Spring 2005

This section is for practical, everyday advice about chambers, chamber design, and general vacuum practices. If you have any questions that we could answer or if you'd like to submit a tip, please e-mail it to us and we'll include it in future updates of this page.

Remember that creating a vacuum inside a chamber will cause a pressure differential on the walls of the chamber. Each square foot of space on the outside wall of a chamber is experiencing approximately 2000 pounds or 1 ton of pressure being exerted on the space. This will cause deflection of the unsupported walls. As the chamber is being used this deflection can, over time, cause material/weld fatigue and can also cause your internal components to move or shift damaging critical alignment requirements. If the deflection is severe enough it can also cause the chamber to implode. Box shaped chambers are much more susceptible to deflection and the problems associated with it. If you have questions regarding the amount of deflection in your design and how to properly stiffen or properly support it, contact Nu Vacuum. We will be able to give you information on the amount of deflection you will experience and how to reduce or prevent it. A few examples of chamber deflection are that a .5” thick plate with an open area of 12” x 18” will have .067” deflection and a .75” thick plate with the same open areas will have .020” deflection. Can your chamber or internal mechanisms withstand this level of deflection?

Double wall chambers like those in furnace applications use water flowing in the space between the walls to cool the chamber. This water has a pressure associated with it and even the best water flow circuits will have a slight positive pressure build up inside the shell or jacket. Additionally, if there is any problem with the water flow and it is blocked or interrupted, the pressure can quickly build up inside the jacket space. Be sure to properly calculate the thickness of the inner wall so that it has some safety built in to compensate for this slight or aggressive positive pressure.

As you design your larger chambers be sure to include some provisions for lifting eyes or lifting lugs. This is important for the manufacturer so that they can move the chamber around and turn/flip it during fabrication/machining and it could also be important for you so that you can better move/position it once it is inside your building. Lifting eyes are good but can sometimes be obtrusive. Lifting lugs (short blocks of material with threads that you can screw in a lift eye and remove it when you don’t need it) are smaller and easier to work around. We have several different designs of lifting eyes and lugs.

Vacuum hinges need to have adequate movement in two directions. One movement is to close the door and the other movement is to allow the door to be uniformly drawn in during pump down. We have many different designs of small and large vacuum hinges. Contact us if you need design support.

Don’t be afraid to add a few extra ports to your chamber design. If you plan on using the chamber for a long period of time or for multiple applications, it is better to have a few extra ports you can use later rather than have to take the chamber out of service to add the needed ports. Taking it out of service at a later date is much more expensive, cumbersome, and time consuming than adding a few ports on now.

Full and half dovetail o-ring grooves are common in vacuum applications. The dovetail is there primarily for two reasons. One is to allow the o-ring to have good compression inside the groove thereby greatly reducing the potential for the o-ring to be pinched or cut by the top of the groove. The second is to help keep the o-ring in place during opening and closing of the port/door. From a technical standpoint full dovetails are best when you have the potential for both vacuum and positive pressure inside and outside the chamber such as with vacuum furnaces. Half dovetails are best if it will be a vacuum only application inside the chamber. From a machining standpoint if you have a round chamber and the final groove will be put in on a vertical turning center it is best to have a half dovetail. If you have a square/rectangular shape and are putting the groove in using a CNC milling center or a horizontal boring mill then it would be best to specify a full dovetail. There are standard groove designs that are used by the vacuum industry. Contact NVSI for a chart of these designs or for help with your o-ring design requirements.

Most of the vacuum fitting/flange manufacturers are bead blasting the exterior and interior of their half nipples. This makes it more difficult for vacuum chamber suppliers to make a chamber with a perfectly shiny exterior. We need to polish off the bead blast and then recreate a shiny finish on each tube. Some customers are opting for a polished inside of the chamber and for a bead blasted exterior of the chamber.

The number of bolt holes/bolts you have sealing a chamber does reach a finite level sealing performance and having lots of bolts/bolt holes does not always improve the sealing potential of your chamber. Contact us and we will do the necessary calculations and tell you how many you need and what size they should be. Remember that every bolt hole is a machining operation and each one adds cost to your chamber.

Having ports on the sidewalls of chambers reduces the deflection of the chamber walls.