Compressor math

[DarkAge]

If a compressor is 2 minutes to fill a 50 liter tank with 10 bar, does that mean that it will provide 250L/min ?

eg:
50 liter * 10 bar = 500L/2 min = 250L/min

As 1 bar is normal atmospheric pressure, 10 bar must be 10 times as much.

Or is there something very wrong in my math ?

 

[rtbcomp]

I suppose that would be the average over the time to fill the tank.

When the tank is empty (at 1 bar) the l/m from the pump will be higher than when the tank is full at 10 bar. Basically the same mass of air is drawn into the pump on each stroke and the same mass is expelled, but as the pressure increases that mass is squashed into a smaller volume.

 

[piman]

Hello Dark Age,

I think your maths are correct, but only if you consume the air at 1 bar pressure, which doesn't suit many applications. If you use it at a more common pressure say between 5 to 8 bar then the practical volume of the tank will be much less than 250 litre per minute?

Alec

 

[DarkAge]

Hello Dark Age,

I think your maths are correct, but only if you consume the air at 1 bar pressure, which doesn't suit many applications. If you use it at a more common pressure say between 5 to 8 bar then the practical volume of the tank will be much less than 250 litre per minute?

Alec

Could you elaborate this, thanks. Isn't air always measured at 1 bar ?

 

[Shox Dr]

Could you elaborate this, thanks. Isn't air always measured at 1 bar ?

For measuring what?

You could have a drill using 90psi at 6cfm and another using 85psi 5cfm

What you need to know/work out is the psi per cfm


or Im I talking crap

 

[DarkAge]

the reason im asking is because if my math is correct than i've got 5 compressors between 1,5 and 4 hp and none of them provide the L/min as mentioned in their spec. Eg the 4 hp in my example should provide 340L+/min.

 

[Shox Dr]

Don;t forget manufacturers state cfm. And not FAD (Free Air delivered) this is the amount of air the compressor can supply on a open valve

 

[Shox Dr]

... so take off anywhere between 25-30% off the stated CFM

 

[shenion]

Do they spec the FAD or displacement?

Displacement in LPM or CFM is just the volume the piston covers, not what you would get out of it.

Here it is the other way around. Compressors are sold by CFM at a given pressure. You can usually never find the displacement. Mine is 9.9CFM at 90 PSI, 10.9 at 40 PSI. I think the displacement is close to 13CFM.

 

[tigler]

It would be a very good thing if trading standards made it compulsory to state ONLY fad and the pressure.

 

[RonA]

The formula you need Is P1xV1/T1=P2xV2/T2 where P=Absolute pressure (working pressure + atmospheric pressure), V=Volume & T=Temperature. the number suffix applies to the condition of the fluid before/after expansion or compression.

 

[rtbcomp]

Or (P1 * V1)/T1 = constant.

The constant depends on the actual gas under consideration.

T1 is absolute temperature (ie degC + 273). I don't see how atmospheric pressure comes into it because this formula applies to a closed vessel.

 

[RonA]

Or (P1 * V1)/T1 = constant.

The constant depends on the actual gas under consideration.

T1 is absolute temperature (ie degC + 273). I don't see how atmospheric pressure comes into it because this formula applies to a closed vessel.

You are right for most practical purposes the temperature is irrelevant & can be discounted but that's the complete formulae I learnt when studying Thermodynamics.

If expansion is involved cooling of the fluid must always be considered & likewise heating of the fluid for compression. Free air delivery takes account of any temperature difference & assumes the fluid is at the same temperature post expansion/compression often quoted as STP.

 

[Rick]

It would be a very good thing if trading standards made it compulsory to state ONLY fad and the pressure.

From my understand free air delivery is what the pump will do at low/no pressure. My compressor at home is 16cfm (4hp) from memory, at 100psi+ it is more like 10cfm. Infact both together (7hp total) are around 16cfm. This with the 2 tanks, 400l, does me fine for most jobs other than blasting. To be honest most air tools these days are useless for home/hobby use. At work I have a 50hp Hydrovane which is great. But for home use an electric counterpart would be better IMO. Other than things that actually need air. i.e. spraying, blow off guns, inflaters, nail guns.

 

[FranV8]

Right chaps,

Volumes of air from comps are specified as the volume of that air at atmospheric pressure. (And 20 Degrees C I think)

Think of it more as the volume of air it consumes from the atmosphere - obviously when this is compressed it occupies a volume far less.

The maths at the top are pretty much correct.

Shenion - reason for the different capacities quoted for yours are the volumetric efficiency, impacted in part by the comression ratio of the machine I guess. As it discharges it'll leave a small volume in the cylinder, at higher pressures this will expand back out more and take up some of the volume being drawn in by the suction stroke.