Heat Transfer (Set 6) Flashcards
As one of the transport phenomena, the concepts in heat transfer are crucial for design, operation, and maintenance of heat transfer equipment such as heat exchangers and insulation.
Reason for operating an evaporator in multiple effect is to secure
both (a) and (b).
In a cooling tower, water becomes cool by
loosing sensible heat.
In Fourier’s law, the proportionality constant is called the
thermal conductivity
The heat transfer co-efficient in film type condensation is ____ that for dropwise condensation.
lower than
Pick out the wrong statement.
Thermal conductivity of a dry material is more than that of the damp material.
Duhring’s plot’ is of use in
evaporation
Which has the lowest Prandtl number?
Liquid metal
Pick out the wrong statement.
Duhring’s plot is used for calculating the concentration of solution.
Prandtl number for most of dry gases is about
0.72
Prandtl number is the reciprocal of
Thermal diffusivity/Momentum diffusivity
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
For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is
Nu = 2/pi x Gz
Multiple effect evaporation accounts for
steam economy
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
In Biot number, the characteristic length used is the ratio of the ____ of the solid.
volume to surface area
Which of the following is the most widely used heat insulating material for pipelines carrying steam?
85% magnesia cement and glass wool.
Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re^0.8 . Pr^n, is not valid, if the value of
Reynolds number for the flow involved is < 2100.
Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?
Triangular pitch
In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid
and the wall are proportional to individual resistances.
The purpose of providing expansion bellows in the shell of tubular exchanger is to
account for the uneven expansion of shell and tube bundles.
A graph between ____ is called Wilson plot.
1/U Vs. 1/V^0.8
Grashhoff number is given by
gD^3.β.Δtρ2/μ^2
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
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
Pick out the wrong statement.
Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
What is the geometric mean of two heat transfer areas A1 and A2?
sqrt*(A1 . A2)
Evaporation by thermocompression results in the
both (a) and (b).
Forced circulation evaporators are normally used for concentrating liquids having
both(a) & (b)
Steam economy in case of a triple effect evaporator will be
> 1
