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Compressor fill rates

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Compressor fill rates

Unread postAuthor: watto » Fri May 16, 2008 10:32 am

How do I work out how long it will take to fill my chamber to a certain pressure at a certain compressor rate? For example, my chamber is about 1.8 liters, how long would it take to get to 130PSI with a compressor that fills at say 18 liters per minute? thanks
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Unread postAuthor: daxspudder » Fri May 16, 2008 10:34 am

well it adds 10x as much air every minute, 1atm being 14psi it will get up to 140 in about a minute, so less than a minute to answer your question 8)
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Unread postAuthor: watto » Fri May 16, 2008 10:37 am

great thanks, could you explain a bit more how I could do it with other variables, I'm not so good at maths, so say a chamber of 6.6782 liters, a fill rate of 34.689 liters per minute and a pressure of 162PSI
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Unread postAuthor: socoj2 » Fri May 16, 2008 10:41 am

daxspudder wrote:well it adds 10x as much air every minute, 1atm being 14psi it will get up to 140 in about a minute, so less than a minute to answer your question 8)


Your kind of wrong.

He may have phrased it poorly but flow will go up and down depending on pressure.

Compressors will have X LPM @ X Pressure

If he is saying the compressor will flow 18 Lpm @ 130 psi, then it will be

( 1.8/18 )*60= 6 seconds

2nd

Assuming 34.689 LPM @ 162 PSI to fill a chamber of 6.6782 liters to a pressure of 162 psi.

6.678/34.689*60= 11.55 seconds

Again if the chamber pressure is lower the times will be lower.

For the record if you wanted to fill an 18 liter chamber with the compressor in the first problem it would take 1 minute.
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Unread postAuthor: watto » Fri May 16, 2008 11:43 am

ok the only part I don't understand is how you get the answers of 6 and 11.55 seconds without including 130 and 162psi in the equation somewhere.
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Unread postAuthor: socoj2 » Fri May 16, 2008 12:20 pm

watto wrote:ok the only part I don't understand is how you get the answers of 6 and 11.55 seconds without including 130 and 162psi in the equation somewhere.


Thats what i am saying. I think you are missing part of the equation.

If you just want to fill a tank from 0-Full with the full output pressure of the compressor then that pressure is irrelevant.

If you want to know how long it will take to fill a chamber to 78 PSI that is 4.6 liters from a compressor that is 26 LPM @ 160 PSI.

Then pressure matters.

Further clarification

If you wanted to fill an 18 liter chamber to 90 psi from a compressor that puts out 18 Liters per minute at 90 psi it would take 1 minute.

If you want to fill a 36 liter chamber to 90 psi from a compressor that puts out 18 liters per minute at 90 psi it would take 2 minutes.

X = Chamber
Y = Output Volume of Compressor
A = Desired chamber pressure
B = Output Pressure of compressor
Z = 60 seconds if you want the answer in seconds leave off if you want it in fraction of minutes

( x/y * A/b ) * Z =

*note this is really oversimplified because as you get over 300-400 psi heat starts to build which changes how much volume can flow and well its a hell of a lot more math.
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Unread postAuthor: daxspudder » Fri May 16, 2008 1:19 pm

what you dont understand sojo, is that he is using a compressor that seems to not have its own pressure chamber, look at what he says in the original post:
How do I work out how long it will take to fill my chamber to a certain pressure at a certain compressor rate? For example, my chamber is about 1.8 liters, how long would it take to get to 130 PSI with a compressor that fills at say 18 liters per minute? thanks


everytime you double the amount of air, you increase pressure by 14 psi... i.e. 2x origional amount of air= 14psi,3x=28,4x=42, etc... in his question, the CHAMBER(of his cannon) is 1.8L, if the compressor fills it with 18LPM, then in one minute it will multiply by 10 (18L/1.8L).... the math your using is pure fiction...
If you want to fill a 36 liter chamber to 90 psi from a compressor that puts out 18 liters per minute at 90 psi it would take 2 minutes.

2 minutes to double the volume of air, creating 14psi, not 90. your math is wrong wrong wrong.


great thanks, could you explain a bit more how I could do it with other variables, I'm not so good at maths, so say a chamber of 6.6782 liters, a fill rate of 34.689 liters per minute and a pressure of 162PSI
to answer this 34.689/60= .578LPS... 6.678/.578=11.554(time to double the volume in seconds), 162/14(air pressure constant)=11.57.... so 11.554*11.57=133.68, so the answer is 2 minutes, 14 seconds.
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Unread postAuthor: socoj2 » Fri May 16, 2008 1:30 pm

the math your using is pure fiction...


Not that the math is pure fiction. I have no clue the kid doesnt have a chamber on his compressor.

If he is using a shop type compressor then it still applies.
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Unread postAuthor: daxspudder » Fri May 16, 2008 1:35 pm

no it doesnt, i work with 4500psi air 5 days a week, Im 100% positive that my math is dead nuts. accept the learning experience, and try it with something with known volumes, and youll see im right.
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Unread postAuthor: socoj2 » Fri May 16, 2008 1:47 pm

K i give how long does it take to fill an 18 liter tank with an output of 18Liters per minute at 100 PSI...

anda 36 liter tank to 100 psi with an Ouput of 18 liters per minute @ 100 psi.
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Unread postAuthor: daxspudder » Fri May 16, 2008 2:01 pm

there is no @XXX PSI with compressors. 18lpm is the volume of air it moves a minute, if its already compressed to xxpsi it doesnt need to be pumped/compress into a container, look up how a compressor works before you waste his time with your posts. i like this site because its great for help/ideas, but when you broadcast bull like that your not helping anyone.
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Unread postAuthor: socoj2 » Fri May 16, 2008 2:28 pm

What my Dewalt Compressor is rated for..

# Free Air CFM @ 90 PSI 5.4
# Free Air CFM @ 125 PSI 4.5
# Free Air CFM @ 135 PSI 4.3
# Free Air CFM @ 175 PSI 3.7

Could you please tell me if i have it regulated to 175 psi, how fast it will fill a 1.5 cubic foot Chamber to 85 psi?
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Unread postAuthor: clide » Fri May 16, 2008 6:20 pm

There is a @ xxx psi on compressor flow ratings, but it doesn't mean that it is providing that flow rate of air at that pressure. It means that it is providing that flow rate of air at standard pressure when the outlet is at that pressure. That is a little confusing so I'll try to restate it. The CFM ratings are for cubic feet of air at standard pressure. The reason for the @ xxx psi is because the compressor's RPMs are slowed at higher output pressure so there is overall less flow rate through.

It may also make more sense to rephrase dax's calculations.

For a 1.8 L chamber at 130 psi there is:
1.8 * 130/14.7 = 15.9 L of air at standard pressure

Now in real life it will fill faster when the pressure is lower, but if you want a conservative estimate you can use the high pressure flow rate.
15.9/18*60 = 53 seconds
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Unread postAuthor: jimmy101 » Fri May 16, 2008 8:52 pm

You also have to take into account thermal affects.

If your compressor doesn't have a resevoir then the air in your chamber will be very hot when it first reaches your target pressure. (Typical shop compressors when they go from 1ATM to ~8ATM will heat up the plumbing between the cylinder and resevoir until it is too hot to touch.)

As the air cools the pressure will drop and you'll have to reconnect the compressor to make up for the temperature drop.

If you are filling a chamber from the resevoir of a compressor then the opposite happens. As the compressed air expands into the chamber it cools. If you disconnect the air source then let things warm back to room temp the pressure will rise above what it was when you disconnected the supply.
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Unread postAuthor: watto » Fri May 16, 2008 9:57 pm

ok thanks for the replys, I'll clarify a few things, firstly
socoj2 wrote:the kid doesnt have a chamber on his compressor.
Correct I'm talking about 12v compressors that dont have a tank but I'm not a kid I'm just bad at maths :D turned 20 last month :cheers:

So I wanna make a wooden box with a couple of car batteries and a heavy duty compressor in, so I can fill this http://www.spudfiles.com/forums/t-d-u-thunder-down-under-4-new-videos-15-05-t14407.html gun out in the bush without having to pump my floor pump for ages. Ive been looking at the flow rates of some and was looking for a simple eqation to estimate how long each would take.

I think clide put it best when he simplified/re-arranged dax's thoughts on the matter... makes sence to me, thanks guys

edit:
ok I'm pretty sure I understand, so if:

X=chamber in litres
Y=desired psi
Z=14.7(what is this the atmosphere thing?)
W=amount of air required to fill chamber
F=fill rate in LPM
M=60
T=approximate time required to get to desired psi

then would I be correct to say:

X*Y/Z=W
and
W/F*M=T
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