HEAT TRANSFER QUIZ

A metal ball of radius 0.1 m at a 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 co-efficient 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

55.5

0.15

0.55

555

TRUE ANSWER : ?
YOUR ANSWER : ?

The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)

∝ √t

∝ t

Independent of 't'

∝ 1/t

TRUE ANSWER : ?
YOUR ANSWER : ?

The inside heat transfer co-efficient in case of turbulent flow of liquid in the tube side in a 1-2 shell and tube heat exchanger is increased by __________ times, when the number of tube passes is increased to 8.

40.8

20.8

20.4

40.4

TRUE ANSWER : ?
YOUR ANSWER : ?

Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube

Graetz number > 100

Exit fluid temperature < wall temperature

Exit fluid temperature = wall temperature

Exit-fluid temperature > wall temperature

TRUE ANSWER : ?
YOUR ANSWER : ?

The ratio of momentum diffusivity to thermal diffusivity is the __________ number.

Stanton

Nusselt

Grashoff

Prandtl

TRUE ANSWER : ?
YOUR ANSWER : ?

For flow over a flat plate, the ratio of thermal boundary layer thickness, 'xt' and hydrodynamic boundary layer thickness 'x' is equal to (where, NPr = Prandtl number)

NPr1/3

NPr-1

NPr-1/3

NPr

TRUE ANSWER : ?
YOUR ANSWER : ?

Double pipe heat exchangers are preferably useful, when

High viscosity liquid is to be cooled

Requirement of heat transfer area is low

A corrosive liquid is to be heated

Overall heat transfer co-efficient is very high

TRUE ANSWER : ?
YOUR ANSWER : ?

Multiple effect evaporators are commonly used in the manufacture of P. Paper Q. Superphosphate R. Sugar S. Fats

P and S

P and R

P and Q

R and S

TRUE ANSWER : ?
YOUR ANSWER : ?

Circulation pump is located below the evaporater to

Avoid frequent priming

None of these

Avoid cavitation

Create more suction head

TRUE ANSWER : ?
YOUR ANSWER : ?

Economy of a multiple effect evaporator depends upon the

Heat balance consideration

Rate of heat transfer

Neither A nor B

Both A and B

TRUE ANSWER : ?
YOUR ANSWER : ?

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