Air compressors in series?

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I have a medium size air compressor, 3 phase, a V4 Clisby (an old, locally made Australian brand probably unknown anywhere else).
It's a one step compressor and I now realize that a 2 step compressor would work more efficiently at the hi volume and pressure I need to sandblast.
I wonder about the feasibility to add another compressor in series?
That would let my old unit work at medium pressure hi-volume, then the second unit would bump the pressure.
That should be more efficient than the usual connection of a second compressor in parallel.
A quick net search didn't turn up any examples, but maybe spray painters and mechanics aren't as technically adventurous as DIYaudio members;)
Anyone here considered it?

David
 
Not audio related, but ... Umm or maybe your sandblasting an amplifier chassis, yeah that's it...

Never heard of a Clisby, I've heard of Quincy though!

Buying a 2 stage compressor would be better but what you're proposing should work as long as the compressor and receiver tank that are used as the second stage can handle the increased pressure. This applies to both the increased inlet pressure and the outlet pressure. I do not think that there are many single stage compressor heads which were designed to have 80 - 100 psi being fed to them, so you may end up with a cracked or exploded head on the 2nd stage compressor.

Be careful that you do not create a bomb out of your second stage receiver tank by over pressuring it beyond its rating plate. If both compressors are single stage then they will probably have relatively low pressure rated receiver tanks.

In addition, To keep from burning up the second stage compressor you will probably need an aftercooler between the 1st and 2nd stage compressors especially if the flow is really high.

Experience tells me that you'll still run into problems. You'll probably spend a lot of time and effort doing this and a good bit of money too. I would think that in the end it would be best to sell the two single stage compressors that you wanted to use for this project and buy the 2-stage compressor you need and be done with it.

Look at the upside, the time you save by buying the compressor you need instead of experimenting with your existing ones will afford you much more free time to build and/or listen to your DIY audio gear.
 

PRR

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...a 2 step compressor would work more efficiently at the hi volume and pressure I need to sandblast. ...

Is "efficiency" really a problem? Is your electric bill limiting your work?

Two air-to-125PSI(8bar) machines in series:

The total air flow (CFM) will still be less than the first machine's free-air delivery. While CFM drops with output pressure, not by much on a healthy piston machine, so un-loading its output with a second machine's suckage gains very little total air flow.

The second machine's exhaust pressure will be no more than its motor's torque against the piston area at that pressure. While you have increased its intake pressure you still have normal air in the crankcase and that is what it is fighting against. (Pressurizing the crankcase would be dangerous.) The machine's torque is already sized for the rated exhaust pressure.

2-stage compression is done-right by using large-and-small stages at lower compression ratio each (mildly smaller losses) and with inter-cooling between (reduced work). These are readily available. 5HP-20HP 2-stage machines are not cheap.

If you need bigger-- guy up the road has a 60HP engine compressor with a for-sale sign. Dunno if it makes high pressure, or just high CFM as for jack-hammering.
 
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As been said earlier, You will runt in to trouble running 2 compressors in series.
Isn't 8 bar enough working pressure för You?
My experience is that one needs a big flow, and when You ask for more than what can be delivered pressure will go down. So 2 in paralell is propably better.
Is there som kind of flow/pressure specified so You know what needs you have ?

Figge
 
Not audio related, but ... Umm...

It's to sandblast my bandsaw renovation.
To cut the steel to widen my hydraulic press.
To bend the aluminium plates for the constrained layer front faces of my speakers.;)

Be careful that you do not create a bomb...you will probably need an aftercooler

Both fair points, I will definitely respect the tank pressure limits but I don't need extraordinary pressure, maybe 7 bar (105 psi for the metrically handicapped), just a lot of volume at that pressure.
I may run the second compressor off the tank of the first, effectively an intercooler.

...will afford you much more free time to build and/or listen to your DIY audio

This project is all part of the fun of DIYaudio.

Is "efficiency" really a problem? Is your electric bill...

Not the electric bill so much, the main incentive is that better efficiency means I can use a smaller and cheaper motor.
Of course, smaller bill is nice and also satisfies my environmental conscience.
But it's partly the aesthetics of a more efficient solution.

Two air-to-125PSI(8bar) machines in series:..

I would not use two identical machines, for the reasons you mention, the second would be optimized for its specific task.

5HP-20HP 2-stage machines are not cheap.

Yes, precisely the point.
Local shop has a nice 10 hp 2 step machine for $3,500 but I haven't found a decent unit for much less.
An extra 3 hp unit added to my current compressor would be pretty cheap because 3 hp motors are pretty much a commodity item, as are compressors to match.
But it looks borderline practical, hence the question.

...compressor with a for-sale...

Not even borderline practical to ship it to Australia!

Best wishes
David
 
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Run them in parallel, NOT series. You need volume for sand blasting not more PSI. And be sure to install check valves in each before they are teed together. I did this for a time, worked fine, but eventually broke down and bought a compressed that would deliver the 20CFM at 125 PSI I needed.
 
You need a bigger compressor or two compressors tied together at the outlets. Sandblasting requires a LOT of air and usually around 80PSI.

At the race shop when doing any kind of sand blasting or even using the glass beading cabinet we turn on both compressors. Even than, frequent breaks to let the compressors recover is to be expected. The shops that do a lot of sand blasting have very large compressors, usually engine driven.


BillWojo
 

PRR

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You want a bigger first stage (and motor), not more stages.

Get a V-8. On one bank, take off all the rocker arms. Put light springs on the exhaust valves. Put one-way valves into four spark-plug bodies and plumb them together. You have a 50-100HP compressor. If it does not come to full 100PSI, take the head off and shave it until the valves kiss the pistons (V-8 fans will know what your engine will take).

(There was once a US company, their main product was tractors with oversize Six and Eight engines, half the cylinders converted to air-pumping. It would drive itself, and a trailer, far off road, and run big air tools there.)
 
So 2 in paralell is propably better...

Run them in parallel, NOT series. You need volume...

You both miss my point.
For any specific volume & pressure it is more effective to have two compressors in series, where each can be optimized for it's particular pressure, rather than add the output of two compressors in parallel where they are both inefficient.
The physics is that air compression is not a linear process.

...that would deliver the 20CFM at 125 PSI I needed.

That about what I had in mind, what power and price?

..requires a LOT of air and usually around 80PSI.

80 psi is a bit low, 100 psi is about 50% faster and seems to be the de facto standard for professional sandblasters.
Jsantoro's 125 seems a bit excessive, perhaps nominal pressure at the compressor rather than actual pressure at the blaster.

...we turn on both compressors. Even than, frequent breaks to let the compressors recover is to be expected...

Yes, just my point, more efficiency means less heat, less stress on the compressors, less interruptions and better work flow, not just smaller power bill.

Best wishes
David
 
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..with an old VW engine. It worked. It was loud.

Yes, a local farmer reminded me of the old VW compressor too.
Noisy, smelly, too much maintenance, V8 even more so.
3 phase induction motors are so trouble free.

...perhaps just hire a diesel unit for a day or so once a year.

Even more sensible would be to skip the hired diesel and just take it to a professional who already has one and pay him to do it.
But it's nice to have the set up when you want to do a job.

Best wishes
David
 
While CFM drops with output pressure, not by much on a healthy piston machine

This prompted me to pull out my compressor's spec. sheet and do the numbers.
CFM drops by about half at 100 psi.
And if I only pump to say 50 psi then I can run the compressor faster.
At 40% more revs I have about twice the volume at half the pressure and the motor will actually be less loaded because compression is a bit more efficient.
Have to check the red-line for the compressor but ~1200 rpm looks reasonable.

..The [second] machine's torque is already sized for the rated exhaust pressure...

Peak torque won't increase because that's set by the peak pressure, and I can increase the torque from the motor, just alter the pulley ratio of the belt drive.
So I don't need different size pistons like a normal 2 step compressor where they run on the same crankshaft and revs.

Now I've done some numbers this actually starts to look practical!

Best wishes
David
 
5HP...$1800 US

Thanks, that's about what I expected for both power and price, more expensive in Australia.

If this was a PSU...And, would additional capacitors help here?

It's not quite equivalent to a PSU, there's adiabatic compression that doesn't match any obvious circuit behavior, at least not in a way very clear to me.
"Additional capacitors" is more tank volume and won't help - the problem is sustained output, a sandblaster uses a lot of air, practically non stop.

I've had a think and now incline to use my V4 as the final step, it is rated for >100 psi, as is the tank.
Just consider the other compressor as away to increase the mass flow, almost like forced induction on an internal combustion motor.
Not a turbo but 'boosted V4' sounds cool, 1/3 of a Rolls Royce Merlin;)
Maybe I need to study car hop-up or aircraft books, the physics of the pre-compressor would be the same as a blower.

Best wishes
David
 
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What CFM at what PSI (to use the US terms, don't know the SI equivalents as used for compressors) do you need? If you can't get that through the first compressor I don't see how the second could do it. My experience with high CFM devices (air routers) suggested that you just need to start with a lot of capacity and air volume. Sandblasting may be similar and need a lot of air but shops here seem to do fine with typical 5 HP air compressors; 14 CFM @ 90 PSI.
 
What CFM at what PSI (to use the US terms, don't know the SI equivalents as used for compressors) do you need?...shops here seem to do fine with typical 5 HP air compressors; 14 CFM @ 90 PSI.

Hi Demian
I need about 100 PSI and preferably at least 16 CFM - there must be some limit where more CFM doesn't help but in practice more is always better.
I did a few calculations and found that at 100 PSI the benefit of 2 step compression is not as much as I expected, about 15% in theory, maybe more or less in practice.
So the cost effective solution is just to buy cheap compressors and manifold them all.
The local discount store has commodity compressors so cheap that I could buy half a dozen for less than one quality 2 step unit.
That's not a solution I like much even if it is economically rational.
First I will try to improve the efficiency with which I use the air, with a better abrasive feeder and perhaps better abrasive.

Best wishes
David
 
I reckon rather than a second compressor...Lots of volume in the receiver...before the compressor needs to start up.

I already have a tank 1.2 m wide, this is not a small portable compressor, it's a fixed, fairly heavy duty professional unit that holds about 4 cubic feet.
A 1 atmosphere drop is about the limit before the blaster becomes unacceptably inefficient, so at 16 CFM that happens in only 15 seconds or so.
What work have you done where this is a "significant portion of the job"?
I wish my jobs were so short, I've never even seen a job like that.
Sand blast work always seems to require sustained full output, it's not like some little touch-up spray job where you hit it for a few seconds, check the finish, think about the next pass...
So I don't think more receiver volume is a useful option.

Best wishes
David
 
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