With the heaters operating, how long would it take to raise the temperature of a full tank of oil from 20°C to 60°C?
A. 3.2 h
B. 6.3 h
C. 7.5 h
D. 9.0 h
An engineer was instructed to design a holding tank for synthetic lubricating oil. Two requirements were that the amount of time necessary to drain the tank and the force needed to lift the drain plug be minimized. In the initial design, the drain plug, which weighed 500 N, rested on the drain hole and was lifted by a thin rod that extended through the top of the tank. The tank was insulated and had 10 electric immersion heaters that each use 5 kW of power. The oil has a boiling point of 220°C, a specific gravity of 0.7, and a specific heat that is 60% that of water. The heat capacity of the tank was negligible compared to the fluids contained in it.

The tank was built and then tested by filling it with water. The air pressure inside and outside the tank was 1 atm. The force required to lift the plug was found to be 5310 N. In testing the heater capability, the tank was filled to the top with water at 20°C. With all 10 heaters operating, the water temperature reached 100°C 15 h later. The technician who conducted the evaluation reported that the full tank of water was completely discharged approximately 30 s after opening the drain.

Rod for lifting drain phug in plug 500 N Immersion heaters