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4.4: Friji na Pampu za joto

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    176200

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    Malengo ya kujifunza

    Mwishoni mwa sehemu hii, utaweza:

    • Eleza jokofu na pampu ya joto na uorodhe tofauti zao
    • Tumia coefficients ya utendaji wa friji rahisi na pampu za joto

    Mzunguko tuliotumia kuelezea inji katika sehemu iliyotangulia wote hurekebishwa, hivyo kila mlolongo wa hatua unaweza kufanywa kwa urahisi kwa upande mwingine. Katika kesi hiyo, inji inajulikana kama jokofu au pampu ya joto, kulingana na kile ambacho ni lengo: joto limeondolewa kwenye hifadhi ya baridi au joto limetupwa kwenye hifadhi ya moto. Aidha jokofu au pampu ya joto ni inji inayoendesha kinyume.

    • Kwa jokofu, lengo ni kuondoa joto kutoka eneo fulani.
    • Kwa pampu ya joto, lengo ni juu ya kutupa joto kwenye eneo fulani.

    Sisi kwanza tunazingatia jokofu (Kielelezo\(\PageIndex{1}\)). Madhumuni ya inji hii ni kuondoa joto kutoka kwenye hifadhi ya baridi, ambayo ni nafasi ndani ya jokofu kwa jokofu halisi ya kaya au nafasi ndani ya jengo kwa kitengo cha hali ya hewa.

    Takwimu inaonyesha schematic ya jokofu au pampu ya joto na mshale wa juu Q subscript c katika T subscript c Wakati hii inakwenda kupitia jokofu au pampu, mshale W huongezwa kutoka kulia na matokeo mshale wa juu ni Q subscript h katika T subscript h.
    Kielelezo\(\PageIndex{1}\): A schematic representation of a refrigerator (or a heat pump). The arrow next to work (W) indicates work being put into the system.

    A refrigerator (or heat pump) absorbs heat \(Q_c\) from the cold reservoir at Kelvin temperature \(T_c\) and discards heat \(Q_h\) to the hot reservoir at Kelvin temperature \(T_h\) while work W is done on the engine’s working substance, as shown by the arrow pointing toward the system in the figure. A household refrigerator removes heat from the food within it while exhausting heat to the surrounding air. The required work, for which we pay in our electricity bill, is performed by the motor that moves a coolant through the coils. A schematic sketch of a household refrigerator is given in Figure \(\PageIndex{2}\).

    The figure shows schematic diagram and working of a refrigerator.
    Figure \(\PageIndex{2}\): A schematic diagram of a household refrigerator. A coolant with a boiling temperature below the freezing point of water is sent through the cycle (clockwise in this diagram). The coolant extracts heat from the refrigerator at the evaporator, causing coolant to vaporize. It is then compressed and sent through the condenser, where it exhausts heat to the outside.

    The effectiveness or coefficient of performance \(K_R\) of a refrigerator is measured by the heat removed from the cold reservoir divided by the work done by the working substance cycle by cycle:

    \[K_R = \dfrac{Q_c}{W} = \dfrac{Q_c}{Q_h - Q_c}\]

    Note that we have used the condition of energy conservation, \(W = Q_h - Q_c\), in the final step of this expression.

    The effectiveness or coefficient of performance \(K_P\) of a heat pump is measured by the heat dumped to the hot reservoir divided by the work done to the engine on the working substance cycle by cycle:

    \[K_P = \dfrac{Q_h}{W} = \dfrac{Q_h}{Q_h - Q_c}.\]

    Once again, we use the energy conservation condition \(W = Q_h - Q_c\) to obtain the final step of this expression.