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Monday, January 30, 2012

Excercise for you physics boffins.

In my new hobby gig as a hose-dragger, I've been practicing SCBA drills. We've been changing bottles in the dark or with a blinder mask on, donning and doffing at speed, going through obstacle courses with wax paper in our mask plates, etc. At the end of each exercises, you go to the compressor and refill the tanks. This is a fairly straightforward process, but one can't just dump the air from the waiting storage tanks in all at once, or the SCBA tanks get hot, and the air doesn't seem to want to go in. I mentioned the issue of the liquid boiling up, and one of the younger firefighters said, "liquid? It's just air. There's no liquid." I tried to explain that, thanks to Charles' Law (and Boyle's Law), gases under enough pressure do go to liquid state*, but he didn't get it.

We use the Scott air packs with carbon fibre tanks, and I hadn't realized just how awesome they were until we did some training with a neighboring agency that uses old Scott air packs with old steel tanks. The carbon fibre tanks are slightly larger than a 3-litre bottle in diameter, and a bit longer. They are filled to 4500 psi. The steel tanks are much larger --about 11" in diameter and a few inches longer-- and weigh about 4 times more. The steel tanks only go to 2500 psi.  Here's the thing: both hold 45 cubic feet of air.

Interestingly, both packs will take each others' bottles, but the more modern carbon fibre bottles have to be bled down to 2500psi before being put on the older packs. Better than nothing, but not good. Given the decreased volume, 2500 psi on one of those is not much.

Supposedly, 45'^3 of air is good for thirty minutes. In my experience, I get about 10 to 15 when laboring. Hey, I'm a big boy.

While refilling the carbon fibre bottles, the same young guy gets really frustrated at how very long it takes to get from 3800 psi to 4000psi, which we've set as the minimum amount that the bottles must be filled. The large electric compressor labors for a long time, long after the storage bottles are depleted.
I told him that I am quite sure that the mass of air stacks with the volume-- it's not a linear progression for amount of air inserted for a given volume over a given time from 2500 to 2700psi, say, as it is to 3800 to 4000 psi.

So here is my question, and Stingray, take note:

For a given cylinder of, say: 6" X 21", what is the amount of volume of air (at STP) being inserted between between 2500 psi and 2700 psi?

Then, what's the volume of air (at STP) being inserted between 4000 psi and 4200 psi?

Express your answer in equivalent cubic feet, at STP.

________________________________
*EDIT: Though, probably not in this case. Consensus seems to be that the pressure isn't high enough to achieve liquid air, in these bottles.

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22 Comments:

At Monday, January 30, 2012 2:10:00 PM, Blogger Alan said...

I was told there would be no math!

 
At Monday, January 30, 2012 2:27:00 PM, Blogger ZerCool said...

Sorry, but the young'n was right. No liquid in an SCBA tank, just a lot of tightly-packed air. CO2 is one of the only gases I'm aware of that is routinely transported as a liquid. (In user-sized containers, that is.)

 
At Monday, January 30, 2012 2:47:00 PM, Anonymous Anonymous said...

I could probably do the math, after looking up various equations and playing with the numbers for awhile, but work would probably like me to do some actual work.

However, that is not why I wrote. I don't think the air goes to actual liquid in the tank. I think it is still just compressed. If it DID go to liquid, I don't think the regulator would work very well.

But you have me intrigued and I am going to do some checking.

D. Scruggs
Capt, Boulder Creek Fire

Sr. Sw. Engineer
Lockheed Martin.

 
At Monday, January 30, 2012 3:57:00 PM, Blogger Matt G said...

Given that I've never actually felt it flop in these tanks as a liquid, y'all are probably right. I had assumed that those tanks were liquid when at fullest, and evaporated in the tank at some pressure.

Okay. Matt was probably wrong about that (there are two pressure regulator step-downs before the gas is breathed, though, so I thought that perhaps that was when it would change.). The valve on the tank is on the bottom, which would tend to cause it to START with the liquid. But hey, it seemlessly goes from 4500 psi to +1 psi (above 1 atmosphere), so maybe it's just that marvelous.

A full tank is notably heavier than an empty one.

 
At Monday, January 30, 2012 3:57:00 PM, Blogger Kevin said...

You left out some necessary information: what is the temperature of the air in the tank? As noted, pressurizing a volume of air causes a temperature increase.

Now, if you postulate holding the gas temperature fixed, the relationship is, in fact, linear per the aforementioned Boyle's Law: P1V1=P2V2. 45'^3 at standard atmospheric pressure (14.696psi) and temperature (288.15°K) to 4000PSI at standard temperature requires approximately 0.17'^3 of volume (to two significant figures accuracy).

Hold the volume constant, and an increase of 200PSI is the addition of approximately 2.2 cubic feet of air at STP.

On a standard reciprocating compressor, the compressor cylinder has a fixed displacement defined by the bore and stroke of the cylinder and piston. The compression ratio is the difference between the bottom-dead-center volume and the top-dead-center volume. For illustration purposes, let's set that to 300:1 - e.g. the volume of the cylinder at BDC is 300x greater than at TDC - and set the displacement at precisely .01 cubic foot. With the discharge valve closed, one stroke would drive the pressure up to 300x atmospheric pressure, or over 4400psi. If the attached tank were at STP, that stroke would move almost the entire 0.1 cubic foot of air into the tank, but as the tank pressure comes up, less and less of that 0.01 cubic foot moves into the tank. At 4,000PSI the pressure differential is only 400psi, but the ratio of volume between the two reservoirs is 17:1. When the valve opens and the pressure equalizes, far less than 0.01 cubic feet of air at STP moves from the compressor into the tank, and as the pressure increases, you can see that this transfer must decrease. That's why it takes a lot longer to move 2.2 SCF of air into the tank at 4000PSI than it does at 2000PSI.

 
At Monday, January 30, 2012 3:58:00 PM, Blogger Matt G said...

I'd like to see a graph of volume inserted versus pressure change in the bottle.

 
At Monday, January 30, 2012 4:00:00 PM, Anonymous Evyl Robot Michael said...

Nope, shouldn't be any liquid in your tanks. At my last job, we filled tanks to 6,000-psi air all the time and cascaded between them, and bled them off at full speed with no liquification. The only reason you'll get liquid in a breathing tank is if the purification filters need to be replaced. If the desiccant/dryer compound is saturated, you'll get liquid water in your breathing air. Try being in a fire and feeling like you are in the process of drowning! Apparently that's what it's like when somebody decides to refurbish your purification cartridges with cotton balls because that's what the old media looked like. Or, that's how the story went at any rate. 80

 
At Monday, January 30, 2012 4:05:00 PM, Blogger Matt G said...

Kevin, your point is well-made, and that's actually the main reason that we're doing it slowly, is to give the heat a chance to dissapate. The bottles heat up significantly, and the carbon fibre doesn't seem to want to bleed off the heat as fast as if it were steel, which is a much better heat conductor. By the time we've pumped 4,000 psi into one of them, they are some warm dumplings. Say, 110F in a 50 degree room, after having spent 15 minutes filling them?

 
At Monday, January 30, 2012 7:19:00 PM, Blogger ZerCool said...

Forgive my brevity (and perhaps brusqueness) earlier - I was hammering that out on a mobile while waiting for the Mrs to get out of work.

I did consider the possibility of things going liquid in the SCBA tank, and came up with one HUGE reason that it's highly improbable:
An SCBA must provide air to the user through 360 degrees of orientation.

In other news:
- wrapped tanks are a godsend for the back. And the low-pro 4500psi bottles are great for maneuverability but actually aren't much lighter than the wrapped 2216s. (They are both, as you noted, MUCH lighter than steel. They also have to be hydrotested more often - every three years instead of every five. A small price to pay for my back.)

- 15 minutes is a pretty average estimate on how long that bottle will last for most. My old department did some consumption drills involving actual work while dressed up, and most of us were in the 15-18 minute range. One member who is in disgustingly good shape (century bike rides are a few hours diversion in the afternoon) went near 40.

I'm presuming your packs have vibra-lert regulators - keep in mind what that means knowing your consumption rate.

(And by the by, I am thoroughly enjoying reading these FD updates. It's a pleasure seeing it from rookie eyes again.)

 
At Monday, January 30, 2012 7:19:00 PM, Blogger BCFD36 said...

Matt,
ANON here again.

1. To get the gas to go to liquid, you have to cool it after it has been heated due to the compression. With a standard SCBA compressor, this doesn't happen and shouldn't happen.

2. You want to fill your bottles slowly. If you don't it heats up the gas a bit. If you fill it TOO fast, it heats it up a lot. If your tank is hot, when it cools the pressure will actually have dropped in the tank. I've seen drops of 50 and 100 lbs after the tank cools down. At that point the transgressor gets re-edumicated.

D. Scruggs
Capt, Boulder Creek Fire

Sr. Sw. Engineer
Lockheed Martin.

 
At Monday, January 30, 2012 7:28:00 PM, Anonymous Anonymous said...

When filling SCUBA tanks (25 years ago), we used to put the tanks in a water bath prior to filling. It was just an old 55 gallon plastic drum we cut down a bit. Heat dissipated faster that way, and the water would cofferdam the blast if (when) someone's old tank would let go, after which we'd have to get a new water bath again.

Don't know if it'd be a good idea for carbon fiber tanks, but I wouldn't want to be the one to field-test the new procedure on 4000 psi tanks. If the tanks weren't fully submerged, they might have different amounts of elasticity or something. In fact, forget I suggested it. Please!

FormerFlyer

 
At Monday, January 30, 2012 7:30:00 PM, Blogger Old NFO said...

Actually you're also getting into thermodynamics and the heat generated by the friction of the compressed air as it's forced into the tank and the viscosity of the molecules as they 'rub' against each other and the sides of the tank (at 59 deg F) (and it's never liquid). Kevin's discussion is correct.

 
At Tuesday, January 31, 2012 12:17:00 AM, Blogger Kristopher said...

Your tanks have liquid in them because the air that was pumped into them was humid. The moisture immediately condenses to water at pressure.

You need a dessication system in your area.

If you can feel water sloshing, you need to have the tanks opened, cleaned out, and inspected for rust if they are steel.

You can avoid this by having a dive shop do a Normoxic fill ( using pure O2 and N2 to make artificial air ).


Only CO2 tanks normally have room temp liquid gas in them, which is why you never leave a CO2 tank on its side ... the liquid CO2 will eat your regulator if it flows through it.

 
At Tuesday, January 31, 2012 8:12:00 AM, Blogger Matt G said...

@ Former Flyer:
I would be hesitant to immerse the carbon fibre tanks; the resin around them might be chipped in places, and I don't want to think about what happens when too much unnecessary moisture gets in there, for the inevitable expansion and contraction cycles, which will include freezing, given that the tanks sit on the trucks, go out in all weather, and get very cold when they've been sucked dry. When filling the tanks, they sit in vertical steel cylinders. If they go, the blast goes up in a cone from the top of the cylinder (mostly). If you're over that, then I guess it was your time. :) Bottles sitting in the vertical steel cylinders is the signal for: "These are empty; refill it, please." I gather that everyone is expected to drop what they're doing and do just that, if they see them in that condition. Especially rookies.

 
At Tuesday, January 31, 2012 8:20:00 AM, Blogger Matt G said...

@ BCFD36:
Dave, I'll own that I made a stupid assumption, and I'll make the point to the young'n that I was wrong. I will then go to the dentist, do deal with the loss of enamel on my teeth. ;)
You're exactly right that it has to work from all positions. During the obstacle course drill that I was on the other day, I was upside down, passing the pack through 11" spaces at oblique angles, etc. I should think that there's a way to convert liquid to a gas in a step-down regulator expansion chamber, then have a second one for when all the liquid air has been made gaseous, though. How do the astronauts do it?

 
At Tuesday, January 31, 2012 8:26:00 AM, Blogger Matt G said...

@ OldNFO:
I didn't forget that part. I actually started a paper back at UT in physics on (and this dates me) the friction of liquid helium in the cooling system of the SuperConducting Super Collider (SSC). I worked on it awhile, and realized that I didn't yet have the math and physics chops to attempt such an ambitious project, and aborted it to write a paper on how the SSC would probably prove the existence of the Higgs Boson. Sadly, we threw the baby out with the bathwater in 1993, and the SSC tunnel is now a 54 mile storage facility for junk. :(

 
At Tuesday, January 31, 2012 3:13:00 PM, Blogger Old NFO said...

Matt, I figured you had considered it, and just 'ignored' it for the moment... Flyer is right, that is why we put steel tanks in a water bath. And I humped those SCBA rigs with the steel tanks for almost 10 years as a volunteer and NEVER got 30 minutes out of one... sigh

 
At Thursday, February 02, 2012 3:18:00 PM, Anonymous Blackwing1 said...

Matt:

Looking at rough approximations of the air density at the four different pressures, you can calculate the difference in mass of air between them. The volume of the tank is about 593.8 cubic inches, equal to about 0.3436 cubic feet. Take the delta in density times the volume, and you have the weight. With air at STP (59F, 60%RH, 14.696 psia) having a density of 0.076182 lb/ft3, divide the weight by the density and you get the STP volume difference in the tank.

As follows:
2500 psia: 12.619 lb./ft3
2700 psia: 13.628 lb./ft3
Weight change = 0.3467 lb.
STP volume change: 4.55 ft3

4000 psia: 20.191 lb./ft3
4200 psia: 21.200 lb./ft3
Weight change = 1.009 lb.
STP volume change: 13.24 ft3

You're getting almost three times as much air in at the higher pressure. At least, that's my shot at it.

How are they getting rid of the water created by compressing the ambient air? You've got to have some kind of drain for the liquid, don't you? Then I'd also guess that you wouldn't want air at saturation inside the tanks, so there's probably some kind of water-vapor dryer on the outlet line, too?

 
At Thursday, February 02, 2012 3:21:00 PM, Anonymous Blackwing1 said...

Of course, all of the above assumed that time was left to allow the tanks to cool to a constant 75F for those densities to be correct.

Can you really tell the difference between a filled tank and an empty one? I'd guess a total difference in weight of only about 7 pounds or so.

 
At Thursday, February 02, 2012 4:54:00 PM, Blogger Matt G said...

We've got a fancy-Dan compressor that actually dehydrates the air.

 
At Thursday, February 02, 2012 4:59:00 PM, Blogger Matt G said...

"Can you really tell the difference between a filled tank and an empty one? I'd guess a total difference in weight of only about 7 pounds or so."
Sure. Those carbon-fibre tanks only weigh about that themselves. I don't think that the 45'^3 of air weighs quite 7 lbs, though. Thought that it would. But it's noticable that during drill when they're trying to trick me by having me put a new tank on an SCBA in a dark room or with a blinder on, I can tell by picking it up that I've got an empty or nearly empty tank.

 
At Friday, February 03, 2012 9:18:00 PM, Blogger Mr. Fixit said...

Matt, I used to be a Scott Tech at my FD. I think I have a chart with lots of the info you are wondering about, but its at work. I'll try to find it next week, since I'm 500 miles west right this minute.

 

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