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The three different types of heat transfer

 
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There are three different types of heat transfer:

1. Conduction occurs when two objects are physically touching. This is the most efficient form of heat transfer. Think of food in a frying pan. The heat is being conducted from the frying pan to the food. Hold your sandwich an inch away from the frying pan and you’ll be waiting a while for grilled cheese.

2. Radiation is energy that travels in a directional wave. When radiation strikes an object it transfers heat to it. Think of sitting around a campfire. The front half of you may be warm but your back might be cold. That’s because the air itself is cold - you are being warmed by radiant heat.

3. Convection is heating the air, some of which heats you. This is the least efficient type of heat transfer and is sadly the way most of our homes are heated.

I think that, in nearly all cases, conduction is ideal. Except occasionally it's inconvenient, so sometimes radiant heat is the most practical option. Maybe there's a time when convection is the best choice but I'm not coming up with one.

What are some good examples of applying each of the different types of heat transfer?
 
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I would say that convection is valuable in the case of an existing space or a new one that needs to have partitions or barriers that block radiation to some areas.

The convection would be an adjunct heat transfer method for the remote parts of the zone.
 
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I wonder if the efficiency of these three types of heating is based more on a cooking or industrial application.  For instance, cooking pancakes with conduction is quite fast.  Cooking marshmallows with radiant heat is pretty fast.  Cooking chocolate chip cookies in a convection oven is slowest.

But, if you're trying to heat a body to a comfortable level, I'm thinking the order becomes reversed.  It's much easier to heat the air in a house to 70 and distribute it evenly so that it touches all your body at the same time.  And the walls, cat, floors, bed, etc.  It's harder to heat your body surface with radiant heat on all sides (cold backside at the camp fire, roasting shins),  It's harder yet to heat a body with conduction unless you are sitting/sleeping on a warm surface.  

I also wonder about the efficiency claims.  I haven't seen the numbers so I could be way off here.  But I tend to think of things in terms of "energy cannot be created or destroyed".  If my house is insulated to R20, does it matter which way it's heated?  If I'm ok using radiant heat to just shine on my body and let the rest of the walls, furniture, water pipes, goldfish, etc drop to 40F, I can save some energy by concentrating it on my carcass.  If I want that radiant energy to heat the walls and floors and goldfish and keep the pipes in the walls from freezing, I suspect I'd be consuming as much energy as if I was using convection.
 
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Also, how can you convert from one type to another?

For instance a radiator converts from conductive to radiant... what other ways are there?  After that I get kind of lost.

Like could you somehow convert convective warm air that has risen to the ceiling into radiant and reflect it back down??
 
Glenn Herbert
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Heat loss from the building envelope depends on the air temperature (leakage), and the interior surface temperature of the exterior walls (conduction: R-factor). The air needs to be brought up to temperature and then moved to the body to be heated, and it must be warmer than the surface of the body, unless the whole body generates heat, when it just needs to be warm enough for heat to not radiate away from the body. In general, convective heating requires a warmer space than radiative heating.

Radiative heating is instant on the exposed surfaces of a body, and delayed some (how much?) on surfaces exposed to re-radiated heat from other parts of the room. Radiative heating lets the air be cooler, reducing the heat loss from the envelope for the same comfort level.

Conductive heating, when applied to the proper areas, makes the body feel warmer even with substantially cooler air. For most people (maybe all), warmth from the floor or a bench can make the body feel comfortable in air around 5 degrees F cooler than with convective heating.
 
Glenn Herbert
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Radiant heat will be least concentrated at the edges of the space, thus heating the exterior walls less than the room contents. Convective heat is often supplied at the exterior walls for uniform comfort, thus heating the exterior walls more than the rest of the room. It is possible to design a convective system to not do that, but that is the norm.

A radiator conducts heat through itself from the source (unless it is an electrically heated surface material, or exposed flame), and radiates to room contents directly, with some heating of air which then circulates.

Convective warm air at the ceiling will warm the ceiling surface, which will radiate some of the heat back to the room while conducting some of it to the outside.


Air is relatively transparent to radiant heat, so for a source to significantly warm air, the air molecules must contact the source surface, then travel to the body and contact it. This makes heating air less effective than radiating or conducting directly to room contents.
 
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Shawn Klassen-Koop wrote:

1. Conduction occurs when two objects are physically touching. This is the most efficient form of heat transfer.



I'm not sure what you mean by efficient.  Radiation can be the most powerful type of heat transfer, to the point that conduction and convection are completely ignored.  Radiative heat transfer is a quadratic equation, so radiation losses can be huge.
 
Glenn Herbert
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Radiation and conduction are complementary: when there is contact for conduction, there is no radiation, and when there is space for radiation, there is no conduction. Which one is more efficient or effective in a given situation depends on variables of surfaces and materials. (Shiny surfaces are poorer radiators, but do not affect conduction one way or the other.)
 
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force convection is natural convection on steroids. That is why I blow on my hot coffee when I want it to cool quickly.

While I am quite familiar with laws of thermodynamics mechanics etc. I rely mostly on guts when designing something. Guts is sort of unconscious mind, laws are the conscious.
 
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