Pre-heat tube on cannister-top burner: reasons why?

I can't think of a good reason for a pre-heat tube on a canister top stove, but I can't imagine what it looks like either. Got a picture?

Canister top stove (Pocket Rocket):

No preheat tube because gaseous butane goes from canister to stove so can be burned as is

If you have an inverted stove, then liquid butane goes to burner so the preheat tube is needed to evaporate butane. Same thing for white gas or kerosene stove.

If you have a preheat tube it adds a small amount of weight and complexity.

I use top stove down to 20 F but it starts getting slow. At temperatures below 20 F (or maybe below 25 F) you have to use tricks like putting canister in hot water or having windscreen designed to warm canister. Or use inverted canister stove or white gas/kerosene.

A preheat tube is not really necessary for temps above freezing (32F.) Below that, they become increasingly useful down to about 20F, as Jerry says. Below that they are necessary.

I never camp in winter anymore, and, I never use a canister stove. I tried several but they either broke, started sputtering & flaring. They were always a bit unreliable. So, as with the past twenty or so stove stove purchases, I went back to my SVEA. A bit heavy at 17oz (weighed on a digital lab scale at 16.97oz) it remains competative with any self contained stove on the market, even today.

Anyway, the preheat tube establishes a "known" temperature/pressure to the gas. It doesn't really matter what type of fuel. At a certain flame height, it gets x amount of heat from the flame. Since temp and pressure are related, this means a relativly constant flow of gas to the jet to mix with air, and to the burner head, given minor fluctuations in input pressure. White gas, Kero, etc require a preheat. (The SVEA uses a combined valve/brass heat exchanger to preheat the white gas into a vapour, same principle in a smaller output stove at ~4500BTU. It does not work for larger stoves all that well.) While Isobutane/propane mixes do not "require" a preheat, they run better with one. The windpro for example works very well, running very consistantly down to 20F.

Any fuel MUST mix with air before it will burn. This means that everything (well, 99.9%, explosives are different) must be in a gas form before it will burn…wood, plastic, wax, gasoline, whatever. So, by maintaing a constant heat on the gas, they can guarantee a constant pressure, hence consistant burns.

This solves the consistancy of burning problem, but does NOT solve the step functions associated with canisters. You get the full 11oz or 7oz canister every time. You cannot adjust it. 7oz isn't that much, but is significant.

I think you missed the word 'cannister', James.

Ha, ha….no. Fuels all burn the same way. Only the boiling point varies. Or heat of vaporization or whatever the fancy technical term for it is these days: wax, Alcohol, lp gas, isobutane, butane lighters, white gas, gasoline, kerosene (liquid paraffin in england,) etc. (Actually it is a curve when graphed.) They all have to gasify, mix with oxygen in the air, then they burn. A preheat tube simply supplies heat to the fuel insuring fuel vaporization. In the case of our isobutane/propane canister stoves, it insures a good vaporization temperature. Necessary? Maybe not, but if you have ever had a sputtering canister top stove, it is highly likely it was freezing up. Nothing you can do except heat it up a little. Preheat tubes prevent the sputtering and give more consistant flame adjustments. That may or may not suit the OP. Generally, a topper will simply use the ambient air temp. Once that stabilizes with the canister being used, then you can get a consistant adjustment. Most people don't bother. Turn it on reasonably high (so any loss of pressure due to heat loss from the gas being drawn off doesn't shut it off) and ignore it for 4-5 minutes. Not the most efficient way to heat water. It works, though.

Obviously no-one else bothered to read the OP

Alan,

I don't see a stove by that designation. Do you have a different name on it anywhere? Picture or link to one?

A pre-heat tube might be handy on a boat or RV where you could have liquid fuel sloshing about. Just guessing though.

No photo yet, but here i a diagram (via link)

http://www.coleman.com/coleman/parts/ProductImage2.asp?product_id=3004-750T

When clicked for enlarged diagram, on part with control knob, you can see the piezo igniter near control, then at back you can see the preheat loop, an upside U of tubing.

Raymond:

its an old discontinued stove.

Hi,

You might try a Query in "Search Forums". I just put in "pre heat tube" and got back 352 places with those words. I lot was written back a few years about this question.

Yes, but these search results are all about remote stoves NOT canister-top stoves.

In a word (or two): improved performance. At least that’s what I think. Much like a Coleman F-1, the preheat loop on an HPX3000 is going to conduct some heat back down to the tank (judging by the position of the pre-heat loop) which will afford some performance improvement.

This is the best photo of the pre-heat loop I could find:

You can kind of see the pre-heat loop in this photo from the web:

Other photos:

HJ
Adventures in Stoving

Hj:

here is disassembled hpx 3004:

Ah. Much better photo.

I still say the same thing. Since it's a top mounted canister stove, they cant be trying to vaporize the fuel — although maybe if you used this stove with a Brunton canister stand (or the equivalent), you could do just that. Since they're not vaporizing the fuel, I believe they must be trying to conduct heat down into the tank to improve performance. Some side by side tests with other stoves to see if there is indeed some cold wx performance gain would be interesting.

HJ

But why bother with a gas carrying tube: to conduct heat they would just need a solid metal rod (cheaper).

Running the gas through a pre-heat loop will virtually guarantee the elimination of any sputtering or flaring due to some liquid fuel being in amongst the gas as the fuel exits the jet.

It is a rather curious design isn't it? I'd have loved to have been a fly on the wall in the design sessions.

One thing you could seriously do though, is pair it with a Brunton canister stand and run with the canister inverted. You'd then have a remote inverted stove for the price of a canister stand.

HJ

That stove design is darn loopy [sorry, really]. I say cut the thing in two with a bandsaw so's we can see the guts!

Maybe they were trying to design their own scramjet stove or something? I can't imagine further heating of vapor would have much of an effect and I can't imagine even with the mass of all that brass it would have a measurable effect on the canister's temperature (heating the fuel on cold days).

So yeah, I guess we're seeing a stove designed following Pizza and Beer night. Or just maybe the burner was adapted from a remote fuel stove.

Shrug,

Rick

If the tube is before the needle valve, it is there to prevent condensation at or after the needle. There will be considerable pressure drop across the needle valve. That means if the gas, siphoned off the top of the tank, is near the boiling point (like during cold weather). When the gas pressure drops across the needle valve, liquid will condense causing sputtering. The pre-heat tube will super heat gas preventing condensation. The stove should be able to run to a lower temperature than a comparable stove without the tube.

I hope that wasn't too technical. I had to try really hard not use the terms "saturation line" or "isentropic expansion"

If the gas is near it's boiling point, there will be little pressure in the canister (by definition), so the "needle" valve (I've yet to see a canister stove with a true needle valve) will be fully open to maximize the gas flow. The only pressure drop will be across the jet. All the same, I was impressed by the technical terms you tried not to use but did.

+1 on the post beer and pizza night design

If the tube is before the needle valve…

It appears from the photo that the pre-heat tube is after the valve — which I think would make more sense in any event (why have gas in the pre-heat tube when the valve is closed?).

I still think there's going to be heat conducted back to the tank although goodness knows exactly why they'd pick this design. Beer and pizza night? Tequila shot night more like. Or maybe they were testing alcohol stoves and had some 190 proof …

I suppose a simple way to test my heat feedback theory is to run the stove for a while and then feel the top of the canister. If the top of the canister is at all warm or even simply room temperature, then there's heat feedback going on. Normally, a canister should be pretty cold after a stove has been in operation for any length of time.

HJ

"I suppose a simple way to test my heat feedback theory is to run the stove for a while and then feel the top of the canister. If the top of the canister is at all warm or even simply room temperature, then there's heat feedback going on. Normally, a canister should be pretty cold after a stove has been in operation for any length of time."

True, that. Absent a windscreen I often find canister temps drop during use, I suppose from evaporative cooling.

I'm still lobbying for further disassembly. Inquiring minds want to know!

Cheers,

Rick

My recommendation to actually use the pre-heat tube to best advantage is to get a Brunton "remote canister kit" that converts a canister-top stove to a remote stove. REI sells them.

With this kit you also should then get a new angled MSR plastic tripod stand (from their new multi-fuel stove) to invert the canister. Then your stove's heat tube will function in colder weather.

That said, I still rely on my MSR Dragongly liquid fuel stove for cold weather B/C it's utterly reliable. Haven't used my ancient (1971) SVEA 123 for over a decade.

Sorry, no further disassembly: might be destructive.