Box Fan vs Computer Fan vs Automotive Fan: What kind of fan do I need?

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
I am planning to build an air cleaner/purifier for my Hack-space as the air is very dusty. It's likely that I'll build a second one afterwards for extra capacity. But I need a fan to pull the air from 2 filters, but I'm having trouble figuring out which type of fan to get.

I want a rather quiet fan with a maximum air flow of 280-640 CFM. The diameter will probably be from 30-50cm. I want it to use a brushless DC motor, but I'll replace it if I have to. I will probably use an axial-flow fan, but tell me if there's a reason I should use a centrifugal fan instead. If I can't find the right large fan, I might resort to using two smaller fans (20-30cm).

I will probably not use a computer fan, as they are generally small and have weak air flow. However, they seem to be the only type of fan designed to be quiet. Computer fans are often below 25dB, while larger fans seem to almost-always be over 40dB (up to 72dB). As far as I can tell, most of the fans on Digi-Key are of this type.

I have found larger fans advertised as "automotive fans". They appear to be mounted easily like computer fans. However, they seem to always be rated for higher air-flow than what I want, while being loud. They're more expensive than computer fans, but some of them are just within budget.

The "box fans" appear to be made as appliances, not as a component for something else. They therefore look harder to mount, and I would imagine that they're heavier.

So what kind of fan should I look for? What search term should I use to find the fan that I want?
What kind of fans do appliance manufacturers use for heaters and purifiers?
 
Last edited:

crutschow

Joined Mar 14, 2008
23,813
Radial (centrifugal) type fans, such as used in furnaces, are typically used if the air needs to go through a filter since they can generate the higher air pressure needed to force the air through the filter while still maintaining a good air flow.

Higher air flow generally means more noise so that's a tradeoff.
If you use the fan in a vent located some distance from your location, that can significantly reduce noise.
Here and here are examples.

Running a oversized fan at a lower speed is one way to reduce noise while still maintaining the desired air flow.
 

wayneh

Joined Sep 9, 2010
16,129
Find a dehumidifier being thrown out or on craigslist. They don't last long but it's almost never the fan that fails. They usually have a pretty good centrifugal fan in them (older models may be axial) and will even have ducting you might be able to easily hack into a filter holder.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
Running a oversized fan at a lower speed is one way to reduce noise while still maintaining the desired air flow.
Yeah that's right. However, it seems that computer fans, which are the smallest, are the only fans that are designed to be quiet. This is probably due to demand from PC enthusiasts. But I want to find a quiet fan that's larger than a computer fan.

I'm only going to use something like a MERV 13 filter. But shouldn't an axial-flow fan be powerful enough if it's as wide as the filter itself? I was thinking of going for axial-flow because it fits nicely in a compact design. The air could flow through the filter before the fan, to protect the fan bearing from dust.

I still don't have my main question answered though. I don't know if the type of fan with the characteristics I want would be marketed as an "automotive fan", "ventilation fan", "industrial fan", "computer fan", etc. So I therefore don't know how where I can find the right fan, or what search term I can use to find it. None of the fans on Digikey seem to be what I'm looking for, so I don't know exactly where to look.
 

crutschow

Joined Mar 14, 2008
23,813
it seems that computer fans, which are the smallest, are the only fans that are designed to be quiet.
The main reason for them being quieter is they run at a slower speed than standard noise fans.
Hence my comment about buying a larger fan and reducing it's speed with a speed control.
 

MaxHeadRoom

Joined Jul 18, 2013
19,466
A 50cm radial fan is quite large and using a typical shaded pole AC motor for this will make it quite noisy, you may find a air exchanger system fan that may do it, the alternative is to use a AC radial and replace the motor with a DC motor and control it with PWM for quietest operation.
Max.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
From research I have been doing, I have found that there's a balance between air flow and static pressure of different fan types, and going from the most air-flow-optimised; Airfoil-blade fan > Sickle-blade fan > Mixed-flow fan > Centrifugal fan. I have a feeling that one of the middle two would be best suited. However, I'm having a hard time finding where to buy large fans (even online).

But if I did go for a centrifugal fan, there's something I must ask. I notice that some of them have a casing around them that blow the air through a hole in the side (first picture). However, some appear to be cased where the air is directed behind the fan (second picture), presumably after being pushed by the fan in all radial directions. Are there any functional differences between these two types that I should consider?


...you may find a air exchanger system fan that may do it,
Thank you. This is the first reply that addresses my main question.

The main reason for them being quieter is they run at a slower speed than standard noise fans.
No need to repeat yourself. As I said, I already knew this before the first time you said that.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
I have found that the two types of fans I have pictured above are called "side blowers" and "inline blowers", respectively. What I meant to ask is if there are performance differences between side blowers and inline blowers. Does one have more airflow, one handle more static pressure? Is one more efficient than the other, or more quiet?
 

BobaMosfet

Joined Jul 1, 2009
893
They are also called 'squirrel fans' or 'squirrel cage fans'. These types of fans are generally used to create pressure in a smaller output dimension. Frankly, if you're going to use this for a room, you want a bigger, slower, quieter fan, that can move enough air (CFM) through filtration (which creates drag) to exchange air at the rate you want, without causing the fan motor to overheat.

Box fans are dirt cheap.
 

MaxHeadRoom

Joined Jul 18, 2013
19,466
I have found that the two types of fans I have pictured above are called "side blowers" and "inline blowers", respectively. between side blowers and inline blowers. Does one have more Is one more efficient than the other, or more quiet?
They are Radial and axial respectively.
Generally the radial moves more air for a given size but is generally a little noisier.
Generally the higher the restriction on a radial fan, the lower the current.
Max.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
They are Radial and axial respectively.
Nope; not true. An axial fan is fan that blows the air straight, along it's rotation axis, without significantly changing the direction of air flow. Most desktop computer fans are of this type. Here's a picture of an axial fan:

In both of the fans in the pictures I previously posted, the air makes a 90-degree turn. It enters in the direction of the rotation axis, and is then directed away from it. The difference between the side blower and the inline blower is that in the side blower, all the air exits in the same direction through a hole at the side of the case. The difference with the inline blower is that the air is pushed in all directions away from the axis, but then redirects towards the back of the case where the hole is.
 
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