HEAT EXCHANGER DESIGNING

HEAT EXCHANGER

FUNCTION :

           

To cool the dissolving solution from 65 °C to 35 °C before crystallization.

Design :

        Two heat exchangers connected parallel having same specifications will work on half of the total flow rates.

To cool fluid from 63 °C to 35 °C with water being heated from 25 °C to 40 °C.

           

T1 = 63 °C = 145.4 °F                        ,           t1 = 25 °C = 77 °F

T2 = 35 °C = 95 °F                 ,           t2 = 40°C = 104 °F

Flo 0w rate of Fluid                                           

Flow rate of water

Hot		Cold	Difference
145.4 95	High Temp. Low Temp.	104 77	41.4°F =  ΔT2 18 °F =    ΔT1

Heat Balance:

Heat to be removed      =          2891802.84

63 °C                          35 °C

40 °C                           25 °C       (Eq. 5.14 By Kern P / 89)

LMTD Correction :

                                        0.395              From fig. 18 by Kern P / 828)

Ft for 1 ─   2 exchanger = 0.525          (Very Low)

(From fig. 19 by Kern P / 829)

Ft for 2 ─   4 exchanger = 0.92

Corrected LMTD         =       LMTD x  Ft

=       28.04 x  0.92

=       25.85° F

Let Tubes available for service are  &1" Δ Pitch &13BWG

(From Kern, Page 128)

Inner diameter  of tubes           =          0.56 in.

(From Kern, Page 843, Table10)

(Page 843 by Kern)

Assuming U D from Kern, Page840

U D                  =          40

Q                     =          U DA (LMTD)

(From Kern, page 89, eq.5.13)

                                                                                     2

Total length of tubes                              =         

                                                            =                                  14247.14 ft

Let length of each tube                          =                                  16 ft

(From Richardson & Coulson, Vol 6, Page 645)

Number of tubes                       =                                

`                                                                       =                                  890.45 tubes

                                                                                                (From Page 842 by Kern)

For  “OD and 1" Δ Pitch for 4 passes.

Nearest No. of tubes                                        =                      882

(Table 9, Page 842 by Kern)

And internal diameter of Shell is 35 in.

(From Kern, page 842, Table 9)

A         =          NtdoL

=          8820.196316  

=          2770.185 ft2

Corrected UD                       40.38

This is nearly equal to assumed value of U D = 40

In our case the flow rate of cold fluid H2O is less than hot fluid (slurry) , also slurry is corrosive therefore slurry is taken in tubes and water is taken in shell.

Let,

Baffle spacing               =          B          =          12 in.

            (25 % cut segmented baffles)

(By Kern Page 156)

Clearance       =      Cl    =      PT    -    OD

                                                            =          1.0     -    0.75 in

                                                            =          0.25 in

Shell Side: (water, cold fluid)

At,

=          90.5 °F

For Water,

µ          =          0.8cp   =          0.8×2.42

                                   

(Page 822 by Kern)

(By Kern, Table 10 Page800)

(Page 804 by Kern)   

                                                De        =          0.73 in.

(From fig 28, Page 838 By Kern)

=          10434.68

                                    JH    =      58

(From fig 28, Page 838 By Kern)

(Eq. 6.15b by Kern)

Let,

(By Kern, page111)

                                                          =          1

=          1.83

Tube side (Hot Slurry Fluid):

(Table 10, Page 843 by Kern)

(Eq. 7.48 By Kern)

Eq. 7.2 By Kern, Page 138)

=          0.0466ft

=                 120.2 °F

µ              =      4Cp

                                                            =          4 2.42

                                                                        =          1332.22

=          48.75

K         =          0.9 K water

                                                                                                (Kern page 161)

=          0.9  0.37

=          0.333

=          343.35

JH     =       3.7

(From Kern Fig.24, Page 834)

(Eq. 6.15, By Kern, Page 111)

Where,

(From Kern, Page 111)

=          2.51

So,

(From Kern, Page105, and Eq. 6.5)

(Eq. 6.71, Page 106 by Kern)

(Eq. 6.10, Page 107 by Kern)

=          0.0026

Pressure drop:

Shell side:

N Re    =          10434.68

                                                f           =          0.0021

Fig 29, Page 839 by Kern)

                                                 S         =          1

                                                Ds        =         

                                                            =          2.917 ft

No. of crosses                                      =          N + 1

                                        =          16

For 2 – Passes

16 * 2         =       32

(Eq. 7.44, Page 147 by Kern)

=6.82 Psi

6.82 Psi < 10 Psi  (Allowable pressure drop)

Tube side (Slurry, hot fluid)

For,

N Re    =          1332.22

f           =          0.00048

(From Kern, Fig26, Page 836)

(From Pak Arab NP Manual)

(Eq. 7.45, Page 148 by Kern)

=       1.5 Psi

Exit and Entrance Losses:

                            

For,

(Fig. 27, Page 837 by Kern)

(Eq. 7.46, Page 148 by Kern)

=       0.105 Psi

(Eq. 7.47, Page 148 by Kern)

=          1.5 + 0.105

=          1.605

1.605 < 10 Psi (Allowable Pressure Drop)

As,         

 And  Both less than 10 Psi

So,

ASSUMPTIONS FOR DESIGN ARE VALID

           

INTRODUCTION AND BASIC CONCEPTS

Heat: -

          Heat is defined as the energy in transit. Heat itself cannot be seen, but its effects can be seen and measured as a property called temperature. Heat transfer occurs whenever two bodies are at different temperatures and brought in contact with each other or, whenever there is a temperature gradient within a body.

Heat transfer: -

                   

 The science of thermodynamics deals with the quantitative transition and rearrangements of energy as heat in bodies of matter. Heat transfer is the science, which deals with the rate of exchange of heat between the hot and cold bodies called source and receiver. The science of heat transfer involves the study of the principles that govern and the methods that determine the rate of heat transfer.

Heat Transfer and Thermodynamics: -

First law deals with the energy balances and deals to the concepts of enthalpy, whereas the second law deals with the availability balances and determines the direction in which the heat energy flow and leads to the concept of entropy. In fact, it is the second law, which says that heat flows from a location of high temperature to a location of low temperature. Then you may be wondering as to what is the need for a separate science of heat transfer. The answer is, thermodynamics deals with the equilibrium processes; the laws of thermodynamics can easily calculate total heat transfer from one equilibrium state to another equilibrium state. However, these laws alone can calculate the rate of heat transfer and the temperature variation with time and position and to do this, we need the laws of heat transfer. 

Fundamental Laws of Heat Transfer: -

Fundamental laws governing heat transfer are enumerated below:

(1)-First law of thermodynamics: - give conservation of energy,

(2)-Second law of thermodynamics: - gives direction of heat flow,

(3)-Equation of continuity: - gives conservation of mass,

(4)-Equation of flow:-Newton’s second law of motion,

(5)-Rate equations governing the three modes of heat transfer,

         (a)-Conduction-Fourier’s law of conduction

         (b)-Convection-Newton’s law of cooling 

         (c)-Radiation-Stefan Boltzmann`s law

(6)-Empirical relations for fluid properties such as specific heat, thermal conductivity, viscosity etc,

(7)-Equation of state for the fluid. 

Analogies with Other Transport Processes: -

                                                                        We know that in nature, flow occurs spontaneously from a higher potential to a lower potential: electricity flows from a higher voltage potential to be lower voltage potential and water flows from a higher datum level to a lower datum level. Similarly, for the aforementioned three transport processes, we can observe following:

(1)-Transport of heat energy---occurs from a higher temperature level to a lower temperature level.

 (2)-Transport of momentum---occurs from a higher velocity level to a lower velocity level.

 (3)-Transport of mass---occurs from higher concentration level to a lower concentration level.

In other words, we can say that the driving potential for heat transfer is the temperature gradient, driving potential for momentum transfer is the velocity gradient and driving potential for mass transfer is the concentration gradient. Therefore, we can say that the governing equations for these processes must have some similarity.

Applications of Heat Transfer: -

                                                                            Heat transfer is an important branch of thermal science, which has applications in diverse fields of engineering.

(a)-Chemical engineering: In process equipments used in refineries, chemical plants etc.

(b)-Electrical engineering: Cooling systems for electrical motors, generators, transformers etc.

(c)-Metallurgical engineering: In furnaces, heat treatment of components etc.

(d)-Mechanical engineering: In boilers, heat exchangers, turbine systems, internal combustion engines etc.

(e)-Nuclear engineering: In the removal of heat generated by nuclear fission using liquid metal coolants, design of nuclear fuel rods against possible burnout etc.

(f)-Aerospace engineering and space technology: In the design of aircraft systems and components, rockets, missiles etc.

(g)-Cryogenic engineering: In the production, storage, transportation and utilization of cryogenic liquids for various industrial, research and defense applications.

(h)-Civil engineering: In the design of suspension bridges, railway tracks, air-conditioning and insulation of buildings.

Modes of heat transfer: -

There are three different modes of heat transfer.

1: Conduction

2: Radiation

3: Convection

Strictly speaking, only conduction and radiation called modes of heat transfer, because these processes occur solely due to temperature difference between the two interaction systems. On the other hand, conviction depends on the transport of mass from one region to another, in addition to the temperature gradient.

In most situations heat transfer occur by a combination of these modes.

A qualitative explanation of these modes of heat transfers presented in what follow:

1: Conduction: -

                               Conduction is the mode heat transfer from a region of high temperature to region of low temperature with in a solid, liquid or gas medium or between different mediums, which are in physical contact with each other.

We know all matter consists of large number molecules, which are in random motion.

The energy possessed by matter can broadly classify as macroscopic modes of energy and microscopic modes of energy.

The macroscopic modes of energy can be estimated in term of macroscopically measurable quantities like velocity and position of matter under consideration.

On the other hand, the microscopic mode energy, namely the translational energy, rotational energy and vibration energy, etc., associated with the molecules constituting the matter cannot be estimated in terms of macroscopically quantities. The total energy processed by all these microscopic modes is called the internal energy of the matter. The internal energy of the matter increases with the increase in temperature. If a temperature gradient exists in the medium, the molecules in the successive layers of the medium will have different internal energies. The molecules with high internal energy will have higher translational kinetic energy. Then transfer of energy from molecules with higher internal energy to the molecules with lower internal energy takes place through collisions in fluids or through diffusion of faster moving electrons from higher to low temperature regions in metals.

2: Radiation: -

                          The mode of heat transfer from a body at high temperature to a body at low temperature, when the bodies are not in direct physical contact with each other or when the bodies are separated from each other in space, is called radiation. The energy transfer through radiation exists even if perfect vacuum exists between the two bodies, which are separated from each other. Radiation is an electromagnetic phenomenon and the radiation, which is propagated as a result of temperature difference, is called thermal radiation. The energy transferred by this mechanism is called the radiant heat. All bodies continuously emit radiant heat, which travels with the speed of light. The intensity of the emission depends upon the temperature and the nature of the surface. Heat transfer by radiation requires no medium for propagation and the motion of the radiant heat in space can be described by wave theory. Heat transfer by radiation becomes increasingly important as the temperature of the body increases.

3: Convection: -

                              The mode of heat transfers as heat by the combined effect of conduction and material transport is called the convection heat transfer. Convection heat transfer from a solid surface to the surrounding fluid takes place through the following steps

(a)-Conduction heat transfer takes place from the solid surface to the adjacent fluid particles, thus increasing the internal energy as well as the temperature of the fluid particles.

(b)-The fluid particles with higher internal energy and temperature to a region of low temperature and mix with the fluid particles possessing lesser internal energy and transfer a part of their internal energy.

Depending on the nature of the forces, which causes the material motion, convection heat transfer is classified as natural convection or free convection and forced convection.

Free-Convection: -

                                      If the material transport and mixing of matter occurs due to density differences caused by temperature gradient in the medium, then the convection heat transfer is called natural or free convection.

Forced-Convection: -

                                            If the material transport and mixing of matter is induced by some external agency like a pump, blower, fan or impeller, then the convection heat transfer is called forced convection.

The effectiveness of heat transfer depends largely on the material transport and mixing of matter. A study of convection heat transfer requires knowledge of the fluid flow characteristics.

Steady-state and Unsteady-state Processes: -

In the analysis of heat transfer problems it is essential to identify the mode of heat transfer as well as whether the process is transient or steady state.

Steady state implies that the properties of the system at any specified location are independent of time and there is no accumulation of energy in the system and hence there is no change in the internal energy of the system. That is, the rate of heat transfer into the system is equal to the rate of heat transfer out of the system.

In a steady state the rate of heat transfer in a system is constant and the temperature at any specified location of a system also doesn’t change with the time.

           

On the other hand, if the process is transient or in a unsteady state, the temperature at any specified location of the system changes with time and the internal energy of the system also changes with the time. In the transient process, the rate of heat transfer into the system is not equal to the rate of rate of heat transfer out of system, that’s why there is accumulation or depletion of energy in the system.