Propane / Butane

[Ralph]

I know that modern vans can switch from butane to propane with the built in regulator but can someone explain to me how it works please?

In the old days you needed a different regulator for each and they worked at different pressures (I think) so how come it works on just one now?

Besides just academic interest I need to convince my wife who believes that our oven works better on butane...

 

 

[Dave Newell]

Hi Ralph, I'll try to explain for you. Butane has a higher calorific value than propane, that is it has more energy potential per unit (usually measured in MegaJoules per cubic metre). To try and match the performance of the two gases they were traditionally supplied at different pressures to the appliances, 28 mbar for butane and 37mbar for propane. Ideally the appliances would be jetted slightly differently as well.

 

With the advent of bulkhead mounted regulators it was decided to use a compromise pressure of 30 mbar rather than have to change the regulator for the different gases.

 

Your wife is quite probably correct that the oven works better on butane because there is more energy potential per unit than propane and as they would both be fed to the oven at the same rate the butane will get the oven hotter faster.

 

Hope this helps, D.

[Clive]

But if your wife uses the oven to cook a nice hot roast dinner up in the mountains in winter the Propane will win because the Butane would have stopped evaporating and the oven given up totally!

 

Enjoy,

C.

[derek500]

I've never quite understood the need to use propane over butane.

 

Here in Spain our domestic hot water and hob both run off butane. We use 12kg butane bottles from Repsol.

 

The bottles are outside in the open on a NE facing wall and connected through the house wall.

 

It gets quite cold here at night in the winter and -8º has not been unknown.

 

We have never had any problems with our appliances not working.

 

When we came to England at Christmas we used the same butane in our MH. A few of the nights were very cold, below freezing, and again we had no problems. At night we kept the Truma set to "heating only, no hot water, thermostat set on 1" to avoid the boiler dumping.

 

I suppose at least with the MH there is some protection from the cold as they are "inside" apart from three ventilation shafts in the locker, so I would expect less problems than I would have at home.

 

BTW, currently in Spain a 12kg bottle is 11.80€. The price is set by the government!!

 

 

[Ralph]

Dave,

 

Your explanation makes perfect sense, thanks.

I had thought that propane had the higher calorific value so she must be wrong but fortunately she doesn't read this forum so I won't have to tell her she was right after all. 8-)

Reading your figures it looks like using propane is a bit of a compromise then as the 30mBar pressure is a lot nearer to butane.

 

Clive,

 

Don't know about being up a mountain but we've certainly had butane freeze on us before I switched. Sods law it only ever happened when we had no electric hookup.

 

 

[Dave Newell]

I deliberately didn't mention about propane working down to lower temperatures because Ralph didn't ask about it. Butane has a boiling point of around 2 degrees Celsius while propane boils at minus 42 degrees Celsius. what this means in reality is that as the ambient temperature falls to around freezing point the butane will gradually reduce in its ability to turn from liquid into gas. Propane however will continue to boil and produce gas all the way down to -42 degrees.

 

We currently still use butane and back in November we camped in Bridgwater in sub zero temperatures but had no problems with the gas. I suspect your Spanish gas is not pure butane but a mixture of butane and propane. As the temperature falls to around freezing the butane stops gassing but the propane will continue to work.

 

D.

[Ralph]

Dave Newell - 2007-01-24 6:46 PM

 

As the temperature falls to around freezing the butane stops gassing but the propane will continue to work.

 

D.

 

Also worth pointing out, which may not be obvious to everybody, that this is affected by the amount of gas left in the cylinder.

When I've had butane freeze it's been because the cylinder was getting low anyway. Perhaps a full cylinder would have been OK at the time.

 

 

[Guest starspirit]

Derek,

 

I seem to recall reading somewhere that the Butane in other countries contains 'other additives' (probably some propane) to ensure that it works without the complication of having to supply two types of gas in different colour cylinders along with all the complications and cost implications.

 

I suppose Butane started here when campers only camped in summer and only us modern lot withour insulated and heated units brought about the need to use the hitherto commercial grade of Butane - or something?

 

So why don't Calor just ditch Butane and reduce the price of Propane do I hear anyone else say?

 

But wouldn't that just be too easy?

[Clive]

We had a Camping Gas (Butane) bottle freeze up inside the Dormobile in the south of France in late summerwhen er indoors was using all of the cooker rings for a sunday lunch. Changed to Propane but use a regulator which has optimised pressure for Propane on our bulk tank.

 

 

The pressure increase compensates for the reduction in calorific value and the ability to fill at the pumps makes it cheaper and more convienient.

 

 

We had four inches of snow by 0700 this morning, it was all gone when we came home at 1700.

C.

[derek500]

Ralph - 2007-01-24 8:45 PM

 

 

Also worth pointing out, which may not be obvious to everybody, that this is affected by the amount of gas left in the cylinder.

When I've had butane freeze it's been because the cylinder was getting low anyway. Perhaps a full cylinder would have been OK at the time.

 

 

Good point Ralph, at home when the water heater 'runs out' I always connect the bottle to the hob to get a bit more value for money!!

 

In the summer I maybe get a day or two's worth, but in the winter I can get a week or so.

 

Our Repsol delivery man does keep a supply of Propane as well - same orange bottles but with a black band. But I've yet to see any of those being sold.

 

BTW. The Repsol man in Spain, is like an English milkman, and we all put our empties out on the pavement the day he comes!!

 

[Ray]

Ralph - 2007-01-24 8:45 PM

 

 

"Also worth pointing out, which may not be obvious to everybody, that this is affected by the amount of gas left in the cylinder.

When I've had butane freeze it's been because the cylinder was getting low anyway. Perhaps a full cylinder would have been OK at the time".

 

We spent a Holiday in Scotland a few years ago where it was very cold at the beginning of May. Each morning we had problems with getting sufficient Butane to vapourize and found the only solution was to swap over to a full bottle to cook breakfast and make tea. The bottles were then switched back and were fine for the rest of the day so it would indicate that the above posting is correct. Hope this tip helps if you have the same problem. PS we filled a flask at night for the ealy morning cuppa.

[Brambles]

Another reason Butane bottles contain some propane is so if they freeze then a partial vacuum is not created as the propane will keep the pressure up - or so I believe and does seem to make sense for safety reasons etc.

[Clive]

Sorry Brambles but I think someone has been telling you cow poo!

 

 

[Brambles]

Cow Poo!! thats methane. Wonder if that would work. Now there's an idea! LOL.

 

 

What I do know is that Butane does comtain some Propane and vica a versa due to the process of separating the gases as a by product of the petroleum industry. I shall rephrase my comments as this has the advantage should the Butane in a bottle freeze, then the propane element will prevent a vaccum forming in the storage tank, and also maintain some pressure. As most who use butane in the winter will have have found a full bottle of butane will work at sub zero or near to zero temperatures.

Is this cow poo as well?

[Dave Newell]

You are unlikely to ever experience butane freezing as its freezing point is minus 138 degrees Celsius!

 

More info available here http://www.world-petroleum.org/education/lpg/index.html

 

When you say the cylinder freezes I think you are mistaking moisture in the air condensing on the cylinder and freezing as the temperaturee drops.

 

For Butane or Propane to boil they have to take energy from the surrounding atmosphere, this causes the cylinder to become very cold. The higher the evaporation rate the more energy is required. This is the reason that putting an insulating jacket on butane bottles will not prevent them getting too cold to boil the butane liquid, in fact it exaggerates the problem by preventing the absorption of heat from the surrounding atmosphere.

 

One possible reason that we have not had problems with butane despite camping in sub zero temperatures is that in the Highwayman the heater flue passes through the gas locker, albeit shielded by an enclosed metal panel but it still passes a certain amount of warmth to the gas locker, presumably enough to keep the liquid butane above boiling point.

 

D.

[Brambles]

When I say freeze I mean Zero degrees Celsius.

[Dave Newell]

Well Brambles at Zero degrees Celsius water freezes but butane and propane are both still very much liquids and under pressure in cylinders so there's absolutely no chance of a vacuum forming inside the cylinder, unlees it was completely empty of course.

 

D.

[Brambles]

Dave, if you get much below the liquified temperatire of Butane the gas part of it will turn to liquid and the pressure in the bottle will fall below that of atmospheric pressure, if it was not for the propane element,not a true vaccum technically.

[Brambles]

You all know fine what I meant by freezing, you are just deliberately winding me up aren't you? :)

[MandyAndy]

Ohhhh Brambles, would they ???? LOL

[Basil]

Brambles - 2007-01-25 9:51 AM

 

Dave, if you get much below the liquified temperatire of Butane the gas part of it will turn to liquid and the pressure in the bottle will fall below that of atmospheric pressure, if it was not for the propane element,not a true vaccum technically.

 

Not quite correct. Firstly Butane's boiling point is actually -0.5 degrees C (not 2 degrees as previously stated) and therefore will still be boiling off down to this point. Below this temperature it does not stay liquid but will continue to evaporate although at reduced pressure. As the temperature falls lower the rate of evaporation reduces until such time as it is equal to atmospheric pressure at which point it will be unable to release into the atmosphere and will remain liquid until such times that the temperature will make it freeze, at no point during these stages will you be achieving even a partial vacuum. The temperature at that point, (and long before) would be far less than you will ever experience in the world of motorhomes I would suggest. This explains why even a partially full cylinder will supply some gas initially.

Regarding changing to a more full up cylinder, if you compress a fluid (gases are in fact fluids) then you raise the boiling point. In the case of Butane if you compress the fluid, Butane's boiling point of -0.5°C will be raised to 38°C by increasing its pressure to 2.6 atmospheres so this is why a full cylinder will still gas because it is still compressed. As the gas is drawn off the pressure that is compressing is released and eventually if the draw off is greater than the boil off it will also cease to make gas available, but you will notice if you stop using it for a period it will again gas next time you use it.

If you are stillunable to get your heads round this look at a pan of water on a gas hob with a lid if you just leave it there the lid will get condense form on it, that is the water gasing off. As you increase the temperature then you can actually see the moisture breaking off the surface of the fluid, the more heat you apply the more the the moisture bubbles off until eventually the rate of release is so great you get bubbles of gas coming up through the liquid and bursting off the surface and gassing into the atmosphere.

 

Bas

 

Bas

[Brambles]

what is the boiling point of butane at 30mbar which is the pressure required orthe approx pressure required to operate appliances correctly.

Could it be 2 deg C ?

Is -0.5 deg C the boiling temp at 1 atmosphere.

 

You have not explained why a full bottle of gas should have a different pressure from one that is partially full. The last post implies a full bottle wil have a higher pressure, which is actually true, but not for the reasons stated as far as I read it, but because of a higher percentage of none butane gases such as propane. It does not matter what amount of pure butane you have compressed, the pressure will be the same at a given temperature. If any liquid/gas in a sealed container which has a temperature below its boiling point at atmospheric pressure, falls to a temperature below that boiling point then the pressure will fall also to a point of equilibrium where the pressure is now below the atmospheric pressure. Open a valve and it will suck in air.

 

edit - some spelling mistakes corrected.

 

 

 

[Brambles]

Basil, you have also implied for the pressure to be below that of atmospheric pressure it is somewhat way below -0,5 deg C. I think that is what you are saying. If it is then it is not correct. below the atmospheric boiling point the pressure will drop, so at -0.4 degrees C the pressure will be below atmospheric pressure hence a partial vacuum as such.

[Dave Newell]

Butane is stored as a liquid at 1.8 Bar which is approximately 28 psi. Even at zero degrees Celsius the pressure will remain at, or near to, 1.8 Bar. If the cylinder temperature is below the boiling point of the butane then when you open the cylinder valve to atmosphere the pressure will drop to near atmospheric and stay there until such time as the temperature increases above the boiling point of the butane at which point it will rise to 1.8 Bar (or thereabouts). The operation pressure of the appliances has got nothing whatsoever to do with the pressure in the cylinder because the former is controlled by the regulator.

 

Propane by contrast is stored as a liquid in cylinders at 6.8 Bar which is approx 102 psi. Because propane has a much lower boiling point than butane (minus 42 degrees Celsius0 it will continue to gas even in Alpine conditions.

 

Brambles, with all due respect to you, if you are going to enter discussions of this nature then it is important to use terminology that enables everyone else to understand your meaning. There is a huge difference between butane's freezing point and that of water so to apply the one to the other just results in confusion.

 

D.

[Brambles]

If the cylinder temperature is below the boiling point of the butane then when you open the cylinder valve to atmosphere the pressure will drop to near atmospheric and stay there until such time as the temperature increases above the boiling point of the butane at which point it will rise to 1.8 Bar (or thereabouts). .

 

If the cylinder containing pure butane (no propane) is below the boiling point then the pressure will fall as the butane condenses back to a liquid. The opening of valve not necessary.

The reason you appear to get what you say, is because of the propane content.

 

J.