"sn4cktimes" (snacktimes)
09/30/2020 at 15:09 • Filed to: None
0
21|
"sn4cktimes" (snacktimes)
09/30/2020 at 15:09 • Filed to: None | 0
| 21 |
Edit: We have the issue solved! Thanks people!
I’m plugging away at some studying for a test, I’ve done ALL my math, and normally get a really good grade on exams. There’s one practice question I can’t seem to figure out and I’m not sure why. Wondering if anybody here knows how to solve it. I’ve tried working it through a Combined Gas Law equation, tried making the denominators the same in both Celsius and Kelvin, making sure that the psig number is converted to psia, using Gay-Lussac’s Law equation, and some general fiddling with the numbers.
I feel like there’s something stupidly simple I’m missing here. For reference the answer should be 14.247:1 . When I use the CGL equation I can get the numbers... but one decimal place off at 1.4247... but I also made up the final volume of “1". So... anybody? I’m obviously making this far more complex than required.
Q: A compressor has an intake pressure of 14.7 psia at 5ºC and an exhaust pressure of 311.5 psig at 160ºC. Calculate the compression ratio.
E90M3
> sn4cktimes
09/30/2020 at 15:25 |
|
It’s been a long time since I’ve done math, but couldn’t you just use the ideal gas law to find the volume of air at 14.7 psi and 5C and 296.8 psi and 160C and then divide?
ShrimpHappens, née WJalopy
> sn4cktimes
09/30/2020 at 15:28 |
|
This doesn’t help you at all, but thermo goddammits thermodynamics was by far my least favorite course. I’m a civil engineer--give me dirt and water!
Highlander-Datsuns are Forever
> sn4cktimes
09/30/2020 at 15:30 |
|
I get 1.4 as well using deg. C. Normalize 14.7 psig to 160 deg C (470 psia ) then compare the two pressures, 470 pisa /(311.5 +14.7)=1.4
Never mind. I should be working and not trying to remember how to do thermo 25 years after I took the class.
dumpsterfire!
> sn4cktimes
09/30/2020 at 15:33 |
|
I'm 30 years out from maths like that, so I thought I'd chime in that I can't help you.
MasterMario - Keeper of the V8s
> sn4cktimes
09/30/2020 at 15:34 |
|
Probably just a difference in rounding here from the answer.
Daily Drives a Dragon - One Last Lap
> sn4cktimes
09/30/2020 at 15:38 |
|
So combined gas law you’re doing (
(P1*V1)/T1) = ((P2*V2)/T2) and you have a second equation that relates V1 and V2 right?
The only thing I can think of without actually doing the question is that intake is given in absolute pressure and output is given in gau
ge
Darkbrador
> sn4cktimes
09/30/2020 at 15:38 |
|
I think you need to disassemble the compressor and look at the pump.
|
sn4cktimes
> ShrimpHappens, née WJalopy
09/30/2020 at 15:42 |
|
I’m a welder/mechanic so not my jam either. Thanks though.
|
sn4cktimes
> dumpsterfire!
09/30/2020 at 15:42 |
|
Yeah, I’m a “ways” outside of prime school years as well.
|
sn4cktimes
> MasterMario - Keeper of the V8s
09/30/2020 at 15:44 |
|
I did it back-asswards! I solved for V1 not V2. That makes sense now. Thanks.
ibRAD
> Highlander-Datsuns are Forever
09/30/2020 at 15:46 |
|
You got farther than I did. I was the ‘guru’ in my thermodynamics class (also 25 years ago) I got out the textbook a few years ago and apparently someone had stolen my copy and replaced it with a replica copy written in some strange foreign language.
|
MasterMario - Keeper of the V8s
> sn4cktimes
09/30/2020 at 15:47 |
|
Doesn’t matter which way you solve it though.
I’m guessing you had a value in the wrong place or something.
|
sn4cktimes
> E90M3
09/30/2020 at 15:48 |
|
More or less MasterMario’s example just showed me my mistake.
|
sn4cktimes
> Highlander-Datsuns are Forever
09/30/2020 at 15:49 |
|
MasterMario’s show and tell got me there. Thanks though.
|
sn4cktimes
> MasterMario - Keeper of the V8s
09/30/2020 at 15:51 |
|
Huh, dunno then. But I managed to boner it. Seems to work for me now.
|
sn4cktimes
> Daily Drives a Dragon - One Last Lap
09/30/2020 at 15:52 |
|
Yes, I think I managed to typo some value in my mathings.
|
sn4cktimes
> Darkbrador
09/30/2020 at 15:54 |
|
Yeah... as a mechanic this wouldn’t really be MY problem IRL . That’s the Engineer’s problem. My problem is getting it to “make air” and not cause down-time.
|
E90M3
> sn4cktimes
09/30/2020 at 16:10 |
|
I also forgot that for pressure it’s psia+psig, like I said been a little. I do have a ChemE degree, but it’s been almost 8 years since I’ve done a lot of calcs.
Future next gen S2000 owner
> sn4cktimes
09/30/2020 at 16:24 |
|
But I managed to boner it.
That’s what she said.
|
sn4cktimes
> Future next gen S2000 owner
09/30/2020 at 16:34 |
|
Ha!
|
sn4cktimes
> E90M3
09/30/2020 at 16:38 |
|
Yeah, I’m better at math now than I was in school. But still some mathematical relationships escape my understanding. I once solved Saturn’s gravitational constant in Physics class... It was not the answer we were looking for. It excited my teacher enough that he went and looked it up in a book and gave me some bonus marks anyway... so mistake = win?