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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
85% magnesia cement and glass wool.
Cotton followed by aluminium foil.
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.
the heat capacity is not constant and there is a phase change.
None of these.
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
> 4000
< 2100
2100-4000
> 10000
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
20.4
40.8
20.8
40.4
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator
Increases
Decreases
Remains constant
may increase or decrease, depends on the vacuum
With increase in temperature, the thermal conductivity of a gas
remains same.
may increase or decrease depending on the type of gas.
decreases.
increases.
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 overall heat transfer co-efficient.
Constant specific heat.
No partial phase change in the system.
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
3.97
180
970
data insufficient, can't be predicted
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 T4
α 1/T
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
3.06 x l05
7.24 x 105
6.12 x 105
9.08 x 105
Kg of liquid evaporated per hour in an evaporator is defined as its
steam load
capacity
NONE OF THESE
Economy
The ratio of momentum diffusivity to thermal diffusivity is the __________ number
Nusselt
Grashoff
Prandtl
Stanton
Duhring rule is important in solving problems on
Humidification
Crystallisation
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
recuperator
Boiler
heat exchanger
regenerator
Double pipe heat exchangers are preferably useful, when
a corrosive liquid is to be heated.
overall heat transfer co-efficient is very high.
requirement of heat transfer area is low.
high viscosity liquid is to be cooled.
In an extended surface heat exchanger, fluid having lower co-efficient
can flow either inside or outside the tubes.
should not be used as it gives very high pressure drop.
flows outside the tubes.
flows through the tube.
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
all (a), (b) and (c).
degree of supersaturation.
rate of heat transfer.
degree of turbulence.
Rate of crystallisation does not depend upon the
turbulence within the solution.
extent of supersaturation.
None of these.
number and active surface area of the crystals.
Loss of heat from untagged steam pipe to the ambient air is by
Convection
All (A), (B) & (C)
Radiation
Conduction
1
2
16
31
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