2 Mark Question and Answers

Ohm Sakthi

Adhiparasakthi Engineering College

Department of Chemical Engineering

CH2304

HEAT TRANSFER

for

V SEMESTER B.Tech. (Chemical Engg.)

Important 2 Marks Q & A

with

PART-B QUESTION BANK

Prepared by:

Ms.G.Saraswathy B.Tech.,M.E.,

Assistant Professor

Ohm sakthi

UNIT 1

HEAT TRANSFER BY CONDUCTION

1. What is the driving force for heat transfer? Why?

Temperature gradient is the driving force for heat transfer. Only when there is a temperature difference between the hot body and the cold body, heat will flow spontaneously from the hot body to the cold body until thermal equilibrium is reached between the two bodies. In other words, the temperature difference between the two bodies drives the heat transfer between them. Hence, it is known as the driving force for heat transfer.

Q = Driving Force (Temp. gradient)

--------------------------------------------

Thermal Resistance

2. Give some examples of heat transfer operations?

* Evaporators

* Dryers

* Furnaces

* Reactors

* Refrigerators

* Radiators

* Shell and Tube Heat Exchangers

3. Define Conduction?

Conduction is the flow of heat that occurs either due to the exchange of energy from one molecule to another without appreciable motion of the molecules (or) due to the motion of free electrons if they are present.

Conduction occurs only in solids and they should be in physical contact with each other.

Eg: Heat flow through brick wall of furnace.

4. Define Convection?

Convection is the mode of heat transfer which occurs in fluids where the molecules are farther apart. When a fluid flows inside a duct or over a solid body and the temperatures of the fluid and solid surface are different, heat transfer between the fluid and the solid surface will take place. This type of heat transfer is called convection. There are two types of convection.

(i). Free convection – which occurs due to buoyancy effects or density difference created by temperature difference.

(ii). Forced convection – Created by external agencies like fan, blowers etc,

The heat transfer by convection is always accompanied by conduction.

Eg : Boiling of water in a vessel.

5. Define Radiation?

The mode of heat transfer called radiation refers to the transfer of energy through space by electromagnetic waves. Radiation is an electromagnetic phenomenon and it requires no medium. In fact, the energy transfer by radiation is maximum when the two bodies exchanging energy are separated by a perfect vacuum.

Eg: Transfer of heat from the Sun to the Earth.

6. Differentiate between forced convection & free convection.

Free Convection	Forced Convection
If the fluid motion is setup by buoyancy effects resulting from the density variation caused by the temperature difference in the fluid, the mode of heat transfer is said to be free or natural convection. It occurs naturally. The rate of heat transfer is slow when compared to forced convection. Eg : Heating of water in a vessel.	It the fluid motion is artificially created by means of external agency such as blower, fan etc, the mode of heat transfer is termed as forced convection. It is created artificially. The rate of heat transfer is high. Eg : Cooling of a fluid by means of over head fan.

7. What is LMTD?

LMTD stands for Logarithmic Mean Temperature Difference. The usual temperature difference ∆T varies with position in the heat exchanger along its length. So, an average ∆T has to be used in the calculation of heat transfer rate Q. In order to estimate the average or mean ∆T, the individual ∆T values at each and every point along the length of the heat exchanger has to be known which is highly impractical to measure. Instead, the logarithmic mean temperature difference which uses only the two ∆T values at the inlet and outlet is convenient to use and also found to approximately give the values close to experimental ones.

Rate of heat flow, Q = UA ∆Tm

Here, ∆Tm = (∆T_A - ∆T_B)

------------------------ is the LMTD value.

ln (∆T_A / ∆T_B )

where ∆T_A = Temp difference between hot and cold fluid on side ‘A’ of heat

exchanger.

and ∆T_B = Temp difference between hot and cold fluid on side ‘B’ of heat

exchanger.

The larger the LMTD, the more heat is transfer.

8. Give some examples of one dimensional steady state conduction type heat transfer.

Heat flow through: * brick wall of furnace.

* Metal sheet of boiler.

* Metal wall of heat exchanger tube

* Reactor surfaces.

9. State the Fourier’s law of heat conduction?

It states that, “the rate of heat flow (dQ) through a uniform material is directly proportional to the area of heat transfer (A), the temperature gradient in the direction of the heat flow (-dT ) and is inversely proportional to the length of the path of flow (dx).

dQ α - A. dT

----

dQ = - K. A. dT

----

This is the mathematical representation of Fourier’s law of conduction.

10. Define Thermal Resistance?

Thermal resistance in heat transfer is the ratio of driving force DT for heat transfer and the rate of heat transfer.

Thermal Resistance = Driving force for heat transfer (Temp. difference)

----------------------------------------

Rate of heat transfer

∆ T ∆ T

Q = -------- R = ---------

R Q

The unit of thermal resistance is K /W.

The resistance of a plane wall of thickness x is given by x / kA.

11. Define Thermal conductance?

The reciprocal of resistance is called conductance which for heat conduction is,

1 Rate of heat Transfer

C = ------- ═ --------------------------------------

R Driving force for heat transfer (Temp. difference)

∆ T

R = ---------

gives C = ---------

∆ T

Its unit is W / K. For a plane wall of thickness x, conductance can be written as kA / x.

12. Define Thermal conductivity?

Thermal conductivity (k) is the quantity of heat passing through a quantity of material of unit thickness with unit heat flow area in unit time when unit temperature difference is maintained across the opposite faces of material.

dQ = - k. A. dT

----

- dQ . dx

k = ----------- When A=1m², dx=1 m, dT =1 K, then k =dQ

A . dT

The unit of thermal conductivity is W / m K

13. What is meant by series of resistances ?

When a wall is formed out of series of layers of different materials it is called composite wall (or) series of resistances.

14. Give the equations for one-dimensional (1D) steady state heat conduction through a plane wall, compsite plane wall, cylinder, composite cylinder and sphere. [ NOTE: Each equation is a separate 2 mark question. Write the equation as below and also explain the terms completely. Better to draw the respective figures also.]

Heat conduction through plane wall:

k A ( T₁– T₂)

Q = ----------------------

where k is thermal conductivity, A is area perpendicular to the direction of heat flow, x is thickness of material through which heat flows, T₁& T₂are the hot face and cold face surface temperatures respectively.

Heat conduction through a composite plane wall:

DT_overall

Q = ----------------------

x₁/k₁ A + x₂/k₂ A + …..

where x_1,x₂ …are the thicknesses of each successive layer of material and k₁, k₂ ….are the respective thermal conductivity values of layer 1, layer 2 and so on.

Heat conduction through a cylinder :

k Am ( T₁– T₂)

Q = ----------------------

(r₂ – r₁)

where, Am = 2πr_mL is the logarithmic Mean Area and r_m is the logarithmic mean radius which is =

Heat conduction through a composite coaxial cylinder ( or ) a series of (cylinder) resistances:

DT_overall

Q = --------------------------

( r₂ – r₁ ) ( r₃ – r₂ )

----------- + -----------+….

k₁ Am₁ k₂ Am₂

Where, Am₁= 2πr_m1 L

Am₂= 2π r_m2 L

Heat conduction through a sphere:

Q = 4π k ( T₁– T₂)

----------------------

1 1

---- - ----

r₁ r₂

15. Draw the equivalent electrical circuit for heat flow through the following systems? (Each is a separate 2 mark question. Draw the figure and explain the analogy).

Fig 1 : One dimensional heat flow through plane wall & its electrical Analogy / Equivalent electric circuit.

Fig 2 : Heat flow through composite plane wall and its Electrical Equivalent.

Fig 3 : Heat flow through Hollow cylinder and its electrical analogy.

Fig 4 : Heat flow through a composite cylinder and its equivalent electrical circuit / analogy.

Fig 5 : Overall heat transfer through plane wall and equivalent electric current.

16. What is Newton’s law of cooling ?

Newton’s law of cooling gives the heat flow by convection. It states that “the heat flux from a solid surface to a fluid is proportional to the temperature difference between the surface and the fluid.”

Thus if fluid mean temp T_∞ is in contact with a solid surface at a temp T_w and if T_W > T_∞, then,

------ ( T_W – T_∞ )

------ = h ( T_W – T_∞ )

Q = h A ( T_W – T_∞)

The constant of proportionality ‘h’ is called film heat transfer co-efficient.

17. Write a note on conduction in fluids?

Conduction is greater only in solids, where atoms are in constant contact. Conduction is not possible or negligible in liquids except in liquid metals like gallium, Indium that have very low melting points and are liquids at room temperature and gases because the molecules are usually farther apart, giving a lower chance of molecules colliding and passing on thermal energy or heat.

18. How does ‘k’ vary with temperature and how to find average ‘k’ value?

For small temperature ranges, the thermal conductivity may be taken as constant but for large temperature ranges, ‘k’ varies linearly with temperature and the relationship is given as k = a+bT where a & b are empirical constants and T is the temperature.

The average k, k may be obtained either by using arithmetic average of individual values of k at surface temperatures T₁ , T₂etc.

i.e. k = k₁ + k₂

------------- ( or )

by calculating the arithmetic average of temperatures i.e. (T₁ + T₂)/ 2 and using value of ‘k’ at that temperature.

19. Write the Analogous parameters in flow of electricity and heat conduction?

Analogous Parameter	Electricity	Heat conduction
1. Rate of flow 2. Driving force 3. Governing factors 4. Resistance	Current (I ) amperes Potential Voltage difference (∆ V) volts Resistivity etc. Electrical Resistance which is a function of resistivity & dimensions of the conductor.	Heat flow ( Q ) watts Temperature difference (∆T) Kelvin. Thermal conductivity etc. Thermal resistance which is a function of thermal conductivity and dimensions of the conducting solid.

Ohm sakthi

UNIT 2

FILM COEFFICIENTS AND THEIR APPLICATION

1. Give examples of conduction with heat source?

There are situations in which heat generation occurs in a conducting medium. A few common examples are: (i) a nuclear fuel element irradiated by high energy neutrons to trigger nuclear fission (ii) an electrical conductor in which heat generation occurs because of a chemical reaction etc.

The heat generation rate may be a function of its position in the solid.

2. What do you understand by two-dimensional steady state conduction?

In many practical situations, real heat conduction may take place along more than one dimension. This is especially true of systems with irregular boundaries and/or with non-uniform temperatures along boundaries.

Such situations may lead to two- or multi- dimensional conduction.

3. Give some common examples of multi- dimensional conduction?

· Heat treatment of metallic parts of different shapes

· Composite bodies

· Cooling of IC engine blocks, chimneys, air-conditioning ducts etc.

4. Name the various methods of analysis used in multi- dimensional heat conduction problems?

(i). Analytical method

(ii). Graphical method

(iii). Numerical method

(iv). Analogical method

5. What is an isotherm?

An Isotherm is defined as a curve on a graph that connects points of equal temperature. It is also called an isothermal line.

6. Define a conduction shape factor?

It is defined as the ratio of total number of heat flow tubes in the cross section and the number of temperature increments between T₁ and T₂ so that T₁-T₂=MDT.

7. Write the relationship between the conduction shape factor and thermal resistance?

Conduction shape factor, S and thermal resistance R_th are related by: R_th = 1/kS where k is thermal conductivity of the material. Thus , the shape factor for any configuration may be obtained after evaluating its thermal resistance.

8. Define: Transient Heat Conduction?

A solid body is said to be in a steady state if its temperature does not vary with time. However, if there is an abrupt change in its surface temperature or environment, it takes some time before the body to attain an equilibrium temperature or a steady state.

During this interim period, the temperature varies with time & the body is said to be in an UNSTEADY STATE or TRANSIENT STATE.

9. Give examples of occurrence of transient heat conduction.

Boiler tubes, rocket nozzles, electric irons, automobile engines, cooling of IC engines, cooling and freezing of food, heat treatment of metals by quenching, heating and cooling of buildings etc.

10. How is the temperature field represented in any transient heat problem?

T=f(x,y,z,t). The solution of an unsteady state problem will be more complex than that of a steady state one because of the presence of another variable time, t.

11. What are the types of transient heat conduction problems?

I. Periodic heat flow type problems – in which the temperature varies on a regular basis. Eg: the variation of temperature of the surface of earth during a twenty-four hour period.

II. Non-Periodic heat flow type problems – in which the temperature at any point within the system varies non-linearly with time.

Ohm sakthi

UNIT 3

CONVECTION

1. What is Dimensional - Analysis?

Dimensional - Analysis is a method by which we combine the independent variables of a problem into dimensionless groups and the experimental data can be very conveniently expressed in terms of these dimensionless numbers.

2. State the Buckingham’s theorem?

It states that, “The number of independent dimensionless groups that can be formed from a set of ‘n’ variables having ‘r’ basic dimensions is (n-r).”

This theorem is useful in determining the number of independent dimensionless groups that can be obtained from a set of variables.

3. Define Hydrodynamic boundary layer (or) velocity boundary layer?

When a fluid moves over a solid surface, the fluid particles at the surface of the solid have the velocity approximately zero. The transition from zero velocity at the surface of the solid to the free stream velocity at some distance away from the solid surface in the direction normal to the direction of flow takes place in a very thin layer called “ Velocity ( or) Hydrodynamic boundary layer ”.

It can also be defined as a region in which the fluid velocity is less than 99% of the bulk fluid velocity or free stream velocity.

4. Define Thermal boundary layer?

When a heated solid body is placed in a fluid stream, due to the difference in temperature between the solid surface and the fluid, heat transfer will occur and a temperature gradient will be set up. This temperature gradient is considered to exist within a layer close to the surface. This layer of fluid close to the surface within which the temperature gradient exists is called as the “Thermal boundary layer”.

5. What is Nusselt Number? What is its significance?

Nusselt Number is a dimensionless number which is given by:

Nu =

where h = heat transfer coefficient

L = Characteristic length

k = Thermal conductivity

Significance: It is the ratio of wall temperature gradient to the temperature gradient across the fluid in the pipe.

6. What is Reynolds Number & what is its significance?

Reynolds Number is a dimensionless number which is given by:

Re =

Where D = Diameter of pipe

u = Velocity of fluid

ρ = Density of fluid

μ = Viscosity of fluid

Significance: It can be described on the ratio of inertial force to viscous force.

7. What is Prandtl Number & write its significance?

Prandtl Number is a dimensionless number which is given by:

Pr =

where Cp = heat capacity of fluid, μ is the viscosity of the fluid and k is thermal conductivity of fluid .

Significance: It is the ratio of momentum diffusivity ( ) to thermal diffusivity ( ) i.e.

8. Write the equation for Stanton Number & write its significance?

Stanton Number is a dimensionless number which is given by:

Nu h

St = ----------- = ----------

Re. Pr ρ u Cp

where h = convective heat transfer coefficient, ρ = density of fluid, u is fluid velocity, Cp = specific heat capacity of fluid.

Stanton Number is the ratio of Nusselt Number to the product of Reynolds Number and Prandtl Number.

Significance: It gives the ratio of rate of wall heat transfer by convection to the rate of heat transfer of bulk flow.

9. Define Peclet Number and give its significance?

Peclet Number (Pe) is a dimensionless number which is given by:

ρ u L Cp

Pe = Re. Pr = ------------

where ρ = density of fluid , u = velocity, L = Characteristic length, Cp = specific heat capacity of fluid, k =thermal conductivity.

Significance: It gives the ratio of rate of heat transfer by Bulk flow to the rate of heat transfer by conduction.

10. What is Graetz Number & write its significance?

Graetz Number (Gz) is a dimensionless number which is given by:

m Cp Pe . d = Pe . d

Gz = -------- = -------- --------

K L L L

where m is mass flow rate, Cp and K are respectively heat capacity and thermal conductivity, L = characteristic linear dimensions for flow.

Significance: It is similar to Peclet number but used in connection with analysis of heat transfer in laminar flow of fluids in pipes.

11. What is Grashoff Number and what is its significance?

Grashoff Number (Gr) is a dimensionless number which is given by:

g L³ ( T_s – T_o)

Gr = -------------------------

where L – Characteristic length, g - Acceleration due to gravity, - coefficient of volumetric expansion, T_s - wall temperature, T_o – ambient fluid temperature, – viscous force.

Significance:

Buoyancy force

It the ratio of --------------------------

Viscous force

Grashof number plays the same role in natural convection as the Reynolds number does in forced convection.

12. What is Biot Number? What is its Significance?

Biot Number (Bi) is a dimensionless number which is given by:

where, L_c – Characteristic length, k – thermal conductivity of solid & h - heat transfer coefficient.

Significance: It gives the ratio of internal thermal resistance of a solid body to its surface (external) thermal resistance.

It is used in analysis of unready state conduction problems.

13. Write Sieder – Tate equation for laminar flow.

Seider and Tate equation for laminar flow:

Nu = 1.86

The above correlation is applicable when,

(a) 0.48 < Pr < 16700

(b) the viscosity ratio is within the range 0.0044 < < 9.75

14. Write Sieder and Tate equation for Turbulent flow?

Sieder and Tate equation for Turbulent flow

Nu = 0.027 Re ^0.8 Pr ^0.33

The condition of applicability of this equation is

(a) 0.7 Pr 16700

(b) Re 10000

15 . Write the Dittus – Boelter Equation?

Dittus – Boelter Equation for turbulent flow through a circular pipe:

Nu = 0.023 Re^0.8 Prⁿ

where n = 0.4 for heating ( Tw > T )

n = 0.3 for cooling ( Tw > T )

The condition for applicability of this equation is

(a) 0.7 Pr 160

(b) D /L 0.1

16. State the Reynolds Analogy?

Reynolds was the first person who observe that “ There exists a similarity between the exchange of momentum and exchange of heat energy in laminar motion and for that reason it has been termed as “Reynolds Analogy “

Nu h

------ = St = ------- = f/2

Re.Pr Pu Cp

This can be used to determine the heat transfer coefficient ‘h’ if the function factor ‘f’ is known.

17. State Prandtl Analogy?

St = -----------------------

1+5 (Pr -1)

Here ‘f ‘is fanning friction factor. It provides a more realistic equation of turbulent flow. It reduces to Reynolds analogy if Pr = 1

18. Chilton - Colburn Analogy – Define?

------------ = j_H= = f/2

Re.Pr^1/3

Where j_H is colburn j factor

Using the well known correlation for the friction factor f = 0.046 Re¯^0.2 for pipe flow, j -factor is given by j_H = 0.023 Re^-0.2

19. What is Rayleigh Number?

Rayleigh Number is the product of Grashof number and Prandtl number.

Ra = Gr x Pr

L³ p² g T Cp μ

= ---------------------- * ----------

² K

L³p² g T μ

Ra = ---------------------

² K

20. What are the types of boiling?

Types of Boiling

Pool Boiling (or) Flow Boiling (or )

Natural convection Boiling Forced convection Boiling

21. Differentiate pool Boiling from flow boiling?

If heat is added to a liquid from a submerged solid surface, the boiling proves is called pool boiling. In this process, the vapours produced may form bubbles which grow and subsequently detach themselves from the surface, rising to the free surface due to buoyancy effects.

E.g. Boiling of water in a kettle.

In contrast, “flow boiling “occurs in a flowing stream & the boiling surface itself be a portion of the flow passage.

E.g.: Heating of fluid flowing through heat exchanger tubes.

22. What is the classification of pool boiling with respect to temperature?

Based on the liquid temperature, the types of pool boiling are:

(1) Sub-cooled or local Boiling where Liquid temperature < Saturation temperature

(2) Saturated or bulk Boiling where Liquid temperature Saturation temperature

23. Name the six regimes of pool boiling curve?

There are six regimes in pool Boiling curve which are,

Zone - I : Free Convection

Zone - II : Bubbles condense in super heated liquid

Zone - III : Bubbles rise to surface

Zone - IV : Unstable film boiling

Zone -V : Stable Film boiling

Zone - VI : Radiation coming into play

24. Define Critical heat flux point or peak heat flux point?

In the pool boiling of a saturated liquid, for excess temperatures T_e) beyond 50°C, nucleate boiling regime ends and film boiling starts. The maximum heat flux point occurs at this transition which is of the order of 1 MW / m². This maximum heat flux point is called CRITICAL HEAT FLUX POINT OR PEAK HEAT FLUX POINT.

The aim is to operate an equipment close to this point by never beyond it. If the heating of metallic surfaces is not limited to this point, the metal may be damaged or it may even melt. That is why it is also known as BURN OUT POINT.

25. Define Burn out Point?

If the heating of metallic surfaces is not limited to this critical heat flux point, the metal may be damaged or it may even melt. That is why this peak heat flux point is also known as BURN OUT POINT.

26. What are the types of condensation?

Film wise condensation Drop wise condensation

In this type, the condensate wets In this type, the vapor condenses

the surface forming a continuous into small liquid droplets of

film which covers the entire surface. vapors various sizes which

fall down the surface in a

random fashion .

Generally occurs on a clean Occurs on surface which is uncontaminated surface contaminated or coated with certain

additives

27. Which condensation gives high heat transfer rate? Why?

The dropwise condensation gives a much higher rate of heat transfer than filmwise condensation.

This is because in dropwise condensation, a large portion of the area of the surface is directly exposed to the vapour facilitating higher heat transfer rates.

But in filmwise condensation, the condensate film covers the entire surface and further grows in thickness as it moves down by gravity. There exists a temperature gradient in the film yielding lesser heat transfer rates.

28. How will you find Thermal layer thickness?

If δ_t is thermal boundary layer thickness and δ is the velocity boundary layer thickness, then

δ_t = -------------

Pr ^1/3

Using Pr 0.01, we get

-------- 0.16

δ_t

This relationship gives the flat plate analysis and slug flow model for heat transfer in liquid metals.

29. What are the factors that influence the heat transfer coefficient during nucleate Boiling?

The factors which affect the nucleate boiling are:

(a) Pressure: It controls the rate of bubble growth and therefore affects the temperature difference causing heat energy to flow.

(b) Heating surface characteristics: The material of heating element has a significant effect on boiling heat transfer coefficient.

(d) Mechanical Agitation: The rate of heat transfer will increase with the increase in mechanical agitation of the fluid.

30. How will you find boundary layer thickness?

The boundary layer thickness (δ) can be found by using the following equation,

Where δ = boundary layer thickness

x = distance from leading edge of plate

Re_x = local Reynolds Number =

Ohm sakthi

UNIT 4

HEAT EXCHANGERS

1. What is a heat exchanger? Mention some of its applications?

A heat exchanger is any device used for effecting the process of heat exchange between two fluids that are at different temperatures.

Uses: Heat exchangers are useful in many engineering processes like those in:-

a) Refrigerating and air-conditioning systems

b) Power systems

c) Food processing systems

d) Chemical reactors

e) Space or aeronautical applications

2. What are the types of heat exchangers?

(i). Direct contact heat exchangers

(ii). Recuperators or surface heat exchangers

(iii). Regenerators

3. What is a direct contact heat exchanger? Give eg.

A heat exchanger in which two fluids exchange heat by coming into direct contact is called a direct contact heat exchanger. Eg: open feed water heaters, desuperheaters, jet condensers etc.

4. What are recuperators? Give eg.

Recuperators are surface heat exchangers in which the fluids are separated by a wall. The wall may be a simple plane wall or a tube or a complex configuration involving fins, baffles and multiples of tubes. For eg: Double pipe heat exchangers, shell-and-tube heat exchangers, condensers, evaporators etc.

5. What are regenerators? Give eg.

A periodic flow type of heat exchanger is called a regenerator. In this type of heat exchanger, the same space is alternatively occupied by the hot and cold gases between which heat is exchanged. Eg: They are used in preheaters for steam power plants, blast furnaces, oxygen producers etc.

6. How will you classify heat exchangers based on fluid flow arrangement?

(i) If both the hot and cold fluids flow in the same direction, the arrangement is called PARALLEL FLOW heat exchanger

(ii) If both the fluids move in opposite directions, the arrangement is called COUNTER FLOW heat exchanger

(iii) If both the fluids move at right angles to each other through the heat exchanger, the arrangement is called CROSS FLOW heat exchanger

7. What is the role of baffles in a heat exchanger?

ü To create a turbulence in the shell-side fluid

ü To enhance the cross flow velocity of shell-side fluid relative to the tubes, baffles are generally provided.

8. What are compact heat exchangers? Give eg.

Compact heat exchangers are special types of heat exchanger s that are compact in size. However their relative heat transfer area is increased by the usage of fins, pins or spiral grooves on their outer surface. Normally a liquid flows through the tubes and a gas with a low heat transfer coefficient flows over the extended surfaces.

9. What is overall heat transfer coefficient?

For a plane wall, U = where i and o represent inside and outside surfaces of wall respectively.

For a cylindrical wall, Uo = and Ui =

Overall heat transfer coefficient takes into account the inside and outside surface heat transfer coefficients of convection hi and ho as well as the thermal conductivity k of convection.

10. Define : Fouling Factor

The surfaces of a heat exchanger do not remain clean after it has been in use for some time. The surfaces become fouled with scaling or deposits which are formed due to impurities in the fluid, chemical reaction between the fluid and the wall material, rust formation etc.

The effects of these deposits is felt in terms of greatly increased surface resistance affecting the value of U. This effect is taken care by introducing an additional thermal resistance called the FOULING RESISTANCE or FOULING FACTOR, R_f.

The unit of R_fis m² K / W.

11. Which heat exchanger requires lesser area – parallel flow or counter flow? Why?

The area requirement of a counter flow heat exchanger is lesser compared to that for a parallel flow heat exchanger. This is because for the same terminal temperatures of fluids and for the same heat transfer rate, the LMTD for a counter flow type is MORE than that for a parallel flow type.

12. Draw the temperature distribution of fluids in (a) condenser and (b) evaporator?

In the case of a condenser, the hot fluid will remain at constant temperature since it undergoes a phase change, while the temperature of the cold fluid increases.

In the case of an evaporator, the cold fluid will remain at constant temperature since it undergoes a phase change, while the temperature of the hot fluid decreases.

13. What is the role of correction factor F in heat exchanger calculations?

The flow conditions in multiple-pass and cross-flow heat exchangers are much more complicated than those in double-pipe and single-pass heat exchangers. For such complex cases, the determination of the mean temperature difference is so difficult that the usual practice is to modify the general equation Q = U A DTm, by including the correction factor F, as follows:

Q = U A F DTm where DTm is the same value as for a counter flow double-pipe heat exchanger with the same hot and cold fluid temperatures as in the more complex design.

14. Define P and R in heat exchanger design?

The two temperature ratios P and R used in the design of multipass and cross flow heat exchanger are defined as follows:

P =

R =

Where to & ti refer to outlet and inlet temperatures of tubeside fluid and To & Ti refer to outlet and inlet temperatures of shellside fluid.

A good design should involve the selection of parameters P and R such that the value of F is always greater than 0.75

15. What is Effectiveness-NTU Method? When it is used?

This method is used for heat exchanger analysis if the terminal temperatures of the fluids are not known. This type of situation is encountered in the selection of a heat exchanger or when the exchanger is to be run at off-design conditions.

This method is based on effectiveness of a heat exchanger in transferring a given amount of heat.

Effectiveness, ε = =

ε = ε The group is called the NTU –Number of Transfer Units.

16. Define NTU? What is its significance?

The group is called the NTU –Number of Transfer Units.

ü NTU is a dimensionless parameter.

ü It is a measure of the heat transfer size of the exchanger.

ü The larger the value of NTU, the closer the heat exchanger reaches its thermodynamic limit operation.

Ohm sakthi

UNIT 5

RADIATION AND EVAPORATION

1. Define Radiation and give two examples?

The mode of heat transfer called Radiation refers to the transfer of energy through space by electromagnetic waves. Radiation is an electromagnetic phenomenon and it requires no medium.

Examples: The tube stills in petroleum refineries, Operation of a furnace etc.

2. Define Absorptivity, Reflectivity and Transmissivity ?

Thermal radiation incident on a body tends to increase its temperature. However, depending upon the nature of the material constituting the body and its surface characteristics, the incident radiation may be absorbed, reflected or transmitted, partly or fully.

The fraction of incident radiation absorbed by a body is called absorptivity (α). The fraction reflected is called reflectivity ( p) and the fraction transmitted through the body is the Transmissivity (τ). These fractions should add up to unity:

α+ ρ + τ = 1

3. Define Black body.

A surface which absorbs light of all wavelengths in the visible range is called Black body. Thus for an opaque black surface, ρ = o ; τ = o ; α = 1

· A black body completely absorbs incident radiation irrespective of their wavelength i.e. α= 1.

· A black body is a perfect emitter, ε = 1 . A black body is a perfectly “diffuse emitter ”

4. Define White Body ?

A surface that reflects light of all wavelengths in the visible range and does not absorb any light preferentially is called a “ White Body ”

Thus for opaque white surface, α= o ; τ = o ; ρ = 1

5. Define Gray body?

A gray body is defined as a substance whose emissivity and absorptivity are independent of wavelength. Thus a gray body is also an ideal body, but its ε and α values are both less than unity.

6. Define Emissivity?

The ratio of the total emissive power of a body (E) to that of a black body (E_b) is called emissivity (ε).

ε = -------

E_b

7. Differentiate between Specular and Diffuse surfaces?

Specular surface	Diffuse surface
If angle of incidence of radiation is equal to the angle of reflection then the reflection is called specular. There is only one reflected radiation.	If the angle of incidence is not equal to the angle of reflection in diffuse surfaces. The incident radiation is reflected uniformly in all directions.

8. What are the characteristics of a black body?

A blackbody is a surface that has the following characteristics:

· A black body completely absorbs incident radiation irrespective of their wavelength i.e. α= 1.

· A black body is a perfect emitter, ε = 1

· A black body is a perfectly “diffuse emitter ”

9. Write the Planck’s equation?

2π h C² λ^-5 K₁ λ^-5

E_b_λ = ------------------------------- = ------------------

exp (h C / λ K_B T ) ^-1 exp (K₂/λ T )^-1

Where,

E_b_λ = Emissive power of monochromatic black body.

T = Absolute Temp of black body.

h = Planck’s constant

K_B = Boltzmann constant

C = Velocity of light

K₁ = 2π C²h and K₂ = h C / k_B

The above equation is known as “ Planck’s law ” or “ Planck’s distribution.”

10. State Planck’s law.

By Planck’s law, monochromatic emissive power E_b_λcan be defined as the amount of radiant energy emitted by a surface per unit area per unit time and per unit wavelength.

(Also write the Planck’s equation here as in Q.No:9)

11. State the wein’s displacement law ?

Wein’s Displacement law can be deduced from Planck’s law that the maximum wavelength λmax corresponding to the peak of the λ Vs E_b_λplot is inversely proportional to the temperature of black body.

In other words, λ max T = constant = 2898 µm K

The above eqn is called “Wein Displacement law ”

12. State the Stefan Boltzmann law ?

A basic Relationship for blackbody radiation is the Stefan Boltzmann law which states that the total emission power of a black body is directly proportional to fourth power of Absolute temperature.

E_b = T⁴

Where, is universal constant = Stefan Boltzmann constant = 5.729x10^-8 W / m² K⁴

13. What is meant by monochromatic emission?

The term monochromatic emission refers to emission of radiation consisting of electromagnetic waves of a single wavelength. The monochromatic emission power (or) spectral emission power E_b_λof a black body as function of wavelength of radiation and is derived in Planck’s law.

14. State Kirchhoff’s law ?

Kirchhoff’s law states that the emissivity of a body which is in temperature equilibrium with its surroundings is equal to its absorptivity.

ε = α (or )

( ε / α) = a constant

15. Write the factors which affect (or) determine the rate of radiant heat exchange between two bodies?

The factors are,

* The temperature of the individual surfaces

* The emissivities of the individual surfaces

* How well one surface can view/see the other

* The absorptivity of the intervening medium

16. Define view factor (or) shape factor (or) Configuration factor?

The fraction of total radiant energy that is emitted by the surface “ i “ and is received or intercepted by the surface “ j “ is called the view factor or Configuration factor or shape factor.

The fraction of total radiation emitted by i &intercepted by j

View factor, F_ij = ------------------------------------------------------------------------------------

Total radiation emitted by “ i “

17. What is called overall Interchange factor?

For heat exchange by radiation of infinitely two parallel plates, the overall Interchange factor can be expressed as

F₁₂ = ---------------------------

1 1

---- + ---- - 1

e₁ e₂

Here, F₁₂ is called overall Interchange factor which is a function of e1 and e₂i.e. emissivities of two plates.

18. Define Radiation shield?

In order to reduce the transfer by radiation between two surfaces, a third surface is introduced in between them. This surface is known as Radiation shield.

For the simple case, when ‘n’ shields are employed each having the same

emissivities as the initial planes,

Qn = ---------- Q

n + 1

where Qn = Q with ‘n’ shield s

Q = Exchange of energy is the initial planes were not separated by

radiation shield.

Q_{before shield} ( T₁⁴– T₃⁴)

---- = --------------------

A 1 1

----- + ---- - 1

e₁ e₃

Q _{after shield} ( T₁⁴– T₂⁴) ( T₂⁴– T₃⁴)

---- = -------------------- = -----------------------

A 1 1 1 1

----- + ---- - 1 ----- + ---- - 1

e₁ e₂ e₂ e₃

1. What is Evaporation?

Evaporation is the process of vapourisation of mainly aqueous solution in order to get a concentrated solution. Concentration of dilute solutions in chemical industries is common. For example, juice of sugarcane has to be concentrated in order to obtain sugar. The concentration is performed by evaporation of water from juice. In evaporation normally the thick liquor is valuable product and the vapours are discarded.

2. Define BPR or BPE ?

BPR- Boillng point rise ; BPE- Boiling point elevation

BPE=Boiling point of solution - Boiling point of pure solvent

BPE is the difference between boiling point of solution and boiling point of pure solvent. BPE is caused due to the increased boiling point when a solute is dissolved in a solvent.

3. State Duhring rule.

Duhring rule is useful for determining BPR or BPE. It states that boiling point of a given solution at a given pressure is linear function of boiing point of water at the same pressure. Thus by plotting boiling point of solution Vs boiling point of water a straight line prevails at same pressure.

4. What is driving force in evaporation?

The difference in temperature, ∆T is the driving force in evaporation. In evaporation, ∆T is the difference between the temperature of the steam (being used for evaporation) and the temperature of solution at which it boils.

5. What are the types of evaporators?

6. What is the mechanism involved in the natural circulation evaporators?

Natural circulation evaporators are those in which the solution circulates due to density difference or buoyancy effect caused by heating the solution in contact with the heating surface.

7. What is the mechanism involved in the forced circulation evaporators?

In forced circulation evaporators, the centrifugal pump forces solution through the tubes at the velocity of 1-6 m/s. the boiling does not start in the tubes due to sufficient static heat. Due to this static heat the solution becomes superheated. Since the velocity of the flashing mixture is high, the separation of vapours and concentrated solution takesplace due to inertial force at impingement baffles. Impingement baffles minimize the entrainment also.

8. Define Capacity of evaporator

Capacity of evaporator is defined as amount of water evaporated per hour.

9. Define Economy of evaporator

Economy of evaporator is defined as number of kgs of water evaporated per kg of steam used.

10. What is the use of multiple effect evaporator?

Multiple effect evaporator is used to enhance the steam economy by utilizing the enthalpies of water vapours obtained from the previous evaporator.

11. How will you determine steam consumption ratio?

The steam consumption ratio can be determined by dividing capacity with economy. It is given by number of kgs of steam consumed per hour.

12. Overall material balance for evaporator?

Kg/h of feed = Kg/h water evaporated + Kg/h of thick liquor.

13. Types of multiple effect evaporator?

Generally, four types of multiple effect evaporators are commonly used,

Ø Forward feed multiple effect evaporator.

Ø Backward feed multiple effect evaporator.

Ø Parallel feed multiple effect evaporator.

Ø Mixed feed multiple effect evaporator.

Ohm Sakthi

PART-B QUESTION BANK

UNIT -1

1. Discuss in detail the three modes of heat transfer with illustrations.

2. Derive the equation for one-dimensional steady state heat conduction equation for a plane wall /flat plate.

3. Problems based on the above derivation in Q.No:2

4. What is thermal conductivity? Write its significance.

5. Derive the equation for one-dimensional steady state heat conduction equation for a composite plane wall or composite flat plate or through a series of resistances.

6. Problems based on the above derivation in Q.No:5

7. Derive the equation for one-dimensional steady state heat conduction equation for a hollow cylinder.

8. Problems based on the above derivation in Q.No:7

9. Derive the equation for one-dimensional steady state heat conduction equation for a composite coaxial cylinder or through a series of cylindrical resistances.

10. Problems based on the above derivation in Q.No:9

11. Derive the equation for one-dimensional steady state heat conduction equation for a hollow sphere.

12. Problems based on the above derivation in Q.No:11

13. Discuss the analogy between heat flow and electricity flow in detail.

UNIT -2

1. Derive the equation for conduction with heat source.

2. Derive the equation for two-dimensional steady state conduction using analytical method.

3. Describe the graphical analysis of two-dimensional systems.

4. Discuss the transient heat conduction problem with examples.

5. Problem based on transient heat conduction.

UNIT -3

1. Derive an expression for forced convection heat transfer under turbulent flow conditions using dimensional analysis.

2. Derive an expression for free convection heat transfer under turbulent flow conditions using dimensional analysis.

3. Discuss the influence of boundary layer on heat transfer?

4. Write the significance of the following dimensionless numbers: (i) Nu (ii) Re (iii) Pr (iv) Gr (v) Gz (vi) St (vii) Pe (viii) Bi

5. Write short notes on the heat transfer in packed and fluidized beds.

6. Write short notes on the heat transfer in molten metals and liquid metals.

7. Discuss the different regimes of pool boiling curve with a neat explanatory diagram. Also write its significance.

(or) Explain the various stages involved in the boiling of saturated liquid.

8. Problem based on equations for forced convection.

9. Problem based on equations for free convection.

10. Problems based on boundary layer thickness.

UNIT-4

1. Give a neat sketch of typical heat exchange equipment indicating its components and functions. (or) Describe the parts and functions of a 1-2 heat exchanger with a neat sketch.

2. Problems based on calculating Overall Heat Transfer Coefficient U with and without fouling resistance.

3. Derive an expression for Logarithmic Mean Temperature Difference (LMTD) for rate of heat transfer per unit area of parallel/counter flow heat exchanger.

4. Problems based on LMTD to calculate area or length of parallel/counter flow heat exchanger.

5. Problems to calculate area of condenser or evaporator.

6. Problems to calculate P,R in multipass or cross flow exchanger and finally to calculate area.

7. Derive an expression for ε-NTU for parallel flow heat exchanger. Also give graphical representation.

8. Derive an expression for calculating the effectiveness of a counter-current flow heat exchanger

UNIT -5

1. Explain the concept of black body. What are its characteristics?

2. Describe the following radiation laws in detail: (i) Planck’s Law (ii) Wein’s Displacement Law (iii) Stefan-Boltzman’s Law (iv) Kirchoff’s Law

3. Explain the grey body concept and derive the Kirchoff’s Law.

4. Derive an expression for radiation heat exchange between two parallel surfaces or planes.

5. Problems based on above derivation in Q.No:4

6. Explain the effect of radiation shield kept between two radiating surfaces.

7. Problem based on calculation of heat transfer rate before and after placing the radiation shield.

8. With a neat sketch, explain the construction and working of an evaporator. (short tube calendria type standard vertical evaporator is preferred). What are its merits and limitations?

9. Discuss the different feeding arrangements in multiple effect evaporator with neat sketches.

10. Problem based on calculating area and capacity of a single effect evaporator.

11. Problem to calculate steam economy.

12. Problem to calculate the boiling points of solutions in a triple effect evaporator.

Pages

Thursday, 4 October 2012

2 Mark Question and Answers

No comments:

Post a Comment

Total Pageviews