CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: Mechanical Engineering Thesis   Next in Forum: Please Help to Advise Information of Purchase Order
Close
Close
Close
21 comments
Member

Join Date: Mar 2017
Posts: 9

PV = NRT

05/31/2017 10:00 AM

We are amplifying our air system for an aplication from 100 PSI to 200PSI. How would i determine the temperature of the 200 PSI? Is it as simple as P1/T1+P2/T2?

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 12220
Good Answers: 774
#1

Re: PV=NRT

05/31/2017 10:15 AM

No, it is not as simple as you would like. The universal gas law has five attributes on either side of the equation. One is a constant and the other four are all variables. Your math not only ignores the other two variables it omits the operator of an equation sign.

__________________
"Don't disturb my circles." translation of Archimedes last words
Register to Reply
Member

Join Date: Mar 2017
Posts: 9
#2
In reply to #1

Re: PV=NRT

05/31/2017 10:31 AM

I meant P1/t1=P2/T2. How would i find the increase in temp then. I am not sure of volume

Register to Reply
Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 12220
Good Answers: 774
#5
In reply to #2

Re: PV=NRT

05/31/2017 10:47 AM

If you're not sure of either the volume of gas or the moles of gas then you cannot predict the temperature.

__________________
"Don't disturb my circles." translation of Archimedes last words
Register to Reply
Guru

Join Date: Jan 2006
Location: Tamworth, UK.
Posts: 1532
Good Answers: 35
#21
In reply to #2

Re: PV=NRT

06/02/2017 5:33 PM

P1 x V1/T1 = P2 x V2/T2 is better to explain the problem you have.

V and T depend on each other. It is like saying A + B = 12, what is A?

The adiabatic compression formulas given above are better, but your 200 psi is the pressure in the tank when the pump stops - unless 200 psi is the minimum - in which case what is the highest pressure? - and if 200 psi is the cut-out pressure when the pump stops - what is the cut-in pressure when it starts?

The actual pressure during pumping will be the same as the tank pressure as it rises to 200 psi, or whatever.

the lower working pressures (whatever they happen to be at the time) are the ones to use in your formula.

If the booster is an air operated pump, then it is likely to slow down as pressure rises (it could go so slow as to stall) so adiabatic compression is unlikely in practice.

Booster pump suppliers could give you some practical working temperatures.

Most important is the pressure safety aspect as others have said

__________________
When arguing, remember mud-slinging = lost ground.
Register to Reply
Guru

Join Date: Jun 2009
Location: South of Minot North Dakota
Posts: 8399
Good Answers: 771
#3

Re: PV=NRT

05/31/2017 10:35 AM

Temperature at what point in the system?

Off the compressor?

In the tank?

Some place else?

Register to Reply
Member

Join Date: Mar 2017
Posts: 9
#4
In reply to #3

Re: PV=NRT

05/31/2017 10:40 AM

I am guessing the highest temperature point would be during compression from 100PSI to 200PSI.

Register to Reply
Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 12220
Good Answers: 774
#9
In reply to #4

Re: PV=NRT

05/31/2017 11:15 AM

That sounds like a good guess. Remember that after this point your "hot" gas will be transferring this heat to any cooler surfaces containing this gas by the laws of thermodynamics.

__________________
"Don't disturb my circles." translation of Archimedes last words
Register to Reply
Guru
Engineering Fields - Control Engineering - Time to take control United States - Member - New Member Engineering Fields - Systems Engineering - New Member Engineering Fields - Mechanical Engineering - New Member

Join Date: Apr 2009
Location: Tampa, Florida, USA
Posts: 2044
Good Answers: 83
#6

Re: PV=NRT

05/31/2017 10:54 AM

"We are amplifying our air system for an aplication"

It might help us help you if you could describe what your air system is.

__________________
J B
Register to Reply Score 1 for Good Answer
Member

Join Date: Mar 2017
Posts: 9
#8
In reply to #6

Re: PV=NRT

05/31/2017 10:58 AM

So we currently have 100 PSI throughout our plant. We need 200 PSI in order to run our valve gate system in our injection molds. So we have a booster pump that pumps up a tank to 200PSI. When the gates need opened or closed the pressure is released from the 200PSI tank and then is built up again by the booster pump.

Register to Reply
Guru

Join Date: Jun 2009
Location: South of Minot North Dakota
Posts: 8399
Good Answers: 771
#10
In reply to #8

Re: PV=NRT

05/31/2017 11:50 AM

Well given that the air in the 200 PSI tank and system will be a few degrees above the ambient temperature while the compressor is running and for a few minutes after that as they cool down back to ambient temperature.

Register to Reply
4
Power-User

Join Date: Aug 2010
Location: Wales, UK
Posts: 348
Good Answers: 17
#12
In reply to #8

Re: PV=NRT

05/31/2017 11:56 AM

I have supplied boosters for the application you describe and the temperature of the boosted air has never been a problem. It is cooling whilst in the tank. I would want to ensure that the components are capable of a working pressure of 200 psi as most are 150 psi (10 Bar) rated.

Best regards,

John

__________________
Best advice I can give as you get older "Never miss an opportunity to relieve yourself"
Register to Reply Good Answer (Score 4)
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.
Posts: 26721
Good Answers: 701
#13
In reply to #8

Re: PV=NRT

05/31/2017 12:46 PM

That activity would be of great interest to the company that supplies burst indemnity insurance cover for the facility.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Guru

Join Date: Apr 2010
Posts: 5762
Good Answers: 578
#15
In reply to #8

Re: PV=NRT

05/31/2017 2:17 PM

OK, what happens when you pump air into a tank can be modeled as adiabatic compression. Two things will happen: the pressure will increase and the temperature will increase. How much temperature rise depends on the ratio of the specific heats for constant pressure and constant volume.

Adiabatic compression is the limiting case where no heat is allowed to escape, and it will give you the worst case for temperature rise.

Here is an example. You can plug in your values for 100 psi and 200 psi.

"Example of adiabatic compression[edit]

The compression stroke in a gasoline engine can be used as an example of adiabatic compression. The model assumptions are: the uncompressed volume of the cylinder is one litre (1 l = 1000 cm3 = 0.001 m3 ); the gas within is the air consisting of molecular nitrogen and oxygen only (thus a diatomic gas with five degrees of freedom and so γ = 7/5); the compression ratio of the engine is 10:1 (that is, the 1 l volume of uncompressed gas is reduced to 0.1 l by the piston); and the uncompressed gas is at approximately room temperature and pressure (a warm room temperature of ~27 °C or 300 K, and a pressure of 1 bar = 100 kPa, i.e. typical sea-level atmospheric pressure).

so our adiabatic constant for this example is about 6.31 Pa m4.2.

The gas is now compressed to a 0.1 l (0.0001 m3) volume (we will assume this happens quickly enough that no heat can enter or leave the gas through the walls). The adiabatic constant remains the same, but with the resulting pressure unknown

so solving for P:

or 25.1 bar. Note that this pressure increase is more than a simple 10:1 compression ratio would indicate; this is because the gas is not only compressed, but the work done to compress the gas also increases its internal energy which manifests itself by a rise in the gas's temperature and an additional rise in pressure above what would result from a simplistic calculation of 10 times the original pressure.

We can solve for the temperature of the compressed gas in the engine cylinder as well, using the ideal gas law, PV=nRT (n is amount of gas in mol and R the gas constant for that gas). Our initial conditions being 100 kPa of pressure, 1 l volume, and 300 K of temperature, our experimental constant (=nR) is:

We know the compressed gas has V = 0.1 l and P = 2.51×106 Pa, so we can solve for temperature:

That is a final temperature of 753 K, or 479 °C, or 896 °F, well above the ignition point of many fuels. This is why a high-compression engine requires fuels specially formulated to not self-ignite (which would cause engine knocking when operated under these conditions of temperature and pressure), or that a supercharger with an intercooler to provide a pressure boost but with a lower temperature rise would be advantageous. A diesel engine operates under even more extreme conditions, with compression ratios of 20:1 or more being typical, in order to provide a very high gas temperature which ensures immediate ignition of the injected fuel."

https://en.wikipedia.org/wiki/Adiabatic_process

Register to Reply
Guru

Join Date: Apr 2010
Posts: 5762
Good Answers: 578
#16
In reply to #15

Re: PV=NRT

05/31/2017 2:40 PM

Actually, this document is a more straight forward example for Adiabatic heat rise from compression. You can plug in 100 psi and 200 psi.

Register to Reply Score 1 for Good Answer
Guru

Join Date: Apr 2010
Posts: 5762
Good Answers: 578
#7

Re: PV=NRT

05/31/2017 10:57 AM

PV=nRT is Avogadro's law.

https://en.wikipedia.org/wiki/Avogadro%27s_law

P is pressure, V is volume, n is the number of moles of gas, R is the universal gas constant and T is absolute temperature (degrees Kelvin). You have four variables, P, V, n, and T. If you know any three, you can find the fourth.

I'm not sure what "amplifying the air system" means.

Register to Reply
Guru
Hobbies - Musician - Engineering Fields - Chemical Engineering - New Member Engineering Fields - Control Engineering - New Member Engineering Fields - Instrumentation Engineering - New Member

Join Date: Jan 2007
Location: Moses Lake, WA, USA, Thulcandra - The Silent Planet (C.S. Lewis)
Posts: 3951
Good Answers: 175
#11
In reply to #7

Re: PV=NRT

05/31/2017 11:52 AM

PV=nRT is not Avogadro's Law (which is V/n=k), it is the Ideal Gas Law.

!

__________________
"Reason is not automatic. Those who deny it cannot be conquered by it. Do not count on them. Leave them alone." - Ayn Rand
Register to Reply
Guru

Join Date: Apr 2010
Posts: 5762
Good Answers: 578
#14
In reply to #11

Re: PV=NRT

05/31/2017 1:41 PM

Yup!

Register to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19301
Good Answers: 1129
#17
In reply to #14

Re: PV=NRT

05/31/2017 2:56 PM

I love Avagadoes....

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply Score 1 for Good Answer
2
Guru

Join Date: Jun 2009
Location: South of Minot North Dakota
Posts: 8399
Good Answers: 771
#18

Re: PV = NRT

05/31/2017 7:55 PM

FWIW, way back in my tech college days I had an applied pneumatics class and one of the convenient rules of thumb for air compressor outlet temperature was on the rough order of 2 degrees F, or a bit less, per PSI over inlet temperature.

That was to say that if you had a air compressor putting out 150 PSI to a system with a 70 degree F inlet temperature if the outlet temperature was anything around or under, 70 + 150 x 2 = 370 F it was probably working okay.

I've checked many an air compressor with an IR thermometer and found that rule seems pretty consistent. Most run less and very few that are working right run higher unless they are being overdriven for their design or have a valve problem.

Given that I would expect you compressor outlet temperature to be about 200 F, or a bit under, whatever the inlet temperature is at most.

Register to Reply Good Answer (Score 2)
Power-User

Join Date: Jul 2008
Location: Adelaide, Australia
Posts: 382
Good Answers: 12
#19

Re: PV = NRT

06/01/2017 12:14 AM

Whilst all the comments seem valid, the temperate is measured from absolute zero, plus 460 + 32 for f or plus 273 for c. So you would need a huge difference in temperature make a significant difference overall.

Think I got that right.

Tony

__________________
The nice thing about Standards is there are so many to choose from.
Register to Reply
Guru

Join Date: May 2016
Posts: 884
Good Answers: 41
#20

Re: PV = NRT

06/01/2017 11:53 AM

PV=nRT is applicable to the instantaneous temperature of the compresed gas. The temperature of system components will depend on time, area, conductivity, convectivity, etc. and will take a lot of detailed modeling to determine.

Of greater concern immediately is whether the increased pressure may lead to an unsafe condition due to potential dieseling of the compressor. If your compressor is actually running single stage at a 13:1 ratio and the air contains any oil, the flash point of the oil could result in diesel ignition of the air/oil mix or possibly carbonizing and leaving hard carbon deposits in the compressor cylinder, piston and valves. If two stage, you may need to add an intercooler between stages or switching to a high temperature oil.

Register to Reply
Register to Reply 21 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

horace40 (1); JBTardis (1); jesw55 (1); Jpfalt (1); krk725 (3); Mikerho (1); PWSlack (1); redfred (3); Rixter (4); SolarEagle (1); tcmtech (3); Tonymech (1)

Previous in Forum: Mechanical Engineering Thesis   Next in Forum: Please Help to Advise Information of Purchase Order

Advertisement