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Pump down time for vacuum pump

Pump down time for vacuum pump

(OP)
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)

where
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

2
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:
http://www.engineeringtoolbox.com/vacuum-evacuatio...

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

RE: Pump down time for vacuum pump

(OP)

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

(OP)

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

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