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Here's a typical outdoor gas stove:

enter image description here

It is fed by a low-pressure gas supply in the form of a camping cartridge or via an LPG cylinder connected to a low-pressure regulator (around 30mbar) like this one:

enter image description here

My question is: is it possible to increase the heat output of the burner by connecting two low-pressure supplies in parallel via a T-piece like this one?

enter image description here

Would this actually result in double the gas flow/double the heat output, or would the stove itself impose some limitation on the maximum pressure of gas able to flow through it (excluding the obvious extreme cases of catastrophically high pressure where the stove or pipes might crack)?

How dangerous is it? Obviously I assume it's not a recommended procedure and it probably has safety considerations, but for example would doubling a low-pressure supply from 30mbar to 60mbar pose a serious risk, or are appliances in general built to withstand far higher pressures than this? I know standard gas hoses, for example, can withstand pressures of 20 bar (100x higher than low pressure canisters).

Is there a way of determining whether an appliance will be able to handle higher flow?

I've seen this done on cheap/flimsy camping stoves so I assume it's not a catastrophic risk especially with better-made burners.

WackGet
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    You’ve misunderstood the video you linked - he didn’t add a second canister to the same burner, he added a second burner (with its own canister) the the same stove. – Jonathan R Swift May 20 '20 at 09:35
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    "is it possible to increase the heat output" of a burner. Yes, if you know what you're doing, which you don't. - "Is it safe to do so" on a *portable* stove. No. – Mazura May 21 '20 at 00:55
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    Perhaps an "outdoor stove windshield" would work, as less hot air will be blown away and wasted. I see reviews saying things like "it cuts my time to boil water by almost half" and "think my burgers cooked a lot quicker." – Andrew Morton May 21 '20 at 13:26

4 Answers4

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is it possible to increase the heat output of the burner by connecting two low-pressure supplies in parallel via a T-piece like this one?

The primary factor is gas pressure at the tee. If the gas pressure drop from the low pressure regulator to the tee is negligible and the first tank & low pressure regulator is able to supply enough gas to maintain 30 mbar then a second tank & regulator also set at 30 mbar will not permit extra flow to the stove. If the pressure drop from the first pressure regulator to the tee is significant, though, then the second gas input will increase total flow to the stove.

The best method for increasing gas flowrate is to increase the low pressure regulator setpoint. You can use equations such as this one to estimate flowrates through the different hoses although parallel flow means you'll have to solve for pressure at the tee intersection iteratively. Or try and make flow paths from each tank symmetrical.

Small threaded fittings often are rated for high pressures well above 10 bar due to scaling laws but you should check, wear eye protection, keep electronics away from your work area, and have a second person nearby with a fire extinguisher in case an uncontrolled fire does occur.

Other issues to be aware of regarding the burner design incide:

  • Air/fuel ratio

  • Gas velocities and Venturi effect (for drawing air into gas stream)

Here is a set of presentation slides with vocabulary that may be useful for your own research. Beware that it's possible to lose ignition due to insufficient mixing of enough air with the fuel. This means you'll probably have to modify stove fittings that affect this mixing process.

baltakatei
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  • Thank you, that was very helpful. So basically I'd have to get an adjustable regulator (or one with a higher output)? – WackGet May 20 '20 at 16:25
  • The flow rate calculator you linked is unfortunately way over my head, so I hope you won't mind me asking a basic question: if hardware is normally rated for very high pressures such as 10 bar, why don't the manufacturers advertise things like stoves as having a far higher maximum heat output? E.g. the cast iron stove in my original post is rated at 9KW with a 30mbar regulator. Why don't they say "can be operated at 27KW with a 90mbar regulator" (or whatever it would be)? Would the design of the stove (pipe diameter, airflow etc.) prevent this? Is it just a marketing thing (27KW is "too hot"?) – WackGet May 20 '20 at 16:47
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    Manufacturers (at least for US and Canada markets) probably restrict their stoves' advertised capabilities in order to meet the requirements of "Outdoor Cooking Gas Appliances (ANSI Z21.58 / CSA 1.6)", currently a [voluntary standard](https://www.cpsc.gov/s3fs-public/VoluntaryStandardsActivitiesFY2019MidYearReport_0.pdf) which could be made a mandatory standard by the [US](https://cpsc.gov) or Canada. To see exactly what these requirements are, [you'll have to pay about 500 USD to find out](https://webstore.ansi.org/Standards/CSA/ANSIZ21582018CSA). See also ZNSI Z21.45, and Z21.54. – baltakatei May 20 '20 at 18:17
  • Yeah that makes sense; thanks for the insight - I've learned several new things today. So ultimately it seems there's little risk if I were to use a slightly more powerful regulator with the stove? For example, using 50mbar instead of 30mbar? I assume the biggest risk would come from overwhelming the weakest part of the system and thus causing a blowout, but from what I'm reading it seems like that shouldn't happen below very high pressures (5-10 bar at the least); is that about right? – WackGet May 20 '20 at 22:28
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    @WackGet, going from 30 mbar to 50 mbar isn't "slightly". If a rating doesn't give a tolerance range, a good rule of thumb is 10%: your stove should work correctly at 27 mbar or 33 mbar. At 50 mbar, even if nothing breaks, the increased output could heat things that aren't supposed to get hot, or the increased gas flow could mix poorly with air, producing soot buildup and carbon-monoxide emissions, or other problems. – Mark May 20 '20 at 22:39
  • DO NOT get an adjustable regulator for the stove unless you want to blow yourself up. It's designed for that pressure, not for more. Buy a new burner rated for the output you want if you want more output. – Tiger Guy May 21 '20 at 02:01
  • @Mark That doesn't seem to be very good engineering. Usually you want a margin of safety of at least x2, don't you? For instance, a bridge rated for 100 tonnes shouldn't fail until more than 200 tonnes is placed on it. – nick012000 May 21 '20 at 02:28
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    @nick012000 there's safety against catastrophic failure and there's safety against foreseeably hazardous operation. Overly large flames would fall into the latter category, presenting a similar hazard to using it in strong winds or with a too-small pot. If you design it to be safe at twice the regulated pressure, it won't work very well at the regulated pressure. However, it might be worth testing in controlled conditions, if there aren't any plastic or soldered parts closer than the end of the hose. – Chris H May 21 '20 at 11:08
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    Blowout and even leakage aren't an issue going from 30 to 50 mbar. note that there are 37mbar regulators for propane in some places, which might be a better option (and easier to get) than 50 mbar. More importantly than a fire extinguisher when testing would be a long enough hose after the valve that you could be sure of shutting off the cylinder if you ended up with large flames. – Chris H May 21 '20 at 11:11
  • Thanks Chris, and everyone. I don't plan on doing anything stupid; was just wondering if using a different cylinder would be possible/effective/safe (I have two butane ones at 29mbar and a propane one at 50mbar). However I take your point about the design of the stove being optimised for its stock pressure so I don't know whether I'll even bother trying it. – WackGet May 21 '20 at 21:11
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No, connecting two tanks together will not double the flow.

The regulators fitted to tanks limit the pressure and flow rate so all you do is double the amount of gas the burner has available ie doubles the time of use.

Do NOT interfere with the regulators.

There are automatic change over systems that have both tanks connected and it switches over when one is empty.

Solar Mike
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  • You can makethe answer even clearer by adding something along the lines: having two bottle does not double pressure in any way. The working pressure is controlled by the regulator. – mart May 20 '20 at 06:33
  • @mart did I put “limit the pressure and flow rate”? – Solar Mike May 20 '20 at 06:35
  • Yes, lets see if the OP understands it. – mart May 20 '20 at 06:48
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    @mart Given the use of English by the OP I don’t think there will be an issue. – Solar Mike May 20 '20 at 06:52
  • I think I get it. So it's *not* like `2 x 30mbar = 60mbar`, but rather more like `2 x 30mbar = double the amount of burn time at 30mbar`? Does it mean that each regulator will effectively force itself to flow at 15mbar? – WackGet May 20 '20 at 16:20
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    @WackGet no, each regulator will work at 30mbar - well one will be more than the other so it will work first... – Solar Mike May 20 '20 at 16:25
  • OK, I see... I obviously need to think harder about pressurised systems. I was picturing a regulator kind of like a hose which just spits out water into a drain regardless of how much water is already running through that drain (so if you have two hoses, you get double the amount of water). But in that example I suppose the drain is not pressurised so the hose flow is not impeded. So if I understand correctly, a regulator simply won't output anything at all if the pressure in the system is already at 30mbar. Thanks for your help. – WackGet May 20 '20 at 16:34
  • One more question - you mentioned automatic changeover systems. Why is an automatic system needed? Can't you just connect two tanks together and they will naturally run down together? Is it just so one tank will get fully emptied while the other remains full? – WackGet May 20 '20 at 16:40
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    @WackGet means you know when one is empty you can get a replacement and not run out. – Solar Mike May 20 '20 at 17:16
  • @WackGet, the advantage of an automatic changeover system is that you can remove the empty bottle while the stove is still on: the changeover valve closes off the side with the empty bottle so no gas leaks out. This is more useful for things like RV furnaces or water heaters that see continuous use than for something like a stove that's intermittently used. – Mark May 20 '20 at 22:34
  • "the purpose of a propane regulator is to 'bottleneck' the propane down" from "over 200 psig" to "under 10 psig", a safe and usable pressure." – [propane101.com](https://www.propane101.com/regulators.htm). **They need to (not do this, by interfering with *anything*)** find a way to increase *flow* not pressure, so as to not literally defeat the 'heart of any propane gas safety system'; the regulator. – Mazura May 21 '20 at 01:18
  • @engineer this proposed link just highlights your knowledge. – Solar Mike Jun 08 '20 at 18:52
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No. The regulator itself will see to that. It's around 30 millibars - 3/100 0f a Bar. The pressure inside a gas cylinder is around 10 Bars - 3,000 times that of what gets into your stove!

It wouldn't matter if you had ten cylinders all feeding the regulator, it could still only deliver that 30 millibars to the stove.

The only way to get more gas, therefore more heat would be to chamge the regulator. There are plenty on the market - 3 Bars is common. but don't do it!

The stove has been designed to run happily on those 30 millibars - components and joints will comfortably cope with that (very low) pressure. Much more, and it gets very dangerous - from an exploding into bits point of view from the pressure, let alone exploding in flames!

tim
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There is a mathematical formula called "The Hanson Theorum" basically it's the calculating the area of a circle, very accurate without the use of Pi.

Imagine a pipe with a $10cm$ in diameter bore, well the calculation is $10 x 10$ (to get the square of the bore) and multiply it by "The Hanson Theorum" which is $0.78$.

The calculation is: $10^{2}\cdot 0.78cm = 78cm^{2}$ of bore area.

If your gas jet is say $0.3mm$ we can create a higher gas flow, by boring that jet out to a slightly larger size with either a specialist drill size or some fine guitar wire... This will increase the gas flow, but at the same pressure. The regulator, should be able to cope with a $20 - 30\%$ greater flow.

So let's assume the jet is:$0.3mm \rightarrow 0.3\cdot 0.3 \cdot 0.78 = 0.07mm^{2}$ of jet bore area

Now if we want to increase the flow rate, by $20\%$ - that means we need to figure out what a jet bore $20\%$ larger is IN AREA, not diameter, the simple maths.

$$ (10+2)\cdot 7 = 0.084 \rightarrow \text{of area} $$ $$ 0.084 = 0.327\cdot 0.327 \cdot 0.78 = \dots \rightarrow \text{when converting back to diameter} $$

My brain is tired.... So you have to increase the jet size from $0.3mm$ to $0.327mm$ in diameter.

OK you can drill them out - with a special drill or you can scrap them out with a piece of 0.009" or $0.23mm$ steel guitar wire... with a diagonally cut end, and a small drilling machine etc.. and just scrape away - you will get the technique... or you can hammed the wire into a square or rectangular shape....

Since you really are ONLY reboring it by a hair or two's increase in diameter.... it's not much and then you can see if your stove works well. Try to make the issue of small shavings in terms of rejetting the stove, rather than way too big.

If you go too big first go, your regulator may not flow that amount of gas AND OR your you may need to enlargen the air mixing stage, to get a hot clean flame, instead of a sooty yellow flame...

Yellow flames also produce Carbon Monoxide which is lethal in significant amounts in confined spaces.... and people under some conditions - like it's freezing cold, they are in a snow storm and the only place to cook is inside the little tent...

My little butane can single burner portable camping stove - on cold days like 10C, the burning rate which is based upon the boiling rate of the gas, which slows down as the evaporating gas, chills the liquid gas, so it cools and boils off at an incrediby slow rate, which means cooking a big meal in a pot takes 10 x as long as cooking on a 35C day.... So I need to fiddle with the jet a little to increase the rate of gas flow and not fiddle around much to make the stove improvements into a major engineering project, rather than a simple improvement.

Teo Protoulis
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Shane Hanson
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    0.78, your Hanson theorem is basically Pi /4 3.1415926 / 4 = 0.785. So you are still using Pi but don't realize. – Solar Mike Jul 06 '20 at 13:44