Info about air compressor (rotary screw)

[ibatechnology]

Dear all,

I just want to confirm about the rotary screw compressor with oil injection one. The compressor that i have is the 3 cubic meter/min and the pressure is 0.8 MPa. So when to operate the compressor at its full capacity i.e at a pressure of 0.8 MPa then the density of a compressed gas calculated at 300 K was 9.3 kg/cubic meter. So mass flow rate was calculate at 3 cubic meter/min and it was 27.9 kg/min.

So my question is simple that is can we increase the CFM above than the rated CFM. As rotary screw have a variable CFM option available but can we increase it above then the specified CFM rating or it can be done only within the specified range. If it is possible then should we also have to look after the mass flow rate to not increase above than the maximum rating .

For instance if it is possible to increase the CFM up to 6 cubic meter /min then I must have to decrease the pressure of the compressor in order to obtain the mass flow rate within the compressor maximum range. e.g. if i selected the pressure of 0.2 MPa then the density becomes 2.3 kg/cubic meter and the mass flow rate becomes 13.9 kg/min. So as you can see mass flow rate is within the compressor maximum range but the CFM is increased beyond its specified range so is it possible or I have to look after both CFM and mass flow rate.

Please reply to solve my query I shall be very thankful to you.

Best Regards,

Ibadullah Safdar

 

[henry Kadzielski]

Good morning.
You already have this. As your output air pressure reduces the volume increases. Basic rule of thumb, if your compressor produces 100cfm @ 100psi (7bar), then it will produce 200cfm @ 50psi (3.5bar)

 

[ibatechnology]

Thanks for your reply.
I am still confused . Let me explain in another way. The compressor i have has the CFM of 3 cubic meter/min (0.05 cubic meter/sec) and pressure rating of 0.8 MPa. So I have calculated density at 0.8 MPa which is 9.3 kg/cubic meter. The mass flow rate was calculated as m=(density)xCFM so mass flow rate becomes 0.465 kg/sec. So this is the maximum mass flow rate the compressor will produce when i set it at pressure of 0.8 MPa.
Now my problem starts from here that is I need the CFM above than the 0.05 cubic meter/sec. Can i increase it beyond that limit as the mass flow rate will only change when i change the pressure but is it possible that the CFM can be increased beyond 0.05 cubic meter.sec.
I hope you will understand what I want to say.
Regards,
Ibadullah Safdar

 

[henry Kadzielski]

You cannot go above the max, unless you can speed up the compressor, then you have other issues eg motor capacity. You can increase the capacity that you are getting by increasing the compressor speed, either by pulley diameters (if belt driven) that would then require a larger electric motor. Also you have to accommodate the extra heat buildup in the compressor with larger oil cooler. I am assuming you have a screw compressor. One very important note to watch out for, if the compressor is already running at it's max speed and you increase it can and will go 'BANG' quickly followed by a big expence. Similar requirements if the compressor is run by a diesel engine, if the compressor is belt driven it is much easier, however newer drives are gearbox, so you are limited to increase the speed of the engine if that is possible

 

[knighty]

not sure about screw compressors, but hydrovanes have a pressure regulator right on the end of the compressor head, it keeps the pressure inside the compressor itself high (120psi for mine) and then discharges any extra into the are receiver

it needs that pressure inside the compressor for the oil return to work properly


so volume of compressed air out of the compressor should remain pretty constant



no idea about screw air compressors - have a few screw refrigeration compressors at work and they have multiple pressure valves built into them

 

[armalites]

One thing that is causing confusion here...

CFM is cubic foot per minute, CFM is not measured in cubic metres per minute

 

[henry Kadzielski]

cfm is cubic feet per minute!

 

[Nick67GT]

if it's an atmospheric intake, how about considering a boosting arrangement to increase the mass flow?