I have one stage vane pump for refrigeration and of course it cant produce enough vacuum, so I added piston compressor in series which makes about 50torr vacuum.
And the big surprise is that this dual stage system works significantly worse than vane pump alone.
If I disconnect compressor from working system , i get better vacuum almost instantly if i connect it back vacuum get worse again.
Any ideas why this happen?
Strange pump behavior
Re: Strange pump behavior - Oil sealing mechanism uses atmospheric pressure to function
Digix,
What is happining is that you are removing the pressure that is needed to push the
vane pump's oil in the the vane cavity.
Oil is needed to provide the dynamic seal between the vane, rotor and the stator,
end plates. This needs the pressure from the outside of the vacuum of the pumping
cavity to supply the oil feed.
When you applied the piston pump to the normal outlet of the vacuum service pump,
the pressure that is used to feed the oil is removed and thus the subject vacuum service
pump runs dry and there is no oil to seal between the vanes and stator ring and thus
it works worse.
Many of the diagrams do not show that the oil also displaces the bit of air in the
exhaust passageway between the cavity and the reed valve. It is this total displcement
that gives the magic of the vacuum service air pump.
Otherwise that bit of air will return to the arriving crescent of space after the passage
of a vane if it was not for the displacive effect of the oil in the cavity.
Hope this helps
Mike Kan
What is happining is that you are removing the pressure that is needed to push the
vane pump's oil in the the vane cavity.
Oil is needed to provide the dynamic seal between the vane, rotor and the stator,
end plates. This needs the pressure from the outside of the vacuum of the pumping
cavity to supply the oil feed.
When you applied the piston pump to the normal outlet of the vacuum service pump,
the pressure that is used to feed the oil is removed and thus the subject vacuum service
pump runs dry and there is no oil to seal between the vanes and stator ring and thus
it works worse.
Many of the diagrams do not show that the oil also displaces the bit of air in the
exhaust passageway between the cavity and the reed valve. It is this total displcement
that gives the magic of the vacuum service air pump.
Otherwise that bit of air will return to the arriving crescent of space after the passage
of a vane if it was not for the displacive effect of the oil in the cavity.
Hope this helps
Mike Kan
Re: Strange pump behavior - Oil sealing mechanism uses atmospheric pressure to function
There's a basic problem with your arrangement. You 've built a two stage pump system, which CAN work, provided the pumps are the proper type.
In this case, they aren't. A typical piston type refrigeration pump has a non-zero dead volume that limits its compression ratio to somewhere around 30:1. This is the result of the exhaust valve's construction.
Since an vacuum pump is actually a compressor, (the exhaust is just above atmospheric pressure) the piston pump's lowest possible inlet pressure would be around 25 Torr. Lower than that and it will not have enough compression to open the valve against the atmosphere.
Therefor, this type of pump will restrict the system, to a fairly high ultimate pressure.
However, it you can find a second rotary vane pump, you have a chance to make a two stage system that could reach a lower inlet pressure than one pumpcould produce. This is because of the point Michael Kan made about the oil filling the dead space so the vane pumps compression ratio can be very high.
Michael's suggestion about the oil supply being curtailed is interesting. I hadn't heard of that issue before.
Dave Cooper
In this case, they aren't. A typical piston type refrigeration pump has a non-zero dead volume that limits its compression ratio to somewhere around 30:1. This is the result of the exhaust valve's construction.
Since an vacuum pump is actually a compressor, (the exhaust is just above atmospheric pressure) the piston pump's lowest possible inlet pressure would be around 25 Torr. Lower than that and it will not have enough compression to open the valve against the atmosphere.
Therefor, this type of pump will restrict the system, to a fairly high ultimate pressure.
However, it you can find a second rotary vane pump, you have a chance to make a two stage system that could reach a lower inlet pressure than one pumpcould produce. This is because of the point Michael Kan made about the oil filling the dead space so the vane pumps compression ratio can be very high.
Michael's suggestion about the oil supply being curtailed is interesting. I hadn't heard of that issue before.
Dave Cooper
Re: Strange pump behavior - Oil sealing mechanism uses atmospheric pressure to function
Michael's theory can be right, since when under atmospheric pressure vane pump produces much more sound than under that weak vacuum.
Unfortunately use of second vane pump will be too expensive option, probably it would be better to buy 2-stage pump instead of second 1-stage in that case.
But in that case how much vacuum i must provide to this vane pump to run it properly in 2 stage system?
Or probably it will be best to leave vane pump as is one stage and try to make some drag pump to put in front of it. since as I see 2 stage vane pump vacuum is not much better than one stage.
Unfortunately use of second vane pump will be too expensive option, probably it would be better to buy 2-stage pump instead of second 1-stage in that case.
But in that case how much vacuum i must provide to this vane pump to run it properly in 2 stage system?
Or probably it will be best to leave vane pump as is one stage and try to make some drag pump to put in front of it. since as I see 2 stage vane pump vacuum is not much better than one stage.
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Frank Sanns
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Re: Strange pump behavior
The best you can hope for with this arrangement can be achieved if the piston pump is the first pump in the series. The first to pump down the atmosphere to a few torr or so. Run a vacuum line to the exaust of your single stage vane pump. This configuration should give you the best chance for sucess. The other way has no chance for a variety of reasons but most notably as previously stated by others is that there is a significant volume of air that a piston pump can never exhaust. This will always limit the attainable vacuum compared to even a single stage vane pump.
Frank S.
Frank S.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
Re: Strange pump behavior
Frank S. wrote:
> The best you can hope for with this arrangement can be achieved if the piston pump is the first pump in the series. .
>
> Frank S.
Are you sure for this? since it looks quite impossible to do anything in that way because if I put piston pump first vacuum pressure will be not enough to open piston pump valves.
also there are no problems with exhaust for piston pump since if required i can put any amount of piston compressors in series, they are free anyway.
> The best you can hope for with this arrangement can be achieved if the piston pump is the first pump in the series. .
>
> Frank S.
Are you sure for this? since it looks quite impossible to do anything in that way because if I put piston pump first vacuum pressure will be not enough to open piston pump valves.
also there are no problems with exhaust for piston pump since if required i can put any amount of piston compressors in series, they are free anyway.
Re: Strange pump behavior
Digix -
I think Frank is saying the Piston pump needs to be on the atmosphere side.
There is one other point to make, for any tandem pump arrangement. the pump's volumetric capacities must match up.
If the atmospheric side pump...("first or last" depending on which way you view it)...is too small, it will restrict the throughput of the high vac side pump. The restricted flow, may NOT be enough to adequately pump down your chamber... due to leaks, outgassing and etc. Thus the pressure could rise, in the tandem pump configuration.
Many refrigeration compressors have a quite small outlet port, because their intended purpose is to pump a small volume at high pressure (10 - 30 atmospheres). Vacuum applications need a large volume at low pressure... one atmosphere.
Dave Cooper
I think Frank is saying the Piston pump needs to be on the atmosphere side.
There is one other point to make, for any tandem pump arrangement. the pump's volumetric capacities must match up.
If the atmospheric side pump...("first or last" depending on which way you view it)...is too small, it will restrict the throughput of the high vac side pump. The restricted flow, may NOT be enough to adequately pump down your chamber... due to leaks, outgassing and etc. Thus the pressure could rise, in the tandem pump configuration.
Many refrigeration compressors have a quite small outlet port, because their intended purpose is to pump a small volume at high pressure (10 - 30 atmospheres). Vacuum applications need a large volume at low pressure... one atmosphere.
Dave Cooper