How to calculate ave scfm for vane compressor 1

[IntrepidLearner]

Hi guys,

Rotary vane compressor with 30kW rated motor. I measured the power consumption over a 24 hour period, and can therefore calculate the average kW consumed for that period.

I also know the servo valve setting. The compressor is rated at 165cfm. How can I determine the average scfm produced during this 24 hour period?

Will knowing the package load be necessary for the calculation?

 

[vzeos]

IntrepidLearner,
If you know that the compressor is rated at 165cfm at some rpm then you can calculate the cfm per revoulution. Then you would need to count the revolutions to get total cfd. You would need to measure the pressure and the temperature to correct to scfd. By the way, a rotary compressor is just a rotary meter running backwards, so you might want to Google "rotary meters" to get some ideas.

To correct to standard flow rate, Q[sub]s[/sub], you need to correct the flowing temperature T[sub]a[/sub] and pressure P[sub]a[/sub] to standard T - P conditions using the combined gas laws:

P[sub]s[/sub]Q[sub]s[/sub]/T[sub]s[/sub]Z[sub]s[/sub] = P[sub]a[/sub]Q[sub]a[/sub]/T[sub]a[/sub]Z[sub]a[/sub]

where the subscript a refers to actual conditions and the subscript s refers to standard conditions, Z is the compressibility.

Solving for Q[sub]s[/sub] and noting that Z[sub]s[/sub]=1 for P[sub]s[/sub] << 100 psia

Q[sub]s[/sub] = T[sub]s[/sub] P[sub]a[/sub]Q[sub]a[/sub]/T[sub]a[/sub]Z[sub]a[/sub] P[sub]s[/sub]

 

[IntrepidLearner]

Thank you, vzeos. Your formula will prove useful. However, I don't believe that it totally answers my question. This will determine the scfd assuming the compressor is:

[1] working at max air output 100% of the time,
[2] has its servo set to the default (or rated) setting.

I could be wrong on this but I believe that if the servo valve is reduced from rated setting, it will consume less power when working at max output and, thus, not generate at the rated cfm (or scfm).

Furthermore, when the demand fluctuates, so too does power consumption and, thus, so too does air flow.

I could be subject to correction, but this is my understanding.

 

[vzeos]

IntrepidLearner,
The method I described does not assume:
[1] working at max air output 100% of the time,
[2] has its servo set to the default (or rated) setting.

The method I described suggests that you find out the rated cf/min @ rev/min, then
(cf/min)/(rev/min)=cf/rev
If your unit has a rev counter, then cf=rev X cf/rev.
The rev counter will count revs regardless of duty.
You will need to install a PT recorder to calculate scf.

 

[IntrepidLearner]

vzeos,

OK, gotcha. Unfortunately, we don't have rev counter. Is there any way to approximate the revs based on a log of the power consumption (taking readings x1/min)?

 

[Artisi]

You should know the motor speed - therefore the total number of revs /24 hrs is the number of revs per minute x 60 x 24.
Use 1480 RPM for 4 pole 50hz
1780 4 pole 60hz
960 6 pole 50Hz
1160 6 pole 60hz

 

[IntrepidLearner]

Sorry, but upon further consideration, I'm still a little confused here. How do you account for when the compressor is in off load and idle states? The rotor will still be making revolutions, but no compressed air is being delivered to load.

Also, will the scfm delivered when demand is high (ie, when compressor is consuming max power) be the same as that when demand is low (where compressor is consuming just over off load power)?

From my understanding, for a 30kW rated motor:
max power = ~37kW (= package load)
off load power = ~27kW

 

[IntrepidLearner]

Can no one help me with my queries above?

 

[dcasto]

Problem is you would have to know the effiency of your campressor and motor package. This includes all electrical, mechanical and thermodynamic properties. If you assume the unit was 80% efficient or something you could back into the volime moved if you had all the suction and discharge conditions and the fluid properties.

It's a big request..........

 

[Artisi]

I think you have to help yourself - fit a flow measuring device to the inlet or discharge and measure the flow over a 24 hr period - fairly simple and straight forward.

By measuring power consumption seems a long way round to come up with an answer as there are too many unknown variables. That is unless of tests on your compressor for every flow/pressure/power consumption variable and then intergrating this information with your daily power reading.

 

[Artisi]

Correction to previous posting -

That is unless you test your compressor -----