RPM or CFM?

[starmanmm]

When choosing a fan for making an spray booth, which is more important... the RPM or CFM rating?

I have been told that if the fan has a higher RPM rating that it doesn't matter what the CFM is.

Example. A fan has a RPM rating of 2550 but a CFM rating of 190; the other fan has a RPM rating of 1550 but a CFM rating of 310.

I've been told that the higher RPM is the key for drawing the air than the CFM.

Here is what I am looking at: One fan is listed at 2550 RPM for an air flow of 390 CFM and the other is listed at a fan speed of 2930 RPM for an air flow of 195 CFM. The smaller CFM has a higher RPM which I am being told would draw the air better due to the speed.

So, I am not sure which one to buy... the 190 CFM or the 390 one?

Thoughts?

[Johnnycrash]

CFM. RPM is just how fast it spins. CFM is how much air it moves. And air movement is what you need to vent. Revolutions Per Minute don't mean squat if it ain't moving the Cubic Feet per Minute.

[seam-filler]

Agreed - CFM is the more important. The difference in the two units you quoted is probably in the design (or even type) of fan the slower one obviously has a better, more efficient design of fan.

[aussie cylon]

Yes....I tend to agree with the CFM being more important.....however....if you want a quieter fan, then RPM would probably be what you're after. Lower RPM with higher CFM would have more pitch on the blades which would tend to slap the air....causing excessive noise. On the other hand, higher RPM with lower CFM would have blades slicing through the air reducing noise considerably. But here we're only talking about a tabletop hobby spray booth, so CFM wouldn't really come into it. I would much rather have a very quiet fan instead of a very noisy one....Again, if we're talking about an industrial sized spray booth for painting cars etc, I would obviously go for CFM......so it's up to you. Pros and cons for both....

[naoto]

Yes....I tend to agree with the CFM being more important.....however....if you want a quieter fan, then RPM would probably be what you're after. Lower RPM with higher CFM would have more pitch on the blades which would tend to slap the air....causing excessive noise. On the other hand, higher RPM with lower CFM would have blades slicing through the air reducing noise considerably. But here we're only talking about a tabletop hobby spray booth, so CFM wouldn't really come into it. I would much rather have a very quiet fan instead of a very noisy one....Again, if we're talking about an industrial sized spray booth for painting cars etc, I would obviously go for CFM......so it's up to you. Pros and cons for both....

Hrm... when it comes with fans for computer cases, you want the higher CFM with lower RPM for reduced noise -- thus the trend toward larger-diameter fans (i.e. larger fan doesn't need to be spun as fast to get the same CFM).
I did once work with an industrial PC -- not only were the fans running at a high rate (because the computer may be placed in places with higher ambient temperatures than normal home/office environment, so it was important to have a *lot* of airflow), but air was running through multiple stages of filters (to make sure particulates and other contaminants don't get into the computer). That thing sounded like a vacuum cleaner (of course, if it was in a factory with a bunch of machinery running, you wouldn't notice it).
A friend of mine had a PC that had rather poor case design: first it needed to use two banks of 40mm fans (because there wasn't enough room for larger diameter fan); second the fans would be fighting convective currents (i.e. case was designed to suck in air from the top and blow out the bottom -- as you know, hot air rises and cool air sinks); third due to the way the drive bays were located, they wouldn't get much airflow - necessitating addition of more fans for the drive bays. That sucker was noisy as heck (though not as noisy as that industrial PC that I previously mentioned). Trying to tune the system for quieter operation by reducing the fan RPMs usually resulted in overheating.

[aussie cylon]

thus the trend toward larger-diameter fans (i.e. larger fan doesn't need to be spun as fast to get the same CFM).

I'm talking about comparing identical sized fans....obviously if you use a larger fan, you get more air flow...

[seam-filler]

True to an extent, but blade pitch, blade surface area and number of blades are equally important to determining airflow as RPM - perhaps even more important.

Think of a variable pitch propeller on an aircraft... If you've got the prop in feathered config (blades at almost 90 degrees to direction of travel) you can rev the engine as hard as you like, but you ain't going nowhere.

[naoto]

thus the trend toward larger-diameter fans (i.e. larger fan doesn't need to be spun as fast to get the same CFM).

I'm talking about comparing identical sized fans....obviously if you use a larger fan, you get more air flow...

As seam-filler has indicated, the design of the fan/impeller is also quite important. Usually they'd be designed to operate within a certain speed range, moving a certain amount of air. If spun much faster or much slower than this design speed will result in less-than-optimal performance. Too fast and you're probably going to get more noise than airflow. Too slow and the fan will probably not move much air to make much of a difference. I'm apt to think that over a longer period of time, fans that running at higher RPMs are probably going to end up being more noisy because of vibration from the fan becoming off-balance as well as wearing of the bearings (in this case, selecting a setup that is easy to service might be a good idea -- which is one of the reasons I'm apt to lean toward a larger desktop computer over an all-in-one or laptop -- upgrades and repair are generally much easier).

And since this discussion is about a paint booth -- you're going to have some filtering -- which would probably contribute somewhat to the noise. And likely the noise would increase with greater amount of air passing through it per unit area. A safety consideration would also include possibility of fire caused by sparking in motor possibly setting off flammable paint fumes.

[seam-filler]

A safety consideration would also include possibility of fire caused by sparking in motor possibly setting off flammable paint fumes.

An excellent point, especially if you're planning to spray more inflammable lacquers & enamels. Centrifugal fans are often used here because the motor will never be in the airflow and can be insulated better.