HEAT EXCHANGERS OBJECTIVE QUESTIONS (GATE, IES, IAS)PREVIOUS 20-YEA...

8. Heat Exchangers

O

^BJECTIVE

Q

^UESTIONS

(GATE, IES, IAS)

Previous 20-Years GATE Questions Types of Heat Exchangers

GATE-1. In a counter flow heat exchanger, for the hot fluid the heat capacity = 2

kJ/kg K, mass flow rate = 5 kg/s, inlet temperature = 150°C, outlet

temperature = 100°C. For the cold fluid, heat capacity = 4 kJ/kg K, mass

flow rate = 10 kg/s, inlet temperature = 20°C. Neglecting heat transfer to

the surroundings, the outlet temperature of the cold fluid in °C is:

[GATE-2003]

(a) 7.5

(b) 32.5

(c) 45.5

(d) 70.0

Logarithmic Mean Temperature Difference (LMTD)

GATE-2. In a condenser, water enters at 30°C and flows at the rate 1500 kg/hr.

The condensing steam is at a temperature of 120°C and cooling water

leaves the condenser at 80°C. Specific heat of water is 4.187 kJ/kg K. If

the overall heat transfer coefficient is 2000 W/m

²

K, then heat transfer

area is:

[GATE-2004]

(a) 0.707 m

²

(b) 7.07 m

²

(c) 70.7 m

²

(d) 141.4 m

²

GATE-3. The logarithmic mean temperature difference (LMTD) of a counterflow

heat exchanger is 20°C. The cold fluid enters at 20°C and the hot fluid

enters at 100°C. Mass fl0w rate of the cold fluid is twice that of the hot

fluid. Specific heat at constant pressure of the hot fluid is twice that of

the cold fluid. The exit temperature of the cold fluid

[GATE-2008]

(a) is 40°C

(b) is 60°C

(c) is 80°C

(d) Cannot be determined

GATE-4. In a counter flow heat exchanger, hot fluid enters at 60°C and cold fluid

leaves at 30°C. Mass flow rate of the hot fluid is 1 kg/s and that of the

cold fluid is 2 kg/s. Specific heat of the hot fluid is 10 kJ/kgK and that of

the cold fluid is 5 kJ/kgK. The Log Mean Temperature Difference

(LMTD) for the heat exchanger in °C is:

[GATE-2007]

(a) 15

(b) 30

(c) 35

(d) 45

GATE-5. Hot oil is cooled from 80 to 50°C in an oil cooler which uses air as the

coolant. The air temperature rises from 30 to 40°C. The designer uses a

LMTD value of 26°C. The type of heat exchanger is:

[GATE-2005]

(a) Parallel flow

(b) Double pipe

(c) Counter flow

(d) Cross flow

Page 62 of 97

GATE-6. For the same inlet and outlet temperatures of hot and cold fluids, the

Log Mean Temperature Difference (LMTD) is:

[GATE-2002]

(a) Greater for parallel flow heat exchanger than for counter flow heat

exchanger.

(b) Greater for counter flow heat exchanger than for parallel flow heat

(c) Same for both parallel and counter flow heat exchangers.

(d) Dependent on the properties of the fluids.

GATE-7. Air enters a counter flow heat exchanger at 70°C and leaves at 40°C.

Water enters at 30°C and leaves at 50°C. The LMTD in degree C is:

[GATE-2000]

(a) 5.65

(b) 4.43

(c) 19.52

(d) 20.17

Heat Exchanger Effectiveness and Number of Transfer Units (NTU)

GATE-8. In a certain heat exchanger, both the fluids have identical mass flow

rate-specific heat product. The hot fluid enters at 76°C and leaves at

47°C and the cold fluid entering at 26°C leaves at 55°C. The

effectiveness of the heat exchanger is:

[GATE-1997]

GATE-9. In a parallel flow heat exchanger operating under steady state, the

heat capacity rates (product of specific heat at constant pressure and

mass flow rate) of the hot and cold fluid are equal. The hot fluid,

flowing at 1 kg/s with C

p

= 4 kJ/kgK, enters the heat exchanger at 102°C

while the cold fluid has an inlet temperature of 15°C. The overall heat

transfer coefficient for the heat exchanger is estimated to be 1 kW/m

²

K

and the corresponding heat transfer surface area is 5 m

²

. Neglect heat

transfer between the heat exchanger and the ambient. The heat

exchanger is characterized by the following relation: 2

ε

= 1 – exp

(–2NTU). [GATE-2009]

The exit temperature (in °C) for - the cold fluid is:

(a) 45

(b) 55

(c) 65

(d) 75

Previous 20-Years IES Questions

IES-1.

Air can be best heated by steam in a heat exchanger of

[IES-2006]

(a) Plate type

(b) Double pipe type with fins on

steam side

(c) Double pipe type with fins on air side (d) Shell and tube type

IES-2.

Which one of the following heat exchangers gives parallel straight line

pattern of temperature distribution for both cold and hot fluid?

(a) Parallel-flow with unequal heat capacities

[IES-2001]

(b) Counter-flow with equal heat capacities

(c) Parallel-flow with equal heat capacities

(d) Counter-flow with unequal heat capacities

Page 63 of 97

IES-3.

For a balanced counter-flow heat exchanger, the temperature profiles

of the two fluids are:

[IES-2010]

(a) Parallel and non-linear

(b) Parallel and linear

(c) Linear but non-parallel

(d) Divergent from one another

IES-4.

Match List-I (Heat exchanger process) with List-II (Temperature area

diagram) and select the correct answer:

[IES-2004]

List-I

A. Counter flow sensible heating

B. Parallel flow sensible heating

C. Evaporating

D. Condensing

Codes:

A B C D

A B C D

(a)

3

4

1

2

(b)

3

2

5

1

(c)

4

3

2

5

(d)

4

2

1

5

IES-5.

The temperature distribution

curve for a heat exchanger as

shown in the figure above

(with usual notations) refers

to which one of the following?

(a) Tubular parallel flow heat

exchanger

(b) Tube in tube counter flow

heat exchanger

(c)

Boiler

(d)

Condenser

[IES-2008]

IES-6. Consider the following statements:

[IES-1997]

The flow configuration in a heat exchanger, whether counterflow or

otherwise, will NOT matter if:

Page 64 of 97