Thread: May need another compressor... How big.. Used or New??

I read the other day a compressor thread on this forum that basically gave the instructions on how to calculate the actual CFM of a compressor. Interesting but I didn't think much of it at the time.

For a while Ive thought the compressor had "slowed down" but again hadn't really thought much about it, then yesterday I noticed my compressor making a strange rattling noise - I instantly thought cylinder head issues as its a cheapo chinese one.

Anyway today I calculated the CFM - 2.8CFM from a 2HP motor (direct drive).

So, assuming its impending doom - how big is big enough for a replacement?

Its one thing to say was my existing big enough - the answer is no, not really. My needs have changed now and I use it more frequently in the garage around the lathe and my car restoration - not big use but enough to say the 3 or 5CFM that I once had isn't really enough. In the future I will need to be able to spray a some old Land Rover panels.

Interestingly I also recall the thread mentioned above that said actual CFM's can be anywhere unto half of the "advertised" CFM. With this in mind, a new 12CFM unit may struggle to drive an 8FM spray gun. Then there's the old chestnut of old Australian vs new Chinese.

I don't have 3 phase so will need to stick to single phase.

Any pointers or recommendations in this minefield I know nothing about?!? Whats a good make, whats to be avoided etc etc.

Thx
Jon

A taiwanese compressor head may give you more life...

I would go for a proper slow revving 2 or 3hp unit...

Thanks RC. Do you mean try and find a replacement head for my existing unit or simply a new Taiwanese version of something similar to what I have.

Also, when you say slow running, do you mean one that has a pulley & belt arrangement rather than a direct drive one?

Thx
Jon

That was probably me that posted the how to calc the CFM for a compressor.

That method calculates the CFM at the average recharge pressure. So if the compressor triggers on at 80psi and turns off at 120 psi, the CFM rating is at 100 PSI.
High air delivery rates at higher pressures are

However, for spray painting much lower pressures than this are needed.
Provided the Free Air Deliver of the compressor is up to it a 6 (real) CFM @ 100 PSI compressor should be able to deliver 8CFM at the pressures needed for spray painting e.g. ~30 psi.
The problem with small compressors is they simply cannot even deliver these sorts of volumes at low pressure.

What is really needed for spray painting is a big tank, this volume will ensure you don't get 90% through a big job and run out of air.

A reputable 3HP with a 120L tank should be capable of doing a fair job of spray painting.

Whether you go cheap and cheerful or for something more upmarket I reckon depends on your wallet, expected lifetime and your use patterns.
If you are only going to be using it occasionally for spray painting and will be happy to get 5-10 years out off it then cheap and cheerful might suffice.
If you are going to be a regular user and you want a 20Yr lifetime then cheap and cheerful is probably not going to cut it.

Getting back to your compressor.
- What pressure does it get to?
- does it leak? you might find a bit of teflon tape is all you need to get the pressure back.
- as far as the noise goes, check the motor fan, they come off the motor shaft very easily and rub on the compressor housing.
- tighten the compressor head bolts and if that does not work I'd pull the heads off and checked the valves. They are usually made of thin SS or ally and if they have broken or corroded they may just need to be replaced.
- change the oil

Originally Posted by ventureoverland

I read the other day a compressor thread on this forum that basically gave the instructions on how to calculate the actual CFM of a compressor. Interesting but I didn't think much of it at the time.

For a while Ive thought the compressor had "slowed down" but again hadn't really thought much about it, then yesterday I noticed my compressor making a strange rattling noise - I instantly thought cylinder head issues as its a cheapo chinese one.

Anyway today I calculated the CFM - 2.8CFM from a 2HP motor (direct drive).

So, assuming its impending doom - how big is big enough for a replacement?

Its one thing to say was my existing big enough - the answer is no, not really. My needs have changed now and I use it more frequently in the garage around the lathe and my car restoration - not big use but enough to say the 3 or 5CFM that I once had isn't really enough. In the future I will need to be able to spray a some old Land Rover panels.

Interestingly I also recall the thread mentioned above that said actual CFM's can be anywhere unto half of the "advertised" CFM. With this in mind, a new 12CFM unit may struggle to drive an 8FM spray gun. Then there's the old chestnut of old Australian vs new Chinese.

I don't have 3 phase so will need to stick to single phase.

Any pointers or recommendations in this minefield I know nothing about?!? Whats a good make, whats to be avoided etc etc.

Thx
Jon

I am very happy with the Stanley 2.5HP / 50L tank compressor. It comes from China, but its quality appears to be well above other Chinese compressors I have sen. If you can wait a few months... In the past Supercheap Auto had it on sale regularly, at least twice a year. I bought it for $399, but before they had it on sale for 449 and 499. The list price is probably a bit too steep at $749. This is a two stage belt drive pump with oil sump (copy of an Italian make), so it performs well. Free air delivery is a believeable 190L per minute. If you sometimes but not always need more air, I would consider to buy two such units. That way you have a joint 100L tank, can run one or both pumps depending on demand, and also have a spare unit to take away from home if needed. Seriously, for home use having two smaller air compressors is much better than one larger unit for the same money.

I would recommend a belt drive compressor over the cheap direct drive that are available. I've found that belt drive compressors last longer that direct drive ones. you cant run a direct drive one off a long extension cord either.

Bob, thanks - If I recall correctly I seem to recall your post about a compressor and mens shed and thereafter the method for calculating CFM. The VFD idea seemed a bit too complicated for me!

Originally Posted by BobL

Getting back to your compressor.
- What pressure does it get to?

On @ 75psi
Off @ 110psi

Originally Posted by BobL

- does it leak? you might find a bit of teflon tape is all you need to get the pressure back.

No. At least not from the pipework and tank - not sure about the head area although it will hold full pressure for a day or so.

Originally Posted by BobL

- as far as the noise goes, check the motor fan, they come off the motor shaft very easily and rub on the compressor housing.
- tighten the compressor head bolts and if that does not work I'd pull the heads off and checked the valves. They are usually made of thin SS or ally and if they have broken or corroded they may just need to be replaced.
- change the oil

Its a cheap direct drive unit so no fan or pulley's. Will definitely have a look at the head this weekend and check the oil (but there was some in it a month or so ago when I checked).


Having done more reading overnight Im thinking that 1 x 100L tank (or thereabouts) would be more than enough for me and if on THE ODD occasions I needed more capacity then Id need to plug in an empty 9Kg gas bottle or two. The other thing I was looking at last night was a used 50L 3HP National Air Compressors unit on Gumtree ($350 and just down the road) although this may seem a bit steep compared to the Stanley unit CBA mentioned at SuperCheap. Any thoughts or experiences on the "National Air Compressors" brand?

THX
J

Which compressor to buy is a popular thread starter and usually gets pages long with people listing their own compressor as a good one.
In my view asking which compressor to buy and hardly even mention what it will be used for and how often, is the same as to ask what to cook for dinner and not to mention what ingredients are available or who are we catering for.

As with any power tool or machine, bigger is better and dear is better than cheap.
So, if you can find one made in Germany or Switzerland 120L and 3hp, belt driven 2 or 3 cylinders cast steel you got it all in one.
Anything of lower quality or smaller size is a compromise you must determine. Lower the quality and you can have similar performance for weekend use. Use it every day all day and you will throw it away in a few years. Smaller size than necessary for the use you will give it (whatever that is) means you will have to interrupt and wait for the compressor to pump up your tank again or in the case of air tools perhaps even make it unusable.

If you ask me "what car should I buy?" I will tell you buy a Landcruiser with a V8.
If you ask someone else he may tell you buy a 2 door smart car.

Originally Posted by ventureoverland

Bob, thanks - If I recall correctly I seem to recall your post about a compressor and mens shed and thereafter the method for calculating CFM. The VFD idea seemed a bit too complicated for me!

VFDs are for 3phase units and for folks that either can pay the $$$ for knowledgable sparkies to install or are comfortable working with mains V. If that's not you then it's best to stay away from them.

On @ 75psi
Off @ 110psi

OK so it's still turning itself off which is good since that means it's less likely to over heat and kill itself.

No. At least not from the pipework and tank - not sure about the head area although it will hold full pressure for a day or so.

That's not too bad for a used cheap compressor.

Its a cheap direct drive unit so no fan or pulley's.[/QUOTE]
Even direct drive motors still have a fan - take the motor cover and have a look underneath

Will definitely have a look at the head this weekend and check the oil (but there was some in it a month or so ago when I checked).

I was fooled about my cheapie having oil in it for quite a few years but eventually I realised I was looking at an oily stain on the sight glass. Then I changed the oil and very little came out and what came out was more like tar than oil.

It sounds like the head or the valves might have a small leak. When you take it apart take some pics, we may be able to see something.

Having done more reading overnight Im thinking that 1 x 100L tank (or thereabouts) would be more than enough for me and if on THE ODD occasions I needed more capacity then Id need to plug in an empty 9Kg gas bottle or two. The other thing I was looking at last night was a used 50L 3HP National Air Compressors unit on Gumtree ($350 and just down the road) although this may seem a bit steep compared to the Stanley unit CBA mentioned at SuperCheap. Any thoughts or experiences on the "National Air Compressors" brand?

Sorry I don't know the national brand.

Instead of using gas bottles, the other alternative would be look out for a compressor that has a dead motor/pump, these come up occasionally for $50 or so on gumtree. I saw one on the side of the road about 10 years ago and kick myself for not grabbing it.

Originally Posted by Marc

Which compressor to buy is a popular thread starter and usually gets pages long with people listing their own compressor as a good one.
In my view asking which compressor to buy and hardly even mention what it will be used for and how often, is the same as to ask what to cook for dinner and not to mention what ingredients are available or who are we catering for.

As with any power tool or machine, bigger is better and dear is better than cheap.
So, if you can find one made in Germany or Switzerland 120L and 3hp, belt driven 2 or 3 cylinders cast steel you got it all in one.
Anything of lower quality or smaller size is a compromise you must determine. Lower the quality and you can have similar performance for weekend use. Use it every day all day and you will throw it away in a few years. Smaller size than necessary for the use you will give it (whatever that is) means you will have to interrupt and wait for the compressor to pump up your tank again or in the case of air tools perhaps even make it unusable.

If you ask me "what car should I buy?" I will tell you buy a Landcruiser with a V8.
If you ask someone else he may tell you buy a 2 door smart car.

Thanks - This is actually a useful post and raises another two questions.

Q1:
Whats the principle difference(s) between 1, 2 and 3 cylinder compressor?

My thinking is either that the unit has multiple small cylinders and they all individually contribute a bit of air to the tank, or that each cylinder successively and sequentially compresses the air a little more. This latter method may keep air temperature down and thus less water in the air???

I assume also 1 big cylinder will be noisier than multiple smaller ones and would probably rev slower.

... or is it horses for courses and makes no difference?


Q2:
It appears that most 2hp - 3hp compressors either cut out at 145psi or 160psi. Usually (from what Ive seen) the 145psi units have a higher flow rate than the 160psi units. I assume that flow is more important than the extra stored pressure? After all, most things run through some sort of regulator.

...or have I got it wrong?


Thx
Jon

Originally Posted by cba_melbourne

I am very happy with the Stanley 2.5HP / 50L tank compressor. It comes from China, but its quality appears to be well above other Chinese compressors I have sen. If you can wait a few months... In the past Supercheap Auto had it on sale regularly, at least twice a year. I bought it for $399, but before they had it on sale for 449 and 499. The list price is probably a bit too steep at $749. This is a two stage belt drive pump with oil sump (copy of an Italian make), so it performs well. Free air delivery is a believeable 190L per minute. If you sometimes but not always need more air, I would consider to buy two such units. That way you have a joint 100L tank, can run one or both pumps depending on demand, and also have a spare unit to take away from home if needed. Seriously, for home use having two smaller air compressors is much better than one larger unit for the same money.

I had a close up look at one of these Stanley units at Supercheap a few weeks ago and they certainly do look on the better side for a chinese unit.
However, for that sort of list price I'd also be looking at something like Pilot air, an Aussie company that uses Italian made pumps.

Two things about the Stanley that I noted was the close spacing on the on-off PSI (115/145) - most of the compressors I have played with have had something like 40 or 45 psi spacing.
The other thing was that it had a 75% duty cycle meaning it should not be used continuously. This is pretty standard for small compressors.

The Stanley manual specifically states the motor/pump should not be running for more than 45 seconds without giving it a 15 second break and that tool use should cease when the compressor triggers itself back on. This also means adding an additional tank to such a compressor will overheat the pump every time it recharges and which I why I don't advocate putting larger tanks on small compressors.
The majority of pump wear occurs when the pump is at its hottest which happens late in the second half of the pump running cycle. Doubling the size of the tank means the pump is recharging for longer than advisable at higher temperatures than normal. Yes the pump has to charge twice as often but those recharges will always occur at a lower average temperatures.
Compressors that can safety have bigger tanks added to them are those that can run continuously.

This also has implications if two small compressors are joined together because to prevent any one pump from overheating it's then advisable to run both pumps simultaneously. This means triggering both compressors of the one pressure switch. Now we talking about 20A which should not really be taken off a single 10A circuit. All in all not as straightforward as it might seem?

Originally Posted by ventureoverland

Q1: Whats the principle difference(s) between 1, 2 and 3 cylinder compressor?

My thinking is either that the unit has multiple small cylinders and they all individually contribute a bit of air to the tank, or that each cylinder successively and sequentially compresses the air a little more. This latter method may keep air temperature down and thus less water in the air???

I assume also 1 big cylinder will be noisier than multiple smaller ones and would probably rev slower.

... or is it horses for courses and makes no difference?

The numbers of cylinders usually affects pump cooling which improves overall efficiency. More cylinders is a simple way to keep the air a bit cooler when being compressed. Better still is sequential compression with some sort of finned heat exchanger between each cylinder. This helps compressors to more easily get to higher PSI and still maintain good CFMs. That aside a deeply finned well cooled single cylinder may be more efficient that a shallow finned poorly cooled multi cylinder compressor.

Q2: It appears that most 2hp - 3hp compressors either cut out at 145psi or 160psi. Usually (from what Ive seen) the 145psi units have a higher flow rate than the 160psi units. I assume that flow is more important than the extra stored pressure? After all, most things run through some sort of regulator.

Correct For most people flow is more important.

+1 for the Stanley from Supercheap. At $750 it's overpriced, but on sale at $500 or less it's good value. Build quality is very good for a Chinese machine. Variations of it are flogged under the Ross brand. The design derives from an Italian manufacturer, so the gauges are only in Bar.

As mentioned, the FAD of 190LPM seems pretty honest. Unlike the Gasweld 15S compressor which has no markings on it for FAD, but the staff swear it does 305LPM. From a 2.2HP motor.

If I could justify it, I'd get the Peerless P14 - a slow-revving 2.75HP unit that does 275LPM FAD - relatively quiet and you can use it on a 10A GPO. But it costs $1,000.

Originally Posted by BobL

Better still is sequential compression with some sort of finned heat exchanger between each cylinder. This helps compressors to more easily get to higher PSI and still maintain good CFMs.

I did read somewhere that at the typical pressures found in the home shop multi stage compressors are less efficient then just single stage compressors..

Originally Posted by BobL

This also has implications if two small compressors are joined together because to prevent any one pump from overheating it's then advisable to run both pumps simultaneously. This means triggering both compressors of the one pressure switch. Now we talking about 20A which should not really be taken off a single 10A circuit. All in all not as straightforward as it might seem?

Practically speaking, needing to run the two off a single pressure switch isn't (always) necessary.

If I am spray painting, I will connect my small direct drive comp to my larger belt drive comp. Now they both start and stop at different pressures, however because of the large air usage during painting, both comps will always trigger during any given cycle, so they both contribute to filling the tanks.

So basically what I am saying is that if one compressor alone can't handle the job, then it's pretty safe to connect in a second without needing them to operate off a single pressure switch. However not quite so safe (well it may be quite safe depending on the comps involved) if the air usage is minimal, as only one comp may do all the work.