Reconsider Example 7–2. Another method that could be explored to keep the engine outer surface temperature below the fire hazard limit of 180°C is to increase the insulation thickness. Increase the insulation thickness by threefold and verify if this meted would keep the temperature below the required limit for the safe operation of the engine. Compare the effectiveness of this method with the one used in Example 7–2.

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Heat dissipated from an engine in operation can cause hot spots on its sur- face. If the outer surface of an engine is situated in a place where oil leakage is possible, then when leaked oil comes in contact with hot spots above the oil's autoignition temperature, it can ignite spontaneously. Consider an engine cover that is made of a stainless steel plate with a thickness of 1 cm and a thermal conductivity of 14 W/m-K. The stainless steel plate is covered with a 5-mm-thick insulation (k = 0.5 W/m-K). The inner surface of the engine cover is exposed to hot air at 350°C with a convection heat transfer coefficient of 7 W/m²-K (Fig. 7-14). The 2-m-long engine outer surface is cooled by air blowing in parallel over it at 7 m/s, in an environment where the ambient air is at 60°C. To prevent fire hazard in the event of oil leak on the engine cover, the engine cover surface should be kept below 180°C. It has been determined that the 5-mm-thick insulation layer is not sufficient to keep the engine cover surface below 180°C. To solve this problem, one of the plant supervisors sug- gested adjusting the blower capacity to provide an increase in the cooling air velocity by 10%. Determine if this is a viable method for keeping the engine cover surface below 180°C. Evaluate the air properties at 120°C.