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Re: Creating vacuum

Posted By: SVP Neon Equipment
Date: Wednesday, 1 August 2007, at 11:01 a.m.

In Response To: Re: Creating vacuum (Jan)


> Mark,

> Thanks for the detailed response. I guess
> the question then is : What is the physical
> difference between the mechanical pumps used
> to service refigeration systems, and the
> mechanical vacuum pumps used in the neon
> industry? Both is typically two stage rotary
> vane, so the concept is the same. Is it
> tolerances?

> My thinking is that a less expensive
> refrigeration pump (say 25 mikron) combined
> with a diffusion pump should give enough
> vacuum for neon tubes (1-5 mikron)

> Jan

Jan,

Unfortunately it doesn't quite work that way. I don't know what the specific differences are as I have never looked into refrigeration pumps to compare them in-depth. So I cannot give you a definitive answer as to what the differences are. But based on the mfgs. stated ultimate base pressures and reports from neon shops who have tried refrigeration pumps, there are no doubt significant differences.

If the refrigeration pump is factory rated for a base pressure of 25 microns it will likely not come close to that in real-world conditions. I would suspect more like 30-35 microns on a good day. Ambient air temperature and quality will affect this. On a hot, humid day it will be worse.

Diffusion pumps will start to function at their rated tolerable fore pressure for a particular pump, but that does not mean they will function very well or near peak performance. The better the base pressure capability is for a mechanical pump, the better the diffusion pump will work and the better the ultimate vacuum will be. Conversely, to a point, the worse the base pressure capability is for the mechanical pump, the worse the ultimate vacuum will be for the diffusion pump and the greater the chance for problematic vapor backstreaming.

Diffusion pumps are compression pumps. They take a small volume of gas load from the intake, which is too low for a mechanical pump to remove effectively, and pushes it toward the exhaust until the pressure in the exhaust of the diff pump is high enough for the mechanical pump to remove. This is a continuous function. So the faster the mechanical pump can draw off the compressed (relatively speaking) gas load from the exhaust of the diff pump, the faster and better the diff pump will function.

If the pressure in the exhaust port of the diff pump is on the order of 15-20 microns, a mechanical pump that has a base pressure of 10 microns will readily remove what is being 'fed' to it by the diff pump. On the other hand, a mechanical pump that has a base pressure of 25 microns will not be able to remove the gas load at a fast enough rate to allow the diff pump to function at its full capacity. The result will be a higher pressure (or not as good of a vacuum) on the intake of the diff pump. This does not mean the diff pump is not working, but it means it is not working as well as it should or could. And this quick example does not even consider the differences in pumping speed curves for the mechanical pumps, which also has a significant effect on overall performance.

To look a little further into it, immediately following bombarding you want the diff pump to start functioning fairly well as quickly as possible in order to turn the manifold into a virtual high vacuum pump and to remove the gas load from the neon unit as quickly as possible. With a better, higher capacity mechanical pump the diff pump will obtain its tolerable fore pressure sooner as well as its optimal pumping speed sooner to accomplish this.

The object is to get the diff pump working as quickly as possible and then to quickly reduce the foreline pressure to the point where the diff pump can function at peak pumping speed and throughput. For small diff pumps this is usually below the capabilities of typical refrigeration pumps.

Mark


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