Compressor Query

MozamPete

MozamPete

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I'm thinking of upgrading my garage compressor - currently I have a 2hp, 50l (13 gal), 115psi, direct drive unit. Probably about 4 cfm.

Once you go above that in size you start to get options not readily available in the small compressors, and I'm not sure of the pros and cons so was hoping to solicit some opinions as to:
  1. Direct Drive vs Belt Drive
  2. Single Cylinder vs 'V' Head
  3. Horizontal vs Vertical (space saver) Tank
  4. New vs Second Hand
I am initially a bit wary of buying a second hand pressure vessel that you can't see the inside of to check for rust (at least not until you get it home, pull some fittings off, and stick a borescope in it). What are other peoples opinions on buying second hand compressors (or more specifically compressor tanks)?

Looking at something in the 100l to 200l, 3hp range so I can still run it off a single phase 220V supply. Use is for a bit of everything - tools/blow down, small sand blast cabinet, occasional spray painting, etc
 
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The questions become maximum pressure and how much volume at what costs. What is the highest working pressure you think you will need? Sand blasters use huge amounts of air at lower pressure, around 90 psi. Two stage compressors are more efficient, especially at higher pressures, but also cost a lot more, and use less electricity per unit of air delivered. Horsepower essentially means nothing because the numbers have been cheated over the years until they are essentially meaningless for comparison. Volume of air delivered at a specific pressure, usually 90 psi here in the US, is a good way to compare compressors. They are also usually rated here at 40 and 90 psi and two stage compressors at higher psi as well, like 120 or 150 psi. Another consideration is how much air is delivered for how much electricity used. Two stage compressors are better for that as well, as they are more efficient. Noise is also a big issue, unless you can put the compressor far away from where you will be working or in a soundproofed room. I like to say that an air compressor is a device for turning money into noise... 8^)

I have been using the same compressor for about 35 years, a Sears Craftsman unit, 20 gallon (80l) tank, two old school horses, 7.8 standard cubic feet per minute (SCFM) at 40 psi, 6.3 SCFM at 90 psi, 150 psi maximum. The motor plate says 12.0 amps at 230v, single phase. It works fine for shop air, and air tools, just keeps up with most smaller air sanders and grinders, and is pretty hopeless for sandblasting anything more than small parts unless you are willing to keep stopping to let it build air.

It has been just fine for my home shop use, and I hope it soldiers on forever.

Edit: Note that when you do the math, the motor on my compressor uses 2760 watts, which when divided by 746 watts/hp becomes 3.7 horsepower (input) compared to the rated 2 hp. They had different horses in those days, Clydesdales instead of Chinese ponies -- or maybe more honest people doing the rating...
 
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My current compressor does most of what I need but struggles with providing the volume needed for the sand blasting and my big air angle grinder (which I don't use much anyway). Also think it would struggle if I attempted any large painting jobs.
The blaster cabinet states it needs 5 to 10 cfm, and with the smallest nozzle in the sand blaster gun (the 5cfm end) the compressor runs constantly and can only maintain about 60psi - gets the job done, but not well. So I'm looking to try and double the cfm I can get if possible.
The 3HP limit is the most I would like pull from a single phase outlet over here and seems to be point where most compressors change to 3 phase (although technically the single phase socket over here is rated at 220V, 16A so is close to 4.5HP, but in the international 230V market that these compressor are manufactured for I think a 10A limit is more normal).
Going by the fact you say a two stage compressor is more efficient, I should be able to get more CFM for the same HP which is probably an advantage as I am looking to get the most I can from a single phase connection.
 
This is one of those things that the biggest that fits your budget and space. bigger is better. And Cast iron and belt drive Is the way to go.
 
You don't need to replace your current compressor, you can just add another one. You say your current compressor is about 4 cfm and does all you need except sandblasting, lets say your blast cabinet needs 10 cfm, you can get a compressor that puts out 6 cfm and hook together with the other one and get 10 (or what ever you need) cfm. that way you can turn on the extra compressor only when you need it.
 
Jut to let you know, I hat those direct drive things. But the 2 stage is great, The older ones can be rebuilt with little problem, and normall last for ever if taken care of.
Tanks, I also am a bit shy of ones I cannot look at, but have also seen some nice ones that I wuld purchase as well.
I figure whatever CFM I need and double it would be what I really want, but then what I can afford is something different, I am currently running a used, hand me down, twin cylinder that is over 20 years old 20 gal craftsman in the shop, It is about had it and cannot keep up any longer. I have a 1966 Kellogg American two stage with a Wisconsin 10HP engine that was my fathers, but have not fired it up in a few years. (it could keep up with a 3/4 or 1 inch impact going full out with not problem. It even kept up with sand blasting a horse trailer years back.
But the thing is so noisy the neighbors complain.
It all depend on what you want, and what you can afford. and if you can afford better there is never a time when you can have to much air.
 
OK next question - if I just got a new compressor pump head, say something rated like 5.5 kw (7.5HP) ,830 lt per min,12.5 bar,920 rpm.
My thinking is I could choose the the pulley size such that a smaller motor would run it but at a lower speed (and hence output). As a piston compressor is a positive displacement pump I think it would be pretty much a linear relationship - halve the rpm and you would get half the flow rate for half the power input. Lower speed could also mean less noise, less wear, less heating of the air, plus it could be uprated in the future with a 3 phase motor if I ever got a place with a 3 phase supply. I could then just replace the compressor head and use my existing motor and tank. Could save some money this way.

What's the flaw in my logic? Can I just run a piston compressor at a lower speed without problem?
 
You can run it at lower speed but I doubt you'll get much, if any, more air out of it than your current compressor, if you use the same 2hp motor.

Here's some info I found:
Top 10 Compressed Air "rules Of Thumb
 
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your least expensive problem solver is just a much larger air storage system--80 gal or bigger is best( I hook several tanks together quickly with quick disconnects and shutoff valves to hold pressure in the larger tanks till I need it) that way I can have one or several small compressors like Eddy says to pump up a 10 gal tank quickly or to refill the pressure in the other tanks when you don't need them----just get some lengths of hose with quick disconnects to connect tanks when needed---the tanks don't have to be close together either for this easy system to work for your shop---shut off valves and no leaks helps tremendously also----Dave
 
Yes I agree. Other than possibly a small improvement in efficiency the air out is directly related to the electrical energy in.
What I should of said is use an existing motor - a 3HP one, not the existing compressor's 2HP one.
 
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