Thermal Design Considerations

Contents Acknowledgments............................................................................................ i List of figure .................................................................................................. iv List of table..................................................................................................... v NOMENCLATURE ...................................................................................... vi Preface .......................................................................................................... vii Chapter 1: Introduction ................................................................................. 1 1.1. Heat exchanger......................................................................................................... 1 1.2. Classification of heat exchanger............................................................................... 1 1.2.1. Fixed tube sheet exchanger............................................................................................ 2 1.2.2. Removable tube bundle ................................................................................................. 3 1.2.2.1. U – Tube ........................................................................................................................ 3 1.2.2.2. Floating head ................................................................................................................. 4 Chapter 2: Thermal Design Considerations ................................................... 6 2.1. Shell.......................................................................................................................... 6 2.2. Tube.......................................................................................................................... 7 2.3. Tube pitch and tubelayout ....................................................................................... 8 2.4. Tube passes............................................................................................................... 9 2.5. Tube sheet............................................................................................................... 10 2.6. Baffles..................................................................................................................... 11 2.7. Fouling Considerations .......................................................................................... 13 2.8. Selection of fluids for tube and the shell side.......................................................... 15 Chapter 3: Thermal Design Process ............................................................. 16 Chapter 4: Design problem .......................................................................... 20 4.1. Problem statement .................................................................................................. 20 4.2. Fluid properties ...................................................................................................... 21 4.2.1. Crude oil ...................................................................................................................... 21 4.2.2. Kerosene ...................................................................................................................... 22 4.3. Assign fluid to shell and fluid to tube...................................................................... 24 4.4. Evaluate heat duty and outlet temperature of crude oil .......................................... 25 4.5. Estimate the overall heat transfer coefficient.......................................................... 27 iii 4.6. Calculate Logarithmic mean temperature different................................................ 30 4.7. Calculate heat transfer area.................................................................................... 32 4.8. Decide appropriate tubes......................................................................................... 33 4.9. Calculate tube – side heat transfer coefficient ........................................................ 35 4.10. Calculate shell diameter.......................................................................................... 37 4.11. Calculate shell – side heat transfer coefficient........................................................ 39 4.12. Overall heat transfer coefficient ............................................................................. 41 4.13. Pressure drop.......................................................................................................... 42 Conclusion .................................................................................................... 47 Reference ...................................................................................................... 48

Acknowledgments

I would like to thank to all of the teachers in the department petrochemical, oil and gas faculty, Hanoi University of Mining and Geology was dedicated teach

me during the time I study and practice at school

I am deeply extending my sincere appreciation to my instructor, Dr Vũ Văn Toàn, for his valuable advice, constant support, commitment, dedication, encouragement and precious guidance, creative suggestions and critical comments, and for his being everlasting enthusiastic from the beginning to the end of the seminar Without his urge, no doubt, this work would not have been possible at all

And finally, I would like to thank my parents, friends who always there encouraging me during 5 years as well as during I do my graduation thesis

Contents

Acknowledgments i

List of figure iv

List of table v

NOMENCLATURE vi

Preface vii

Chapter 1: Introduction 1

1.1 Heat exchanger 1

1.2 Classification of heat exchanger 1

1.2.1 Fixed tube sheet exchanger 2

1.2.2 Removable tube bundle 3

1.2.2.1 U – Tube 3

1.2.2.2 Floating head 4

Chapter 2: Thermal Design Considerations 6

2.1 Shell 6

2.2 Tube 7

2.3 Tube pitch and tube-layout 8

2.4 Tube passes 9

2.5 Tube sheet 10

2.6 Baffles 11

2.7 Fouling Considerations 13

2.8 Selection of fluids for tube and the shell side 15

Chapter 3: Thermal Design Process 16

Chapter 4: Design problem 20

4.1 Problem statement 20

4.2 Fluid properties 21

4.2.1 Crude oil 21

4.2.2 Kerosene 22

4.3 Assign fluid to shell and fluid to tube 24

4.4 Evaluate heat duty and outlet temperature of crude oil 25

4.5 Estimate the overall heat transfer coefficient 27

4.6 Calculate Logarithmic mean temperature different 30

4.7 Calculate heat transfer area 32

4.8 Decide appropriate tubes 33

4.9 Calculate tube – side heat transfer coefficient 35

4.10 Calculate shell diameter 37

4.11 Calculate shell – side heat transfer coefficient 39

4.12 Overall heat transfer coefficient 41

4.13 Pressure drop 42

Conclusion 47

Reference 48

List of figure

Figure 1.1: Classification of shell and tube heat exchanger 2

Figure 1.2: Fixed tube sheet heat exchanger 3

Figure 1.3: U - tube heat exchanger 4

Figure 1.4: Floating head heat exchanger 5

Figure 2.1: Shell of a heat exchanger 6

Figure 2.2: Tube of a straight tube heat exchanger 7

Figure 2.3: Heat exchanger tube layout 8

Figure 2.4: Straight tube heat exchanger one and two pass tube side 9

Figure 2.5: Tube passes arrangement 9

Figure 2.6: Stainless tube sheet 10

Figure 2.7: Different type of heat exchanger baffles 11

Figure 2.8: Baffle cut 12

Figure 2.9: Baffle cut and fouling in shell 12

Figure 2.10: The fouling in heat exchanger 13

Figure 4.1: Specific heat of hydrocarbon liquid 25

Figure 4.2: Overall heat transfer coefficient 28

Figure 4.3: Correction factor F 31

Figure 4.4: Tube side heat transfer factor 35

Figure 4.5: Shell bundle clearance 38

Figure 4.6: Shell side heat transfer factor 40

Figure 4.7: Tube side friction factor 42

Figure 4.8: Shell side friction factor 43

List of table

Table 2.1: Typical values of fouling coefficients and resistances 14

Table 2.2: Guidelines for placing the fluid in order of priority 15

Table 4.1: Typical overall heat transfer coefficient 27

Table 4.2: Constant for calculating shell diameter 37

Table 4.3: The common tube per pass 45

NOMENCLATURE

Cp,h Specific heat of hot fluid kJ/kg.oC

Cp,c Specific heat of cold fluid kJ/kg.oC

Th,i Hot fluid temperature at inlet oC

Th,o Hot fluid temperature at outlet oC

Tc,i Cold fluid temperature at inlet oC

Tc,o Cold fluid temperature at outlet oC

U Overall heat transfer coefficient W/m2.oC

ΔTm Logarithmic mean temperature different oC

Nt Number of tubes

Np Number of tube pass

ht Tube – side heat transfer coefficient W/m2.oC

hs Shell – side heat transfer coefficient W/m2.oC

Ua Assumed over all heat transfer coefficient W/m2.oC

U0 Calculated over all heat transfer coefficient W/m2.oC

Preface

Heat exchangers are systems of thermal engineering in which its applications are occurred in different industries Heat exchangers are the basic or heart of once organized plant since it transfers energy to the processing plant This paper describes about thermal design considerations and the thermal design of shell and tube heat exchangers

Chapter 1: Introduction

1.1 Heat exchanger

Heat exchanger is a device that is used to transfer thermal energy no mixing between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid For the heat transfer to occur two fluids must be at different temperatures and they must be thermal contact Heat exchange involves convection

in each fluid and conduction through the separating wall Heat can flow only from hotter to cooler fluids However, they are not only used in heating applications but are also used in cooling applications, such as refrigerators and air conditioners

1.2 Classification of heat exchanger

Many types of heat exchangers can be distinguished from on another based

on the direction the liquids flow In such applications, the heat exchangers can be and be parallel-flow, cross-flow, or countercurrent In parallel-flow heat exchangers, both fluid involved move in the same direction, entering and exiting the exchanger side by side In cross-flow heat exchangers, the fluid paths run perpendicular to one another In countercurrent heat exchangers, the fluid paths flow in opposite directions, with each exiting where the other enters Countercurrent heat exchangers tend to be more effective than other types of exchangers

Heat exchangers are also typically classified according to transfer processes, number of fluids, surface compactness, construction features and heat transfer mechanisms Amongst of all type of exchangers, shell and tube exchangers are most commonly used heat exchange equipment

Figure 1.1: Classification of shell and tube heat exchanger

1.2.1 Fixed tube sheet exchanger

Mostly, it is used in high pressure and high temperature applications Fixed tube sheet heat exchangers are the one that

industries and refinery services, as there is absolutely no chance for intermixing of fluids This type of heat exchanger is employed where even slightest intermixing of fluids can’t be tolerated A fixed tube sheet

are secured at both ends to tube sheets welded to the shell

The principal advantage of the fixed tube sheet construction is its low cost because of its simple construction In fact, the fixed tube sheet is the leastexpensive construction type

The disadvantage of fixed tube sheet heat exchanger is shell pass cannot be cleaned with the mechanical method and can be cleaned with chemical method only Their maintenance process is difficult

The Fixed tube

temperature difference between the shell and tube is small The temperature difference is slightly great but the pressure of shell pass is not high with media in the shell pass not easy to scale

.1: Classification of shell and tube heat exchanger

Fixed tube sheet exchanger

Mostly, it is used in high pressure and high temperature applications Fixed tube sheet heat exchangers are the one that are very much used in process chemical industries and refinery services, as there is absolutely no chance for intermixing of fluids This type of heat exchanger is employed where even slightest intermixing of fluids can’t be tolerated A fixed tube sheet heat exchanger has straight tubes that are secured at both ends to tube sheets welded to the shell

The principal advantage of the fixed tube sheet construction is its low cost because of its simple construction In fact, the fixed tube sheet is the leastexpensive construction type

The disadvantage of fixed tube sheet heat exchanger is shell pass cannot be cleaned with the mechanical method and can be cleaned with chemical method only Their maintenance process is difficult

The Fixed tube -sheet heat exchanger is applicable to all services where the temperature difference between the shell and tube is small The temperature difference is slightly great but the pressure of shell pass is not high with media in the shell pass not easy to scale

.1: Classification of shell and tube heat exchanger

Mostly, it is used in high pressure and high temperature applications Fixed

are very much used in process chemical industries and refinery services, as there is absolutely no chance for intermixing of fluids This type of heat exchanger is employed where even slightest intermixing of

heat exchanger has straight tubes that

The principal advantage of the fixed tube sheet construction is its low cost because of its simple construction In fact, the fixed tube sheet is the least

The disadvantage of fixed tube sheet heat exchanger is shell pass cannot be cleaned with the mechanical method and can be cleaned with chemical method

changer is applicable to all services where the temperature difference between the shell and tube is small The temperature difference is slightly great but the pressure of shell pass is not high with media in

Figure 1.2: Fixed tube sheet heat exchanger

1.2.2 Removable tube bundle

A fixed tube sheet heat exchanger is the cheapest because of the ease of fabrication This heat exchanger requires periodic cleaning, replacement of tubes etc and inside of the tubes can be easily cleaned by mechanical means (by forcing wire brush or worm) and cleaning of the tubes from outside require removal of the tubes bundles from the heat exchanger, in addition to the above cited difficulty many heat exchangers are provided with removable tubes bundles So as to make removal of the tube bundles possible and to allow for considerable expansion of the tubes, a removable tube bundle exchanger is used

The advantage of U – tube exchanger is it allows for differential thermal expansion between shell and tubes as well as between individual tubes and capable

of withstanding thermal shock High heat transfer surface area forgiven shell and tube size Shell side can be steam or mechanically cleaned Bundle can be removed for shell side cleaning and maintenance

The disadvantage: individual tube replacement is difficult The exchanger cannot be made single-pass on tube side, so true countercurrent flow not possible Draining tube side is difficult in vertical (head-up) position Tube side can be cleaned by chemical means only

Figure 1.3: U - tube heat exchanger

on a floating head side Shell cover at the floating head end is larger than the other end so as to enable the tubes to be placed as near as possible to the edge of the

fixed tube sheet The tube sheet along with floating head is free to move and take the differential thermal expansion between the shell and the tubes bundle

The advantage of floating head type is the tube of the exchanger is removable for inspection and mechanical cleaning of outside of the tubes It is widely used in chemical industry and suitable for rigorous duties associated with high pressure and temperature and also with dirty fluids

Figure 1.4: Floating head heat exchanger

Chapter 2: Thermal Design

Considerations

The flow rates of both hot and cold streams, their terminal temperatures and fluid properties are the primary inputs of thermal design of heat exchangers Thermal design of a shell and tube heat exchanger typically includes the determination of heat transfer area, number of tubes, tube length and diameter, tube layout, number of shell and tube passes, type of heat exchanger (fixed tube sheet, removable tube bundle etc), tube pitch, number of baffles, its type and size, shell and tube side pressure drop etc

2.1 Shell

Shell is the container for the shell fluid and the tube bundle is placed inside the shell Usually, it is cylindrical in shape with a circular cross section, although shells of different shapes are used in specific applications and in nuclear heat exchangers to conform to the tube bundle shape Shell diameter should be selected

in such a way to give a close fit of the tube bundle The clearance between the tube bundle and inner shell wall depends on the type of exchanger Shells are usually fabricated from standard steel pipe with satisfactory corrosion allowance

Figure 2.1: Shell of a heat exchanger

2.2 Tube

Tube OD of ¾ and 1‟ are very common to design a compact heat exchanger The most efficient condition for heat transfer is to have the maximum number of tubes in the shell to increase turbulence The tube thickness should be enough to withstand the internal pressure along with the adequate corrosion allowance The tube length of 6, 8, 12, 16, 20 and 24 ft are preferably used Longer tube reduces shell diameter at the expense of higher shell pressure drop Finned tubes are also used when fluid with low heat transfer coefficient flows in the shell side Stainless steel, admiralty brass, copper, bronze and alloys of copper-nickel are the commonly used tube materials

Figure 2.2: Tube of a straight tube heat exchanger

2.3 Tube pitch and tube

Tube pitch is the shortes

tubes The tubes are generally placed in square or triangular patterns in the tube sheets The square layouts are required where it is necessary to get at the tube surface for mechanical cleaning The triang

given space The tube spacing is given by the tube pitch/tube diameter ratio, which

is normally 1.25 or 1.33 Since a square layout is used for cleaning purposes, a minimum gap of 6.35 mm (0.25 in) is allowed between

Figure 2.3: Heat exchanger tube layout

Tube pitch and tube-layout

Tube pitch is the shortest center to center distance between the adjacent tubes The tubes are generally placed in square or triangular patterns in the tube sheets The square layouts are required where it is necessary to get at the tube surface for mechanical cleaning The triangular arrangement allows more tubes in a given space The tube spacing is given by the tube pitch/tube diameter ratio, which

is normally 1.25 or 1.33 Since a square layout is used for cleaning purposes, a minimum gap of 6.35 mm (0.25 in) is allowed between tubes

Figure 2.3: Heat exchanger tube layout

t center to center distance between the adjacent tubes The tubes are generally placed in square or triangular patterns in the tube sheets The square layouts are required where it is necessary to get at the tube

ular arrangement allows more tubes in a given space The tube spacing is given by the tube pitch/tube diameter ratio, which

is normally 1.25 or 1.33 Since a square layout is used for cleaning purposes, a

2.4 Tube passes

The tube passes vary from 1 to 16 The tube passes of 1, 2 and 4 are common in application The number of passes is chosen to get the required tube side fluid velocity to obtain greater heat

formation Increasing the number of tube pass that will increase velocity of fluid of tube side The partition built into exchanger head known as partition plate (also called pass partition) is used to direct

Figure 2.4: Straight tube heat exchanger one and two pass tube side

The arrangement of tube pass can be illustrated in the figure below

Tube passes

The tube passes vary from 1 to 16 The tube passes of 1, 2 and 4 are common in application The number of passes is chosen to get the required tube side fluid velocity to obtain greater heat transfer coefficient and also to reduce scale

Increasing the number of tube pass that will increase velocity of fluid of tube side The partition built into exchanger head known as partition plate (also called pass partition) is used to direct the tube side flow

Figure 2.4: Straight tube heat exchanger one and two pass tube side

The arrangement of tube pass can be illustrated in the figure below

Figure 2.5: Tube passes arrangement

The tube passes vary from 1 to 16 The tube passes of 1, 2 and 4 are common in application The number of passes is chosen to get the required tube

transfer coefficient and also to reduce scale Increasing the number of tube pass that will increase velocity of fluid of tube side The partition built into exchanger head known as partition plate (also

Figure 2.4: Straight tube heat exchanger one and two pass tube side

The arrangement of tube pass can be illustrated in the figure below

2.5 Tube sheet

The tubes are fixed with tube sheet that form the barrier between the tube and shell fluids The tubes can be fixed with the tube sheet using ferrule and a soft metal packing ring The tubes are attached to tube sheet with two or more grooves

in the tube sheet wall by “tube rolling‟ The tube metal is forced to move into the grooves forming an excellent tight seal This is the most common type of fixing arrangement in large industrial exchangers The tube sheet thickness should be greater than the tube outside diameter to make a good seal The design of tube sheets is a fairly precise and complex process; the exact number of tubes needs to

be established and a pattern of holes calculated to spreads them evenly over the tube sheet surface Large exchangers may have several thousand tubes running through them arranged into precisely calculated groups or bundles

Figure 2.6: Stainless tube sheet

2.6 Baffles

Baffles are used to increase the fluid velocity by diverting the flow across the tube bundle to obtain higher transfer coefficient Baffles also serve as support s for the tubes during operation and help in preventing vibration from flow induced eddies The distance between adjacent baffles is called baffle-spacing The baffle spacing of 0.2 to 1 times of the inside shell diameter is commonly used Baffles are held in positioned by means of baffle spacers Closer baffle spacing gives greater transfer coefficient by inducing higher turbulence The pressure drop is more with closer baffle spacing

Figure 2.7: Different type of heat exchanger baffles

Baffle cut is the height of the segment that is cut in each baffle to permit the shell side fluid to flow across the baffle In case of cut segmental baffle, a segment (called baffle cut) is removed to form the baffle expressed as a percentage of the baffle diameter

Figure 2.8: Baffle cut

Baffle cut, either too much or too little, creates velocity changes in the side fluid Baffle cuts can vary between 15% and 45% and are expressed as ratio of segment opening height to shell inside diameter The upper limit ensures every pair

shell-of baffles will support each tube A baffle cut shell-of 20 to 25% provide a good heat transfer with the reasonable pressure drop The following figure compares too much flow restriction, just right, and too little flow restriction for a shell and tube exchanger

Figure 2.9: Baffle cut and fouling in shell

2.7 Fouling Considerations

Fouling is generally defined as the accumulation and formation of unwanted materials on the surfaces of processing equipment, which can seriously deteriorate the capacity of the surface to transfer heat under the temperature difference conditions for which it was designed The most of the process fluids in the exchanger foul the heat transfer surface Therefore, net heat transfer with clean surface should be higher to compensate the reduction in performance during operation Fouling of exchanger increases the cost of construction due to over sizing, additional energy due to poor exchanger performance and cleaning to remove deposited materials A spare exchanger may be considered in design for uninterrupted services to allow cleaning of exchanger

Figure 2.10: The fouling in heat exchanger

The effect of fouling is allowed for in design by including the inside and outside fouling coefficients in ca

heat-transfer resistances, rather than coefficients Typical values for the fouling coefficients and factors for common process and service fluids are given in the following table

Table 2.1: Typical values of fouling coefficients and resistances

The effect of fouling is allowed for in design by including the inside and outside fouling coefficients in calculation Fouling factors are usually quoted as transfer resistances, rather than coefficients Typical values for the fouling coefficients and factors for common process and service fluids are given in the

Typical values of fouling coefficients and resistances

The effect of fouling is allowed for in design by including the inside and

lculation Fouling factors are usually quoted as transfer resistances, rather than coefficients Typical values for the fouling coefficients and factors for common process and service fluids are given in the

Typical values of fouling coefficients and resistances

2.8 Selection of fluids for tube and the shell side

The routing of the shell side and tube side fluids has considerable effects on the heat exchanger design Some general guidelines for positioning the fluids are given in the following table It should be understood that these guidelines are not ironclad rules and the optimal fluid placement depends on many factors that are service specific

Table 2.2: Guidelines for placing the fluid in order of priority

Tube-side fluid Shell-side fluid

Fouling fluid

Less viscous fluid

High-pressure steam

Hotter fluid

Chapter 3: Thermal Design Process

1) Obtain the required thermo physical properties of hot and cold fluids

2) Assign fluid to shell side or tube side

3) Perform energy balance and find out the heat duty (q̇) of the heat exchanger 4) Assume a reasonable value of overall heat transfer coefficient (Ua)

5) Determine Logarithmic mean temperature different (ΔTlm), correction factor (F)

6) Calculate heat transfer area (A) required:

∆T ∗ F ∗ U7) Design tube

 Decide the tube diameter (di, do), tube pitch (Pt), tube length L and tube layout

 Calculate number of tubes (Nt) required to provide the heat transfer area (A)

 Decide the number tube pass (Np)

 Calculate tube side fluid velocity

 Calculate Nussult number

 Calculate bundle diameter (Db)

D = d ∗ N

K

n1 and K1 can be look up in appendix

 Determine the shell clearance (Cb)

 Calculate shell diameter (Ds)

D = D + C

 Assign baffle cut and baffle space

 Calculate shell side cross flow area

A =p − d

p ∗ D ∗ B

 Calculate the shell equivalent diameter

 Triangular pitch arrangement

 Calculate Renoldous number

ht: tube – side heat transfer coefficient

hs: shell – side heat transfer coefficient

do: outside diameter of tube

di: inside diameter of tube

Rt: fouling factor of fluid in tube side

Rs: fouling factor of fluid in shell side

kw: thermal conductivity of the wall

10) Check pressure drop If it is too high, adjust design until it is pertinent

11) Compare the calculated and assumed overall heat transfer coefficient If it is close, tabulate results Otherwise, adjust design and do loop until the difference between the calculated and assumed is small

Chapter 4: Design problem

4.1 Problem statement

Design a shell-and-tube exchanger for the following duty

60 kg/s of kerosene (42 API) leaves the base of a kerosene side-stripping column at 190oC and is to be cooled to 130oC by exchange with 75 kg/s light crude oil (Bach Ho Crude oil 36.6 API) coming from storage at 50oC The kerosene enters the exchanger at a pressure of 5 atm and the crude oil at 5 atm A pressure drop of 1 atm is permissible on both streams Allowance should be made for fouling by including a fouling factor of 0.00054 (W/m2oC)-1 on Crude oil stream and 0.000352 (W/m2oC)-1 on the Kerosene stream

Hot flow (Kerosene)

Pressure drop: 1 atm Fouling factor: 0.00054(W/m2oC)-1