HEAT TRANSFER

1 HEAT TRANSFER
Multiple Choice
1. Water is boiling in a pan on a stove at sea level. During 10 minutes of boiling, it is
observed that 200 grams of water has evaporated. Then the rate of heat transfer to the
water is:
A. 0.84 KJ/min
B. 45.1 KJ/min
C. 41.8 KJ/min
D. 53.5 KJ/min
ANS: B

2. A group of 50 persons attend a secret meeting in a room which is 12 m wide by 10 m

long and a ceiling height of 3 m. The room is completely sealed off and insulated. Each
person gives off 150 kCal per hour of heat and occupies a volume of 0.20 m3. The room
has an initial pressure of 101.3 kPa and temperature of 16°C. Calculate the room
temperature after 10 minutes. Use R = 0.287 kJ/kg-K and Cv = 0.171 kCal/kg-K.
A. 33.1 °C
B. 37.7 °C
C. 38.7 °C
D. 31.7 °C
ANS: A

3. Determine the heat extracted from 2000 kg of water from 25°C to ice at -10°C.
A. 821,120 KJ
B. 621,150 KJ
C. 165,150 KJ
D. 921,150 KJ
ANS: D

4. A creamery must cool 20,000 liters of milk received each day from an initial
temperature of 29°C to a final temperature of 2°C in 5 hours. If refrigeration losses
amount to 10 percent of the cooling load, what must be the capacity of their refrigerating
machine? Note: specific heat of milk is 3.9 kJ/kg-K and SG = 1.05.
A. 38.5 TOR
B. 36.5 TOR
C. 37.5 TOR
D. 39.5 TOR
ANS: A
5. How many tons of refrigeration is required to produce 10 metric tons of ice per
day at -10°C from raw water at 22°C if miscellaneous losses are 15% of the chilling and
freezing load?
A. 17 TOR
B. 20 TOR
C. 15 TOR
D. 24 TOR
ANS: A

6. Fish weighing 11,000 kg with a temperature of 20°C is brought to a cold storage and
which shall be cooled to -10°C in 11 hours. Find the required plant
refrigerating capacity in tons of refrigeration if the specific heat of fish is 0.7 kCal/kg -
°C above freezing point and 0.30 kCal/kg -°C below freezing point. The freezing point is
-3°C. The latent heat of freezing is 55.5 kCal/kg.
A. 25.26
B. 15.26
C. 14.38
D. 24.38
ANS: D

7. If the load on water-cooled condenser is 150,000 Btu/hr and the temperature rise of
the water in the condenser is 10°F, what is the quantity of water circulated in gpm?
A. 30
B. 40
C. 20
D. 50
ANS: A

5. A piston-cylinder device whose piston is resting on top of a set stops initially contains
0.50 kg of helium gas at 100 kpa and 25°C. The mass of the piston is such that 500 kpa
of pressure is required to raise it. How much heat must be transferred to the helium
before the piston starts rising?
A. 1557.13 KJ
B. 1657.13 KJ
C. 1757.13 KJ
D. 1857.13 KJ
ANS: D
6. The load on a water-cooled condenser is 90,000 Btu/hr. If the quantity of water
circulated through the condenser is 15 gpm, determine the temperature rise of the water
in the condenser.
A. 12°F
B. 14°F
C. 16°F
D. 18°F
ANS: A

7. Four thousand liters per hour of distillates are to be cooled from 21°C to -12°C and
12% of wax by weight is separated out at 15°C. The specific heat of oil is 2 kJ/kg°C
and S.G. is 0.87. The specific heat of the wax is 2.5 and the latent heat of fusion is 290
kJ/kg. Allow 10% for the losses, find the capacity of the refrigerating machine.
A. 40 TOR
B. 21.08 TOR
C. 70 TOR
D. 31.08 TOR
ANS: D

2 Single and Composite Wall

Multiple Choice
1. Calculate the heat transfer per hour through a solid brick wall 6 m long, 2.9 m high,
and 225 mm thick, when the outer surface is at 5°C and the inner surface 17°C, the
coefficient of thermal conductivity of the brick being 0.6 W/m-K.
A. 2,004.48 KJ
B. 3,004.48 KJ
C. 2,400.48 KJ
D. 3,400.48 KJ
ANS: A
2. 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 1200°C while that of the
outer. surface of steel is 50°C. The thermal conductivities of the wall material in W/m-K
are; firebrick = 10; insulating brick = 0.26; and steel = 45. Neglecting the film resistances
and contact resistance of joints, determine the heat loss per m 2 of wall area.
A. 1.93 m2
B. 2.93 m2
C. 1.55 m2
D. 2.55 m2
ANS: A

3. A refrigerator is 2 m high, 1.2 m wide and 1 m deep. The over-all heat transfer
coefficient is 0.532 Wm2°C . How many kilograms of 0°C ice will melt per hour if the
inside temperature is maintained at 10°C while the surrounding air temperature is at
35°C?
A. 3.40 kg
B. 1.29 kg
C. 1.60 kg
D. 2.70 kg
ANS: C

4. Calculate the heat loss per linear ft from 2 in nominal pipe. (2.375 in. outside diameter)
covered with 1 in. of an insulating material having an average thermal conductivity of
0.0375 Btu/hr-ft-oF. Assume that the inner and outer surface temperatures of the
insulation are 380oF and 80oF respectively.
A. 110 Btu/hr-ft
B. 116 Btu/hr-ft
C. 120 Btu/hr-ft
D. 126 Btu/hr-ft
ANS: B

5. An insulated steam pipe located where the ambient temperature is 32°C, has an inside
diameter of 50 mm with 10 mm thick wall. The outside diameter of the corrugated
asbestos insulation is 125 mm and the surface coefficient of still air, ho = 12 W/m2 -K.
Inside the pipe is steam having a temperature of 150°C with film coefficient hi = 6000
W/m2 -K. Thermal conductivity of pipe and asbestos insulation are 45 and 0.12 W/m-K
respectively. Determine the heat loss per unit length of the pipe.
A. 110 W
B. 120 W
C. 130 W
D. 140 W
ANS: B
6. A cubical tank of 2 m-sides is constructed of the metal plate 12 mm-and contains
water at 75°C. The surrounding air temperature is 16°C. Calculate the overall heat
transfer coefficient from water to air. Take the coefficient of thermal conductivity of the
metal as 48 W/m-K, the coefficient of heat transfer of water is 2.5 kW/m 2 -K and the
coefficient of heat transfer of the air is.16 W/m 2 -K.
A. 15.84 W/m2°C
B. 14.84 W/m2°C
C. 16.84 W/m2°C
D. 13.84 W/m2°C
ANS: A

7. A hollow sphere has an outside radius of 1 m and is made of polystyrene foam with a
thickness of 1 cm. A heat source inside keeps the inner surface 5.20°C hotter than the
outside surface. How much power is produced by the heat source? The thermal
conductivity of polystyrene foam is 0.033 W/m-°C.
A. 200 W
B. 216 W
C. 300 W
D. 316 W
ANS: B
8. A pipe 200 mm outside diameter and 20 m length is covered with a layer, 70 mm thick
of insulation having thermal conductivity of 0.05 W/m-K and a thermal conductance of
10 W/m2 -K at the outer surface If the temperature of the pipe is 350°C and the ambient
temperature 15°C, calculate the external surface temperature of the lagging.
A. 32.6°C
B. 22.6°C
C. 42.6°C
D. 53.6°C
ANS: A

9. One side of the refrigerated cold chamber is 6 m long by.3.7 m high and consists of
168 mm thickness of cork between outer and inner walls of wood. The-outer wood wall is
30 mm thick and its outside face temperature is 20°C, the inner wood wall is 35 mm
thick .and its inside face temperature is -3°C. Taking the coefficient of thermal
conductivity. of cork and wood as 0.042 and 0.20 W/m-K respectively, calculate the heat
transfer per second per m2 of surface area.
A. 5.318 J
B. 4.318 J
C. 6.318 J
D. 3.318 J
ANS: A

10. Calculate the heat loss per linear foot from a 10 in. normal pipe ( outside diameter =
10.75 in. ) covered with a composite pipe insulation consisting of 1 ½ in. of insulation I
placed next to the pipe and 2 in. of insulation II placed upon insulation I. assume that
the inner and outer surface temperatures of the composite insulation are 700 OF and
110OF respectively, and that the thermal conductivity of material I is 0.05 Btu/hr-ft-OF
and for material II is 0.039 Btu/hr-ft-OF.
A. 126.13 Btu/hr-ft
B. 123.13 Btu/hr-ft
C. 120.13 Btu/hr-ft
D. 323.13 Btu/hr-ft
ANS: D
3 Radiation
Multiple Choice
1. A steam pipe having a surface temperature of 250°C passes through a room
where the temperature is 27°C. The outside diameter of the pipe is 100 mm and
emissivity factor is 0.8. Calculate the radiated heat loss for 3 m pipe length.
A. 1434.47 W
B. 3746.35 W
C. 2852.45 W
D. 3546.45 W
ANS: C

2. Consider a person standing in a breezy room at 20 °C. Determine the total rate
transfer from this person of the exposed surface area and the average outer surface
temperature of the person are 1.6 m2 and 29 °C, respectively. The emissivity of a person
is around ε = 0.95. The convection heat transfer coefficient is 6 W/m2·K.
A. 86.40 watts
B. 81.70 watts
C. 198.1 watts
D. 168.1 watts
ANS: D

3. A 10 cm diameter spherical ball whose surface is maintained at a temperature of

70°C is suspended in the middle of a room at 20°C. If the convection heat transfer
coefficient is 15 W/m2-°C and the emissivity of the surface is 0.8, determine the total
heat transfer from the ball.
A. 23.56 watts
B. 32.78 watts
C. 9.22 watts
D. 43.45 watts
ANS: B

4. A 4 m2 asphalt pavement with emissivity of 0.85 has a surface temperature of 50°C.

Find the maximum rate of radiation that can be emitted from the surface.
A. 2,068.32 watts
B. 2,078.32 watts
C. 2,088.32 watts
D. 2,098.32 watts
ANS: D

5. How many watts will be radiated from a spherical black body 15 cm in diameter at a
temperature of 800°C?
A. 5.34 kW
B. 4.34 kW
C. 6.34 kW
D. 3.34 kW
ANS: A

6. The hot combustion gases of a furnace are separated from the ambient air and its
surrounding which are a 25˚C, by a brick wall 0.15 m thick. The brick has a thermal
conductivity of 1.2 W/m-K and a surface emissivity of 0.80. Under steady state
conditions and outer surface temperature of 100 ˚C is measured. Free convection heat
surface temperature of 100 ˚C is measured. Free convection heat transfer to the air
adjoining this surface is characterized by a convection coefficient of 20 W/m 2-K. What is
the inner temperature in ˚C?
A. 623.7
B. 352.5
C. 461.4
D. 256.3
ANS: B

7. A thin square steel plate, to 10 cm on a side, is heated in a blacksmiths forge to a

temperature of 800°C. If the emissivity is 0.60, what is the total rate of radiation of
energy?
A. 900 watts
B. 450 watts
C. 350 watts
D. 700 watts
ANS: A

8. An uninsulated steam pipe passes through a room in which the air and walls are at
25˚C. The outside diameter of the pipe is 70 mm, and its surface temperature and
emissivity are 200˚C and 0.80 respectively. If the coefficient associated with free
convection heat transfer from the surface to the air is 15 W/m2-K, what is the rate of
heat loss from the surface per unit length of pipe?
A. 997.94 W/m
B. 897.94 W/m
C. 797.94 W/m
D. 697.94 W/m
ANS: A

9. The temperature of the flame in the furnace is 1277˚C and the temperature of its
surrounding is 277˚C. Calculate the maximum theoretical quantity of heat energy
radiated per minute per square meter to the surrounding surface area.
A. 19,321.65 kJ
B. 18,321.65 kJ
C. 17,321.65 kJ
D. 16,321.65 kJ
ANS: A

10. A plate-type solar energy collector with an absorbing surface covered by a glass plate
is to receive an incident radiation of 800 W/m 2. The glass plate has a reflectivity of 0.12
and a transmissivity of 0.85. The absorbing surface has an absorptivity of 0.90. The area
of the collector is 5 m2. How much solar energy in watts is absorbed by the collector?
A. 2500
B. 2880
C. 3510
D. 3060
ANS: D

4 Heat Exchanger
Multiple Choice
1. A counter-flow heat exchanger is designed to heat a fuel oil from 45°C to 100°C while
the heating fluid enters at 150°C and leaves at 115°C. Calculate the arithmetic mean
temperature difference.
A. 40°C
B. 50°C
C. 60°C
D. 70°C
ANS: C

2. An economizer receives hot gas (1.13 kJ/kg-K) and water in ratio of 1.5 kg gas/kg
water. The gas enters at 454°C and leaves at 179°C, the water enters at 48°C. Find
LMTD for parallel flow.
A. 127.62°C
B. 132.49°C
C. 130.45°C
D. 124.78°C
ANS: A

3. A liquid to liquid counter flow heat exchanger is used to heat a cold fluid from 120°F
to 310°F. Assuming that hot fluid enters at 500 °F and leaves at 400°F calculate the log
mean temperature difference for the heat exchanger.
A. 132°F
B. 232°F
C. 332°F
D. 432°F
ANS: B
4. What is the log mean temperature difference in deg. C of a double-pipe counter flow
heat exchanger where a fluid enters at 90 deg. C and exits at 50 deg. C. Inside the inner
pipe another fluid enters at 20 deg. C and exits at 60 deg. C.
A. 0
B. 75
C. 30
D. 35
ANS: C

5. An oil heater heats 100 kg per minute of oil from 35°C to 100°C in a counter flow
heat exchanger. The average specific heat of the oil is 2.5 kJ/kg°C. Exhaust gases
used for heating enter the heater with an average specific heat of 1 kJ/kg°C, a
mass flow rate of 250 kg/min and an initial temperature of 200°C. The over-all heat
transfer coefficient is 75 W/m2°C. Determine the heating surface in m 2.
A. 36.11°C
B. 41.72°C
C. 32.72°C
D. 25.34°C
ANS: A

6. A counter flow bank of boiler tubes has total area of 900 ft 2 and its over-all efficiency
of heat transfer is 13 Btu/hr-ft2-°F. Calculate the heat transferred if the log mean
temperature difference is 1380°F.
A. 16,146,000 Btu/hr
B. 17,147,000 Btu/hr
C. 18,148,000 Btu/hr
D. 15,145,000 Btu/hr
ANS: A

7. A fuel oil of 20oAPI is to be heated in a heater which makes two passes thru heater
tubes and the heating fluid makes one passes but the flow is cross flow through the
heater due to baffles inside the shell.
Quantity of oil to be heated 3000L/hr
Temperature of oil entering heater tubes 21°C
Temperature of oil leaving heater tubes 95°C
Heating fluid, steam enter at 105°C and leaves as condesate at 105°C. Assume specific
heat of oil to be 2.093 kJ/kg-K. Find the heating surface area if the over-all coefficient of
heat transfer is taken as 140 W/m2°C.
A. 24.76 m2
B. 30.75 m2
C. 23.75 m2
D. 32.54 m2
ANS: A

8. A two-pass surface condenser is to be designed using overall heat transfer coefficient

of 480 Btu/°F-ft2 of outside tube surface. The tubes are to be 1 inch outside diameter
with 1/16 in walls (or 7/8 in. inside diameter). Entering circulating water velocity is to be
6 ft/s. Steam enters the condenser at a rate of 100,000 lb/hr at a pressure of one psia
and an enthalpy of 1090 Btu/lb. Condensate leaves at saturated liquid at one psia.
Circulating water enters the condenser at 85 deg. F and leaves at 95 deg F. Note: 1 psia
condensate has temperature of 101.7 deg. F. Wet steam entering becomes condensate at
101.7 deg. F with hf = 69.72 Btu/lb. Calculate the total number of tubes to be used in
each pass.
A. 15,400 tubes
B. 10,450 tubes
C. 11,456 tubes
D. 18,200 tubes
ANS: D

9. A two-pass surface condenser is to be designed using overall heat transfer coefficient

of 480 Btu/°F-ft2 of outside tube surface. The tubes are to be 1 inch outside diameter
with 1/16 in walls (or 7/8 in. inside diameter). Entering circulating water velocity is to be
6 ft/s Steam enters the condenser at a rate of 100,000 lb/hr at a pressure of one psia
and an enthalpy of 1090 Btu/lb. Condensate leaves at saturated liquid at one psia.
Circulating water enters the condenser at 85 deg. F and leaves at 95 deg F. Note: 1 psia
condensate has temperature of 101.7 deg. F. Wet steam entering becomes condensate at
101.7 deg. F with hf = 69.72 Btu/lb. Calculate the total length of the tube to be used in
ft.
A. 742,000 ft
B. 324,000 ft
C. 123,000 ft
D. 543,000 ft
ANS: A

10. A two-pass surface condenser is to be designed using overall heat transfer coefficient
of 480 Btu/°F-ft2 of outside tube surface. The tubes are to be 1 inch outside diameter
with 1/16 in walls (or 7/8 in. inside diameter). Entering circulating water velocity is to be
6 ft/s. Steam enters the condenser at a rate of 100,000 lb/hr at a pressure of one psia
and an enthalpy of 1090 Btu/lb. Condensate leaves at saturated liquid at one psia.
Circulating water enters the condenser at 85 deg. F and leaves at 95 deg F. Note: 1 psia
condensate has temperature of 101.7 deg. F. Wet steam entering becomes condensate at
101.7 deg. F with hf = 69.72 Btu/lb. Calculate the total length of the tube to be used in
ft per tube.
A. 20.4 ft
B. 80.8 ft
C. 17.4 ft
D. 15.5 ft
ANS: A