S3 Flashcards
What is the steam economy in case of a single effect evaporator system?
<1
If average heat transfer coefficient is ha and the local coefficient at the end of the plate is hi, then in case od heat transfer to a fluid flowing over a flat plate, heated over its entire length
ha=2hi
jH factor for heat transfer is not a function of the ______ number.
Both b and c
B. Nusselt
C. Grashof
The Fourier number is used in the analysis of problem involving heat transfer by
transient conduction
The energy radiated from a surface Q at absolute temperature T is related as
Q α T^4
Baffles are provided in heat exchangers to increase the
heat transfer coefficient
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
In case of a vertical tube evaporator, with increase in the liquor level, the
capacity of the evaporator is decreased
The ratio of total radiating power to the absorptivity of the body depends upon the _______ as per Kirchoffs law.
temperature of the body
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
decreases
A 2-4 heat exchanger involves
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
With an increased temperature, the thermal conductivity of steel
decreases
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids?
Sieder-Tate equation
The main function of baffles provided in a shell and tube heat exchanger is to
enhance turbulence
In a single effect evaporator, the economy is
<1
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
Which of the following is not concerned with the heat transfer?
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
In counter flow compared to parallel flow,
both a and b
A. LMTD is greater
B. Less surface area is required for a given heat transfer rate
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
Crystallization of solids from a homogenous solution is an _______ process
exothermic
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
Value of Prandtl number for water ranges from
5 to 10
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
conduction
Among liquids, water has a comparatively high value of thermal conductivity, due to its
partial ionization
Reynold’s analogy states that
St= f/2
For evaporation of viscous solution multiple effect evaporator, the preferred feeding scheme is
backward
The sum of reflectivity and absorptivity for an opaque body is equal to
1
The film thickness for laminar dilm condensation on vertical surface ______ from top to bottom
cumulatively increases
In case of multipass shell and tube heat exchanger, providing a baffle on the shell side ______ the heat transfer rate
increases
The local surface conductance for laminar film condensation on vertical surface is
** α1/t**
Heat transfer coefficient for liquids increases with
increasing temperature
A diathermaneous substance _______ the thermal radiation completely
transmits
A hot body will radiate heat most rapidly, if its surface is
black and rough
Evaporation of 1kg of water from a solution in a single effect evaporator requires about ______ kg of steam
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
Analogy between mass and heat transfer is not applicable in case of
both b and c
B. Thermal or pressure mass diffusion
C. Viscous heating or chemical reaction
A ______ surface has the maximum thermal emissivity out of the following
black and rough
Radiant energy received by a body is proportional to
d^2
Mechanical recompression evaporation is used in the production of
distilled water
A measure of the extent to which viscous heating is important relative to the heat flow resulting from the impressed temperature difference is represented by the ______number.
Brinkman
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
The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
arrest the entrained liquid
What is the thermal conductivity of a perfect heat insulator?
zero
Fouling factor used in the design of a multipass shell and tube heat exchanger is a
factor of safety
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
Prandtl and Reynolds analogy are same, when Prandtl number is
1
Heat transfer occurs by natural convection because change in temperature causes difference in
density
The ratio of kinematic viscosity to thermal diffusivity is called the ____ number
Prandtl
