Ever since I have been in this industry the salesman have been saying there is only so much CFM you can get through a 2 inch hose.
it seems obvious that bigger is better up to a certain limit. What is that limit?
The following is primarily for Andy and Ron, others wade in at your own peril.
Andy, there is a limit, it is determined by available atmospheric pressure and air velocity through the hose. I deal with some of these principles every day, but until I started playing what-if with the flow calculator I never put everything together in my head. Thanks for getting me to think this through.
Unfortunately what follows still applies only to an open length of hose, but it's a start. It doesn't directly answer Ron's question, assuming he meant a cleaning set up including wand, but it does set a limit. The blower for maximum performance in an actual cleaning situation would be smaller than this.
I created a special new flow calculator screen to go with the discussion:
This screen is again with 100' of 2" hose. I increased the cfm as high as I could without the calculator crashing, I got to about 305 cfm. Notice the velocity v2 - this is how fast the air is exiting the hose as it's going into the blower. Let me tell you, 52,500 feet per minute is screaming! In fact, the speed of sound is not far away, 67,560 fpm. Any higher than 305.03 cfm and there wasn't enough atmospheric pressure to move the air, the velocity became infinity, and the calculator crashed. Things get pretty goofy as you approach the speed of sound, and I don't pretend to understand the physics, for that you need a real rocket scientist!
So, at first blush you might be tempted to say that 305 cfm is the most flow you can get through a 2" hose - but not so fast.
Cfm is a volumetric measurement - a cubic foot is a volume. If I have a box that is 12" by 12" by 12" it would enclose a cubic foot of air. How much air (by mass/weight) is in there? Ah - it depends. If I'm at sea level it will contain more air than at the top of Mount
Everest, because at lower pressures the molecules spread out. Put another way, if I fill a balloon with one cubic foot of air at sea level then take it up the mountain the balloon will get bigger. As the pressure is lowered a given mass of air will expand to fill a larger volume. Ever buy a bag of chips in LA then drive up into the mountains and had the bag blow up? same principle.
Here's the rub - the 305 cfm that the calculator came up with is air at atmospheric pressure - basically like the air around you now. But as the air enters the open hose the pressure starts to drop. It drops evenly with distance until it's at the lowest pressure going into the blower. As the pressure drops, the air expands, or occupies a greater volume.
Blowers are rated using acfm, or actual cfm. This means the volume is measured as the air flows into the blower, at the lowest pressure, meaning also at the highest volume!
Look again at the flow calc screen. Notice that the vacuum at the blower inlet is reading 21.86" hg. It took that much vacuum to achieve our max flow - but - this means the air pressure went from 29.92 (standard atmospheric pressure) to 8.05" hg at the blower inlet - a factor of 3.76 to one. This means that the air
volume increased by the same factor (assuming the temperature remains the same, different discussion). So, the 305 cubic feet of atmospheric air
entering the hose expanded to 3.76 times it's size, or 1147 cubic feet by the time it entered the blower. Since the blower is rated in acfm, our hypothetical blower would have to be rated at 1127 cfm/21.86" hg to achieve maximum flow through the 100' of 2" hose (on earth, anyway).
So, What is the maximum flow? if you're talking about acfm as measured at the blower inlet, it's 1147 (or thereabouts). if you're measuring at the hose inlet it's 305 cfm (which is also called scfm - standardized to sea-level atmospheric conditions).
If you're looking to find a truckmount that will achieve terminal flow through 100' of
open hose you probably won't find one at the show. But it's a moot point - the really interesting question is what's the answer for a complete cleaning setup with hose and wand, sitting on carpet? Which wand? Which carpet? How much hose?, 2 or 2-1/2"? Probably why there are so many different TM designs out there....