Vacuum Horsepower and Table Size

      Rough ideas about vacuum pump sizing and setup for CNC hold-down purposes. April 21, 2011

Question
I'm examining all the options to add big vacuum so I can run a bleeder board on my 50" square table. It looks like a Roots can pull a couple more inches of vacuum than a regenerative blower with a lot of flow for the horsepower. I'd be interested to hear from anybody who's running one. How many HP, what size table, etc.

Forum Responses
(CNC Forum)
From contributor R:
I use a 15HP Roots on my 4x8 table with 5 zones. Works good if the spoil board is kept surfaced. You also need the bed to be set up right with a gasket and the edge of the spoil board sealed. The blower is very loud and exhausts hot air. Mine is under my CNC - not a good place for it.



From the original questioner:
Wow! 4x8 with 15HP! That's definitely what I'd call efficient! Do you have a gauge to monitor vacuum? Also, did you get the vac system with the machine, buy it packaged and ready to run, or engineer and assemble it yourself? Thanks - I hope your machine's making money today.


From contributor R:
I do have a gauge. With a fresh surfaced spoil board and a sheet of melamine the blower will pop on the safety relief valve intermittently till I start to cut through the melamine. That is at 14". When I run the spoil board for a while I need to use the zone valves more to keep enough vacuum for small parts. The blower was from my CNC manufacturer but the table and piping was self-engineered. Some research, taking ideas from others. The table works well.


From contributor A:
5 x 10 table. 20hp Roots, lightweight MDF for a spoil board. Holds nested cabinet parts very well, even with 4x8 sheets and not closing off the open space on the table. Noisy as hell though.


From contributor J:
20 hp - 5x12. No problems nesting. Keep spoil board surfaced.


From contributor C:
I have 2 vacuum systems that have a Roots blower feeding into a backing high pressure pump. Anyone ever try one of these? One unit is rated at 400cfm, max pressure 26hg, blower has 5hp, backer has 6hp.


From the original questioner:
Thanks for bringing this up. Can you give us a little more info? Are these 2 systems just stuff you have lying around? Do you have a router to try them on? I have, in fact, considered linking up my 5 horse Becker vane pump with a Roots booster, but I still don't know if I can use any and all Roots blowers for this, and what the control requirements are as far as whether the pumps can start simultaneously, relief valves, etc.


From contributor E:
I use a Sutorbilt running on a 7.5hp motor. I use it on a 5x8 table. It gets just under 15" at around 170 cfm (I believe most typical lobes only do around 15" max and cfm rises with HP increase). I made a relief valve that keeps it around 14". Did a full sheet of 5" diameter circles from Sintra the other day with no problem. It is loud and hot, however. I use this setup because I have neither the 3 phase nor amperage for a converter to run much of anything better.


From contributor C:
From what research I have done, how these multi stage pumps work is based on Bolye's Law, i.e. if you compress a gas (air) by X factor you decrease the volume by the same. So the blower on top takes in say 300cfm, compresses it to 8psi (16"hg) and the backing pump only has to move 300/8 =37.5 cfm to keep up. That said, with your 50x50 table, a 5hp Busch pump would do pretty good for parts down to 1sq ft. The oil-filled vane pumps are pretty quiet.


From contributor P:
The Roots style such as the screw vacuum pumps are the way to go in my opinion. Much more positive displacement, and they are louder for sure. The rotary vane units are okay - less expensive, but don't seem to be able to pull as high of vacuum. You get what you pay for.

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