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Bolton_Alan
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Gas Volume - Total Air from a Stack

04/09/2009 7:33 AM

Gents, a question from a colleague,

He has 80 cubic metres of air at 170 degrees from one source, combining with 30 cubic metres of air at 80 degrees from another source exiting from a common stack. What is his total volume of air coming out of the stack

cheers

Al

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PWSlack
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#1

Re: Gas volumes

04/09/2009 8:39 AM

Assuming it to be an ideal gas (air isn't) 111m3 @ 145degC as a first approximation.

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sb
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#2
In reply to #1

Re: Gas volumes

04/09/2009 9:01 AM

Where is the calculations ?

PV/RT ?

So GAs Rule

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PWSlack
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#3
In reply to #2

Re: Gas volumes

04/09/2009 10:17 AM

<...Where is the calculations ?...>

In the bin!

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yeonhoyu
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#4

Re: Gas Volume - Total Air from a Stack

04/10/2009 12:25 AM

Provided that air's pressure is almost ATM and there is no change in pressure while mixing in stack.

Temperature change = (170x80+80x30)/(80+30)= 145.5 degree.

So, 170 degree 80 m3 air is cooled down to 145 degree while 80 degree 30 m3 air is
heated. Cooled gas will withdraw and hot gas will expand. How much?

Vol = 80x(273+145)/(273+170) + 30x(273+145)/(273+80) = 111.001 m3

Hope it will help you!!
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Anonymous Poster
#5
In reply to #4

Re: Gas Volume - Total Air from a Stack

04/10/2009 9:23 AM

...and that's how another annoying homework was completed!
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sb
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#6
In reply to #5

Re: Gas Volume - Total Air from a Stack

04/11/2009 3:25 AM

No this OP does not seem to be a college guy (at least from his previous posts )

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editorgbanalysts
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#7
In reply to #4

Re: Gas Volume - Total Air from a Stack

04/11/2009 11:46 AM

I am curious here about the inconsistency between the volumes (80 + 30) used for calculating the mixed gases Temperature and the final volume (111.001) calculated with the mixed gases temperature. I have the curious feeling that the two volumes should be the same.

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sb
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#8
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Re: Gas Volume - Total Air from a Stack

04/13/2009 12:04 AM

These are very rough approximations. As PWS already informed.

The actual accurate will be non-ideal gas and iterative.

As the first step, let us mix the gases as liquid

so the final temp

T'= (v1T1+v2T2)/(v1+v2)

= (80* (170+273)+30*(80+273))/(80+30) = 418.45oK=145.45oC

Now based on this approx T calculate back the individual volumes as

V1/T1 = V2/T2

For the first stream

80/(170+273) = V'1/(273+145.45) or V'1= 75.56

Similarly for the other one, V'2=35.56, the addition comes out to be 111.1

Then you have to do a re-iteration based on this modified approximation. But since the variation is not high (110-111.1 is only about 1%), we can almost take it as final value (espacially since the main base of calculation - ideal gas - itself is wrong)

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editorgbanalysts
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#9
In reply to #8

Re: Gas Volume - Total Air from a Stack

04/13/2009 12:48 AM

The temperatures of the two gases are so far apart that I thought the implicit approx on the density in the energy balance was too rough, but it does not matter I guess. I got 141 for the temperature and 117 for the volume, but that is also first approx so I thought I might ask. However, being less than a 10% error perhaps it does not matter.

But thanks for the further clarification.

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#10
In reply to #8

Re: Gas Volume - Total Air from a Stack

04/14/2009 2:26 AM

Wow!

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Bolton_Alan
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#11

Re: Gas Volume - Total Air from a Stack

04/14/2009 3:29 AM

Thankyou all for your input into this question, an approcimation is adequate for our needs and the information will be passed on to my colleague.

Once again, thanks

Al

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