Heat Transfer Miscellaneous

Heat Transfer Miscellaneous

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Heat-Transfer

Heat Transfer Miscellaneous
  1. Cold water flowing at 0.1 kg/s is heated from 20°C to 70°C in a counter flow type heat exchange by a hot water stream flowing at 0.1 kg/s and entering at 90°C. The specific heat of water is 4200 J/(kgK) and density is 1000 kg/m3 . If the overall heat transfer coefficient U for the heat exchange is 2000 W/(m2 K), the required heat exchange area (in m2 ) is








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    Ch = Cc = 0.1 × 4200 = 420 J/s
    Cc (Tc1 – Tc2) = Ch (Th1 – Th2) (70 – 20) = 90° Th2
    LMTD = θ1 = θ2 (∵ θ1 = θ2)
    Cc (Tc2 – Tc1) = UA(LMTD)
    or A = 0.525 m2.

    Correct Option: B


    Ch = Cc = 0.1 × 4200 = 420 J/s
    Cc (Tc1 – Tc2) = Ch (Th1 – Th2) (70 – 20) = 90° Th2
    LMTD = θ1 = θ2 (∵ θ1 = θ2)
    Cc (Tc2 – Tc1) = UA(LMTD)
    or A = 0.525 m2.

  1. In a concentric counter flow heat exchange, water flows through the inner tube at 25°C and leaves at 42°C. The engine oil enters at 100°C and flows in the annular flow passage. The exit temperature of the engine oil is 50°C. Mass flow rate of water and the engine oil are 1.5 kg/s and 1 kg/s, respectively. The specific heat of water and oil are 4178 J/kgK and 2130 J/kgK, respectively. The effectiveness of this heat exchange is








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    Tc,i = 25°C;
    Tc,o = 42°C;
    mc = 1.5 kg/s;
    Cc = 4.178 kJ/kg.k
    Th,i = 100°C;
    Th,o = 50°C;
    mh = 1 kg/s;
    Ch = 2.130 kJ/kg.k
    mcCc = 1.5 × 4.178
    = 6.267 kW/°C = Cmax
    mhCh = 1 × 2.130
    = 2.130 kW/°C = Cmin

    effectiveness = ε =
    Ch(Th,i - Th,o)
    Cmin(Th,i - Tc,i)

    =
    100 - 50
    		= 0.666
    100 - 25

    Correct Option: A

    Tc,i = 25°C;
    Tc,o = 42°C;
    mc = 1.5 kg/s;
    Cc = 4.178 kJ/kg.k
    Th,i = 100°C;
    Th,o = 50°C;
    mh = 1 kg/s;
    Ch = 2.130 kJ/kg.k
    mcCc = 1.5 × 4.178
    = 6.267 kW/°C = Cmax
    mhCh = 1 × 2.130
    = 2.130 kW/°C = Cmin

    effectiveness = ε =
    Ch(Th,i - Th,o)
    Cmin(Th,i - Tc,i)

    =
    100 - 50
    		= 0.666
    100 - 25

  1. The heat transfer process between body and its ambient is governed by an Intemal Conductive Resistance (ICR) and an External Convective Resistance (ECR). The body can be considered to be a lumped heat capacity system is








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    Lumped heat capacity can be applied when Bi < 0.1
    Bi hl/k, for desired condition i.e. Bi < 0
    k > h, ICR is small

    Correct Option: D

    Lumped heat capacity can be applied when Bi < 0.1
    Bi hl/k, for desired condition i.e. Bi < 0
    k > h, ICR is small

  1. Biot number signifies








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    Biot number = Rconduction/Rconvective

    Correct Option: D

    Biot number = Rconduction/Rconvective

  1. When the fluid velocity is doubled the thermal time constant of a thermometer used of measuring the fluid temperature reduces by a factor of 2 (T/F)








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    It is not necessary that h would become a half if velocity becomes double.

    Correct Option: D

    It is not necessary that h would become a half if velocity becomes double.