You are missing the fact that the reservoir temperature used for cooling/heating is not necessarily the temperature you are trying to reach. Say you had a cup of water at 20 C that you want cooled to 7 C. You can put it in a 5 C fridge but that takes a long time. I you use a -20 C freezer, you can create a 40 C temperature difference, which allows you to reach 7C much faster.
However a gas stove can create a temperature difference of several hundred degrees. Even if your goal is 100 C (which is 6 times further than 7 C), the rate of heating is large enough (>>6) that you get there sooner.
The point is it’s very easy to get a 2000 degree flame and therefore a ~1900 degree temperature difference with a room temperature object but the lowest cooling temp that is practical is probably only about -10 which only gives you a difference of less than 100 degrees with a room temperature object.
It’s a bit OT, but this should be corrected. Microwave ovens do not work by radiative heat transfer. When you talk about radiation in the context of heat transfer (i.e. conduction, convection and radiation), you’re talking about heat transfer between two bodies driven by their temperature difference and requiring no physical contact (as in the case of conduction and convection). The broiler element in a conventional oven works partially by radiative heat transfer.
A microwave oven does not have a hot element. It emits microwaves which excite water atoms and the increased vibration of the water causes the increase in temperature. So you can say that microwave energy is “radiation” but it is not a radiative heat transfer mechanism.
Going back a number of posts, and slightly off-topic:
If that were true–why would anyone buy one? One of my friends has a convection oven. This sucker’s amazing (the oven that is); it cuts the cooking time and the temperature needed each by about a quarter. See, it’s a regular oven with the addition of a fan; the fan blows the hot air from the source up to where the food is, instead of waiting for it to rise on its own.
You are a little off on the application of these definitions.
Conduction- heat transfer from direct contact of two items.
Convection - heat transfer between a moving fluid and a solid.
Radiation- the exchange of thermal radiation energy between two or more bodies. Thermal radiation is defined as electromagnetic radiation in the wavelength range of 0.1 to 100 microns (which encompasses the visible light regime) (Radiation Theory in Heat Transfer)
A standard oven makes use of radiation from hot coils and the heated inside of the oven, and small natural convection currents. A convection oven adds a fan to increase the fluid flow (the air motion) to increase the heat transfer rate to your food. They cook much faster than ordinary ovens. Likewise convection cooling works well in those wine chillers.
Microwaves do not use radiant heat transfer to heat food. They use microwave radiation, which is a different wavelength band than standard radiant heat transfer (cm length rather than micron) and do not rely on temperature differential. http://imagers.gsfc.nasa.gov/ems/micro.html
No, I wasn’t ignoring **micco]/b]'s answer. Just displaying an astounding talent for timing the Submit button so that I duplicate the last two answers.
Okay, call me quite mistaken on that one… :smack: As far as the question of ‘if so, why would anyone buy one…’ I thought that they were slower but BETTER for certain purposes… speed isn’t everything, especially when it comes to food!!
To get stuff cold in a hurry, you’re looking for a blast freezer. These things operate at around minus 60-80 degrees and as the name suggests, blast the food with high-speed cold air, adding the wind chill effect to the overall scheme.
Food gets very cold very fast, but the freezer is very expensive. Chef Morimoto (of Iron Chef fame) has one in his latest restaurant. IIRC, they said it was a roughly $20,000 appliance. Also, a tad dangerous - if you just reach in for something without gloves, you’ll have badly frosbitten hands in a couple seconds.