## Heat Transfer Miscellaneous

#### Heat-Transfer

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

1. 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

1. 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

1. 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

1. 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)

1. 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.