Upright gas canister vs inverted

[andrewa]

Recently acquired an MSR Windpro on a close out from prolitegear. It's a stove using a remote upright gas canister.

Having read a lot about inverted canister stoves, I was hoping for an easy conversion to inverted for snow use. Sure enough a half turn (with pliers) of the connection allows this, without any gas leakage.

The newer Windpro 2 allows canister use either way.....

My question is , given that inverted canisters/liquid feed allegedly work better in cold, is there ever any advantage in using an upright canister?.

Andrew A

[Maelgwn]

Generally there is better heat control with canister upright. With the inverted canister the valve is controlling liquid, so is mostly closed and harder to make fine adjustments. (i.e. half a turn from off to full)

[alanoutgear]

All gas stoves burn a vapour/air mix, even the liquid withdrawal (LW) stoves. I have a number of stoves including vapour and liquid withdrawal. The liquid that leaves the cannister in the LW stoves must be vapourised before it reaches the burner head for proper combustion, otherwise you will get flaring as the liquid gets through the metering jet, then vapourises and mixes with air to reach the flammability range. Liquid to vapour expansion is around 1:170 as I recall, so a little bit of liquid makes a lot of vapour to mix with air. LW stoves generallly have a preheating tube sometimes called a generator in the US, before the jet and this needs to track through the flame zone to heat up and vapourise the liquid as it passes through to the jet.

Once a vapour withdrawal stove body heats up sufficiently it is possible to gently invert the cylinder and convert to LW (clearly this only applies to remote cannister stoves and not cannister top stoves). This all depends on the stove design, and the amount the stove body heats up to vapourise the liquid inside the body. If you want to try this, do it outdoors and be careful, because the liquid will not be in contact with heat for as long as it would be with a purposely designed LW stove. I've done this with my Optimus Stella+, and it works. I simply invert the cannister on it's head on an angle after the stove has been burning for a few minutes and heated up. Most of the fuel is withdrawn as liquid but not as much as if the cannister was fully inverted.

The real advantage of LW stoves is constant pressure almost until the cannister is empty, so the flame doesn't drop off as the vapour pressure in a vapour withdrawal stove decreases with use. The cannisters of LW stoves also don't cool and become less efficient as the vapour is withdrawn and acts as a refrigeration system for the cannister, which is why vapour withdrawal stove cannisters get cold and clammy in heavy use. Also LW stoves are less affected by cold down to several degrees below zero for propane/butane mixes (not for butane alone though). You don't get much notice of an empty LW cannister either, just a minute or two as the residual vapour burns off after all the liquid has been withdrawn.

I prefer LW stoves because I have found them to be more reliable and less affected by ambient conditions. Gram counters don't like the extra weight of the preheating tube and remote fuel line though.

[Mark F]

The benefit of liquid withdrawal stoves are:

The canister does not need to be pressurised in order to deliver fuel to the burner. At cold temperatures butane does not boil so in a pure butane canister (the old Campin GAz pierce type) there is not enough pressure to feed to burner once the temperature drops below about freezing.

In mixed gas canisters the propane with a much lower boiling point provides the pressure and the fuel so at low temperatures with an upright canister, you are burning mostly propane rather than the butane. The result of this is that the canister ends up comprising almost all butane and ceases to work even though it may be a quarter full.

In an LW system the propane forces the liquid blend to the burner in the same proportion as in the cyclinder so the propane is not preferentially depleted and the canister will continue to work until it is empty.