Heat Transfer (Set 6)

Question 1

Reason for operating an evaporator in multiple effect is to secure

Answer 1

both (a) and (b).

Question 2

In a cooling tower, water becomes cool by

Answer 2

loosing sensible heat.

Question 3

In Fourier’s law, the proportionality constant is called the

Answer 3

thermal conductivity

Question 4

The heat transfer co-efficient in film type condensation is ____ that for dropwise condensation.

Answer 4

lower than

Question 5

Pick out the wrong statement.

Answer 5

Thermal conductivity of a dry material is more than that of the damp material.

Question 6

Duhring’s plot’ is of use in

Answer 6

evaporation

Question 7

Which has the lowest Prandtl number?

Answer 7

Liquid metal

Question 8

Pick out the wrong statement.

Answer 8

Duhring’s plot is used for calculating the concentration of solution.

Question 9

Prandtl number for most of dry gases is about

Answer 9

0.72

Question 10

Prandtl number is the reciprocal of

Answer 10

Thermal diffusivity/Momentum diffusivity

Question 11

In a heat exchanger with steam outside the tubes, a liquid gets heated to 45°C, when its flow velocity in the tubes is 2 m/s. If the flow velocity is reduced to 1 m/s, other things remaining the same, the temperature of the exit liquid will be

Answer 11

more than 45°C

Question 12

For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is

Answer 12

Nu = 2/pi x Gz

Question 13

Multiple effect evaporation accounts for

Answer 13

steam economy

Question 14

In case of a supercooled solution, which is on the verge of crystallisation, the free energy of the solution as compared to that of the solid is

Answer 14

more

Question 15

Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around ____ metres/second.

Answer 15

1

Question 16

In Biot number, the characteristic length used is the ratio of the ____ of the solid.

Answer 16

volume to surface area

Question 17

Which of the following is the most widely used heat insulating material for pipelines carrying steam?

Answer 17

85% magnesia cement and glass wool.

Question 18

Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re^0.8 . Pr^n, is not valid, if the value of

Answer 18

Reynolds number for the flow involved is < 2100.

Question 19

Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?

Answer 19

Triangular pitch

Question 20

In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid

Answer 20

and the wall are proportional to individual resistances.

Question 21

The purpose of providing expansion bellows in the shell of tubular exchanger is to

Answer 21

account for the uneven expansion of shell and tube bundles.

Question 22

A graph between ____ is called Wilson plot.

Answer 22

1/U Vs. 1/V^0.8

Question 23

Grashhoff number is given by

Answer 23

gD^3.β.Δtρ2/μ^2

Question 24

The overall heat transfer co-efficient for a shell and tube heat exchanger for clean surfaces is U0 = 400 W/m2.K. The fouling factor after one year of operation is found to be hd0 = 2000 W/m2.K. The overall heat transfer co-efficient at this time is

Answer 24

333W/m2.K

Question 25

In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m2.s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m2/kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is

Answer 25

5000

Question 26

Pick out the wrong statement.

Answer 26

Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.

Question 27

What is the geometric mean of two heat transfer areas A1 and A2?

Answer 27

sqrt*(A1 . A2)

Question 28

Evaporation by thermocompression results in the

Answer 28

both (a) and (b).

Question 29

Forced circulation evaporators are normally used for concentrating liquids having

Answer 29

both(a) & (b)

Question 30

Steam economy in case of a triple effect evaporator will be

Answer 30

> 1