FAQ
Wednesday, July 2nd, 2008FREQUENTLY ASKED QUESTIONS
Q: I was told that I needed a 6,000 CFM fan for my old dust collector and it would need a 15 HP motor. I found a 6,000 CFM fan in my Grainger catalogue that looked just like the one I was quoted and it only needed a 2 HP motor. What’s going on?
A: Fans are rated based on two factors.
1. CFM, which is the volume measured in Cubic Feet per Minute (CFM) of air flow that a particular fan will produce.
2. SP, which is a measure of resistance measured in inches of water gage and referred to as static pressure (SP). This factor will give you the amount of resistance that a particular fan will produce its rated volume against. In the case of a dust collection system, SP refers to a negative pressure (vacuum) that is generated as the fan tries to pull air through a dirty filter. The dirtier the filter, the higher the SP. Once the SP exceeds that for which the fan is rated the CFM will begin to diminish, sometimes drastically.
The fan you were quoted was probably sized for pulling 6,000 CFM of air through a dirty filter media as well as through ducting and various elbows, hoods etc. that make up your particular system. In order to ensure you had the full 6,000 CFM during actual use the SP that was calculated for all this resistance resulted in a motor size of 15HP.
Most system vary with regard to HP requirement since every installation is different unless you are using a product like the Envirosystems AirWall units which do not required ducting and therefore can have the same fan for every installation. The AirWall units already have sufficient HP to provide full volume even when the filters are full and ready to begin pulsing.
The fan you found in Grainger will probably only delivers 6,000 CFM at an SP of between 1 and 1.5 In. S.P. (w.g.). While this would be fine for moving air through a room, it is useless for dust collection since even a clean dust collection filter of any type will have a resistance of almost that much before they even have any dust in them. I can appreciate your concern about fan cost as well as energy cost for the high HP fan, but if your particular system, due to filter type, ducting, etc. requires an S.P. of 6 to 8 inches, then the HP requirement for that fan may very well be 15 HP. While there are slight differences in how much HP is required by various types of fans to achieve the same volume per unit of resistance, the basic concept of HP per CFM at a given SP is fairly constant and are calculated using what are referred to a fan laws.. If you would like to do more research on this you can reference the following universal fan laws. You may find several sources of information regarding them on various web sites.
FAN LAW FORMULA
CFM varies Directly with RPM (CFM 1 / CFM 2 = RPM 1 / RPM 2)
SP varies with the SQUARE of the RPM (SP 1 / SP 2) = (RPM 1 / RPM 2)2
HP varies with the CUBE of the RPM (HP 1 / HP 2 = (RPM 1 / RPM 2)3
Just remember that it is important, any time you are getting quotes for dust collection systems or any other air movement device, which you ask for the SP at which your fan volume is being rated. This will help eliminate the possibility of getting a system that works very well at first but after a few weeks or months doesn’t seem to be moving enough air. All 6,000 CFM fans are not equal.
Q: How do I calculate how large my dust collector needs to be?
A: This is a very good question and one we address several time a day. The first thing you must determine is how fast you want or need the air to move through the area you are collecting dust from. Depending on the type of dust and the size of the room, this could be anywhere from 50 feet per minute (FPM) to over 160 FPM. For the sake of explanation let’s settle on 50 FPM.
The next thing you must determine is the size of the area you wish to ventilate. Again, for the sake of explanation let’s settle on an entire room with dimensions of 20 ft. wide by 10 ft. high by 50 ft. long.
For the purpose of determining air flow there are only two dimensions that are important; the width and the height. The length is relatively un-important unless it is extremely long, in which case there may be some consideration given to that fact. In our case, 50 ft. is not an issue. To calculate how large a system we need we will multiply the width by the height. A 20 ft. width times a 10 ft. height will give us a 200 Sq. Ft. cross section.
We now multiply this cross section by the air movement we want in FPM. Our 200 Sq. Ft. cross section multiplied by 50 FPM will give us a required air volume of 10,000 Cubic Feet per Minute (CFM). Once you understand how this works it is quite simple. We simply take the cross section area and move it through the room the number of feet we want it to move each minute and then total the volume of space that took up.
