Duct Vacuums with 2 impellers vs 1 impeller, does it actaully double the suction?

Sorry, not even close. Placing 2 vacuum impellers inside of one cabinet in theory seems like a good idea, unfortunately thew laws of physics don’t quite agree.

Its true it will increase the suction but only slightly. The general theory of suction in simple terms will state that when using 2 impellers side by side you will gain an extra 50% in CFM, meaning that if you have one impeller that can generate 2000 CFM’s then the second impeller will add an extra 1000 cfm’s worth of suction, now adding a 3rd and 4th impeller only marginally increases the suction. The 3rd impeller would increase your suction by only 25% (500 cfm) and the 4th impeller by only 12.5% (another 250 cfm). But manufacturers of these systems will use the “free air” 2500 cfm ratings of each impeller to give the illusion that the vacuum is generating 5000 cfm’s when in fact it probably only is pulling a meagre 3750cfms at best at that is only of the impellers are correctly spaced and the inlet opening is distanced at exactly the right spot and of course without any filters in place that would cause restriction of the airflow.

The small benefit you get from having 2 impellers and 2 motors in one vacuum is definitely not worth the extra weight and size you would not have to contend with. The best solution would be to simply have 2 – 110 volt vacuums, if you tie the 2 vacuums together at the duct and run 2 hoses down to each vacuum then you would get truly double the suction, plus you would have the added benefit of being able to tie in the vacuums in 2 different points in the ducts when needed ( you could have one tied into the main trunk line and the 2nd vacuum tied into a smaller off chute or into a reheat coil VAV box ). Also 2 – 110 volt vacuums will be smaller and lighter and easier to manoeuvre around the job site, there is nothing more menacing then trying to fit a massive duct vacuum in a cubicle.