Roots Pump

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Chris Giles
Posts: 31
Joined: Mon May 14, 2018 10:46 pm
Real name: Chris Giles

Roots Pump

Post by Chris Giles » Mon Jan 07, 2019 11:31 pm


I have a rather unusual backing pump which I have just got working. In case anyone else finds one I will tell you what I had to do to fix it up and I’d appreciate any input on how this will differ in practice from a rotary vane pump. I do also have an Edwards E1M5 RV pump, as yet untested which I need to get the oil for.

The pump is described here.
It is a dry multi-stage Roots pump with a fast pumping rate of 8.2CFM but relatively weak final vacuum: 22.5 microns / 0.03 mbar stated in the datasheet. I will be using it as a backing pump to a Pfeiffer Turbo which says it only requires 5 mbar from the backing pump. Pfeiffer do sell this range of pumps in pumping stations with their turbos so I believe this should be fine.

When I got the pump it was a bit of a punt as the case was battered and rusty. The pump assembly was loose in the outer case. I had to repair some of the cabling and used 4 M6 bolts to secure the pump in the case. Also it is a 3-phase model with a Harting industrial connector for the AC input which was damaged. I was fearing the worst overall but actually the unit is quite new and looks pristine internally. See below:
It has a VFD controller inside which is capable of running on US or European line voltage:
The VFD rectifies the mains then generates the AC required for the motor at the required frequency to allow different motor speeds. The 3 input lines go into a 3-phase bridge rectifier, which is just like the standard 4-diode arrangement but with an extra pair of diodes for the 3rd phase (there is no neutral) I have simply connected 2 of the lines together, on the grounds that the UK 240 VAC single phase is approx. the same as one of the phases from US 3-phase (220V) which it can operate on. I replaced the AC connector and it works! It does appear to need “warming up” at a lower speed for a couple of minutes, otherwise it shuts down with what is probably an over-current error. Perhaps this is because its not properly configured to run at this lower voltage. Anyway, after this point it seems to run happily at max speed (100Hz).

I have rigged it up with a 50cm KF25 hose and an MKS 901P to test it. It gets to 0.1 mbar in about 10s then after 2 mins it settles at about 3.5x10-2 mbar (with purge gas inlet sealed off roughly). I notice that as it is turned off, the pressure reverts back to atmospheric almost immediately. As expected, there are no oil fumes from the pump. There is no user-serviceable oil. It is pretty loud though - sounds like a mixture between a jet engine and motorbike!

So what I’m wondering is:
- Are there any practical differences I would find between using this vs an RV pump as a backing for a turbo?
- Given that it is compatible with the Pfeiffer turbos is there any downside to the low vacuum?

Also could anyone tell me what type of gas connector this adaptor is? It is going from a G1/4. I just want to put a tap on it to seal it firmly:

I realize an RV pump would need a foreline valve to stop the oil backstreaming. Now I see with this pump I would also need a valve to shutoff the foreline in case the pump stops for any reason, otherwise there could be a very fast pressure rise coming into the exhaust of the turbo. I am thinking of using a solenoid valve and setpoints in the MKS 901P sensors so shut it automatically.


John Futter
Posts: 1342
Joined: Thu Apr 22, 2004 2:29 am
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Re: Roots Pump

Post by John Futter » Tue Jan 08, 2019 6:05 pm

Normally a roots type pump is used with an ordinary backing pump to significantly speed up the pumping speed
they are also used in industrial cutting lasers to circulate the lasing gas through an intercooler

Chris Giles
Posts: 31
Joined: Mon May 14, 2018 10:46 pm
Real name: Chris Giles

Re: Roots Pump

Post by Chris Giles » Tue Jan 08, 2019 10:26 pm

Thanks John.

Yes, I read in some other posts that Roots pumps were an addition to an ordinary type. However, this one is definitely marketed as a stand-alone backing pump by Pfeiffer. In some models of their HiCube Turbo Stations they use this range of pumps where they advertise a faster pump-down time. This one uses the model i have, so it must be fine for that. I've just read the manual for that Turbo Station and i see that does have a normally closed solenoid valve in the foreline to protect the turbo so I will plan to add that.

I just found this in some of the marketing blurb:
The multi-stage ACP roots pump is the dry alternative to the conventional
rotary vane pump. As the ACP does not include gaskets between rotor
and stator and the rotors run without contact, no gasket wear results in
the pumping system as it does with other dry pumps. Therefore, this
pump is suitable for all vacuum applications for which maximum
cleanliness is required. In normal use, the maintenance intervals are up
to four years apart.

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