Can fan RPM affect power consumption in heat exchangers?

[Lester Tiongson]

Hi Guys,
I just want to ask if there is a relation between fan RPM and the fan power consumption.
Because in some specifications I see, Power consumption is not consistently increasing as you increase the Fan RPM.
If we lower the RPM , will we still achieve the same airflow?
if we always set at low RPM, will it take longer time to transfer heat?
Then If it takes longer time, will the power consumption then get bigger and not smaller?
Please correct me if I’m wrong,
Thanks for your time

 

[russ_watters]

Welcome to PF!
Generally, airflow is proportional to rpm and power is a cube function of rpm (though it does depend a little on the particulars of the fan). These are called the affinity laws:
http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html

And yes, heat transfer will generally be lower at lower airflow.

 

[Mike_In_Plano]

In addition, it depends on the motor. Brushless motors are usually very efficient over a wide range of speed. Tapped induction motors are about the worst - think 20%... Thus air handlers using tapped induction motors are energy hogs, while those with BLDC motors are amazingly efficient, especially if the ducting is well designed.

 

[Lester Tiongson]

Thank you Sirs for your comments.

The reason I'm asking this is that I have a project to design a heat exchanger.
However , the fan spec we used has a power consumption of 145W, but customer only requires 95W per fan.
The problem is we already ordered it and we are looking for solutions.
Is it possible to lower the power consumption from 145 to 95W?

 

[Bandersnatch]

Could you perhaps add a resistor to the circuit to reduce power, and either exchange or extend the vanes to compensate for the decrease in airflow (you get more flow from large, slow moving fans)?

 

[russ_watters]

Is it possible to lower the power consumption from 145 to 95W?

Certainly - just use a speed controller. But without knowing the details of the fan or heat exchanger it is impossible for us to know what the impact on the application will be.

 

[Lester Tiongson]

What details do you need?

 

[russ_watters]

I'd need to see the fan curve, the fan's dimensions, the heat exchanger's physical configuration and any test data on fan and heat exchanger performance.

Also, it is possible that the 95W was pulled out of the air and doesn't actually mean anything or it is possible that the 95W is actually based on a specific design -- you could always ask the customer what the basis is.

 

[Lester Tiongson]

I attached the fan specifications. I will also send you the core of the heat exchanger that we use. We searched for the fan that can overcome the core's pressure drop (static pressure). It was recommended by our supplier tot meet that requirement.
For the 95W basis, actually in this system we need to have two fans, one for sucking the warm air from inside of the cabinet and also one for sucking cool air from outside cabinet. Total power consumption of two fans need to be 190W. and it will be powered by a DC 48V batteries. Currently total power consumption of two fans is 280W.

 

[jack476]

It sounds like you're asking about computer fans (I've seen similar questions around the internets before), which are a bit more complicated than just a DC motor (which would, for the most part, simply act and dissipate power like a resistor by p = VI = V²/r), they are PWM-controlled and so can use differing amounts of power based on how hard your computer has to work to keep cool. They should tell you exactly what kind of power they will be drawing (compared to the rest of the PC tower, not a whole lot, really), and here's a program I often have used for estimating that kind of thing: http://www.extreme.outervision.com/psucalculatorlite.jsp Fans are in the right-hand side of the page, about halfway down. Hope that helps.

 

[russ_watters]

I attached the fan specifications. I will also send you the core of the heat exchanger that we use. We searched for the fan that can overcome the core's pressure drop (static pressure). It was recommended by our supplier tot meet that requirement.
For the 95W basis, actually in this system we need to have two fans, one for sucking the warm air from inside of the cabinet and also one for sucking cool air from outside cabinet. Total power consumption of two fans need to be 190W. and it will be powered by a DC 48V batteries. Currently total power consumption of two fans is 280W.

Sorry I let this one die. I'm moving it to mechanical engineering...

Do you have any additional details about the heat exchanger you can provide? In particular, how do you know the pressure drop that you have in the spec sheet? (354 pa).

Now, the thing that strikes me is that the airflow requirements and fan specs don't match very well:
Required Airflow: 650 m3/hr
Required Pressure: 354 pa

But on your fan curve, a pressure of 354 pa corresponds to an airflow of 950 m3/hr. So your options are:
1. Reduce the voltage to reduce the airflow.
2. Pick a different fan.
3. Make sure your spec was right.
4. Keep the extra airflow for safety factor.

However, assuming the same fan efficiency at lower speed (not guaranteed), reducing the airflow to 650 m3/hr at the same pressure won't give you the needed wattage -- you'll only drop it by the ratio of the airflows.