Air flow rate in a closed pipe

In summary, the conversation discusses the difficulties in finding an equation to determine the flow rate of air through a train braking system. The equation being used currently is questioned due to the low flow rate it provides. Suggestions are made to determine the speed of sound in the pipe and to account for changes in back pressure and air density.
  • #1
iscariotrail
1
0
I can't seem to find an equation that fits the situation I am facing. Either that, or I just don't believe the answer the current equations are providing.

I am trying to develop the size for an air compressor to fill up a train braking sytem. I have equations for most of the system, but I can not find a relationship for the flow rate of the air through the entry point as the system fills with air.

The train pipe is 8,000 feet long, 1.25" Schedule 80 steel. It is initially at 0 psig, while the connection point is 90 psig. Considered isothermal, I've been using the equation:

P12 - P22 = [ M2RT / gA2 ] * [fL / D + 2 Ln (P1/P2) ]

Solving for the equation, I'm coming up with:

M = 0.1673 LB/s
Q = 17.1 CFM
v = 32.1 ft/s

The flow rate just seems too low, and that's why I question the equation I'm using. When the 90 psig is connected to the 0 psig, air should be rushing in (I'm thinking between 150 & 200 CFM), right?

Any thoughts or suggestions? Would be much appreciated.
 
Engineering news on Phys.org
  • #2
Initially with an 80 psig air supply pressure, the flow is probably limited by the speed of sound (i.e choked flow). So determine the speed of sound in the pipe and multiply by the cross sectional area.

Eventually, as the back pressure increases, you will exit choked flow. There are several formulas for testing for choked flow, but an ininital check is that the back pressure is less than 50% of inlet pressure.

After you exit choked flow, just use basic fluid mechanics: DP = f (Leq/d) rho vel^2 but time step it to account for increase in back pressure and changes in air density.
 

1. What is air flow rate?

The air flow rate in a closed pipe refers to the amount of air that is passing through the pipe per unit time. It is typically measured in cubic feet per minute (CFM) or cubic meters per hour (m^3/h).

2. How is air flow rate calculated?

Air flow rate can be calculated by multiplying the cross-sectional area of the pipe by the velocity of the air flow. The cross-sectional area is usually measured in square feet or square meters, and the velocity is measured in feet per minute or meters per second.

3. What factors affect air flow rate in a closed pipe?

Several factors can affect the air flow rate in a closed pipe, including the diameter and length of the pipe, the smoothness of the pipe's interior surface, and the presence of any obstructions or bends in the pipe. Other factors such as temperature, pressure, and humidity can also have an impact on air flow rate.

4. How does air flow rate impact the performance of a system?

The air flow rate in a closed pipe is a critical factor in the performance of a system. Too low of an air flow rate can result in inadequate ventilation or cooling, while too high of an air flow rate can lead to energy waste and potential system damage. It is important to maintain an optimal air flow rate for efficient and safe operation of a system.

5. How can air flow rate be controlled in a closed pipe?

Air flow rate can be controlled in a closed pipe through the use of dampers, which can adjust the size of the pipe opening to regulate the amount of air flowing through. Other methods include using fans or blowers to increase air flow, or using valves to restrict air flow. Proper sizing and installation of these components is crucial for effective control of air flow rate.

Similar threads

Replies
6
Views
2K
  • Mechanical Engineering
Replies
19
Views
2K
Replies
5
Views
909
  • Aerospace Engineering
Replies
10
Views
562
  • Mechanical Engineering
Replies
14
Views
3K
  • Mechanical Engineering
Replies
6
Views
256
  • Mechanical Engineering
Replies
5
Views
12K
Replies
1
Views
1K
  • Mechanical Engineering
Replies
2
Views
1K
  • Mechanical Engineering
Replies
13
Views
950
Back
Top