compressor pressure and 3 stage filter

[sixtwoeight]

Morning, I ve just finished the pipework and joining 2 compressors together to give me a possible 28cfm. Each compressor has a working pressure of 10 bar - so I guess ive got 20 bar in pipework.

I have a 3 stage filter at the end of the pipework (about 15m away from compressors) This filter has a maximum inlet pressure of 18 bar. I know I will get some pressure drop over that distance but am worried that 20bar will damage the filters which are not cheap.

I thought to overcome this I could just fit a regulator in the pipework before the filter, but looking at the regulators they only have a maximum inlet pressure of 10bar some are even less. I could fit 2 regulators, one at the oulet of each compressor and turn the pressure down a bit on each??

Anyone have any experience, views or recommendations?

Thanks

 

[brewdexta]

The capacity is cumulative but the pressure of both together will still be 10 bar so I think your good to go.

 

[normspanners]

The capacity is cumulative but the pressure of both together will still be 10 bar so I think your good to go.

+1


I would use one pressure switch to control the two compressors because you will find that only one will switch on and be the dominant one, unless you take a large amount of air, but they will then have to play catch up, but definitely have a delay timer to the second compressor that is switchable to save a massive current inrush starting two motors at the same instant, but when big capacity is not needed it can be switched off, to save noise. When the pressure is reached both will cut out at the same time.

 

[rtbcomp]

In practice with two compressors you only need to set the pressure switches a few psi apart. If the 2nd compressor doesn't switch off then you need that air anyway so it doesn't matter.

With an electrical delay setup you also need a solenoid valve(s) to dump the air from the delivery pipe(s).

 

[normspanners]

The problem for me and the other people I have done this for, the noise is the big factor, when you are waiting to listen for a noise etc in the workshop the second compressor then takes twice as long to achieve that last few psi. and that seems to go on for a loooong time ( boiling pot syndrome) especially with those cheap pressure switches with the massive differentials,on a small comp pumping a high pressure into a huge area.
The other way to do it with two control switches is to have a check valve between the two receivers with the second tank being the lower pressure blowing though the check valve as the pressure drops in the first tank.
Regarding one pressure switch all you have to do is link the check valve unloader pipes. which is normally just a length of 5mm plastic. that will unload both feed pipes, as long as the delayed compressor check valve is ok it should not fill in the few seconds of the delay

 

[rtbcomp]

........
Regarding one pressure switch all you have to do is link the check valve unloader pipes. which is normally just a length of 5mm plastic. that will unload both feed pipes, as long as the delayed compressor check valve is ok it should not fill in the few seconds of the delay

That has implications for single compressor running.

The idle compressor would have to have its pressure switch set to "on" in order to close its unloader valve, which means it would have to be isolated from the mains (you'd probably have an isolator fitted anyway so not much of a problem)

If the unloader pipes were just connected together the running compressor would still try to fill the idle tank via the unloader pipe. You could fit a non-return valve in the unloader pipe to prevent this.

 

[eil]

I put a timed delay on the contactor of one of my two coupled up compressors to stagger the start up, but in practise have found that I can do without it as the pressure switches even though they are set to a very similar pressure cut off/on is enough to stager the start up`s. I would try it as is first, before modifying as every situation is different. Are the two receivers the same size on yours.

 

[sixtwoeight]

Thanks for replying.
My poor little brain is struggling to follow some of that!!
I can understand the potential problem - I think.

My 2 compressors are exactly the same model - if I were to start them up say 5 minutes apart wouldn't they then be staggered so the pressure in the first one will drop 5minutes ahead of the other one and so on. Maybe im missing the point......?

At least I can now see that 2 compressors with 10 bar will not produce 20 bar in the pipework - so my filters are safe.

Thanks

 

[rtbcomp]

If the tanks are connected the pressure will be same in each, (air will flow from the high to low pressure, evening things up) so if switches are set the same they will switch off at the same time.

You need to set the switches a couple of psi apart.

 

[eil]

Its not the cutting off that's the problem its the cutting on, all you need is 3 or 4 seconds between each start-up, it would be best to adjust the switches when you have the application that needs most air.

 

[rtbcomp]

True, when first starting switch one on then the other, then on the difference in switch settings will give the delay.

In my setup I switch one compressor on, walk 20 feet to the other and switch that on, sufficient delay there!

 

[normspanners]

OK there are swings and roundabouts here, BUT you cannot get away from the fact that in normal operation,ie air leaks, light use etc with two pressure switches one compressor will allways be the dominant one and it will allways switch on first and cut off first (unless heavy air use) so therefore will take twice as long to build to pressure because the area is twice the size , the second compressor cannot switch on because if the first switch is set to say 9.1bar cut in and the second is set to 9bar the pressure will only go down to 9.1 when the first compressor will switch on then the pressure will only go up from the 9.1 so will not go down to 9bar (unless heavy use) then you have endure the noise of one compressor for twice the time. You do NOT save anything by only having one compressor running because a tank of air costs the same the only way to make it cheaper is to uprate from single to three phase.

 

[rtbcomp]

You could have a pair of check valves, one on each tank outlet, so that each pump only fills its own tank. This will get over the "will take twice as long to build to pressure because the area (Volume) is twice the size" problem.

Anyway, what is "twice as long" to go from 9 bar to 9.1 bar?

 

[normspanners]

You could have a pair of check valves

I have already meantioned that but you would only need one from the 9bar to the 9.1 cut in tank pressure

Anyway, what is "twice as long" to go from 9 bar to 9.1 bar?

you are not increasing the pressure from 9bar to 9.1 that was only an example of the cut in pressures that the two pressure switches could be adjusted to (not the switch differential BTW)
The cut out pressure would be higher 10 bar maybe (and that would have to be a good quality pressure switch to get that small a diferential) most of the cheap ones have a 3bar differential which means that the first compressor would be filling twice the volume of the two tanks, therefore taking twice as long.

One way I have done it was to have the main workshop air line connected to the high pressure tank and a blocking check valve connected to the lower pressure tank.The normal small air usage comes from that first tank, that compressor cuts in at a higher pressure and ONLY fills that tank because of the blocking check valve, and never drops below 9.1 bar. the second tank is set to cut in at 9bar so you have always got at least 9bar on the low side of the check valve with 9.1 on the other higher pressure side.
So all the normal air leaks / small usage is serviced by the one compressor set to cut in at 9.1bar that is only filling the higher pressure tank and will soon switch off.
UNTILL you have a large air demand that drops the overall system pressure below 9bar At that point the higher pressure compressor is allready running and the back up tank is allready filled to 9bar so will pass though the check valve into the main system, but at the point it drops below 9bar the second compressor cuts in (this also creates a delayed switching of the pumps) but boosts the overall pumping CFM by combining the two compressors, the lower pressure compressor will cut out first at 10 bar and the main compressor will continue for a short time till it reaches 10.1bar.
The lower pressure 10 bar tank will drop though usage to 9 bar but not go below that, and be sat there ready to back up any high demand.

 

[rtbcomp]

The reason I suggested two check valves was for single compressor running - I seem to have got it from somewhere that this was suggested as an option, although reading the thread I can't see where, maybe another thread.

Even so, I would favour the simpler approach, no extra plumbing to connect the unloading pipes, no extra cost for a delayed contactor and no check valves. Just put a lever ball valve on each tank. Then you can use either compressor or both.

 

[normspanners]

My system WAS simple in a busy garage situation where no one person was in control of the system it looked after its self, the noise was kept to a minimum the beauty was that they would never have any down time of air tools because they would always have a plumbed in compressor that would give air automatically if either failed, ok, maybe a little lacking in pumping capacity but at least they could use the lifts that use air and power tools etc.