How do we get cooler fill air on HOT days?

[ti325v]

Hi Cleavitt:
See my post about the guy with mch silent-.
I have lots of experience with the cooling coil concept
.http://www.scubaboard.com/forums/co...roblems-mch-16-ets-minisilent-compressor.html
Very difficult to cool the air anywhere near enough to overcome the heat of compression generated by fast filling.
I have checked into the thermodynamics, and have had success, in cooling my prefilter air, but the delta T from ambient (20*) down to lets say 5* will require massive amounts of cooling by conduction. I need 15 meters of tubing to drop from 42* to 26* and most of the cooling occurs in the first 5 meters, where the temperature difference greatest. As the air approaches the temperature of the coolant the heat has less 'drive" to the temperature of the coolant.
fill slower...pv=nrt.

 

[Hickdive]

You can certainly cool the air after it leaves the compressor. But it will only heat up again when it compresses into the cylinder.

 

[ti325v]

I just that !

 

[Hickdive]

BBB though...

 

[Suzuki416]

Sadly the 2 cheapest options are:

1. Put more pressure in to account for the pressure drop because ambient is hot & air is hot.

or

2. Fill slower.

The 2nd is cheaper & easier for everyone.

 

[lloyd_borrett]

G'day,


Sure I expected the obvious answers of "put more air in" and "fill much slower". Both are easy, but impractical.


If you're already normally filling cylinders to the max pressure allowed so that customers get nice fat fills, then "put more in" just isn't a safe option.


Certainly "fill much slower" is easily to do, but it's not very customer service friendly. It's on the hotter days that the dive shop is the most busy and most divers will only wait just so long for their air fills.


Options like the Bauer and L&W refrigerated air cooler units to cool the air between the compressor and the filters are something new to me and they look like a possible partial solution. Does anyone know a ball park figure for the cost of those units or have experience of them in use?


We'd already been thinking of cooling the air between the storage banks and the final filters. I appreciate the simple yet practical solutions already in use that some have shown us here.


Any other ideas, or examples of setups, would be much appreciated.


Best regards, Lloyd Borrett.

 

[ti325v]

Do some research into the amount of heat generated by adiabatic compression of that much air to that pressure. It heats up 17 Kg of aluminum 20*c easily enough with a moderate fill speed...how many calories is that ?
How many tones of cooling you need to remove that by how many tanks you are filling ?
Solution, inject liquid air and maybe it might be cold enough...You aint gonna win, add more whips and fill slow. cheap guaranteed to work.
Do the math. If I had time I would do it just for fun. Then you gotta work out how you are going to cool that air.Pulling heat out of fast moving air is not easy.
If you have to do them one at a time put a pmv on each whip with a flow restrictor, set up 20 whips. 10 minute fills, if 200 bar 12 liter tanks, set he pmv`s for 3300, will cool to about 3000.
if you are going to fill ten or 20 at a pop, better have a heck of a pump, or some serious bank system.

 

[Suzuki416]

As mentioned, if you fill 'quickly' - speed is relative, then you are fighting a losing game - however, I didn't say overfill.
What I did say was put a bit more in.

Basically, you ARE NOT compensating for the heat buildup caused by filling fast so in essence you are giving a low fill.


I'd at least water bath them to the necks - although I don't like this in general as water gets everywhere, it gets very slippy quickly, as is causes more issues than it solves.
Your tanks are rated to X psi @ X temp - think its 15C. If your pressure at that temp is not max psi then you are short filling... ergo - put more in.

---------- Post added June 29th, 2013 at 09:16 AM ----------

Lloyd,
your other problem is the ambient air temp of your bank - I am assuming that you fill from the bank ?.

how large is your bank,
Is there any way of cooling the room ?

Thinking outside the box, How about placing your bank cylinders into a large plastic bag & mounting them up to the necks into a water bath permanently. This way the air bank temp is at the ambient water temp.
Also this way the banks are 'not wet' so don't have to worry about those issues too..

 

[lloyd_borrett]

Thanks everyone. I'm certainly getting plenty of food for thought.

Please let me clarify a few things.

We already fill as slow as is practically possible so that divers get the best fills we can provide without them having to wait more than 30 minutes all up. So we don't try and fill at a fast rate, and there isn't much leeway to fill significantly slower when things are hot and busy.

If divers let us keep their cylinders to cool down and top up, we do this. But for the majority of divers this isn't an option and they wear the drop in pressure when things cool down. But on the hot summer days the drop is greater than we like and we'd like to improve the outcome for our divers.

We already fill to as high a pressure as we can to compensate for the pressure drop when things cool down. A typical steel cylinder here is rated to 232 bar. Throughout the year we'll typically fill to 240 to 250 bar so that when things cool down the divers get a 225 to 230 bar fill. But on the hot days things cool down to even lower pressures and we'd like to avoid this. Putting more in by going beyond 250 bar isn't an option.

An air cooling system between the compressors and filters into the storage banks would probably help only minimally given that we can't do too much to reduce the exposure of the storage banks to the ambient heat. The best we can do to cool the room is by using some large fans to push air through it.

The idea of cooling the air between the storage banks and the final filters is probably a more practical way to go. So we'll investigate this further.

We have one CompAir Reavell 10 cuft/m compressor in operation and another on standby. We're planning to add two Ingersoll Rand 15 cuft/m compressors soon and have all four compressors in operation. But we'll be dedicating one of the 15 cuft/m compressors to a new air storage bank system and air fill panel dedicated to hydros in a separate work area.

Our air bank system consists of a main bank of six 350 bar 50 litre cylinders with second and third banks of 350 bar 50 litre cylinders each. We're planning to add two 350 bar 250 litre cylinders and two 350 bar 50 litre cylinders to this bank system soon.

We currently have two air fill whips and two nitrox fill whips on the fill panel. It's unlikely we'll add extra whips to this panel in the short term.

We are considering dedicating one of the 10 cuft/m compressors to a new EAN32 bank system, and filling the bank system with EAN32 via a membrane or nitrox stick. Then add a two or four whip panel dedicated to EAN32 fills.

Best regards, Lloyd Borrett.

 

[Suzuki416]

If you put the bank cylinders into an air conditioned room you will have less ambient heat to deal with...

This may be a cheaper way to solve this.

Are they in the same room as the compressors or outside ?

---------- Post added July 3rd, 2013 at 01:49 PM ----------

just reread your post - moving the bank to an area where it isn't subject to so much ambient heat may be cheapest in the long run.

Yep - I'd be uncomfortable putting in more than 250bar so thats not going to happen..