Bank size?

[TravisD]

So the shop I've been teaching at closed up recently, and I've 'ended up' with the compressor/fill station. This weekend I spent hookup up the bank bottles (compressor isn't quite hooked up yet, pending electric).

As part of the 'package' I have 28 bank bottles - 23 are 4500 PSI, and 5 3500 PSI that have been dedicated to banking 50% nitrox. Due to space limitations, it looks like my optimal air bank size is 10 bottles, that I've configured as sets of 5, 3, and 2, 5 of course being the 1st part of the cascade, 2 being the last 'high pressure" bank.

Expected use here is "friends and family". I'm not trying to get into the air fill business. Compressor is a ca. 2001 Bauer 13 CFM. I want to be able to run the compressor when conditions are right (e.g weather isn't to hot/humid, and most importantly when it won't piss off the neighbors).

I could probably squeeze a couple more bottles in if I had to, but other than the "more is better" principle I'm not sure I'll see much actual benefit. I already have some interest in some of the extra bank bottles, to hopefully offset some of the cost of this endeavor.

Also ended up with a large (26968) Haskel booster too, mostly used previously for putting O2 into the Nitrox banks so as to not return 1/2 full bottles to the gas company. I'm probably going to switch over to a Nitrox stick instead. (I don't have any personal need to boost O2 really, other than maybe my first aid kit bottle...)

So is ~4k cu ft. of bank 'plenty'?

 

[Tracy]

For personal use, definitely.
It is more than 90% of dive shops.

 

[rx7diver]

Some things to consider:

1. You'll need to run the compressor enough (often enough and long enough each time) to keep it healthy.

2. Will you be filling doubles or filling singles for your family and friends? LP (e.g., 2,250 + 10%) cylinders or HP (e.g., 3,500 psig) cylinders?

3. The fewer bank bottles you have in your cascade system, the fewer you'll have to have periodically hydro'd, etc. As a start, can you run the numbers for eight 4,500 psig cascade bottles configured in four, two-bottle banks? With this setup, how many fills can you get before you need to run the compressor to get the bottles all back to full, and approximately how long must the compressor run to achieve this?

rx7diver

 

[nadwidny]

So is ~4k cu ft. of bank 'plenty'?

Sounds legit. I run a bank of 4x3500 with a similar sized compressor. So far so good for fills for friends and family.

 

[TravisD]

Most people I hang with are on either HP singles (80/100/130) or AL80's. I'm basically constrained to 4 banks - and one of those is the Nitrox bank - due to 4 bank inlets on the fill panel. (I don't want to deal with turning cylinder valves on and off all the time, other than the Nitrox bank since it's the easiest to reach).

I'm not overly concerned with Hydro costs, that's a once-a-decade thing. I'm also not trying to 'minimize' the bank size here - I'm basically constrained to a specific space, and 15 total bottles (10 air, 5 Nx) fit the available space nicely. I could squeeze in a couple more if it really made sense, but it'll make compressor maintenance more difficult.

I was mainly concerned if I was going to regret not having more capacity (I know, I know...) or if I should re-allocate the split differently - 4-3-3 instead of 5-3-2 for instance.

Thanks for the feedback! Can't wait to get home fills going soon...

 

[Cio]

4K is plenty.
If you have e a 3-2-1 bank config, you will have access to ~ 2K (50% of total)
Bank config is considered optimum @ 3-2-1 (or 6-4-2, 12-8-4 ect...)

 

[TravisD]

Ah, that's the math that has been escaping me. Thanks Cio! Seems like I'm close-enough. I might try and sneak in an other bottle if things work out, but not prioritizing it.

 

[rob.mwpropane]

Does anyone actually hydro bank bottles? Or maybe they hydro once and then forget about it.

 

[Tracy]

Does anyone actually hydro bank bottles? Or maybe they hydro once and then forget about it.

I never install them without a fresh hydro. From that point on, outside of a fire department, I have never seen them come out until the shop goes out of business.

 

[rx7diver]

Does anyone actually hydro bank bottles? Or maybe they hydro once and then forget about it.

The only cascade system I am intimate with is the one that used to be owned by the univ that offered the scuba course I took (in 1986). Old, military surplus, water-cooled, 3-stage (IIRC), Worthington compressor connected to six 3,500 psig bottles that he would fill to max fill pressure of ~2,500 psig usually. Although only select TA's (I was one) were permitted to operate the compressor and fill cylinders, scuba students often were in close proximity to the compressor and cascade bottles and fill whips (which were sited in the scuba gear room).

The scuba coach/prof would hydro his bottles quite often, cycling through them every three-five years or so (IIRC).

I don't know if the university's risk management office stipulated this schedule, or if this schedule was due to the coach's own natural risk aversion. (He would often speak of a fill station accident that occurred at a univ in a neighboring state, that maimed or killed someone.)

Note: The above is recalled from ancient memory, so ...

ETA: Just searched and found an old post (Help Settle a Tank Filling Discussion) I made, which is consistent with what I wrote above.

rx7diver