How a Polycold or MegaCold Works In Vacuum Coating to Increase Production
Water vapor trapping with a Polycold was a concept developed by Dale Meissner in the 1970's as a way to speed pump down of vacuum chambers and to reduce the base pressure to enhance vacuum coating. In pumping a vacuum system, you can either remove molecules of gas from the chamber or as in the case of water vapor change the state of the gas by condensing (trapping) the water vapor molecules on a cold surface.
Some Science Principles
When pumping a chamber from atmosphere, between ~10-3 to ~10-8 Torr, 65% to 95% of molecules in the chamber are water vapor. Whenever a vacuum system is opened to air, the internal surfaces of the chamber will be covered with layer upon layer of water molecules. The thickness of the layer will depend on the relative humidity of the air, the exposure history, etc. As vacuum pressure is lowered in the chamber this water vapor releases from the walls and surfaces of the chamber and then must be pumped or trapped to further reduce the pressure in the chamber.
A mechanical trap like a Polycold or MegaCold, uses refrigerant to cool a stainless steel or copper coil in the chamber. Ability of this coil to trap water vapor is mostly based on the amount of cryogenically cooled surface area in the chamber. As water vapor is released off the walls or other internal chamber surface, they must be either pumped or trapped to further reduce the vacuum pressure in the chamber. If you increase the amount of coil in the chamber the ability to trap water vapor molecules as they "bounce " around the chamber increases.
High Vacuum Turbo or Diffusion Pump Vs. Polycold or MegaCold Unit
Once a chamber is into the millitorr range, the high vacuum pump starts becoming effective to lower the vacuum pressure. In molecular flow, these gas molecules must intersect the opening of the turbo or DIP and be pumped away. This takes time as the pump opening is small.Many times these pumps are on elbows and not directly on the chamber. This results in up to a 50% loss of pumping efficiency due to conductance. So lets do the math.....
A 50,000 l/s high vacuum pump reduced in efficiency by 50% provides maybe 25,000 l/s in pump speed. So to get to 100,000 l/s, you need 4 of these pumps which is not practical due to cost and space concerns.
A Polycold PFC 1100HC or MegaCold 50Ain conjunction with a 5/8" coil design of about 120 ft in the chamber can produce in excess of 100,000 l/s of water pumping speed or 4X the high vacuum pump. Compared to the small inlet of the high vacuum pump, the coil in your chamber is a great target for the water vapor molecules to hit! So without the Polycold, it could affect the time to pump the chamber 4x or more!
Conclusion
Polycolds or MegaColds are vital to quickly evacuate vacuum chambers for production applications. In addition to speed the elimination of water vapor molecules in the chamber reduces the vacuum base pressure and in many applications improves the quality of vacuum coating. Increasing the surface area of the copper or stainless coil with the proper size Polycold or MegaCold, can result in increased production.
