View Full Version : Balance piston to female rotor

03-04-2013, 12:21 PM
Does anyone know the importance of having a balance piston fitted to a female rotor. As with Mycoms, they are not fitted. Other machines such as Howdens include them. The female rotor generally spins slower and has less surface area on discharge. Is this just a safety measure to prolong the thrust bearings? Interested to hear some feedback.

03-04-2013, 05:44 PM
Not refrigerant but the principles`s the same. On Screw air compressors the balance piston does just what it says, it balances the trust of the screw being pushed back by the air being compressed thereby taking the load off the thrust bearings. The Balance piston on an oil flooded machine is controlled by oil pressure at the same pressure as the air being compressed. On oil free compressors it`s controlled by air pressure.

04-04-2013, 11:09 AM
Thanks for your reply but it really hasn't cleared the fact that some manufacturers include them and some don't. Take a booster for instance, due the the low pressures that they are subjected to, some boosters don't even come or need balance pistons as the thrust bearings are enough to hold the rotors as opposed to a high stage/single stage compressor. I am trying to understand the purpose of a balance piston on a female rotor as the thrust load on it is fairly minimal ( I think). Regards.

04-04-2013, 12:57 PM
All of the above is correct, the balance piston 'balances' the thrust loads and prolong thrust bearing life, and female rotors do have a lower thrust load than the male. If you look at Howden machines you will see the female piston has a smaller diameter in recognition of the differing loads. Howden supply compressors with balance pistons to suit the general duty range of that frame size, but it is possible (at additional cost) to have piston diamaters calculated to a specific duty condition (common on gas processing units) to maximise bearing life. As to other manufacturers I cannot comment, but in the absence of a balance piston there will probably be an additional or larger thrust bearing to compensate. hope this helps!

04-04-2013, 10:56 PM
Hi, Mat-NH3 :)

welcome to RE forums ...

All answers are correct ... some manufacturers produce compressors with thrust balance piston&sleeve installed on male rotor only, some on both rotors and some with additional thrust balance piston incorporated into input shaft seal arrangement (Howden). All depend on compressor duty.

Do not know about other Mycom screws, but V series are equipped with balance piston&sleeve on the male screw i.e. driving shaft.

As I remember Stal screws were equipped with balance piston&sleeve and with very rigid axial thrust bearings .... overhaul time recommended by manufacturer were 40000 hours for high stage compressors for busters even longer time.

New compressors are of a little bit different construction like smaller thrust bearings and smaller thrust balance pistons (recommended overhaul time is shorter), but for sure we can say balance pistons&sleeves are there to extend the life of axial thrust bearings.

Final construction and execution depend on manufacturer of screw compressor and for us is not easy to say exactly why.

I know this is not an exact answer to your question, but I agree with you .... axial forces on the female rotors are very small and that is the reason to not install balance piston&sleeve.

Maybe some other members can put more light on this issue.

Best regards, Josip :)

05-04-2013, 12:56 PM
Hello, balance piston for female rotor is used for high-condensing high pressure, such as heat pump .......

10-04-2013, 12:16 PM
It appears that only Howden have balance pistons on female rotor that I know of. There may be a few that have pressure on end of female rotor like Dunham Bush, but comparing different brands of compressors can bring more questions than answers. Some manufacturers even have variable pressure on male rotor balance piston in high pressure applications. In heavy duty application tilted pad thrust bearings are used.

21-04-2013, 10:29 AM
Just to confuse things further.
The Gram GSV compressor and the Frick TDSH compressor are more or less identical internally. The Frick has balance pistons on both rotors and the Gram only on the male. To compensate for this the thrust bearings on the Gram are much larger.