Vacuum Sizing Issues

This discussion of vacuum pump needs includes a nice explanation of the principles of vacuum pressure, and some tips on ganging multiple pumps to the same hold-down table. May 4, 2011

I have the opportunity to go partners on a CNC router. However, my building has three 175 amp meters, and the electrician is saying that the 40hp vacuum draws too much for any one meter. Just out of curiosity, how much vacuum are some of you running? I see that Omnitech advertises one of their packages with only a 10hp vacuum, yet I have seen small parts get tossed even with 40hp. How much is too little?

Forum Responses
(CNC Forum)
From contributor M:
Workholding a on a spoilboard is a process that involves many variables and there is no simple and easy answer to how much is enough. Horsepower is a rough indicator of power, but different styles of pump work better for evacuating the plenum quickly or making up for lost seal and others for holding a "harder" vacuum. So many factors come into play... condition of the spoilboard, material warp, smoothness of the material being held, system air leakage, spoilboard material and thickness, sealed edges, tool sharpness and diameter, and many others. These factors are sometimes more important than horsepower.

I ran an Omnitech Selexx pal with a 9 horsepower Becker pump. Overall a very nice machine. It was a 5 by 10 table and we cut mostly pre-finished veneer core. If I had the entire table covered it worked reasonably well, but this meant covering a strip one foot on the left and 4 feet at the end with scrap. It was a painful process 40 times a day. Zoning of the table would have been very useful for workholding, and not too expensive an addition for the manufacturer, but it was not an option. No amount of pressure will hold down the corners of a moderately warped sheet, so for VC you have to be very careful with suppliers. If you are running melamine primarily or MDF core, it is an advantage. Plywood often is more warpy and difficult.

Many people run with less - some even use edge clamps and no vacuum at all. But I think the 10 HP on a spoilboard is, in general, bare minimum.

From contributor D:
More often than not, people want more vacuum than they have. If you have a 40hp pump, which was probably matched to the rest of your machine layout, going lower I can only imagine will cause grief. There will always be exceptions for certain applications.

From contributor G:
You could run two or three pumps 10 to 20 hp that would allow you to up the CFM as needed and hook them into the three separate meters. That way they start one at a time and keep the starting amperage down.

From contributor P:
We run two 10hp pumps, the second connected to a vacuum switch which turns it on when the vacuum drops too low. The system works very well.

From contributor T:
Vacuum holds by using atmospheric pressure to push down against the vacuum created. The deeper the vacuum, the more the hold down pressure. As a reference, atmospheric pressure is greatest at sea level where the air column is tallest and 14.7 psi at sea level is the highest pressure possible by vacuum. Vacuum is measured on a scale called inches of mercury ("hg). But there is no practical way to produce a perfect vacuum in a shop (complete removal of every molecule of air = 30"hg), and not many shops are at sea level. So you need to factor the possible vacuum pressure. The pressure will determine the smallest size part possible to hold by vacuum, without using special finesse in programming. So you will need to consider the size of the parts.

There are several vacuum sources and they have different theoretical maximum vacuum pressures. The three best pump styles in terms of maximum pressure are rotary vane, rotary screw, and liquid ring. Specifications vary a little, but the theoretical peak for these pumps is 29.9"hg. So consider what kind of pump to use.

The other measure of a pump is flow, in either cubic feet per minute, CFM, or cubic meters per hour C3M/Hr. There is a small difference in efficiency between styles, but it's small enough to be insignificant (it is possible to calculate the proper size pump for a requirement, but it is necessary to quantify a number of variables, above mentioned and others like materials used).

From the original questioner:
Thanks for the responses. We have determined that pump (40hp Travaini) pulls about 90 amps while it's running, and that we have enough juice after all.

From contributor P:
The more, the better! You will find it difficult to have too much vacuum, especially if you are running small parts. There are a lot of variables as to what you will actually need. It seems that many companies tend to underrate the need for vacuum volume, or use a "best of situation" example. If a manufacturer says you only need a 10hp system, you probably actually need 20hp or better. Of course, remember table size if a factor. We have two 40hp units on a 4x24 twin table and it is marginal on many of the small parts we run. My advice? Get the biggest, baddest vacuum system you can afford. Don't cheap out - you will lose production.