Vacuum pump

[edtihcc]

When using a vacuum pump to maintain vacuum in a distillation, is it true that upon establishing a vacuum, it should be running at lower speeds as the process goes because there is less vapour produced?

if it runs at full speed all the time to maintain the pressure, what could be the problem?

 

[pablo02]

It depends upon the type of pump motor and the controls...

If driven by a regular motor, no, it will run at the same speed no matter what the vacuum -- the motor amps should probably drop somewhat as the vacuum increases... in this case, also check the presence of a control valve, as the vacuum increases, it should be closing (?? maybe opening ?? see note below)...

If it is a variable speed motor, then yes, the speed should be dropping off, depending upon the control settings...

At some point, the pump will reach a point where it will only maintain a given vacuum, depending upon a number of factors --

a) if a constant speed and no control valves, you need to know what vacuum it should be able to maintain (also check amp loading of pump); failure to maintain that expected vacuum means you have a leak of of some proportion

b) if a constant speed and a control valve, if the valve remains open [?? or throttled closed ??] (again check motor amps), then suspect a control problem or a leak [always assume the controls are working untill you prove otherwise] -- I am not that familiar with a system using a control valve in this application -- I am not sure if it should open or it should close...

c) on a variable speed, check the speed vs vacuum -- if the speed does not come down (and vacuum correspondingly) again, check the system for leaks and then the control systems..

 

[25362]

An alternative with "rough" vacuum (absolute pressure 1-760 torr):
The operation consisting in evacuating a system in a reasonable lapse of time, and then to maintain a vacuum by exhausting air leakages or other non condensables coming in with the feed or generated in the process, as well as when circulating a gas from vacuum to pressure, as with a battery of rotary vacuum dewaxing filters, is also done by having a set, of say 3 or 4, equal vacuum pumps.
Thus one can reduce the number of working pumps as the process becomes steady.
Control instrumentation can be used to adjust the varying vacuum to a desired level.
With booster ejectors, sometimes it is practical to install two unequal units, say 2/3 and 1/3, to enable saving steam in similar conditions.

 

[DrFluor]

Depending on the pumping system you are working with, there may be either an internal or external gas ballast system on the pump. With an internal ballast, the valving is often essentially check valves built into the pump that bleed some reject gas (from the high pressure side of the pump) back to the feed side of the pump. Depending on the process gas (or contaminants) these valves can get stuck. Excessive leak-back will cause the pump to run full out even as you establish vacuum. Similarly an oversized external ballast loop can lead to excessive gas recirculation, and the motor working harder.

 

[Torricelli]

Dear Edtihcc,

The best system to maintain a vacuum is normally a combination of Condenser + Vacuum pump.
The condenser is sized to handle the vapours and the pump to handle the non-condensables (air leakage, nitrogen, C1-C4,..).
If the evacuation is too long, a second pump (or an ejector) is installed in addition running in parallel during the evacuation.

To answer your question, - and when using a pre-condenser, the pump will run at full speed during the evacuation (to get the shortest evacuation time).
During the destillation you will control the pressure with a by-pass or with a frequency controller.

Give us more details about your application in order to get more ideas.

ET

http://www.vacuum-guide.com