# Electricity consumption of a fan in a cooling tower

1. Aug 3, 2013

### Solibus

Hi everybody!

I'm currently working on a project where a wet cooling tower is involved. The wet cooling tower is an induced draft system. I know the volume of air handled and also the properties of the air entering and leaving the system.

I want to determine the electricity consumption of the fan depending on these parameters.

I have found two formulas. The first comes from the .pdf "SPX - Cooling tower fundamentals", at the page 58, they present several formulas and there is one to calculate the power output of a fan: spxcooling.com/pdf/Cooling-Tower-Fundamentals.pdf‎

aph = ( Q * ht * D ) / (33000 * 12)

where:
aph -> air horsepower
Q -> volume of air handled
ht -> total pressure differential
D -> density of water at gauge fluid temperature

But this formula does not determine the electricity consumption of a fan. But the power output of one. Do you know if I can use an efficiency coefficient to calculate the electricity consumption?

Then, I found this website (http://www-old.me.gatech.edu/energy/beth/four.htm#A2), it gives an equation for the fan horsepower:

hp = A * cfm^B

where:
cfm -> air flow rate in the tower
A and B -> constants which correspond to a particular tower box size

What do you think about this equation? It may work, but I have looked for these two coefficients and I still haven't found them.

2. Aug 4, 2013

### siddharth23

So currently your problem is once you need the efficiency. Once you get that, you get the answer, right?

3. Aug 4, 2013

### Solibus

I agree with you. However, I'm not sure this formula is the best one and I have no idea about this efficiency. It's for this reason, I posted on this forum. Because, I'm pretty sure that I'm not the first person who wants to calculate the electricity consumption of a fan. There is probably a well-known method to do it.

4. Aug 6, 2013

### Solibus

Is my question too vague? Thank you for the feedback.

5. Aug 6, 2013

### sanka

Not sure if I completely understand your original question, but if you just need to calculate the power consumed by the fan(s), maybe you should just use the fan laws where the power is given by;

Power α ρN^3D^5

So if you know the power consumed by the fan at the nominal rotational speed (should be given in fan catalog), then using the formula above, you can determine the power (in watts) for a given speed which corresponds to the air flow rate through your system.