Monday, April 16, 2018

A Short Primer On Vacuum

Most processes operate on the principle of pressure, steam, air, hydraulics, IC engines and even explosives. This results in vacuum being a less understood concept. Air has volume and because of gravity has weight. The total weight of air from the earths surface to the edge of space acting on the earths surface at sea level is 14.7 psi (pounds per square inch). A press. gauge calibrated to 0, in the vacuum of space, when brought to earth would read 14.7 psi. Ok so thats easy. The important number here is 14.7 psi that is the maximum amount of press that can be exerted on the exterior surface of a vessel that has been evacuated to a perfect vacuum. Doesn't sound like much right. As in the example below a 3"X 4.5" pop can, has a surface area of around 56.5 sq. in. X 14.7 psi = 830.5 . So if you evacuate the air from the can, or condense the steam as the case may be, thats 830.5 lbs. acting on the surface of that can to crush it. In real life it will be a bit less since perfect vacuums are harder to develop but thats still an awesome amount of power exerted by nothing more than the weight of air.

I could go on but thats the basics. Vacuum has and is being used to do work. As shown below, the earliest steam engines operated on the principles of vacuum not press, and vacuum is often used as a  clamp for glued assemblies. When overlooked, vacuum has also caused much damage as pictured in the second example below.

As an operating engineer my background is right but I always did better doing than teaching so here is a excerpt from "Mechanical Movements" on the early development of the steam pumping engine using vacuum.



And here is an example of what can happen if you don't understand or disregard vacuum and how it is developed. Pictures and commentary are on the Metalworking.com Drop Box Files site.