Posted By Steve E
I need some advise on Propane gas bottle leaking.
I went to my friends unit to say hello and as it was cold he had his propane heater on, i noted that the bottom 12 inches of the 5 foot high propane canister had frosting around it.

I know this is not a good thing but I've never had any dealings with gas canisters before.

Any advice on what or why this is and what should be done would be much appreciated.

Posted By Stefan Daunt
The decompressing gas inside the cylinder cools the tank which causes moisture from the atmosphere to condense on the outside of the tank and freeze.

You can use the frost line as a rough guide as to how much propane is left in the cylinder if it is not fitted with a gauge.

Posted By Guderian
Perfectly normal, as explained. Expanding gas cools down, hence the moisture from the air freezes on the outside of the bottle.

Some seriously poor advice given too .....

Posted By Ron Hunter
Science in action. A decrease in pressure within a fixed volume results in a drop in temperature.

Posted By ScotsAM
Ah, Good old Charle's Law

Posted By Andy Petrie
Sorry to get picky but the pressure will remain pretty static within the bottle, it's the liquefied gas evaporating into the space left when the gas is used that results in the temperature drop, as a change in state requires much more heat than a change in pressure or temperature.

It's like when you boil a kettle, it doesn't take long to get the temperature to a 100c, but to actually boil it requires massive amounts of heat to change the water from the liquid to vapour phase (it would take ages to actually boil the kettle dry). It's just happening at much lower temperatures with the heat coming from the atmosphere.

Posted By John Richards
"Sorry to get picky but the pressure will remain pretty static within the bottle, it's the liquefied gas evaporating into the space left when the gas is used that results in the temperature drop, as a change in state requires much more heat than a change in pressure or temperature"

Correct.
But at some point the [liquid] gas temperature will decrease such that the liquid will turn gaseous more slowly. That happens with a high withdrawal rate. The more gas you use, the lower the temperature and, as the liquid cools down the less gas you get.

Posted By Raymond Rapp
Apologies for my previous posting I misinterpreted your thread. However, I am glad others have provided correct advice...feeling rather humble.

Ray

Posted By Steve E
Thank you for all your help.

Posted By Ian Dale
Not sure where you are working but butane freezes at -138°C! I think I would be more worried with other things frezing!
Ian Dale

Posted By MP
Back to the LPG freezing thread, how many times have we seen frost on the bottom of a cylinder with a chap helpfully playing his torch on the cold area "to warm it back up"

Now, this may be hi-jacking the thread but....

No disrespect to the original poster Steve, but does his statement maybe point to the root cause of a lot of the negative press H&S gets.

"I know this is not a good thing but I've never had any dealings with gas canisters before."

Steve has done the right thing and sought further advice, but how many folk just "know it is not a good thing" and make up daft rules and bans etc.?

Posted By Ian Futcher
I think the technical term for the process of evaporating something off in a confined bottle is "adiabatic expansion"? When you get adiabatic expansion, the energy for the evaporation has to come from somewhere, so it comes from the heat capacity of the lquid, resuling in a temperature drop. Something to do with entropy?

Or are my grammar school physics lessons mis-remembered?

Ian

Posted By Jay Joshi
Ian is spot on--This is termed as the Joule-Thompson Effect:-

Refer to :-
http://en.wikipedia.org/wiki/Joule-Thomson_effect

Posted By ScotsAM
I think that's the combined gas law.

Posted By John J
To sum up, its perfectly normal

Posted By Richard Paul-Jones
Some years ago I attended a gas safety course where frosting on LPG cylinders was identified as a danger signal. In a large application of LPG such as a temporary catering facility where there are numerous appliances daisy chained together it is possible for the gas demand at the start of the chain to be so high that there is none left for the end of the chain. The danger is that a burner is turned on at the end of the chain and works until other burners are turned on nearer the supply. The end burner goes out due to lack of gas, but with the valve still open. As the other burners are then turned off the end burner will start discharging unlit gas.

Of course it takes a particular set of circumstances for this situation to lead to an explosion: there must be a long chain, the pressure in the system must be too low for all burners to work simultaneously, the end burner must have been turned on and forgotten, the other burners must have been used and turned off, there must have been a build up of gas and a source of ignition – so it’s unlikely, but far from impossible.

As the degree of frosting is, among other things, dependent on the rate that gas is vaporised and drawn from the cylinder, the faster the flow the more likely and heavier the frosting. Heavy frosting is a signal of high demand, and a hint that a quick check of the facility might be in order.

Paranoid, moi? Never...

Posted By Kirsty Davies2
Agree with Andy!

Posted By ScotsAM
As do I. Though not as much as my own post!

Posted By Andy Petrie
Group Hug!