Math... need... Help!

NYSpeed Automotive NYSpeed Off Topic
1

Okay.
So I’m sizing compressors.
Crunching numbers here, but this doesn’t seem right at all.
Does anyone know if there’s a calculator to figure this out?
I tired digging one up, but all I can find are ones you have to pay for.

Using conversions from http://www.pnconline.com/conversion.htm#S

cubic foot per hour = 0.124676 x gallon per minute
cubic foot (US) = 7.481 x gallon

I’m going to be using an item requiring 21SCFM @ 80PSI
The Compressor I have sized is 60 Gallons @ 165PSI
Producing 15.8CFM @ 90PSI

However, I am mainly looking at the run time… if the pump is NOT RUNNING.
So more or less, that last line of info I gave is not needed, but I gave it anyway… In case someone wants to factor that in (provided it turns on at approx 2/3 tank pressure or so)…

So, here’s my math.
However, I really, really don’t see how there is 7 cubic feet in one gallon. I really think I’m misunderstanding that. I also don’t see how one gallon fills 7 cubic feet… soooo… Yep!
But I did it anyway.

60Gal x 7.481 = 448.86SCF (Tank max cap)

448.86SCF / 21SCF = 21.37minutes (run time)

But, that’s not accounting for the time that the tank is below 80PSI.
Since 80 is half of 165 (almost), I figure that would equate to about a 10 minute continuous run time.
Is… this right?
It doesn’t seem right… So that’s why I’m posting here asking for input!

2

you can fit x cubic feet into a gallon by pressurizing it.

otherwise, good luck

3

The compressor should have specs saying what its max cfm is at its max duty cycle.

4

Yeah, but I’m trying to figure out what it is, provided the compressor is not running.

5

this reminds me, where the hell is newman

6

california…

that doesnt work because when the units cancel there is nothing left. its just a number. you would need to have minutes somewhere is the equation

this sounds like a calculus/derivititave/rate of change problem.

7

well obviously there’s no minutes…
o.O
I’m sure there’s some complicated formula to figure it out.
But I don’t know it :stuck_out_tongue:

8

1 cubic foot = 7.48 gallons. i think you have that backwards as you said here

it should be
60Gal/7.481 = 8.02SCF (Tank max cap)

8.02SCF / 21SCF = .38minutes (run time) so 23 seconds?

if i correctly understand what you are doing

9

Yeah, that’s what I was figuring.
But if you take a 12"x12"x12" box… and hold it up to a milk jug…
I don’t think it really takes 7 and a half milk jugs to fill such a box.
That’s why I’m confused as fuck.

edit;
actually i just went and did it…
I guess it seems possible.
meh.

10

Do you actually have the compressor to run tests with?

I can tell you right now 21 cfm’s at 80 is alot of air.

I have an excel document I made based on a page:
http://www.truetex.com/aircompressors.htm

Which if I plug in your numbers gives me just over 130 seconds before the pressure is under 80psi.

But I don’t think this is accurate because it is supposed to be the scfm the pump is pumping and the time is supposed to be for the pressure to increase from 80 to 165, not discharge.

23 seconds sounds alot more reasonable.

11

why do the compressors need to be off? what exactly are you running that needs that much cfm?

12

No sir, I’m sizing them right now. I want to try and make sure I get one big enough.
I don’t want to dump 1k in a compressor, only to find out it can’t even keep it running… Then I’m stuck with a 1k compressor, that will sell for maybe half of that, then I have to upgrade to something higher.
Rather do it right the first time.
Unless I can hook two compressors together… ooh… not a bad idea…
lol.

and yeah, it is a lot of air -.-

ArcAir K-3

https://www.mythermadyne.com/thc/en/US/adirect/thc?cmd=catProductDetail&_bcs_=2000106400301064003https%3A%2F%2Fwww.mythermadyne.com%2Fthc%2Fen%2FUS%2Fadirect%2Fthc%3Fcmd%3DcatDisplayStyle%26OP%3D_bcc_%26catKey%3D600892%26_bcs_%3D-1%2508%2523%2523%2508%2523%2523%2508https%253A%252F%252Fwww.mythermadyne.com%252Fthc%252Fen%252FUS%252Fadirect%252Fthc%253Fcmd%253DcatLanding%2508%2523%2523%2508true%2508%25070%2508600892*%2508Arcair%2508https%253A%252F%252Fwww.mythermadyne.com%252Fthc%252Fen%252FUS%252Fadirect%252Fthc%253Fcmd%253DcatDisplayStyle%2526DispStyle%253DcatLanding%2526catKey%253D600892%2508%2523%2523%2508false%2508%2507100%2508K-3%2508Search%253DK-3%2508%2523%2523%2508%2523%2523%2508false%2508%23%23true20001064003K-3+Extreme+Straight+Handle+Manual+Gouging+Torch+-+Medium+Duty%23%23%23%23false&showAddButton=true&srcApp=&productID=01064003&tabName=OverView

                                                                                                                                                                                                             Flow Rate: 21 cfm (0.60m3/min)         Pressure: 80 psi (5.6kg/cm2)

edit;
and they can be on.
that’s fine.
just tougher to calculate.
and i don’t know when it turns on, even.

13

I’d go bigger than that one you are looking at I think we can all agree from the numbers.

If it SAYS it will do 16 @ 90 there’s no way it will ACTUALLY do 21 @ 80.

14

so its like a plasma cutter but with air instead? kind of like a cutting torch?

15

Do you have to buy new? What about this one:

http://buffalo.craigslist.org/bfs/1429859430.html

This dude wants $1k

t-30 7.5hp shows 24acfm @ 175 psi
They list for $2k new
New and specs -->http://www.bigbluhammer.com/products/air_compressors.htm

16

Yeah, that’s why I was wondering how long the 60 gallons would give of run time.

Kinda. It’s more used for gouging though. It basically is a reverse stick welder?
Instead of the rod welding it, it superheats it up and the air blows it away. It’s mostly used to remove weld. Or to cut through multiple plates stacked on top of each other. (Where as a plasma cutter or oxyfuel torch would not be able to burn through any of it, since it would only heat up the first plate and not even be able to get the lower plates to cutting temp)
It is LOUD.
When I say loud… I mean, FUCKING LOUD.

Plasma cutters used compressed shop air too, though.
Just a lower amount, usually like 5-10CFM @ 90psi.

Damn, that’s a pretty good deal.
Only thing I don’t like about used ones, are the pumps typically are rated around 8k hours. Depending on the model (IR ones are usually 20k).

But I would def jump on that honestly.
Except it’s 3 phase.
My welder (which I would end up running it off of if I were to take it mobile), is only 1phase. =(

(that’s also the FP model, which is 2.4k new :])

Going to look in to being able to rewire it to 230/1/60.

17

right, I knew that. lol. never heard of such a thing.

18

Never? :
It makes life easy when you fuck up :stuck_out_tongue:

19

Soooo.
Do air compressors go on sale on black friday?
lol.
Probably not any of the good ones anyway. -.-

20

I didn’t see anyone catch it, but:
THERE ARE 7.481 GALLONS IN ONE CUBIC FOOT
Not, 7 cu/ft in a gallon. That should help you out