Itās a fair bit more complicated that that. The overall static pressure of the system, the cumulative frictive loss of a given airflow, the actual pressure drop created by the fan, and the volume and weight of the material being transported, all need to be known in advance, to calculate the size of the fan needed on the other end (the collector size). That size is calculated by determining the system parameters, and then using the desired air velocity within the duct. Air velocity is the ultimate requirement. If it moves too slow, the chips fall out of the stream, and accumulate in the duct, or are simply not pulled out of the machine in the first place. If you open any of the machines, and look inside them, youāll see the direct result of too-low velocity. If youāve used the jointer, and had to stop and clean out the duct and hose because it was clogged, you can thank low air velocity as well.
Thereās a pretty good explanation of the basic parameters to a system Here.
In order to correctly size a collector, an Engineer needs to first calculate the overall pressure conditions of the system as designed, and then determine what the worst case condition would be (i.e. highest expected number of users at stations at the far end of the system normally). This will dictate the size of the collector (with a clean exhaust filter).
Since there is no existing design, no one has done this, and therefore it logically follows that no one knows how big it needs to be. The salesman just did a wild-ass guess that it would be big enough. Then he chummed up a nonsense estimate on an external collector, which included a lot of made up numbers which favored selection of his product.
Speaking as an Engineer who has three years working in our woodshop, and having a working familiarity of our useage, current system performance, and future needs, Iām going to make an educated guess that the system needs to be about twice as large as the unit proposed by Andrew. But thats just a guess.
Conditioning all this, is that the Building Inspection Plans examiner that reviews our permit application, is going to require engineering drawings, and very likely require them to be stamped by an engineer. The Fire Marshall (who will also review the application) will likely want the same thing.