Rotary vane vs liquid ring performance ?

[CharlieVT]

Maybe someone can help explain what I am seeing as I experiment with my vacuum pumps.

I have an old rotary vane dairy pump which I have modified run oil through with an oil reclaimer. It pulls about 22"Hg. I have no idea what CFMs it is capable of.

I have two identical liquid ring pumps I have been trying to put into service. I have spec sheets for these pumps, they are rated for 20-25"Hg. and up to 30CFM. I rebuilt one of the liquid ring pumps with a new seal kit. Everything looked okay during disassembly, and the new seal kit didn't improve the performance.

With the liquid ring pumps isolated from my releaser and tubing system I can get 22-23"Hg indicated at the pump. As soon as I connect the liquid ring to the releaser/tubing system the vacuum drops to 14-15"Hg at the pump and never climbs back. When I subsitute the old rotary vane pump and connect to the exact same system will quickly pull the vacuum in the system down to over 20"Hg.

I can't figure out why the liquid ring pumps won't pull the vacuum down like the old rotary vane pump will. The liquid ring pumps are clearly capable of over 20"Hg when isolated from the system; I'm wondering if the liquid rings pumps just aren't delivering enough CFMs to pull the system down, but I am surprised that an old rotary vane pump can pull the system down to better than 20"Hg when the liquid rings cannot.

Anybody have ideas?
Thanks in advance for your replies.

[Buffalo Creek Sugar Camp]

Single stage liquid ring pumps typically do not pull many cfms when you get to high vacuum. The vane pumps seem to pull more cfm's, even at high vacuum. I have replaced a few ring pumps that were single stage with vane pumps that have coolers and have seen an improvement

[CharlieVT]

Single stage liquid ring pumps typically do not pull many cfms when you get to high vacuum. The vane pumps seem to pull more cfm's, even at high vacuum. I have replaced a few ring pumps that were single stage with vane pumps that have coolers and have seen an improvement
Thanks, that's a very helpful reply.

[Walling's Maple Syrup]

You have leaks in your tubing system and/or releaser. If you "deadhead" the pump and get 22-23" and you drop that much vac. when you open the woods there is no doubt you have leaks in the system. That said, Jeremy is right. Vane pumps have more of an ability to overcome those leaks than a ring pump, thus explaining the higher vac. levels with the vane pump.
Neil

[BreezyHill]

Rotary Vane pumps are much more efficient at evacuating to achieve a vacuum than a piston pump, due to the simplicity of the design and the frequency that the pump completes a cycle.
In a four vane pump there are four areas that preform at one time. At any given point of a revolution of the pump one segment is just starting, #2 is at 25% cycle, #3 is at 50% cycle and #4 is just starting to evacuate the compressed air. Since most vane pumps run at 600 to 1000 rpm then a total of 2400 to 4000 cylinders of air are removed per minute. Where as a piston pump would need to run at speeds twice as fast to achieve the same quantity of dumps per minute or be of a much larger cylinder size.
In a vane pump the laws of physics help its performance, as the vanes are thrust outward to the housing to function; where the piston pump is always working against natural forces as it works to compress air in a space and a valve assembly is forced open to release the compressed air.
Rotary vane carried a stigma because most pumps put into maple service were old worn out pumps that had already lived their life; but if you rebuild the pump or buy a new pump and modify it slightly to meet the stresses that it needs to endure a rotary vane is able to out preform a piston pump very well.
While if one has the equipment and the knowhow to turn a rotary into a liquid ring pump you can have the ultimate pump. Once this is achieved a you have a rotary vane pump that has the high vacuum characteristics of the liquid ring piston pump in a unit that is efficient in rapidly removing large volumes of air from a space.

So in your case you have a tubing system that has a leak. Several tiny leaks will be difficult to locate where a tap that has been removed is easy.

Spots to look for leaks are: saddles, mains connections, taps, and tubing connections.
Saddles that don't have the multi ring saddle pads and the locking bolt are more prone to loosening in the freeze thaw cycle
mains connection are also affected by the freeze thaw cycle and may need a clamp to be retightened from time to time.
taps...too deep and the tree can split causing a season long tiny leak.
Too loose and they sit there leaking til you seat them.
Tubing connection leaks are often the result of spreading the tubing too much prior to sliding on the connection.
Check the mains for rodent damage and deer damage.

Simply put you have learned a valuable lesson on pumps. Now search out those leaks and you will be fine.

Good Luck!
Ben

[wiam]

Um...."liquid ring" and "piston" pumps are completely different

[CharlieVT]

Um...."liquid ring" and "piston" pumps are completely different

Yes, I was a little confused by Ben's post. Helpful in intent, but a little confusing. No one had previously posted about piston pumps. Sometimes we need to define our terms, but I wonder if the statement: "...the liquid ring piston pump..." doesn't suggest some confusion about the nature of different pump types.

I actually have all three types, an old dairy piston pump, an old dairy rotary vane pump, and a couple single stage liquid ring pumps. I aquired all these when I got the contents of a sugarhouse. I've rebuilt all three and have a pretty good idea how they work. I am becoming an expert in old junk. ;)



The early reply which stated that the liquid ring pumps don't pull a lot of CFMs at high vacuum makes sense. I have come to understand that while the liquid ring has certain advantages, it is more sensitive to leaks and has trouble maintaining high vacuum when it has to move high CFMs at the same time.

Thanks to everyone who has taken the time to reply.

[BreezyHill]

Sorry to confuse some of you but when I say piston I was referring to the LR pump. Running on only a few hours of sleep lately. But those that have looked at the pumps likely got the grasp.

One mans junk is anothers treasure. Just have to know how and what to modify and you can make a super pump inexpensively.

I would sermize the pump was onlyu a single stage or in need of a good rebuilding if the LR could only hit 22-23 choked off.

While when I spoke of liquid ring on a rotary it is remanufacturing the rotary pump to supply oil evenly to the vanes to produce a liquid ring or seal of oil to the vanes and housing. This enables the rotary vane to become a super star in its own right.

Check out the rules in nascar that prohibit the rotary engine in racing. That's an interesting read if you have ever seen a mazda RX motor in action.

Sorry again for confusing you!