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Kitchen Notes: Heat Transfer and Cooking
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ELEM
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PostPosted: Mon May 18, 2015 3:51 pm    Post subject: Does water stay hotter longer depending on how it was heated Reply with quote

I am not an engineer, but I have a question for one. A natural gas utility advertises that "WITH NATURAL GAS, WATER HEATS UP TWICE AS FAST AND STAYS HOTTER LONGER."

The “heats up twice as fast” claim is obviously imprecise but makes some sense, given that a gas burner comes on instantly at full heat while an electric burner heats gradually. However “heats up” is pretty vague; “heats up” to what extent? Also, all else being equal the quantity of water should affect the relative times to a particular temperature because once both cooking methods hit full output the electric burner might at some cross-over point be more efficient with less heat escaping. At least it’s debatable.

But “stays hotter longer”? Could it be that water heated by a gas flame somehow stays hotter longer than water heated on an electric burner? Different molecular reaction? If the claim is somehow true, would the difference in heat maintenance disappear once the water hits the boiling point?
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Dilbert



Joined: 19 Oct 2007
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Location: central PA

PostPosted: Mon May 18, 2015 5:59 pm    Post subject: Reply with quote

this is about stove tops I presume?

the 'heats up faster' bit is as you suggest fairly clear. not only is a gas flame 'instantly' hot - compared to the time lag for a spiral coil to heat up - but gas burners can deliver more BTU/time than the classic electric coils.

the big residential gas burner can put out about 22,000 BTU/hr - the (lossless) equivalent of near 30 amps at 220v nominal. so you have instant 'lots more heat' on the bottom plus combustion gases running up the sides of a pan.

it takes one BTU to raise one pound of water one Fahrenheit degree - so pint's a pound the world round,,,, two quarts of water for pasta, roughly 4 pounds, starting off at 75'F going to 212'F, delta T 137 F' x 4 pounds = 548 BTU needed - if the entire 22,000 BTU/hr gas burner were absorbed (it's not....) it would take 1.49 minutes to boil. lossless obviously does not apply - it takes longer - not all the heat is absorbed by the pan/water....

a large spiral electric coil will vary, but wattage wise they are in the 2100-3000 range. and the 3000 watts is roughly half the "power" of the 22,000 BTU/hr burner.

alternate heat sources - induction / radiant - alter the picture slightly - but "power in vs power out" still applies and gas will still win.

"stays hotter longer" - there must be some fine print there somewhere.
makes no sense - and certainly the fuel/heat source makes zip difference to how fast a pot cools down once removed from the heat.
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ELEM
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PostPosted: Mon May 18, 2015 6:47 pm    Post subject: Natural gas vs. Electric heating and heat retention Reply with quote

Dilbert -

Thank you for the quick and clear response.

There is no fine print in the ad on "stays hotter longer". I assumed from the context that the gas company was referring to stovetop cooking. On reflection, although it seems less likely, perhaps it was referring to hot water heaters. If so, there too a gas hot-water heater would presumably heat faster than an electric but I doubt that gas fueled hot water tanks are typically better insulated than electric ones to support "stays hotter longer."

The whole claim seems a good reason to take advertising with a few grains of salt (suggesting a discussion of boiling point elevation).

Best,
ELEM
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Dilbert



Joined: 19 Oct 2007
Posts: 1175
Location: central PA

PostPosted: Tue May 19, 2015 6:10 pm    Post subject: Reply with quote

well, if the gas company is advertising their own hot water heaters, it is possible those devices are "better insulated" than the usual. but you're right, there's nothing more inherent about gas hot water heaters over electric to 'keep hotter longer'

with the minor exception of a pilot light. most gas appliance in the US have switched to electronic ignitions - no more pilot lights.....
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JonR0
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PostPosted: Fri Jan 08, 2016 8:04 pm    Post subject: Heat Capacity of Foods (and combinations thereof) Reply with quote

I was seeking more about the heat capacity of foods, in fact having asked Siri for the heat capacity of a casserole. Burr's article does a decent job of describing heat transfer, and categorizing the speed and effect of various approaches to cooking. With graduate courses in Convection, Conduction and Radiation heat transfer under my belt, though, it didn't serve my needs that well. He felt he needed to use some metaphor to make heat transfer easier to understand. It may have worked for some, but clouded the topic for me.

Any help in finding an article on the heat capacity of foods would serve my interest better, and be appreciated. My hope is to determine how long to heat a leftover dish to serving temperature, knowing its mass, the mass of its container, and the desired serving temperature. This shouldn't be nearly as complex as determining how long to cook a dish in which phase and chemical changes occur.
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Simon
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PostPosted: Thu Nov 17, 2016 4:34 am    Post subject: Thank you very much for this wonderful piece Reply with quote

Hi Michael,

thank you for making the effort to put this piece together. I was putting a piece of content together for my own website to explain the difference in pan materials used in Germany. Your article acted as a good entry point and I hope it was alright that I linked to your article as a source.

Greetings from Germany,

Simon from https://pfannenhelden.de
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PaulT
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PostPosted: Fri Dec 02, 2016 5:34 pm    Post subject: reheating food Reply with quote Delete this post

JonR0 said, in part:
>Any help in finding an article on the heat capacity of foods would serve my interest better, and be appreciated. My hope is to determine how long to heat a leftover dish to serving temperature, knowing its mass, the mass of its container, and the desired serving temperature. This shouldn't be nearly as complex as determining how long to cook a dish in which phase and chemical changes occur.<

This is what I would refer to as "the wrong question". Knowing the mass of food, and mass of container, is only somewhat useful. And you're missing the characteristics of the heating medium completely! Look at your desired end conditions: All of the food is at or above the desired reheat temperature. It's been decades since my last heat transfer course, but to get to your end conditions, look at the path of the heat from the medium to the middle of the food. Significant thermal resistance comes from the container (if it's ceramic/glass type - a thin metal foil tray would offer virtually no thermal resistance compared to the food) and then the food itself. The heating of the food would occur almost completely through conduction, unless it is a liquid that can move, or the food is stirred one or more times. Stirring can only be done for something like a pasta in sauce, or mashed potatoes, and not for something like a cake or quiche that has to have its structure retained.

Now look at the heat path through the food. A container that is cubical will have the longest path to the center (there aren't many spherical containers), while the same mass of food shaped in a rectangular solid with a depth that is much smaller will result in there being not only more surface area exposed to the heating medium but a shorter path to the center of the smallest dimension. If you assume that the medium has unlimited heat capacity (heats the surface of the container/food to its temperature instantly and maintains that temperature) then the heating of the food depends only on how long it takes to get the center up to temperature, and the shorter the path the quicker that will occur. A hotter medium will also accelerate this, up to the point that the outer layer is damaged from the heat before the center gets to temperature.
So knowing the mass of the food, and the container, is secondary. One must know/learn how fast the medium can transfer heat and its temperature, and how quickly the food conducts it based on its shape, constituents, and thickness. If you have numeric values for these, then it's a simple conduction problem.

It's almost never a real world question of "how much power/how many BTU's of gas are burned", the question is how long does the heating medium that has its characteristic energy consumption rate have to be applied to heat the variable container size of variable heat capacity food. This is in part because with the possible exception of microwave heating, the losses of energy to the environment are large in comparison to the amount of heat actually transferred to the food. One number that appears on the 'net is 1.8 J/g/degree C for pasta (referenced http://www.physicspages.com/2015/07/13/heat-capacity-of-pasta/) but this presumably refers to the uncooked hard noodle, not the final one with significant water absorbed, cheese and sauce added, etc.

All this said, IMO the best way to determine the time to reheat something is experience. Obtain a food thermometer, place its indicating point in the thickest part of the food, place all in the heating medium, and record the temperature over time.

For food which can be stirred, one can apply heat until the average temperature is the serving temperature, then stir to even out the temperature of the entire serving. This will be much sooner than just waiting until the coldest part has reached serving temperature. It also moves cooler food into contact with the hot container, increasing the average rate at which heat is absorbed, since the heat transfer is related to the temperature difference.

Now if you really want the heat capacity of the food for some reason, you can place a uniformly warmed portion of it into a known quantity of water, wait for the two to come to equilibrium, and calculate from the rule of mixtures. But I maintain that the more useful value in the real world is the rate of heat transfer within the food.
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