼ inch and when are the effects noticeable. I determined, from calculations shown below, the theoretical max air flow for ¼ inch is 13.35 CFM and for 3/8th inch, max air flow is 30.05 CFM. These numbers represent the max air flow out of a compressor with the regulator set to 90 PSI assuming no losses to friction or other inefficiencies. (In reality for a 50’ hose we could expect roughly 10% loss.) What does this mean? First if you have a compressor with a high output you immediately restrict your max air flow to 13.35 CFM if you have any quarter inch fittings, hoses or other connections in your set up. Second, you still can benefit from a 3/8th inch hose if your compressor can’t output more than 13.35 CFM. If you have a large tank, as long as your output pressure remains above 90 PSI, with a 3/8th inch hose you still can get a flow above 13.35 CFM. Though you will only be able to maintain that flow rate until your tank pressure drops below 90 PSI. In conclusion if you don’t have any tools, or anticipate buying any, that require close to or more than 13.35 CFM it is a safe bet to stick with the standard ¼ inch hose and fittings. But if you need to run bigger impact wrenches or other high flow tools, it would probably be a worthwhile investment to upgrade to 3/8th. I am not familiar with any, but if you had a tool that required a high volume of air flow only for an instant, with a 3/8th inch hose you may be able to get by using an under powered compressor if you have a large enough tank. (How long it would work is a calculation for another day.) These calculations were made assuming isentropic flow, at sea level, with an ambient temperature of 70℉. Again in reality you can assume around a 10% loss. You can calculate other flow rates using the same equation, just substitute in your hose diameter and operating pressure. PS all calculations were done in SI units then converted back to standard. Let me know what you think or if my math or logic is wrong.


7 replies so far

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JoeinGa

7481 posts in 1470 days


#1 posted 03-03-2016 11:25 PM

I use air tools like impact guns and air sanders, and have been known to paint a car, so the bigger hose is a must.

But for the average woodworker guy using the occasional air nailgun or blowing off the lathe, the smaller will do fine.

-- Perform A Random Act Of Kindness Today ... Pay It Forward

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shipwright

7168 posts in 2261 days


#2 posted 03-04-2016 12:00 AM

I’ve used 1/4” for years, even on big paint jobs because the line is so much easier to handle. I do however plumb my shops with lots of 1/2” copper line so my hoses are never very long, max maybe twenty feet.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees. http://thecanadianschooloffrenchmarquetry.com/

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joejinky

8 posts in 514 days


#3 posted 03-04-2016 12:11 AM

If you can, it is good to install air tanks at the END of air lines, along with their own water drains if you don’t have a drier on the compressor exit. A bladder near your tool that is fed by the compressor line will give you a volume of air to work from, rather than relying strictly on the pressure in the hose. As you use the air in the bladder, the compressor line will replenish the air volume and you will not notice a drop in air pressure nearly as much as being hooked directly to an air hose alone.

-- I must be doing something right. I still have all of my fingers!

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MrUnix

4222 posts in 1662 days


#4 posted 03-04-2016 12:37 AM

These numbers represent the max air flow out of a compressor with the regulator set to 90 PSI assuming no losses to friction or other inefficiencies. (In reality for a 50’ hose we could expect roughly 10% loss.)

Not sure what you are referring to about the 10% loss at 50 feet or how you came to that number… But if you go to the air flow calculator over at Gates, you can calculate loss by hose size and length at various pressures and CFMs. As an example, I punched in the numbers for a 20 foot length of 1/4” hose at 90 PSI and 13CFM. For 1/4”, that comes back with a little more than 15psi loss (.78psi/ft), while a 3/8” hose has just a bit less than 2psi loss (.09psi/ft). At 50 feet, that 1/4” hose will have a drop of almost 40psi at the end, and a 3/8” hose would only have about 5.5psi drop.

Sure seems to me that, even for fairly short runs, there is a pretty substantial difference between 1/4” and 3/8”... maybe I’m missing something though.

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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shipwright

7168 posts in 2261 days


#5 posted 03-04-2016 12:52 AM

If your compressor is putting out 125 psi and you are spraying at around 50-80 using a regulator at the gun you can afford quite a drop.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees. http://thecanadianschooloffrenchmarquetry.com/

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Jim B

57 posts in 693 days


#6 posted 03-04-2016 01:31 AM

10% was just a very rough estimate of loss of CFM. The gates calculator seems pretty hand for finding PSI loss though.

View woodbutcherbynight's profile

woodbutcherbynight

2427 posts in 1872 days


#7 posted 03-04-2016 01:44 AM



I use air tools like impact guns and air sanders, and have been known to paint a car, so the bigger hose is a must.

But for the average woodworker guy using the occasional air nailgun or blowing off the lathe, the smaller will do fine.

- JoeinGa

Same

-- Live to tell the stories, they sound better that way.

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