Effects of backstreaming on a diffusion-pumped system

[rkum99]

Sorry, if this should have been in an homework section.

Anyway, I was wondering what effects backstreaming has on the pumping speed
of a diffusion-pumped system. I can see how you would not want backstreaming
to maintain clean system, but would it also affect the net speed?

Also, does anyone know how to calculate the volumetric flow rate at a point along
the vacuum system. I know how to calculate it in theory, but how would one go
about solving something of this sort in practice (with an actual vacuum system).

Thanks for the help,
me.

 

[nsaspook]

Also, does anyone know how to calculate the volumetric flow rate at a point along
the vacuum system. I know how to calculate it in theory, but how would one go
about solving something of this sort in practice (with an actual vacuum system).

Thanks for the help,
me.

There is always some increased level of backstreaming as the hot oil vapor jet contacts a greater amount of air molecules as flow is increased that usually requires a baffle or cold trap to condense the vapor back into the pump. Having to pump that escaped vapor back into the pump does slightly effect the max possible pumping speed further along the system. At some point you start to have excessive backsteaming caused by excessive pump pressure that exceeds the actual pumping speed of the oil vapor at the pump causing a hell of a mess.

Some practical methods of calculating vacuum flows and pumping speed can be found here.
https://www.lesker.com/newweb/menu_techinfo.cfm?section=condcut&init=skip
https://www.lesker.com/newweb/menu_techinfo.cfm?section=pumping&init=skip

 

[rkum99]

Thanks a bunch, this will be useful.:)

 

[nsaspook]

Thanks a bunch, this will be useful.:)

The rate of oil migration from backstreaming is also determined by the vacuum flow regime of the molecules. At low flow rates and high vacuum the pumping system is in molecular flow so the random movements of oil vapor molecules can easily cause them to migrate into a vacuum chamber.

http://www.pfeiffer-vacuum.com/know...es-of-flow/technology.action?chapter=tec1.2.6

 

[rkum99]

One more question:

Say I wanted to calculate the conductance of the system using the diameters of the pipes, do
I need to consider the conductance through the diffusion pump, mechanical pump, cold trap, etc.
I would assume that I need to, but the website seems to imply that I only need to take into
consideration the conductance of the connections.

If this is the case, would a cold trap that is not in use be considered as a pipe?

 

[nsaspook]

One more question:

Say I wanted to calculate the conductance of the system using the diameters of the pipes, do
I need to consider the conductance through the diffusion pump, mechanical pump, cold trap, etc.
I would assume that I need to, but the website seems to imply that I only need to take into
consideration the conductance of the connections.

If this is the case, would a cold trap that is not in use be considered as a pipe?

Normally the traps are directly on top , short and are the same diameter of the diffusion pump inlet so their effect on conductance is small under normal flow conditions. The mechanical pump system conductance is isolated by the compression ratio of the diffusion pump inlet to the foreline so as long the correct level of foreline pressure is maintained it's effect on hivac system conductance is small.