Originally Posted by
Jeff Roltgen
In general, an increase in HP is necessitated due to the presence of a larger impeller. For instance, the Murphy Rogers has a 16" impeller with a 9" inlet. 2,400 free air cfm, as I see is similar to a current 5hp3ph model offered by Grizzly. At half the price, their next model down is 5HP single phase, yet still spinning a reasonably larger 15" impeller from the dreaded 12".
If you have any way of viewing a contending machine in person, you may be shocked at the sheer mass of a 5HP 3ph setup. These units are made to run incessantly, need almost no maintenance, and I picked up this old one for almost free, which included 8 and 9" trunk runs of spiral pipe. Many times, due to 3ph designation, they are promptly dismissed, making for dismal resale value.
As we've all discussed here, you now know a simple $80-$230 static PH converter or VFD is the secret to making one run, so take advantage if you can, and see if one of these old buggers is looking for a new home.
Last thing I'll mention, yet of great importance: If you land a unit of this size, you really do need 8-9" main trunks, reducing to 6-5" branches as you reach 15-20 ft from DC, reducing to 4" only for the smallest machinery. In other words, not advisable to connect a system of 4" duct to the mouth of an air moving monster like one of these - you'll likely reduce it's performance to that of a 3HP 12"impeller at the 9" to 4" reducer, = nothing gained. (Never underestimate the power-foiling capabilities of an under-sized duct. )
Still, those old units are around, and many times sitting with a pile of spiral duct...
Jeff