# A Airflow/speed in a pipe due to pressure differentials

1. Sep 12, 2016

### Shayne T

I was just wondering is there any way to calculate what the airspeed would be, inside of a pipe with a known diameter and length, that lay with each end exposed to a known, different pressue than the other.

Was wondering if such a set up would be feasible for generating clean electricity by setting up a generator within the pipe that would spin due to the airflow. Im sure somebody has already thought of this and realized it was less feasible than current methods, but I was just curious, as I cannot find any information on the internet that discusses this.

2. Sep 12, 2016

### Simon Bridge

Bernoulli's equation?

3. Sep 12, 2016

### Staff: Mentor

Air pressure varies with altitude. Therefore every vertical pipe has a pressure difference between the ends, but the airspeed is zero.

A horizontal pipe with different pressures at each end will experience air flowing from the higher pressure to the lower pressure. But that air will flow even without the pipe. We call it wind. So drop the pipe, keep the generator, and call it wind power.

4. Sep 12, 2016

Not if you want an accurate answer. Viscosity is important in pipes, as it is by far the most important source of pressure drop.

This would be true out in the atmosphere, but there are situations where @Shayne T's idea would make a little more sense, for example, if you have two separate rooms that are generally at different pressures but need not be. You could put such a generator between them to equalize the pressures (partially) and recover some energy from the process.

You could start with the Hagen-Poiseuille equation. You could also use the Bernoulli equation as suggested by @Simon Bridge provided that you make accommodations for the viscous losses using the Darcy-Weisbach equation. That approach would also allow you to use a head loss term to represent the power extraction.

5. Sep 12, 2016

### Simon Bridge

... also, googling the terms will provide more insights and save typing.