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How Many Cycles?

02/19/2017 2:56 AM

Greetings, and I hope this first post makes sense!

I have machine that uses compressed air. Rather than using a compressor for this machine, we are considering nitrogen gas since it's use is infrequent.

The specifics are a 40 Cu Ft Nitrogen tank, filled to nominal 2200 psi. at room temperature. The regulator controlled delivery pressure is 120 Psi. The machine cylinder volume is estimated to be 20 Cu In, but since it is a dual acting cylinder I would assume it will use twice that volume, so 40 CuIn (.03CuFt) per cycle is probable.

So now the question. How many cycles can I reasonably expect until the cylinder pressure drops to the 120 psi operating pressure?

Any help on calculating this would be appreciated so we can determine if this is a sensible direction to go!

Thanks

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#1

Re: How many cycles!

02/19/2017 4:06 AM

As the cylinder approaches empty, you still need to have pressure in it greater than 120 psi; arbitrarily say 200 psi. That gives you a 2000 psi range to work with, x 40 ft3 x 1728 in3/ft3. Dividing by (120 psi x 40 in3), that gives 28,800 cycles. However, any incidental leaks might eat heavily into that.

Moreover, 40 ft3 sounds like a honkin' big cylinder. Are you sure it's not 40 scf? If so, the cycles will reduce by a factor of about 2000/15.

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#3

Re: How many cycles!

02/19/2017 5:10 AM

A JC size tank holds about 40 cubic feet of nitrogen and is roughly 6" dia by 24" tall.

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#12

Re: How many cycles!

02/19/2017 2:34 PM

40 scf, not 40 actual cf.

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#19

Re: How many cycles!

02/20/2017 7:11 AM

GA

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#2

Re: How many cycles!

02/19/2017 5:06 AM

Factor the tank and regulator costs into the equation and you will quickly find that buying a small cheap air compressor is a better use of your money.

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#5

Re: How many cycles!

02/19/2017 6:00 AM

Thanks for jumping in!

Agreed, but we already have an empty tank (may require testing), and a regulator in the shop. We also have a small pancake compressor but it's noise will cost us more in hearing protection than it is worth. This machine is a terminal crimper, and it is used in spurts, maybe for a few days to build up stock, and then not used for a few months. If the tank yield is reasonable, the gas consumption cost likely would be minimal.

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#6

Re: How many cycles!

02/19/2017 7:25 AM

Why do you need hearing protection to run a air compressor for a few minutes at a time?

Does your safety inspector not know basic sound exposure regulations regarding intensity and duration?

Don't your technicians know how to add a simple intake muffler to a air compressor?

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#4

Re: How many cycles!

02/19/2017 5:48 AM

Thanks Tornado for the amazingly quick response!

Here are the details I gave, but I could easily be wrong!

The dual acting cylinder measures 3" long x 3" diameter (neglecting end plates and piston) which I think works out to be about 40 CI (.03CF). The tank is listed as 40 CF

Perhaps my reference to 'Cylinder' for both tank and the operating cylinder is the confusion.

Does this help?

Many thanks

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#9

Re: How many cycles!

02/19/2017 12:16 PM
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#7

Re: How many cycles!

02/19/2017 8:02 AM

there are many small compressors under a couple hundred bucks that are actually very quiet.

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#8

Re: How many cycles!

02/19/2017 12:07 PM

By the basic numbers if its a .03 cu/ft stroke volume and double acting and operates at 120 PSI (~8.16 ATM) that works out to,

.06 x 8.16 = ~.5 or about 80 cycles working capacity on a 40 cubic foot source.

Methinks the air compressor is a more practical and realistic choice.

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#18

Re: How many cycles!

02/20/2017 4:09 AM

You said "Methinks the air compressor is a more practical and realistic choice."

I (and many others here I am sure) would agree....

Also, when to order the new Nitrogen cylinder is the next "topic" I would guess!!!

All a waste of time....he should use a small but quiet compressor....and if there is a large receiver available, and its fittings are properly sealed, maybe the pressure will hold over several usages...who knows?

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#10

Re: How many cycles!

02/19/2017 12:28 PM

A 20 cubic inch motor would displace ~328 cc's, this is huge to be air driven...are you sure you don't mean .20 cubic inches?

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#11

Re: How Many Cycles?

02/19/2017 2:21 PM

Wow..Such responses!

I am now in a vortex with all these numbers and getting really dizzy!

In the most basic thinking.... Tell me what is wrong with this logic.

The cylinder dimensions are the exterior dimensions of the cylinder, which will be more than the actual cylinder volume since wall thickness, end plates and rod area are not being considered.

If the cylinder is 3" diameter and the stroke is 3"

then V=(Pi * (R^2)) --- V= (3.14 * (1.5 * 1.5)) = 21.2 Cu In

The gas tank is 40 Cu Ft @ 2200 PSi

then Cu In=(CuFt * 1728) -- 40 * 1728=69120 Cu In

Since the cylinder is double acting, it will require 42.4 Cu In per cycle

then 69120/42.4=1,630 operations to empty the tank.

At some point before the 1,630 cycles, the pressure will drop to an unusable level. This probably is not a reasonable approximation since each cycle changes the tank pressure and volume a bit. I guess the error is assuming that the volume of the tank verses the volume being used will be linear over the pressure change.

At 2200 psi the tank would contain 40 CuFt, but as the pressure drops, the volume also changes...but by how much?

I don't believe that CFM is of interest since there is no 'recovery' from the cylinder as in a compressor.

Since we do have a compressor, there is always an option to use a 100 ft. hose and put it outside, but right now it is felt that the nitrogen would be a more acceptable solution. As said, we have the empty tank, regulator, line, fittings... so the quick answer might be to fill the tank, lash it all up and just see how it goes,

...but this is more educational...I think?

Side note:

My forte is electronics, so I am dragging through this with locked brakes!

Thanks All!

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#13

Re: How Many Cycles?

02/19/2017 3:24 PM

I'm not sure how cycles equates to RPM's but seems this motor must run very slowly to last more than a minute per tank.....

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#14

Re: How Many Cycles?

02/19/2017 4:11 PM

This is not a 'MOTOR' it is a pneumatic cylinder!

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#17

Re: How Many Cycles?

02/19/2017 6:29 PM

OK well with the nitrogen you won't have to worry about water...I would go with a few cylinders though...unless time is not an issue....

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#15

Re: How Many Cycles?

02/19/2017 5:36 PM

Sort of.

You're forgetting that at 120 PSI you essentially have 8.16 times as much air compressed into the same volume as is at normal 0 static ambient pressure.

so 42.4 x 8.16 = 346 cubic inches of air used per cycle.

1728/346 = 5 cycles per cubic foot or theoretically ~200 cycles per tank.

Add in airline and valving losses with each cycle and you might be lucky get 150 cycles out of a tank.

Toss in time and resources required to top off a bottle form a larger one (K @ ~228 or T @~304) that in itself will see a moderate loss of volume and pressure and you start playing a rather quick game of rapidly diminishing returns for the overall work involved.

Maybe 150 cycles for the first tank. 100 - 120 for the next refill and 60 - 80 for the next and so on.

Burn through 2 - 3 K or T size tanks at typical welding gas supply dealer refill costs and a small air compressor and 100+ feet of 1/4" - 5/16" air line starts looking pretty cost effective.

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#16

Re: How Many Cycles?

02/19/2017 6:19 PM

Hummm.....The devil is in the details!

Thanks all!

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#20

Re: How Many Cycles?

02/20/2017 7:20 AM

Surprisingly for TCM the above calculation does not include the 2200 psi of the tank.

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#21

Re: How Many Cycles?

02/20/2017 8:22 AM

Okay? what relevance does the pressure the 40 cubic feet of gas in the storage tank is under play in the equation?

Are you assuming the tank/cylinder is 40 cubic feet (~300 US gallons liquid capacity) in volume and pressurized to 2200 PSI rather than it being a common JC size high pressure cylinder as I based my calculations on?

compressed-gas-tanks/tank-sizes

Just for a heads up I used to work at a welding supply center and by common terminology used in that industry the cylinder volume ratings are based on what their contained gas would be at STP. Not what the actual physical dimensions of the tank/cylinder is.

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#22

Re: How Many Cycles?

02/20/2017 9:39 AM

You're probably right, I did assume that a 40 cubic foot tank meant a 40 cu foot tank. He did mention in one post that they happened to have such a tank on sight, and, I can't imagine that that would have been worth mentioning if he'd been talking about a 2 foot cylinder.

Incidentally (though I can't claim I knew this), over here one of those 2 foot cylinders would be called a 9 liter gas bottle.

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#27

Re: How Many Cycles?

02/21/2017 3:31 AM

I may have gotten it right on the tank pressure end (2200 psi), but Tcmtech got the 120 psi side better than I did. In both cases, it is the difference between scf (standard cubic feet) versus acf (actual cubic feet).

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#23

Re: How Many Cycles?

02/20/2017 10:15 AM

If nitrogen is used in a confined area, you must also consider oxygen displacement.

Spring a leak and you could have a dangerous situation in the workplace

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#24

Re: How Many Cycles?

02/20/2017 12:06 PM

if your crimping machine's air cylinder is not rated for nitrogen you could dry out its seals and cause it to need a rebuild. learned this the hard way.

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#25

Re: How Many Cycles?

02/20/2017 2:39 PM

Good thought about the seals being affected by the drier nitrogen. Since the manufacturer recommends a Filter/Regulator/Lubricator in the supply line the drying issue should not be of a concern.

Also the amount of nitrogen to be exhausted in the size of the room it will be used in is probably negligible. The room is also joined to two other rooms with free ventilation, but I'll keep that danger in mind.

The crimping machine, for anyone interested, is an imported table top unit, that is designated AM-10, and it is widely available from many suppliers on-line.

Since there seems to be several opinions about how long a 40 Cu Ft tank might last with this project, and I have just about everything needed, I'll get the tank filled, and give it a try, then post back the score.

If the facts and the theory don't agree, throw out the facts!

Side Note:

I am on other forums and I must say that the response here has been outstanding!

Thanks!

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#26

Re: How Many Cycles?

02/20/2017 6:33 PM

Kind of amazing what sort of help you can get from technicians and engineers if you just give them some realistic numbers and intentions for a a design to work with.

(kinda rare to get that level of input to work with here.)

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#28

Re: How Many Cycles?

02/23/2017 11:28 AM

Hi I just looked at that AM-10 unit and for the price of \$230 US dollars, I seriously doubt that the controls are of any quality, So expect some leakage into this.

On the size of units air cylinder don't forget that to figure volume You (or Us) also have to figure Rod size on the return stroke. So using your number of 20 Cu In. it may only use 15 on return stroke. & the return stroke may not need 120 psi. to return it or it may have spring assist.

Rating is .4 - 1 MPA for that unit.

Simply put your .03 cu.ft. at 1 atmosphere takes (8 Times as much air) 8 x 14.7psi = 117.6 (within your usable pressure range) so each cycle is now .24 CuFt. or only 4 cycles per cubic ft. thus 40 foot tank 160 strokes or 80 foot SCUBA tank +320 as they are +3000psi.

with expected leakage do 120 cycles at once maybe ok.

do 50 and take a lunch & leave the tank on. plan on doing the rest after the tank is refilled.

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#29

Re: How Many Cycles?

02/25/2017 7:41 PM

Sorry about the spacing, did not find a way to fix it from a cut and paste!

I found an online gas calculator here: http://www.cmwelding.com/technical/gas-calculator-gas-phase

:

:

Tank 40 Cu Ft / 2200 PSI

Temperature (STP): 70 deg F (at Standard Temperature & Pressure)

Product: Nitrogen (N2)

Pressure: 2200 psig

Water Volume: 40 cu.ft.

Temperature: 70 deg F

Density: 10.67126 lbm/ft3

Density (STP): 0.07244 lbm/ft3

Compressibility: 147.306

Product Volume: 5892.25 scf

Product Weight: 426.85 lbs

:

:

Cylinder = 3" bore, 3" stroke, Dual acting

Temperature (STP): 70 deg F (at Standard Temperature & Pressure)

Product: Nitrogen (N2)

Pressure: 120 psig

Water Volume: .2 cu.ft.

Temperature: 70 deg F

Density: 0.66512 lbm/ft3

Density (STP): 0.07244 lbm/ft3

Compressibility: 9.181

Product Volume: 1.84 scf

Product Weight: 0.13 lbs

:

:

Cycles = Tank Product Volume / (Cylinder Product Volume*2)

Cycles = 5892 scf / (1.84scf*2) = 1601 Cycles Approximation!

Opinions!?

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#30

Re: How Many Cycles?

02/25/2017 8:56 PM

Please confirm Your Supply tank/(Cylinder) size. Is it smaller or larger than a SCUBA Tank?

Your above calculation has once again used supply tank as being large enough to hold 40 Gallons of water & I thought we settled this.

If it is as large as a person, then #1 answer wins.

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#31

Re: How Many Cycles?

02/25/2017 11:14 PM

No! The Cu Ft refers to the volume of STP gas compressed into the cylinder at a given pressure, not the cylinder's physical dimensions. A cylinder of gas, compressed to higher pressure, contains more gas volume at STP.

Using the calculator I linked to:

A 40 Cu Ft tank of nitrogen compressed to 2200 PSI has 5892.25 Cu Ft of gas at STP

The same cylinder compressed to 3000 PSI has 7750.55 Cu Ft of gas at STP

The cylinder didn't change it's size

This cylinder is overall 22.5" tall and has a 7" diameter. Certainly it cannot contain 40 Cu Ft of water, at STP, or any other pressure.

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#32

Re: How Many Cycles?

02/26/2017 1:28 AM
 Re: How Many Cycles? 02/25/2017 7:41 PM Sorry about the spacing, did not find a way to fix it from a cut and paste! I found an online gas calculator here: http://www.cmwelding.com/technical/gas-calculator-gas-phase Tank 40 Cu Ft / 2200 PSI Temperature (STP): 70 deg F (at Standard Temperature & Pressure) Product: Nitrogen (N2) Pressure: 2200 psig Water Volume: 40 cu.ft. Temperature: 70 deg F Density: 10.67126 lbm/ft3 Density (STP): 0.07244 lbm/ft3 Compressibility: 147.306 Product Volume: 5892.25 scf Product Weight: 426.85 lbs.

Just wondering are these not the Above specs are from your last post. Input error?

Water Volume: 40 cu.ft. & Product Weight: 426.85 lbs Should be a hint something is wrong.

your weight should be less than 30 pounds. not 426.85 lbs.

another note; If you have a 40 cuft tank THAT size it will hold, lets just say 40 CuFt. at design pressure

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#33

Re: How Many Cycles?

02/26/2017 2:23 AM

Here comes that vortex again! In an effort to simplify the complicated, the simple has become complicated! I'll need to revisit this whole thing with a load of caffeine, but, I will be back!

Thanks

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