Last time the discussion was about throughput and conductance in vacuum systems. This time we will look at the pressure profile throughout the vacuum system in a slightly different way than it was shown last time. The first thought might be that once the vacuum system is under vacuum carrying out the process, the lowest pressure will be in the vacuum chamber and that the highest pressure will be at the primary pump exhaust which will be atmospheric pressure. As we see from Fig. 1, this is not quite correct.

Fig. 1 shows how the pressure changes through the system and actual values of P pressure and S speed are given in the table, Fig. 2. The pressures shown assume that the chamber has been evacuated (pumped down) to the process pressure needed and conditions are stable.

Last month we discussed Gas Molecules and Gas Flow and at the end of the article mentioned the term Conductance. This time we will talk a bit more about conductance in vacuum system piping and why it has to be taken into consideration in the design of a typical vacuum furnace or similar vacuum system.

Firstly though, we will discuss Throughput. Have you ever wondered why vacuum pipes and connections are of several different sizes on any vacuum system? I would suggest that most users don’t really give it any thought. It is what it is. So let’s look at the sections of a vacuum system and again try to visualize those gas molecules, which are so tiny we can’t see them, and understand the conditions at different places in the system.

If you are a homeowner with a garden and a lawn, watering and mowing are regular tasks that need to be carried out. When you are watering your flower bed or vegetable patch at home you turn on the hose tap and see the water coming from the spray nozzle. You can then direct the water to the places where it is needed. Similarly, when mowing your lawn you can see what area you have already cut and can direct the mower to cut the next area of long grass. These tasks are made easier because you can see what you are doing. Trying to water or mow with your eyes covered would be much more difficult.

When you are evacuating a vacuum system, one of the biggest problems is that you can’t see what you are evacuating. Gas molecules are so small that you cannot see them. So how do you know when they have been moved out of the system and how do you know the best way to move enough of them to allow you to complete your particular process? In this article we will try to understand what gas molecules are and how they behave at different pressures in a vacuum system. If you have a mental picture of the molecules using your imagination, it can possibly help you to understand how your vacuum system works and solve problems with its operation if something goes wrong.

On any machine that has a rotating shaft there will be a shaft seal of one type or another. If the machine is a simple electric motor, for example, the seal may be used just to retain the lubricant in the bearings and to prevent dust and dirt from entering the bearing. This type of seal generally needs little or no maintenance for small motors from ¼ to perhaps 10 HP.

The shaft seals in any pump that the electric motor drives are ones that do need maintenance and replacement, whatever type of pump it is. If the pump moves liquids, such as a centrifugal water pump, it is important that the seal doesn’t leak although in some applications a small amount of leakage can be tolerated. If the liquid being pumped is hydraulic fluid, it may be at high pressure and the seal would be designed to withstand that pressure without failing.

The correct use of the Gas Ballast valve on a mechanical oil sealed rotary vacuum pump has always been seen as “black science” or just plain guesswork. It is a very simple device and when used correctly can keep a vacuum pump working well even though it may be used on a very wet process and the oil becomes contaminated with condensed vapors.

One series of vacuum pumps I worked with had a gas ballast valve that had no stop when opened. If you unscrewed it enough the ballast knob would come off in your hand allowing maximum air to enter the pump, the pump to become noisier and a blast of oil mist to come out of the exhaust. I always thought that if a lab technician ever did that they would replace the knob and never ever touch the gas ballast valve again.

In many vacuum systems, especially those where the chamber is large, has a large internal surface area and the chamber load adds extra surface area, the pump down can be slowed substantially when the chamber pressure drops to the range where the water vapor molecules on the surface desorb and have to be pumped away.

Pressure and temperature determine when this vapor desorbs, but at ambient temperature around seventy two degrees Fahrenheit or twenty degrees Centigrade the vapor desorbs from about 50 Torr down to about 0.1 Torr. The vapor pressure of water at ambient temperature is about 18 Torr, so that is where maximum desorption may occur.

Vacuum gauges measure the pressure readings in the range from atmospheric pressure down to some lower pressure approaching absolute zero, which is not attainable. Some gauges read the complete range with low resolution and others can only read a portion of the range but with better resolution, usually used for the lower pressures.

There are three groups of vacuum gauges based on the method of operation, mechanical, thermal conductivity and ionization. For this discussion we will only talk about the thermal conductivity and ionization gauges because purely mechanical vacuum gauges are generally not used on vacuum furnaces.