Compressor filling rate; does it change as tank pressure increases?

[Wallowa]

Does a Hp Scuba compressor fill rate decrease as the tank pressure increases? Will going from 500psig to 1,500psig take less time than going from 2,000psig to 3,000psig?

Thanks...

 

[The Chairman]

Yes. That's true for all compressors.

 

[tbone1004]

Does a Hp Scuba compressor fill rate decrease as the tank pressure increases? Will going from 500psig to 1,500psig take less time than going from 2,000psig to 3,000psig?

Thanks...

yes but with caveat that it will depend on what your PMV is set to. The filtration system requires backpressure to work properly and those will often be set to around 2000psi so the fill rate from 0 to 1000 is the same as 1000 to 2000. After the PMV pressure is exceeded then you have the compressor slowing down naturally and then after about 3000psi you are also fighting Z-factor for non-linear compressibility of the gas which makes it even slower than normal. Lots of other variables that come in with why they produce less per stroke at higher pressure, but it is definitely a consistent result that higher pressure=less volume. We are usually talking <10% though, so it's all in the "close enough" range.

You can see this called out in low pressure shop compressors that will often give CFM ratings at 60, 90, and 100psi. In high pressure compressors they don't typically give those ratings and will just give the charge rate at their rated pressure.

It should also be noted that the actual CFM output of the compressors will be much more variable with high pressure compressors based on the intake temperature and relative humidity. The less dense the gas coming in is *hot*, the lower the output volume will be, the more water in the incoming gas *high humidity*, then the more water needs to be taken out, which also lowers the output volume per stroke.

 

[Wookie]

You guys need more horsepower if your compressor is slowing down.

You need new rings and valves if your fill rate is measurably decreasing.

In theory, your fill rate will suffer as pressure rises because more gas is slipping past the rings. In reality, the difference between 2000 (PMV setpoint) and 3500 (hot fill pressure) or 4500 (bank pressure) or 6000 (SCBA bank pressure) should be negligible in a well maintained compressor.

 

[The Chairman]

should be negligible in a well maintained compressor.

It's simple physics. While it's less noticeable and could be seen as negligible on large, commercial compressors, that's not so with a small home compressor, even if it's "HP". Your experience will lie somewhere between those two points.

 

[Wookie]

It's simple physics. While it's less noticeable and could be seen as negligible on large, commercial compressors, that's not so with a small home compressor, even if it's "HP". Your experience will lie somewhere between those two points.

I have spares if someone needs them.

The in-between kind.

 

[Wallowa]

I have spares if someone needs them.

The in-between kind.

Thanks for input...my PMV is set at 1,800psig and for me the logic is that as pressure increases in bottle it will resist the incoming flow of gas from compressor; how much it will "impede" or slow down incoming flow rate will be subject to a ton of variables...compressor with "X" cfm rating will not deliver that best flow rate when pushing against higher and higher pressures in bottle..I am guessing that the cfm rating is at a specified constant rpm and while rpm may not drop flow rate may...again, I am guessing but I would assume stated flow rate is determined at specific rpm and with whip open and no back pressure.

I of course could be wrong!

 

[The Chairman]

The solution is always more fill bottles.

 

[Wallowa]

The solution is always more fill bottles.

Ha! Or two compressors! Speed is not the question for me and neither is filling more that 4 bottles during one 'session'...a cascade system introduces issues of it's own and in the end compressor time is compressor time so filling storage bottles with X cfm will still take X mins...true below PMV set point the larger storage bottles will contain more cfm delivered at optimum flow rate; but above that they will require more time due to more cfm in bottle as increasing pressure slows cfm... or so I think!

Why the hell do I overthink this stuff! Don't answer that, a rhetorical question!

 

[tbone1004]

You guys need more horsepower if your compressor is slowing down.

You need new rings and valves if your fill rate is measurably decreasing.

In theory, your fill rate will suffer as pressure rises because more gas is slipping past the rings. In reality, the difference between 2000 (PMV setpoint) and 3500 (hot fill pressure) or 4500 (bank pressure) or 6000 (SCBA bank pressure) should be negligible in a well maintained compressor.

it's not slowing down in terms of pump RPM, just slowing down in terms of volume moved per stroke. It is definitely something that all compressors are subject to. The difference in our application is what I would consider negligible, <10% in a well running pump to 3000psi, but it is still there. It gets considerably worse above 3000psi but that's due to z-factor and you can't do anything about that.