Thinking of my own compressor
I had been considering getting a compressor for my diving for some time. Basically I dive nitrox and trimix almost exclusively and had been mixing my own gasses already for several years. I would mix the gasses then lug them to the dive shop, then bring them home again to analyze them, and then I would load them back into the van to head off diving.
Initial Research
I started looking at the various compressor models available to the home user. Basically I was looking for a single phase electric compressor since three phase is not available in a residential home, and I figured the gas model would be more trouble because of worries about carbon-monoxide and noise from the gas engine.
There are a number of manufacturers that make small single phase compressors these are:
- Bauer - made in Germany
- Coltri - made in Italy
- Alkin - made in Turkey
- Ingersol-Rand, Worthington etc
Bascially I wanted to find something that had an automatic pressure cut off, and automatic drains so I could let the compressor run with minimal supervision.
Here is a link to the Coltri MCH-6 which is typical of these small comressors
Coltri MCH-6
The auto drain/pressure switch option is about a $1200 option on most small comressors.
I also found this page which was an excellent summary of how compressors and filtration works.
How Compressors and Filters Work
Basically I narrowed my initial specification to be a unit between 4 and 6 cfm with auto drains and presure switch.
I dive double 95cu tanks so filling these at let's say 5 cfm would take about 40 minutes with a 5 cfm compressor.
Found one
I started reading the compressor sections of various dive forums like
The deco stop and
Scuba Board to get as much information as I could about compressors and peoples experiences with them.
Eventually I found a Bauer Capitano with auto drains and pressure switch in North Carolina. This unit was also to come with two 4500 psi cascade bottles and a Bauer P1 filter system. Normally these compressors come with the Bauer P0 filter which is much smaller (3800 cu ft capacity rather than 15000cuft). This unit has a 5HP single phase motor and is rated at 6cfm.
The seller indicated that the unit has been professionally serviced and the oil had been regulary changed. He also indicated the compressor could pump to 4700 psi and was currently running at the sellers house.
I got in touch with the seller and arranged to head down to North Carolina to have a look at this beast and see it run.
We decided to combine this trip with a visit to the Blue Ridge Parkway and off we went in the direction of North Carolina.
When we arrived the seller was already in the process of taking the compressor apart and other than seeing it run with no pressure I was not able to see it run other than to verify it did in fact turn over and the motor worked. This made me a little suspicious, but because this package was still a good deal I decided to take a chance on it. Everything I have read indicated there is nothing that can go wrong with these units that is not fixable.
So we loaded the compressor, filter system, and cascade bottles into the van and headed home. This thing is a beast and weighs about 250lbs. We had fun getting it in and out of the van to say the least.
Let's check this thing out
Once I got the compressor home I began work to get the beast up and running. The first thing I did was give the whole outside of the compressor a good cleaning. The compressor had been sitting outside and there was quite a bit of dirt and grease on all the cooling coils. Both the interstage coils and the final stage cooling coils.
To clean the coils I used carburetor cleaner. This stuff is commonly available and does an excellent job of cutting grease. The nice thing is the solvent then evaporates so it does not leave a residue on the compressor. I went through several cans of cleaner and a lot of rags, but eventually got the compressor coils clean and grease free. This is important because the compressor needs to be able to cool properly both for the quality of your air fills and for the life of the compressor.
I then picked up some white paint and touched up the frame of the compressor and the mounting brackets.
All in all this made the compressor look at lot better.
Starting to hook things up
My next task was to get the compressor placed in the garage and get all the tubes and pipes hooked up. I also needed to get electricity to the compressor which draws 22 amps at 220 volts.
I already had a sub panel in the garage so it was not too bad to run some conduit and #10 wire to the compressor to allow the electrical connections to be made.
next was hooking up and mounting the filter panel. The panel weighs about 50 pounds and needed to be mounted to the wall. It also required the air output from the compressor to be connected to it. At the other end I had to hook up the hose to the pressure cutoff switch and then a line to a fill hose so I could connect to a scuba tank. I also connected the drain from the moisture separator to the auto drain valve on the compressor.
I also need to pickup some Chemlube 751 so I could change the oil in the compressor. Chemlube 751 is the standard oil used in Bauer breathing air compressors. The stuff is used because it has a very high flash point to minimize the chance of any fumes getting into the breathing air when the compressor is running and generating a lot of heat.
I changed the oil over. The old oil had a bit of dirt in it, but otherwise looked in decent shape.
The next order of business was to try powering up the compressor...
Bang!
So I finally got around to powering up the compressor. Not to fill any tanks, but just to see if it worked and whether it would pump reliably to a reasonable pressure.
I powered up the compressor and sure enough I saw the pressure gauge begin to rise. When it hit 2000 psi I thought I'd go an grab a screwdriver to adjust the pressure cut of switch which I knew the previous owner had set to about 4700 psi. I intended to turn it down to something lower like say 4000 psi. Just as I stepped away from the compressor there was a loud bang! and an incredibly loud hissing noise. After recovering from the initial shock I reached over and turned off the compressor. My ears were ringing but nothing serious had happened. ...Note to self wear hearing protection when running the compressor!
After having a look to see what had occured it turns out the stainless steel line running from the interstage separator to the drain valve had come apart and slipped out of the compression nut. I had a look at the nut and it did not seem to have as many threads as the nut on the other side. I decided to replace the whole line with new line, nuts and compression glands. Cost was a reasonable $12.
I was then able to run the compressor up to 4000 psi and verify the pressure switch cuts out properly.
I also began to notice that the mechanical timer that runs the auto drain valve was shot. It was in the equipment bay on the side of the compressor and water had been leaking in a little. It was corroded and was no longer working.
This was also a clue to begin looking at the filters to see if any damage had been caused. The only way to activate the drain valve is electronically and I knew there was no manual drain on the compressor. The previous owner had obviously been running the compressor with the drains not working.
I took apart the final separator and had a look at it. I had to unscrew a fitting which was threaded into the aluminum housing on the top of the separator. It was really stiff and had a bunch of white pipe sealant on it which I thought was odd.
When I took the core of the separator apart I noticed that one of elements was cracked. It's the left most part in the photo...
Turns out Bauer only sells the separator filter as a whole assembly so it cost me $230 to replace this assembly. On putting the separator back together I noticed why the pipe sealant was on the hose leaving the separator. It has been cross threaded and the threads were damaged causing the copper gasket (which was intended to seal the fitting) not to seat correctly. I had to tap the fitting out to a 1/4 NPT pipe thread, replace with fitting with a 1/4 npt fitting and seal it as normal with teflon tape.
Things are beginning to come together now!
After cleaning out a number of components on the filter system and replacing the filters in the separator, and final filter and cleaning and checking the interstage separator I was finally beginning to find fewer and fewer issues with this system.
I had to have the two high pressure cascade bottles hydro tested and tumbled and had to replace one of the CGA 347 valves because some of the last few threads were damaged. The cascade bottles had to be tumbled to remove corrosion in the bottom of the tank (likely due to the separator issue mentioned earlier). The tanks passed hydro and I was able to pick up some fitting to hook it all together.
I now had the beginning of a nice fill station.
I also connected a three way switch to the outside of the equipment bay on the compressor to allow the drains to be triggered manually.
Nitrox Stik
The next order of business was to add a Nitrox Stik to the compressor. Basically this device will mix oxygen with the incoming air so that nitrox can be generated and compressed by the compressor.
Basically the Stik is a tube with baffles inside (to mix the gas thoroughly) and at one end oxygen is injected. At the other end an oxygen analyzer monitors the mixture so it can be adjusted to exactly the right oxygen percentage on the fly.
Basically the nitrox stik can be made at home using some 2 inch PVC pipe, some pipe fitting and 1 1/2 inch abs temporary pipe caps (to act as baffles inside the two inch pipe).
It is also important to make sure to not cause too much resistance to the compressor intake since this will affect compressor life and efficiency.
In the photo you can see I added two Stiks. One for helium and one for oxygen. By using two in this fashion I can continuous blend either nitrox or trimix.
Here is a good link that described how to go about continuous blending trimix using two stiks in series:
Shadow Dweller Blog
Auto Drains
My next order of business was to get the auto drains working. Setting an egg timer for 15 minutes and running outside to flip the drain switch get's old pretty quickly.
I needed to get a timer that would allow the drains to run every 15 minutes or so for about 10 seconds.
I did some checking around and found that there are some devices called "multifunction timers" that are available for various industrial control applications. I headed into the local electrical wholesale house and began looking into these timers.
They had a din rail mounted timer 700-HRF (shown in photo) that had exactly the function I was looking for. Basically it has an adjustable on and off cycle. Both the on and off can be adjusted from second/minutes/10 minutes/hours. The timebase on the dial can also be adjusted and by turning the dials on the front both the on and off timer can be selected. I picked up the timer, socket and a piece of din rail for about $150. I also replaced a relay which is used to ensure the drain moves to the open position when the compressor it shut down.
After some wiring and changing my manual drain switch from a three way to a four way switch (so I can still trigger manually if I wish) the auto drains were working correctly.
I must say it's nice not having to baby sit the compressor as much. It's still important to keep and eye and ear out to ensure the drains are working but it's nice not having to be anal about it and run out there continually.
Moisture Monitor
After running the compressor for a while I began to think about when to change the filter cartridge. Basically with the P1 filter I'm supposed to be able to process 15000 cuft of air. The thing is that this 15000 cuft depends to some extent on environmental conditions. It depends a little on humitity, but not much since the moisture seperators are very efficient at removing moisture from the compressed air. The main thing that affects filter life is temperature. Higher temperatures reduce the life of the filter.
The filter contains three chemicals to handle three aspects of filtration.
- Dessicant to remove moisture from the air
- Granulated Carbon to remove oil and organics from the air
- CO Catalyst to convert any carbon monoxide present into CO2
Basically there is enough of these three items in the cartridge to handle 15000 cuft of air. The thing is that if the dessicant stops working it also makes the carbon and CO catalyst useless since they do not work when wet.
In order to double check my air quality I installed a visual moisture indicated in line after my final filter. This allows me to see when excess humidity is present in the compress gas and alerts me that my filter needs changing. Basically the disk in the monitor has three sections for 10, 20 and 30% relative humidity. Basically each section will change from blue to pink when enough moisture is present to trigger that section of the disk. When the disk begins showing that 20% relative humidity is present I know it's time to change the filter, or if I hit the 15000 cuft capacity of the filter.
It's nice to have something that gives a double check that the filters are still working well!
Created on 07/27/2007 09:34 AM by bloedorn
Updated on 08/22/2008 05:16 PM by bloedorn
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