Originally Posted by AmishElectricCompany
CFM = Cubic Feet per Minute. It is a measure of velocity into or out of a space.
Think of it this way: You have two cars drag racing - the timer at the end is a measure of velocity, or how long it takes the cars to travel a quarter mile. If both of the cars are identical, they cross the finish line at the same time...they don't go twice as fast.
The only way to get more CFM is to add a stronger motor, not more of them. Even then, your suction is only ever going to be as strong as your single most powerful motor. Let's say you have
- x2 motors, each rated for 100 CFM. Max CFM = 100
- x5 motors, one is rated for 150 CFM, the other four are 100 CFM. Max CFM = 150
I have been watching this thread since it started. This post has bothered me in particular since it appeared.
I do not claim to be an air flow expert, but I know a little physics. @AmishElectricCompany
is right - CFM stands for cubic feet per minute. He is incorrect about "velocity."
Cubic feet is a measure of volume. One cubic foot is a box that is 12 inches x 12 inches x 12 inches. If a shop vac can move 100 CFM, it means that it can move (suck or blow) 100 "boxes" worth of air every minute.
(Yes, let us assume a "standard temperature and pressure.")
If you attach two 100 CFM shop vacs to a system, they will try to suck 200 CFM. That's when turbulence comes into play. Air has viscosity and other properties that affect how it flows in a system. The shop vac filters will slow some of the flow, too. Car engine designers think a lot about this problem - How do you get a lot of gas and air to flow and distribute very quickly into a small chamber before you light the spark?
Just because you have 200 CFM of capacity in your two shop vacs doesn't mean that the shop vacs will be able to suck the entire 200 CFM of air. For a lot of reasons.
You can move your 100 CFM of air by pushing a 12 x 12 box of air forward one foot every 0.6 seconds:
* Imagine a 12 x 12 box with a flat 12 x 12 piston inside, with the 12x12 piston pushing forward 1 foot. That's one cubic foot. If you do that every 0.6 seconds for one minute, you will get 100 CFM (cubic feet per minute).
* Imagine a 4 x 4 box with a flat 4 x 4 piston inside, but this time with the 4x4 piston pushing forward 9 feet to get one cubit foot of air. If you do that every 0.6 seconds for one minute, you will get 100 CFM.
Note the air speed in the two examples. The air in the 4x4 example must move 9 times faster than the air in the 12x12 example to achieve the same 100 cubic feet per minute. To get the air to move faster, you must increase the air pressure.
I hope you understand the point above. You can get 100 CFM from a wide opening with low pressure (low air speed), or a narrow opening with high pressure (higher air speed). Note the popularity of HVLP "high volume low pressure" sprayers - they give you a lot of CFM because the hose is big. My air compressor is far more powerful. One reason it can't deliver the same CFM as an HVLP sprayer is because it pushes the air through a much smaller hose. It can't build up enough pressure in the tank to push the air that fast. It would explode first.
Turbulence and filters interfere with air flow too. That is one factor that makes it difficult to analyze @mhhickma
The question has come up about what happens if you gang the two shop vacs together in series. I don't know. What I do know is that there are multi-stage systems that do exactly that. You can buy 2, 3, 4, and even 5 stage HVLP sprayers for example, which move more air as you add stages. The stages are arranged in series, with the output of one feeding the input of the next. The output of one stage is higher pressure air, which is then compressed by the next stage into even higher pressure air, etc. The design of the air flow between the stages is important - they don't simply feed one directly into the next.
Bottom line prediction:
If mhhickma attaches two shop vacs to his dust collection system, it should draw more air than a single shop vac. How much more air will actually move is too hard to determine. The system may cause the shop vacs to work harder, increasing wear. You know that sound when the motor works harder because the hose is partially blocked. The noise would bother me. Even with ear protection, my shop vac is annoying.