Question

In an oil cooler, oil enters at 160°C. if water enters at 35°C flows parallel to oil, the exit temp of oil and water are 90°C and 70°C respectively. By increasing the length of heat exchanger the min temp to which the oil can be cooled is

• A. 90°
• B. 97.5
• C. 76.7°
• D. 81.2°

Answer 1

The correct option is C 76.7°

Energy balance :

Energy supplied by hot fluid = energy absorbed by cold fluid

${\dot{m}}_0.C_{P0}(T_{hi}-T_{he})={\dot{m}}_w\times C_{pw}(T_{cc}-T_{ci})$

$\frac{{\dot{m}}_0.C_{p0}}{m_w.C_{pw}}=\frac{T_{ce}-T_{ci}}{T_{hi}-T_{he}}=\frac{70-35}{160-90}=\frac{35}{70}=0.5$

Achieveing minimum temperature of hot fulid or achieveing maximum temperature of cold fluid.



Energy balance:

${\dot{m}}_0.C_{p0}(T_{hi}-T_{he})={\dot{m}}_w.C_{pw}(T_{ce}-T_{ci})$

$\frac{{\dot{m}}_0.C_{p0}}{{\dot{m}}_w.C_{pw}}=\frac{T_{ce}-T_{ci}}{T_{hi}-T_{he}}=\frac{T-35}{160-T}$

$80-0.5T=T-35$

$1.5T=115$

$\Rightarrow ={76.67}^{\circ }C$