Thursday, November 26, 2020

GATE Chemical Engineering Questions and Answers-Heat Transfer

Hot liquid is flowing at a velocity of 2 m/s through a metallic pipe having an inner diameter of 3.5 cm and length 20 m. The temperature at the inlet of the pipe is 90C. Following data is given for liquid at 90C

Density    = 950 kg/m3;     Specific heat = 4.23 kJ/kg C;            

Viscosity = 2.55×10-4 kg/m.s; Thermal conductivity = 0.685 W/m C

The heat transfer coefficient (in W/m2 C) inside the tube is

    (A)     222.22

    (B)     111.11

    (C)     22.22

    (D)     11.11

                                                                                                            GATE 2008

         Answer:          (D)

 

A metallic ball (ρ = 2700 kg/m3 and Cp = 0.9 kJ/kg C) of diameter 7.5 cm is allowed to cool in air at 25C. When the temperature of the ball is 125C, it is found to cool at the rate of 4C per minute. If the thermal gradients inside the ball are neglected, the heat transfer coefficient (in W/m2 C) is

        (A)             2.034

        (B)       20.34

        (C)       81.36

        (D)       203.4

                                                                                                            GATE 2008

         Answer:          (B)

 

Two plates of equal thickness (t) and cross-sectional area, are joined together to form a composite as shown in the figure. If the thermal conductivities of the plates are k and 2k then, the effective thermal conductivity of the composite is

GATE 2008

        (A)          3k/2

        (B)       4k/3

        (C)       3k/4

        (D)       2k/3

                                                                                                            GATE 2008

         Answer:          (B)

 

Transient three-dimensional heat conduction is governed by one of the following differential equations (α – thermal diffusivity, k – thermal conductivity and Ψ – volumetric rate of heat generation)

GATE 2008

                                                                                                                        GATE 2008

         Answer:    (D)

 

Common statement for next two questions

A slab of thickness L with one side (x = 0) insulated and the other side (x = L) maintained at a constant temperature T0 is shown below.

GATE 2009

A uniformly distributed internal heat source produces heat in the slab at the rate of S W/m3. Assume the heat conduction to be steady and 1-D along the x-direction.

 

The maximum temperature in the slab occurs at x equal to

        (A)    

        (B)    L/4

        (C)    L/2

        (D)    L

                                                                                                                        GATE 2009

         Answer:    (A)

 


The heat flux at x = L is

            (A)    0

            (B)    SL/4

            (C)    SL/2

            (D)    SL

                                                                                                                        GATE 2009

         Answer:    (D)

 

For the composite wall shown below (Case 1), the steady state interface temperature is 180C. If the thickness of layer P is doubled (Case 2), then the rate of heat transfer (assuming 1-D conduction) is reduced by

GATE 2009

    (A)    20%

    (B)    40%

    (C)    50%

    (D)    70%

                                                                                                GATE 2009


Answer:    (B)

 

A double-pipe heat exchanger is to be designed to heat 4 kg/s of a cold feed from 20 to 40C using a hot stream available at 160C and a flow rate of 1 kg/s. The two streams have equal specific heat capacities and the overall heat transfer coefficient of the heat exchanger is 640 W/m2.K. Then the ratio of the heat transfer areas required for the co-current to counter-current modes of operation is

(A)    0.73

(B)    0.92

(C)    1.085

(D)    1.25

                                                                                                GATE 2009

 

    Answer:          (C)

 

A well-insulated hemispherical furnace (radius = 1 m) is shown below:

GATE 2009

The self-view factor of radiation for the curved surface 2 is

(A)    ¼

(B)    ½ 

(C)    2/3

(D)    ¾

                                                                                                GATE 2009 

Answer:          (B)

 

The Prandtl number of a fluid is the ratio of

(A) Thermal diffusivity to momentum diffusivity

(B)  Momentum diffusivity to thermal diffusivity

(C)  Conductive resistance to convective resistance

(D) Thermal diffusivity to kinematic viscosity

                                                                                                GATE 2009 

                    Answer:    (B)

 

During the transient convective cooling of a solid object, Biot number → 0 indicates

(A) Uniform temperature throughout the object

(B)  Negligible convection at the surface of the object

(C)  Significant thermal resistance within the object

(D) Significant temperature gradient within the object

                                                                                                GATE 2009

 

                Answer:    (A)

 

Common statement for next two questions

Hot oil at 150C is used to preheat a cold fluid at 30C in a 1 - 1 shell and tube heat exchanger. The exit temperature of the hot oil is 110C. Heat capacities (product of mass flow rate and specific heat capacity) of both the streams are equal. The heat duty is 2 kW

 

Under co-current flow conditions, the overall heat transfer resistance (I/UA) is

            (A)    0.4 C/W

            (B)    0.04 C/W

            (C)    0.36 C/W

            (D)    0.036 C/W

                                                                                                            GATE 2010

             Answer:    (D)

 

Under counter-current flow conditions the overall heat transfer resistance (I/UA) is

(A)    0.4 C/W

(B)    0.04C/W

(C)    0.36 C/W

(D)    0.036 C/W

                                                                                                            GATE 2010

             Answer:          (B)

 

The view factor matrix for two infinitely long co-axial cylinders, shown in the figure below, is

GATE 2010
GATE 2010

                                                                                                            GATE 2010

             Answer:          (A)

 

The figure below shows steady state temperature profiles for one dimensional heat transfer within a solid slab for the following cases:

P: uniform heat generation with left surface perfectly insulated

Q: uniform heat generation with right surface perfectly insulated

R: uniform heat consumption with left surface perfectly insulated

S: uniform heat consumption with right surface perfectly insulated

GATE 2010

Match the profiles with appropriate cases.

(A)    P-I, Q-III, R-II, S-IV 

(B)    P-II, Q-III, R-I, S-IV

(C)    P-I, Q-IV, R-II, S-III

(D)    P-II, Q-IV, R-I, S-III

                                                                                                            GATE 2010

 Answer:          (A)

 

Which one of the following statements about baffles in a shell and tube heat exchanger is FALSE? Baffles

    (A)    Act as a support to the tube bundle

    (B)    Reduce the pressure drop on the shell-side

    (C)    Alter the shell-side flow pattern

    (D)    Help in increasing the shell-side heat transfer coefficient

                                                                                                         GATE 2010


Answer:          (B)

 

The ratio of the thermal boundary layer thickness to the concentration boundary layer thickness is proportional to

    (A)    Nu

    (B)    Le

    (C)    Sh

    (D)    Pr

                                                                                                         GATE 2010


Answer:          (B)

 

The ratio of Nusselt number to Biot number is

(A) Conductive resistance of fluid/conductive resistance of solid

(B)  Conductive resistance of fluid/convective resistance of fluid

(C)  Conductive resistance of solid/conductive resistance of fluid

(D) Unity

                                                                                                         GATE 2010

 

Answer:          (A)

 

Heat is generated uniformly within a solid slab. The slab separates fluid 1 from fluid 2. The heat transfer coefficients between the solid slab and the fluids are h1 and h2 (h2 > h1) respectively. The steady state temperature profile (T vs, x) for one-dimensional heat transfer is CORRECTLY shown by

GATE 2011

                                                                                                                           GATE 2011

                 Answer:          (A)

 

An aqueous sodium chloride solution (10 wt%) is fed into a single effect evaporator at a rate of 10000 kg/hr. It is concentrated to a 20 wt% sodium chloride solution. The rate of consumption of steam in the evaporator is 8000 kg/hr. the evaporator capacity (kg/hr) and economy are

    (A)    5000, 0.625

    (B)    10000, 0.625 

    (C)    5000, 1.6

    (D)    10000, 1.6

                                                                                                         GATE 2011

                Answer:          (A)


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