By now, most maintenance departments should have a plan in place for schedules repairs and maintenance during the summer months, especially if the plant has a shutdown for vacation. The part of the vacuum furnace system that I know best is the vacuum system, so I will concentrate on this.

The summer maintenance schedule should cover all the vacuum pumps on a vacuum furnace. For some it will just be mechanical pumps, the rotary piston pumps and the Roots booster (or blower); for others it will include the oil diffusion pump or pumps and the small rotary vane holding pump, if used. By Howard Tring

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. By Howard Tring

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. By Howard Tring

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. By Howard Tring

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

If you have a typical vacuum furnace it is normal to have at least three electronic vacuum gauge heads mounted on the system to monitor the level of vacuum at selected positions. These gauge heads send signals back to the controls system and the vacuum readings are used to ensure that the vacuum pumps are working correctly and that the process chamber is at the correct low pressure (vacuum) for the specific process. To many casual observers the readings and names of the measuring units being used are like a foreign language, and they may well be because many names were derived in Europe. Let’s take a look at the different vacuum measurement units in use around the world and where the names came from. By Howard Tring

Since oil sealed mechanical vacuum pumps started to be commercially available in the 1920s there have been many companies making this type of pump. The oil flooded rotary piston vacuum pump runs at a slow rotational speed and is a heavy duty design. It has been manufactured for about 85 years.

In the early 1900s these pumps were relatively small at up to 40 cfm capacity. By the late 20s and early 30s the industrial revolution was gaining steam and the manufacturers designed larger capacity pumps to suit new applications. By the 1940s rotary piston vacuum pumps were offered in capacities up to 700 cfm. They are known for the ability to keep on running despite ingesting all sorts of contaminants. Kinney claims to have introduced the world’s first rotary pump in 1904, two years after the company was founded. The Kinney document I have just says rotary pump but I presume it was a rotary vane design as other documents say the rotary piston pump design was introduced in about 1909. In 1926 their popular model was the VSD-8811, later to be the KS-47. Current large models are called the ”KT” series. By Howard Tring