$500 Compressor Kit

[joebob24]

How many people would be interested in a $500 kit that you could assemble with basic tools that would produce a compressor capable of something like 3-4 SCFM at 3500 PSI? Motor, filtration and mounting not included. Just trying to gauge interest at this point.

 

[Cio]

@joebob24
As someone mentioned previously, get an interstage water separator, this can be anything but you will need it.

Also, are you drying/filtering it after the final stage of compression?

 

[joebob24]

@joebob24
As someone mentioned previously, get an interstage water separator, this can be anything but you will need it.

I might need it someday, but I don't think I do right now. I am only getting a few drops of water out in the separator every 15 minutes or so and another few drops in the filter housing after running for 2 hours. The air that I am using enters the system at a dewpoint of 34f @120 psi, which only contains around 1 ml of water per AL80. I don't think the coltri mch6 has any interstage water removal at all. It is one of the things I plan on adding when I start testing with wet air though.

Also, are you drying/filtering it after the final stage of compression?

I have a cheap water/oil separator after the final stage followed by a 14" breathing air filter:

Coltri Breathing Air Filter SC000440-MHC, X302414


The system is at 17 hours of runtime right now. Other than a few external oil leaks, it seems to be chugging along nicely. The oil leaks should be pretty easy to stop, I just need to tear it down and add an o-ring groove to one of the parts.

It is very consistently at 2 cubic feet per minute at around 600 RPM. Multiplying that out, it has put out somewhere around 2,000 cubic feet already. Honestly, I am kind of surprised I haven't run into more issues. I keep expecting it to throw a rod out the side of the case any second.

 

[Gone for diving]

How does the load on the connecting rod compare, at 120 psi in its non modified version, vs with the hp piston, at 4000ish psi?

Did you do any math that way?

 

[joebob24]

How does the load on the connecting rod compare, at 120 psi in its non modified version, vs with the hp piston, at 4000ish psi?

Did you do any math that way?

The compressor head is rated to 150 psi with a 2.5" piston, which works out to 736lb on the rod. The final piston is only 3/8", which works out to 441 lb, so in theory it should be fine. That is just the load at the top of the stoke though. I haven't actually done the math for all crank angles. The design compression ratio is closer to 10 to get 150psi, so that means that it it only seeing that load in the last bit of stroke where it has really good mechanical advantage. With a ratio of 6 to 1 on the final stage it starts to load higher sooner. One of these days, I'll calculate it out, but I think it still should be lower than the rated load. I did only pay $100 for it though and you do get way you pay for most of the time. If I don't count my time, it has already paid for itself in air and it has been a really fun project so far, so if it does throw a rod I will still probably be happy enough.

If it makes it to 100 hours, I'll probably do a full teardown and see how it is wearing.

 

[Cio]

I might need it someday, but I don't think I do right now. I am only getting a few drops of water out in the separator every 15 minutes or so and another few drops in the filter housing after running for 2 hours. The air that I am using enters the system at a dewpoint of 34f @120 psi, which only contains around 1 ml of water per AL80. I don't think the coltri mch6 has any interstage water removal at all. It is one of the things I plan on adding when I start testing with wet air though.





I have a cheap water/oil separator after the final stage followed by a 14" breathing air filter:

Coltri Breathing Air Filter SC000440-MHC, X302414


The system is at 17 hours of runtime right now. Other than a few external oil leaks, it seems to be chugging along nicely. The oil leaks should be pretty easy to stop, I just need to tear it down and add an o-ring groove to one of the parts.

It is very consistently at 2 cubic feet per minute at around 600 RPM. Multiplying that out, it has put out somewhere around 2,000 cubic feet already. Honestly, I am kind of surprised I haven't run into more issues. I keep expecting it to throw a rod out the side of the case any second.

You are putting ridiculously wet air into your tank. Typical scuba air should be -40F dew point at pressure. You are putting saturated air in your tanks, this is not good. and should be addressedASAP. All you need is desiccant in a pressure vessel. What kind of filtration do you have?

 

[Gone for diving]

The compressor head is rated to 150 psi with a 2.5" piston, which works out to 736lb on the rod. The final piston is only 3/8", which works out to 441 lb, so in theory it should be fine.

You'll be fine, I see no problems, with that,
Is 600 rpm the original rated speed?
Seems a bit low, to me.
The main thing is to make sure the lubes is being thrown up enough.

What are you using for oil SAE 30?

I really like this in general, you have done a good job for your first prototype.

 

[rjack321]

I cringe at the volatiles coming off harbor freight oil.

 

[Tom_Ivan]

How does the load on the connecting rod compare, at 120 psi in its non modified version, vs with the hp piston, at 4000ish psi?

Did you do any math that way?

Huh? different thread?

 

[Gone for diving]

Huh? different thread?

Was answered very well in this quote,

Reason for asking was to see how much load the Chinese compressor crank and conecting rods are seeing,
He definitely did a bit of math,

The PCP Chinese compressors do the same thing, and I am sure they are loaded higher.

The compressor head is rated to 150 psi with a 2.5" piston, which works out to 736lb on the rod. The final piston is only 3/8", which works out to 441 lb, so in theory it should be fine. That is just the load at the top of the stoke though. I haven't actually done the math for all crank angles. The design compression ratio is closer to 10 to get 150psi, so that means that it it only seeing that load in the last bit of stroke where it has really good mechanical advantage. With a ratio of 6 to 1 on the final stage it starts to load higher sooner. One of these days, I'll calculate it out, but I think it still should be lower than the rated load. I did only pay $100 for it though and you do get way you pay for most of the time. If I don't count my time, it has already paid for itself in air and it has been a really fun project so far, so if it does throw a rod I will still probably be happy enough.

If it makes it to 100 hours, I'll probably do a full teardown and see how it is wearing.

 

[joebob24]

You are putting ridiculously wet air into your tank. Typical scuba air should be -40F dew point at pressure. You are putting saturated air in your tanks, this is not good. and should be addressedASAP. All you need is desiccant in a pressure vessel. What kind of filtration do you have?

Read the rest of the post you quoted. I was only talking about the inlet air moisture content, which is what would be removed by an interstage moisture trap. The air isn't even saturated yet between stages because it is coming in pretty dry. It does saturate after the final stage, which goes thorough a separator then this filter:

It is a 14" breathing air filter from filtertechs for the final filtration:

I haven't posted much about the filtration, because it really isn't the interesting part of the project.

Is 600 rpm the original rated speed?

The compressor head is rated to 1000 rpm. I just need to get a new pully, so I can get it up to that speed and hit the target 3 cfm. Heat generation is a real concern though, so I have been working on verifying temperatures before I speed it up much.

What are you using for oil SAE 30?

I am using coltri ST 755 oil. Mostly just because one of my dive buddies has a coltri compressor and I wanted a full synthetic oil that could handle high heat and I knew was rated for breathing air.