Heat Transfer Flashcards
In a cooling tower, water becomes cool by
losing sensible heat
The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.
lower than
Thermal conductivity of a dry material is more than that of the damp material.
Wrongggg!
Duhring’s plot is used for calculating the concentration of solution.
Wronggg!
Prandtl number for most of dry gases is about
0.72
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
more than 45°C
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
more
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
1
Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling ?
Triangular Pitch
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
333 W/m2.K
In a laboratory test run, the rate of drying was found to be 0.5 x 10-3kg/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
5000
Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
Wronnggg!
Steam economy in case of a triple effect evaporator will be
> 1
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent
7
The maximum is the emissive power of a surface at a temperature T1occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.
Wronggg
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 co-efficient from the right face is 10 W/m2.K. At steady state, the temperature of the right face in °C is
48.7
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
tube diameter
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
1-1 heat exchanger
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
5000-15000
One kilogram of water at 0°C is changed to superheated steam of one atm pressure and 300° C. The major heat consumption in the process will be to
evaporate the water.
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
high
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is
16.5 kW/m2
In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.
logarithmic
A black body when hot, emits heat radiation of __________ wavelengths.
all
A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities arekAandkBand the specific heat capacities are CPA and CPB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when
kA< 0.5kB
Heat transfer by conduction in the turbulent core of a fluid flowing through a heated pipe is negligible, if the value of Prandtl number is
0.6
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
3.66
Radiator of an automobile engine is a __________ type of heat exchanger.
direct contact
For turbulent flow in a tube, the heat transfer co-efficient is obtained from the Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co-efficient will change by a factor of
6.1
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 W.m-1.°C-1respectively. The ambient air is at 30°C and the heat transfer co-efficient 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
165.4
Value of Nusselt number [Nu= (hD/k)] for the heat transfer by conduction from a droplet or a spherical particle to a surrounding stagnant film is
2
In an interphase heat transfer process, the equilibrium state corresponds to equality of temperature in the two phases, while the condition for equilibrium in an interphase mass transfer process is equality of
concentrations
Calburn analogy is applicable for the value of Prandtl number from
0.6-120
Multiple effect evaporators are used to
increase the steam economy & the capacity.
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
decrease
With increase in temperature, the total emissivity of conductors
increases
Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.
falling film
