Thread: Compressors 101?

Despite claims made to the contrary, 20 cfm is definitely 3 phase territory. The most a quality single phase can do is about 12 CFW with a tail wind and recent grease and oil change while cheaper units can only do 7-8 CFM.

My cheap chinese 2.5HP unit is now about 7 years old and refuses to die but I don't use for high volume air delivery - mainly I use it to get the swarf and lube off taps and dies. I did have to retighten the head bolts and refurbish the foil valves a couple of years back but it sounds like it is still good for another 7 years.

G'Day Timless,
To get 20cfm you'll need at least a 5hp motor driving; it's 4cfm per hp as a rule of thumb....
5hp is a pretty big domestic motor and 3ph would be the way to go.
It maybe worthwhile looking at a petrol powered compressor with some good sound proofing & muffler??
Cheers, crowie

Air Compressor 3 Phase good condition
Cash and pick up only $50.00

I already have the 3 phase on in the shed.... this is the same cost as a second hand Chinese cheapie - I'm just wondering if the running costs and noise will drive me broke and insane is all.

This new Chinese single phase CLAIMS to be 19 cfm...

air commpressor19 CFM 240V 3HP motor 100L tank .hevy duty dubel belt ,
market value $900 am asking just $600 .

Then there's a new Atlas Copco...single phase for about the same cost.

Brand new atlas copco compressor 3 hp ideal for small workshop or home workshop rrp $1299 can deliver I also have a 2 hp unit offering at $650 negotiable

The $50 buck 3 phaser seems to be a no brainer bargain....so i must be missing something obvious - remembering the old axiom - "there are no free lunches in this world".

Sooo - what am I missing here?

Should I be racing off to collect this $50 bargain before it gets away?

My recommendations are as follows

1. Belt Driven
2. Twin or Triple heads pump
3. Cast Iron Pump not Alloy

So going on those, I would ditch the $50 jobbie.
My 17cfm triple is still going strong after 20yrs or more with quite a few yrs of those being in an industrial situation.

Timeless

It sounds as though you have some basic knowledge of air compressors from a previous life.

Big consumers of air are air driven tools and to a lesser extent paint spraying. A large 1PH compressor (3HP) will often have to be situated close to your power supply or the voltage drop will prohibit easy starting particularly from cold. If you have 3PH available I would go down that track and buy a second hand machine. Of course you would want to see it running to evaluate.

If your main use is a nailing gun, almost anything will do as they has a very low air consumption. Just a comment on the noise as it is my belief that the compressor makes the noise as opposed to the electric motor.

The important spec of any air compressor is the Free Air Delivery. It is the equivalent to HP at the rear axle on a car.

Regards
Paul

Thanks Fellas - I'd honestly forgotten how much I like air powered tools in my previous life and how cheap they can be for the work they do..

The 2nd hand 3 phase one is just a 1 hp motor and something like 6-8cfm, according to its owner.

I might grab it today and see how it goes initially - if its no good (not enough air / too loud) - well for $50 I can always fill it with concrete and use it as a boat mooring!

What I saved on air, I can put towards proper dust extraction maybe or some more tools. Swings and roundabouts. It's all about the Journey - I keep telling myself this.

Cheers & beers fellas - its amazing how much one bloke can forget... the old saying if you don't use it, you lose it.

Originally Posted by Timless Timber

This new Chinese single phase CLAIMS to be 19 cfm...

One thing we established fairly early on in these forums was that manufacturers claims regarding machinery, especially those out of china, are not worth the ink used to print them.

Yes Indeed Bob there seems to be fair degree of fudge factor in the gear coming out of China - but in fairness to the Chinese the guy building it, he might have had a dodgy bead on his abacus when he worked out the HP and CFM rates!.

Originally Posted by crowie

G'Day Timless,
To get 20cfm you'll need at least a 5hp motor driving; it's 4cfm per hp as a rule of thumb....
Cheers, crowie

You need horsepower to get cubic feet per minute plus a big enough receiver to carry the volume for high air use...

******************************************************************************************************************
Rules of thumb:

• A good single-stage compressor, per true HP input, will deliver about 4 true CFM at 100 psig. It is not uncommon to see this more like 3 CFM per HP.
• A two-stage compressor is more efficient, typically by 15 percent, versus a single-stage compressor. You might see 4 to 5 CFM per HP at 100 psi, instead of 3 or 4 CFM.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Air compressors for the home or small shop have been advertised and sold with ridiculously inflated horsepower ratings. This was especially true before the year 2004, when the US government said it would enforce more honest ratings (see the end of this essay), and it seems to be returning to the marketplace after a year or two of spotty compliance. The specs and stickers on the unit likely do not tell the truth, and add confusion instead of critical information to buying decisions. But you do not need a testing laboratory to calculate true horsepower or CFM delivered. I will explain below how to estimate these ratings from pressure readings and elapsed time measurements.
The way to measure true power is to measure the time it takes to pump the reservoir tank of known volume from a known starting pressure to a known ending pressure. Then you can figure the true CFM from the difference in starting and final pressures, times the volume of the tank, divided by the time it took to pump up. You can also time the pump-up cycle from the cut-in to the cut-out pressure, since that's how one usually runs a compressor. These true performance measurements are impossible to fake.


The above is a quote from - Evaluating True Horsepower and CFM Ratings of Air Compressors

Well the little old 3 phase $50 compressor now lives at my place!

I think it cost $50 in diesel to go get it!

The hose, tyre gauge/filler, water separator / regulator and blow down gun/nozzle are worth more than that!

I realize that its a bit large and heavy (cast iron) to be used anywhere but in the shed... so in due course I'll try and pick up a cheap $50- Chinese single phase one - and take it out onsite when I need to use an air nailer etc.

Can't hurt to have 2.

For now this old one will do for the shed!.

Might even treat it to a birthday and change the oil and clean the filter etc

It will probably break down in shock if I do!

See what tomorrow brings..... slowly slowly catchee monkey.

Originally Posted by Timless Timber

Well the little old 3 phase $50 compressor now lives at my place!
I think it cost $50 in diesel to go get it!
.
.

If you need an free air delivery measurement I can probably do that with my gear.

Cheers
Bob

Thanks for that Bob,
At this point I haven't even plugged it in yet, for all I know right at this moment - it might even run backwards.
I'll take a closer look this morning, it was all too hectic here last night.... to get anywhere near the shed UN-fortunately.

Edit:
This morning I ventured into the shed to play with the compressor...
Wouldn't you know it, it has a 10 amp 4 pin 3 phase plug while my outlet is 5 pin 32 amp outlet, so I can't even plug it in to test it!
I'll have to source another 32 amp plug end and swap them over!.
I also managed today to pick up a wet and al oxide dry wheel grinder off gumtree, for $75.
On the way home I swung by a garage sale, and picked up a brad air nailer and 2 x 25 meter extension leads for $100.
That's this fortnights play dough done and dusted...after the compressor, grinder, nailer and leads.

Slowly it's all coming together!.

Picked up a 32 amp 5 pin 3 phase male plug for the compressor today.

Now it works a treat, starts and runs fine etc... and after a few thread tape applications to various air blow down guns and tyre gauges as well as the air regulator inlet, it even holds air!.

I'm indeed a happy chappie with the progress made today!.

Not all days go this well.