Heat Transfer (Set 4) Flashcards
As one of the transport phenomena, the concepts in heat transfer are crucial for design, operation, and maintenance of heat transfer equipment such as heat exchangers and insulation.
Intermittant tube cleaning is possible to be done in case of a ____ evaporator.
basket type
Harmonic mean temperature difference is given by
(2. ΔT1. ΔT2)/(ΔT1 + ΔT2)
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is
thin walled having the value of Ao Ai/< 2.
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for
cube
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 BTU/hr. ft.2°F
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?
coke breeze
Thermal diffusivity is given by
k/p.Cp
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?
1
Fourier’s law of heat conduction applies to ____ surfaces.
a. isothermal
b. non-isothermal
both (a) and (b)
____ chart is known as transient heat conduction chart.
Dirhing’s
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
Temperature difference between the two surfaces of the wall.
Which of the following has the minimum thermal conductivity?
Nitrogen
In a heat exchanger, one transfer unit means
a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then
hf < hd
Pick out the wrong statement:
Heat sensitive materials should be concentrated in high pressure evaporators.
For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a ____ evaporator.
vacuum
Circulation pump is located below the evaporater to
create more suction head.
The statement that “maximum wavelength of radiation is inversly proportional to the temperature” is ____ law.
Wien’s displacement
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
all (a), (b) and(c)
At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below :
The thermal conductivity of material I
is smaller than that of II.
Duhring rule is important in solving problems on
evaporation
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
0.75 D
Multiple effect evaporation is generally recommended, when the
large scale evaporation of liquor is needed
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.
4^0.8