# Fan Power Dissipation

1. Mar 5, 2015

### al_garnett

Hey, I'm trying to find a fan that will dissipate 1471.5W. The fan is going to be driven via a belt connected to a driving spool and not driven by an electric motor.

How do i go about calculating the required fan size?

Is there anywhere i can look at fan specifications for fans that are not driven by an electric motor?

Thanks

2. Mar 5, 2015

### Simon Bridge

Does it have to be exactly 1471.5W or would 1472W be fine? Or 1471.499W?

There are many variables besides fan size (diameter), like the number of blades and their pitch, that determine the energy used.
The energy used by the fan at a particular speed depends on how much it changes the speed of the fluid that it is expected to push about (and losses).
You could have a look at electric fan specs and use the blades off one that is closest - either buy them or use them as a starting point for your design.

3. Mar 5, 2015

### al_garnett

Im essentially lowering a 150kg mass 20m to the ground by using the fan to dissipate energy.
The mass will be attached to a rope that is wrapped around a spool.
As the mass descends the rope will be pulled from the spool causing it to rotate. The spool will then drive the fan via a belt.
The mass has a gravitational potential energy of 29,340J, calculated using E=mgh.
Subsequently, by setting the time required for the descent, 20 seconds, and using the equation Power=E/time I find the power input to the fan 1471.5W.
Basically i need to find a suitable fan to dissipate this amount of energy so that the mass will descend at 1m/s.
If anyone could help it would be greatly appreciated.

4. Mar 5, 2015

### Staff: Mentor

Airflow times pressure (divided by efficiency) equals power....just make sure you have the units right.

5. Mar 5, 2015

### Jobrag

You should look at a drawing of the chime mechanism of a mechanical clock, they use a contraption rather like a paddle wheel from an early steamship to slow down the striking weight, it should be easier to "tune", I don't think that this approach will give you a particularly repeatable result, simple things like air pressure will effect the result.

6. Mar 6, 2015

### Simon Bridge

Im with jobrag - experiment with a paddlewheel.
You can also slow the fall by having it lift another weight.