Stove Fuel Physics

[Panope]

I measured the time required to boil 2 cups of water on 4 different stove tops. Here are the results:

-------Stove--------------Heat Rating--------Time to Boil 2 Cups

Alcohol, Origo--------------(7,000 BTU)-----------5:25
Propane, GE-Household-----(9,500 BTU)-----------3:40
Electric, GE-Household------(6,150 BTU)-----------3:40
Butane, Burton-portable----(7,400 BTU)-----------2:20

Noteworthy was the inconsistent boil times vs. BTU rating of the stoves. For example,the Propane stove is rated at 9,500 BTU and the Electric stove is rated at 6,150 BTU yet both stoves had identical boil times.

Also, the Butane stove was more than twice as fast as the Alcohol stove yet they have similar BTU ratings.???

I used the same kettle for each test and the starting water temperatures were similar. Lowest altitude test (Alcohol) was 0 feet above sea level and the highest test (Electric) was 400 feet above sea level.

Does anyone have a good explanation for the above discrepancies?

Steve

https://www.youtube.com/watch?v=QoJzEDz0dqk

[mstrebe]

Variations in experiments always come down to 2 things:

1) Uncontrolled variables

2) Bad data.

Assuming your data about BTU output of the various stoves is good, then the question is: "What are your uncontrolled variables?"

The most obvious one is how much thermal energy is being lost to the environment rather than being directed into the pot? Some possible answers include:

1) Height of the pot above the burner.

2) Ambient temperature

3) Ratio of heat radius to pot radius

4) Thermal conductivity of the different pots (if they are different--this is the easy variable to control for)

Essentially, there are many reasons why one configuration may be more efficient at delivering heat to the water than other configurations.

It's really hard to setup proper test conditions to determine this kind of variation, but I would argue that the BTU production is unimportant, and the test that is important is the one you've already performed: Which of these stoves in typical conditions heats my coffee fastest?

[Seymore]

Good test.

I'll stick my neck out and say the manufacturers' ratings are questionable. The test is definitive.

Shouldn't the stoves be rated in Btu/minute?

[StuM]

Efficiency. Heat loss around the cooking container compared to heat being captured by the container.

There is much more heat loss around the kettle with alcohol/propane/butane cooking than with electrical. Hold you hand beside the kettle and feel the convected heat from the flames curling round the bottom of the kettle compared to the lost heat when the kettle is sitting directly on an appropriately sized electrical element.

Compare the flame size with the butane v alcohol and see how much is burning directly under the kettle and how much is continuing to burn as it passes out from under the kettle. You should find that the butane burn is more concentrated under the kettle so less heat is lost that with the alcohol.

Try it again with an induction cooker rated at the same wattage as your electric stove (if the kettle is not suitable for induction cooking, compare the two using a suitable pot). The induction cooker will be quicker again because there is less waste heat escaping around the pot.

[Panope]

Originally Posted by mstrebe

............ but I would argue that the BTU production is unimportant, and the test that is important is the one you've already performed: Which of these stoves in typical conditions heats my coffee fastest?

I agree that the BTU ratings did a poor job at predicting performance. I wonder if the ratings are useful for predicting fuel/electricity consumption?

Thanks for the insights,

Steve

[Panope]

Originally Posted by Seymore

Good test.

I'll stick my neck out and say the manufacturers' ratings are questionable. The test is definitive.

Shouldn't the stoves be rated in Btu/minute?

Thanks Seymore.

I'll leave the 'BTU/minute question alone is it might develop into the similar argument as the 'kw vs. kwh' debate that rages on a different thread.

Steve

[Panope]

Originally Posted by StuM

Efficiency. Heat loss around the cooking container compared to heat being captured by the container.

There is much more heat loss around the kettle with alcohol/propane/butane cooking than with electrical. Hold you hand beside the kettle and feel the convected heat from the flames curling round the bottom of the kettle compared to the lost heat when the kettle is sitting directly on an appropriately sized electrical element.

Compare the flame size with the butane v alcohol and see how much is burning directly under the kettle and how much is continuing to burn as it passes out from under the kettle. You should find that the butane burn is more concentrated under the kettle so less heat is lost that with the alcohol.

Try it again with an induction cooker rated at the same wattage as your electric stove (if the kettle is not suitable for induction cooking, compare the two using a suitable pot). The induction cooker will be quicker again because there is less waste heat escaping around the pot.

Stu: I'll try different stoves if they become available to me. But I am just satisfying my curiosity so I will not be terribly motivated to seek them out.

I'll check out the flame characteristics that you mentioned and do the 'hand beside kettle test' next time.

Steve

[leftbrainstuff]

The BTU rating is measured in a controlled draft free environment. I would also expect the test to be lab quality and use a calorimeter for determining heat capacity, not a kettle. Both of these factors dont simulate a real world galley.

You also don't mention the condition of the stoves or the source of your fuel.

Not uncommon to see great variability in quality and performance of these fuels. Individual suppliers will blend based on availability, cost and supply agreements. Contaminants such as moisture will reduce the flame quality dramatically.

As a rule propane will give the best performance all things being equal. Some love induction cookers. We get over 6 mths from two 20lb propane bottles and we cook and bake daily.

Your test, which I applaud you for, allows you to compare actual stoves but does not allow you to determine fuel performance alone.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app

[mstrebe]

Originally Posted by Seymore

Good test.

I'll stick my neck out and say the manufacturers' ratings are questionable. The test is definitive.

Shouldn't the stoves be rated in Btu/minute?

You would think. BTU is defined as the amount of heat necessary to raise 1 pound (?) of water 1 degree F. It's just about the exact amount of heat produced by completely burning a typical wooden kitchen match.

It's usually used to rate space heaters and air conditioners, which are considerably more efficient than stoves because unless they're poorly designed, the entropic waste heat of a heater _is_ it's product, so they can be 100% efficient. In situations where efficiency is important (ability to direct the heat to a target), you have to know both the BTU production and the efficiency to know anything.

Frankly, as the OP's testing points out, it's kind of a useless measure for stoves.

It does, however, tell you how much fuel you're consuming. So the OP's test of minutes to boiling / BTU tells you which stove is most efficient.

You won't beat an inductive cooktop for either efficiency or safety on a boat. I wish someone made a 12V model. I had a friend who would drop a washer into his ceramic coffee cup and put that on his inductive cooktop. You want to see efficiency, that's it: Only the washer got hot, the coffee cup insulated it, and it would boil water in just a few seconds with the cup taking a long time to warm up. Perfect for coffee.

[Pegu Club]

So the max difference here is one hundred and eighty seconds.....
Not much of a time difference imo. The real question to me is how much fuel is consumed.
I use an origo non pressure alcohol stove. I understand that other mediums raise the temp quicker, no question, but heck I have nothing but time when i am chilling out on the boat, so the the difference to me is no big deal. The clincher for me was the virtually no maintenance stove.
like all things boat, there are comprimises on everything, most every stove will cook your food effectively, it just depends what does it for you.

[LakeSuperior]

Originally Posted by leftbrainstuff

The BTU rating is measured in a controlled draft free environment. I would also expect the test to be lab quality and use a calorimeter for determining heat capacity, not a kettle. Both of these factors dont simulate a real world galley.

Heat or thermal capacity (watts/meter^3-deg C) is the ability of a material to store heat. Consequently, I don't think you meant what you wrote. BTU (BTU/hr) output is not a heat capacity but rather a measure of how much heat the burner can deliver per unit time.

Doing some quick googling it seems that the burner ratings are derived and not measured.

For a gas type burner the rating is calculated from the type of fuel used (caloric content) and the rate at which it is burned. In the case of an electric burner it is simply calculating the resistive (ohmic, Joule) heating, I^2xR.

These ratings are not directly related to how fast a pan of water can be brought to boil because of the reasons already mentioned.

[StuM]

Originally Posted by Seymore

Good test.

I'll stick my neck out and say the manufacturers' ratings are questionable. The test is definitive.

Shouldn't the stoves be rated in Btu/minute?

Stoves are actually rated in BTU/hr. Yes, it's another place that units are used incorrectly all the time, you very rarely see BTU/hr spelled out.

But since the stove ratings are almost solely used for comparison purposes and not for subsequent calculations, it is not really worth getting bothered about. (I can only fight so many battles at once )

[StuM]

Originally Posted by mstrebe

I had a friend who would drop a washer into his ceramic coffee cup and put that on his inductive cooktop. You want to see efficiency, that's it: Only the washer got hot, the coffee cup insulated it, and it would boil water in just a few seconds with the cup taking a long time to warm up. Perfect for coffee.

Ooooh, I'm gonna try that! It appeals to the geek in me

[hpeer]

what temp was the water when you started?

I'd like to do a compare with our kerosene stove and a butane camp stove.

[newhaul]

I like my gasoline stove 12,000 but max perks coffee almost as fast as I can get dressed ( that's a 10 cup perk pot)