Stove Fuel Physics

[newhaul]

Originally Posted by StuM

Incidentally, 1lb of propane contains about 21,600 BTU or energy.

So a 30lb (13Kg) cylinder contains about 648,000 BTU and should give you about 54 hours of cooking at full bore on your 12,000 BTU/hr stove.

Or it's good for about 216 of your 12 cup pots of perk coffee. Assuming about $30 per cylinder refill cost (some places will be quite a bit cheaper - in many parts of the world, (like here) it is a lot more expensive), that works out at as a fuel cost of about 14 cents a pot or 1.2c a cup.

Stu its even cheaper than that here a 30lb cylinder holds 5 us gal of propane and I can get it for just under 2 bucks a gal at the local feed store so would be approx ten bucks a tank. So would be about .004 cents per cup. Love my gas

[Seymore]

It would be nice to see the OP, Panope, run his test, with his kettle, on a kerosene stove and a pressurized alcohol stove. And Hpeer would like to see his test done on a boat's propane stove.

Our perfect rigorous thinking is that Panope's kettle is not duplicable elsewhere.

[TeddyDiver]

Originally Posted by Roy M

I got this from businessdictionary.com:

Amount of heat required to raise the temperature of one pound of water (at or near 39.2 degrees Fahrenheit) by one degree Fahrenheit in practical terms, the amount of heat generated by one lighted stick of match. One Btu is equal to about 252 small calories or 0.252 kilocalories, 778.17 foot pounds, or 1055.06 joules. One pound of air-dried wood generates about 7,000 Btu, a gallon of liquid propane (a hydrocarbon) about 92,000 Btu, a gallon of fuel-oil about 140,000 Btu, one barrel of gasoline about 5.25 million Btus, an average ton of coal about 20 million Btu, and one kilowatt-hour of electricity about 3,400 Btu. Despite its name, this non-metric unit is used more often in the US, Canada, and Caribbean than in Britain (or rest of the world) where calorie is preferred. Used also as a unit of measurement for natural gas prices (1,034 Btu = 1 cubic-foot of natural gas). One Btu per hour equals 0.293 watt and is represented by the symbol Btu/h (not Btuh).

Read more: What is British thermal unit (Btu)? definition and meaning

Thanks for sharing some 'real' physical units. Outside the US Btu's and Fahrenheits are rare klingon dialect

[StuM]

Originally Posted by Wotname

Hey, while you are in number crunching mode, how many cups of coffeee do I get from 5 litres of kero - ignoring any heat I might get from the alcohol pre heat

Since you ask

The heat content of kero is 12.8kWh per kg or 12.8 x .82 = 10.5kWh per litre. So you have about 52.5 kWh of energy available.

1 kWh = 859 kilocalories so you have about 45,000 kcal available

1 calorie raises 1ml of water by 1°C

Assume a 20°C start temperature with a 250ml coffee mug you need 80 x 250 calories = 20Kcal per cup.

So at 100% efficiency it would be 45,000/20 = 2,250 cups
Assume a similar 30% efficiency to propane stoves, that would end up as about 675 cups.

To look at it another way:
You get about 170 x 250ml cups per kg of kero.
Newhaul gets 200 cups of an unspecified size per kg of propane, but I suspect that "perk cups" are quite a bit less than 250 ml.

Bottom line is that there doesn't seem to be much in it on a weight for weight basis between kero and propane

[Panope]

Seymore, I agree that I should get aboard some different boats with my kettle and test other marine stoves. I'll ask around the marina next week.

Steve

[Snowpetrel]

It would also be interesting to see if a black kettle would absorb heat faster than the same polished one?

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[StuM]

Originally Posted by StuM

To look at it another way:
You get about 170 x 250ml cups per kg of kero.
Newhaul gets 200 cups of an unspecified size per kg of propane, but I suspect that "perk cups" are quite a bit less than 250 ml.

Bottom line is that there doesn't seem to be much in it on a weight for weight basis between kero and propane

Just went looking and apparently a standard "coffee cup" in the US as used in percolators etc is a miserly 6oz (177ml) and much the same in most other places. So Newhaul's 200 cups is the same as only 140 of Wotname's assumed 250ml standard metric/aussie cups.

[Wotname]

Originally Posted by StuM

Since you ask
.......
You get about 170 x 250ml cups per kg of kero.
.......
Bottom line is that there doesn't seem to be much in it on a weight for weight basis between kero and propane

Lazy bottom Wottie says

[Wotname]

Originally Posted by StuM

Just went looking and apparently a standard "coffee cup" in the US as used in percolators etc is a miserly 6oz (177ml) and much the same in most other places. So Newhaul's 200 cups is the same as only 140 of Wotname's assumed 250ml standard metric/aussie cups.

Yeah but...
Wotties first COD (cup of the day) is 300 ml

[newhaul]

Originally Posted by Snowpetrel

It would also be interesting to see if a black kettle would absorb heat faster than the same polished one?

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

What is the black on the pot most likely the polished pot would be more efficient. Soot on a pot is actually a great insulator. Results would depend more on what metal the pot is made from. My pot is aluminum and coffee perks significantly faster than it does with my same sized stainless pot. YMMV.

[LakeSuperior]

Originally Posted by newhaul

What is the black on the pot most likely the polished pot would be more efficient. Soot on a pot is actually a great insulator. Results would depend more on what metal the pot is made from. My pot is aluminum and coffee perks significantly faster than it does with my same sized stainless pot. YMMV.

Not so fast. The two important heat transfer mechanisms in the heating the pot problem are convection and radiation. Convection is the hot gases impinging on the bottom and rolling up the sides of the pot transferring heat to the metal and then, via conduction through the metal, to the water.

Radiative heating is the thermal photons incident on and absorbed by the bottom of the pot. These thermal photons are emissions created in the hot gases and at the hot metal surface of the burner parts. If the emissivity of the pot is high then much of the radiative heat is absorbed by the metal and conducted to the water. If the surface pot is of low emissivity or reflective then the radiative energy is not absorbed but is reflected resulting in a longer time to boil the water.

The emissivity of the pot is typically not necessarily a function of the visible color. Metals tend to be reflective and oxides, sots, and coatings/paints are absorptive independently of the visible color for burner temperatures. An aside here, the rules are somewhat different for solar radiation.

Consequently, aluminum naturally has an oxide layer which is absorptive and stainless tends to be much more reflective than aluminum and will heat the water slower. This correlates with your experience.

A sot layer would have to be fairly thick to have much impact on the heat transfer through reduction in the conduction from the outer sot surface through the metal to the water.

The amount of heat transferred by both the radiation and convection mechanisms may be fairly equal.

[newhaul]

Originally Posted by LakeSuperior

Not so fast. T

A sot layer would have to be fairly thick to have much impact on the heat transfer through reduction in the conduction from the outer sot surface through the metal to the water.

The amount of heat transferred by both the radiation and convection mechanisms may be fairly equal.

I disagree with your statement of soot not having an effect on efficiency also what do you mean by thick layer.
Here is a quote ;" soot has five times the insulating value of asbestos, the heat transfer loss rises dramatically as the layer of soot builds up. For example, 1/8" (3.2mm) of soot - which can build up in only two weeks - results in a heat loss of 47%, with an increased fuel consumption need of 81/2%."

[LakeSuperior]

Originally Posted by newhaul

I disagree with your statement of soot not having an effect on efficiency also what do you mean by thick layer.
Here is a quote ;" soot has five times the insulating value of asbestos, the heat transfer loss rises dramatically as the layer of soot builds up. For example, 1/8" (3.2mm) of soot - which can build up in only two weeks - results in a heat loss of 47%, with an increased fuel consumption need of 81/2%."

I agree with your numbers above however I have never seen a 1/8 inch of soot on a coffee pot and I have a pot I have used directly on the campfire for 20 years. If you are depositing soot at the rate of 1/8 inch per two weeks you need to look at your air fuel burn ratio. You may be generating a substantial amount of carbon monoxide.

[newhaul]

Originally Posted by LakeSuperior

I agree with your numbers above however I have never seen a 1/8 inch of soot on a coffee pot and I have a pot I have used directly on the campfire for 20 years. If you are depositing soot at the rate of 1/8 inch per two weeks you need to look at your air fuel burn ratio. You may be generating a substantial amount of carbon monoxide.

I agree with you . Those numbers and stats are from the HVAC trades specificly on oil fired main steam boilers but it does illustrate the effects. The only stove I have on my boat that would even produce soot is the 2 burner gasoline stove.
Which by the way is the highest btu per gallon fuel that a stove can use

[StuM]

Originally Posted by newhaul

The only stove I have on my boat that would even produce soot is the 2 burner gasoline stove.
Which by the way is the highest btu per gallon fuel that a stove can use

BTU/gal

Gasoline: 124,000
Kerosene: 135,000

(Diesel: 139,000. While not a boat stove, I can remember the use of diesel drip cookers in military field kitchens.)

Just sayin'

Edit: A 139,000 BTU/gal marine stove: http://www.go2marine.com/product/200...with-oven.html