Mechanical Engineering Review Online

1. Determine the thermal conductivity of a material, in W/m-deg C, that is used to a 5 m² test panel, 30 mm thick with a temperature difference of 25^oF between surfaces. During the 5 hrs of test period, the heat transmitted is 480 kJ.

a. 0.02235

b. 0.01152

c. 0.09831

d. 0.08118

2. Sea water for cooling enters a condenser at 30^oC and leaves at 45^oC. The condenser temperature is 50^oC. Determine the log temperature difference.

a. 9.01 deg C

b. 10.82 deg C

c. 12.38 deg C

d.8.71 deg C

3. Steam initially is saturated at 250^oC passed through a 15 cm steel pipe for a total distance of 180 m. the steam line is insulated with material having k=12 W/m-K and thickness of 2.5 cm. The ambient temperature of 28^oC. Calculate the surface film conductance of air if the moisture at the discharge end is 12% and steam rate is 0.15 kg/s. (At 250^oC: h_f=1085 kJ/kg, h_fg=1716.2 kJ/kg, h_g=2801.5 kJ/kg)

a. 2.8 W/m-^oC

b. 3.2 W/m-^oC

c. 1.2 W/m-^oC

d. 3.3 W/m-^oC

4. At an average temperature of 120^oC, hot air flows through a 6 m pipe with outside diameter of 85 mm and pipe thickness of 1 cm. The temperature of the tube along its entire length is 25^oC. If the heat transfer is 600 W, determine the convective film coefficient, in W/m²-^oC.

a. 1.34

b. 2.58

c. 3.11

d. 4.80

5. The hot combustible gases of a furnace are separated from the ambient air and its surrounding, which are 30^oC, by a brick wall 130 mm thick. The brick has a thermal conductivity of 1.33 W/m-K and surface emissivity of 0.85. At steady state conditions the outer surface temperature is 130^oC. Air surface film conductance is 18 W/m²-K. What is the total heat transmitted for 25 square meter area.

a. 66.6 kW

b. 78.3 kW

c. 60.8 kW

d. 58.2 kW

6. A counterflow heat exchanger is designed to heat fuel oil from 28^oC to 130^oC while the heating fluid enters at 140^oC and leaves at 108^oC. The fuel oil is 28^oAPI and has a specific heat of 2.4 kJ/kg-K and enters the heat exchanger at the rate of 1100 gallons per hour. Find the heating surface area, in m² if the overall coefficient of heat transfer is 430 kcal/hr-m²-K.

a. 13.3

b. 14.9

c. 15.8

d. 16.1

7. A 6 in (schedule 80) and 20 ft long, uninsulated B.I. pipe conveys steam at 390^oF with an average ambient temperature of 90^oF. If the cost of the fuel is P230/106 Btu with the net energy conversion efficiency of 80%, what is the annual cost of the heat lost? Let the surface coefficient at steam side, hi=1000 Btu/hr-ft²-^oF and for air side, ho= 2 Btu/hr-ft²-^oF. (For 6 in pipe Schedule 80, for steam use: Do=6.625 in, Di=5.761 in. For Iron: k=30 Btu/hr-ft-^oF)

a. 63122.5

b. 80014.3

c. 58311.1

d. 52163.5

8. Calculate the energy transfer rate across 7” wall of firebrick with a temperature difference across the fiber brick with temperature difference across the wall of 60 deg C. The thermal conductivity of the firebrick is 0.68 Btu/hr-ft-oF at the temperature interest.

a. 300.2 W/m² 

b. 311.6 W/m²

c. 397.0 W/m²

d. 352.1 W/m²

9. Steam, initially saturated at 2.0 MPa, passes through a 10 cm standard steel pipe for a total distance of 150 m. the steam line is insulated with a 5 cm thickness of 85% magnesia. For an ambient temperature of 24^oC, determine the quality of steam which arises at its destination if the mass flow rate is 0.130 kg of steam per second. Properties of steam: At 2.0 MPa and 213.15oC: hf=908.79 kJ/kg, hfg=1890.7. Thermal conductivity, k for 85% magnesia = 0.069 W/m-K, h_o for still air is 9.45 W/m2^-K.

a. 80.2%

b. 93.5%

c. 85.8%

d. 94.3%

10. At an average temperature of 110^oC, hot air flows through a 2.8 m long tube with an inside diameter of 50 mm. The temperature of the tube is 18^oC along its entire length. Convective film coefficient is 20 W/m²-K. Determine the convective heat transfer from air to the tube, in W.

a. 700.3

b. 809.3

c. 750.5

d. 820.3

11. What is the heat transfer in the glass surface area of 0.80 m² having an inside temperature (room) of 28^oC and 15^oC outside temperature (surrounding), The thickness of glass is 0.008 m. The thermal conductivity is 1.9 W/m-K,

a. 1.3 kW

b. 3.5 kW

c. 4.3 kW

d. 2.4 kW

12. A room is 12 ft long and 10 ft wide and 8 ft high. The inside and outside temperatures are 9 deg F and 60 deg F, respectively. Calculate the heat transmitted through the walls if the coefficient of heat transmission is 0.13 Btu/ft²-deg F-hour.

a. 2430.8 Btu

b. 2884.1 Btu

c. 2333.8 Btu

d. 2671.3 Btu

13. 280 kJ of heat flow by conduction from the outside to the inside of a cold storage in one hour. If the temperature and all other conditions are the same, what is the heat flowing through the cold storage in 2.50 hours?

a. 700

b. 650

c. 600

d. 550

14. The hot combustion gases of a furnace are separated from the ambient air and its surrounding which are 28^oC, by a brick wall of 0.16 m thick. The brick has a thermal conductivity of 1.3 W/m-K and surface emissivity of 0.82. Under steady state conditions and outer surface temperature of 100^oC is measured. Free convection heat surface temperature of 100^oC is measured. Free convection heat transfer to the air adjoining this surface is characterized by a convection coefficient of 18 W/m²-K. Determine the inner temperature, in ^oC.

a. 300.2

b. 323.3

c. 405.8

d. 355.2

15. What is the external heating surface area, in square feet of a tube with following dimensions: tube inside diameter=4 in, wall thickness=1/2 in, and length of 18 ft.

a. 18.3

b. 25.1

c. 28.7

d. 23.6

16. A heat exchanger is designed with the following specifications:

Hot gas temp: 1150 deg C
Cold gas temp: 48 deg C
Unit surface conductance on the hot side: 240 W/m²-K
Unit surface conductance on the cold side: 295 W/m²-K
Thermal conductivity of the metal wall: 110 W/m-K
Determine the maximum thickness of the metal wall between the hot gas and cold gas so that the maximum temperature if the wall does not exceed at 550 deg C.

a. 8.3 mm

b. 12.3 mm

c. 10.6 mm

d. 18.2 mm

Answer Key (Handwritten)