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Heat pumps

  1. Oct 2, 2005 #1
    I'm having real trouble understanding the working process of a heat pump. A liquid with a certain temp flows in a system and gets vaporized, then it is compressed so that pressure and temp increase. Then it is cooled by a substance with lower temp so that it becomes a liquid again, and this other substance gets warmer. Now the liquid flows through let's say sea water and gets even colder...? Finally the liquid flows back into the vaporizer and starts on a new round. That's basically how I've understood the process, but I'm not at all sure about it. Can someone verify?

    My second problem is when one is not after heat, but cold. One starts to use refrigerants. How do these substances work? Do they work the opposite way of water and boil at -30C and freeze at +30, or what? I'm totally confused here...
     
  2. jcsd
  3. Oct 2, 2005 #2
    Whether heating or cooling application, still you require refrigerants. In cooling application, the atmosphere becomes the sink(where you reject the heat) and the controlled space a source(where you absorb the heat). In heating application, the controlled space becomes sink and the atmosphere a source. Diverting valves are used in both gas and liquid lines to route the fluid to the either of the heat exchangers.
     
  4. Oct 3, 2005 #3

    russ_watters

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    Staff: Mentor

    Your cycle doesn't sound right: sounds like you have two heat rejection points.

    Put simply, a heat pump is an air conditioner with the two coils reversed. To make it as simple as possible, consider an air-source heat pump. With an air-source heat pump in air conditioner mode, the hot gas goes to a coil outside at, say, 130F and is cooled/condensed. Then it is expanded, which cools it to around 40F and goes into the coil in your air conditioner to cool the air.

    If you reverse the flow of refrigerant, the coil inside becomes hot and the coil outside becomes cold.

    edit: being from Norway, I guess you're probably not all that familiar with air conditioning. I hope my explanation worked for you. If it helps, a refrigerator uses the same cycle as an air conditioner: the coil in the back gets warm and the coil inside gets cold.
     
  5. Oct 3, 2005 #4
    indeed,

    as russ_watters said, don't forget to release heat into the building and to capture heat later in the cycle when the fluid has been cooled under the heat source temperature

    the cycle is as follows, for example:

    - the fluid is at -10°C
    - the fluid flows in heat-exchangers pipes 1 meter under your garden ground
    - it is heat up to the ground temperature, say 10°C
    - note here that the fluid gained some energy E1
    - the fluid is directed to the house with ambient temperature at 19°C
    - note that it cannot heat up the house till now
    - the fluid is compressed and gains a further amount of energy E2
    - because of the energy received, it is heated up to 30°C
    - now the fluid can heat up the house by going through the convectors
    - its temperature decreases to 20°C
    - note that its pressure has not yet changed significantly
    - now the fluid goes through a turbine (or a valve) where its pressure goes back to the low value
    - it is important to note that the turbine (or valve) should produce some work in order to take energy off from the fluid
    (assuming here an ideal gas, more discussion otherwise)
    - now the fluid is at -10°C and the cycle can repeat

    It is important to observe that the energy consumption is E2, while the available heating energy is E1+E2. Therefore, it is possible to have an efficiency significantlty higher that 100%. Note also that radiatiors would need higher temperatures (say 60°C) than convectors and this would decrease the efficiency since you would need a higher compression to reach 60°C.

    Now I have this question: what is a "refrigerant" fluid ?
     
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