Question

A continuous flow water heater (geyser) has an electrical power rating $=2$ $kW$ and efficiency of conversion of electrical power into heat $=80\%$. If water is flowing through the device at the rate of $100$ $cc/sec$, and the inlet temperature is ${10}^0$,then the outlet temperature will be:

• A. ${12.2}^{o}C$
• B. ${13.8}^{o}C$
• C. ${20}^{o}C$
• D. ${16.5}^{o}C$

Answer 1

The correct option is B ${13.8}^{o}C$

Given:
$P=2$ $kW$
Efficiency $\left(E\right)=80\%$
$V_{in}=100$ $cc/sec$
$T_{in}={10}^{o}C$
Density of $H_{2}O=1$ $g/c{m}^3$
$\therefore$ Mass of $H_{2}O=100$ $g$
Efficient power $=\frac{80}{100}\times 2$ $kW=1.6$ $kW=1600$ $W$
This power goes into increasing $T$ of water
$\therefore H=mC\Delta T$
$\Delta T=\frac{H}{mC}=\frac{1600}{4.186\times 100}={3.8}^{o}C$
$T_f=T_{in}+\Delta T={13.8}^{o}C$