SA-6 Compressor Balanced Stage Pressures and Temperatures With Other Considerations

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tbone1004

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@2airishuman splash lube compressors can typically go down to about 2/3's of nameplate RPM before it's an issue. The Junior is spec'd to run between 1500 and 2300rpm depending on power availability.



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This thread is a split from another thread about 12vdc air compressors. The thread wandered into an interesting and separate topic about Rix compressors and the issues of pressure balancing and temperature management. That discussion will continue here.
 
Just a couple of points really to register my interest in small scuba compressors and this very interesting great topic.
But first for fun. Sitting on my office desk is a very small oil free and oil less High Pressure 4500 psi bottle filling air compressors (That are capable of compressing from ambient air) and are available in both 12 and 24 and 28 volt DC Just no one has thought yet that the yacht market would be a suitable application.

The “bottle” you lot call “tanks” is the little round silver thing on the end of my finger and its purpose is, once filled to 4500 psi to be attached to a device that really does kill tanks.
I will hook it up filling a standard 80 and find out tomorrow how long it takes but I expect most of the working day.
I dare say I have a better cheaper solution we are working out for a client with a catamaran mounting the scuba compressor upside down hanging it from the ceiling in the engine compartment and has a similar requirement but not as interesting. But for now just to show there are small HP 24 volt compressors around.

The air intake filter is the small 28mm polyester white disc on the middle right side
The power socket is the small Anphenol socket lower left and the long silver and black
"coffee" flask under my hand is the HP zeolite filter assembly.
Weight is under that of a small dog 11 kilos (24 lbs)
Power wise using a 19 to 32VDC range and 28VDC nominal
Draw is 6 amps worse case (at 269 bar) and 10 amp for one (1) second on start
(Peak current on start up is 25 amps for 15 seconds when the gas heater is operative)
Current between 15 seconds and 2 minutes is 10 Amps Maximum
Current over 2 minutes and 39 minutes is 9 Amps maximum
Current after 39 minutes running drops to 6 Amps worse case
The gas heater current is 3 Amps at 28 VDC
Operational range sea level to 10,000 ft
Excursions to 20,000 ft
Survival Sea level to 60,000 ft (18288m)
(all in height altitude in case anyone is foolish enough to ask verification.
Attitude: Roll Pitch 30 degrees
Design philosophy is the same 3 stage swash plate principle as the more commercial SA-6 160 LPM 6 SCFM design just smaller.

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Nice, Ian. I think you should productize it. What is the output volume per minute? Is it loud?

Not sure I can be that specific on an open forum but volume wise it is so small its measured in grams per second G/s or lbs per hour lb/hr hoverver noise is negligible. Filling a scuba cylinder 11L water capacity standard 80 size to 207 bar 3000 psig (from empty) your talking all day to fill.
And to be fair it was for illustration example purpose only and never intended for scuba applications just one specific application that we need not get into.

The intriguing question to me at least is if we can match a suitable scuba compressor to this solar powered application and for that we need to choose a block that we can reduce the motor size the current draw together with the RPM
And this is where you leave recreational scuba blocks such as the Bauer far behind and use compressor blocks that can be reduced both in motor size and RPM without damaging the running gear due to oil starvation

Going from 1500 RPM and running down to say 500 to 900 RPM for say a 3600 psi filling pressure to cover most applications

1.0 scfm flow with a 1.0 BHP draws 9.1 amps at 115/1/60 (115 volt 1 single phase and 60 Hz)
1.6 scfm flow with a 1.5 BHP draws 13.5 amps at 115/1/60 (115 volt 1 single phase and 60 Hz)
Also
1.6 scfm flow with a 1.5 BHP draws 6.8 amps at 230/1/60 (230 volt 1 single phase and 60 Hz)

2.5 scfm flow with a 2.0 BHP draws 18.1 amps at 115/1/60 (115 volt 1 single phase and 60 Hz)
2.5 scfm flow with a 2.0 BHP draws 9.0 amps at 230/1/60
And for us Eurosceptics
2.5 scfm flow with a 2.0 BHP draws 7.5 amps at 230/1/50

3.5 scfm flow with a 3.0 BHP draws 13.5 amps at 230/1/60
3.5 scfm flow with a 3.0 BHP draws 9.6 and 8.6 amps at 208/230/3/60
3.5 scfm flow with a 3.0 BHP draws 9.6 and 4.8 amps at 230/460/3/60

For the larger Yacht typically using a marine generator maximum flow would be around 6.5 SCFM
at say 4500 psi to enable bank charging/filling

6.5 scfm flow with a 5.0 BHP draws 14.4 and 13.0 amps on a 208/230/3/60 supply
6.5 scfm flow with a 5.0 BHP draws 11.3 and 13.0 amps on a 200/208/3/60 supply
6.5 scfm flow with a 5.0 BHP draws 22.5 amps on a 230/1/60 supply

Finally again for the Brits in Europe
6.5 scfm flow with a 5.0 BHP draws 19.0 amps on a 380/3/50 supply
 
@2airishuman splash lube compressors can typically go down to about 2/3's of nameplate RPM before it's an issue. The Junior is spec'd to run between 1500 and 2300rpm depending on power availability. 1500 for 2hp 110v motor, 2100 for 3hp 220v motors, and 2300 for the 6hp gas motor.

And by contrast the Rix SA-6 oil free spec to run 1450 RPM and deliver 160 LPM (than the Bauer Junior II 100 LPM)
you can run to below 500 RPM if you like with zero damage or consideration just reduced flow balanced to any motor size and output
 
Your back pressure regulator will determine the min draw. If it is set at 2700 than it makes no difference how full you fill to up to 2700 the draw will be the same. If you lower it to 2000 then you will reduce the electrical load until tank pressure goes above 2000.


Excellent point, for the typical oil lubricated compressors used in scuba diving.

Yet with the oil free SA-6 example I made with or reduced motor size and
a lower sub 1000 RPM the BPR setting is also reduced pro rata to 30-50 bar 400-700 psig

Although the BPR setting is primarily to increase filter chemical life expectancy and protect the floating head piston.
With the SA-6 filter life is extended by virtue of being oil free and the piston rod contact pressure needed is only around 20 bar. Yet as neither of which pressure is available with either oil free or oil lube compressors designs on start up the option to lower the BPR setting while running with the oil less designs is of considerable advantage while charging with micro motors.
 
Excellent point, for the typical oil lubricated compressors used in scuba diving.

Yet with the oil free SA-6 example I made with or reduced motor size and
a lower sub 1000 RPM the BPR setting is also reduced pro rata to 30-50 bar 400-700 psig

Although the BPR setting is primarily to increase filter chemical life expectancy and protect the floating head piston.
With the SA-6 filter life is extended by virtue of being oil free and the piston rod contact pressure needed is only around 20 bar. Yet as neither of which pressure is available with either oil free or oil lube compressors designs on start up the option to lower the BPR setting while running with the oil less designs is of considerable advantage while charging with micro motors.

There is more to it than that. the rpm and bpr sets the sweet spot of sorts. the compressor is designed to run ballanced at a certain rpm and BP stetting. I am not sure what the rpm may do for it but the BPR sets the pressure for the last stage to match the force ( psi and area of the piston) of he other cylinders. At that pressure the compressor runs smooth and does not vibrate. below or above that pressure the compressor will vibrate. It is best to have that sweet spot n the upper 2000's so that at the top off of the tank the final stage does not have the larger force on it. The compressor will last longer if it is designed with a ballanced bp at say 2800 rather than 1500. The bpr has little to do with the final stage floating piston chattering as it will chatter until the BP gets to maybe (like my SA6) 50-100 psi and is good after that. Any bpr immediately after the final stage is good for that. However they are not normally put there. They are put after the first external moisture separator to pull the water out. In that use,,,,, the higher the BP the better,, as it mechanically removes the water and not chemically in the filter stack. Compressor rpm ,,, as I have seen determines the output of 3 or 6 cuft/min. I think that on that SA6 the BPR is right after the internal final moisture separator,,,,, however that separator is a very small volumn so the clatter stops after a couple of seconds. The larger the separator the longer the chatter if your first BPR is placed after it.
 
@iain/hsm what is your ideal BPR setting for the filters and third stage? I have a pair of SA6's that I'm building now. One is going to have a 1.5hp motor run off of a VFD for travel use, the other will have a 3hp motor for stationary use. Went with the 3hp 220v for noise, but also from the Rix engineers recommending to run considerably slower for better life. Compressor will be filling 6x 4500psi banks, but rather infrequently and most of its life will be running 3000-4500psi refilling the banks.

@KWS no need you can't put a pair of BPR's in. One set low to let the compressor come up to pressure quickly, then the other for the filters
 
@iain/hsm what is your ideal BPR setting for the filters and third stage? I have a pair of SA6's that I'm building now. One is going to have a 1.5hp motor run off of a VFD for travel use, the other will have a 3hp motor for stationary use. Went with the 3hp 220v for noise, but also from the Rix engineers recommending to run considerably slower for better life. Compressor will be filling 6x 4500psi banks, but rather infrequently and most of its life will be running 3000-4500psi refilling the banks.

@KWS no need you can't put a pair of BPR's in. One set low to let the compressor come up to pressure quickly, then the other for the filters
I agree I was pointing out what a BPR would accomplish in various positions. One can put a valve between the compressor and the mechanical separator prior to the filter stack, and slowly open it when the compressor starts or get some air to prime the final stage prior to startup. that is what I do. On my bauer I come out of he compressor with a T WHERE IT JOINS TO THE MOISTURE SEPARATOR. the 3rd side of the T goes to a hp air source like a 6 liter tank. I open that and it puts a couple of hundred psi back pressure on the last stage and partially fills the moisture separator. Hit the start button no noise. If you run the compressor a lot you dont have to do that as the mechanical separator never looses pressure to below 50 psi.
 
I agree I was pointing out what a BPR would accomplish in various positions. One can put a valve between the compressor and the mechanical separator prior to the filter stack, and slowly open it when the compressor starts or get some air to prime the final stage prior to startup. that is what I do. On my bauer I come out of he compressor with a T WHERE IT JOINS TO THE MOISTURE SEPARATOR. the 3rd side of the T goes to a hp air source like a 6 liter tank. I open that and it puts a couple of hundred psi back pressure on the last stage and partially fills the moisture separator. Hit the start button no noise. If you run the compressor a lot you dont have to do that as the mechanical separator never looses pressure to below 50 psi.

Another good point. Commonally known as "Pressure Priming" but in this application (Solar power) the Piston rod loading on start needs to be a low as possible if not removed completly hence the BPR pressure reduction to marry up with rod loading and current draw with the limited power supply capability. We need to come down in power draw to match up with the solar supply capability in this application.

Changing position of the BPR is also a useful tool in mechanical water seperation, On the SA-6 an option to use a pressure ported Nupro pressure relieving valve after the 2nd stage water separator can control the 2nd stage pressure to the 3rd stage higher than the nominal charging rate for the stages. In addition to a light pressure priming.

Your final point about never loosing below 50 psi I'm unsure how you achieve this, you can pressure prime before start using the residual pressure in the filter tower granted. But the non return valve on the inlet side of the tower prevents you from using the pressure decay method from the BPR and the NRV inlet side prevents any leak back. So how and where do you get this 50psi from to load the 3rd stage floating piston before start I guess is my question .
Unless your starting free floating (knocking) and using the 2nd or 1st stage developing pressure to prime the 3rd stage head. Im just unsure from your description. Iain
 
@iain/hsm what is your ideal BPR setting for the filters and third stage? I have a pair of SA6's that I'm building now. One is going to have a 1.5hp motor run off of a VFD for travel use, the other will have a 3hp motor for stationary use. Went with the 3hp 220v for noise, but also from the Rix engineers recommending to run considerably slower for better life. Compressor will be filling 6x 4500psi banks, but rather infrequently and most of its life will be running 3000-4500psi refilling the banks.

@KWS no need you can't put a pair of BPR's in. One set low to let the compressor come up to pressure quickly, then the other for the filters

1. The ideal BPR pressure setting for normal use using mains power with an oil free SA-6 with a standard 18 inch filter tower its 1000 psig minimum to 1500 psi maximum anything more and its a diminishing return as far as extending the filter life is concerned.

Now by contrast those with the oil lubricated Bauer compressors you will need it much higher at 200 bar 2900 psi
as per the Bauer Junior II example we have on the post because you really need to extend the filter chemical as much as possible because of the oil mist carry over you get off small oil lubricated compressors.

Further using oil in the compressor the filters are compromised by having to remove the oil smell using Charcoal
and even then that chemical needs to be kept dry at all time or else the collected or what is called adsorbed oil
can be desorbed released back out in one lump when the filter water vapour chemical (the zeolite) is saturated.

By contrast with your SA-6 there is no charcoal so you have more water removing chemical, you have additional cooling because your not trying to cool the cup full of hot oil in the block

2. For your 3rd stage on the SA-6 you have three choices, A. use a single standard BPR after the final stage and use either a 500 -1500psi back pressure setting, or a reduced pressure say 20 bar as in this position you are using it only to reduce the free floating (knocking) time before the line pressure builds up.

B. Or fit a single BPR after the chemical filter to increase the "dwell time" of the water vapour across the adsorbing zone in the chemical tower and for this 1500 psi woud be as much as you need to remove the water vapour dewpoint The only issure not covered using this method is the increase in time to initially fill the tower with pressure increased the knocking time before the back pressure is sufficiant to hold the floating piston on the rod hence the free flow knocking time is increased charging the tower each time the filter is changed.

C. Use the single BPR after the filter use a check valve before the filter and presure prime the 3rd stage from the 2nd stage filter tower blank plug. Now what you need to understand is you must prime from the 2nd stage up into the 3rd stage inlet valve. Not directly into the 3rd stage head or the 3rd stage discharge valve will stop any pressure from loading the 3rd stage piston. Clear as mud I guess.

Also from the Rix engineers recommending to run considerably slower for better life

The Rix enngineer is wrong. Running slower doesnt give a better life...... First off define better.
It's another messed up piece of junk advice, not deliberate I'm sure but a 'Better Life"
This is a term better used for those of us who believe in a better life to follow if you catch my drift.

By implication you are made to think its a longer life and if so we are back to the sort of understanding made by Michael about the 17 year old Bauer.

And in the same vein as the 17 year old Bauer if you run your Rix at half speed for twice as long
you no better off than I am filling twice as much in half the time.
Running slower you do because you need to and if you need to fit a smaller motor.

Incidentally halving the pressure to 1500 psi in the scuba tank also works and that really does increase the comoressor life but no one wants to hear that as a suitable solution now do they.

Now your problem will be your required 4500 psi pressure and to explain that we are going further off topic
 
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