When you think of common maintenance problems that affect your vacuum furnace and pumping system, backstreaming is one of the more-reported problems with diffusion pumps.
A small amount of backstreaming will occur in all diffusion pumps. While this is acceptable for many applications, more critical applications require a baffle be installed to prevent particles from leaving the diffusion pump and reaching the main process. Using a subrefrigerated cold trap between the diffusion pump and main valve can significantly reduce the oil’s ability to backstream into the furnace.
What is backstreaming?
The pumping system is designed to pump in a certain direction. Backstreaming occurs when the pumping fluid flows opposite of that desired direction. This results in the pumping fluid entering the vacuum furnace chamber rather than going in the desired direction of flow back into the base of the pump where it is recirculated. When the pressure in the chamber continues to rise (after crossover), the fluid that is still in transition has a higher potential to become pulled into the vessel via the pressure differential.
What issues can backstreaming cause?
Backstreaming can result in contamination of the hot zone or parts. The oil can also get into any braze material, resulting in poor brazing and bonding. Backstreaming can also affect the entire pumping system, not just the pumps themselves.
What are possible causes of backstreaming?
- Abnormal crossover into high vacuum—normal crossover is 80 microns or less.
- Operating above one micron for extended periods of time
- Low or high oil levels
- Overheated diffusion pump—the ideal water temperature should be 60-80 °F (16-27 °C); the water outlet maximum temperature is 125 °F (52 °C). Causes of overheating include:
- Low or no water flow
- Inlet water being too hot (above 90 °F / 32 °C)
- Leaks in diffusion pump or holding pump circuit
- Low heat input to the diffusion pump, cause by:
- Defective heaters
- Blown fuse or disconnected wires
- Too much oil
- Excessive amount of water
- Water temperature too low
- Wrong or improperly wired replacement heaters
- Diffusion pump thermocouple exposed to cold air
- Backfilling chamber before the main valve is fully closed
- Poorly operating or undersized backing pump
- Changing the manufacturer of the pumping fluid used. This can affect the operating temperature, making it too hot or cold.
Note: Do not use copper wires or lugs to connect heaters as they will oxidize and fail. Only use nickel wires and lugs. As a short-term solution, stainless steel lugs can also be used.
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