Compressed Nitrogen: Calculating Pipe Flow Time

In summary, the conversation is about determining the time it would take for a 300 cubic foot tank of nitrogen air, regulated to 5 psi with a flow rate of 10 CFM, to empty. The equation used to estimate this time is the Ideal Gas Law and the initial estimate is approximately 34.1 minutes. However, factors such as pressure drop and the length and area of the piping may affect the depletion rate of the tank. The mass in the tank is calculated to be 2600 kg and the discharge rate is 15.7 kg/m. Consideration is given to using liquid nitrogen instead.
  • #1
Nicholas Hankins
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Hello, first time on this site and wondered if anyone could help me on what the proper equation or equations would be to answer this question.

I have a 300 cubic foot tank of nitrogen air that will provide nitrogen to a rectangular pipe at 5 psi with a flow rate of 10 CFM to atmosphere at the end of the rectangular pipe. The question is how long would it take the 300 cubic foot tank to empty?
 
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  • #2
Nicholas Hankins said:
Hello, first time on this site and wondered if anyone could help me on what the proper equation or equations would be to answer this question.

I have a 300 cubic foot tank of nitrogen air that will provide nitrogen to a rectangular pipe at 5 psi with a flow rate of 10 CFM to atmosphere at the end of the rectangular pipe. The question is how long would it take the 300 cubic foot tank to empty?
Welcome to the PF. :smile:

I think we need the starting pressure in the tank as well. It is regulated down to 5psi for the flow, right?

Also, this sounds a bit like a schoolwork question, but I'm guessing it is for your work?
 
  • #3
Yes it is for work. And also yes it will be regulated down to 5 psi for the flow. The Tank will actually hold 341 Cubic feet at 3500 psi.
 
  • #4
Nicholas Hankins said:
Yes it is for work. And also yes it will be regulated down to 5 psi for the flow. The Tank will actually hold 341 Cubic feet at 3500 psi.
It seems like you could get an initial estimate just using the Ideal Gas Law. Are you familiar with that equation?

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/idegas.html
 
  • #5
Yes I am. I don't know if I am looking at this problem the right way though.
 
  • #6
I am not sure how to get an estimate with that equation with respect to time?
 
  • #7
Nicholas Hankins said:
I am not sure how to get an estimate with that equation with respect to time?
You have an outflow rate at 5psi and an initial stored volume of gas...
 
  • #8
okay. the initial time it would take from my calculation with a 341 cf tank at 10 cfm is 34.1 minutes. Am I correct though that the pressure drop in the regulator and the length and area of the piping change the depletion rate of the tank as well?
 
  • #9
What is the mass in the tank to begin with? What is the mass flow rate out?
 
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  • #10
from what I have calculated it is 10.63 Kg in the tank with a mass flow rate out of the rectangular pipe is 3.55 Kg/m
 
  • #11
Nicholas Hankins said:
from what I have calculated it is 10.63 Kg in the tank with a mass flow rate out of the rectangular pipe is 3.55 Kg/m
So, from this, it seems that, if the flow rate did not decrease as the tank emptied, it would take about 3 minutes to empty the tank. Is that a good enough answer for you?
 
  • #12
Nicholas Hankins said:
from what I have calculated it is 10.63 Kg in the tank with a mass flow rate out of the rectangular pipe is 3.55 Kg/m
That 10.63 kg can't be right. I get about 2600 kg. I haven't checked your discharge rate yet, but maybe that's wrong too.
 
  • #13
I get a discharge rate of 15.7 kg/m. Based on an initial mass of 2600 kg, it would give a (very rough) emptying time on the order of about 3 hours.
 
  • #14
yes it does thank you. I thing using liquid nitrogen would be better for this use.
 
  • #15
how did you get your 2600 kg?
 
  • #16
Nicholas Hankins said:
how did you get your 2600 kg?
I used the ideal gas law, with a pressure of 3500 psi.
 
  • #17
Oh Okay. Thank you very much for your help.
 

1. What is compressed nitrogen?

Compressed nitrogen is a gas that has been compressed and stored under high pressure. It is often used in industrial and medical applications as a source of inert gas.

2. How is compressed nitrogen used to calculate pipe flow time?

Calculating pipe flow time involves determining the rate at which compressed nitrogen flows through a pipe. This can be done by using the ideal gas law and Bernoulli's equation to calculate the pressure drop along the pipe.

3. What factors affect the flow time of compressed nitrogen through a pipe?

The flow time of compressed nitrogen through a pipe is affected by several factors, including the diameter and length of the pipe, the pressure and temperature of the gas, and any obstructions or restrictions in the pipe.

4. How accurate are the calculations for compressed nitrogen flow time?

The accuracy of the calculations for compressed nitrogen flow time depends on the accuracy of the input parameters and assumptions made. It is important to consider any potential sources of error and to use the most precise measurements possible for the calculations.

5. What are some common uses for compressed nitrogen in industry?

Compressed nitrogen is commonly used in industry for a variety of purposes, including purging, pressurizing, and blanketing processes. It is also used as a cryogenic refrigerant and for inert gas welding and cutting.

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