Max compressor Pressure for Continuous Blending Nitrox

[hawksbill]

If you have experience I would like your input on continuous blending. I am expecting to recieve a HP compressor soon. It is capable of up to 4500 psi and I may want to fill cascade bottles to that pressure. What should be my concerns of continuous blending with a HP oil (Mobil Jet) lubricated compressor to 4500psi? I am rereading Oxyhacker but havent come across max operating pressure for continuous blending. Hope to blend to 40% at 6 cfm into 300-400 cubic ft bottles so my main consideration is for what is going on withinin the compressor. I havent bought the big bottles yet and would like to know what is accepted practice here.

Thanks,

 

[Peter_C]

We run our compressor up to it's max rating of 5,000 psi., while blending 32%. When the banks cool they are right around 4,600.

I am not familiar with the Mobil Jet oil. For oil we use Nuvair 455 Nitrox & Compressor Accessories - Oil If changing types and or brands, the oil needs to be changed, then run the compressor for awhile and change the oil again. Then after a few more hours change it again. Now it is ready to be back on normal service intervals.

 

[pescador775]

Pressure is not a problem but pressure causes heat which oxidizes oil or worse. Since you are in Alaska, perhaps compressor cooling is just an afterthought. Use 455 oil unless the compressor is located in the weather or if running a recond military compressor. Impossible to say more as your post focuses on pressure but doesn't say much else.

 

[hawksbill]

The compressor is a Noah-Howden 4 stage, 6.0 CFM from Shelden Sporting Goods. It has a coalescing filter ahead of a Bauer type filter with replaceable cartridge. It will be operated outdoors but I intend to protect it from rain. The ambient air conditions are likely to be 30-65 degrees F and 70-100 % humidity. If more information would help please let me know.

 

[Scuba Duck]

Heat/Temperature is the issue, but the Temperature is related to the Pressure. First I would like to introduce you to two equations. The ideal Gas Law, and the Adiabatic Compression/Expansion law.

PV=nRT. Ideal Gas Law.

PV^1.4 = Constant. Adiabatic Compression/expansion Law. 1.4 is the ratio of specific heats for air (gamma).

We can do a little re-organization of these laws to formulate the following equations:

(P1V1)/(P2V2) = T1/T2

V2=(P1/P2 * (V1^1.4))^1/1.4

Remember all the temperatures need to be on an absolute scale, either Rankine (F+460) or Kelvin (C+273)

So lets say you didn't remove any of the heat from the compression process, how hot would the gas come out at 3000, and 4500 psi?

In the above equations, we have the ratio of the input to the ouput pressures so P1/P2 = 200 (for 3000 psi) or 300 (for 4500 psi). We are also concerned about the ratio of the volumes of the gas (V1/V2), so we'll assume that V1 =1, and solve for V2

From the adiabatic compression, V2=(1/200 * 1^1.4)^1/1.4
V2 = .0224 , or 2.3% of its previous volume for the 3000 psi compression

Similarly, for the compression to 4500 psi, V2=(1/300 *1^1.4)^1/1.4
V2= .017 or 1.7% of its previous volume.

Lets assume you start at room temperature, 72 degrees Farenheight, or 532 degrees Rankine.

Plug this back into the ideal gas law equations, and...

For 3000 psi compression
P1/P2*V1/V2 = T1/T2 ==> 1/200 * 1/.0224 = 532/T2
T2=2340 R or 1880 F. That's more than hot enough to melt aluminum.

For 4500 psi compression
1/300*1/.017=532/T2
T2 = 2713 R or 2253 F. That's about the temperature that steel melts at.

Now the above equations assume the compression is adiabatic, that no heat is removed. I was trying to point out how important the heat removal is. That unless the heat is properly removed, the temperatures will skyrocket in your compressor.

Add the presence of additional oxygen in the air (Nitrox) because of continuous mixing, and the degredation/oxidation (of oil) occurs even faster, and the issue of heat removal become more important. Most corrosion rates are a function of temperature and oxygen content, as the temperature and oxygen content increase, the rate of degredation/oxidation/corrosion increases.

Lessons learned: make sure you have good flow of air around your compressor. Keep it clean, remove any dust, or oil that may accumulate on the interstage cooling coils. Your compressor will last longer and be happier if you keep it in an air conditioned room.

 

[frogman62]

Hawksbill
Go to "compressor team" and read messages # 1009, 317, 310, 305, 302. Good discussion of jet II oil and recommended oil for Meggitt (formerly Noah Howden) compressors.
Jim Shelden

 

[hawksbill]

Derek, Thanks for laying out the Adiabatic compression/expansion law for everyone. Not sure I had seen this formula before and though some of the symbols lost me, I did follow what you were saying and the potential temps are a good reminder why we always open the O2 valve slowly. Thanks too for the "Nitrox Cost Breakdown" and "Tank Fill Cost Spreadsheets" from your website that I ran across last weekend.

Jim, Thanks for referencing all of the compressor team messages relating to this. I recieved the compressor today and you have built it up very nicely. Plan to be blending by this weekend.

Robert

 

[pescador775]

There is a recent discussion of oil on Scubaboard as requested by a member of Compressor Group. This individual is a paintballer who uses a military recond compressor. Those remarks may or may not apply to your situation. I noticed that this individual has also made some statements at compressor group which may or may not apply to divers. FYI:

For diver air supply and ambient temp range of 0F-100F, use JET II or JET 254, Mobil only as no other brands or types of turbine oil have been verified to comply with OSHA requirements for respiration. These oils are recommended specifically for some military and other apps for Kidde compressors and other associated brand names of this compressor family.

 

[hawksbill]

Thanks Pescador, I really value your input on the board. My original question on pressure has evolved to interest in what is the most suitable oil for this compressor when pumping nitrox.

In using Mobil Jet II for pumping 40% nitrox in this compressor do you think there any reason to change the oil any more frequently than the reccomended 30 hour interval? Would you anticipate any accelerated breakdown of the oil that would justify changing it any sooner or should I just treat it the same as if pumping air?

 

[pescador775]

Use of Mobil JET oils for breathing air compressors is "off label" so no data are available other than provided by the company for turbine apps. However, this information is fairly extensive and allows us to draw some inferences. Firstly, these are very high quality oils which have undergone years of development. They use a synthetic polyol ester base including additives selected on the basis of being the most effective available for protection of moving parts. Until about six years ago, some of these additives were toxic. Mobil charged their suppliers with eliminating the toxicity. Using chemical wizardry this has been largely carried out within the limits specified by OSHA. These are the only oils which I am aware that have been tested extensively on animal subjects and the result documented in detail and protocol and data open sourced.

With regard to diver's compressors these oils seem well suited to protect the compressor through a wide temp range without producing noxious gases. The flash points are very high ranging from 490F to 510F. The anti oxidant character is very good. WRT this latter property Mobil has made efforts to improve anti-oxidation by use of additives. This evolution should catch the attention of divers who are planning or in the process of running oxygen enriched gas through their military air compressor. Oxygen enriched gas, heat and oil can be a troublesome combination. So long as temps are within a reasonable range the only consequence of this type of operation would be accellerated deterioration of the oil. However, under extremes of temp it has been documented that compressor oils can break down and produce carbon monoxide, toluene and other organic gases. If these bypass the filter through some fault in the system the results can be a tankful of bad air.

Maintaining reasonable temps and regular oil changes are the primary defense. On the other hand, some divers take the position that the type of oil is also very important and prefer that which offers the best profile for resisting deterioration in the face of high O2 and high temp. Commonly, so called food grade oils are preferred. This opinion evolved after reviewing tests of these oils in something called an "oxygen bomb". Bomb is just an old term for "can". The oil is subjected to high temps in a pure oxygen environment. The results give an oxidation number and the results for food grade oils (PAO) happen to be far better than for esters. Parallel to this, Mobil has produced a modified JET II oil called JET 254 which is intended to survive under high temperature service with reduced degradation. This is done by adding anti oxidant chemicals to the mix. I suspect the this oil, which is also non-toxic, would be more suitable for use in compressing NITROX. The test data seem to support this. Keep an eye on the compressor oil, have it tested if you like, but I would suggest that the oil be visually and tactially examined after 20 hours. Check for burnt smell, sludge and color. Try the Mobil 254 if you like and make your own decision.