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Heat Transfer
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Which of the following is the most widely used heat insulating material for pipelines carrying steam
Cotton followed by aluminium foil.
85% magnesia cement and glass wool.
Tar dolomite bricks followed by asbestos.
Log mean temperature difference (LMTD) cannot be used, if
heat transfer co-efficient over the entire heat exchanger is not constant.
there exists an unsteady state.
None of these.
the heat capacity is not constant and there is a phase change.
Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by This is valid for the value of NRe equal to
2100-4000
> 4000
> 10000
< 2100
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
40.4
20.4
20.8
40.8
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator
Remains constant
may increase or decrease, depends on the vacuum
Decreases
Increases
With increase in temperature, the thermal conductivity of a gas
may increase or decrease depending on the type of gas.
increases.
remains same.
decreases.
Out of the following four assumptions used in the derivation of the equation for LMTD , which one is subject to the largest deviation in practice
Constant specific heat.
No partial phase change in the system.
Constant overall heat transfer co-efficient.
Constant rate of fluid flow.
Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around __________ BTU
data insufficient, can't be predicted
180
3.97
970
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
αT
α 1/T
α T4
It is desired to concentrate a 20% salt solution
(20 kg of salt in 100 kg of solution) to a 30% salt solution in anevaporator.
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
7.24 x 105
6.12 x 105
3.06 x l05
9.08 x 105
Kg of liquid evaporated per hour in an evaporator is defined as its
steam load
capacity
Economy
NONE OF THESE
The ratio of momentum diffusivity to thermal diffusivity is the __________ number
Nusselt
Stanton
Prandtl
Grashoff
Duhring rule is important in solving problems on
Crystallisation
Humidification
Distillation
Evaporation
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
Boiler
regenerator
recuperator
heat exchanger
Double pipe heat exchangers are preferably useful, when
requirement of heat transfer area is low.
high viscosity liquid is to be cooled.
a corrosive liquid is to be heated.
overall heat transfer co-efficient is very high.
In an extended surface heat exchanger, fluid having lower co-efficient
flows outside the tubes.
flows through the tube.
should not be used as it gives very high pressure drop.
can flow either inside or outside the tubes.
In a forward feed multiple effect evaporator unit
None of these.
no pump is required to withdraw the product from the last effect.
viscosity of liquid is highest in first effect.
transfer from effect to effect is done by pumps.
Crystal size in a continuous crystalliser depends upon the
degree of supersaturation.
all (a), (b) and (c).
degree of turbulence.
rate of heat transfer.
Rate of crystallisation does not depend upon the
None of these.
turbulence within the solution.
extent of supersaturation.
number and active surface area of the crystals.
Loss of heat from untagged steam pipe to the ambient air is by
Radiation
Conduction
All (A), (B) & (C)
Convection
1
2
16
31
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