Question

A counter flow shell - and - tube excitor is used to heat water with hot exhaust gases The water $(C_p=4180J/kg°C)$ flows at a rate of 2 kg/s while the exhaust gas (1030J/kg°C) flows at the rate of 5.25kg/s. If the heat transfer surface area is $32.5m^2$ and the overall heat transfer coefficient is $200W/m^2°C$, what is the NTU for the heat exchanger?

• A. 1.2
• B. 4.5
• C. 2.4
• D. 8.6

Answer 1

The correct option is A 1.2

$m_w=2kg/s$

$C_{pw}=4180J/kgK$

$C_{pg}=1030J/kgK$

$m_g=5.25kg/s$

$A_s=32.5m^2$

$U=200W/m^{2}K$

$\left({\dot{m}}_w{C}_{pw}\right)=2\times 4180=8360W/K$

$({\dot{m}}_g.C_{pg})=5.25\times 1030=5407.5W/K$

$C_{min}=({\dot{m}}_g.C_{pg})=5407.5W/Kg$

$NTU=\frac{UA}{C_{min}}=\frac{200\times 32.5}{5407.5}=1.2$