FAQ's heat transfer

1. FAQ’S on Heat Transfer
Prepared by- Mr. Vishal B. Thakare
Assistant Professor, PARUL UNIVERSITY
(M.Tech Chemical, SVNIT Surat)
vishalbt88@gmail.com

2. Q.1 What are the different modes of heat transfer?
Conduction, convection and radiation are the modes of heat
transfer.

3. Q.2 What is the function of baffles in shell and tube heat
exchanger?
Baffles provide support to tubes and create turbulence in shell
side which increase shell side heat transfer coefficient.
Q.3 Why corrosive liquid should pass through the tube?
Tubes can be made of corrosion resistant material and
inversely increasing the life of shell.
Q.4 Why high pressure fluid flows on tube side?
If high pressure fluid pass from tube sides, tubes have to be
designed for high pressure. This reduces shell side thickness as
well as cost.

4. Q.5 Write different types of convection
Natural convection- In natural convection mixing occurs due to
density difference
Forced convection- In forced convection mixing occurs due to
agitation or by pump.

5. Q.6 What do you mean by tube clearance and tube pitch in shell
and tube heat exchanger?
Shortest centre to centre distance between adjacent tube is called
tube pitch. Shortest distance between two tubes is called
clearance.
The triangular pitch gives large number of tubes in shell than
square pitch and also create large turbulence in shell side fluid. But
it is difficult to clean.

6. Q.7 What is the maximum baffle spacing in shell and tube heat
exchanger?
The maximum baffle spacing in shell and tube heat exchanger is equal
to the diameter of the shell.
Q.8 Why film wise condensation is used in industries?
In film wise condensation heat resistance is uniform and it easy to design.
Q.9 Write application of cross flow heat exchanger
It is specially used for gas-liquid system to cool or heat the gas.
Q.10 What is difference between equilibrium state and steady state.
Equilibrium state- Properties are uniform throught and they do not vary
with time.
Steady state- Properties are not changing at a point or at specified
location and do not vary with time.

7. Q.11 Which are the flow patterns of heat exchanger?
i)
 
2
1
21
T
T
Ln
TT
Tlm





8. ii)
 
2
1
21
T
T
Ln
TT
Tlm





9. iii)
Q.12 How can you increase the steam economy of evaporator?
Steam economy can be increased by using multiple effect
evaporator and vapour recompression.
Cross flow Heat Exchanger

10. Q.13 Which is better heat conductor, diamond or silver?
Thermal conductivity of diamond at room temperature is
2300W/m.K and silver thermal conductivity is 429 W/m.K so
diamond is a better heat conductor than silver.
Q.14 What do you mean by boiling point elevation/rise.
The difference between the boiling point of a solution and that of
pure water at any given pressure is know as boiling point
elevation/rise.
Q.15 Write Stefan Boltzmann law?
It states that the total energy emitted by a black body is directly
proportional to the fourth power of its absolute temperature.

11. Q.16 What do you mean by heat duty of a heat exchanger?
Heat duty of a heat exchanger is defined as the heat that is
transferred from hot fluid to cold fluid per unit time.
Q.17 Write driving force for heat transfer
Temperature difference is driving force for heat transfer.
Q.18 Define fouling factor-
When heat transfer equipment are designed by taking into account
the deposition of dirt and scale by introducing resistance Rd known
as fouling factor.
Unit-m2.K/W

12. Q.19 What is the use of extended surface or fins in heat transfer? Mention the
different types of fins.
Fins increases the heat transfer rate. Mainly fins are of three types
i) Annular fin ii) Pin fin
iii) Rectangular fin

13. Q.20 For what purpose Wilson plot is used.
Wilson plot is used for the determination of heat transfer
coefficient.
ho= 1/intercept
hi= 1/[1/Ui-1/ho]

14. Where
Q = Rate of heat flow, j/s
A= Heat transfer surface,m2
Z= distance measured normal to surface length of conduction, m
dT/dZ= Temperature gradient K/m
- Sign= Indicate decrease in temperature in Z direction
dZ
dT
KAQ 
Q.21 What is Fourier's law of heat conduction?
The rate of heat flow is proportional to area measured normal
to the direction of heat flow and temperature gradient in that
direction.

15. Q.22 Write Dittus-Bolter and Sider tate equation
i) Dittus-Bolter equation is used for turbulent flow
NNU = 0.023(NRe)0.8(NPr)a
a= 0.4 for heating & 0.3 for cooling
ii) Sider tate equation is used for laminar flow
NNU = 1.86[(NRe)(NPr)]0.33(μ/μw )0.14
μ/μw = viscosity correction factor is needed for highly viscous
fluid. For high viscous fluids there will be a marked difference in
viscosity at bulk temperature and at wall temperature.

16. Thermal conductivity(K)
It is also called as proportionality constant, property of material and
transport property of material.
Larger the value of K, higher will be the amount of heat conducted.
Order of thermal conductivity for various phase are
Solid > liquid > gases
2.3-420 0.09-0.7 0.006-0.6W/m.K
Metal having higher thermal conductivity are
Silver [K= 420W/m.K]
Red Copper [K= 395W/m.K]
Gold[K= 302W/m.K]
Aluminum[K=210 W/m.K]

17. Newton's law of cooling-
According to this law heat flux (i.e heat flow per unit area) is equal
to temperature difference between wall temperature and fluid
temperature.
Where
Q/A= Heat flux W/m2
Tw= Wall Temperature
Tf= Fluid temperature
h= heat transfer coefficient W/m2.K
1/h= heat resistance
Tf)h(Tw
A
Q


18. Capacity-
It is defined as the number of kilogram of water vaporized or
evaporated per hour.
Evaporator Economy(steam economy)
It is defined as the number of kilogram of water evaporated per
kilogram of steam fed to evaporator.
Radiation shape factor or view factor-
It is the fraction of radiation energy emitted from one surface and
intercepted by the other surface directly without intervening
reflection.

19. Guide line for directing fluid-
i) Corrosive fluid should be admitted through tubes to save the cost
of expensive alloy shell.
ii) Viscous liquid should be introduced on the shell side as baffles
induces the turbulence
iii) To avoid expensive high pressure shell the high pressure fluid
should pass through tube.

20. Range-
It is the actual rise or fall of temperature of fluid.
Range of hot fluid- Th1-Th2
Range of cold fluid- Tc1-Tc2
Approach-
It is the terminal point temperature difference between hot and
cold fluid.
for counter current flow Th1 - Tc2 is approach

21. The overall resistance to heat flow from a hot fluid to a cold fluid is
made up of three resistance in series
i) Resistance offered by film of hot fluid
ii) Resistance offered by metal wall
iii)Resistance offered by film of cold fluid.

22. EXERCISE
Q.1 Write long form of TEMA
Q.2 Draw the diagram of 1-2 shell & tube and double pipe heat
exchanger.
Q.3 When should we apply the effectiveness- NTU method?
Q.4 What do you mean by thermal diffusivity?
Q.5 List the various dimensionless number that are useful in heat
transfer with its significance?
Q.6 Give the steps of designing of heat transfer.