In a typical industrial vacuum system it is possible to trap pockets of atmospheric air when manufacturing the system and each time a cyclic system is unloaded and reloaded. One example is  fixtures that are bolted to brackets on the chamber wall. If the securing bolts are placed in blind tapped holes, gas can be trapped in the bottom of the hole. With a regular bolt in a blind tapped hole, the only escape for gas molecules is along the thread form where there is space in the root and crown areas of the thread. If a blind hole is unavoidable, bolts that have a hole through them will allow the air to escape easily during the evacuation part of the process. There is a company in the USA that specializes in these types of bolts and screws. Using tapped through holes eliminates this problem.

If a fixture is held in place with a bolt and a nut or nut and washer, air can be trapped in the space around the bolt in the clearance hole. Again the air will slowly bleed away during the evacuation and extend the pump down time. Even mating surfaces can trap air under them which will cause a slower evacuation because the trapped air will only release slowly. Another example of trapped gases in the heat treat industry is when porous insulation materials are used. Air will be adsorbed into the porous material, which has a very large internal surface area, at atmospheric pressure and will then be slow to release during the evacuation due to low conductance of gas from the very tiny spaces in the material.. By Howard Tring

As stated last month, in Part 1, this article talks only about one and two stage “medium vacuum” oil sealed rotary vane vacuum pumps that can produce a catalog ultimate vacuum of about 1 x 10^-2 Torr (0.01 Torr or 10 microns) for a one stage model and about 1 x 10^-3 Torr (0.001 Torr or 1 micron) for a two stage model..

Smaller vacuum pumps such as those used in the heating, ventilating and air conditioning industry (HVAC) are not included as they are often only for intermittent use and do not have the design features built into the laboratory sized continuous running vacuum pumps used in industry and science. Larger rotary vane vacuum pumps, ones that require ball or roller bearings to support the weight of the rotor are not included either. Although they have many similar features to the laboratory sized vacuum pumps, they also have a variety of options to suit different applications. By Howard Tring

This article talks only about one and two stage “medium vacuum” oil sealed rotary vane vacuum pumps that can produce a catalog ultimate vacuum of about 1 x 10^-2 Torr (0.01 Torr or 10 microns) for a one stage model and about 1 x 10^-3 Torr (0.001 Torr or 1 micron) for a two stage model.

Oil sealed rotary vane vacuum pumps are used in the vacuum heat treating and vacuum furnace industry as a holding pump at the exhaust side of the oil diffusion pump. It keeps the exhaust line pressure low enough to prevent stalling of the oil diffusion pump while the larger mechanical pump is roughing (evacuating from atmospheric pressure) the main vacuum chamber. In the early days, pre-second world war, there were also oil sealed rotary cam pumps designs, such as the “world famous Cenco ‘Hyvac’ two stage vacuum pump and the Nelson Pump Co. ‘Nevaco’ vacuum pump (a). Although the Hyvac pumps and other Cenco models are still manufactured today by HyVac Products, PA, the Nelson Pump Company was bought by Ace Pump, TN, and their vacuum pumps are no longer made. By Howard Tring

This article continues the series of Five Reasons that vacuum is used in science and industry. The first “To Provide a Working Force” was published in December 2013. In addition to writing about the reason itself I will try to give one or more examples of the reason in practical applications. If you know of other applications that fit the reason, I will be pleased to hear from you.

2. To Remove Active & Reactive Constituents.

For many vacuum applications the process is a cyclic one. The workload is placed in the chamber, the chamber is evacuated and the process takes place. The chamber is then let back up to atmospheric pressure and the process workload is removed. These cycles can be as short as a few seconds in the case of coating a small communications crystal (Fig.1) to give it a specific frequency, or it can take several hours in the case of a heat treating cycle that may include evacuation, heating, soaking, cooling and eventually back to atmospheric pressure. In this type of cycle the main “constituent” being removed is “air”. By Howard Tring

I wish a Happy New Year to all readers of Vacuum Pump Practice. This article is about some simple physical phenomena that have a bearing on the vacuum processing industry. It will also provide the background information for next month’s article, so don’t forget to check back in February.

States of Matter – All matter consists of atoms, and some atoms combine with others in a chemical reaction to form molecules. For example, water consists of two atoms of hydrogen combined with one atom of oxygen (H₂O). Some gases such as argon (Ar), helium (He) and neon (Ne) are unlikely to combine with a similar or dissimilar atom, while others such as hydrogen, nitrogen and oxygen and usually combine with an identical atom forming a molecule (H₂, O₂ and N₂) and are called diatomic. By Howard Tring

This month I’m going to take a step away from vacuum pumps and systems and write about general applications that use vacuum in the process. There may be some applications you have heard about and some, hopefully, that may be new to you. Whenever a vacuum (a pressure lower than the surrounding atmospheric pressure) is used in a process it will generally fall into one of the Five Main Reasons for using Vacuum. In some cases a process may use vacuum for two of the five reasons. This month I will discuss the first of these reasons, in no specific order.

1. To Provide a Working Force.

First, a short explanation of the two vacuum measuring units used in this article. We know that standard atmospheric pressure is 14.7 lbs. in^-2 and that your real life atmospheric pressure varies up and down a few percentage points from the standard depending on a) weather conditions in your area and, b) your altitude above sea level. By Howard Tring

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. By Howard Tring

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. By Howard Tring

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. By Howard Tring

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. By Howard Tring