FAQ-"Can it withstand the pressure?"

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Brian McDermott
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FAQ-"Can it withstand the pressure?"

Post by Brian McDermott » Tue Jul 31, 2007 3:48 pm

People often ask if their chamber material or wall thickness is adequate to withstand the "vacuum pressure" it will be holding. There is a significant difference between pulling a vacuum on a vessel and pressurizing it to a high pressure.

When you pressurize something, the pressure inside approaches infinity. You are limited by the tensile strength of the weakest part of your chamber. Conversely, when you evacuate something, THE PRESSURE INSIDE APPROACHES ZERO! Thus, the maximum stress your chamber will have to withstand is 15 psi pushing in from the *outside.* That is the pressure differential between the atmosphere and a perfect vacuum.

Once again, even with an infinitely perfect vacuum in the chamber, the maximum pressure it will have to withstand is 15 psi. That's it.

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Re: FAQ-"Can it withstand the pressure?"

Post by Wilfried Heil » Tue Jul 31, 2007 11:43 pm

The next question would be: How thick does the shell have to be to withstand 15 PSI without crushing and imploding?

A 0.3 mm salad dish seems to be barely enough and 1-3 mm stainless steel is already more than needed.

From a mechanical standpoint, it would be better to have the pressure inside of the fusor rather than around it. Do we really need a vacuum?

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Frank Sanns
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Re: FAQ-"Can it withstand the pressure?"

Post by Frank Sanns » Wed Aug 01, 2007 3:47 am

Interestingly, many people are worried when they pull to stronger vacuums. They think going from 10E-2 to 10E-7 is going to really stress the vacuum chamber even though 99.99? of the stresses are already there by 1 torr.

The other factor though is the total FORCE on the hemispheres. Sure there is 15 lbs PER SQUARE INCH but even a small 6 inch fusor has surface area of 2 pi r^2 or 57 square inches of surface area per hemisphere. With 15 psi times 57 square inches that is 848 lbs total force. For an 8 inch fusor the force on each hemisphere is 1508 lbs. Small numbers add up fast when you have multipliers like surface area.

I remember a time when early in my career, my supervisor decided we needed more air flow in our elephant trunk hood system. He told me to go down and shut off all of the other trunks so the air flow would go up on the critical trunk that we were using. When I closed all but the last of the trunks, the rectangular duct that feed the trunk that was around three feet across collapsed right on down the line. I mean 60 feet of duct squeezed like a soda can. Boss was not too please with himself and calculations showed that the air pressure outside of the duct (atmospheric) was just a few ounces per square inch higher than the vacuum inside. The surface area of the duct was something like 30,000 square inches for a total force to collapse the duct on the order of 7,500 lbs! Now you know why thermopane windows do not have a high vacuum for insulation in them.

Frank Sanns

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Richard Hull
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Re: FAQ-"Can it withstand the pressure?"

Post by Richard Hull » Wed Aug 01, 2007 6:00 pm

Brian this was a superb FAQ and I am glad you did this and gave a good explanation that should serve forever here.

Frank noted the large forces on the total fusor body. However, they are evenly distributed over a spherical surface which is one of the strongest known and the best to have such forces applied against. Thus, the thickness needed in a spherical device is incredibly reduced from say a square box fashioned from similar materials at any given pressure.

Again nice FAQ and thanks for your efforts.

Richard Hull
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Re: FAQ-"Can it withstand the pressure?"

Post by Wilfried Heil » Wed Aug 01, 2007 10:24 pm

The common misconception is that vacuum "sucks" and that nature abhors a deeper vacuum proportionally more.

In reality, the only force which acts on the fusor is the atmospheric pressure on the outside, minus the very small remaining pressure inside the vessel. If the chamber can withstand the atmospheric pressure of 1 bar, then it will be sturdy enough for any vacuum level.

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Re: FAQ-"Can it withstand the pressure?"

Post by Verp » Thu Aug 02, 2007 2:53 am

I am reminded of an idea I had for testing vacuum chambers. 1 atmosphere is roughly 10 meters or 33 feet of water. if you have a deep enough body of water on hand, just send your sealed but un-evacuated chamber 20 meters or 66 feet below the surface. If it stands up to the pressure, it should have a reasonable safety margin under vacuum.

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