Welding gas tank sizes

I order tanks from my supplier by: "Little", "Small", "Medium", "Big" and "Really Big" I hope that clears it all up for you!

Most people would have a fit if they found out they had to pay a lot of money, and lug around a 50 Kilo cylinder to get

10 Grams of Hydrogen, for instance. It might not be such a shock with Argon, but there still is very little in that big bottle. Only the gases that condense to liquids under pressure get a sizable mass in the bottle.

Jon

No, atmospheric pressure. The tank contains 135 cu ft of free gas, in compressed form.

Shades of the air compressor war-thread. Remember? Air compressors are rated in cubic ft/minute of free air, not compressed air. Some esteemed r.c.m colleagues could never grasp that.

An 80 cubic foot scuba tank has a volume of 0.4 cu ft, containing air compressed to about 200 bar.

IO was refering to that volume stuffed into the tank.

But you are indeed correct.

Gunner

When I was in college I learned that if I read a passage in e.g. a physics book say 8 times and didn't understand it, it wasn't my fault, it was the author's. I simply moved on to another author covering the same topic.

It wasn't so much that we couldn't grasp it, Richard. I think rather it was that some people couldn't write it up in such a way that it made any sense. I went looking for an alternative writeup explaining your position. Problem is I couldn't find it.

I'm not trying to be confrontational. I'm still puzzled over that business.

GWE

Welcome to the USA where we have lots of seemingly arbitrary schemes of measure, most of which we got from the Brit's at some point and have perverted to better fit our needs for many years now. As far as patronizing twats w/ condescending tones go welcome to RCM. I would ponder that an Egg would have a shell thick enough to deflect a few barbs. Egg by the way nice web page I stumbled across it looking for belt grinder plans/ideas.

Enjoy

AndrewV

thanks!

Indeed! Bookmarked!

Thanks Egg

Gunner

If there are two alternative conclusions to choose from (for example, the volume specification being compressed vs free), then I don't see how imperfect explanations should spoil the analysis. One alternative leads to absurdities, the other fits various observations. Case closed.

Yes, but which is which? :-)

GWE

Case closed? Sounds more like "I'm right, just accept it". You are right, but let's see if we can clear the mystery for GWE. I don't think it's a political or religious issue here. GWE, pls say again what doesn't make sense to you.

Thanks, Don. Let me briefly restate the issue: air compressors are specified usually at two given CFM / psi pairs. For example, 18 CFM @ 90 psi and 19.6 CFM @ 45 psi. It has always been my understanding that these specifications are actual CFM delivered at the specified operating point. Mr. Kinch says no, those are not ACFM numbers, those are SCFM numbers, and to convert to ACFM numbers you have to do some algebra. I understand that SCFM numbers are specified at 14.7 psia, 60 degrees F, and 0% relative humidity. I just don't know why he says every spec is SCFM, and if that is true, how to convert to ACFM.

By the way, I looked up the actual Quincy spec for my particular compressor, a D-325 with 5 hp motor. Quincy is aware of the confusion and explicitly says their spec is ACFM at 175 psi. In case anyone wants to independently verify this, you can either to to quincycompressor.com and fill out their obnoxious form to be able to download it, or you can look at the one I stuck here:

The specification I'm citing is on page 7, for the D325 two stage pump driven by a 5 horse motor at 796 rpm.

I just don't see how anyone can look at that spec and tell me it isn't the CFM at 175 psi they're talking about.

The West Coast application engineer for I-R air compressors also replied to this thread, and said that IR specs are also ACFM at the specified pressure. This made no difference to Mr. Kinch, whose beliefs are carved in granite and just about as transparent.

If I'm wrong, I'm perfectly willing to accept that. I'm wrong all the time, and it doesn't affect me very much. I just don't like being told to accept hand-waving and to believe it on faith.

Grant Erwin Kirkland, Washington

I'm probably going to regret this ...

First off, it's energetically impossible to produce 17.4 CFM of air at

175 psi with 5HP.

Second, by comparing the specs for single stage compressors to the 2- stage compressors on that page it's obvious ACFM refers to flow at atmospheric conditions. The single stage D210 is rated at 100 psi and 2- stage units are rated at 175 psi. The ACFM per HP (approx 3.3 ACFM/HP) is more or less constant across the whole range, despite the difference in ouptput pressure.

I don't know why Quincy uses ACFM (without defining the precise conditions) rather than SCFM.

This page compares ACFM and SCFM.

Ned Simmons

The "actual" versus "standard" in ACFM vs SCFM refers to slight variations in performance due to actual ambient pressure and temp being different from the standard.

I am not sure what the dispute is with my analysis. Are you suggesting that ACFM means output volume, not input volume?

The Quincy document you cited, on page 5, for your model D-325, sez that the compressor delivers 18.64 ACFM at 175 psig given max rpm.

Now if you think that means that there is 18.64 cubic feet of flow *out* of the compressor, then that's wrong. The compressor takes *in* 18.64 cubic feet of free ambient air at atmospheric pressure, compresses it, and delivers a very much smaller output volume at high pressure. The output volume is roughly (175-14.7)/14.7 = 11 times *smaller* than the input volume due to the compression of a factor of 12 from atmospheric pressure. Thus the output volume is roughly 18.64/11 = 1.7 cubic feet per minute.

On expansion, this compressed output volume of 1.7 cubic feet per minute ultimately expands and exhausts back to 18.64 cubic feet per minute of free air again, performing 5 HP of work by virtue of the expansion.

Look at the specs again on page 6 for the model 325. At 796 rpm the cylinder displacement is 22 CFM and the output is 17.40 ACFM. The small difference is the piston compression inefficiency (headspace, imperfect valves, etc). You're taking in 17.40 cu ft per minute of free air and compressing it to about 1.6 cu ft/min output volume at up to 175 psi, not outputing a compressed volume of 17.5 cu ft/min.

That's about as clear as I can make it.

If you think some I-R engineer doesn't agree, one of you is confused.

Just a hack's opinion, but it would seen that the guys that quote ACFM would have to have a pile of different motor pulley combinations available, or some pretty fancy sensor technology built in to the machines to compensate for the variables in use that all seem to affect the AFCM/SFCM differences. Or they would, as stated above, have to define some conditions at which those ACFM were being delivered. Or have I missed something.

For the sake of the discussion, just how much difference does AFCM vs. SFCM make in the grand scale of things? Much? Not much? (not enough math in me to sit down and pound my way through the numbers)

As the saying goes, the great thing about standards, is that there are so many to choose from!

Cheers Trevor Jones

OK. One thing I do understand thoroughly is Kirchoff's law which states that the amount of free air going in has to equal the amount of free air coming out, since the machine obviously isn't manufacturing air.

So this entire thing has been because of the misunderstanding about whether the air hadn't yet expanded or not? Wow. Amazing how simple it really is. When someone takes the time to explain what they're talking about.

Thanks, Richard. I really did get a ton of use from your soda water Web page. That was a real gem.

Grant

Ned cleared up ACFM vs SCFM: minor differences in barometric pressure and outlet air temperature. The air coming out of a compressor is about always somewhat warmer than the temp defined for SCFM. I'd wondered about that too, thanks Ned.

Richard addressed the other points correctly but I'll summarize a bit anyway, merely saying what he said in a slightly different way.

The displacement CFM for your pump is 22 CFM. If it takes in 22 cubic feet of ambient air and compresses it to 175 PSI, the process of compressing it must make the volume of compressed air much less, right? It's being compressed! To raise a volume of air at atmospheric pressure (14.7 PSIA) to 175 PSIG or 190 PSIA, if the temperature were constant it would need to be compressed to 14.7/190 (7.73%) of original volume. Boyle's law, which basically says that compressing air to smaller volume (at same temperature) increases pressure. More on Boyle's law at

The temperature isn't constant in an air compressor, which may be why they now use ACFM rather than SCFM, but that clearly is not where your issue lies. It's a minor detail in the context of this discussion.

Your pump delivers 17.4 ACFM, which is clearly a whole lot more than

7.73% of the 22 CFM it takes in, more like 80%. It clearly can't be delivering 17.4 cubic feet of 175 PSIG compressed air per minute if it's only taking in 22 CFM and then compressing it to 175 PSIG, because it must compress it (reduce volume) significantly to raise the pressure. It's delivering air at 175 PSIG which will become 17.4 cubic feet per minute of exhaust air after it expands again to atmospheric pressure while running thru a air tool or whatever. (Not strictly correct because of some temperature issues, but it's the general idea.)

Let's look at the dual specs you mentioned: 18 CFM at 90 PSIG and

19.6 CFM at 45 PSIG. The CFM's differ little even though the pressures differ significantly. A pump must compress air a lot more to get to 90 PSIG than to get to 45 PSIG, so the delivered volume of compressed air at pressure must be correspondingly less.

Are we making any sense yet?

Actually, it is not Kirchovian. Some "air" disappears, since part of the water vapor (humidity) is condensed out and stripped from the output. Winds up as liquid puddled in your tank, or captured by a drier, or spitting out the delivery line if you have a faulty piping design.

Realistically it's even worse than that.

Heat is rejected by the compressor, cooler and tank. The air that comes out of the exhaust of the tool is cold, even with an inefficient tool. This implies that less work is being extracted from the air than was originally put into it.

At our factory, we reckon that you get about 1hp out of air tools for 4hp into the compressors.

Mark Rand RTFM