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4

Air Flow Into A Tank At Partial Vacuum

fluid mechanics

6 replies to this topic
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#1 rajaking666

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Posted 25 July 2024 - 03:03 PM

Dear all,

I am struck with a problem- to calculate how much air is sucked into a tank (tank volume = 3m3) at 0.3barA when the vent valve is opened.  I started with an Bernoulis principle, dP pressure drop is directly proportional to mass flow rate (air sucked into the tank), however, it is obvious that pressure drop decreases as time increases, this means mass flow rate also decreases. The flow rate decreases and reaches zero when pressure inside the tank is equal to ambient pressure. So this is a non-steady state dynamic case. Can you please suggest a way forward. Thank you.

#2 latexman

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Posted 25 July 2024 - 06:53 PM

Treat it as two states. 0.3 Bara pressure and calculate n1 in PV = nRT. Then, 1 Atm pressure and calculate n2 in PV = nRT. Then subtract n2 - n1.

I assume you know T.

#3 fallah

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Posted 25 July 2024 - 07:54 PM

I am struck with a problem- to calculate how much air is sucked into a tank (tank volume = 3m3) at 0.3barA when the vent valve is opened.  I started with an Bernoulis principle, dP pressure drop is directly proportional to mass flow rate (air sucked into the tank), however, it is obvious that pressure drop decreases as time increases, this means mass flow rate also decreases. The flow rate decreases and reaches zero when pressure inside the tank is equal to ambient pressure. So this is a non-steady state dynamic case. Can you please suggest a way forward. Thank you.

Hi,

At first be aware the time of tank pressuring till having a pressure equal to the ambient, depends on the vent size and associate components (valve, flange, pipe,...).

Then if you would want to have the incoming air flow rate at any moment, you should do a dynamic simulation, but you can go through a simple way by breaking up the pressure increment to several intervals such as 0.3 bara-0.4 bara, 0.4 bara.-0.5 bara,... till reaching the 1 bara and for each interval calculate the amount of the air has been added to the tank volume using the method provided by latexman.

Now if you have measured the time of pressure increment for each interval you can calculate the relevant average incoming air flow rate by dividing the amount of added air to the measured time of interval.

#4 Pilesar

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Posted 25 July 2024 - 08:36 PM

The problem statement 'to calculate how much air is sucked into a tank (tank volume = 3m3) at 0.3barA when the vent valve is opened' is a question of quantity, not flow rate. Use the ideal gas law to determine the quantity of air in the tank at initial conditions. Then use the ideal gas law to determine the quantity of air in the tank at final conditions of atmospheric pressure. The difference is the quantity of air that enters the tank when the vent is opened.

#5 breizh

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Posted 25 July 2024 - 11:26 PM

Hi,

PV=nZRT

BTW initial and final temperatures are missing.

Breizh

#6 Bob11

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Posted 01 August 2024 - 05:21 AM

Hi, I agree that it's a dynamic and non-steady case. I suggest using Bernoulli's equation, but keep in mind the changing pressure inside the tank. A good method would be numerical modeling or computer simulation to accurately determine the air flow changes over time.

Remember to consider atmospheric conditions, as they can affect the results.

#7 katmar

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Posted 01 August 2024 - 09:31 AM

I'm with Latexman and Pilesar on this one. The way you have worded your question, the flow rate is irrelevant.

Assuming that the compressibility factor for air at ambient temperature and the given pressures is 1.00 is more accurate than you will ever be able to measure the volume of the tank.  This makes the ideal gas equation the proper answer.