From me:
"...On a single blower system, you will still notice a difference in one wand when you lift one wand off of the floor. "
From You:
"I can also just remove one hose from one wand, lay it down, and still clean with that hose open and the other wand now cleaning, with very little vacuum loss. "
So, we're in agreement, then... :wink:
Here's the crux... There has to be more airflow attempted than the two wands can "allow", for there to be minimally- noticeable vacuum loss with one off of the floor, disconnected, etc. If there is, then, in theory, there is enough being pulled through the open hose, to act as a "plug", of sorts. So, for this to work, there needs to be a minimum reached, in terms of blower size... or more accurately, airflow capability provided by the blower (which opens the door for over- driven blowers to "qualify").
...But..... (And this isn't directed at you or anyone else, Bob... it's just me thinking out loud, so to speak...)...
If there is more air being pulled than can go through, something else occurs... resistance to that airflow that is beyond the normal resistance introduced by the hose alone.
That resistance is also known by another name: "robbed inches of lift". In other words, that resistance actually "robs" you of lift, by consuming available lift in the process of the air passing through.
So... It really doesn't matter how big the motor is, as long as it has enough power to turn the blower at full RPM under a full load. It really doesn't matter how big the blower is, past the point of it being able to reach the maximum CFM the hose / wand / system plumbing configuration allows for. Because once you get past that maximum, available lift is consumed rapidly, in the process of trying to pull "more"... and regardless of whether the blower can pull 100 or more "extra" CFM, it only has 15" or so of available lift to do it.
What this means is that, if you have 15 inches of lift available, and your system is reading 8" before a wand hits the floor, then you really only have 7" to work with...
This gets back to Greenie's big push for 2.5" "trunk" hose, and why it works... reduced restriction to the airflow = reduced "consumed" inches of lift.
All of this is looking at the vacuum side of the suction part of a truck mount. But the pressure / exhaust side is as important, however commonly overlooked. Simply put, any restriction on the exhaust has a similar effect to restriction on the vacuum side. Why..? Because, just like blowers have limited inches of lift, they have limited pressure capability as well... but pressure restriction on a system is not something that is measured... or set up to be measurable... even though it is, as important as vacuum restriction.
In other words, inlet restriction is only half of the story.
But now, let me point to another factor... an intentionally- hidden one by several manufacturers in our industry...
If you look closely (really closely) at most exchanger systems, you'll find that there has been intentional vacuum restriction built into their systems. Look at the plumbing. It's virtually always on the vacuum side... not the exhaust. (Because the motor exhaust is often plumbed into the blower exhaust, and motor back pressure is not only critical, but E.P.A. mandated now...)
How is this done...? It's done by smaller diameter plumbing... It's either actually smaller piping... or via several unnecessary turns... or, in some, specially- fabricated blower intake plumbing that has an "orifice" of sorts, hidden in it's construction. What this does, is reduces airflow and makes the blower have to work harder for the airflow it pumps... In other words, it makes the blower strain... and in straining, it makes it run hotter, which means more heat available for heat transfer.
... but it also makes the blower's net CFM decrease... In other words, the net is more like a blower that's the next size down. So, when a prospective buyer looks at the system and is hung up on a particular blower size, he, in effect, is being cheated... because the blower he sees, is effectively not the one he gets to clean with as an owner of the system.
Take for instance a system that has a #47 blower, plumbed down from 3" (7 sq. in.) to 2.5" (4.9 sq. in.) just under30% reduction of cross- section for air flow......
...or a #45, plumbed down from 2.5" (4.9 sq. in.) to 2" (3.14 sq. in.) .. over 30% reduction.
(Man, I got on a bit of a rant here... Just some ideas I've been considering and observing over the last few years, while looking at heat exchange technology, as others do it...)