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    Sunday, February 22, 2015

    What will be the change in room temperature if door of refrigerator kept open?

    What will be the change in room temperature if the door of the refrigerator kept open? This question is often asked at a mechanical engineering job interview and also in engineering examination.

    Ans: NO, Rather, the temperature of the room will increase over time, not instantly.


    To understand this, let us first understand the working of the domestic refrigerator. It contains four different equipment namely compressor, condenser, capillary tube and evaporator and it also has refrigerant or working fluid/medium which continuously circulating through all these four different equipment to enable heat transfer and thus cooling. The refrigerant while passing through different parts, changes phase and this enable cooling in a domestic refrigerator.

    Construction and Working of domestic refrigerator

    The evaporator is a heat exchanger that removes the heat from stuff or foods to be cooled by evaporating (Evaporation condition: BP < Evaporation temperature) the low-pressure liquid refrigerant while passing through it. At low pressure, Boiling Point (BP) of refrigerant is low, well below room temperature, thus when exposed to room temperature, liquid refrigerant starts evaporating by taking heat from foods, thus foods get cooled). Once the complete evaporation of refrigerant takes place, It has now no more capacity to absorb the heat from foods. And again it needs low-pressure liquid form to gets evaporated.

    Refrigerant can only get condensed (liquid form) in the condenser (at room temperature condition). But at room temperature condition, the refrigerant is well above its BP, so it cannot be converted to lower phase i.e. liquid phase and will remain in vapor form (Room temperature > BP of refrigerant). 

    Hence, it needs higher BP than room temperature so that condensation can take place at room temperature. BP is a function of pressure, higher the pressure of the refrigerant, higher the BP and vice versa. Thus, Compressor is used to raise the pressure of low-pressure vapor refrigerant. At the outlet of the compressor, vapor refrigerant is in high pressure and high-temperature condition. Due to high pressure, BP of refrigerant is now well above the room temperature and it can now easily condensed in the condenser by rejecting heat to room temperature air (Condensation condition: BP > Condensation temperature). The amount of heat is now summation of heat taken at evaporator plus heat of compression at the compressor.

    Thus, at the outlet of condenser, the refrigerant is at high pressure and is in liquid form. But again, to get lower boiling point well below the room temperature, this high pressure should be again reduced to low pressure condition of evaporator. To reduce the pressure, high-pressure liquid refrigerant is now passed through the capillary tube which has high length and lowers inside diameter. Thus while passing through the capillary tube, due to friction, the pressure is reduced and partial cooling takes place as boiling point reduction starts (along with evaporation).

    Now, low-pressure liquid refrigerant (having BP is well below the room temperature) enters the evaporator and starts evaporating at room temperature by taking the heat of foods to reduce its temperature. And CYCLE continues. 

    Let us analyze the situation. 

    Q1 = Heat absorb at evaporator
    Q2 = Heat rejected at condenser
     W = Work of compressor

    The relation is now, Q2 = Q1+W
    Thus, Q2 > Q1

    In closed-door situation, Q2 > Q1 (as Q1 will be absorbed from evaporator compartment only)
    Thus, in a closed-door situation, there is no change in the room temperature as usual.

    In the OPEN door situation, Q1 will be an addition of heat taken from foods and the entire room, which eventually increases the Q2. So, there will be an increase in the temperature of the room if door is kept open. 

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