Small compressors...?

[Evans]

I won't beat around the bush trying to make up excuses for my obvious nickel-rocketry.

Instead I'll just get to the point:

What is the least I will pay for a simple air compressor that won't get me killed?

It doesn't have to be robust or portable or convenient. It certainly doesn't have to be fast. Could take all week to fill my tanks, as long as they're ready by the weekend. Could be gas or electric (it would be used in or near my garage and never moved). I am looking for a financial baseline. It just has to be safe. Any references would be appreciated.

As long as I'm asking questions, just how much do they have to be baby-sat while they're running? If you need to be watching them every minute, then fill speed might become more of an issue, I suppose.

And I guess even better, are there any compressor-related FAQs or web pages or whatever people could point out?

Thanks

 

[Genesis]

The Alkin W31 is a tough act to beat.

Roughly $2500 new from AirTex. Available in either 220V Electric (single phase) or gas. I recommend the electric if its going where you have power - no CO ingestion risk, and a lot quieter.

Tending depends on your humidity. If its humid, you have to dump the condensors every 10-15 minutes. If its drier (like it is right now in Florida - RHs were under 40% yesterday) I was able to go 20 minutes and not have much in the condensators at all.

Beware going too long between dumps - if liquid water gets where it doesnt' belong you can either hydrolock the third stage (boom!) or get a fire/explosion in the filter stack (boom!)

If you're willing to go used/Mil Surplus, then the little Kidde 4-stage units are a good bet, but you need to pay attention as most available don't come with a filter stack, and those tend to be a bit expensive. Those little things will outpump a W31 easily, but you'll have to add your own filtering setup to it.

 

[pescador775]

Thanks for the candor. Ordinarily, it is so hard to answer questions of this kind. Still, the question amounts to "I don't drive, nor do I understand cars, but can you recommend a cheap one". It's tough.

The price point should be about $1000-1500. That will get a servicable compressor of about 3 cfm, sometimes more. It should be electric, 220v/single phase. (Do you have in garage?). Otherwise, a lightweight gas would be second choice.

Are you in a hurry to buy? Will cost you. If patient, look for a used Bauer Junior, low hours. A Bauer Utilus 10 might also do fine depending on condition. Every three months or so a real bargain shows up on E BAY. I saw a Junior go 'buy it now' for $1200 in April. It lasted 3 hours.

You can pay less or more but there are trade offs as with anything. However, less than a year ago, I saw a MIL surplus (Navy EOD) gas driven Cornelius, 4 cfm go for $675. Even so, if you are not mechanical and knowledgable would be best to steer away from MIL surplus, IMO.

Yes, you should keep an eye on the compressor while operating. They have to be drained after each fill or every 30 minutes. This should be no problem for any but the most scatterbrained among us.

I don't know of any FAQs on compressors but I might be able to help with specific questions of a technical nature. If you PM me let me know here first. I don't check mail very often.

 

[Genesis]

with used dive compressors, especially older ones, is that you have to pay close attention to the condition of the filter/condensator chamber.

Specifically pertaining to Bauers, there was a recall on some of those condensator/stack parts in the past. I don't know the specifics, but if you get one of the bad ones......

They also have a cycle life, after which they should be replaced.

Anyway, the bottom line is that just like a scuba tank the condensor/filter stack is under full tank pressure, and it has enough gas in it to do a LOT of damage if it comes apart.

You can indeed get some good bargains, but for older models you have to be at least somewhat careful, as if you end up buying a filter/condensator replacement you might spend $500 on that, and now suddenly that $1500 "deal" on a used compressor isn't so good.

Finally, beware of one that has been taken on board a boat. Most of the vendors don't use stainless for their cooling tubes (Alkin being a notable exception) and those cooling tube systems can get a bit expensive to replace. Also, salt corrosion in general is nasty-bad-stuff. Best bet is to be able to inspect before purchase, of course.

Units that aren't too old (5 years or less) with a reasonable number of hours on the clock can, however, be a very good value.

 

[stephan01]

pescador775

I have read some of you replys on the Kiddie and you seem to know the most . I PM you about getting one worked on if possible. I have two 4 stage units that will not pump up. I know nothing about these units so I'm at a loss. I would like to build my own unit.

stephan01@hotmail.com

 

[jbichsel]

to get my own compressor. I will set up a storage bank in my garage.

As to draining, when I lived in Texas, Dallas area, the dive shop I DM'd at, drained about every 20-30 minutes of run time. We had a large bank, so only needed to drain 1-3 times per day.

However, humidity there was high.

What would recommendations be for Colorado, where the humidity is usually single digits to teens, unless it is raining or snowing?

 

[pescador775]

The drain schedule should be set for a particular system irrespective of humidity. In other words, set your schedule for the highest relative humidity likely to be encountered and just make it a habit. For the typical combined filter/condenser like Bauer's PO I suggest draining every 30 minutes or each tank fill which ever comes first. This is allowable even though the combined or unitary filter/condenser has small water capacity, because this type of compressor has an interseparator which precleans the air before it reaches the filter/condenser. Other types of compressors, such as the Stewart Warner Mil surplus, normally come with an original condenser of SW manufacture plus an add on, after market filter. The condenser has very large capacity and SW recommends 2 hours as the maximum drainage schedule. This is also true for the Walter Kidde condenser for portable applications which also has large capacity.

I am now convinced that the separate condenser and filter is most efficacious even on small compressors, particularly if the backpressure valve is fixed to the condenser and upstream of the filter. Some folks think that the valve should be fixed downstream of the filter and so pressurize both cans. However, I believe that the maximum condensate should be removed to reduce entry of humid air into the filter. The filter dessicant chemical is typically a molecular sieve. Sieves do not depend on the concentration of moisture to be efficient. They act on a molecular flow basis and should be more efficient if not loaded down with water and oil emulsion generated by high back pressure and physical proximity to the mechanical separator. Filters are really intended for polishing the air and not assuming the burden which should be reserved for mechanical separation.

 

[Genesis]

I bet I could get significantly better life out of my filter stack if I put in a high pressure condensator, and put the priority valve on the output of THAT, rather than on the filter cannister output.

That would cause most if not all of the moisture/oil condensate to be held before it got to the filter stack, which should result in a significant increase in stack life.

Anyone got a 5000psi working pressure condensator laying around at a reasonable price?

 

[oxyhacker]

I've always been told, by Bauer and LF, that media filtration also works better at higher pressures. Remember that high pressure in the media not only concentates the pollutants but it also reduces gas velocity and increases dwell time.

I think there are two separate issues here. Of course one should remove as much moisture as possible before it reaches the media stack, by increasing separator efficiency. But once you've done that, any moisture and oil that makes it past the separator is going to end up either in the media or in the tank, so I'd rather load down the media with it, than let it get by.

Hey, can you believe that someone is selling mil-surplus Kiddes on ebay with optional filter canisters, but no separators, as suitable for filling SCUBA tanks!

pescador775 once bubbled...
I am now convinced that the separate condenser and filter is most efficacious even on small compressors, particularly if the backpressure valve is fixed to the condenser and upstream of the filter. Some folks think that the valve should be fixed downstream of the filter and so pressurize both cans. However, I believe that the maximum condensate should be removed to reduce entry of humid air into the filter. The filter dessicant chemical is typically a molecular sieve. Sieves do not depend on the concentration of moisture to be efficient. They act on a molecular flow basis and should be more efficient if not loaded down with water and oil emulsion generated by high back pressure and physical proximity to the mechanical separator. Filters are really intended for polishing the air and not assuming the burden which should be reserved for mechanical separation.

 

[oxyhacker]

Doesn't your Alkin have one already? Normally the HP separator is between the output and the media stack so one priority valves serves them both. Or am I missing something?

Genesis once bubbled...
I bet I could get significantly better life out of my filter stack if I put in a high pressure condensator, and put the priority valve on the output of THAT, rather than on the filter cannister output.

That would cause most if not all of the moisture/oil condensate to be held before it got to the filter stack, which should result in a significant increase in stack life.

Anyone got a 5000psi working pressure condensator laying around at a reasonable price?