Compressor Operation

[spectrum]

There are always a few threads running about good fills, fill cost and all of that usual stuff. I frequently hear that "the compressor works harder to provide the higher pressure air". I am looking for factual answers here, it you need to preface you comment with i think or I believe I'd rather you post elsewhere this time.

My understanding is that the compressor takes air up through the stages compressing it to it's design pressure and then discharges it to a cylinder or bank. As an example, if it put out .05 cubic feet at 4000PSI feet per stroke it will expand to .2 cubic feet when it lands in a 1000 PSI cylinder and the pressure will rise accordingly. (Boyles law and assume ideal gas behavior). As the pressure in the cylinder or bank rises there will be less expansion and a slower rise in pressure which exactly what you see if you watch a hot fill or bank replenishment. The compressor is working longer but the load, stress, wear and tear (assuming continuous duty) is the same per cubic foot of air compressed regardless of pressure. I realize that this means more KW but am i missing anything else?

Thanks,
Pete

 

[Wookie]

You are not thinking about heat of compression as it transfers to the components of the compressor. When the compressor is cool, the tolerances are tight, resulting in more air compressed per stroke. When the whole mess heats up, tolerances widen, resulting in potential losses past pistons, valves, and rings. Older Bauer K-14's don't have rings on the final piston, so the losses are more than a newer K-15 or a Mako of the same CFM. Additionally, as the oil heats up, it becomes thinner, resulting in less lubrosity, probably not effecting the compressor in the short term, unless it gets too hot, but it certainly allows tolerances to open up in the long term.

 

[oldschoolto]

I know you didn't want a layman in here Pete... But I'll tell you that a compressor does not have a valve train that controls the air flow.. They use a reed valve system... So the air is freely moving at the start of the compression of air in to the cylinder being filled and the flow of air out of the finale stage has no back pressure... As the pressure in the cylinder being filled increases the air flowing out of the finale stage can't freely flow... Back pressure... Now the piston must start really working to get the gases out on the compression stroke... More work equals more load and more load equals more heat and stress... So it's working harder....

I wish I knew all the smart guy terms....

Jim...

---------- Post added November 25th, 2012 at 10:28 AM ----------

The oil not only lubes the compressor parts, But the bigger ones there is a oil pump that lubes the bearings, part of the oils job is to fill the space between the bearing and the crankshaft and cushion the rod against the crank... Thinner oil, Less cushion .... And Tolerances get tighter with heat if it's a aluminum piston in a steel cylinder... As the piston ring wears it's end gap it's larger and lets more air pass thou the gap... Unless your using thermo piston that is made to expand very little ... Now we can put a much tighter tolerance on the cylinder bore( .0015 ) instead of the normal ( .004 to .005 ) ...

Jim....

 

[Bob DBF]

I wish I knew all the smart guy terms....

Jim..

And if you did, using them might not clarify the issue. Aside from that, I have heard a lot of BS from people using those smart guy terms to obfuscate an issue of which they had little understanding and wished to present themselves as expert.




Bob
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There is no problem that can't be solved with a liberal application of sex, tequila, money, duct tape, or high explosives, not necessarily in that order.

 

[oldschoolto]

So that means you understood my post Bob....

Jim...

 

[spectrum]

I knew they were reed valves but hadn't considered how that would change the dynamics of at least the finial stage. Good point. It's obvious now that you put it in front of me!

As for running long, tolerance and lubrication it seems that would at least theoretically be negated by a continuous operation rating, or at least staying in the duty cycle. Of course in absolute terms the easier it runs the better.

Good stuff so far!

 

[Wookie]

I've not seen reed valves in high pressure compressors except in the first stage suction of very old Bauers or any Ingersoll Rands, but I haven't worked on all compressors, either. The valve train very much controls flow, however, not in that it throttles flow, but the valves ensure that the flow goes the right way through the compressor. Think of the valves as a series of check valves, allowing air into the cylinder on the intake cycle, and preventing back flow on the compression cycle. One of the main causes of failure of compressors to attain rated flow is that the valves wear out, or the springs weaken or break, or the compressor heats up and the valve tolerances change, resulting in back leakage of compressed air into the interstage before the cylinder. If you don't have an interstage gauge, sometimes the safety valve will lift to protect the cooling coil. It's about the most frustrating thing a compressor mechanic has to chase down, which is why if you have more than a few hundred hours since the last valve change, a technician will automatically service the valves first, especially if there is evidence of carbon.

Most early stage pistons are made of aluminum as oldschoolto stated, with rings. Larger Mako compressors have rings on the fourth stage piston with the tighter tolerances stated above. Small Makos and Bauers have a floating final stage piston that sounds like the compressor is coming apart when it starts, until it builds pressure. Those pistons have very tight tolerances, I have no idea what the allowable is, but if you can see a scratch along the length of the piston, it's time to throw it away.

---------- Post added November 25th, 2012 at 11:25 AM ----------

So that means you understood my post Bob....

Jim...

We all got Bob's post.

 

[frogman62]

The military KIdde's have a restricted flow orifice in the 4th stage because the 2nd, 3rd and 4th stage pistons are not attached to the crankshaft. (2, 4, 6, and 12 CFM units) They are held against the scotch cam by stage pressure --They always have back pressure without a back pressure valve. Only the 1st and second stages have rings ( note: 6 and 12 CFM later models have synthetic rings on 3rd stage) 3rd and 4th stage pistons and sleeve's are both metal to metal very close tolerance. The output is very constant from low pressure to high pressure because of the flow restriction design of the 4th stage. These 4 stage compressors in good condition easily pump to 5000PSI. The original govt design criteria was 3000PSI operation with a 4 to 1 safety factor. When I sell a converted military compressor, I usually run it for up to one hour during testing to 6000PSI.
When they get worn (1500 to 2000 hours) they have considerable blow by but will still pump 3000PSI without difficulty.
The output of air and oil vapor increases so draining the moisture trap more often helps prolong the filter media.
Heat , with the correct oil (mobile Jet II) does not become a problem until you begin filling multiple tanks at 3 PM in 100 degree temperatures and 80% humidity. Everything else suffers in these conditions
Jim Shelden

 

[NAM001]

your load will remain cnstant on the compressor till your output exceeds the back presure regulator. thus if your reg is set at 2700 and you fill lp tanks the compressor load is constant through out the fill of the tank. If on the other hand you are using the same on a 3500 hp tank the load is constant till it gets to 2700 and then the load increases on the ocmpressor from 2700 to 3500.

 

[oldschoolto]

Like I said in the first line of my post, I'm a layman... I think I made a very simple example why a compressor is working harder as air pressure increases after the air is sent out of the last stage of a compressor... Never worked on or looked inside a High pressure compressor... I have rebuilt a few shop air compressors ....The systems of controlling air flow is the same if you use a plate valve or a reed valve as they both use different pressures on each side of the valve to control it... Unlike a valve train that will hold the valve open or closed regardless of the different pressures on the valve...

Jim...