Question

Oil is cooled by water in a counter flow heat exchanger. The oil flow rate is 2000 kg/h, entering at ${107}^{o}C$ and leaving at ${30}^{o}C$. Its mean specific heat is 2.51 kJ/kg.K. Water enters at ${15}^{o}C$ and its exit temperature is not to exceed ${80}^{o}C$. The heat transfer coefficient is expected to be $1.5kW/m^{2}K$

The effectiveness of the heat exchanger is

• A. 0.163
• B. 0.5
• C. 0.837
• D. 0.678

Answer 1

The correct option is C 0.837

Energy Balance :
${\dot{m}}_o.C_{po}.(T_{hi}-T_{he})=\dot{m_w}.C_{pw}\left(T_{ce}\right)-T_{ci}$

$0.5556\times 2510(107-30)=\dot{m_w}.C_{pw}.(80-15)$

$\dot{m_w}.C_{pw}=1652.01W/K=C_{max}$

$\dot{m_0}.C_{po}=1394.556W/K=C_{min}$

Effectiveness of heat echanger

$(\in )=\frac{C_{min}(T_{hi}-T_{he})}{C_{min}(T_{hi}-T_{ci})}$

$=\left(\frac{107-30}{107-15}\right)$

$\in =0.8369$