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Heat rejection.

  1. Jan 3, 2007 #1
    I am having a hard time understanding the term "heat rejection". I always envision that the heat is radiating and do not understand the term. Why not "heat ejection"?
     
  2. jcsd
  3. Jan 3, 2007 #2
    heat rejection is where you have a perfect insulator that does not accept the transient energy (as in most insulators) heat resistant material however resists or only accepts a small percentage of the heat transmitted
     
  4. Jan 3, 2007 #3

    russ_watters

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    Staff: Mentor

    I'm not sure I understand the question. The words "reject" and "eject" are two forms the same word, so it doesn't really make much difference which you use except that people use "reject" by convention. Grammatically, the word "reject" is probably preferrable because of the implication that something rejected is useless (definition #4). However, while basic thermo, heat of rejection is really useless, when you get into more complex applications, even heat of rejection can be reused.
     
    Last edited: Jan 3, 2007
  5. Jan 3, 2007 #4
    Leaving aside your numbering of your definitions of the word, and how "useless" :frown: you think heat rejection is..

    Surely heat ejection would imply that the heat is (spontaneously) ejected out from within the material (presumably cooling it down). It sounds like something you might design an engine to do.

    Rejection implies something was directed at you, and you turned it away. This better describes the concept here of heat almost being reflected by an insulator.
     
  6. Jan 4, 2007 #5
    Sorry for being so brief. Here are two examples that I have heard engineers talk about.
    1.) An engineer was asked what happened to the power used to spin a centrifuge. His answer is that all of the power would be "rejected" as heat into the environment.
    2.) In refrigeration it is common to refer that the heat is rejected at the evaporator coils. I would think that the heat would be rejected at the condenser coils.
     
  7. Jan 4, 2007 #6

    russ_watters

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    I'm not sure what you mean by spontaneous. All cycles require an input of work or heat, otherwies there would be no need for them (that's why making cold water in the winter is called "free cooling" - you use only a cooling tower, not a chiller). The point, though, is that if you are using a heat engine to generate mechanical work, you utilize as much of the heat energy as you can and whatever is left gets ejected/rejected. For that application, I don't know that either term is really better.

    For an air conditioner, however, the heat itself is the desired output work and you really do reject it (at the condenser, wrharper - heat is absorbed at the evaporator). You have heat in your refrigerator that you don't want so you use the refrigeration cycle to discard it.
     
  8. Jan 5, 2007 #7
    Thank you, coming from an electrical background this terminology was foreign to me. On a evaporator would it be correct to say that the exterior surface of the coil is absorbing heat from the environment and the interior surface is rejecting heat into the refrigerant? Then if that is true then one could say that the condenser coil is doing the same thing. I know the basics of refrigeration but was confused by the swapping of this terminology in some of my reading.
     
  9. Jan 5, 2007 #8
    Little fast on my post. If the first statement is true then the condensor would would be absorbing heat from the refrigerant and rejecting heat into the environment.
     
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