Making my Rix SA6 better

[tbone1004]

Alright @iain/hsm here's the thread.

This thread is specific to a pair of Rix SA6's that I'm building.

A few important notes and history

• These are older military compressors that were in bad shape and I'm fully rebuilding.
• Both compressors are going to be slowed WAY down. I have bank bottles, so I have no need for it to run at 6cfm to directly fill bottles. Slower speeds also means less power consumption when travelling or modifying the garage power supply.
  • Compressor #1 is going to be run with a 1.5hp, 220vac 3p motor using an inverter VFD to be able to run off of a standard 110v power outlet in the US.
  • Compressor #2 is going to be run with a 3hp, 220vac, 3p motor using a standard VFD to run off of a standard 220v dryer outlet.
  • Both VFD's are going to be set in constant current mode so they will slow down as the pressure goes up to keep stable power consumption when travelling.

• Both compressors will have their stock fans removed and replaced with electric fans. This will provide continuous cooling at full speed since they are running considerably slower than their design parameters and since heat buildup is of serious concern since they are officially only rated for 30mins duty cycle.
• Both compressors will have nitrox sticks using 2" Koflo static mixers part number 2-40C-4-12-2. Shutoff solenoids will be tied to the VFD.
• Both compressors will have auxiliary filtration both for coalescing and filtration. The military didn't care about dew point since they were using AL bottles, but these are going into steel banks and steel bottles, so the condensers on the stock pumps are woefully inadequate. Not knowing the age and history of the condensers, I'm just not going to use them. Will replace with new units from Parker to get to Grade D air, then put a hyperfilter on there for CO to CO2 conversion and CO2 removal.
  • 2nd and 3rd stage will have Water Separators with auto-drains
  • After the 3rd stage WS, there will be a 1um coalescer, then a pair of .01um coalscers, then an adsorber.
  • After all of that will be a hyperfilter to convert any CO that's in the area to CO2 then scrub it. I know the Rix won't generate any CO, but you never know what's in the air around you.

Things that I want Iain's opinions on.

• Intake filter
  • I was planning on using a Solberg since they're readily available and about the only option for aftermarket silencer/filter combo's. You don't seem to like them since they're metal, but for indoor environment how much of a risk is there?
• Modifications to make this reliable at 3000-4500psi.
  • The engineers at Rix seem to have a 30 minute duty cycle in their mind and that the sales guys only increased the OPV pressures when they increased the pressures from 3000 to 5000. I am pretty sure there should be something done for balancing to get it to behave that high.
  • Want to run at 3000 minimum since that's what the banks are going to be run at, but more importantly I want the coalescer PMV at 3000psi minimum for dew point since it is going to be filling steel bank bottles. Since the original pressure was 1500psi, it leads me to believe that the balance pressure is lower than 3000. I understand it's more work to keep it at that pressure, but the filters are happier, coalescers are happier, and my bottles will be happier with drier air.
• Cylinder temperature and pressure monitoring.
  • Want to have interstage pressure monitoring to be able to cut the compressor off if any stage goes above setpoint. Per Rix, at 4500psi outlet pressure, that's 100psi on the first stage and 620 on the second stage. Likely set a 120/650psi cutoff. Final output obviously being the 4500psi that the banks are going to be filled to. May lower that to 4000psi or so if there is any good reason to.
  • Rix recommended 450f temperature cutoff. We consider 400f the max operating temperature for PTFE in industrial applications since that is where initial degradation occurs. I will likely set the cylinder temperature cutoff around 370f. I will likely have air temp monitoring in all cylinders by replacing the plug with a T so I can get pressure and temperature at the heads, and then since I have a 4th input on the controller, I'll put a surface probe near the bearings to keep the grease from getting too hot and running out.
• EAN50 pumping
  • Since this is going to be used for technical diving, it would be ideal if it could pump EAN50. I know you've modified these things for oxygen use, and that would be ideal, but what should be done with a max speed of 800rpm to pump EAN50?

​

 

[SurfLung]

Hey T-Bone, this poor little post has been sitting here un-answered for too long. I know you addressed it to Iain but it has "RIX" in the title and keeps drawing my attention. So, I thought I'd ask some questions and maybe get some discussion going? I've seen here and on the VDH forum where you always seem to want to slow your compressor down. At first, I think you were doing it to save on wear and tear. But then above I see you're limited on electric power and also considering low power for travel. So I'd like to give you my take and see what you think of it.

1. Wear and Tear... After re-building my rings and heads, I've learned about the "self sacrificing" parts of the RIX SA compressors. The teflon rings wear out instead of the pistons and cylinders. And in the heads, the O-Rings melt or cook off and release pressure before the metal can fail. These are two easily fixable wear and tear aspects, so why be concerned with wear and tear... Especially if the compressor is running within it's specifications?

2. Running fast or running slow... Doesn't it take the same number of revolutions to pump the volume of air required to fill your bank bottles? Isn't that the same or awfully close to the same wear and tear as running at full speed? I question whether all of the complication and modification is worth it when it might be the same or similar wear and tear?

3. Limited Power and Travel Portability - Isn't that what having a gas powered compressor is all about? You can run it anywhere regardless of whatever electric power is available? Why not just convert these SA6s to gas power so you can run them anywhere at full speed?

4. Noise - One of my favorite things about filling from bank bottles is that you don't have to sit there and listen to a compressor running. The quicker a compressor can fill those banks (and tanks) the sooner you can turn it off and enjoy the peace and quiet. So why would you want to slow it down and run continuously?

Okay, I rest my case(s)... Probably revealed my many ignorances... Hope you know I meant no offense.

 

[tbone1004]

1. the Rix is not designed for continuous duty at it's current speed. Since I don't fill tanks directly from compressors if I can help it and have a fairly large bank setup, I need it to be able to run for many hours at very high pressure. Those banks will almost never get much below 3500psi which is very different than the original design for that pump. Wear at tear when you are filling a single tank that is at 1000psi up to 3300psi, then change to another tank and fill maybe 4-5 of them at a total of maybe 300cf is very different than trying to fill a cascade system from 3500psi to 4500psi with a total of 600cf in one go.

2. wear and tear is largely determined by heat, especially with teflon which degrades significantly after 450f, but starts degrading around 400f. Making sure the pump is less than that temperature when it is fully heat soaked will make sure that it doesn't wear excessively. These compressors were designed to run hard and fast to keep the bottles filled as quick as possible. Frequent maintenance on them was not a concern, perks of the military.
The complication is a lot less for me given the fact that I run the engineering and maintenance departments at a manufacturing site where I have a team of guys that do this all day every day. Fairly simple, but the questions are largely to get @iain/hsm 's experience with this specific pump.

3. They are currently gas powered and being converted to electric. Problem with gas is emissions. Emissions in terms of gases and having to route them away from the intake, and emissions of noise. Generators are much quieter than the gas motors on these compressors, and an extension cord is much easier to route than an intake pipe, especially since this one has a nitrox mix stick. Having a generator is also much more versatile when travelling. It is not quite as efficient obviously, but it is much more versatile. With the VFD, you can always turn it down a bit if you need to use the generators power for something else. Gas powered means having to transport gasoline and dealing with all of that as well. If/when I run these off of a generator, it will be a dual fuel generator set up to run primarily on propane. A lot harder to do that on the pump directly.

4. When they run slower they run quieter. The slower they run, the quieter they are. Running around 1cfm for a 6cfm pump should be significantly quieter than the SA6 running full out which is actually quite loud. The big intake filter should help considerably as will the variable speed electric fan.

 

[SurfLung]

Thanks for trying Tbone. However, it makes less and less sense to me the more you try to explain. I know it's my lack of experience and knowledge on the subject. And I know your purpose in starting this thread is to get some answers of your own. So, I'm going to bow out of this conversation. Good luck with this project.

 

[tbone1004]

@SurfLung until Iain shows back up, which for me is largely clarification, what makes less and less sense? Whole point of this forum is to help everyone gain a better understanding of why we do the things we do. I know this is a special use case, but there are reasons for doing it. Please ask away as we may all learn something

 

[rjack321]

Have you looked at the starting amps for your 110V VFD to 3p 220 1.5hp? (I have no idea how to calculate this)

I "get" why you want electric motors on a portable. But you end up with a massively heavy generator to accomplish the startup load. I'm not even sure my 3.5hp 220V 1p motor can be started by my 8500W generator because the largest size breaker on the generator is 20amps. Its also stupid heavy (~300lbs and guzzles gas). A 6.5hp gas engine is massively lighter and if you're CBing nitrox like I am anyway you need a stick so its not hard to have a 8-10ft snorkel on the inlet which is more than adequate to achieve zero CO in inlet gas. And for all practical purposes doesn't weigh anything. Your plan might be optimal on paper but I have ruined more than one set of rotors in my truck (2010 Tundra with tow package and oversized rotors) hauling too much weight so anything to bring down the total tonnage is something to be looked at carefully.

If you ever do go mobile/portable, you are going to want to run at full speed. Nothing sucks like staying up all night in a campground (pissing off the neighbors) filling tanks. If your fill rate is 1/2 or 1/3rd than your depth consumption rate that's not going to be a good time.

 

[tbone1004]

@rjack321 vfd's can control starting amps. Unlike a normal "soft starter" this can maintain the low RPM until it stabilizes. It can be set to constant current mode as well which will give a slow ramp up, then slow the RPM's down as pressure goes up. It can "start" at 1rpm if you tell it to and take 10 minutes to get up to speed if you wanted it to. Can also start up near instantly like a traditional "soft starter".

If/when this travels, it will be in a trailer, not in the back of my Expedition. It's loaded enough with just the Rix and the other stuff I need to cave dive. When I rebuild my enclosed trailer, the compressor will be mounted inside of it. I am still unsure about how much of the bank will be in the trailer vs. the garage, but I have long whips anyway.
On the noise thing, again, that's part of the advantage of going electric with VFD's, you can slow it down to where the noise is unobtrusive and let it go as long as it needs to, or you can speed it all the way up if noise doesn't matter, and if at a campground, don't need to run a generator. Typically the generator is quieter than the gas motors on most compressors, but if you are at a campground, you can run it off of shore power.

In either instance, it will be in the garage for now and we will see what the noise and actual speed looks like once it's done. Should be another 5-6 weeks before it's spinning since I'll be travelling for the next 3 weeks.

 

[rjack321]

I don't camp at campgrounds with power. Nearest power to me is 2+ hrs by logging road, which if "deactivated" are impassable to trailers. A 6.5hp gas engine is the maximum efficiency both per unit power and weightwise. The 10ft snorkel is bulky but doesn't weight anything. The mixing stick hangs from a tree.

I have a 1000W Yamaha generator (27lbs dry) for camping, but it's getting to the point where I need solar power. The reason being that some of my batteries can take up to 10 or 12 hrs to charge if fully depleted (e.g. 180wh suit heat and 160wh light batteries). So by the time I'm out of the cave and hiked back to the truck and actually at home base might be 8-9pm. There's no way to refill and recharge to actually dive the next day anymore. And I don't want to leave a generator unattended overnight or to go hiking or dry caving for 6-8 hrs on the rest day. So I am looking at solar options, but they are still more weight and bulk, especially with a deep cycle battery to even out the generation vs use curves.

Part of the reason I have a CCR (two actually) is because the "line laid per unit prep time" was way out of wack on OC.

 

[tbone1004]

@rjack321 this is going to be used primarily for CCR filling. Gas prices in cave country are just too cheap to drag a compressor around to fill OC. For the CCR it's all about the convenience which is why I have the Haskel in the Pelican case.
Your special use case is certainly different than mine, but I'd argue if you are already carrying one generator, why carry two?

When I camp, I have a Goal Zero Yeti 400 that I use. Standalone it's enough for most things and the inverter is the right size to charge my batteries. If I'm going somewhere for any length of time that I need to keep my Engel running and won't be driving/have access to shore power, I can throw a couple 100ah AGM deep cycles in the truck. Each of those will run the fridge for a day each which gives me some extra freedom. They're heavy but they're cheap.

Have you thought about getting something like a military m101 that is designed to go behind HMMWV's and similar? Can't imagine anything your pickup would be able to go through that those trailers wouldn't follow.

 

[rjack321]

I only have to 1000w generator its light and fairly quiet so we can think and plot survey data and stuff like that. I fill suit gas and the booster drive gas off the compressor. And some limited amounts of OC gas for buddies.

My big 8500w generator stays at home and powers a chunk of my house in winter storms. Its too big and loud to take camping. I am not sure it would start my 3.5hp 1p motor off the 20amp 220V plug anyway? It might, never tried that. Takes me about 75mins to change the electric to gas, the mounting holes are the same and I have a dedicated belt/pully for each.

I have tried a group 24 deep cycle battery in the past, but it died recently powering my spot welder and I'm not sure I want to replace it. It would help lengthen my overnight charging times and I do have some reserve on the 1000W gen to add a 100w lead acid charger. Or run my truck and recharge with some jumpers (I have a 160amp alternator). But its one more thing (and heavy)