Monday, November 23, 2020

GATE Chemical Engineering Questions and Answers-Heat Transfer


A multiple effect evaporator as compared to a single effect evaporator of the same capacity has

(A) Lower heat transfer area

(B)  Lower steam economy

(C)  Higher steam economy

(D) Higher solute concentrations in the product

                                                                                                           (GATE-1998)

 

Answer:  (C)

 

The advantage of using a 1 – 2 shell and tube heat exchanger over a 1 – 1 shell and tube heat exchanger is

(A) Lower tube side pressure drop

(B)  Lower shell side pressure drop

(C)  Higher tube side heat transfer

(D) Higher shell side heat transfer

                                                                                                           (GATE-1998)

 

Answer:  (C)

 

For a counter-current heat exchanger with `T_h^i=80℃,T_c^0=60℃,T_h^0=50℃andT_c^i=30℃` and the temperature difference between the two streams being the same everywhere along z, the direction of flow of the hot fluid, the temperature profile should satisfy

        (A) d2T/dZ2>0

        (B) d2T/dZ2=0

        (C) d2T/dZ2<0

        (D) dT/dZ=0    

                                                                                                                (GATE-1999)


Answer:  (B)




In a pipe flow, heat is transferred from hot wall to the liquid by                         

(A) Conduction only

(B)  forced convection only

(C)  forced convection and conduction

(D) free and forced convection

                                                                                                           (GATE-1999)

 

Answer:  (C)

 

As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer coefficient                                                                                                            

(A) Continues to increase

(B)  Continues to decrease

(C)  Goes through a minimum

(D) Goes through a maximum

                                                                                                           (GATE-1999)

 

Answer:    (D)

 

Gibb’s phase rule finds application when heat transfer occurs by                                               

(A)    conduction

(B)    convection

(C)    radiation 

(D)    condensation

                                                                                                           (GATE-1999)

 

Answer:    (D)

 

Heat transfer occurs by natural convection because change in temperature causes differences in

(A)    Viscosity

(B)    density

(C)    thermal conductivity

(D)    heat capacity

                                                                                                           (GATE-1999)

 

Answer:  (B)

 

The Grashof number is defined as the ratio of                                                    

(A)     Buoyancy to inertial force

(B)      Buoyancy to viscous forces

(C)      Inertial forces to viscous forces

(D)     Buoyancy to surface tension forces

                                                                                                           (GATE-1999)

 

Answer:  (B)

 

For turbulent flow in a tube, the heat transfer coefficient is obtained from Dittus-Boelter correlation. If the tube diameter is halved and the flow is doubled, then the heat transfer coefficient will change by a factor of

(A)    1 

(B)    1.74

(C)    6.1

(D)    37

                                                                                                           GATE 2000

 

Answer:  (C)

 

The Sieder-Tate correlation for heat transfer in turbulent flow in a pipe gives Nu α Re0.8  where Nu is the Nusselt number and Re is Reynolds number for the flow. Assuming that this relation is valid, the heat transfer coefficient varies with the pipe diameter (D) as                                                                    

(A)    D-1.8

(B)    D-0.2

(C)    D0.2 

(D)    D1.8

                                                                                                           GATE 2001

 

Answer:    (B)

 

Heat transfer by natural convection is enhanced in systems with                                               

(A)     High viscosity

(B)      High coefficient of thermal expansion

(C)      Low temperature gradients

(D)     Low density change with temperature

                                                                                                           (GATE-2001)

 

Answer:  (B)

 

The overall heat transfer coefficient for a shell and tube heat exchanger for clean surface is U0=400 W/m2K. The fouling factor after a year of operation is found to be hdo = 2000 W/m2K. The overall heat transfer coefficient at this time is                                                

(A)    1200 W/m2K

(B)    894 W/m2K

(C)    300 W/m2K

(D)    287 W/m2K

                                                                                                           (GATE-2001)

 

Answer:    (C)

 

The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by                                                                  

(A)     Painting the surface black

(B)      Painting the surface white

(C)      Giving the surface a mirror finish

(D)     Roughening the surface

                                                                                                           (GATE-2001)

 

Answer:    (C)

 

In thermal radiation, for a black body                                                                             

(A) α = 1, ε ≠ 1

(B)  α ≠ 1, ε = 1

(C)  α ≠ 1, ε ≠ 1

        (D) α = 1, ε = 1

                                                                                                           (GATE-2001)

 

Answer:    (D)

 

If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid

A.     Remains unchanged

B.     Increases

C.     Increases or decreases depending on number of shell passes

D.     Decreases

                                                                                                      GATE 2002

 

Answer:    (D)

 

Air flows through a smooth tube, 2.5 cm diameter and 10 m long, at 37°C. If the pressure drop through the tube is 10000 Pa, estimate

a. The air velocity through the tube and the friction factor

b. The heat transfer coefficient using Colburn Analogy

jH=(St)(Pr)0.67

Where St is the Stanton Number and Pr is the Prandtl Number. Gas constant, R = 82.06 cm3 atm/mol.K Darcy friction factor, f = 0.184Re0.2. Other relevant properties of air under the given conditions: viscosity = 1.8 × 10-5 kg/m s, density = 1134 kg/m3, specific heat capacity, C1, = 1.046 kJ/kg °C, thermal conductivity = 0.028 W/m °C.

                                                                                                      GATE 2002

 

Answer: (a) 47.34 m/s, (b) 178.224 W/m2°C

 

A double pipe countercurrent heat exchanger is designed to cool 3500 kg/hr of benzene flowing in the inner pipe from 80°C to 35°C. Water enters at 20°C and exits at 37°C in the annular space. The inside pipe has an inner diameter of 3.5 cm and wall thickness of 3.56 mm. The outer pipe has an inner diameter of 5.25 cm and is insulated. Neglecting the wall resistance to heat transfer from the inner pipe, and assuming the individual film heat transfer coefficient for water to be 6600 W/m2 °C. calculate:

a. the individual heat transfer coefficient for benzene flowing in the inner pipe.

b. the overall heat transfer coefficient based on inside diameter of inner pipe.

c. the total length required for the heat exchanger.

Nu = 0.023 (Re)0.8(Pr)0.3

Where Nu is the Nusselt Number and Pr is the Prandtl Number. Average properties of benzene: viscosity = 4 × 10-4 kg/m s, thermal conductivity = 0.147 W/m °C, specific heat capacity = 1880 J/kg °C, density = 837 kg/m3.

                                                                                                      GATE 2002

 

Answer:    (a) 1428.37 W/m2°C, (b) 1210.65 W/m2°C, (c) 23.24 m

 

A 10 cm diameter steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m°C). It loses heat to the surroundings at 30°C. Assume a heat transfer coefficient of 8.0 W/m2 °C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cm, the rate of heat loss from this insulated pipe will be

A. greater than that of the un-insulated steam pipe

B. less than that of the un-insulated steam pipe

C. equal to that of the un-insulated steam pipe

D. less than the steam pipe with 5 cm insulation

                                                                                                      GATE 2002

 

Answer:    (D)

 

1000 kg of liquid at 30°C in a well-stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and overall heat transfer coefficient to the liquid is 1500 W/m2 °C. Assuming negligible heat loss to surrounding and specific heat capacity of the liquid to be 4 kJ/kg °C, the time taken for the liquid to reach desired temperature will be

A. 15 min

B. 22 min

C. 44 min

D. 51 min

                                                                                                      GATE 2002

 

Answer:    (D)


A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and the specific heat capacities are CPA and CPB, for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B when

A. CPA > CPB

B. CPA < CPB

C. kA < 0.5kB

D. kA > 2kB

                                                                                                      GATE 2002

 

Answer:    (C)


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