Thinking of buying a compressor...

[Braunbehrens]

oxyhacker once bubbled...
Airspeed Press has such a book in the works but it won't be out for a while.

Continuous mixers work fine for trimix - you just have to add a tee or another port for the He. Only problem is, you really ought to have an He analyzer which adds another $1000 to the tab

BTW, the compressor/mini-cascade/cont mixer shown in the Gallery on the Airspeed website
http://www.airspeedpress.com/gallery.html
cost under $1300

I'm looking forward to that book, Vance! Sounds like a good read.

I thought He analyzers are down to $500, but I could be wrong. They are definitely nice to have. Maybe you need to figure out how we can homebrew one ;-)

cont mixing is fine and dandy, but still no good for the home mixing tech. How the heck am I going to get 3000 psi of O2 into my stages?

Also, what about the argon?

You still need a big cascade for those, and then it's basically at the point where if you have a good local tech shop it's not worth it.

I just doubled up on all my tanks, so I don't have to go to the LDS so often. I figure I can drop an empty set and pick up a full set and be on my way. It's cheaper than setting up a fill station...and a lot safer.

 

[pescador775]

Spray, are you still looking? What did you get?

Sorry, I didn't see this thread. You got waylaid by a lot of tech talk which probably doesn't apply to you, although PP blending might, in time.

A lot of bang for the buck is the ALKIN. This Turkish made compressor is available in the US. The ticket would be the 3.5 cfm model. It's a bit on the heavy side at 110 pounds but should outlast the Coltri 3/1. Price is comparable.

For a super trick portable compressor at a reasonable price see PROTECAIR. They make a Yanmar Diesel powered 4 cfm compressor with all the bells and whistles for about the same as above, $2500. Weight, 120 lbs. This rig uses a military surplus (new) Stewart Warner pump which is normally good for 3000 hours before overhaul. The intrinsic (cash) value of this unit is about twice that of a Maxair. In other words, if they charged $5000 it wouldn't necessarily be out of line.

 

[pescador775]

The adventurous among us may be tempted to save money by building a compressor vice purchasing a turn key set up. Generally, a compressor consists of a motor or gas engine, pump, condenser/filter, relief valve, back pressure valve, lines, filler whip and frame. Saving money can be an iffy thing and 'building your own' can be a satisfying, or frustrating project.

It's been said (by 'hacker') that if you buy a surplus compressor it's a 'throwaway'. In other words, if it breaks you basically have no choice but to discard it for lack of parts or whatever. There is enough truth here to caution the average Joe not to attempt this. However, the mechanically minded may find the economics intriguing. For example, it is possible to buy a complete, working surplus pump for about the same price as a replacement piston for a name brand product.

Roughly half of the surplus units bought from a wholesaler are non functional. Let's look a bit closer at this issue. The majority of problems seem to crop up with the Walter Kidde series of pumps originally used in aircraft, on the flight line and in flame thrower chargers. Although these compressors use materials and construction of extremely high quality they have some quirky features and minor deficiencies which can frustrate the uninitiated. The most common is the 'stuck piston syndrome'. The third and fourth stages of the 890228M Kidde Rotax aircraft compressor are actually 'plungers'. These are solid metal billets made of chrome steel and they reciprocate in cylinder barrels which have very hard sleeves. The clearances (select fit) are approx .0005! Over time, with disuse, these 'pistons' can freeze in the barrels. When the pump is started up the second stage relief valve blows off and the owner thinks his bargain surplus unit is shot. Generally, nothing could be farther from the truth. The problem can be corrected by removing the heads and unscrewing the barrels. Using a plastic mallet, the pistons can be freed up, oiled and replaced. At this point, the owner should have checked the valves and springs for crud. Clean in acetone, Ensolv or brake cleaner. Make sure to torque the heads when reassembling. This is all quite simple with a manual (available on EBay, etc).

The Kidde four stage is capable of 4 cfm (true number) and 4500 psi without breaking a sweat. It is the military spec construction and four stage design which allows this kind of performance in a tiny, high rpm machine, day after day. I have seen these units test run to 8,000 psi and hold pressure for 15 minutes without leaking. If high output and extremely light weight are required this is the only one I am aware of to do the job.

Anyway, if anyone wants to start a dialogue, I'll check in from time to time.

 

[Spray25]

Wow, lot's of good info in those last two posts. I am still looking. No big hurry as the budget is tight right now. I have noticed the Alkin, but haven't done a lot of research into them yet.

I have considered the DIY approach, but I don't know if that is for me. I am VERY mechanically inclined, but know nothing about compressors. All the talk of military surplus being junk scares me even more. I see these units sell on Ebay for very good prices, but would not know how to get replacement parts etc. or which ones are better than others.

Pescador, feel like teaching a rookie the ropes?

 

[pescador775]

I don't mind helping but I have to figure out what the starting point is. The statement made by someone that military surplus is 'junk' is, like most generalities, partly true. However, I can tell you that the *cost is no object* construction of some of these compressors is pleasing to the eye of an engineer. The junk thing happens when the surplus dealer starts banging these little compressors around. Bent cylinder fins, bent cooling fins, and the like. All these cosmetics can be corrected. Occasionally though, there will be a warped intake valve or some such thing. Also, the head clearances are very crucial to proper function. This problem is rare but all can be corrected with a caliper, torque wrench, monkey wrench (unscrew barrels) and a few hex key sockets (torque heads). Mostly, all that is needed is a good cleaning of the heads.

Proper oil is crucial to these units. I've seen everything including peanut oil used in Kidde pumps. However, the best oil I have seen is Braco 885, made by Castrol. Not only does it have lubricity and very low viscosity crucial to the tight clearances but the stuff is also a solvent which clears out the carbon.

There are two main schemes used for compressor rotation and hence power source. These are right hand and left rotation. The Stewart Warner and Kidde 3 stage use left hand rotation. The Kidde Rotax 4 stage aircraft compressor is right hand rotation. These are not easily reversible because of problems with the oil pump and fan. Therefore, the Kidde 3 and SW are best for direct drive. The Kidde 4 is best for belt drive.

More later.

 

[pescador775]

A year ago, I saw a brand new Bauer Utilus 10 sell for about $1500. This compressor is similar to the currently marketed Junior/Junior 2 models. These compressors produce 3.5 cfm @ 4500 psi and weigh about 110 pounds. They are all 3 stage design. Filtration is a function of an interstage separator and a large condensator/filter with a cartridge insert. The package also includes back pressure valve and blow off valves on the stages and also a final. These compressors are very well made although not to the standard set by some military surplus equipment. Even so, by using large barrels and low rpm, they are able to achieve respectable performance while giving up a slight weight penalty. Bauer compressors do not have the expensive stainless steel, brass or monel valves and seats of the military compressors. Bauer makes up for this by ease of maintenance. The second and third stage valves of the Bauer last about five years. Failure is signaled by blow off. Basically, the valves don't wear out, they rust up. However, a person handy with tools who can read a manual can change out the valves in about 2 hours, or less with experience and all the right tools. One thing you will need to get is a German screwdriver or a gunsmith's screwdriver. The screws which hold the carry handle of the compressor are the straight slot type, like on a Browning shotgun. You will also need a torque wrench and a set of metric hex sockets. Otherwise, the usual metric wrenches. Some of the large Bauer compressors require special pin indexed spanners but not the Utilus 10, etc. Overall, these can be good value for the careful shopper.

 

[pescador775]

Measuring the sea level output of a compressor is fairly simple but there are a couple of caveats. When filling a tank for purposes of making a measurement of cfm it is important to remember that you are not only filling a tank but also the condenser and filter of the compressor. Therefore, to get an accurate measurement let the condensator/filter fully pressurize before opening the tank valve. It is at this point that you begin timing the fill. Write down the pressure and time at regular intervals to later determine if the output varies with pressure. After filling, let the tank cool off and measure the final pressure. Divide this number by the measured time to get the real number for the output, e.g. 4 cfm, 5 cfm, etc.

Don't bother to delve too deeply into this subject by reading industry literature. Compressor manufactures use tricky definitions and gimmicks to inflate performance numbers. For instance, it used to be that any physicist or engineer could understand the term SCFM (standard cubic feet per minute) This defined free air delivery at standard conditions. No more, the mfgrs invented their own definition which relates to a partial fill of a "80" (actually 77.4) cf tank. About the only thing you need to understand is inlet conditions. Basically, this just means that performance is normalized to sea level pressure and room temp. If you test at high altitude or unusual temps the results need to be adjusted for the specific conditions.

 

[Genesis]

They ALL (the manufacturers) use "tricky" definitions Pesc.

This much I found out by reading literature - I didn't need to actually test, but doing that (simply reading!) just makes the point even more poignant!

For example, the Max-Air is claimed to have a 3.5cfm FAD (free air delivery) but a 4.2cfm "charging rate" (from the aforementioned 500 to 3000 psi in an AL80 - really 77.4)

But, if you look at the real numbers, it doesn't work like that.

They quote 22 minutes to charge that 77.4 tank from 500 - 3000 psi - and that 3000 psi is its "final temperature", which of course is a hot fill and will cool down!

So you'd think we would have 64.5 cfm of air delivered in 22 minutes. That's 2.93 cfm of actual delivery - not 3.5! But its even worse than that, because that bottle is really only charged to 2700-2800 psi once it cools, so the real delivery rate is roughly 2.8 cfm - not 2.93!

All the others on the market, by the way, do the same thing, including Bauer and Alkin. Get used to it, because you WILL find this to be the case with essentially ALL of the so-called "numbers" these guys publish for compressors until you get into the really big (e.g. 10cfm+) units.

Is it a big deal? Well, that depends. Its a truth-in-advertising thing, but beyond that, not really. Perhaps the CPSC or FTC might be interested in this little bit of lying, but I doubt it.

The other thing is to be very careful with high-RPM units. Beyond the noise issues there is a serious wear problem with the higher-RPM models as well, along with a significant uptick in heat produced. This may not be too big of a deal in a cooler climate, but it definitely is here in Florida!

 

[pescador775]

Interesting comments, Genesis. As a consumer, I would be ticked if I bought a car with the understanding that it would accellerate and top out at certain speeds to later find out that the performance was only about 80% of what was claimed. Same with compressors and I resent their chicanery.

The heat issue is a function of design, including rpm, materials and cooling design. Small compressors make extensive use of aluminum, which has high dissipation,. The four stage military compressors, some of which turn as high as 3750 rpm, use aluminum heads, barrels, crankcases and the finned tubing also made of aluminum or copper. Due to the four stage design, these compressors are rated for operation in ambient temps as high as 160F. (Failure is a serious matter since they are originally used in fighter aircraft). In contrast, most commercial units are rated only for 100F regardless of rpm. Remember, the higher the rpm, the faster the fan turns. Also, heat is a function of volume of air compressed, not rpm. Most of these units produce 4cfm or less.

Lucky you, I miss florida and as soon as the kids are out of college I will be heading that way for some clear water and the aroma of coppertone.

 

[Genesis]

but for the consumer/industrial units, its a different matter.

When you get into the larger units (10cfm and up) the ratings seem to be more in line with reality. Also, in the smaller units, pressurizing the purification system is of real importance and does affect the times significantly; for example, my hyperfilter stack holds close to 6cf at 3000 psi! So, it takes me a full couple of minutes to pressurize that, and then there's another 3cf or so in the primary stack and hoses; that's 9cf of gas I have to compress before anything useful flows into the cylinder (and as such it grossly affects my rate computations unless I first pressurize the stacks before starting the stopwatch!)

Of course on the second and subsequent cylinders this is much less of a consideration.... but for the first one you do, it makes the numbers look "funny".

The primary heat issue that I've seen is with the smaller units like the Max-Air 35 and the Bauer Junior - both of which have explicit warnings about filling multiple cylinders back-to-back or filling a bank in their manuals. Alkin doesn't mind; they say that the weak spot, if any, is in the motor - not the block. Worst case you might end up replacing the motor at some point in time with a compressor-duty rated (1.0SF) motor - they're cheap enough ($100 or so just about anywhere.) After having run mine for a couple of hours at a crack to crank tanks, I can tell you that they're quite conservative in terms of heat production - the motor doesn't get uncomfortably warm at all, and the unit's maximum temperature at the third stage outlet, as measured with an IR thermometer, is 150F. At the inlet to the purifier its typically 3-5F over ambient, which speaks well of their cooling tube design.

The biggest problem for many of the larger units is that they not only get cost prohibitive but are also very much portability and boat-usable prohibitive. A typical 8kw boat genset can start the Alkin, but it will NEVER start a Bauer Oceanus or a Max-Air 55. No chance. My maximum output surge current on my 8kw Kohler is 45 amps (@ 240V) and my maximum steady-state current is 33 amps. The Alkin requires 18 amps to run at full load; I can start it if I'm careful with what else is running on the boat. Once its running, I can actually have the AC on in at least the salon and be ok. There are other issues with a compressor on a boat, not the least of which is where you're going to put it. In the engine room is a BAD idea due to heat, among other things (120F is pretty common in the ERs of most pleasure craft, especially with the genset up and the mains off - the generator is making lots of heat, and the mains aren't turning over any air! That just happens to be the scenario when you'd like to charge tanks, does it not?)

To give you an example, the "Clark Yacht" 55MA, which has the ludicrous claim of a 5.8cfm displacement (but 15 minutes to fill a 77.4cf bottle, hot, from 500-3000 psi - so a real output of 4.2cfm!) requires, according to Clark, a twenty kw genset to successfully start it. That's a 5hp motor and is probably similar in start-up requirements to either a Bauer Capitano or Oceanus! There ain't no way in Hades the typical boat genset can start that unit.