DC vs. AC voltage fans

[kjreiniger]

I have two 4-5" dia. AC fans in parallel on my tank that turn on when the temperature reaches 81 degrees. The area that houses the tank can get really hot, especially during the summer. To prevent the tank from overheating, I was considering adding 2-4 smaller fans (approx. 2" dia.) that turn on with the lights. Since the AC fans are pretty loud in parallel and barely blow in series, would it be better to use DC fans for this operation? Is it true that DC fans are quieter/more efficient?

 

[kjreiniger]

I guess a good starter question would be for anyone who has used DC fans, are they quiet when they are running and do they provide a sufficient amount of air flow?

 

[kjreiniger]

bump

 

[scoobyrs25]

Well, they are the same thing since it is DC by the time the current actually hits the fans. I got a single 4" fan from OC Aquatic that is very quiet and pushes way more air than any other fan I have ever used. I have it on one side of my 7' long canopy. On the other side is a hole the same size. I can feel the air pushing out the other side as if the fan was actually in that other hole. I have it come on at 81 degrees and no longer run a chiller.

 

[Onesaltydawg]

DC fans use a transformer which is the typical big black box at the power outlet...these are far from energy efficient...Case in point they draw current even when the power switch is turned off per SCE...A quick test is touch the box if it is warmer it is consuming electricity cause your bill to go up. I hope this helps.

 

[kjreiniger]

good point. I am actually most interested in the quietness and amount of airflow over the power consumption. So far it sounds like they are best for this application. Just for clearification, I have AC fans which run AC throughout with the lack of the little black box.

 

[Onesaltydawg]

<a href=showthread.php?s=&postid=12769921#post12769921 target=_blank>Originally posted</a> by kjreiniger
good point. I am actually most interested in the quietness and amount of airflow over the power consumption. So far it sounds like they are best for this application. Just for clearification, I have AC fans which run AC throughout with the lack of the little black box.

And when they are powered off by a timer or unplugged, you are not using electricity.

 

[kjreiniger]

True. When a DC fan is on, does it make more or less noise than an AC fan?

 

[Onesaltydawg]

I am not sure since I have only used only AC fans

 

[RussM]

DC-driven fans in general have more motor torque than a comparably-sized AC-driven fan. This allows

a) smaller diameter motors
b) larger and more fan blades
c) can operate efficiently at lower speeds.

This results in higher airflow and less noise. Typical 80mm/3" AC fans, for example, have noise levels between 27-35dB; you can get high-volume PC cooling fans that are super quiet, in the 15-20dB range. Temperature-sensing variable-speed DC fans (like the IceCap fans) are also readily available, but at lower prices.

I will be replacing my AC canopy fans w/ 12vdc models soon. I'll probably order them from www.windychilly.com.

 

[dsc80]

Like RussM mentioned, look at the dB rating of a fan for how much noise it makes, and CFM (cubic feet per minute) to gauge how much air it'll move (how much cooling). Manufacturers typically underrate their dB and overrate their CFM, so take the numbers with a grain of salt.

In general, larger fans at lower RPMs will give you equivalent CFM as a smaller fan, but with much less noise.

You can get fans from most PC cooling sites, and an additional AC/DC converter to power them from a wall outlet.

 

[flat broke]

You can look at the dB ratings, but unless you know the test criteria that were used to obtain the reading, it may be kind of pointless. I have been running a single 120v fan from Grainger http://www.grainger.com/Grainger/items/3VU71 that Wrightnow turned me on to when I got started, and it's been great. Now as far as noise, the installation is key. If the air flow has to make any abrupt direction changes close to the fan, the noise is far more noticeable. As an example, mine fires down onto the water and then out the back of my canopy. Mine is noticeably louder than Mark's (which is pretty damn quiet), but the noise is the noise of the airflow, not the fan motor itself. His is mounted on one end of his tank and blows across the length with no obstuctions close to the inlet or outlet of the fan. Yes, DC motors are more efficient, but the transformers you would require to power them aren't.

One other item to consider is the CFM rating of the fan. A small fan with the same CFM as a larger one is likely to run at a higher RPM which will more than likely induce more motor an airflow noise.

Chris

 

[kjreiniger]

<a href=showthread.php?s=&postid=12779211#post12779211 target=_blank>Originally posted</a> by flat broke
Now as far as noise, the installation is key. If the air flow has to make any abrupt direction changes close to the fan, the noise is far more noticeable.
Chris

Makes sense.......just like putting elbows in line with a pump, the more you have, the more flow you have transfered into turbulence/noise. I currently have the fans in the back of the tank, approx. 3" from the wall which would lead me to believe that a large deal of noise could be coming from the air being forced against the wall. The DC fans that I add would run from one side of the tank to the other across my T-5 lighting.

Also, I think I was originally shooting for four 2" fans but I'm thinking I should go with two 3-4" fans and make sure that they are not going to be at a high rpm so I can run them without worrying about the noise level. Great help guys.