Question about pressure variation when using plasma treater

We have metallizer. When operating plasma treater there is a variation or rise in evaporation zone pressure throughout the metallizing cycle. Theoretically the pressure of winding should rise as plasma treater is located in winding section, but the winding zone pressure is not varying, what is the reason behind this?

Answer

This sounds to me to probably be related to a problem of cooling.

I would monitor any cooling systems that you are using. If you have different cooling water to the plasma treater, the deposition drum, protective shields and the evaporation boats I would monitor each separately. I would check both the input temperature and output temperature.

Typically once you have pumped out your system the residual gas is almost all water that is out gassing from all surfaces including from the unwinding roll from trapped air and from water contained within the polymer.  Once the roll is unwinding there will be an increase in the gas load because of the release of the trapped air as the roll keeps presenting fresh surfaces. 

As you suggest this is a fairly constant process and so you would expect everything to remain constant throughout the deposition run.

If you have a problem with a cooling system what happens is that initially everything is cool as the process has only just started and there may be a large thermal mass that has to be warmed up. If you are recirculating the water (or coolant, which may be a water /glycol mix or something similar) there is also the thermal mass of the water.  What happens is that the water passes through something that is heating up and so the water is heated up, but usually to a lower temperature than whatever they are passing through. The coolant then returns to the chiller that returns the coolant to a constant starting temperature.  If the heating load is higher than the chiller capacity you will progressively see that the coolant is no longer returned to the same starting input temperature but this temperature gradually creeps up.  As the coolant temperature creeps up it no longer cools as well as it ought to and so the internal surfaces become hotter.  If these surfaces have any stray deposition on them they will have a very high surface area and so can contain a lot of water. As the temperature increases water can be released from the surface thus raising the pressure.

Most systems have cryopanels that will pump away the water very well but it does depend on the water having access to the cryosurfaces.  It is common to see systems with only cryopanels in the unwind zone whereas it has been shown by Telemark Cryogenics  Ltd as presented at AIMCAL that it is better if the cryopanels are distributed across different zones.  The deposition zone will always heat up the web and internal surfaces more that anything occurring in the unwind zone. The deposition zone also has the greatest potential for having coated surfaces that can mop up very large quantities of water.

If the problem does not appear what the plasma treater is not used it could be that some of the cooling systems are linked and that it is only when the plasma treater is used that the chiller cannot keep the temperature low enough.  Even if the coolant systems are not linked the plasma treater does add another heat load within the system. It is possible that the hot plasma treater is radiating heat to another surface that is the releasing the water.

Another option is that the extra heat is reaching the cryopanel surface and that over time the surface of the ice no longer remains frozen but stays as a liquid with a correspondingly higher vapour pressure.  The cryopanels are always a balancing act as the core remains cold but the growth of the ice can affect the total capacity of the panel. If the water is released within the system very quickly the ice may be very porous and this is then has a worse thermal conductivity than if the ice was built up slowly and was very dense.  The dense ice will allow more water to be collected than the porous ice. If there is too much water within the system over time the ice will build up to a thickness where no more water can be condensed and so the system pressure will rise.

Finally it is possible you have a small water leak that is normally closed but under heating opens up slightly and leaks water out.

I hope this helps you sort out the problem.