INDIABIX-HEAT TRANSFER Flashcards
Air is best heated with steam in a heat exchanger of ___
>plate type.
>double pipe type with fin on steam side.
>double pipe type with fin on air side.
>shell and tube type.
> double pipe type with fin on air side.
Black liquor generated during paper manufacture is concentrated in a ______
»single effect evaporator.
»single effect evaporator followed by a crystalliser.
»multiple effect evaporator.
»multiple effect evaporators followed by a crystalliser.
> > multiple effect evaporator.
When vaporisation takes place directly at the heating surface, it is called
>film boiling
>nucleate boiling
>vapour binding
>none of these
> nucleate boiling
Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity the economy of the evaporator respectively)
a) C/E
b) E/C
c) CE
d) 1/CE
a) C/E
Reynold’s analogy states that _____
Nst α f
Nst α NRe
NNu α f
NRe α f
Nst α f
With increase in temperature, the thermal conductivity of non-metallic amorphous solids
»decreases
»increases
»remains constant
»first decreases up to certain temperature and then increases
increases
Fourier’s law applies to the heat transfer by
>convection
>radiation
>conduction
>all (a), (b) & (c)
> conduction
In the equation Q = UAΔt; Δt is
> geometric mean temperature difference.
arithmetic mean temperature difference.
logarithmic mean temperature difference.
the difference of average bulk temperatures of hot and cold fluids.
> logarithmic mean temperature difference.
With increase in porosity, the thermal conductivity of a solid substance
>increases
>decreases
>remains unchanged
>may increase or decrease; depends on the solid
> decreases
For an ideal black body
absorptivity = 1
reflectivity = 1
emissivity = 0
transmissivity = 1
absorptivity = 1
In Joule’s experiment, an insulated container contains 20 kg of water initially at 25°C. It is stirred by an agitator, which is made to turn by a slowly falling body weighing 40 kg through a height of 4 m. The process is repeated 500 times. The acceleration due to gravity is 9.8 ms-2. Neglecting the heat capacity of agitator, the temperature of water (in °C) is
40.5
34.4
26.8
25
34.4
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
>increases
>decreases
>is not affected
>may increase or decrease; depends on the feed
decreases
The steam ejector is used to
remove condensate from the steam pipelines.
create vacuum.
superheat the steam.
none of these.
create vacuum.
For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
increases
decreases
remains constant
passes through a maxima
passes through a maxima
Hot water (0.01 m3 /min) enters the tube side of a counter current 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 of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately
32
37
45
50
32
In a heat exchanger, floating head is provided to
facilitate cleaning of the exchanger.
increase the heat transfer area.
relieve stresses caused by thermal expansion.
increase log mean temperature gradient.
relieve stresses caused by thermal expansion.
Leidenfrost point is a term concerned with the
a)condensation of the saturated vapor on a cold surface.
b)concentration of a corrosive solution by evaporation.
c)heat transfer between two highly viscous liquids.
d)boiling of a liquid on a hot surface.
d)boiling of a liquid on a hot surface.
A process stream of dilute aqueous solution flowing at the rate of10 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
counter flow with process stream on shell side.
counter flow with process stream on tube side.
parallel flow with process stream on shell side.
parallel flow with process stream on tube side.
counter flow with process stream on shell side.
In case of evaporators, liquid entrainment results primarily due to
high vacuum in the evaporator.
high evaporation rate.
foaming of the solution.
high heat transfer rate.
foaming of the solution.
Baffles in the shell side of a shell and tube heat exchanger
a)increase the cross-section of the shell side liquid.
b)force the liquid to flow parallel to the bank.
c)increase the shell side heat transfer co-efficient.
d)decrease the shell side heat transfer co-efficient.
c)increase the shell side heat transfer co-efficient.
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 are equal in both the 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 on shell side.
d) shell and tube heat exchanger with air on shell side and steam inside tubes.
b) finned tube heat exchanger with air outside and steam inside.
The number of kg vaporised per kg of steam fed to the evaporator is defined as
capacity
rate of evaporation
economy
rate of vaporisation
economy
Economy of a multiple effect evaporator is not influenced much by the
boiling point elevations
temperature of the feed
rate of heat transfer
ratio of the weight of the thin liquor to thick liquor
boiling point elevations
Increasing the liquor level in the evaporator results in the
decreased capacity.
increase in liquor film co-efficient.
decreased effect of hydrostatic head.
increased true temperature drop.
decreased capacity.
The heat flux (from outside to inside) across an insulating wall with thermal conductivity, K= 0.04 W/m.°K and thickness 0.16m is 10 W/m2. The temperature of the inside wall is - 5°C. The outside wall temperature is
25°C
30°C
35°C
40°C
35°C
Which of the following is concerned with both heat and mass transfer?
Lewis relationship
Nusselt number
Kutateladze number
Froude number
Lewis relationship
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
higher economy
lower capacity
both (a) & (b)
lower economy
higher economy
