4 replies to this topic

[nmh]

If a positive displacment pump/roots blower used for conveying plastic pellets is used as a vacuum pump, will it have the same conveying capacity as roots blower?

Edited by nmh, 04 May 2011 - 08:14 AM.

[Art Montemayor]

NMH:

We are discussing a positive displacement device. This means POSITIVE VOLUMETRIC DISPLACEMENT – not positive mass displacement.

Therefore, if you currently have a Root blower operating in a pneumatic transport operation, you must be using it to compress atmospheric air and supply it to the pneumatic ductwork. That would be the normal way it is used.

Roots blowers can also be used to generate a partial vacuum. I have used them in that application in the past. However, in that application the density of the suction gas is very different from that when the blower is using atmospheric air. Although the volumetric displacement is the same (assuming the same volumetric efficiency – although not true) the mass of gas being “pumped” is much less.

Depending on your conveying “capacity” measurements and how it is operated, the amount of material you can convey will probably be less in the second case.

[nmh]

Hi Art Montemayor,

Thank you for making it much clear for me. Yes, we are using a roots blower to compress the atmospheric air and then supply the compressed air to the pneumatic line.

You mentioned that the density of suction gas (air) is different in vacuum conveying, Is it because of the fact that the amount (volume) of air available is less during vacuum conveying? can you please explain me this?

Regards
Nmh

Edited by nmh, 04 May 2011 - 02:19 AM.

[Art Montemayor]

Nmh

Yes, that is what I meant. Since the density of the air entering the blower is less in vacuum conveying, you obviously are moving less mass in a given time. You are theoretically displacing the same amount of VOLUME - but not mass. Additionally, do not forget that a Roots blower can compress atmospheric air up to as much as 1.5 barg. That means a pressure increase (Delta P) of 1.5 bars. But when the blower is put into vacuum service, the maximum Delta P is theoretically only less than 1 bar - and moving less mass flow rate of air.

That is why I believe you would require a larger blower for vacuum conveying than the one for pressure conveying.

[nmh]

Hi Art Montemayor,

Thank you for your input. Now its clear for me.


Regards
Nmh