Tuesday, November 24, 2020

GATE Chemical Engineering Questions and Answers-Heat Transfer

A canteen requires hot water for its dish-washing. For this purpose, the canteen draws 0.4 kg/s of water at 298 K and heats it to 353 K in a fired-heater. The wastewater leaving the dishwasher is at 333 K. In order to save energy, it is proposed to recover heat from the waste water to partially heat up the incoming water in a counter-current exchanger as shown schematically below:

counter current heat exchange

Assume that there is no loss of water in the dishwasher and a minimum approach temperature of 10 K should be maintained in the exchanger.

a. Determine the maximum temperature to which the incoming water can be heated in the exchanger.

b. If the overall heat transfer coefficient in the exchanger is 1200 W/m2 K and the specific heat capacity of water is 4186 J/kg K, compute the area of the heat exchanger.

c. If the cost of the exchanger in 1982 was 2 × A0.41 (in lakh rupees), where A is the exchanger area in m2 and the Marshall and Swift cost indices in 1982 and 2000 are 315 and 400 respectively, determine the cost of the exchanger in the year 2000.

                                                                                                      GATE 2002

 

            Answer:          (a) 323 K, (b) 3.488 m2, (c) 4.238 lakhs

 

A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed because

A. total heat transfer area of all the effects is less than that in a single effect evaporator system

B. total amount of vapour produced per kg of feed steam in a multieffect system is much higher than in a single effect

C. boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system.

D. heat transfer coefficient ma single effect is much lower than that in any effect in a multieffect system


                                                                                                      GATE 2003

 

            Answer:          (B)

 

The units of resistance to heat transfer are

        A.        Jm-2K-1

        B.         Jm-1K-1

        C.         Wm-2K-1

        D.        W-1m2K

 

                                                                                                      GATE 2003

 

            Answer:          (D)

 

A process stream of dilute aqueous solution flowing at the rate of 10 kg s-1 is to be heated. Steam condensate at 95°C is available for heating purpose, also at a rate of 10 kg s-1. A 1-1 shell and tube heat exchanger is available. The best arrangement is

    A.     counterflow with process stream on shell side

    B.     counterflow with process stream on tube side

    C.     parallel flow with process stream on shell side

    D.     parallel flow with process stream on tube side

 

                                                                                                      GATE 2003

 

Answer:          (B)

 

The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 Wm-1°C-1 respectively. The ambient air is at 30°C and the heat transfer coefficient between the outer surface of wall and air is 20 W m-2 °C-1. The rate of heat loss from the outer surface in W m-2 is

A.     165.4

B.     167.5

C.     172.8

D.     175

 

                                                                                                      GATE 2003

 

Answer:          (A)

 

Steam is to be condensed in a shell and tube heat exchanger; 5 m long with a shell diameter of 1 m. Cooling water is to be used for removing the heat. Heat transfer: coefficient for the cooling water, whether on shell side or tube side, is same. The best arrangement is

A.     vertical heat exchanger with steam on tube side

B.     vertical heat exchanger with steam on shell side

C.     horizontal heat exchanger with steam on tube side

D.     horizontal heat exchanger with steam on shell side

 

                                                                                                      GATE 2003

 

Answer:          (B)

 

A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter ‘d’. For a fixed mass flow rate, the tube side heat transfer coefficient for turbulent flow conditions is proportional to

A.     d0.8

B.     d-0.2

C.     d-1

D.     d-1.8

 

                                                                                                      GATE 2003

 

            Answer:          (D)

 

Air is to be heated by condensing steam. Two heat exchangers are available: (i) a shell and tube heat exchanger, and (ii) a finned tube heat exchanger. Tube side heat transfer area is equal in both cases. The recommended arrangement is

    A.     finned tube heat exchanger with air inside and steam outside

    B.     finned tube heat exchanger with air outside and steam inside

    C.     shell and tube heat exchanger with air inside tubes and steam shell side

    D.     shell and tube heat exchanger with air on shell side and steam inside tubes

 

                                                                                                      GATE 2003

 

Answer:          (B)

 

For a given ambient air temperature with increase in the thickness of insulation of a hot cylindrical pipe, the rate of heat loss from the surface would

    A.     decrease

    B.     increase

    C.     first decrease and then increase

    D.     first increase and then decrease

                                                                                                      GATE 2003

 

Answer:          (D)

 

In force convection, the Nusselt number Nu is a function of

        (A)     Re and Pr

        (B)      Re and Gr

        (C)      Pr and Gr

        (D)     Re and Sc

 

                                                                                                      GATE 2004

 

Answer:          (A)

 

 

For an ideal black body

        (A)     Absorptivity = 1

        (B)      Reflectivity = 1

        (C)      Emissivity = 0

        (D)     Transmissivity = 1

 

                                                                                                      GATE 2004

 

        Answer:          (A)

 

The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/(m.K) and the heat transfer coefficient from the right face is 10 W/(m2.K). At steady state, the temperature of the right face in °C is

            (A)    77.2

            (B)    71.2

            (C)    63.8

            (D)    48.7

 

                                                                                                      GATE 2004

 

            Answer:          (D)

 

A metal bar of radius 0.1 m at uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 Kg/m3 and 0.4 KJ/(kg.K), respectively. The heat transfer coefficient is 50 W/(m2.K). Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is

            (A)    555 

            (B)    55.5

            (C)    0.55 

            (D)    0.15

 

                                                                                                      GATE 2004

 

Answer:          (D)

 

Hot water (0.01 m3/min) enters the tube side of a concurrent shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of density 800 kg/m3 and specific heat 2 KJ/(kg.K) enters at 20°C. The log mean temperature difference in °C is approximately

            (A)    32

            (B)    37

            (C)    45

            (D)    50

 

                                                                                                      GATE 2004

 

Answer:          (A)

 

It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporization is 2100 KJ/Kg and the specific heat of solution is 4 KJ/(kg.K). The rate at which heat has to be supplied (in KJ/min) to the evaporator is

            (A)    3.06×105 

            (B)    6.12×105

            (C)    7.24×105 

            (D)    9.08×105

 

                                                                                                      GATE 2004

 

Answer:          (A)

 

The thermal boundary layer is significantly thicker than the hydrodynamic boundary layer for

        (A)     Newtonian liquids

        (B)      Polymeric liquids

        (C)      Liquid metals

        (D)     Gases

 

                                                                                                      GATE 2005

 

Answer:          (C)

 

An electrically heated element is submerged in a pool of water at its saturation temperature. As the temperature of the element increases, the maximum heat transfer coefficient is observed

        (A) In the free convection regime

        (B)  Between the nucleate boiling and partial nucleate boiling mixed with unstable film boiling regimes

        (C)  In the incipient nucleate boiling regime

        (D) In the stable film boiling regime without significant radiation effects

 

                                                                                                      GATE 2005

 

Answer:          (B)

 

Baffles are used in heat exchanger is order to

        (A) Increase the tube side fluid’s heat transfer coefficient

        (B)  Promote vibration in the heat exchanger

        (C)  Promote cross flow and turbulence in the shell side fluid

        (D) To prevent shell expansion due to thermal effects

 

                                                                                                      GATE 2005

 

Answer:          (C)

 

In film type condensation of liquid along a vertical tube, the thickness of the condensate layer increases towards the bottom. This implies that the local heat transfer coefficient

        (A) Increases from top to bottom

        (B)  Decreases from top to bottom

        (C)  Remains constant from top to bottom

        (D) First increases and then decreases from top to bottom.

 

                                                                                                      GATE 2005

 

Answer:          (D)

 

A black body at a high temperature TH transfers energy by radiation to a black body at a lower temperature TL. Initial TH = 1850°C, TL = 500°C and the net rate of heat transfer is 25 W. After some time, when TH = 1500°C and TL = 750°C, what is the net rate of energy transfer

            (A)    8.73 W

            (B)    9.60 W 

            (C)    13.89 W

            (D)    11.01 W

 

                                                                                                      GATE 2005

 

            Answer:          (D)


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