Thread: Air Compressor Purchase....

Thanks for humouring me guys, but I must admit I don't see why both compressors would need any electrickery or linked/identical cut-in/out switches.

Essentially, what we are talking about is two compressors both aiming for approximately the same pressure. Let's say compressor A reaches its target pressure before compressor B and cuts out. What happens? Compressor B would simply keep running and put a bit of extra pressure into both tanks, raising them both to above compressor A's cutoff point. This shouldn't create any problems, providing both tanks/safety valves are built to hold that pressure. Then, in use, presumably compressor B would kick in first as the tank pressure dropped. But compressor A would only remain inactive as long as the pressure remained above its trigger level. As soon as the pressure dropped below that, then it too would kick in.

I understand malb's points regarding efficiency (although I'm a bit lost by the 4HP=5KVA argument), but if it is simply a case of getting the most blow for your buck I still reckon my suggestion has some merit...

Think about it. Once it gets up to pressure, unless you drop below the ON pressure of both compressors, only the one compressor would ever be operational so the only advantage is getting up to pressure quicker or dropping the overall pressure so much that continuous running occurs at pressures below the on switch setting for both compressors, which is not good for either compressor.



In case 1, the pale blue ON-OFF pressure ranges overlap. Thus once both compressors are up to or over pressure they will turn off. To turn back on the pressure has to drop below the pressure of both pumps, but that never happens because P1 switches on first and raises the pressure again. The combined operational range of the two (in orange) simply becomes the pressure range of P1

Same happens in case 2 where there is one comp (P2) with the on-off band inside the range of the other (P1). Once up to pressure P2 is the only pump that works because P1 never gets outside it's operational pressure band so it never activates.

What happens is, unless you some how connect up both pumps to trigger at the same time, one pump is hardly used and the other dies prematurely

And to think I just wanted to know what spec compressor to buy.

Sorry BobL, but I have to disagree.

We are talking about running two compressors 'in parallel' for high demand applications like spraying, right?

So if one running compressor isn't able to keep up with the tool, the system pressure WILL fall, eventually to below the 'lowest' compressor's trigger point, at which point it too will fire up and so both compressors will be running. There is simply no way for the second compressor to NOT run - unless the first compressor to kick in can replace all the air the tool uses (which it would for low-consumption tools but not for something like spraying).

Regarding premature aging of a compressor, I think that is something of a side issue - obviously, if these compressors were being called upon to do a lot of work (eg daily workshop use) then you should really be buying a decent tool that is up to that kind of duty. I'm talking more about the weekend warriors for whom a 2HP compressor would probably do the vast majority of jobs, but who might need a bit more grunt to do a bit of spraying from time to time. To include that extra capacity in their initial purchase could multiply their outlay by several times, instead of just doubling it which is what I'm suggesting.

Originally Posted by Dusty Dave

Sorry BobL, but I have to disagree.

We are talking about running two compressors 'in parallel' for high demand applications like spraying, right?

So if one running compressor isn't able to keep up with the tool, the system pressure WILL fall, eventually to below the 'lowest' compressor's trigger point, at which point it too will fire up and so both compressors will be running. There is simply no way for the second compressor to NOT run - unless the first compressor to kick in can replace all the air the tool uses (which it would for low-consumption tools but not for something like spraying).

Regarding premature aging of a compressor, I think that is something of a side issue - obviously, if these compressors were being called upon to do a lot of work (eg daily workshop use) then you should really be buying a decent tool that is up to that kind of duty. I'm talking more about the weekend warriors for whom a 2HP compressor would probably do the vast majority of jobs, but who might need a bit more grunt to do a bit of spraying from time to time. To include that extra capacity in their initial purchase could multiply their outlay by several times, instead of just doubling it which is what I'm suggesting.

If you deliberately run the compressors continuously that will just end up killing them both prematurely. The same with adding large tanks, cheap compressors simply are not designed to do this sort of thing, they are designed to have regular breaks.

And for the rest of the time when you are not running continuously, apart from start up, only one compressor will ever be running - this is the one that will most likely die first. You'd be better off leaving them disconnected from each other and alternating their use.

If you have a squiz at the Peerless Products website http://www.peerlessproducts.com.au/ and most helpfully the air tools guide http://www.peerlessproducts.com.au/a...flow_chart.pdf you'll get an idea about the sort specs you need in an air compresser to run various tools.

Personally, I rate the P17.........it might be $900 but it'll outlast plenty of the so called cheapies.