In a laundry, $67\backslash$deg C dirty wash water is dumped into the drain, and $70\backslash$deg C clean water is required. Presently $15\backslash$deg C water is heated in an electric hot water heater, and the electricity cost is given by AEE. The water is required at a rate of $5000$ Kg$/$h$,12$ hours per day, $312$ days$/$yr$.$ To conserve energy it is proposed to install a heat exchanger to preheat the feed to the electric water heater. The interest rate to amortize the investment over $10$ years is$10\%$ per annum. Taxes and insure are expected to have a fixed cost of $$500$ per annum plus $$50/$yr per square meter of heat exchanger surface Design the heat exchanger and the corresponding net annual savings.