Thread: Combining two air compressors

1/4" fittings will probably kill the flow too much, the compressors will also be running pretty much non-stop to keep it going

The 1/4" hose exit is one problem but it should still be able the deliver 8CFM. My concern is that cheaper tool manufacturers under rate the CFM requirements and cheap Compressor manufacturers over rate their compressor recharge ability so the chances of two of those meeting your requirements are slim.

To work things out in detail we'd need to know.
- the PSI requirement of your sander?
- the recharge pressure of the compressors

One small thing I don't like about those compressors is the drain does not appear to be at the very bottom of the tank - this means unless the tank is tipped over it will never completely drain.

The recharge trigger is usually in the 90-100psi range, some can be adjusted but I wouldn't count on it with a cheap one.

Thanks Elan and Bob for the replies, this is the one i am looking at buying two of https://www.ebay.com.au/itm/New-Elec...e/273409733242 with the ebay discount it comes out to about $510 for 2, the equivalent 4 pump on a single tank is like $2000 from Sydney Tools so i am trying to do it cheap'ish

these little compressors turn on around the 90 to 95 psi and off at 115psi all fittings will be changed to nitto 3/8" with 3/8" hose hose length could be up to 20 meters but i will use 10 meters if i have to

tools i use will need 90psi at the tool looking to use a full range of drills, die grinders, sanders, spray painting guns etc..

this is the first sander i will be using https://www.ingersollrandproducts.co...ders/315g.html

I don't think those compressors will survive for too long with tools like that; it's going to be running flat out pretty much from the moment you pull the trigger until you stop.

If you're planning on running high consumption tools for extended periods, I really think you should be looking for a proper compressor for the job. Yes, they're expensive, but you only have to buy good tools once.

Biggest job using the straight line sander will be sanding flat roughly 2 square meters (boat transom needs to be sanded flat) one other big job will be spray painting a whole boat at 35psi to 40psi, the die grinding will be momentarily job all thought the IR die grinder uses less cfm than the straight sander

Thanks for the info.

Assuming the Lumik manufacture's claims are correct (which I seriously doubt) when using the dual compressor arrangement between 115 and 90 Psi, there is 288*(115-90)/15 = 480 total litres to play with.

With the sander using 8 CFM (226.5 L/m) the combined 100L tank will last for 2.1 minutes before the first compressor triggers and the second one will quickly trigger because 140L/min delivered by one compressor is not enough to keep the sander going.

With both compressors running @ 280 L/m this means the compressors will have 280-226.5 = 53 L/m spare capacity so while you are sanding it will take 5.4 minutes to recharge the tank.
The duty cycle is thus 2.1 min off and then 5.4 minutes on, so the off/on ratio is 0.4. I can't see a cheap compressor lasting long under these conditions.

Now compare that to a Pilot Air 3HP, 145 PSI, 100L tank compressor with a 339 L/m FAD.

This has 288*(145-90)/15 = 1056 total litres to play with and at 8CFM that gives you 4.7 minutes of air before it starts a recharge
The Pilotair has a 339 L/m FAD so it has 112.5 spare L/m so while you keep sanding so it will recharge the tank in 2.6 minutes.
The duty cycle is thus 4.7 off and 2.6 minutes on or an off/on ratio of 1.8

Comparing the two ratios shows the 3HP pilot air is 1.8/0.4 or 4.5 X better that the twin Lumiks.

This means 4.5 times less wear and given the build quality of the Lumik is DIY and the Pilotair is at least a semi-pro unit I'd say the Lumiks will die pretty quickly.

If it was me doing that job I'd be looking at a minimum of 100L 4HP 16CFM 145 PSI unit
Under these conditions the duty cycle works out to be 4.7min off and 1.27 mins on or and off/on ratio of 3.9 which is ~10 better than the twin Lumiks.

And like I've said several times said this assumes the manufacturers data is valid.
In practice teh numbers will work out to be worse than this. What is likely to happen is the sander will use more than 226 L/m and the Lumiks won't deliver 280 L/m - as soon as the compressor delivers less than the sander requires the sanding performance will severely deteriorate and eventually the comps will both run out of enough pressurised air and you will have to stop working - been there done that - it's very frustrating.

Note the manual says the sander actually needs 9CFM under load.

Thanks heaps for the calculations Bob, i currently have a Pilotair 10.8cfm compressor but its dying when it builds up pressure it stalls and it slows right down after it turns on, do u think i should fix it or buy a new machine?

This is the old girl pilotair 10.8cfm as mentioned do u think shes worthy of a new electric motor?

Originally Posted by gazza2009au

Thanks heaps for the calculations Bob, i currently have a Pilotair 10.8cfm compressor but its dying when it builds up pressure it stalls and it slows right down after it turns on, do u think i should fix it or buy a new machine?

Bit hard to diagnose that sort of problem long distance. It should be jumping out of it's skin when you first turn it on and only start slowing down when it reaches 90 or so PSI.
Can you put an ammeter on the motor to see what current its pulling during the charging/re-charghing cycle?

I dont have any 240v amp meters on hand i could buy one if they are cheap, the guy i bought the compressor off said he used it for commercial fence building and home handy man spray painting

I'll see now if jaycar sell the same capacitor and i'll swap it out if i can get one to see if that is the problem

I pulled the capacitor out its a 50uf +- 10 i put it on the multimeter it should have read from 40-60 i had to use 2000k setting to get a reading off 900 which settles around 200 but goes up to 240 and down to 180 so the capacitor is likely the problem

I'll order a new one now from ebay

Here's a weird one. I once had a compressor that would start pumping OK but then when it got to ~90 PSI it seemed to pump very slowly for about 30 secs and then it would start pumping again and complete the charge normally.

This had me baffled until I started seeing some black powder around the tank and pulley. It turned out the belts were slipping slightly and then they would warm up and get sticky and finally they would grip enough to complete the pumping.

I already tried that Bob i tried tight, loose and inbetween i now have it with 10-15mm play in the belt still doesnt fix it