Vacaero

Manufacturers of heat treating and brazing vacuum furnaces and controls, complete hot zone and vacuum furnace retrofits, thermal spray coatings, plasma, HVOF and paint coating services.

VAC AERO Service Experience Trust
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Gas Ballasting of Mechanical Oil Sealed Rotary Vacuum Pumps

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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.

Mechanical Booster Pumps for Vacuum Systems

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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.

Hot Zone Selection for Vacuum Brazing of Superalloys

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The Thermal Processing Divisions of VAC AERO International have provided repair services for damaged components from land-based and aerospace gas turbine engines. Engine manufacturers, operators, overhaul centers and commercial airlines are just a few of the customers that depend on these services. Many hot section engine components are fabricated from nickel-based superalloys. These materials cannot be repaired by traditional techniques, such as welding, without causing significant reductions in mechanical properties. As a result, VAC AERO developed proprietary vacuum brazing techniques to repair cracks, wear, and other service-induced damage.

The extent of damage to the engine components is often severe. Therefore, the brazing process involves the use of large amounts of brazing filler metal to make the necessary repairs. When subject to high temperature under vacuum, volatile metallic and organic constituents vaporize from the filler metal. While a portion of these volatiles is removed from the furnace chamber by the vacuum pumping system, the balance tends to condense within the chamber, much of it depositing on the hot zone insulation. In addition, excess molten braze alloy occasionally drips from the workload onto the heating elements and insulation, despite the use of drip trays. These deposits can have a detrimental effect on the performance of the furnace. As a manufacturer and user of vacuum furnaces, VAC AERO needed a hot zone design to withstand these aggressive brazing applications. BY JEFF PRITCHARD

Vacuum Gauges Used on Vacuum Furnaces

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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.

Understanding Vacuum Measurement Units

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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.

Backstreaming

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In simplest terms, backstreaming is the movement of pumping fluid back into the vacuum furnace chamber, that is, oil vapor molecules attempt to reverse course and move up and back toward the vacuum vessel, opposite to the direction of the desired gas flow. Backstreaming is not limited to the pumps themselves, but encompass the entire pumping system (e.g. plumbing, valves, baffles, and traps). The oil type and characteristics play a role as well. In all cases, the result of backstreaming, namely the contamination of the work chamber or workload, is totally unacceptable and often catastrophic.

Backstreaming is often due to; incorrect start-up or shutdown procedures – the far most common operator mistake as far as the writer is concerned, exceeding maximum pump throughput capacity for long periods of time and exceeding the critical discharge pressure in the foreline. Users of vacuum furnaces should be sure that the vacuum system is equipped with all the appropriate interlocks to prohibit vacuum valve cycling above specified pressures that can cause these effects to occur, which will help protect your system, especially whenever it is left unattended.

Saving Money by Maximizing Furnace Uptime Productivity

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“It was only a tiny drop of water, now and then,” lamented the homeowner. “How was I to know that all those little drops would add up to a huge water bill?” The same can be said of a heat treat furnace that is always down for this reason or that. Avoiding the hidden costs associated with equipment downtime is the key to saving money. Maximizing furnace productivity requires a proactive approach, which must continue throughout a unit’s operational lifetime. This requires careful planning and anticipation of problems. The process should begin even before the purchase of a piece of equipment by matching equipment and supplier capabilities with production and process needs. Buying good, well-built, high-quality equipment and operating and maintaining it properly will avoid most hidden costs.

For example, suppose a work center is scheduled to run for a 435-minute shift. However, the work center experiences 30 minutes of unscheduled downtime. The available time equals 435 minutes (scheduled time) minus 30 minutes (downtime), or 405 minutes. The availability is 405 minutes divided by 435 minutes or 93%. Not bad, or so you think. Now, let’s look at performance. Performance represents the speed at which the work center runs as a percentage of its designed speed. In other words, parts produced times ideal cycle time divided by available time. Continuing our example, if the available time is 405 minutes and the standard rate for the part being produced is 40 units per hour (or 1.5 minutes per unit), then the work center produces 242 total units during the shift. If the time to produce the parts (242 units times 1.5 minutes per unit) is 363 minutes, then the performance is 363 minutes divided by 405 minutes or 90%. Again, not bad, or so you think.