Heat Transfer Flashcards
Reynold’s analogy states that
Nst α f
Nst α f
Fourier’s law applies to the heat transfer by
conduction
conduction
For an ideal black body
absorptivity = 1
absorptivity = 1
The thermal efficiency of a reversible heat engine operating between two given thermal reservoirs is 0.4. The device is used either as a refrigerator or as a heat pump between the same reservoirs. Then the coefficient of performance as a refrigerator (COP)R and the co-efficient of performance as a heat pump (COP)HP are
(COP)R = 1.5; (COP)HP = 2.5
(COP)R = 1.5; (COP)HP = 2.5
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 shell side.
Baffles in the shell side of a shell and tube heat exchanger
increase the shell side heat transfer co-efficient.
increase the shell side heat transfer co-efficient.
The average heat transfer co-efficient for laminar film condensation on vertical surface is inversely proportional to (where, ΔT = Temperature drop across condensate film )
(ΔT)^1/4
(ΔT)^1/4
A 10 cm dia steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m2.°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be
less than that of the uninsulated steam pipe.
less than that of the uninsulated steam pipe.
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
Schmidt number.
Schmidt number.
Finned tube heat exchangers
give larger area per tube.
give larger area per tube.
Which one gives the monochromatic emissive power for black body radiation ?
Planck’s law
Planck’s law
For what value of Prandtl number, St = f/2?
1
1
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
for fluids in tubulent flow.
for fluids in tubulent flow.
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
0.2 D
Stefan-Boltzman law which describes the radiation heat transfer states that, it is proportional to (where, t = temperature in °C T = absolute temperature in ° K )
T^4
T^4
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is
4.36
4.36
The thermal radiation emitted by a body is proportional to Tn, where T is its absolute temperature. The value of ‘n’ is exactly 4 for
all bodies
all bodies
It is not preferable to use superheated steam in evaporators, because of its very
low film co-efficient
low film co-efficient
It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporisation is 2100 kJ/kg and the specific heat of the solution is 4 kJ/kg.K. The rate at which the heat has to be supplied in (kJ/min) to the evaporator is
3.06 x l0^5
3.06 x l0^5
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)
NPr^1/3
NPr^1/3
Economy of a multiple effect evaporator depends upon the
heat balance consideration
heat balance consideration
The Graetz number is concerned with the
heat transfer in laminar flow.
heat transfer in laminar flow.
In forced convection, the heat transfer depends on
Re, Pr
Re, Pr
Stefan-Boltzman law applies to __________ body.
black
black
Which of the following is directly concerned with the heat transfer ?
Which of the following is unimportant in forced convection ?
Grashoff number
Grashoff number
Planck’s distribution law is valid for __________ bodies.
coloured
coloured
What is the steam economy in case of a single effect evaporator system ?
< 1
< 1
If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length
ha = 2hl
ha = 2hl
The energy radiated from a surface Q at absolute temperature T is related as
Q ∝ T^4
Q ∝ T^4
Baffles are provided in heat exchangers to increase the
heat transfer co-efficient
heat transfer co-efficient
200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000 seconds. The drying occurs in the constant rate period with the drying rate as, Nc = 0.5 x 10-3 kg/m2.s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m2/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)
0.1
0.1
A 2-4 heat exchanger involves
both counter and parallel-flow of the fluids.
both counter and parallel-flow of the fluids.
In a liquid-liquid heat exchanger, for the same process temperature, the ratio of the LMTD in parallel flow to the LMTD in counter flow is always
< 1
< 1
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?
Sieder-Tate equation
Sieder-Tate equation
LMTD correction factor which is to be applied for a cross-flow heat exchanger increases with increase in the number of shell passes. Its value for a single pass cross flow heat exchanger is
<1
<1
Which of the following is not concerned with the heat transfer?
Schmidt number
Schmidt number
Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around __________ BTU.
180
180
The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m2. The approximate surface temperature (K) is
(Stefan-Boltzman constant = 5.67xl0-8 W/m2.K4)
1000
1000
A multiple effect evaporator has a capacity to process 4000 kg of solid caustic soda per day, when it is concentrating from 10% to 25% solids. The water evaporated in kg per day is
24000
24000
Value of Prandtl number for water ranges from
5 to 10
5 to 10
Reynold’s analogy states that (where, St = Stanton number f = friction factor)
St = f/2
St = f/2
A hot body will radiate heat most rapidly, if its surface is
A __________ surface has the maximum thermal emissivity out of the following.
black & rough
black & rough
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ 1/t
∝ 1/t
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
1-1.3
1-1.3
A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is exposed to a fluid stream with a convective heat transfer coefficient of 100 W/m2.K. The Biot number is
0.025
0.025
Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be
0.45
0.45
A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter ‘d’. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to
d^-0.2
d^-0.2
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
The ratio of momentum diffusivity to thermal diffusivity is called the __________ number.
Prandtl
Prandtl
The critical radius ‘r’ of insulation on a pipe is given by
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe )
r = k/h0
r = k/h
r = k/h0
As per Kirchoffs law, the ratio of the total radiating power to the absorptivity of a body depends on the
temperature of the body only.
temperature of the body only.
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
decreases.
decreases
__________ paint has the maximum absorption coefficient.
Black
Black
What is Nusselt number ?
hD/k
hD/k
Which of the following is correct ?
Driving force = Rate x Resistance
Driving force = Rate x Resistance
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
four times
four times
For what value of Prandtl number, the Col-burn analogy is valid ?
0.6 to 120
0.6 to 120
In a forward feed multiple effect, the pressure build up will be
least at the outlet of the last effect.
least at the outlet of the last effect.
For the same heat transfer area and the terminal conditions, the ratio of the capacities of a single effect evaporator to a triple effect evaporator is
1
1
If Prandtl number is greater than the Schmidt number, then the
thermal boundary layer lies inside the concentration boundary layer.
thermal boundary layer lies inside the concentration boundary layer.
A composite flat wall of a furnace is made of two materials ‘A’ and ‘B’. The thermal conductivity of ‘A’ is twice of that of material ‘B’, while the thickness of layer of ‘A’ is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material ‘A’ is
160
160
Natural convection is characterised by
Grashhoff number
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
4.364
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)^3
(ΔT)^3
Graetz number is given by
mCp/kL
mCp/kL
An evaporator while concentrating an aqueous solution from 10 to 40% solids evaporates 30000 kg of water. The amount of solids handled by the system in kg is
4000
4000
Choose the correct equation.
Nu = (Re) (Pr) (St)
Nu = (Re) (Pr) (St)
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
1/4
1/4
Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100°C and the inside heat transfer co-efficient is 3k/l. If the wall temperature on the outside is held at 25°C, what is the inside wall temperature in degree centigrade ?
81.25
81.25
Agitated film evaporator is suitable for concentrating __________ liquids.
viscous
viscous
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law.
parabolic
parabolic
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.
thin walled having the value of Ao Ai/< 2.
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
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
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
400
400
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
0.15
0.15
1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and the overall heat transfer co-efficient to the liquid is 1500 W/m2.°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be
51 min
The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (ΔT) between wall and outside bulk. The proportionality is given as
(ΔT)^3/4
(ΔT)^3/4
If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will
increase by 2^16
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.
5 minutes
5 minutes
A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time
t = 800 s
t = 800 s
Three material A, B and C of equal thick-nes and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of __________ °C.
70
70
A wall has two layers of materials A and B; each made of a different material. Both the layers have the same thickness. The thermal conductivity of materialA is twice that of B. Under the equilibrium, the temperature difference across the wall is 36°C. The temperature difference across the layer A is __________ °C.
12
12
Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.
Prandtl & Grashoff
Prandtl & Grashoff
In the free convection regime of pool boiling, the heat flux is proportional to
Δt^5/4
Δt^5/4
Fresh orange juice contains 12% (by weight) solids and the rest water 90% of the fresh juice is sent to an evaporator to remove water and subsequently mixed with the remaining 10% of fresh juice. The resultant product contains 40% solids. The kg of water removed from 1 kg fresh juice is
0.7
It is not recommended to use a 1-2 shell and tube heat exchanger for a particular heat duty, whenever the LMTD correction factor is
< 0.75
< 0.75
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.
7
7
In natural convection heat transfer, the correlating parameter is the
Grashoff number
Grashoff number
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
48.7
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
5000-15000
5000-15000
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
always between h1 and h2
always between h1 and h2
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
16.5 kW/m2
In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)
D/5 and D
D/5 and D
Which is the most suitable for the concentration of foamy & frothy liquors ?
Long tube vertical evaporator
Long tube vertical evaporator
A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and 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
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
0.6
The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface)
2K/h0
2K/h0
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
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-1 respectively. 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-2is
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
Calburn analogy is applicable for the value of Prandtl number from
0.6 to 120
0.6 to 120
Prandtl number for most of dry gases is about
0.72
0.72
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
1
1
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
more
In Biot number, the characteristic length used is the ratio of the __________ of the solid.
volume to surface area
volume to surface area
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
333W/m2.K
333W/m2.K
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
5000
5000
Steam economy in case of a triple effect evaporator will be
> 1
> 1
