11. Moist warm air is to be cooled and dehumidified to a wetbulb temperature of 30ºC using water of 25ºC in a dehumidification column. The air enters the tower with a dry-bulb temperature of 85ºC and wet-bulb temperature of 45ºC at 4800 kg/m2 h rate. The overall gas phase mass transfer coefficient is estimated to be 2150 kg/ m3 h. The flow rate of water is 1.4 times the minimum. Calculate height of the tower.
12. A 10 mm diameter electrical conductor is covered by an insulation of 2 mm thickness. The conductivity of the insulation is 0.08 W/m-K and the convection coefficient at the insulation surface is 10 W/m2 -K. Addition of further insulation of the same material will
a) Increase heat loss continuously
b) Decrease heat loss continuously
c) Increase heat loss to a maximum then decrease heat loss
d) Decrease heat loss to a maximum then increase heat loss

13. A pipe of outside diameter 20 mm is to be insulated with asbestos which has a mean
thermal conductivity of 0.1 W/m degree. The local coefficient of convective heat to the surroundings is 5 W/square meter degree. Find the critical radius of insulation for optimum heat transfer from pipe?

a) 10 mm b) 20 mm c) 30 mm d) 40 mm

14. If the thickness of the material through which heat is transferred is increased by a factor of two, then the rates of heat transfer:
a. Increases by a factor of 4
b. Decreases by a factor of 4
c. Increases by a factor of 2
d. Decreases by a factor of 2

15. In conduction process the molecules of the solid pass the heat from one to the other:
a. Without moving themselves from their positions
b. With moving themselves from one place to another

c. Without any medium
d. None of the above

16. Heat transfer by radiation mainly depends upon
a. Its temperature
b. Nature of the body
c. Kind and extent of its surface
d. All of the above

17. A nickel-coated heater element with a thickness of 15 mm and a thermal conductivity of 50 W /m. K is exposed to saturated water at atmospheric
pressure. A thermocouple is attached to the back surface, which is well insulated. Measurements at a particular operating condition yield an electrical power dissipation in the heater element of
6.950 × 107 W /m3 and a temperature of To = 266.4o C

18. A thin plastic membrane is used to separate Helium from a gas stream. Under state conditions, the concentration of helium in the membrane is known to be 0.02 and 0.005 kmol/m3 at the inner and outer surfaces, respectively. If the membrane is 1 mm thick and the binary diffusion coefficient of helium with respect to the plastic is 10-9 m3/s, what is the diffusion flux?

19. Oxygen is maintained at pressures of 2 bars ans 1 bar on opposite sides of a rubber membrane that is 0.5 mm thick, and the entire system is at 25 C. What is the molar diffusion flux of O2 through the membrane? What are the molar concentrations of O2 on both sides of the membrane (outside the rubber)?