How do we get cooler fill air on HOT days?

[lloyd_borrett]

G'day,

When the temperature is in the 35 to 45 deg C (95 to 115 deg F) range here on busy summer days the dive shop compressors work very hard. The result is very hot air through the whole system.

Even though diver's cylinders are filled to a safe high pressure, later they cool down significantly and divers aren't getting the great final fill pressures we are proud to be able to deliver most of the year.

What do people here suggest be done to mitigate the situation?

I guess having the cylinders in a cool water tub while being filled might help.

I've seen references to refrigerated dryers systems from the likes of Hankison and Norgren, but I have no idea as to how they work, where they go it the system, and the results likely to be received. Could someone please educate me?

I've also seen references to after coolers for air compressors, but I'm not sure if these are for use in high pressure 300+ bar setups.

Seems to me even if the air is cooled down after leaving the compressor, it can still warm up quite a bit in the storage banks.

Cooling it again just before the final filters might help as well.

Best regards, Lloyd Borrett.

 

[steveharriss]

All the advice I have taken on board suggests that putting cylinders into water is a bad idea which gains you very little advantage. I'll try and find a link to something that explained this clearly. You've either got to feed the compressor cooled air, or alternatively, cool it after compression. You'll get plenty of good advice on here so I won't try to add my inexpert opinion to that.

http://www.psicylinders.com/inspectors/library/19-filling-cylinders-in-water-time-to-review

 

[TheTrickster]

We double fill them, not sure if you have the time there for that.
But we fill on say Friday PM and then top them up again on Sat AM. But even then, but the time you get in the water, they are down to 210bar.

 

[Suzuki416]

on a hot day I used to put a bit more in...

 

[flots am]

When the temperature is in the 35 to 45 deg C (95 to 115 deg F) range here on busy summer days the dive shop compressors work very hard. The result is very hot air through the whole system.

Even though diver's cylinders are filled to a safe high pressure, later they cool down significantly and divers aren't getting the great final fill pressures we are proud to be able to deliver most of the year.

What do people here suggest be done to mitigate the situation?

Air conditioning in the shop, filling the banks at night when it's cooler, and telling people they can pick up their tanks the next day.

You really don't want to be running a hard-working hot compressor anyway. It's not great for the air quality.

flots.

 

[Wookie]

Filling on a hot day should have a negligible effect on the final pressure. Filling slowly is the key. Fill no faster than 300 PSI/min 20 BAR/min for a hot fill. Slow it down to 50 PSI/Min 3 bar/min for a good solid fill. The water bath won't help enough to matter. If your fill station operators can't fill slowly, you can get a orifice for the fill whip so they cant fill too fast.

Cooling the inlet air isn't much of a help either. The heat of compression is difficult to overcome unless you cool all stages of the compressor. What it will help is your filter life because you'll shake the moisture out of the air by lowering the dewpoint. Various companies make water cooled interstage coolers for whatever compressor you have. Again, you won't see much cooling effect, but your filters will last much longer.

 

[lloyd_borrett]

All the advice I have taken on board suggests that putting cylinders into water is a bad idea which gains you very little advantage.

Well doing this is certainly problematic with a higher risk ogf getting water into cylinders, and that's why we've always preferred to avoid doing this.

---------- Post added June 25th, 2013 at 06:13 AM ----------

We double fill them, not sure if you have the time there for that.
But we fill on say Friday PM and then top them up again on Sat AM. But even then, but the time you get in the water, they are down to 210bar.

We do that too for the divers that this works for. But the majority of people want to wait for their fill a while and then leave.

---------- Post added June 25th, 2013 at 06:17 AM ----------

on a hot day I used to put a bit more in...

We always try to get as fat a fill as we can within the limits. Thus our divers expect fat fills. But on hot days the final outcome is not nearly as good.

---------- Post added June 25th, 2013 at 06:25 AM ----------

Air conditioning in the shop, filling the banks at night when it's cooler, and telling people they can pick up their tanks the next day.

We already do all of that as much as we can. But on hot and busy days with so many divers coming in the compressor ends up working a lot.

We have a second CompAir Reavell 5406 compressor that we plan to add into the mix next summer. Increasing the size of the storage banks might also help a bit.

But when the ambient temperature is so high for a long period, we suspect we need to improve the way the compressors are cooled, plus have an effective way to cool the output air.

 

[swamp diver]

Bauer and L&W compressors make a refrigeration dryer for HP compressors. The pressure dew point is raised on compression and you need to cool the air down below this temperature in order to condense the water vapour to an aerosol and then remove it.

I think both units will cool the air to a nominal temp of +3 C but in reality at 40 C ambient it will be somewhere under 10 C. The extra water removed will prolong the life of your filters substantially and the air downstream of the dryer will be much cooler.

Most hospitals these days don't allow refrigeration dryers since there is a small risk that the refrigerant gas could leak into the breathing air at 1 atm. For a diver this would be almost certainly fatal because the gases would be very narcotic. Bauer gets around this risk somehow by isolating the two circuits so one cannot leak into the other. Not sure about L&W.

BAUER COMPRESSORS - Compressors - Breathing Air - Accessories - B-KOOL

L&W Compressors

 

[ti325v]

Lets go back to the original question,hot fills...and the subsequent pressure drop as the cylinder cools.
This is as I understand it, and experience it, simply a matter of filling speed.
The faster you fill them the more you heat them up, the more the heat, the higher the pressure, the more pressure lost as the cylinder cools.
pv=nrt.

When I was running two whips, filling directly from the compressor I had to top off my fills. When I increased it to 4 whips filling speed decreased, cylinder temperature decreased, and the need to top off my fills went away. I fill my 200 bar tanks to 207, they cool to 200. Every single one every single day.

 

[Cleavitt]

...

Seems to me even if the air is cooled down after leaving the compressor, it can still warm up quite a bit in the storage banks.

Cooling it again just before the final filters might help as well.

...

If you are filling from banks then cooling at the compressor isn't going to help you avoid hot fills. It will help increase filter life, but once the gas is in the storage banks it's going to warm up to ambient temp fairly quickly. The gas would need to be cooled between the banks and the whips. Ideally, I'm thinking the cooling loop would need to be located before any restrictions or flow control valves to get the best results, but I might be over thinking. I have never heard of this being done, but in theory it might cancel out *some* of the heat generated during the filling process and let you fill at a faster rate with the same end results.

You could experiment with this by adding a long coil of stainless hard line between your banks and fill panel/whips. Place the coil in a bucket and fill it with ice water. Fill some tanks with and without ice and see what you get for final pressures. I have no idea how much this will help, but if it doesn't work at least you have only invested some stainless tube and a bucket. If it does work well then you can consider replacing the ice bucket with something more sophisticated. In the end it probably all comes down to how much time the gas spends in the cooling loop.