mid sem exercise

8/10/2019 Mid Sem Exercise

1/4

Question 1

In a power plant, pipes are used for transporting superheated vapour. Superheated vapour

is flowing at the rate of 0.3 kg/s inside a pipe with 5 cm diameter and 10 m in length. The pipe is

located in the power plant which is at 20oC and the pipe surface has a uniformly temperature of

100oC. If the temperature drop between the inlet and exit of the pipe is 30oC, and the specific

heat of the vapour is 2190 J/kgK, determine the heat transfer coefficient as a result of

convection between the pipe surface and the surrounding as given in Figure 1.

(5 Marks)

Figure 1


2/4

Question 2

En. Karim assigned you for his furnace renovation project. The furnace wall dimension is 2.7-m

high and 19-cm wide. The aim of the project is to add insulation to the furnace wall to prevent

the inner heat to escape to the atmosphere.

Your analysis is based on section by section. One section of the furnace wall consists of a 15-cm

wide and 25-cm high horizontal bricks (k=0.72 W/m.K) covered with 2-cm thick plaster layers

(k=0.22 W/m.K) on both side of the wall. The upper and bottom side of the bricks are covered

with 1-cm thick plaster layers (k=0.22 W/m.K). The insulation layer is 2-cm rigid foam (k=0.026

W/m.K).

The temperature inside the furnace is 627oC and the outdoor temperatures is 27oC, while the

convection heat transfer coefficients on the inner and the outer sides are h1=3 W/m2.K and

h2=8 W/m2.K, respectively.

Once the insulation done, the furnace will operate 365 days non-stop throughout the year. The

installation work will cost RM70 for material and RM30 for labor. As an engineer assigned to do

the task, determine:

(Assuming one dimensional heat transfer and solve the problem based on the wall area (27cm x 1

m) as shown in the figure to represent the entire furnace wall)

a) draw the complete thermal resistance network diagram according the figure (1 mark)

b) the heat transfer Q within the wall without insulation, (2 marks)

c) the heat transfer Q within the wall with rigid foam insulation, and (3 marks)

d) how long does it take to get back the installation cost from the energy

it saves if the price of natural gas is RM1.10/therm

( 1 therm =105,500 kJ of energy content). (4 marks)


3/4


4/4

Question 3

A hot surface at 110C is to be cooled by attaching 3 cm long (L), 0.25 cm diameter (D)

aluminum pin fins with thermal conductivity, k is 237 W/mC to it, with a center to center

distance (C) of 0.6 cm. The temperature of the surrounding medium is 25C, and the heat

transfer coefficient on the surfaces is 40 W/m2C. Determine:

a) the rate of heat transfer from the surface for a 1 m X 1.5 m section of the

plate

b) the overall effectiveness of the fins.

Discuss your answer in (a) and (b) if the copper fins (k = 386 W/mC) used

in the analysis instead of aluminum ones.

Figure 3

L

D

C

mid sem exercise

Documents