Compressor Operation

[frogman62]

All of the kidde and most military compressors have spring loaded valves that keep the valve closed except when air is passing through. If the compressor is stopped with full load pressure the valves act as positive check valves and do not allow air flow back through the exhaust valves.
However with pressure on each cylinder, air will bypass the piston rings and bleed back into the crankcase and usually exit through the compressor air intake. After the high pressure filling is complete the compressors should be run for a short period under no load conditions to allow for cooling and to vent any moist air from the system. This can be done with manual systems by opening the moisture drain valve for 1 minute before shutting off the compressor motor
Jim Shelden

 

[NAM001]

You are absolutely correct in that load increases as the back presure increases. No doubt about it at all. Scuba compressors and filtering systems need a high back presure to work effeciently, expecially the filtering. To achieve this a regulator is placed on the out put to keep the system discharge presure at a minimun. Rix systems i think is 1500 psi. There are many schools of thought and appliation. Many run back presure at 2500 and above. Unlike your standard air compressors you reference, many of the scuba hp compressors have a final piston that is not connected to the connecting rod and erquires back presure to maintain contact with the piston and the rod end . If youstart a compressor you hear a lot of rattling an clunking. That is final stage noise that will be there till a high enough pach presure is built up to keep the final stage piston seated on the push rod. If an empty tank was connected at that time is would take quite a while for the noise to stop. The back presure reg makes this happen in the first few seconds of the compressor running. Hence with a constant back presure on the fnal stage, and that compressor load is rooted on the final stage back presure, The compressor load is constant throughout the majority of the tank filling process because of the back presure regulator is simulating a near full tank being charged. I can not speak for all hipacs but my rix and bauer are like that, as is many others. Even with all that said,, your premis is still true. as a hipac with a reg on it has more load that one with out one. Its just a matter of how you look at it. Hope this explanation of the hp compressor workings helps you. here is some more info on them written in laymans terms. I know it made it easy for me to understand it. Neptuno's World Articles - Understanding SCUBA Compressors and Filtration

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...

 

[Wookie]

You are absolutely correct in that load increases as the back presure increases. No doubt about it at all. Scuba compressors and filtering systems need a high back presure to work effeciently, expecially the filtering. To achieve this a regulator is placed on the out put to keep the system discharge presure at a minimun. Rix systems i think is 1500 psi. There are many schools of thought and appliation. Many run back presure at 2500 and above. Unlike your standard air compressors you reference, many of the scuba hp compressors have a final piston that is not connected to the connecting rod and erquires back presure to maintain contact with the piston and the rod end . If youstart a compressor you hear a lot of rattling an clunking. That is final stage noise that will be there till a high enough pach presure is built up to keep the final stage piston seated on the push rod. If an empty tank was connected at that time is would take quite a while for the noise to stop. The back presure reg makes this happen in the first few seconds of the compressor running. Hence with a constant back presure on the fnal stage, and that compressor load is rooted on the final stage back presure, The compressor load is constant throughout the majority of the tank filling process because of the back presure regulator is simulating a near full tank being charged. I can not speak for all hipacs but my rix and bauer are like that, as is many others. Even with all that said,, your premis is still true. as a hipac with a reg on it has more load that one with out one. Its just a matter of how you look at it. Hope this explanation of the hp compressor workings helps you. here is some more info on them written in laymans terms. I know it made it easy for me to understand it. Neptuno's World Articles - Understanding SCUBA Compressors and Filtration

In addition to the back pressure regulator keeping the final stage piston seated on it's drive rod end, the back pressure regulator (or Priority Valve as its known) significantly lengthens the life of the final filter. I do not understand why, I'm sure someone can explain it better than I can. I never used to worry about the efficiency of mine because I now have Makos, and the final piston is not floating, but I was going through final filters at 10-15 hours until I repaired my priority valves.

 

[NAM001]

Wookie

Normally what fails first in the filtering is the desicant becoming saturated. the monoxicon needs neg 50 dew point or lower to work. with the back presure reg set high 2000 to 3000 the mechanical moisture seperation removes more water, which you drain every 15 min or so. leaving the desecant less to do and making it last longer.

In addition to the back pressure regulator keeping the final stage piston seated on it's drive rod end, the back pressure regulator (or Priority Valve as its known) significantly lengthens the life of the final filter. I do not understand why, I'm sure someone can explain it better than I can. I never used to worry about the efficiency of mine because I now have Makos, and the final piston is not floating, but I was going through final filters at 10-15 hours until I repaired my priority valves.

 

[CraigAClark]

As has been mentioned, the fact that compressors work harder at the top end is simply a matter of building down stream back pressure. I know I haven't been around for over a year, but I know I've posted this previously. The fill rate of a "scuba" compressor, by most manufacturers, is based on filling a known volume from 500 psi to 3000 psi, timing that fill and dividing the TOTAL volume by the fill time. The compressor after cooler is hooked directly to the plenum (cylinder), no final separator or filtration volume to consider. Dang I used one of them fancy words. The compressor fill rate in reality is a fake number. You got your Bauer Mariner II rated at 8.4 scfm, fill rate. I believe a lot of folks think this is what the compressor is putting out from start to finish. That isn't the case. With no down stream pressure it may be putting out 12 scfm and as that down stream pressure builds that flow rate is continually dropping and at the top end (5000 psi) may be down to 2-3 scfm. If you have interstage gauges, you will notice that they don't go to a specific pressure and stay there, they rise with the final pressure. You may not notice a rise in the 1st stage pressure. Generally the gauges used for compressor stage pressures are cheap. I hope this is understandable to all.

 

[frogman62]

Craig, Thank you for the post. The description is excellent and needed repeating. Many of us who have been around for more years than we like to admit and have had dive shops where we fill large storage tanks, know that filling rates (when we never go much below 2500 in any tank and fill to 5000PSI) are never what the manufacturer describes.
Jim Shelden

 

[iain/hsm]

To be fair I think most manufacturers are honest in stating each models flow rate.
Or at least try to give an honest answer to the question.
What is difficult is the differences between "Standard pressure and temperature" to either European or America referenced standards.
Here's the link:

Standard conditions for temperature and pressure - Wikipedia, the free encyclopedia

Hence for scuba the filling or charging rate, time from 500 to 3000 psi is given for filling an 11 litre (80cfm) cylinder at 21C (70F)
ambient is a simpler means to compare like for like.

What is also not properly understood with compressor potential purchasers is the huge differences when it comes to work load and service life
and the huge difference in compression ratios, piston diameters, rod loadings, RPM and piston stroke for each of the various makes and models
available when claiming the same flow.

Further the design "balance" of a model should be based on number of matched diameter stages over each pistons compression ratio for a given
discharge pressure and flow. Calculated against the gas inlet and discharge temperature and a balanced rod load including calculating in flow and blow-by.

Not just the "catalogue knowledge' of how long to fill a scuba tank.
Sadly what you get is the cheap fix of available parts cobbled together to make the biggest "bang" for the cheapest "buck" with huge interstage
rod load differences and temperatures.

Also by way of example it should be the actual calculated gas temperature at the discharge valve that should be disclosed, being always much
greater than that "measured" surface or "skin" temperature at say the discharge tubing due to heat losses and dissipation.

For example I use an industrial design compressor (not a scuba model) a 38 scfm flow gas compressor 4 stage fully balanced 4 inch stroke at a low 440 RPM
for a 2755 psi discharge pressure with a 116 psig inlet pressure. Using 14.73BHP to produce 38 SCFM at 2755 psig
Using a Gas inlet temp of 122F and an ambient air temperature of 99F
Piston diameters for a 2.2 compression ratio (1.99 on the 4th stage at 2755 psi)
1st 2.63" @272.84 psig
2nd 1.75" @617.89 psig
3rd 1.13" @ 1376.99 psig
4th 0.75" @ 2755 psig

Gas temp and Rod loadings
1st 269.05 F @1475.83 lbs
2nd 265.29 F @1334.43 lbs
3rd 265.19 F @1242.37 lbs
4th 244.82 F @1222.55 lbs

Compression ratio is finely tuned at 2.25, 2.42. 2.24 and 1.73
You can also see the fine balanced design and that the 4th stage to balance out needs a higher discharge pressure of 3000 psi than 2755 psi

Now if you change the design scope and say make it a three stage
1st 2.63" @429.68 psig
2nd 1.38" @1051.81 psig
3rd 0.88" @ 2755 psig
Gas temp and Rod loadings
1st 365.61 F @2324.2 lbs
2nd 277.13 F @1561.04 lbs
3rd 293.93 F @1655.8 lbs

You can see the compression ratio is all over the place at 3.64, 2.47 and 2.35 and the temperature range is greater.

Now the same calculations on most of our scuba models would give very surprising results.
With gas temperatures and rod loadings not for the faint hearted.
IMHO most scuba models are designed for a short hard life. Do the maths. Iain Middlebrook