II. PROBLEMAS Prob. 01. Furnace Efficiency Improvement. It is common practice in industry to recover thermal energy from the flue gas of a furnace. One method of using this thermal energy is to preheat the furnace combustion air with a heat exchanger that transfers heat from the flue gas to the combustion air stream. Design such a heat exchanger assuming that the furnace is fired with natural gas at a rate of 5 MW, uses combustion air at a rate of 90 standard cubic feet per second, and is 75% efficient before heat recovery is employed. Using the first law of thermodynamics, determine the temperature of the flue gas leaving the furnace before the heat exchanger is installed. Then determine the best design for the heat exchanger and calculate the outlet temperatures for both streams. The most important considerations will be capital cost of the heat exchanger, its maintenance costs, and the pressure drop on both the air side and the flue gas side.