Compressor questions

[iain/hsm]

I haven't bought it yet, that will probably happen tomorrow. It's a custom made Stewart-Warner with a Honda gas engine. But, I don't think the new Colti or Alkin portable gas powered compressors are any different. I one I'm buying is the one I asked about in this thread.

BTW, I just joined the Yahoo group, but since this was being discussed here, I thought I'd ask here. Now that I think about it, I think the owner said you just open the coaloessor (sp?) vent valve and let the compressor run, while changing tanks....does this sound right? Should I every decide to bank air, I know I'd have to do something better.

Waterskier.
Re opening the separator drain valve IMHO. No, I don’t think this is entirely correct. From the photo of the compressor the filter system has been set up wrong.

1. If there is a non return valve fitted before the inlet of the chemical filter or after the separator (and there should have been) then bleeding the separator will have no effect. With the one exception that to restart the compressor off load.
2. If there is no non return valve fitted then bleeding the separator will also drain the chemical filter and the massive reverse pressure loss will, at best degrade the chemical life and at worse break it down to dust.
3. However you can see that as there is no pressure maintaining valve on the chemical filter outlet side so every time you change cylinders the filter has to be drained down anywhy. This will drastically reduce the chemical life of the filter.
4. I would scrap the separator and all those old scrap fittings too old and suspect. Pressure rating is one thing, but the unknown pressure cycle of a 40+ year old pressure vessel another. Get to see inside the thing first, look for rust corrosion.

Frankly this is basic ABC compressor work and should have been done.
Your point of a new Coltri or Alkin being any different? well I would hope they would address the filter efficiency and capability and fit a BPR valve, at least LOL. But at least you get a manual, spares, and a network of service agents, with the reassurance that you are not the only “filling monkey” using the things.
If you are thinking Nitrox buy a big fire extinguisher, in fact buy two and put one next the door on the fastest way out. Iain

 

[Waterskier1]

Iain,

I am not very familiar with HPA. Please do not read this as argumentative, but rather to help me learn.

1. If there were (and I am not sure if there is or not-I don’t have first-hand experience on what some of the devices look like but thought the brass looking device just before the input to the filter is) a non return valve, then why would bleeding the separator have no effect? If the compressor was running, wouldn’t all the pressure it created got out the bleed (drain) valve? If it bleeds off the load for starting, I would think it would bleed off excess pressure too. I believe the owner told me that he runs the compressor until it reaches 3000 psi, then opens the drain until moisture. I thought he also told me that he left it open while changing tanks, and then again after the fill process for a couple minutes to ensure no moisture left behind. I can't recall for sure, but he had a reason for the two gagues. One for filter = tank pressure, and the other for input to the filter (compressor output) pressure. If there were no back flow valve, wouldn't both always be equal?


2. Agree; back flow from the tank through the filter would occur if no non return valve (which I think is the same as a “check valve”).


3. I thought the same thing, but the owner told me that this was a new filter and the cartridges are new material. He said that with the older ABsorption-type keeping the filters under pressure was very necessary. But, with this newer ADsorption sealed cartridge, it is not as critical. I think a pressure maintaining valve is a good idea unless you are only filling one tank at a time. He said that the filter is brand new, as is the cartridge (PP/N X65240).

A couple other questions.


1. The owner said that he doesn’t need to change oil in the compressor, he just adds to it as it is used. One thing I have read over and over is that it is critical to change the compressor oil – especially if CB Nitrox. Are these old military compressors different from the Coltri and Alkin portables in this respect?


2.There does not seem to be inter-stage water separator valves as I have seen mentioned for other compressors. Do these older military compressors not need these? The only routine valve to drain moisture seems to be the one at the bottom of the separator.


3. Your comment about NITROX, was this addressed to CB or PP blending. What are your concerns? Again, please-I am not being argumentative, but want to learn.

 

[cool_hardware52]

The owner said that he doesn’t need to change oil in the compressor, he just adds to it as it is used.

Kinda reminds me of my first Pickup Truck

Tobin

 

[iain/hsm]

Re your point 2 “No inter-stage water separators being fitted to this compressor”

In order to reduce water vapour and to achieve the correct dew point (dryness) all “modern” compressor systems will use some form of chemical dehydrator of the final filter tower. The effectiveness of your chemical and its filter life is (to the large part) dependant with its chemical volume or mass. Molecular sieve for example will only adsorb 15% to 30% of its own weight, depending on the type or brand of chemical.
The tower in your picture holds 1lb (0.45kg) of chemical, some of which will be used up with activated charcoal or carbon for oil adsorbsion.

However in order to maximize the chemical life on “breathing” air compressors all modern “breathing air” compressor designers will use the inter-stage cooling coils and fan cooling effect as a means to cool the gas, and in doing so condense any water vapour into the water condensate towers. This water is then collected in the inter-stage towers and drained as required. This is known as mechanical separation. The advantage is it’s free and does not use up any of the filter chemical, so increases the potential filter life by loading it with less water in the first place.

The need for mechanical separators is to remove bulk-condensed water vapour.

However the lack of inter-stage water separators in these Cornelius Kidde etc compressors tells me it they were never designed for breathing air in the 1st place.

What you will not be aware of is that all these 40+ year old ex military Cornelius, etc were designed not for breathing air but for other uses, including flame throwers.
Gives you some idea how long ago flame thowers were used in a war zone!!!.

However in order not to condense water in the inter-stage cooling coils you have to increase the inter-stage gas temperature but not enough to make liquid water, while reducing what is called the “Approach Temperature” of the inlet to the next highest stage.

As these compressors were never designed for breathing air the designers therefore would allow an increase in gas temp so as not to produce water that could damage the compressor valves by a process known as “Hydraulicing” but allow this high water vapour to continue around the compressor only allowing it to condense after compression hence the big tower at the end of the compression process.

The major pitfall with these scrap Cornelius, Stewart-Warner, Kidde, etc blocks is that if they are now going to be sold on the second hand market as being “good” for breathing air or God forbid CB Nitrox my concerns are then predominantly at the point at which the increased gas temperature, (inter-stage) being too high to condense water, but nearer to the auto ignition point of the oil at which it meets any increased oxygen concentration on the discharge side.

This is known in the trade as "Mixing on Elm Street" part of "Learn or Burn"
Regards Iain.

 

[pescador775]

Cornelius (Stewart Warner) 3.5 cfm and 4.0 cfm compressors were designed originally for use by Navy frogmen and civilian divers. I personally inspected one of these compressors being used by the EOD in the mid-1990's. Some were also produced for use in scientific/industrial applications. Also, Stewart Warner landed some military contracts for flame throwers and ground support equipment which subsequently caused surplus pumps to fall into the hands of modern day divers and others. Many, if not most, were brand new. The lack of interstage separators is irrelevant. The purposes of these separators are to protect floating plungers from unusual wear and to reduce water in blowby gas. It has nothing to do with air quality although that point could be argued in some very rare cases. Generally, it just means that the coalescer has to be designed for high efficiency. The SW coalescer can go as long as two hours between drain intervals according to the Navy diver operation manual. The SW has no floating pistons or plungers and little blowby. The cooling system of the compressor is adequate for its intended purpose with all aluminum construction and large angled fins plus finned tubing. It has a one liter sump. The 4 cfm is one of the toughest compressors ever built.

There are many successful conversions of the 4 cfm and of its smaller sisters. However, I must admit that the one represented here by waterskier looks a little weird. Nothing against it but I would feel uneasy about a novice buying it. The particular el cheapo engine is unusual being more commonly seen on the smaller SW unit. The remark about oil is unsettling.

 

[rcontrera]

I would be surprised if a gas engine compressor had auto shutdown. Did you buy it from Compressed Air Specialties or Brownies? Hard to tell what they might be installing on Yacht compressors.

Pesky is right ... auto shutdown is not standard on gasoline engines. It adds about $180 to ours and I am sure that anyone that is building your system that is using a Honda engine can hook one up for you as well.

 

[iain/hsm]

Cornelius (Stewart Warner) 3.5 cfm and 4.0 cfm compressors were designed originally for use by Navy frogmen and civilian divers. I personally inspected one of these compressors being used by the EOD in the mid-1990's. Some were also produced for use in scientific/industrial applications. Also, Stewart Warner landed some military contracts for flame throwers and ground support equipment which subsequently caused surplus pumps to fall into the hands of modern day divers and others. Many, if not most, were brand new. The lack of interstage separators is irrelevant. The purposes of these separators are to protect floating plungers from unusual wear and to reduce water in blowby gas. It has nothing to do with air quality although that point could be argued in some very rare cases. Generally, it just means that the coalescer has to be designed for high efficiency. The SW coalescer can go as long as two hours between drain intervals according to the Navy diver operation manual. The SW has no floating pistons or plungers and little blowby. The cooling system of the compressor is adequate for its intended purpose with all aluminum construction and large angled fins plus finned tubing. It has a one liter sump. The 4 cfm is one of the toughest compressors ever built.

There are many successful conversions of the 4 cfm and of its smaller sisters. However, I must admit that the one represented here by waterskier looks a little weird. Nothing against it but I would feel uneasy about a novice buying it. The particular el cheapo engine is unusual being more commonly seen on the smaller SW unit. The remark about oil is unsettling.

I’m sorry for being sceptical here or emotive or cause any offence but the term “were originally designed for “navy frogman” and civilian divers? Could you be more specific?

I went back 40 years in Navships, 0994-001-9010, US Navy Diving Manual etc plenty of interesting stuff but nothing on these compressors. Do I have to go back further?
What was the specific NSN number of this Navy diving manual you reference.

Re your comment “The lack of inter-stage separators being irrelevant” IMHO you are missing the point, it is the lack of them that make it precisely relevant to diving and this nitrox requirement. Look the simple compression ratio of a 3 stage compressor from ambient pressure to say 3500psi (240bar) P1 V1 over Temp. Do the maths on the heat temperature and approach gas temperatures to avoid water condensing on the approach side and hydraulic effects inter piston/valve requires a higher gas temperature. These scrap no separator compressors were simply not/never designed for breathing air applications. Let alone any possibility of elevating the oxygen concentration. Do you have any oil analysis pre and post just to prove that point?

One saving grace I guess would be with the rapid oil oxidation at least the thing would seize (or blow up) and as you cant get any new spares for these compressor at least the problem would in some way be solved LOL

 

[iain/hsm]

Pesky is right ... auto shutdown is not standard on gasoline engines. It adds about $180 to ours and I am sure that anyone that is building your system that is using a Honda engine can hook one up for you as well.

One minor point if you want to hook up a standard pressure switch from the compressor wired up to a Honda petrol/gas engine The Honda must have the oil alert option fitted in the first place. The Honda oil alert option normally shuts the engine down when low oil level is detected in the engine sump. The modification is to hook up the O/L engine wires to a standard HP pressure switch (such as a Barksdale pressure switch) the pressure switch then fools the motor into thinking that low oil has occured and shuts it down. iain.

 

[Waterskier1]

First, I want to thank all who have contributed here. I am still going to purchase this compressor - I think it will work great for my boat - easily portable. I never planned to mix Nitrox on the boat. I will evaluate that possibility when I get the compressor, look over it's condition, take some temp reading, etc. But, the Nitrox capability was an "extra". For the price ($1500 shipped to my door), the primary purpose of air on my boat should be well served. And, I think I could probably resell it for most of my investment should I desire to get something better later.

That said, I believe the original question on this thread is still unanswered.

I understand how all that works, as I've built a couple of 60 and 26 gallon shop compressors for myself and friends. I'm interested in how these applications of pressure switches and unload valves specifically apply to high pressure scuba compressors.

Pesky is right ... auto shutdown is not standard on gasoline engines. It adds about $180 to ours and I am sure that anyone that is building your system that is using a Honda engine can hook one up for you as well.

One minor point if you want to hook up a standard pressure switch from the compressor wired up to a Honda petrol/gas engine The Honda must have the oil alert option fitted in the first place. The Honda oil alert option normally shuts the engine down when low oil level is detected in the engine sump. The modification is to hook up the O/L engine wires to a standard HP pressure switch (such as a Barksdale pressure switch) the pressure switch then fools the motor into thinking that low oil has occured and shuts it down. iain.

I understand that there are ways to install automatic shutdown to gasoline powered compressor to shut down the engine when a set pressure is reached. But, unless it has electric start - doubtful on most portables - you would have to restart the engine every time the pressure dropped back down below the set point (or whatever hysteresis value you set). This is fine for a Overpressure (Emergency) Shutdown, but I'm not sure I would want that to happen every time I fill tanks. Since this is not standard on gasoline powered compressors, what is the method?

Correct me if I'm wrong. I don't know how slow a 3 -- 4 CFM compressor is, but is it not possible that you might slow down the flow into the tank to prevent overheating sufficiently enough for the compressor to "catch up" and even go over 3200 pounds without the tank being completely full? I wouldn't want the engine to shut off, and then have to run over and restart it during a slow (cool) fill. Additionally, when I shut off the output, to disconnect the whip from on tank to another, I really wouldn't like to have to restart the engine again as the pressure built up during the whip transfer. I realize that there are work-arounds for this, but my question is what is the common mode for gas powered compressors to handle this? I am sure that the one I bought does not have the shutdown feature; I don't think it has the low oil sensor either. Yes, I could design a circuit that would shut it down, but is that the necessary way to approach the problem. Or, is it acceptable to simply open the separator valve, and let the air bleed out there, while transferring tanks?

 

[rcontrera]

One minor point if you want to hook up a standard pressure switch from the compressor wired up to a Honda petrol/gas engine The Honda must have the oil alert option fitted in the first place. The Honda oil alert option normally shuts the engine down when low oil level is detected in the engine sump. The modification is to hook up the O/L engine wires to a standard HP pressure switch (such as a Barksdale pressure switch) the pressure switch then fools the motor into thinking that low oil has occured and shuts it down. iain.

Actually, we just hook it up to the off/on switch on a standard engine.