INTMECH LECTURE 6

Calculate the energy transfer rate across 6 inches wall of firebrick with a temperature difference across the wall of 50oC. The thermal conductivity of the firebrick is 0.65 BTU/hr-ft-oF at the temperature of interest.

• 285 W/m2

• 369 W/m2

• 112 W/m2

• 429 W/m2

Answer explanation

A vertical furnace wall is made up of an inner wall of firebrick 20 cm thick followed by insulating brick 15 cm thick and an outer wall of steel 1 cm thick. The surface temperature of the wall adjacent to the combustion chamber is 1200oC while that of the outer surface of steel is 50oC. The thermal conductivities of the wall material in W/Mk are: firebrick (10), insulating brick (0.26), and steel (45). Determine the heat loss per sq. m. of wall area.

• 1.93 W/m2

• 2.93 W/m2

• 1.55 W/m2

• 2.55 W/m2

Answer explanation

A composite wall is made up of an external thickness of brickwork 110 mm thick inside which is a layer of fiberglass 75 mm thick. The fiberglass is faced internally by an insulating board 25 mm thick. The coefficient of thermal conductivity for the three are as follows:
The surface transfer coefficients of the inside wall is 3.1 W/m2-K while that of the outside wall is 2.5 W/m2-K. Take the internal ambient temperature as 10C and the external temperature is 27C. Determine the heat loss through such wall 6m high and 10m long. (HW)

• 230.10 W

• 330.10 W

• 430.10 W

• 530.10 W

How many Watts will be radiated from a spherical black body 15cm in diameter at a temperature of 800C?

• 5.34 W

• 4.34 W

• 6.34 W

• 3.34 W

Answer explanation

A liquid to liquid parallel flow heat exchanger is used to heat a cold fluid from 120℉ to 310℉. Assuming that the hot fluid enters at 500℉ and leaves at 400℉, calculate the (a) log mean temperature difference and (b) arithmetic mean temperature difference for the heat exchanger.

• 201.34F; 235F

• 201.34F; 245F

• 210.34F; 235F

• 210.34F; 245F

Answer explanation

A Carnot refrigeration cycle absorbs heat at -12C and rejects it at 40C. Calculate the coefficient of performance of this refrigeration cycle.

• 5.02

• 7.02

• 0.2

• 12.02

A Carnot refrigeration cycle absorbs heat at -12C and rejects it at 40C. If the cycle is absorbing 15 kW at the -12C temperature, how much power is required?

• 5.02

• 3.33

• 0.33

• 2.99

A Carnot refrigeration cycle absorbs heat at -12C and rejects it at 40C. If a Carnot heat pump operates between the same temperatures as this refrigeration cycle, what is the performance factor?

• 5.02

• 6.02

• 0.02

• 1.02

A Carnot refrigeration cycle absorbs heat at -12C and rejects it at 40C. If a Carnot heat pump operates between the same temperatures as this refrigeration cycle, what is the rate of heat rejection at the 40C temperature if the heat pump absorbs 15 kW at the -12C temperature

• 8.22

• 1.799

• 17.99

• 20.99

A refrigeration system operates on the reversed Carnot cycle. The minimum and maximum temperatures are -13F and 162F, respectively. If the heat rejected at the condenser is 95 BTU/s, find the power input required in BTU/min.

• 1503

• 1603

• 1703

• 1803