The Physics
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Opus in profectus

Refrigerators

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Problems

practice

  1. Determine the following quantities for a window mounted, residential heat pump described in the table below:
    1. the heat…
      1. extracted from the environment per hour when the device is operated in heating mode
      2. exhausted to the environment per hour when the device is operated in cooling mode
    2. the real coefficient of performance…
      1. in heating mode
      2. in cooling mode
    3. the ideal coefficient of performance for a 22 °C inside temperature and…
      1. a −2 °C outside winter temperature
      2. a +34 °C outside summer temperature
    4. the velocity of the air blowing into the room when the device is operated in either mode
    Residential heat pump
    heating mode cooling mode both modes
    heating capacity
    7.9 MJ/hour    		 cooling capacity
    10 MJ/hour    		 volts rated
    115 volts
    heating amps
    7.6 amps    		 cooling amps
    7.8 amps    		 air circulation
    8.1 m3/minute
    heating watts
    743 watts    		 cooling watts
    812 watts    		 width
    66 cm
    moisture removal
    0.90 liter/hour    		 height
    40 cm
    depth
    74 cm
  2. Write something else.
  3. Write something different.
  4. Write something completely different.

numerical

  1. One millimole of a monatomic, ideal gas is run through a four-step cycle as shown in the table below.
    1. Sketch the cycle on a PV graph.
    2. Determine the absolute temperature of the gas when it is in states A, B, C, and D.
    3. Determine ΔU, Q, and W for each segment of the cycle (A→B, B→C, C→D, D→A) and for the cycle as a whole (ABCDA). Include an algebraic sign with each answer. (Zero values do not get a sign.)
    4. Determine the real coefficient of performance assuming the cycle is used as a refrigerator.
    5. Determine the ideal coefficient of performance for this refrigerator.
    6. How are the answers to the last two questions in apparent conflict? Why are they?
    A four step closed cycle
    state A B C D
    P (kPa) 100 624 312 100
    V (m3) 24 × 10−6 8 × 10−6 8 × 10−6 16 × 10−6
    T (K)
    path A→B B→C C→D D→A ABCA
    descrip­tion adia­batic iso­choric adia­batic iso­baric whole cycle
    ∆U (J)
    Q (J)
    W (J)