Help Calculating CFM

[Turbo]

I recently acquired 2 sip compressors.

I was servicing the 50l one & while I had the cylinder head off I decided to check how close the figures were for the cfm & fad that were printed on the plate.

The figures I calculated were not what I expected, can I see what you get to see if I am I going mad?

The bore of each piston measured at 60mm, the stroke was 40mm, there are 2 pistons that each go up & down once per revolution. On no load the pump turns at 1163rpm.

 

[rtbcomp]

The volume of a cylinder is base area x height, in this case stroke is the height

So swept vol = pi x r^2 x h = pi x 30 x 30 x 40 cu.mm = 113097 cu.mm per cylinder, = 226194 cu.mm for both cylinders together, for each revolution.

There are 1163 revs per minute, so the swept volume is 1163 x 226194 cu.mm per minute = 263063622 cu.mm/min = 263063.622 cu.cm/min = 9.290004137477654 cu.ft/min

Call it 9 cfm.

 

[knighty]

maybe the difference between air in and air out is what's messing you up?

9cfm in at atmospheric pressure... but not a lot out at 90psi

 

[rtbcomp]

maybe the difference between air in and air out is what's messing you up?

9cfm in at atmospheric pressure... ...

Volume of air in is not necessarily the same as swept volume, it depends on the resistance to airflow on the inlet side.

 

[hotponyshoes]

Your maths looks fine.
What does the plate say then?

Efficiency will be one factor to consider,
It will not be able to draw in the full 100% of the bore capacity on each stroke due to restriction in the air filter, pipe work etc.
It will not push out 100% on each stroke either.

Another thing to consider is heat, as the piston/bore heats up the incoming air will expand preventing the full charge being drawn into the pump.
The air will be heated even more when compressed.

I would expect the quoted free air delivery rating on the pump to be somewhere close to your calculation although allowing 5-10% for intake restriction losses might take this down to 8-8.5cfm?

If they are selling it on specs they might be listing the displacement area, they will count both the down and up strokes so they might be quoting as double what you are getting!

As the output pressure builds this will drop.
Should be 100% (not allowing for losses) at atmospheric, 50% of this at one bar and so on.

Obviously the rpm will drop under load as well. If they give a figure for cfm at X?psi then using the free cfm you can work out the efficiency.

Odds are, it's a copy of another matching and they are copying the ratings from that, or the ratings on the pump are from the pump supplier, and that's using the motor and pulley etc that they recommend and sip have done something slightly different.

 

[Turbo]

The volume of a cylinder is base area x height, in this case stroke is the height

So swept vol = pi x r^2 x h = pi x 30 x 30 x 40 cu.mm = 113097 cu.mm per cylinder, = 226194 cu.mm for both cylinders together, for each revolution.

There are 1163 revs per minute, so the swept volume is 1163 x 226194 cu.mm per minute = 263063622 cu.mm/min = 263063.622 cu.cm/min = 9.290004137477654 cu.ft/min

Call it 9 cfm.

I worked it out the same way & got the same figure! I also tried other formulas I found online & they all gave results around 9 cfm.

From what I understand the cfm figure quoted is the theoretical amount of air the compressor sucks in under no load, so the figure we both came to should be somewhere close to that. But the compressor sticker quotes 13 cfm! Is there an invisible 3rd cylinder I can't see?

It does quote a max FAD of 9.1, which to my mind should be what they quote for the cfm!

The other figure quoted is a FAD of 7.8 cfm, which is the most useful figure as it should be the amount of actual air to supply your tooling.

I'm surprised at a reputable company like SIP quoting misleading figures, if you're buying a 3hp belt drive compressor quoted at 13cfm you would expect it to supply more than 7.8 cfm!

Just shows that you need to be very wary when buying compressors!

Here's a link explaining the difference between cfm & fad.

https://www.airsupplies.co.uk/cfm-explained

The good news is that even with the lower than expected fad figures the pump is able to supply my very air hungry sander & that was before the service. I have had the head off & cleaned a load of gunk from the valves, did some porting plus cleaned an air filter that was caked solid so it should be much improved!

 

[rtbcomp]

They could be quoting the output from the tank filled to 100 psi for 1 minute into fresh air for all we know.

Generally speaking I use the HP figure when comparing compressors, cfm figures are ambiguous and misleading.

 

[Morrisman]

Generally allow at least 20% inefficiency when running a full chatter. I spent my 23 years offshore working on 2000psi compressors, supplying air to large noisy devices we towed behind, and I lost track of how many times I had to explain to the various line managers that a 1800cfm compressor will never, ever, produce 1800cfm, no matter how much they demanded it to.

First stage piston on a 1600cfm 2000 psi Burckhardt compressor, 24” diameter.

Me, perched on top, servicing 3rd stage.

 

[Burdekin]

Are you quoting motor speed? If so what is the pulley ratio to the pump?

 

[rtbcomp]

Are you quoting motor speed? If so what is the pulley ratio to the pump?

...On no load the pump turns at 1163rpm.

That sounds about right for the pump.

 

[Burdekin]

That sounds about right for the pump.

For a 60hz 2 pole motor direct drive it works out about what a direct drive would be quoted as so just thinking maybe the pump seed might be not correct. Smaller pump than original?

 

[Turbo]

Everything on this compressor looks original & I measured the pump pulley speed with a tachometer. I also measured the pulleys & calculated the pulley speed at 1083 rpm but the problem with this is I was measuring the outside of the pulleys & not where the belt sits so it will not be 100% accurate.

 

[knighty]

is ireland on 60hz?

 

[Turbo]

is ireland on 60hz?

NORTHERN Ireland is 50hz like the rest of the UK plus ROI. In fact both countries are linked electrically supplying each other with power when needed.

 

[knighty]

Burdekin posted about 60hz above... I wondered how that tied into the conv?

 

[Burdekin]

Burdekin posted about 60hz above... I wondered how that tied into the conv?

I just thought maybe they could have used 60hz to give a maximum compressor output. Realise U.K. is 50hz but some other countries are 60hz. 4 pole 50hz is 1500rpm v 1800rpm 60hz and 3000 v 3600 for 2 pole.

 

[Turbo]

The motors on both compressors are completely identical - make, hp, etc apart from the speed. The 50l is 2660rpm & the 150l is also something similarly odd but neither is a nice round figure like 2800 or 3000rpm you normally see!

That's why I use the digital tacho, it gives me a reading without any measuring or calculating!