Compressor faulty, or am I stupid?

[hotponyshoes]

I'll check my Clarke industrial but won't be until the weekend when I'll have a chance.

150l in 4.3 minutes? ..very roughly 35l per minute on average. Is that 1.25 CFM (Google!) or is my maths totally wrong. I really only think in metric if I'm honest!

His 10.5cfm compressor should pump 297lpm.

125psi in the tank is 8.6 bar.

150l tank at 8.6 bar is 1290l

1290 / 297 = 4.3 minutes to fill.
Sounds like it's working OK to me?

 

[Pigeon_Droppings2]

I'll check mine at the weekend...I have the clarke industrial XET19. I find the CFM is really only a rough guide...what we're really interested in is what it can push out at the rates pressure!

 

[hotponyshoes]

Air hose size makes a big difference as well.
If you have 1/4" hose the air needs to hit 100mph to give 3cfm.
Up the hose to 3/8" and that drops to 45mph.

Stick your head out a car window at those speeds and you'll see why it makes a big difference

 

[Cook1e]

Air hose size makes a big difference as well.
If you have 1/4" hose the air needs to hit 100mph to give 3cfm.
Up the hose to 3/8" and that drops to 45mph.

Stick your head out a car window at those speeds and you'll see why it makes a big difference

Haha I've done more than 100mph on my bike, so I know what you mean!

So of course my 15m of 8mm hose will drop some pressure, but from what's been said, that's probably a good thing.

 

[Cobbler]

Air tools are generally rated at 25% use, 75% idle, soma 3cfm grinder uses 12cfm continuously so no wonder your compressor struggles

 

[pint]

Air tools are really inefficient, the whole system wastage is dismal.

I rarely get the air die grinder out now i have an m12 die grinder

 

[Cook1e]

Air tools are really inefficient, the whole system wastage is dismal.

I rarely get the air die grinder out now i have an m12 die grinder

Ah, but ...

That's one reason for air tools in the first place.. it's probably more than a couple of hundred watts.
Loads of power for the size of the tool as the power is generated remotely by a massive compressor

 

[hotponyshoes]

Thanks, I'll be interested in the comparison.

I think 150l at 125psi will be a bit more than at atmospheric pressure. If the relationship's linear (a LONG time since I did O-level physics), it's about 10.4 CFM, which is pretty close to spec.

The compressor might be up to spec but that spec is not enough to run the grinder (continuously)

Basic reason why not is (simple version)

Say you have a single piston 150l compressor,
Piston/Cylinder have a 1 litre swept volume.
It has a 10:1 compression ratio.
The motor drives it at 100rpm.

Start it up with an empty tank.
Piston goes down, draws in 1l of air from outside.
Piston goes up, compresses that air to 1/10 its original size. That increases the pressure of the air to 10bar.
Exhaust valve opens and the air goes into the tank.

The compressor does not actually pump the air into the tank. The high pressure air in the cylinder just expands into the low pressure space in the tank until the pressure between both is equal.

So, after 1min & 30secs it's pumped 150l of air. That air has been compressed to 10bar in the piston but re-exapanded into the space in the tank so you now have 150l of air in the tank at 1bar of pressure.

That's not enough so the motor keeps running.
Except now, you have an issue, when the exhaust valve opens at the top of a stroke there is already 1bar of pressure pushing back against the valve. So, from your 10bar of air in the piston, only 9bar can push through into the tank. The other 1bar will remain in the piston and re-exapand to take up 1/10 of the space when the piston goes back down.
So you are now running at 90% efficiency.
So the compressor will need to run for 10% longer to increase the pressure in the tank to 2bar. Ie, the first 1bar will take 1'30".
The next 1bar (to reach 2bar) will take 1'40"

And this goes on. So after 2'10" you'll have 2bar in the tank but you'll only be able to use 8bar from the 10bar in the cylinder on each stroke so you drop down to 80% efficiency and so on as the pressure climbs.

By the time you reach the 90psi (6bar) you need for the grinder the compressor will only be running at 50%.

So, for this fictional compressor,
Tool needs 3cfm @ 90psi = 85lpm
But at 50% this compressor can only deliver 50lpm.

Hence why the 10.5cfm compressor can't run a 3cfm tool.
The same as the 100lpm compressor can't run a 85lpm tool.
You need to use more numbers than just the marketing ones!

 

[hotponyshoes]

Air tools are really inefficient, the whole system wastage is dismal.

I rarely get the air die grinder out now i have an m12 die grinder

They are really inefficient indeed. As with the op, a 3hp motor is not actually capable of running a 1/2hp(?) tool!

But they have plenty of other advantages over a cordless tool so still very popular when efficiency of the power supply is not of prime concern

 

[Cook1e]

The compressor might be up to spec but that spec is not enough to run the grinder (continuously)

Basic reason why not is (simple version)

Say you have a single piston 150l compressor,
Piston/Cylinder have a 1 litre swept volume.
It has a 10:1 compression ratio.
The motor drives it at 100rpm.

Start it up with an empty tank.
Piston goes down, draws in 1l of air from outside.
Piston goes up, compresses that air to 1/10 its original size. That increases the pressure of the air to 10bar.
Exhaust valve opens and the air goes into the tank.

The compressor does not actually pump the air into the tank. The high pressure air in the cylinder just expands into the low pressure space in the tank until the pressure between both is equal.

So, after 1min & 30secs it's pumped 150l of air. That air has been compressed to 10bar in the piston but re-exapanded into the space in the tank so you now have 150l of air in the tank at 1bar of pressure.

That's not enough so the motor keeps running.
Except now, you have an issue, when the exhaust valve opens at the top of a stroke there is already 1bar of pressure pushing back against the valve. So, from your 10bar of air in the piston, only 9bar can push through into the tank. The other 1bar will remain in the piston and re-exapand to take up 1/10 of the space when the piston goes back down.
So you are now running at 90% efficiency.
So the compressor will need to run for 10% longer to increase the pressure in the tank to 2bar. Ie, the first 1bar will take 1'30".
The next 1bar (to reach 2bar) will take 1'40"

And this goes on. So after 2'10" you'll have 2bar in the tank but you'll only be able to use 8bar from the 10bar in the cylinder on each stroke so you drop down to 80% efficiency and so on as the pressure climbs.

By the time you reach the 90psi (6bar) you need for the grinder the compressor will only be running at 50%.

So, for this fictional compressor,
Tool needs 3cfm @ 90psi = 85lpm
But at 50% this compressor can only deliver 50lpm.

Hence why the 10.5cfm compressor can't run a 3cfm tool.
The same as the 100lpm compressor can't run a 85lpm tool.
You need to use more numbers than just the marketing ones!

"You need to use more numbers than just the marketing ones"

That's exactly why I'm here guys - I've learned a lot, even though I don't much like what I've learned...

Thanks for all the info. I'll try running the grinder at lower pressure - I'm only using it to polish the side cases on the bike engine, so it might handle it at lower power/rpm.

The alternatives - bigger compressor (no 3-phase), electric grinder (too big to get in the spaces) - are not practical or affordable.

 

[hotponyshoes]

Air tools are really inefficient, the whole system wastage is dismal.

I rarely get the air die grinder out now i have an m12 die grinder

Plus, I'd guess convenience, portability and usability were your reasons for choosing the m12 not efficiency?

As there will be some losses in the charger so a mains corded tool must be more effort run in terms of electrical consumption?

 

[hotponyshoes]

"You need to use more numbers than just the marketing ones"

That's exactly why I'm here guys - I've learned a lot, even though I don't much like what I've learned...

Thanks for all the info. I'll try running the grinder at lower pressure - I'm only using it to polish the side cases on the bike engine, so it might handle it at lower power/rpm.

The alternatives - bigger compressor (no 3-phase), electric grinder (too big to get in the spaces) - are not practical or affordable.

Try running at a lower pressure .

15m is a long hose even at 8mm.
The bigger the better. It's quite common to run hard pipe around the walls of the workshop, 1" water pipe is a good size and cheap. Or, use the biggest hose you can for as much of the run as you can. A 1/2" hose 13m long then 2m of 8mm at the end so it's nice to work with.

Electric grinder with a flex-shaft?

Or, add extra air tanks to your compressor. (Gas bottles are cheap/free)
The compressor still won't run the tool constantly so won't be any help if you want to polish for 2hrs straight.
But if you are only trying to polish for 10mins at a time but running out of air after 8mins another tank will see that out then recharge while you do something else.

 

[Smallfry]

Air tools are generally rated at 25% use, 75% idle, soma 3cfm grinder uses 12cfm continuously so no wonder your compressor struggles

ISTR reading something exactly on these lines some years ago, the they are rated at about 25% of their ACTUAL consumption.

Die grinders an all other air powered tools are only any good in industrial settings, where there is a HUGE compressor running somewhere.

Solution. Buy a mains powered die grinder. Battery ones are carp too, if you want to use it for more than a couple of minutes.

 

[Cook1e]

Try running at a lower pressure .

15m is a long hose even at 8mm.
The bigger the better. It's quite common to run hard pipe around the walls of the workshop, 1" water pipe is a good size and cheap. Or, use the biggest hose you can for as much of the run as you can. A 1/2" hose 13m long then 2m of 8mm at the end so it's nice to work with.

Electric grinder with a flex-shaft?

Or, add extra air tanks to your compressor. (Gas bottles are cheap/free)
The compressor still won't run the tool constantly so won't be any help if you want to polish for 2hrs straight.
But if you are only trying to polish for 10mins at a time but running out of air after 8mins another tank will see that out then recharge while you do something else.

Shorter hose is the most practical, but if I'm going to reduce the pressure, it won't be an issue, will it? No room for an extra tank.

Flexi drive occurred to me, but I'd have to buy the electric grinder to go with it - not VFM for the one job.

 

[Cook1e]

Lower pressure and a little patience is the answer. My initial concern/confusion was simply that I thought the compressor was sick.

 

[hotponyshoes]

ISTR reading something exactly on these lines some years ago, the they are rated at about 25% of their ACTUAL consumption.

Die grinders an all other air powered tools are only any good in industrial settings, where there is a HUGE compressor running somewhere.

Solution. Buy a mains powered die grinder. Battery ones are carp too, if you want to use it for more than a couple of minutes.

I get that there are so many variables that it's probably impossible to get a standard specification but I'm sure the manafactures just make the numbers/ratings up to beat whatever numbers the competition are using!

I've got several cordless Die Grinders that are fine, I've also got enough batteries and chargers that I can run continuously if I needed to.
I've got both air and cordless but, no mains powered ones!

There are also air tools that are fine for hobby use. Rivet guns are one, will run from a £99 50l compressor unless you are super-fast at reloading rivets and cheap.
I've never seen a mains powered version (they might exist) and a cordless one of the same power as a cheap air one is £££

 

[hotponyshoes]

Shorter hose is the most practical, but if I'm going to reduce the pressure, it won't be an issue, will it? No room for an extra tank.

Flexi drive occurred to me, but I'd have to buy the electric grinder to go with it - not VFM for the one job.

If the tool will still do the work you want at a lower pressure then that's the answer.

If not, see what pressure you need on the last regulator in the line to run the tool.
Then, see how low you need to turn the pressure to allow the compressor to keep up.
Then, post up those 2 numbers and we can work out what you need to do!

 

[Smallfry]

I get that there are so many variables that it's probably impossible to get a standard specification but I'm sure the manafactures just make the numbers/ratings up to beat whatever numbers the competition are using!

I've got several cordless Die Grinders that are fine, I've also got enough batteries and chargers that I can run continuously if I needed to.
I've got both air and cordless but, no mains powered ones!

There are also air tools that are fine for hobby use. Rivet guns are one, will run from a £99 50l compressor unless you are super-fast at reloading rivets and cheap.
I've never seen a mains powered version (they might exist) and a cordless one of the same power as a cheap air one is £££

You are right about that of course, what I meant to say was "motorised" air tools. Things that spin continuously.

Rivet guns, air nailers grease guns etc, have pressure cylinders, and are OK.

 

[not done it yet]

There's a difference between cfm and free air delivery.
The cfm is the theoretical amount of air the compressor is capable of. The free air delivery is the actual and will be lower.
Also depends on the size of hose and quickfits you have.

Not quite. CFM is CFM however it might be claimed. The difference is the real ‘free air’ volume and the displacement CFM of the compressor pistons - which do not take into account of any losses. For the displacement value to be the same as free air, there would need to be zero volume of air above the piston(s) at TDC, which is unattainable. Think here of low and high compression variants of ICEs.

 

[RWD3M]

I think 150l at 125psi will be a bit more than at atmospheric pressure. If the relationship's linear (a LONG time since I did O-level physics), it's about 10.4 CFM, which is pretty close to spec.

It might just be the way I'm reading this, but the statement above confuses me. You do know what CFM stands for right? And huge apologies if I've completely misread what you've said here, I likely have