Compressor faulty, or am I stupid?

[Cook1e]

Hi, newbie here - as per title, please bear with me if I'm being stupid...

I have a Wolf air Dakota 150 compressor, rated at 10.5 CFM free air. I'm using it with a PCL APT702 die grinder, which has a stated 3CFM air consumption. Yet the compressor can't keep up with the die grinder and I have to wait for the tank to recharge before I can continue.

A while ago, I changed the reed valves in the top of the compressor, after I fired it up one morning but it was not building pressure.

Am I confused re CFM? I know there are various ways of stating it, free air (10.5 for mine), but I have also seen it stated as 14CFM, which I know is some kind of marketing overstatement, but I don't understand anything beyond this...

If it's my compressor at fault:
1) How can I measure or estimate what it's actually delivering?
2) What's likely to be wrong with it?

Thanks in advance for the incoming wisdom

 

[Bornfree]

Air tools are extremely hungry. An air grinder will make my 7.5hp 3ph 270litre compressor run nearly continuously.

 

[tigdlo]

What pressure are you running the die grinder at?

The die grinder will use more air the higher the pressure and the compressor will produce less the higher the pressure.

 

[Bravheart]

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.

 

[Pigeon_Droppings2]

I would do a practical test and empty the tank and then time how long it takes to fill then do some rough maths from that given the tank size.

 

[knighty]

if you empty the compressor tank then time how long it takes to fill up again.... someone here will have the same compressor and be able to tell you if it's right ?

or... people have similar compressors and will be able to tell you if it looks about right


EDIT: doh we both typed at the same time :-o

 

[Cook1e]

I just checked and the compressor filled the 150 litre tank (receiver?) to about 125psi in about 4'20".
The regulator on the compressor is set to about 125psi and the regulator on top of the water trap is set to 100psi. There's a long (15 metre) 8mm ID hose between the water trap and the die grinder, so I don't know what pressure drop that will cause.

 

[RonMig]

That grinder spec says air consumption at load is 3cfm, or 85litres per minute....

But your 150litre supply will demand more air before you use the 85 litres....

So I'd say it's normal....

 

[Cook1e]

That grinder spec says air consumption at load is 3cfm, or 85litres per minute....

But your 150litre supply will demand more air before you use the 85 litres....

So I'd say it's normal....

So what am I misunderstanding? Why can't 10.5CFM supply keep up with 3CFM consumption?

 

[hotponyshoes]

It's generally x7 for 90psi air tools.
So for a 3cfm die grinder you'll want a compressor with 21cfm fad.

If the grinder needs 125psi then it's x9 - x10

 

[hotponyshoes]

So what am I misunderstanding? Why can't 10.5CFM supply keep up with 3CFM consumption?

Because 'free air' means 0 back pressure.
Pump at 1 bar (16psi) and you double the back pressure which halves the output flow

 

[knighty]

you could turn down the pressure to the grinder... see how low you can go but it still works

even if the compressor doesn’t keep up, it should last longer before you have to stop

 

[Cook1e]

Because 'free air' means 0 back pressure.
Pump at 1 bar (16psi) and you double the back pressure which halves the output flow

Crikey! So the 3hp electric motor on the compressor can't run the maybe few hundred watts the grinder uses. Around 90% loss of power. I know pneumatic tools are not very efficient, but this is crazy

Thanks for the education

 

[Cook1e]

you could turn down the pressure to the grinder... see how low you can go but it still works

even if the compressor doesn’t keep up, it should last longer before you have to stop

I'll give that a go tomorrow, thanks for the tip

 

[hotponyshoes]

Free air rates are a bit like engine sizes on cars.
A 3.0l is obviously bigger than a 2.0l but that doesn't mean that it's more powerful!

 

[hotponyshoes]

Crikey! So the 3hp electric motor on the compressor can't run the maybe few hundred watts the grinder uses. Around 90% loss of power. I know pneumatic tools are not very efficient, but this is crazy

Thanks for the education

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

 

[Cook1e]

Free air rates are a bit like engine sizes on cars.
A 3.0l is obviously bigger than a 2.0l but that doesn't mean that it's more powerful!

All other things being equal, a 3 litre engine will be nearly half as powerful again than a 2 litre.

Of course, if the 2 litre has higher compression ratio or a turbo or whatever, it could be more powerful than the 3 litre.

Are you saying that different designs of compressor can be more powerful for the same hp motor?

 

[hotponyshoes]

All other things being equal, a 3 litre engine will be nearly half as powerful again than a 2 litre.

Of course, if the 2 litre has higher compression ratio or a turbo or whatever, it could be more powerful than the 3 litre.

Are you saying that different designs of compressor can be more powerful for the same hp motor?

I'm saying you can't just go on the 2.0 or 3.0 alone.
As you just said, all other things being equal then the 3.0 will be more powerful.

But, just like you can find 2l engines with higher outputs than 3l engines, obviously not all other things are equal...

The problem is, compressor manafactures use the biggest numbers possible for advertising as bigger is better in terms of output.
The air tool makers use the smallest numbers possible so the tools look economic to run.
The gap between these sets of numbers is so wide that the truth needs to be stretched by a factor of 7 to fill it

 

[Pigeon_Droppings2]

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!

 

[Cook1e]

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!

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.