Proper sizing for DC ducting. How right is Bill Pentz?

[pprobus]

I'm mostly trying to get my head around this. You do recommend the 6" pipe pretty clearly in your writings, Yes, you need 7? duct to move the 1000 CFM or 6? duct and oversized blower wheel. so I am just itching to figure out how that works when not all, but most single stage dust collectors use 5" fittings like the Central Machinery does... Is necking the port down from 6" to 5" at the DC an option? If not then how does that work? On most of these machines, you CAN make a custom flange to go into the impeller housing, but it would require a LOT of sheet metal work to adapt the inlet ring and outlet of the impeller housing to accept 6" hose... Although it is an easy fix to install a 6? flange on the face of the HF blower, upgrading a HF to 6? pipe is a waste of time and money. That blower and impeller do not move ample air to support 6? duct, so you will end up with clogging in the vertical runs and piles in the horizontal runs unless you use minimum duct and keep very clean open filters.

This begs the inevitable question then. If, in your opinion, after all of the research you have done, the HF DC and similar models are not workable dust collection solutions, why list them at all in your writings other than to say DO NOT DO THIS?

Your assertions on pleated filtration versus a simple bag are certainly based on solid, well established engineering principles (Look at an automotive air filter for an example of using pleated media to expand the amount of surface area available to filter through, thus increasing air flow for a given filtration level).

A cursory Googling of duct size / cfm charts shows a WIDE array of highly conflicting data. Most of this data is aimed at negligible static lift systems (HVAC) and nearly useless, there are the charts you seem to quote, and other charts showing, well vastly different data. Going back to the automobile analogy, while I am certain the amount of static lift has to be somewhat higher, typical carburetor fitted big block V-8 engines were frequently fitted with 650 to 850 CFM 4BBL carburetors, which pulled air through an enclosed can type air cleaner housing with an approximately 3" intake hole for fresh air. From a "Feel of the hand" perspective, a Pontiac 455 CID V-8 Holley 750CFM 4BBL intake pulls with a perceptably lower feeling of suction than my Harbor Freight 2HP Dust collector. This is however a mere pereception issue, and has no basis is scientific method or measurement.

This leaves us to ponder, what setups were tested, and were air quality samples taken before / after the dust collection system was run, and before / after dust producing events took place?

I am not qualified to answer most of what you said, however, I can take a stab at the carb analogy and it is exactly what Bill was saying about the shop vac vs. the DC when it comes to 2.5" dust ports.  A car engine has a surprising amount of negative pressure when it is in the intake phase.  If I remember my HS auto shop class, its something in the high 20's" (28" comes to mind) of mercury class which is far more than a shop vac.  Therefore, the suction of a car engine alone can provide the CFM through such a small opening, not to mention particle size, don't forget when you are running a dust collector you are not only pulling in the finest materials but also the coarse materials.  The finest materials maybe finer than an air-fuel mixture, however, mixed into that is the coarser particles that are certainly larger than the air-fuel mixture of an auto engine and its because of this that you need the larger ductwork and ports to reduce the friction developed in the larger particles that slow down the airflow and ultimately slows down the fine particulates so that they escape and are not captured by the DC.

At least that is how I see what Bill is saying and has been saying on his web site for years.

Paul