Portable compressors / air quality ?

[kd5drs]

I am looking into buying a portable gasoline compressor. I have looked for techincal information / companies that sell filtration systems but have had only limited success. Price is a major consideration but safety overrides that. There are numerous aftermarket high pressure compressors that advertise "scuba" applications with "appropriate filters". This looks to be the less expensive route to take. Almost all our diving will be fresh water shallow <40 deep. Max air 35 is the least expensive "designed for scuba " compressor that I have seen. Airtimeair has one for $1100 but I'm sure it needs a filtersystem. Since these mostly are rebuilt I suspect they will probably be "dirty" and cause excesive filter use. Anyones advice who is using one of these or built one yourself would be appreciated.
KD

 

[Rick Murchison]

MaxAir uses funny math in their advertising... sorta makes me wonder if they do the same in their engineering department.
Rick

 

[omar]

For a little perspective.

It is going to cost you. No way around it.

I have spent almost as much on filtration as I did on my rebuilt compressor. Granted it provides oxygen compatible air but it is worth it to me.

If you don't want to use filtration than you will have to spend the money on a RIX or other oil-less compressor.

It does not matter if you are breathing air at 10 feet or 200 feet it needs to be free of contaminants. Period.

omar

 

[pescador775]

Man oh man, the average diver seems to be super leery of everything and anything. You would think that compressors are pumping out asbestos or (horrors) nicotine. The main 'contaminent' that must be separated out is water. This is followed by the small amount of residual oil vapors. Oh yes, don't forget carbon monoxide from gas engines. This must be avoided and can't be filtered out despite claims about chemical catalysts like hopcalite.

I wouldn't buy one of those Italian made MaxAirs that use chicken wire for the fan guard. Some of those ex military compressors are cheaply priced and very nice indeed. Avoid the one with the 1 1/2 hp military engine. You need 5 horse power, either Honda or new MIL SPEC. These compressors come equipped with condensators. All you need is their ridiculously inexpensive after filter canister which uses a Laurence Factor cartridge. These are packed with multisorb and will knock out any oil mist/odor. The resulting grade E air is cleaner than anything found in one of our cities. You guys breath that stuff 24/7, right?

Before you order your compressor check if it has a 'snorkel' type intake. If not, just ask for one, they have. You attach a six foot, 1" clear plastic hose with a paper element air filter which is hose clamped to the end. This filter is available from Lausen products and is normally used as a small engine intake filter.

Make sure you drain the condensator (small tank on the compressor) every 1/2 hour or every fill. To extend the life of the valves and condensator, run the compressor under NO load (drain open) for 30 secs after the last tank is filled and before shutting down.

One more thing, these military compressors do not come with a back pressure valve. This simple device is absolutely necessary to ensure that all water is wrung out by the condensator. Inquire. Install the valve on the condensator. Have the valve set to 1200-1800 psi.

Most military equipment of this type use #4 JIC flared connectors. It would be best if your fill hose, which will probably be ordered separately, has these connectors, at least on the compressor side. The Stewart Warner backpressure valve, a common military type will have this type of fitting. For some reason, the honchos that sell surplus compressors seem to like 1/4 inch pipe thread on their hoses. This setup is inconvenient for a portable. The flared hose ends can be instantly disconnected countless times and still form a reliable seal. The only place you need a 1/4 inch pipe fitting is on the condensator/where the back pressure valve screws in. This is a permanent install, no removal necessary. Even here, the later condensators often avoid pipe thread and use a JIC compatible thread (3/8-24 O ring).

This sounds complicated but it is not. Some running around will be necessary to locate the few extra parts. The overall cost is less than half that of a Maxair. Parts and service are available for most surplus compressors, especially the Stewart Warner types.

Anyway, don't obsess about air purity; follow this advice if you decide to get a used unit for less than half price but be prepared for few hassles at the outset. Use common sense in operating your compressor regardless of origin and history.

 

[omar]

CO (a colorless, odorless gas) ranks as the most dangerous compressed air contaminant. Headaches, dizziness, unconsciousness or death can occur from exposure to elevated CO levels. Carbon monoxide has first priority as its effects are cumulative and very small concentrations can cause problems.

All oil lubricated compressors, regardless of age and maintenance record, will contaminate the airstream to some degree with lubricant oil mists. The heat caused by compression will cause thermal breakdown of oil which will generate CO. Also, if a compressor begins to overheat to the point that the lubricant oil begins to burn or "smoke", even small amounts of lubricant oil can produce lethal concentrations of carbon monoxide.

Hopcalite is a proven catalyst in breathing air filtration systems for CO removal. Because of this high pressure compressor filters are often equipped with hopcalite which converts CO into much less toxic CO2. The alternative to this is a continuous CO monitor.

Both CGA G-7.1 Grades "D" & "E" list a 10 ppm maximum CO content and a 5 mg/m³ (0.005 mg/L = 5 ppm w/v) maximum for oil mist/vapor

A 5 HP gas engine is the minimum power needed to drive a compressor. If you take one to altitude forget it. It will take considerably longer to get the job done and in some cases you won’t get the max high pressure rating of the unit.

I mix EANx and Trimix, some by PP and some by continuous blending. The extra filtration is a necessary requirement for my uses.

omar

 

[pescador775]

kd, I urge you not to pay any attention to Omar's remarks. Modern diester compressor oils have a flash point of 450 deg +. This is hot enough to melt solder. Your compressor will not produce carbon monoxide unless you are getting your oil from MacDonalds.

A 4 cfm compressor uses approx 3 hp. However, gas engines are not to be run at more than 80% max, so a 5 hp is ideal. Please do not concern yourself with Omar's strange statements. A five hp engine is standard for this size unit.

 

[omar]

It is obvious that you have no experience using compressors at altitude.

These are basic engineering fundamentals:

3% engine hp output/1000 ft altitude (loss due to altitude). We have found that it somewhat more than this (experience vs theory).

1% engine hp output/10°F above OR below 60°F

As for the rest of it. You might as well tell him to ignore reality like you have.

omar

 

[pescador775]

Once again, I must urge readers to disregard the foregoing. In response to Omar's latest, internal combustion engines do lose some power at altitude. However, the reduction in output exhibited by air compressors at altitude is due to the fact that the ambient air itself is thinner. The first stage of the compressor takes in less air mass with each stroke. This requires less energy from the engine which compensates the reduction in engine power but doesn't cure the problem of lower output from the compressor. The lower output cannot be corrected by normal means. An auxilliary low pressure compressor to feed the input would be required (supercharger). This is not practical. Running the compressor at higher rpm is also not practical.

It often happens that a novice asks a simple series of questions that deserve a straight answer. He is often blown off by someone saying dire things will happen. When a pertanent answer is finally given the 'expert' shows up with theoretical objections that have little basis or applicability to the average divers experience. The resulting, often contentious, discussion buries the original poster and leaves him more confused than ever. I hope this is not the case here.

Pesky

 

[omar]

A 4 cfm compressor uses approx 3 hp. However, gas engines are not to be run at more than 80% max, so a 5 hp is ideal. Please do not concern yourself with Omar's strange statements. A five hp engine is standard for this size unit.

We were talking about the power required to drive a compressor. To suggest otherwise is disingenuous at best. The loss of power for a gas engine at altitude is the reason for the reduction in output. Yes, you do have less air mass on the intake stroke for a reciprocating compressor, but the reduction in work is minor compared to the power loss of the engine.

A standard 5hp gas engine is inadequate to drive a compressor at altitude. I know I have experience with this.

I also know from experience that an oil-lubricated compressor delivering breathing air for workers is required to have a high temperature alarm and/or a continuous CO monitor. If there is no CO monitor an air sample and analysis program adequate to ensure that the compressor is not putting out dangerous levels of CO is required.

So the trade off for a personal unit is do lots of sampling or put in adequate filtration. As you pointed out the filter media is cheap. The towers, valves and gauges cost.

If anyone doubts this call a reliable breathing air compressor manufacturer.

omar

 

[pescador775]

Omar, 'The reliable manufacturers', meaning the same parties who lie about the output of their compressors? At least the vendors of surplus units state truthfully what the compressor output is under standard inlet conditions. When they say '3.5 cfm' that's what it is.

To KD: I have plenty of experience building and using surplus compressors. These military units cost a lot to manufacture, that's why you don't see them on the civilian market except as military sales. Taking one apart is a religious experience (G). Needle bearings and expensive machining. The Stewart Warner pump (alone) costs about $4,000 to make. God knows what the 4 stage Kidde costs today. They are used in aircraft and cost about that much in the 1960's. They offer more output, higher rpm's and light weight than any commercial unit. The commercial 'made for divers' compressors are compromises of cost and weight. Of these, the Max Air and others made by Coltri Sub are the lightest and Bauer is probably the heaviest. The four stage Coltri is not the highest quality. The Baurer Utilus 10/Junior is pretty nice but weighs 35 pounds more than the Coltri or typical Kidde or Stewart Warner setup.

I would check out Pro Tec Air (website). The small company has been in this business for years. The owner died a couple of years ago but the son has taken over. Right now, they are selling some units on EBay as 'insanelyjane'(!). I honestly don't know anything about airtimeair but they may have some kind of relationship with Protec. At least the pictures they show on their site are direct copies of Protecs'. I mean, for $2500 you can buy a diesel powered 4 cfm Stewart Warner, brand new. (Bauer's 4 cfm 'Oceanus' weighs a ton and costs $4600). Even if you have to buy $100 filter set and a $100 whip for your surplus unit, this is so much more compressor than any thing built by the commercial guys for the money. Once again, I would check with Protec first. Try to check around for info on Airtimeair. They might be first rate, I don't know. They seem hungry and don't speak English very well (I know how that feels).

KD, on this carbon monoxide thing. If you live in LA or some other smokey place it seems advisable to use a canister/cartridge with CO catalyst 'hopcalite', plus 'vaporshell' and charcoal. This is a standard item and will convert 1 or 2 ppm of CO, enough for incidental presence of this gas. It will not protect you from exhaust gas so make sure you gin up a remote intake hose. Just a few feet of hose and watch the wind direction when filling. Remember, you need a condensator (water trap) and a final filter. The condensator is standard, you buy the filter canister with cartridge.