Friday, November 27, 2020

GATE Chemical Engineering Questions and Answers-Heat Transfer

Oil at 120C is used to heat water at 30C in a 1-1 co-current shell and tube heat exchanger. The available heat exchange area is S1. The exit temperatures of the oil and the water streams are 90C and 60C respectively. The co-current heat exchanger is replaced by a 1-1 counter-current heat exchanger having heat exchange area S2. If the exit temperatures and the overall heat transfer coefficients are same, the ratio of S1 and S2 is

(A)            

(B)              1.1

(C)              0.91

(D)             0

                                                                                                         GATE 2011

 

    Answer:          (B)

 

In film type condensation over a vertical tube, local heat transfer coefficient is

(A)             Inversely proportional to local film thickness

(B)              Directly proportional to local film thickness

(C)              Equal to local film thickness

(D)             Independent of local film thickness

                                                                                                         GATE 2011

 

    Answer:          (A)

 

Consider two black bodies with surfaces S1 (area = 1 m2) and S2 (area = 4 m2). They exchange heat only by radiation. 40% of the energy emitted by S1 is received by S2. The fraction of energy emitted by S2 that is received by S1 is

(A)             0.05

(B)              0.1

(C)              0.4

(D)             0.6

                                                                                                         GATE 2011

 

    Answer:          (B)

 

In a counter-flow double pipe heat exchanger, oil (ṁ = 2 kg/s, Cp = 2.1 kJ/kg°C) is cooled from 90C to 40C by water (ṁ = 1 kg/s, Cp = 4.2 kJ/kg°C) which enters the inner tube at 10C. The radius of the inner tube is 3 cm and its length is 5 m. Neglecting the wall resistance, the overall heat transfer coefficient based on the inner radius in (kW/m2.K) is

(A)             0.743

(B)              7.43

(C)              74.3

(D)             2475

                                                                                                      GATE 2012

 

    Answer:          (A)

 

Heat generated at a steady rate of 100 W due to resistance heating in a long wire (length = 5 m, diameter = 2 mm). This wire is wrapped with an insulation of thickness 1 mm that has a thermal conductivity of 0.1 W/m K. The insulated wire is exposed to air at 30C. The convective heat transfer between the wire and surrounding air is characterized by a heat transfer coefficient of 10 W/m2.K. The temperature in °C at the interface the wire and the insulation is

(A)             211.2

(B)              242.1

(C)              311.2

(D)             484.2

                                                                                                                        GATE 2012

 

    Answer:          (C)

 

For the enclosure formed between two concentric spheres as shown below (R2 = 2R1), the fraction of radiation leaving the surface area A2 that strikes itself is

GATE 2012

                                                                                                      GATE 2012

 

    Answer:          (D)

 

The one-dimensional unsteady state heat conduction equation in a hollow cylinder with a constant heat source q is

GATE 2012

            If A and B are arbitrary constants, then the steady state solution to the above equation is

GATE 2012

 

                                                                                                GATE 2012

 

    Answer:          (B)

 

If the Nusselt number (Nu) for heat transfer in a pipe varies with Reynolds number (Re) as Nu∞Re0.8, then for constant average velocity in the pipe, 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 2012

 

    Answer:          (B)

 

A solid sphere with an initial temperature Ti is immersed in a large thermal reservoir of temperature T0. The sphere reaches a steady temperature after a certain time t1. If the radius of the sphere is doubled, the time required to reach steady state will be

(A)             t1/4

(B)              t1/2

(C)              2t1

(D)             4t1

                                                                                                      GATE 2012

 

    Answer:         (D)

 

For heat transfer across a solid fluid interface, which one of the following statements is not true when the Biot number is very small compared to 1?

(A) Condition resistance in the solid is very small compared to convection resistance in the fluid

(B)  Temperature profile within the solid is nearly uniform

(C)  Temperature drop in the fluid is significant

(D) Temperature drop in the solid is significant

                                                                                                      GATE 2012

 

        Answer:            (D)

 

In a double pipe counter-current heat exchanger, the temperature profiles shown in the figure were observed. During operation, due to fouling inside the pipe, the heat transfer rate reduces to half of the original value. Assuming that the flow rates and the physical properties of the fluids do not change, the LMTD (in ˚C) in the new situation is

GATE 2013
 

(A)                0

(B)                20

(C)                40

(D)                indeterminate

                                                                                                      GATE 2013

 

    Answer:          (C)

 

A hole of area 1 cm2 is opened on the surface of a large spherical cavity whose inside temperature is maintained at 727°C. The value of Stefan-Boltzmann constant is 5.67×10-8 W/m2 K4. Assuming black body radiation, the rate at which the energy is emitted (in W) by the cavity through the hole, up to 3 digits after the decimal point, is _____________.

                                                                                                            GATE 2013

 

    Answer:          5.67 watt

 

Calculate the heat required (in kJ, up to 1 digit after the decimal point) to raise the temperature of 1 mole of a solid material from 100˚C to 1000˚C. The specific heat (Cp) of the material (in J/mol-K) is expressed as Cp = 20 + 0.005T, where T is in k. Assume no phase change. _________.

                                                                                                                  GATE 2013

    Answer:          21.7 KJ

 

In a pool boiling experiment, the following phenomena were observed

P. Natural convection

Q. Film boiling

R. Transition boiling

S. Nucleate boiling

What was the correct sequence of their occurance?

(A)                P, Q, R, S

(B)                S, R, Q, P

(C)                Q, R, P, S

(D)                P, S, R, Q

                                                                                                      GATE 2013

 

    Answer:          (D)

 

The effectiveness of a heat exchanger in the ε-NTU method is defined as

GATE 2013

 

                                                                                                      GATE 2013

 

Answer:    (D)

 

The bottom face of a horizontal slab of thickness 6 mm is maintained at 300°C. The top face is exposed to a flowing gas at 30°C. The thermal conductivity of the slab is 1.5 W m-1 K-1 and the convective heat transfer coefficient is 30 W m-2 K-1. At steady state, the temperature (in °C) of the top face is _____________.

GATE 2014
 

                                                                                                                  GATE 2014

 

               Answer:    271.07 °C

 

An oil with a flow rate of 1000 kg/h is to be cooled using water in a double-pipe counter-flow heat exchanger from a temperature of 70°C to 40°C. Water enters the exchanger at 25°C and leaves at 40°C. The specific heats of oil and water are 2 kJ kg-1 K-1 and 4.2 kJ kg-1 K-1, respectively. The overall heat transfer coefficient is 0.2 kW m-2 K-1. The minimum heat exchanger area (in m2) required for this operation is ___________.

GATE 2014

 

                                                                                                            GATE 2014

 

                Answer:    3.85 m2

 

A brick wall of 20 cm thickness has thermal conductivity of 0.7 W m-1 K-1. An insulation of thermal conductivity 0.2 W m-1 K-1 is to be applied on one side of the wall, so that the heat transfer through the wall is reduced by 75%. The same temperature difference is maintained across the wall before and after applying the insulation. The required thickness (in cm) of the insulation is _____________.

GATE 2014

 

                                                                                                            GATE 2014

 

Answer:          17.14 cm

 

Steam economy of a multiple effect evaporator system is defined as

(A) Kilogram of steam used per hour

(B)  Kilogram of steam consumed in all the effects for each kilogram of steam fed

(C)  Kilogram of steam used in all the effects for each kilogram of water vaporized per hour

(D) Kilogram of water vaporized from all the effects for each kilogram of steam fed to the first effect

                                                                                                      GATE 2014

 

Answer:          (D)

 

In a complete opaque medium, if 50% of the incident monochromatic radiation is absorbed, then which of the following statements are correct?

(P)       50% of the incident radiation is reflected

(Q)       25% of the incident radiation is reflected

(R)       25 % of the incident radiation is transmitted

(S)       No incident radiation is transmitted

(A)       P and S only

(B)       Q and R only

(C)       P and Q only

(D)       R and S only

                                                                                                            GATE 2014

 

Answer:          (A) 


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