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KONDUKTIVITAS

1. A liquid food (specific heat=4.0 kJ/(kg 0C) flows in the inner pipe of doublepipe heatexchanger. The liquid food enters the heat exchanger at 20 0C and exist at 600C. The flow rate of

the liquid food is 0.5 kg/s. In the annular section, hot water at 900C enters the heat exchanger

and flows countercurrently at a flow rate of 1 kg/s. The average specific heat of water is 4.18

kJ/(kg0C). Assumes steady state conditions

a) Calculate the exit temperature of waterb) Calculate log-mean temperature differencec) If the average overall heat transfer coefficient is 2000W/(m20C) and the diameter of the

inner pipe is 5cm, calculate the length of the heat exchanger

d) Repeat the above calculation for parallel-flow configuration.2. A large block of fish (length = 80 cm, width = 60 cm, and thickness = 40 cm) is being tempered

(heated) so that it can be easily cut into smaller blocks. Initially the uniform temperature of the

fish block is -25C. The fish block is placed in a room at 20C. Determine the center temperature

of the block after 20 hours. The properties of the fish are k = 0.55 W/mC, specific heat = 3800

J/kg C, density = 980 kg/m3. The convective heat transfer coefficient is 50 W/m2C. (Note: the

temperature range in this example does not involve any phase change from ice to water,

therefore it is correct to use the temperature-time charts. When a phase change occurs then we

will need to consider a different approach to solve this problem

3. To cool hot edible oil, an engineer has suggested that the oil be pumped through a pipesubmerged in a nearby lake. The pipe (external diameter = 15 cm) will be located in a horizontal

direction. The average outside surface temperature of the pipe will be 130C. the surrounding

water temperature may be assumed to be constant at 10C. The pipe is 100 m long, and

assume no movement of water. Estimate the convective heat transfer coefficient from the

outside pipe surface into water. Furthermore, determine the rate of heat transfer from the pipe

into water.

4. It is desired to limit the heat loss from a wall of polystyrene foam to 8 J s-1 when thetemperature on one side is 20C and on the other -18C. How thick should the polystyrene be?

[ 17 cm ]

5. Calculate the overall heat-transfer coefficient from air to a product packaged in 3.2 mm of solidcardboard, and 0.1 mm of celluloid, if the surface air heat transfer coefficient is 11 J m

-2s

-1C

-1.

[ 7.29 J m-2s-1 C-1]

6. The walls of an oven are made from steel sheets with insulating board between them of thermalconductivity 0.18 J m

-1s

-1C

-1. If the maximum internal temperature in the oven is 300C and the

outside surface of the oven wall must not rise above 50C, estimate the minimum necessary

thickness of insulation assuming surface heat transfer coefficients to the air on both sides of the

wall are 15 J m-2

s-1

C-1

. Assume the room air temperature outside the oven to be 25C and that

the insulating effect of the steel sheets can be neglected.

[ 10.8 cm ]

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7. Some people believe that because of' its lower thermal conductivity stainless steel is appreciablythermally inferior to copper or mild steel as constructional material for a steam-jacketed pan to

heat food materials. The condensing heat transfer coefficient for the steam and the surface

boiling coefficient on the two sides of the heating surface are respectively 10,000 J m-2s-1C-1

and 700 J m-2s-1 C-1. The thickness of all three metal walls is 1.6 mm. Compare the heating rates

from all three constructions (assuming steady state conditions). [ mild steel 2% worse than

copper, stainless steel 4.5% worse than copper ]

8. A long cylinder of solid aluminium 7.5 cm in diameter initially at a uniform temperature of 5C, ishung in an air blast at 100C. If it is found that the temperature at the centre of the cylinder rises

to 47.5C after a time of 850 seconds, estimate the surface heat transfer coefficient from the

cylinder to the air. [ 25 J m-2

s-1

C-1

]

9. A can of pumpkin puree 8.73 cm diameter by 11.43 cm in height is being heated in a steamretort in which the steam pressure is 100 kPa above atmospheric pressure. The pumpkin has a

thermal conductivity of 0.83 J m-1

s-1

C-1

, a specific heat of 3770 J kg-1

C-1

and a density of 1090

kg m-3

. Plot out the temperature at the centre of the can as a function of time until this

temperature reaches 115C if the temperature in the can prior to retorting was 20C.

[ at 70 min, 111C; at 80 min, 116C ; 79 min for 115C ]

10.10. A steam boiler can be represented by a vertical cylindrical vessel 1.1 m diameter and 1.3 mhigh, and it is maintained internally at a steam pressure of 150 kPa. Estimate the energy savings

that would result from insulating the vessel with a 5 cm thick layer of mineral wool assuming

heat transfer from the surface is by natural convection. The air temperature of the surroundings

is 18C and the thermal conductivity of the insulation is 0.04 J m-1

s-1

C-1

. [ 83% ]

11.11. It is desired to chill 3 m3of water per hour by means of horizontal coils in which ammonia isevaporated. The steel coils are 2.13 cm outside diameter and 1.71 cm inside diameter and the

water is pumped across the outside of these at a velocity of 0.8 m s-1

. Estimate the length of

pipe coil needed if the mean temperature difference between the refrigerant and the water is

8C, the mean temperature of the water is 4C and the temperature of the water is decreased

by 15C in the chiller.