Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Pump down time for vacuum pump

Pump down time for vacuum pump

Pump down time for vacuum pump

I am getting conflicting results on finding the pump downtime for a diaphragm vacuum pump for air.

There is a 60 cu. inch vessel that I need to evacuate to 600 mbar abs. in about a second.

The current pump that I have has the following ratings

Free flow = 12 liter/min
Max vacuum = 500 mbar abs.
Initial pressure = 1000mbar abs. (atmospheric pressure)

I'm still awaiting the pump curves from the manufacturer.

The first reference I find is at this site

T = (V / D) * loge(A / (A - B))

T is pumping time, in minutes.
V is volume of tank, in cubic feet.
D is free running displacement of vacuum pump.
A is deadhead vacuum of pump (with inlet blocked) rating.
B is desired vacuum level in tank, in ˝Hg.

This basically takes the ratio of the dead head vacuum to the desired vacuum.

The second reference I find here

t = V / q ln(p0 / p1)

t = evacuation time (s)
V = enclosed evacuated volume (m3, cu.ft)
q = volume flow rate capacity of the vacuum pump (m3/s, cu.ft/s)
p0 = initialization pressure - normal atmospheric pressure (mbar, mmHg)
p1 = end vacuum pressure (mbar, mmHg)

and this takes the ratio of the initial pressure to the final pressure.

What would be the correct way to compute the pump down time?

RE: Pump down time for vacuum pump

How frequently will you be doing this? The better solution, probably, would be to have a tank or volume of pipe that is is already evacuated and open a valve to this. You will be able to use a much smaller vacuum pump. Also, your equations ignore important factors like the conductance of the piping and valve between the vessel and vacuum pump. To get high flow you need large pipe and valves, but this will add to the volume that must be evacuated.

How accurately do you need to hit 600 mbar? Or do you mean less than 600 mbar?

RE: Pump down time for vacuum pump

Quote (spannerInTheWorks)

What would be the correct way to compute the pump down time?

Both ways should give you similar results.

Your tank contains 0.98 liter of air at atmospheric pressure.
Your pump at best can suck 0.20 liter per second.
You only have one second.

At a glance, it seems that you will need to remove more than 20% of the initial mass in order to achieve that vacuum; hence, a bigger pump or an auxiliary tank, as suggested above.

A rough calculation using this calculator tells me that your pump will need around 2.5 seconds to reach 600 mbar:

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

RE: Pump down time for vacuum pump


Quote (How accurately do you need to hit 600 mbar? Or do you mean less than 600 mbar? )

+/- 50 mbar would do.

Yes I agree with you that the equations don't take into account the pipe dimensions and the valve. However I needed a rough estimate for pump sizing.

RE: Pump down time for vacuum pump


Quote (Lnewgban)

Both ways should give you similar results.

I'm getting different result from what you got using the first equation that I had presented in the original post.

T = (V / D) * loge (A / (A - B))

V = 0.03472 cu. ft.
D = 0.42 scfm

gives me
T = 0.133 min which in seconds comes to
T = 8 sec

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


eBook - Simulation-Driven Design With SOLIDWORKS
Simulation-driven design can reduce the time and cost of product development. In this engineering.com eBook, we’ll explore how SOLIDWORKS users can access simulation-driven design through the SOLIDWORKS Simulation suite of analysis tools. Download Now
eBook - Integrating the Engineering Ecosystem
Aras Innovator provides multiple options for integrating data between systems, depending on the scenario. Utilizing the right approach to meet specific business requirements is vital. These needs range from authoring tools, federating data from various and dissimilar databases, and triggering processes and workflows. Download Now
Research Report - Simulation-Driven Design for SOLIDWORKS Users
In this engineering.com research report, we discuss the rising role of simulation and the paradigm shift commonly called the democratization of simulation. In particular, we focus on how SOLIDWORKS users can take advantage of simulation-driven design through two analysis tools: SOLIDWORKS Simulation and 3DEXPERIENCE WORKS. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close