# What happens to teh air pressure when a fan is used?

1. Jun 2, 2005

### infamous_Q

Ok. so say you have a fan that is moving air, at 1 atm, at about 10 m/s. what exactly happens to the pressure? does it change at all or...?

2. Jun 3, 2005

### morry

Well, according to Bernoulli, the faster a fluid flows, the lower the pressure. Does that answer your question?

3. Jun 3, 2005

### Q_Goest

A fan increases the static pressure on it's outlet and can also decrease the static pressure on the inlet slightly.

4. Jun 3, 2005

### FredGarvin

Along what Q_Goest mentioned...

Even though very slight in comparrison, a fan is technically a compressor. The static pressure rise is usually in the order of inches of water. A lot of jet engine manufacturers (including my company) will sometimes refer to the fan as a low pressure compressor.

5. Jun 3, 2005

### infamous_Q

hmm...so if you had something at like 30psi (just a number here..) and used a fan to increase the speed through a straight tube, the pressure would drop...
a) is there anyway to increase this pressure again (with the air still moving at the speed it was before
b) how could i calculate how much it drops?

6. Jun 4, 2005

### quark

Put an additional blower inline. Depending upon your final downstream pressure(if atmosphere) there may be choked flow in the pipeline. Crane Technical Paper 410 is a good resource for calculating pressure drops in pipe lines and accessories and worth to have it at 40USD.

7. Jun 4, 2005

### Cliff_J

infamous - you're crossing contexts here, morry's post isn't clear on this either as you are not in a steady state condition.

If you have air moving in a tube and that tube changes size then the air's speed and pressure would change to correspond to it.

If you add energy to the air with a fan you can increase both its speed and pressure.

As the air encounters resistance from the tube walls (and turbulence) some of its energy will be lost and thus the pressure will drop as a result. The longer and smaller the tube the larger this effect would be.

8. Jun 4, 2005

### Clausius2

Bernoulli cannot be applied along a streamline which passes through a fan in motion, unless your reference frame is attached to the rotor.

As Qgoest and CliffJ have said, the fan increases both velocity and pressure air. But it is certain too that the whole stuff is embeded in an uniform pressure atmosphere. The jump of pressure is very slow, and such pressure energy will be dissipated along the fan weak.

9. Jun 7, 2005

### manulal

I think the simplest answer will be " the pressure will increase if we take the fan and it's surrounding as a closed system as per energy conservation law