Shoebox Tubing Replacement

[AlanMcD]

@Revoman - easy to do, as I log everything . . . it's the only good way to know when we have issues developing, before they become a problem.

One key varaible that needs to be known when comparing Shoebox run times is the input pressure - changing that directly impacts the amount of air that gets compressed on each cycle of the crank, pretty much proportionally. I run mine at ~105 PSI, and of course all gauges have errors in their reporting, so nothing is truly direct.

My Max with the 13 tooth cog fills my 9L tank (88 cuft) at a rate of 7.6 psi per minute of run time. Scaling that to your 73CF tank, that would be about 9.2 psi per minute, so pretty close. What is your input pressure?

 

[Revoman]

Alan!​

My input pressure is ± 110PSI.

mike

 

[AlanMcD]

Since I hadda top off a tank today, a Tiger Shark (73CF) from Brancato, I went ahead and timed it out using an 18-tooth pulley.
Ambient temperature = 69°, Humidity = 16%, Start at 3,700PSI, end at 4,400PSI (I'm adjusting the shut-off down to 4,300PSI by the way).
Took 1.1 hours, or 66 minutes, this using an hour meter connected to the motor powerline.
It's a little subjective as there is no way of knowing EXACTLY how much pressure in/out, just using the gauges on the tank.
That said, at 1.1 hours that's approximately 700PSI equaling: ± .09429 minutes per 1bar fill rate, or ± 9.429 minutes per 100bar fill rate.
This includes 'filling' the high-pressure moisture filter that starts to kick into the tank at around 2,200PSI ( ± 152bar), so it takes a little longer due to that fact.
I haven't hadda chance to compare that to the 15-tooth pulley on my other Box but will do that when I get a chance.
YMMV of course!
View attachment 518917
Picture of the moisture filter, not used in this position, but vertical.

mike

Our input pressures are pretty close, but you feed yours 4.7% more air than I feed mine, if the gauges are and flow are perfect and comparable (we know the gauges almost certainly are not, and who knows on the flow rate - mine has a regulator very close to the Shoebox and it pulsates as the pumps cycles, so there is pressure drop occurring).

But I think you have a math error in your fill rate - I see it as your 700 psi fill is only a 48.3 bar fill, and at 66 minutes that is 0.73 minutes per bar, or 73 minutes for 100 bar fill to your filter and 73 cuft tank.

On mine I have a 9.2 psi per minute fill, or 0.63 minutes per bar and thus 63 minutes for a 100 bar fill to my 88 cuft tank with no filter. The lack of filter is an unknown difference, and who knows what the true volumes of the tanks are - I think it would be better to work in true internal volume as the whole "cuft" thing is really just a convention and approximation.

Something is not adding up, as mine is filling a bigger tank faster to start with. I think we we are missing something . . . probably with the tank volumes more than anything.

 

[Revoman]

Alan!

Hmmm....I did NOT include any tank CUFT in the calculation, PLUS, for some reason I switched the naming from PSI to bar in the write-up. It should ALL have been in PSI....700PSI at 66 minutes, should have read .0943PSI per minute and 9.43 minutes per 100PSI. This not including any tank CF, input pressure, or filters.
I'm guessing that all information should come from a single source with the same set up in order to make it a fair time trial, for THAT particular set up and tank CF, input pressure and filter CF....too complicated for me!

mike

 

[AlanMcD]

OK, that helps . . . but it is still not correct, at least as I see it. And even though you did not "include" tank size in the math, it is inherent in the results as all the data (and thus the rate) is for a specific tank size, with filter . . .

700 psi in 66 minutes is 10.6 psi per minute (you had the inverse of that), and I like that unit - psi/min - but it is for a 74 cuft tank with an input filter that leads to higher volume than just the tank.

Mine was 7.6 psi per minute, but the tank was bigger and the input pressure was a little lower. Adjusting for those two factors I get:

7.6 x 88/73 x 110/105 = 9.6 psi per minute, so about 90% of what you were getting, but that does not take into account the volume of the filter, which we don't know but could guess is 1 or 2 cuft at pressure. One would lower the result to 9.5 and two would lower it to 9.3 . . .

Pretty close in value between the 13 and 15 tooth cogs - about what the math would predict which is 87% (if we assume 2 cuft for the filter, the estimated ratio of our two compressors would be 88%). I don't think that difference in speed either way would be worth a swap, if one had either one installed . . . I'm sticking with the 13 cog drives in mine.

Hope all is well, and have a great day!