How to Calculate Work to produce a Vacuum?

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    Vacuum Work
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Discussion Overview

The discussion revolves around calculating the power requirements for an electric motor used to evacuate a vacuum to a specific level, focusing on the mechanics of a pneumatic cylinder and the associated pressures. The scope includes technical reasoning and mathematical considerations related to vacuum generation.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant seeks assistance in determining the motor requirements for evacuating a vacuum, specifying dimensions and target pressure.
  • Another participant suggests that the work required to reduce pressure is analogous to the work needed to generate pressure, indicating that modeling from first principles may be complex.
  • A different participant argues that the area of the cylinder rod is not significant if it is open to atmospheric pressure, calculating the necessary force based on differential pressure.
  • This participant also notes that torque cannot be calculated without further details on how the rod will be coupled to the motor.
  • Concerns are raised about the ongoing costs associated with maintaining the vacuum due to potential leakage rates, which will influence motor power requirements.
  • The original poster acknowledges the complexity of the topic and mentions the method of connecting the motor to the cylinder via a crank mechanism.

Areas of Agreement / Disagreement

Participants express differing views on the significance of certain factors, such as the cylinder rod area and the complexity of modeling the vacuum generation process. The discussion remains unresolved with multiple competing perspectives on how to approach the calculations.

Contextual Notes

Limitations include assumptions about ignoring losses due to friction and the need for further clarification on the motor-coupling mechanism. The discussion also highlights the dependence on specific definitions of pressure and vacuum levels.

Arnak
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Hi Folks,

Could someone please help me?:blushing:

I want to work out the power requirements for the torque or wattage required in an electric motor when evacuating a vacuum to a certain level.

The tube to be evacuated is 200 mm x 50 mm and the evacuation would be done using a pneumatic cylinder of 50 mm bore and 50 mm stroke.

The vacuum to be achieved and maintained is 2.5 psi or 5" Hg.

The pneumatic cylinder will be driven by an electric motor but how to calculate the motor requirements?

To keep it simple I will ignore losses due to friction etc.

Could someone could please help me with the formula to work that out?:confused:

Thanks,

Arnak
 
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It's the same as the work required to reduce pressure.
Opposite of the work required to generate the pressure.

Depends a great deal on how the pressure is being changed.
It will not be easy to model from first principles - you should probably just look it up.

See how other people solve the same problems.
http://www.gastmfg.com/vphb/vphb_s4.pdf
http://lpc1.clpccd.cc.ca.us/lpc/tswain/chapt3.pdf
 
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I believe the cylinder rod area is unimportant if the rod end of the cylinder is open to atmospheric pressure. The 50mm pump cylinder will have 2.5 psi on one face and 14.5 psi atmospheric on the other. That makes a differential pressure of 12 psi. The area of a 50mm piston is about Pi square inches so you need to apply a force of Pi *12 psi = 37.7 pounds force to the rod.

Torque cannot yet be calculated because you have not explained how you will couple the rod to the motor.

The ongoing cost of maintaining the 2.5 psi absolute pressure will be determined by the leakage rate you must make up.

The motor power requirements will be decided by the time you have available to pull the initial partial vacuum.
The rate of air removal must be faster than the leakage rate you need to make up.
 
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Hi,

Thanks for all the advice, complicated subject isn't it.8-((

I will look up the links and see what I can learn.

The connection to the motor will be via a 2.5" throw crank to the motor shaft.

Good point about the leakage rate.8-))

Arnak
 

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