Engineering Ques. Calculating time to pressure up vessel?
I have a pressure vessel (autoclave) that is 3.05m in length and has a diameter of 1.37m.
I have two air compressors that each have the capability to output 28CFM.
I am intending to use both of these air compressors to pressurise the vessel upto a maximum pressure of 100psi.
Can anyone tell me how long this will take? Assume the vessel is 100% airtight, and also that the two compressors are ran constantly.
If possible, can you put down how this was calculated as it will help me understand whether these two air compressors are going to be suitable enough
in my time constraints. Ideally, I need the vessel fully pressured up at 100psi within 2 hours. Is this possible?
Many thanks for any help.
Nick
Is that thing designed for that.?
100 PSI (7 bar) is a hell of a lot of air pressure for something that size you are going to stand next to.
Cheers
Fred W B
[Edited on 4/11/10 by Fred W B]
does that have a relief valve on it?
vessel volume = 166 CF
required air pressure = 7 times atmospheric
required air volume = 7*166 = 1163
airflow = 2*28 CFM
time to pressurize = 21 minutes
This assumes that the compressors will be able to put out 28CFM @ 100psi each. Which they probably won't as it is normal to quote capacity at 0
pressure. But for two hours to get up to pressure, you're in the right ballpark.
I am with sebastiaan.
My number came out at 19mins 30secs.
If the assumptions are accurate then the pressure will increase linearly with time. My guess is that this will not happen.
Turn it on, and plot the pressure versus time, (every 5 mins).
I cant really give you an answer you have to work on it a bit. volumetric flowrate is directly proportional to the squareroot of the pressure drop.
therefore at the beginning of pressurisation pressure drop would be 100psi. but as time goes on the pressure drop reduces as the autoclave starts
getting pressurised therefore the velocity of flow into the container would reduce ( using Darcy's equation) therefore you are better off
working out in steps using pressure increments of say 10 each step to give you an idea of timing
hope this helps.
The 28 cfm will be free air delivery, under no pressure, you need to know how much volume the compressor can displace at pressure.
The simplest way is probably by experiment. Find the volume of the compressor's tank, measure how long it takes to get from atmospheric pressure
to 100psi, then multiply that by how many times bigger the autoclave is.
From good old boyle's law...
Va = PcVc/Pa
Va = volume at atmospheric pressure
Vc = compressed volume
Pa = atmospheric pressure (absolute, so 1 bar)
Pc = compressed pressure (absolute, so your pressure + 1 bar)
Your volume is 4.5m3, absolute pressure (100PSI + atmospheric) = 7.89 bar, so...
Va = 35.5m3 free air required
28CFM x 2 = 95m3/hr
So time is 22.4 minutes
This assumes an isothermal process, though, i.e. the compressed air doesn't rise in temperature. In reality you will put a lot of work into the
gas by compressing it, so it will heat up a lot. This model assumes all that heat is lost, so overestimates the time.
An adiabatic calculation using
Va = Vc(Pc/Pa)^(1/k)
Where k is the ratio of specific heats, which is 1.4 for air, gives the answer assuming no energy loss in the system...
Va = 19.7m3
So time is 12.4 minutes.
In reality, you'll loose a fair bit of heat depending on how well the system is insulated (fairly well I guess as the vessel is an oven), but not
all, so the real answer would be somewhere inbetween.
Then add leaks, both external and internal (within the compressors themselves), worsening as you reach pressure and your time increases depending on
how knackered the compressors are. If they are rated to provide well in excess of 100PSI and in good condition, time shouldn't increase too much.
If that's the case you should have a very good chance of doing the job in 2 hours.
Hope that helps.
P.S. I would echo the safety concerns of others. Compressed air in that volume is potentially explosive and hugely dangerous. I hope your vessel is
appropriately rated!
Thanks very much so far guys!!!
Will post up the real world findings when I go ahead!!!
Regards,
Nick
Can you can use boyles law (P1xV1 = P2xV2) to predict the output? if you can then the out put is down to 8CFM (2x4CFM) @100psi.
I reckon around 90 minutes
at 56 CFM all the way it would take 21mins
at 8 CFM all the way it would take 145mins
average = 83mins + a bit for efficiency losses.
Davie
ps. wait until you are pumping gas at 3000psi and 1000 cfm then it gets a bit scary.
[Edited on 4/11/10 by daviep]
Not really - it's wrong because you've already accounted for the change in volume when you calculate you need to deliver 35.5m3 into a 4.5m3
vessel.
It's a characteristic of the compressor how much performance is lost as the pressure increases. If you consider the ideal case of a positive
displacement pump with perfect sealing and enough torque that it maintains a constant speed in all conditions, that compressor wouldn't loose any
performance at all.
I'm using a compressor for some testing at work that doesn't loose any CFM until around 7 bar, thereafter it tails off until it's at
about 85% at its rated 10bar.
A knackered old thing with bollarked seals and a dickie motor might never reach its rated pressure as it's output has dropped to 0 before then
just fighting it's internal/external leaks.
That makes sense.
quote:
Originally posted by Fred W B
Is that thing designed for that.?
100 PSI (7 bar) is a hell of a lot of air pressure for something that size you are going to stand next to.
Cheers
Fred W B
I used to strap 232 bar to my back
[Edited on 4/11/10 by Fred W B]
quote:
Originally posted by snapper
quote:
Originally posted by Fred W B
Is that thing designed for that.?
100 PSI (7 bar) is a hell of a lot of air pressure for something that size you are going to stand next to.
Cheers
Fred W B
I used to strap 232 bar to my back
[Edited on 4/11/10 by Fred W B]