DESIGN AND FABRICATION OF ONE SHELL TWO PASS HEAT EXCHANGER
SESSION 2008 – 2012 SUBMITTED BY:
KALEEM ULLAH
(UW-08-ME-002)
BILAL KHAN
(UW-08-ME-003)
SAJID ALI
(UW-08-ME-013) SUPERVISOR: MR.S.H SHAHID
WAH ENGINEERING COLLEGE MECHANICAL ENGINEERING DEPARTMENT
In the name of ALLAH, the most Gracious, most Compassionate
Recite with name of your Lord, Who created, He created man from the Clot of blood, Recite, for your Lord is most generous, Who taught writing by the pen, Taught man what he knew not!
Al-Quran
UNDERTAKING I certify that research work titled “DESIGN AND FABRICATION OF ONE SHELL TWO PASS HEAT EXCHANGER” is our own work. The work has not been presented elsewhere for assessment. Where material has been used from other sources it has been properly acknowledged.
MECHANICAL ENGINEERING
Supervisor: Mr.SH Shahid
Signature:
Date:
Department of Mechanical Engineering Wah Engineering College, University of Wah WahCantt
DESIGN AND FABRICATION OF ONE SHELL TWO PASS HEAT EXCHANGER
DEDIATED TO “We dedicate our project to our parents, teachers, and all those who helped us by any means in accomplishing this project and achieving the required outcomes
ACKNOWLEDEMENT Initiating with the name of Almighty ALLAH, the Lord of Universe, who is the entire source of all knowledge and wisdom endowed to mankind. The authors wish to express their deepest thanks and gratitude to Mr.SH Shahid, lecturer and their project advisor, for fruitful theoretical discussions, sharing his practical experience and his interest in this project. He generously helped us in the accomplishment of this report. His valuable suggestions, tricks and techniques made the tedious task of completing this project much easier and on scheduled time without which it would have been a far more arduous labor. No words can describe our gratitude to our parents and their moral support and prayers, who helped us to meet our targets.
Authors KALEEM ULLAH (UW-08-ME-BE-002)
BILAL KHAN (UW-08-ME-BE-003)
MUHAMMAD SAJID ALI (UW-08-ME-BE-002)
PREFACE Heat exchanger is important equipment used in industries such as process industry, power, transportation, petroleum, air conditioning, refrigeration, cryogenic, heat recovery, alternative fuel, and manufacturing industries. This project was specially chosen for our initial interest to work in process industry. The thesis contains the basic knowledge of heat transfer, heat exchanger, with special emphasis on the designing of one shell two pass heat exchanger.
Authors
TABLE OF CONTENTS CHAPTER # 1 IDENTIFICATION OF PROBLEM ....... Error! Bookmark not defined. 1.1 Statement of Problem .......................... Error! Bookmark not defined.
CHAPTER # 2 LATEST THEORITICAL ASPECT ................................................... 17 INTRODUCTION ................................................................................. 17 2.1 Application ........................................... Error! Bookmark not defined. 2.2 Heat transfer process........................... Error! Bookmark not defined. 2.3 Fouling Factor ...................................... Error! Bookmark not defined. 2.3.1Analysis of heat exchanger................ Error! Bookmark not defined. 2.3.2Heat capicity rate ............................... Error! Bookmark not defined. 2.3.3 Selection of heat exchanger............. Error! Bookmark not defined. 2.3.3.1Heat transfer rate ........................... Error! Bookmark not defined. 2.3.3.2Cost................................................... Error! Bookmark not defined. 2.3.3.3Pumping power ............................... Error! Bookmark not defined. 2.3.3.4Size and weight................................ Error! Bookmark not defined. 2.3.3.5Type .................................................. Error! Bookmark not defined. 2.3.3.6Material ........................................... Error! Bookmark not defined. 2.3.3.7Othere considration ........................ Error! Bookmark not defined. 2.4 1-2 type heat exchanger ....................... Error! Bookmark not defined. 2.5.Basic components of 1-2 type heat exchanger.................................... 2.5.1Tubes ................................................... Error! Bookmark not defined. 2.5.2Tube sheet ........................................... Error! Bookmark not defined. 2.5.3Shell and shell side nozzles ................ Error! Bookmark not defined. 2.5.4Channel covers .................................. Error! Bookmark not defined.
2.5.5Pass Divider ........................................ Error! Bookmark not defined. 2.5.6Baffles.................................................. Error! Bookmark not defined. 2.6 TEMA shell and tube type heat exchanger designationError! Bookmark not defined. CHAPTER # 3 DESIGN OF 1-2 TYPE HEAT EXCHANGERError! Bookmark not defined.
CHAPTER # 4 DESIGN CALCULATIONS..................... Error! Bookmark not defined. CHAPTER # 5
CHAPTER # 6 CHAPTER # 7 Autocad Drawings ................................................................................. 45
CHAPTER # 8 CONCLUSIONS .................................................................................... 50 7.1 Conclusions ....................................................................................... 51 REFRENCES ................................................................................... 52
LIST OF FIGURES Figure 2.1 Schematic representation of the ammonia synthesis processError! Bookmark not defined. Figure 2.2 Schematic representation of urea synthesis .....Error! Bookmark not defined. Figure 2.3 Some chemicals of interest in urea production Error! Bookmark not defined. Figure 3.1 DXU heat exchanger ........................................Error! Bookmark not defined. Figure 3.2 Cross-sectional view of DXU ...........................Error! Bookmark not defined. Figure 4.1 Design constraints ............................................Error! Bookmark not defined. Figure 4.2 The design procedure for shell and tube heat exchanger Error! Bookmark not defined. Figure 4.3 Heat Transfer factor for cross flow tube banks Error! Bookmark not defined. Figure 4.4 Tube row correction factor Fn ..........................Error! Bookmark not defined. Figure 4.5 Window correction factor ................................Error! Bookmark not defined. Figure 4.6 By pass correction factor .................................Error! Bookmark not defined. Figure 4.7 Coefficient for Fl .heat transfer ........................Error! Bookmark not defined. Figure 4.8 Friction factor for cross flow tube banks ..........Error! Bookmark not defined. Figure 5.1 The 1-2 Heat exchangers ..................................Error! Bookmark not defined. Figure 5.2 Temperature correction factor: one shell pass; two or more even tube 'passes ............................................................................................Error! Bookmark not defined. Figure 5.3 Temperature correction factor: two shell passes; four or multiples of four tube passes .................................................................................Error! Bookmark not defined. Figure 5.4 Temperature correction factor: divided-flow shell; two or more even-tube passes ............................................................................................Error! Bookmark not defined. Figure 5.5 Temperature correction factor, split flow shell, 2 tube passError! Bookmark not defined. Figure 5.6 Bundle dia clearance.........................................Error! Bookmark not defined. Figure 5.7 Heat Transfer factor for cross flow tube banksError! Bookmark not defined. Figure 5.8 Friction factor for cross flow tube banks ..........Error! Bookmark not defined. Figure 5.9 Tube row correction factor Fn ..........................Error! Bookmark not defined. Figure 5.10 Window correction factor ...............................Error! Bookmark not defined. Figure 5.11 Coefficient for Fl .heat transfer .....................Error! Bookmark not defined. Figure 5.12 By pass correction factor ................................Error! Bookmark not defined.
Figure 5.13 Coefficient for Fl‟, pressure drop ...................Error! Bookmark not defined. Figure 5.14 baffle geometrical factors ...............................Error! Bookmark not defined. Figure 5.15 Bypass factor for pressure drop Fb „ ..............Error! Bookmark not defined. Figure 6.1 slip-on nozzle flange........................................Error! Bookmark not defined. Figure 6.2 : weld neck nozzle flange .................................Error! Bookmark not defined. Figure 6.3 self-reinforced long weld neck nozzles ............Error! Bookmark not defined. Figure 6.4 "swept" forged nozzles .....................................Error! Bookmark not defined. Figure 6.5 Nozzle with liner ..............................................Error! Bookmark not defined. Figure 6.6 standard roller expansion ..................................Error! Bookmark not defined. Figure 6.7 roller expansion for bimetal..............................Error! Bookmark not defined. Figure 6.8 explosive expansion.........................................Error! Bookmark not defined. Figure 6.9 typical tube end welds. .....................................Error! Bookmark not defined. Figure 6.10 explosive tube end welding. ...........................Error! Bookmark not defined. Figure 6.11 back-bore end welding ...................................Error! Bookmark not defined. Figure 6.12 thick-wall bellows. .........................................Error! Bookmark not defined. Figure 6.13 thin-wall bellows ............................................Error! Bookmark not defined. Figure 6.14 test flange and gland for floating-head exchangerError! Bookmark not defined. Figure 6.15 hydrostatic testing of split-backing-ring floating-head exchanger ......... Error! Bookmark not defined. Figure 6.16 Ring-type flanges............................................Error! Bookmark not defined. Figure 6.17 weld neck flange .............................................Error! Bookmark not defined. Figure 6.18 6.18 clad flange ..............................................Error! Bookmark not defined. Figure 6.19 lap-type flange ................................................Error! Bookmark not defined. Figure 6.20 bolt extension sleeve.......................................Error! Bookmark not defined. Figure 6.21 bolt with spring washers .................................Error! Bookmark not defined. Figure 6.22 Types of flanges .............................................Error! Bookmark not defined. Figure 6.23 bolt load and gasket reaction ..........................Error! Bookmark not defined. Figure 6.24 forces and lever arms for integral flange in operating condition............ Error! Bookmark not defined. Figure 6.25 most common pass arrangements for multi pass channelError! Bookmark not defined. Figure 7.0.1 Stress in Cylinder ..........................................Error! Bookmark not defined. Figure 0.2 Hoop Stress in Thin Cylinder ..........................Error! Bookmark not defined. Figure 0.3 Longitudinal Stress in Thin Cylinder ..............Error! Bookmark not defined.
Figure 0.4 Butt joint ...........................................................Error! Bookmark not defined. Figure 0.5 welding arrangement of the vessel ...................Error! Bookmark not defined. Figure 0.6 Longitudinal joint with two cover plates ..........Error! Bookmark not defined. Figure 0.7 thin sphere under internal pressure p ................Error! Bookmark not defined.
LIST OF TABLES Table 2.1 Composition of the gas stream after each process step† . Error! Bookmark not defined. Table 2.2 Ammonia specifications ...................................Error! Bookmark not defined. Table 2.3 Urea granule specifications ................................Error! Bookmark not defined. Table 5.1 Heat Transfer Coefficients .................................Error! Bookmark not defined. Table 5.2 Conductivity of metals .......................................Error! Bookmark not defined. Table 5.3 Typical overall coefficients ...............................Error! Bookmark not defined. Table 5.4 Typical pitch .....................................................Error! Bookmark not defined. Table 5.5 Assumed & Calculated Overall Heat Transfer CoefficientError! Bookmark not defined. Table 5.6 Thermal Design Summary ................................Error! Bookmark not defined. Table 6.1 Principal pressure vessel codes ........................................................................ 35 Table 6.2 Steel type selection at various temperature rangesError! Bookmark not defined. Table 6.3 Description of plate materials ............................Error! Bookmark not defined. Table 6.4 Bolting data ........................................................Error! Bookmark not defined. Table0.1 Physical Properties of S.S 304 ...........................Error! Bookmark not defined. Table 0.2 Mechanical Properties of S.S 304 ......................Error! Bookmark not defined. Table 7.0.3 Boiler Code .....................................................Error! Bookmark not defined. Table 7.4 Mechanical Design Summary ...........................Error! Bookmark not defined.
Assumption The shell fluid temperature is an isothermal temperature at any crosssection. There is equal amount of heating surface in each pass. The overall heat transfer coefficient is constant. The rate of flow of each fluid is constant. The specific heat of each fluid is constant. There are no phase changes involving evaporation or condensation in any part of heat exchanger. Heat losses are negligible. The fluid properties such as temperature and velocity at inlet and outlet remain the same. The kinetic and potential energy changes are negligible because there is is little or no change in the velocity of fluid. Material of construction for shell is transparent. Material of construction for tube is stainless steel. Axial heat conduction along the tube is usually insignificant and can be considered negligible. 18BWG (Birmingham wire gauge) with mm tube pitch are used. According to TEMA standards Baffle spacing should not be less than 1/5 of(shell diameter),so Baffle spacing B = 90mm
NOMENCLATURE OF THE TERMS USED IN DESIGN Baffle spacing
B
Tube pitch
PT
CHAPTER # 1
IDENTIFICATION OF PROBLEM OF DESIGN PROJECT
PROJECT DEFINITION: To Design and fabricate a 1 shell and 2 tubes pass heat exchanger. The basic concept of this project has been taken from a heat exchanger of similar design contained in the catalogue of TQ, the supplier of laboratory equipment for technical teaching. Modification have been incorporated, wherever required, to suit local material availability and fabrication limitation keeping in view economical aspect. THE PROJECT CONSISTS OF THE FOLLOWING MAIN ASPECT:
1. 2. 3. 4. 5.
Design of shell and tube Design economy and optimization Design validation Material selection Fabrication and testing
CHAPTER # 2
LATEST THEORITICAL ASPECT
INTRODUCTION “Heat exchanger is a device that facilitates the exchange of heat between two fluids that are at different temperature while keeping them from mixing with each other. “It differs from mixing chamber that it does not allow to two fluids to mix they are separated by a wall Or A heat exchanger is a heat-transfer devise that is used for transfer of internal thermal energy between two or more fluids available at different temperatures. In most heat exchangers, the fluids are separated by a heat-transfer surface, and ideally they do not mix. Common examples of heat exchangers familiar to us in day-to-day use are automobile radiators, condensers, evaporators, air pre heaters, and oil coolers.
APPLICATION: Heat exchangers are used in the following application. 1. Power plant 2. Petroleum 3. Heating, ventilating, refrigeration and air conditioning system (HVRAC) 4. Heat recovery system. 5. Chemical process industry.
TYPE OF HEAT EXCHANGERS Different heat transfer applications require different types of hardware and different configurations of heat transfer equipment. The attempt to match the heat transfer hardware to the heat transfer requirements within the specified constraints has resulted in numerous types of innovative heat exchanger designs.
DOUBLE-PIPE HEAT EXCHANGER The simplest type of heat exchanger consists of two concentric pipes of different diameters, as shown in Figure, called the double-pipe heat exchanger. in parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction. In
counter flow, on the other hand, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions.
COMPACT HEAT EXCHANGER Another type of heat exchanger, which is specifically designed to realize a large heat transfer surface area per unit volume, is the compact heat exchanger. The ratio of the heat transfer surface area of a heat exchanger to its volume is called the area density. A heat exchanger with 700 m2/m3 (or 200 ft2/ft3) is classified as being compact. Examples of compact heat exchangers are car radiators (1000 m2/m3), glass ceramic gas turbine heat exchangers (6000 m2/m3), the regenerator of a Sterling engine (15,000 m2/m3), and the human lung (20,000 m2/m3). In compact heat exchangers, the two fluids usually move perpendicular to each other, and such flow configuration is called cross-flow. The cross-flow is further classified as unmixed and mixed flow, depending on the flow configuration, as shown in Figure. In (a) the cross-flow is said to be unmixed since the plate fins force the fluid to flow through a particular inter fin spacing and prevent it from moving in the transverse direction (i.e., parallel to the tubes). The cross-flow in (b) is said to be mixed since the fluid now is free to move in the transverse direction. Both fluids are unmixed in a car radiator. The presence of mixing in the fluid can have a significant effect on the heat transfer characteristics of the heat exchanger.
A gas-to-liquid compact heat exchanger for a residential air conditioning system
REGENERATIVE HEAT EXCHANGER Another type of heat exchanger that involves the alternate passage of the hot and cold fluid streams through the same flow area is the regenerative heat exchanger. The static-type regenerative heat exchanger is basically a porous mass that has a large heat storage capacity, such as a ceramic wire mesh. Hot and cold fluids flow through this porous mass alternatively. Heat is transferred from the hot fluid to the matrix of the regenerator during the flow of the hot fluid, and from the matrix to the cold fluid during the flow of the cold fluid. Thus, the matrix serves as a temporary heat storage medium. The dynamic-type regenerator involves a rotating drum and continuous flow of the hot and cold fluid through different portions of the drum so that any portion of the drum passes periodically through the hot stream, storing heat, and then through the cold stream, rejecting this stored heat. Again the drum serves as the medium to transport the heat from the hot to the cold fluid stream.
SHELL AND-TUBE HEAT EXCHANGERS The most common type of heat exchanger in industrial applications is the shell-and-tube heat exchanger, shown in Figure. Shell-and-tube heat exchangers contain a large number of tubes (sometimes several hundred) packed in a shell with their axes parallel to that of the shell. Heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes through the shell. Baffles are commonly placed in the shell to force the shell-side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes. Despite their widespread use, shell and- tube heat exchangers are not suitable for use in automotive and aircraft applications because of their relatively large size and weight. Note that the tubes in a shell-and-tube heat exchanger open to some large flow areas called headers at both ends of the shell, where the tube-side fluid accumulates before entering the tubes and after leaving them. Shell-and-tube heat exchangers are further classified according to the number of shell and tube passes involved. Heat exchangers in which all the tubes make one U-turn in the shell, for example, are called one-shell-pass and two tube- passes heat exchangers. Likewise, a heat exchanger that involves two passes in the shell and four passes in the tubes is called a two-shell-passes and four-tube-passes heat exchanger.
(one-shell pass and one-tube pass)
1 SHELL 2 PASS HEAT EXCHANGER
HEAT TRANSFER PROCESS Heat exchanger includes two flowing fluids separated by wall. 1. Heat first transmitted from fluid to wall by convection. 2. Through the wall by conduction. 3. From the wall to cold fluid by convection.
The total thermal resistance associated with heat transfer process involves two convection resistance and one conduction resistances shown in the above figure. so the total thermal resistance become ⁄
Where
So the heat transfer through the pipe is ⁄
In the analysis of heat exchangers, it is convenient to combine all the thermal resistances in the path of heat flow from the hot fluid to the cold one into a single resistance R, and to express the rate of heat transfer between the two fluids as
where U is the overall heat transfer coefficient, whose unit is W/m2 · °C, which is identical to the unit of the ordinary convection coefficient h. Canceling T, Equation reduces to
Representative values of overall heat transfer coefficient in heat exchanger
Type of heat exchanger Water to water Water to oil
Units U,
⁄ 850-1700 100-350
Water to gasoline or kerosene Feed water heater Steam to light fuel oil Steam condenser Freon condenser(water cooled) Ammonia condenser(water cooled) Alcohol condenser(water cooled) Gas to gas Water to air finned tubes(water in tubes)
300-1000 1000-8500 200-400 1000-600 300-1000 800-1400 250-700 10-40 30-60 400-850 30-300 400-4000
Steam to air in finned tubes(steam in tubes)
FOULING FACTOR After a period of operation the heat-transfer surfaces for a heat exchanger may become coated with various deposits present in the flow systems ,or surfaces may become corroded as a result of the interactions between the fluids and the material used for construction of the heat exchanger. in either event, this coating represents an additional resistance to the heat flow, and thus result in decreased performance. The overall effect is usually represented by a fouling factor, or fouling resistance, Rf. which must be included along other thermal resistances making up the overall heat-transfer coefficient. The most common type of fouling is the precipitation of solid deposits in a fluid on the heat transfer surfaces. Another form of fouling, which is common in the chemical process industry, is corrosion and other chemical fouling. In this case, the surfaces are fouled by the accumulation of the products of chemical reactions on the surfaces. This form of fouling can be avoided by coating metal pipes with glass or using plastic pipes instead of metal ones. Heat exchangers may also be fouled by the growth of algae in warm fluids. This type of fouling is called biological fouling and can be prevented by chemical treatment. The fouling factor is obviously zero for a new heat exchanger and increases with time as the solid deposits build up on the heat exchanger surface. The fouling factor depends on the operating temperature and the velocity of the fluids, as well as the length of service. Fouling increases with increasing temperature and decreasing velocity. The overall heat transfer coefficient relation given above is valid for clean surfaces and needs to be modified to account for the effects of fouling on both the inner and the outer surfaces of the tube. For an unfinned shell-and-tube heat exchanger, it can be expressed as ⁄
(Source: tabular exchange manufacturing association 7th edition)
Fluid
,(
Distilled water, sea water, river water, boiler feed water TEMPRATURE Below 50c Above 50c Fuel oil Steam(oil free) Refrigerants(liquid) Refrigerants(vapor) Alcohol vapor air
⁄
)
0.0001 0.0002 0,0009 0.0001 0.0002 0.0004 0.0001 0.0004
Fouling can be reduced by: •
keeping velocities sufficiently high to avoid deposits
•
avoiding stagnant regions where dirt will collect
•
avoiding hot spots where coking or scaling might occur
•
avoiding cold spots where liquids might freeze or where corrosive products may condense for gases
SELECTION OF HEAT EXCHANGER The heat exchanger selection depend upon many factor in which some are:
HEAT TRANSFER RATE This is the most important quantity in the selection of a heat exchanger. A heat exchanger should be capable of transferring heat at the specified rate in order to achieve the desired temperature change of the fluid at the specified mass flow rate.
COST Budgetary limitations usually play an important role in the selection of heat exchangers, except for some specialized cases where “money is no object.” An off-the-shelf heat exchanger has a definite cost advantage over those made to order. However, in some cases, none of the existing heat exchangers will do, and it may be necessary to undertake the expensive and time-consuming task of designing and manufacturing a heat exchanger from scratch to suit the needs. This is often the case when the heat exchanger is an integral part of the overall device to be manufactured. The operation and maintenance costs of the heat exchanger are also important considerations in assessing the overall cost.
PUMPING POWER In a heat exchanger, both fluids are usually forced to flow by pumps or fans that consume electrical power. The annual cost of electricity associated with the operation of the pumps and fans can be determined from Operating cost _ (Pumping power, kW) _ (Hours of operation, h) _ (Price of electricity, $/kWh) where the pumping power is the total electrical power consumed by the motors of the pumps and fans. For example, a heat exchanger that involves a 1-hp pump and a -hp fan (1 hp _ 0.746 kW) operating 8 h a day and 5 days a week will consume 2017 kWh of electricity per year, which will cost $161.4 at an electricity cost of 8 cents/kWh. Minimizing the pressure drop and the mass flow rate of the fluids will minimize the operating cost of the heat exchanger, but it will maximize the size of the heat exchanger and thus the initial cost. As a rule of thumb, doubling the mass flow rate will reduce the initial cost by half but will increase the pumping power requirements by a factor of roughly eight. Typically, fluid velocities encountered in heat exchangers range between 0.7 and 7 m/s for liquids and between 3 and 30 m/s for gases. Low velocities are helpful in avoiding erosion, tube vibrations, and noise as well as pressure drop.
SIZE AND WEIGHT Normally, the smaller and the lighter the heat exchanger, the better it is. This is especially the case in the automotive and aerospace industries, where size and weight requirements are most stringent. Also, a larger heat exchanger normally carries a higher price tag. The space available for the heat exchanger in some cases limits the length of the tubes that can be used.
TYPE The type of heat exchanger to be selected depends primarily on the type of fluids involved, the size and weight limitations, and the presence of any phase change processes. For example, a heat exchanger is suitable to cool a liquid by a gas if the surface area on the gas side is many times that on the liquid side. On the other hand, a plate or shell-and-tube heat exchanger is very suitable for cooling a liquid by another liquid
MATERIAL The materials used in the construction of the heat exchanger may be an important consideration in the selection of heat exchangers. For example, the thermal and structural stress effects need not be considered at pressures below 15 atm or temperatures below 150°C.
But these effects are major considerations above 70 atm or 550°C and seriously limit the acceptable materials of the heat exchanger. A temperature difference of 50°C or more between the tubes and the shell will probably pose differential thermal expansion problems and needs to be considered. In the case of corrosive fluids, we may have to select expensive corrosion-resistant materials such as stainless steel or even titanium if we are not willing to replace low-cost heat exchangers frequently
BASIC COMPONENET It is essential for the designer to have a good working knowledge of the mechanical features of STHEs and how they influence thermal design. The principal components of an STHE are: • Shell • shell cover • Tubes; • Channel • channel cover • Tube sheet
TUBE The tubes are the basic component of the shell and tube heat exchanger, providing the heat transfer surface between one fluid flowing inside the tube and other fluid flowing across the outside of the tubes. The tube may be seamless or welded and most commonly made of copper or steel alloys. Other alloys of nickel, titanium, or aluminum may also be used for specific applications. The tube may be either bare or extended surface on the outside. Extended or enhanced surface tube s are used when one fluid has a substantially lower heat transfer coefficient then the other fluid .doubly enhanced tubes that is , with enhancement both inside and outside are available that can reduce the size and cost of the exchanger. Extended surfaces (finned tubes) provide two to four times as much heat transfer area on the outside as the corresponding bare tube, and this area helps to offset a lower outside heat transfer coefficient. Tubes should be able to withstand the following 1. Operating temperature and pressure on both sides 2. Thermal stresses due to the differential thermal expansion between the shell and the tube bundle 3. Corrosive nature of both the shell-side and the tube-side fluids There are two types of tubes: straight tubes and U-tubes. The tubes are further classified as 1. Plain tubes 2. Finned tubes 3. Duplex or bimetallic tubes 4. Enhanced surface tubes
TUBE SHEET Depending on the service, and metal required, tube sheets, less than about 100 mm thick are made from plate, but at greater thicknesses or for high-integrity services, they are made from forged disks. When tube sheets in other than carbon or low-alloy steels are required, the use of clad plate should be considered. This may provide a cost saving, particularly at the larger diameters aid thicknesses. A clad tube sheet will consist of a carbon or low-alloy steel backing plate, having a thickness suitable for the design temperature and pressure, with a layer of the required tube sheet metal bonded to it. The cladding will be about 9.5 and 3 mm thick for multi-and single tube-side passes, respectively. Although the clad metal may be applied by weld deposition, explosionclad tube sheets are widely used because the process provides a high-integrity bond with no contamination of the cladding from the backing metal. In addition, the explosion-clad process provides a much greater combination of cladding/backing metals than the weld deposition process. The use of clad tube sheets becomes essential when a single metal that is corrosion resistant to both shell- and tube-side fluids is not available or is too costly. TEMA specifies tolerances for tube whole diameter, ligament width, and drill drift. Hole diameter tolerances provide for "standard fit" and "special close fit" of the tubes, the latter fit being recommended for tubes that are susceptible to work hardening. Holes are drilled, reamed, and the edges chamfered slightly to prevent damage during bundle assembly. Each pair of tube sheets of double tube sheet exchangers should be drilled together, or one jig drilled from the other, and held together as a matching pair throughout all stages of manufacture. Minimum tube sheet thicknesses are provided by TEMA, and for class R the total thickness less corrosion allowance should not be less than the tube outside diameter. Some fabricators may standardize on greater minimum thicknesses in order to reduce distortion during tube end attachment
CHANNEL COVER
SHELL The shell barrel must be straight and have no out-of-roundness, as a tightly fitting tube bundle must be inserted in it. Most shell and head barrels greater than about 450 mm in inside diameter are rolled from plate, and a complete shell barrel may comprise several smaller barrels, or strakes, welded together end to end. If there is any out-of-roundness, individual
strakes are rerolled after welding the longitudinal seams. The longitudinal seams of adjoining strakes are always staggered. The inside diameter of a rolled shell should not exceed the design inside diameter by more than 3.2 mm (1/8 in) as determined by circumferential measurement. All internal welds must be made flush. When welding large nozzles to the shell "shrinkage" may occur at the nozzle/shell junction and effective measures, such as the use of temporary stiffening, must be taken to avoid it. Shrinkage reduces the shell diameter at the nozzle/shell junction so that the baffle diameter must be reduced accordingly. The increased clearance between baffle and shell may result in reduced thermal performance. Standard pipe less than 450 mm in diameter is usually available, and this will be used for the shell and head barrels instead of rolled plate. Depending on the fabricators roll capacity, at thicknesses of the order of 80 mm and greater or large thickness/diameter ratios, it may be necessary to use forged instead of rolled barrels. When an expensive barrel metal is required for corrosion resistance purposes only, the barrel is formed from the selected metal if its thickness is less than about 15 mm. Above this the use of clad metal should be investigated, as it may provide a cost saving. The clad metal will usually comprise a steel plate, having a thickness suitable for the pressure and temperature conditions with a layer of the required corrosion resistant metal, about 3 mm thick, bonded to it. The cladding may be applied by explosive, roll bonding, or weld deposition methods. TEMA specifies minimum shell and head barrel thicknesses, which depend on barrel diameter, metal and TEMA class.
SHELL AND SHELL SIDE NOZZLE
TUBE-SIDE CHANNEL AND NOZZLE
BAFFLE Baffles are used to support tubes, enable a desirable velocity to be maintained for the shell side fluid, and prevent failure of tubes due to flow-induced vibration. There are two types of baffles: plate rod Plate baffles may be single-segmental, double-segmental, or triple-segmental, as shown in Figure.
The Baffle spacing is the centerline-to-centerline distance between adjacent baffles. It is the most vital parameter in STHE design. The TEMA standards specify the minimum baffle spacing as one-fifth of the shell inside diameter or 2 in., whichever is greater. Closer spacing will result in poor bundle penetration by the shell side fluid and difficulty in mechanically cleaning the outsides of the tubes. Furthermore, a low baffle spacing results in a poor stream distribution as will be explained later.
ANALYSIS OF HEAT EXCHANGER Heat exchangers are commonly used in practice, and an engineer often finds himself in a position to select a heat exchanger that will achieve a specified temperature change in a fluid stream of known mass flow rate, or to predict the outlet temperatures of the hot and cold fluid streams in a specified heat exchanger. In upcoming sections, we will discuss the two methods used in the analysis of heat exchangers. Of these, the log mean temperature difference (or LMTD) method is best suited for the first task and the effectiveness–NTU method for the second task as just stated. But first we present some general considerations
Heat exchangers usually operate for long periods of time with no change in their operating conditions. Therefore, they can be modeled as steady-flow devices. As such, The mass flow rate of each fluid remains constant, The fluid properties such as temperature and velocity at any inlet or outlet remain the same. The fluid streams experience little or no change in their velocities and elevations, and thus the kinetic and potential energy changes are negligible. The specific heat of a fluid, in general, changes with temperature. But, in a specified temperature range, it can be treated as a constant at some average value with little loss in accuracy. Axial heat conduction along the tube is usually insignificant and can be considered negligible. The outer surface of the heat exchanger is assumed to be perfectly insulated, so that there is no heat loss to the surrounding medium, and any heat transfer occurs between the two fluids only. Under these assumptions, the first law of thermodynamics requires that the rate of heat transfer from the hot fluid be equal to the rate of heat transfer to the cold one. That is,
) ) where the subscripts c and h stand for cold and hot fluids, respectively, and
=
=outlet temperatures
In heat exchanger analysis, it is often convenient to combine the product of the mass flow rate and the specific heat of a fluid into a single quantity. This quantity is called the heat capacity rate and is defined for the hot and cold fluid streams as
MECHANICAL DESIGN NOMENCLATURE
SCOPE OF MECHANICAL DESIGN A designer must know how to apply certain codes and standards, which are available for heat exchanger. In mechanical designing of heat exchanger, first of all the designer decides the type of heat exchanger. Following are the important points, which should be kept in mind while doing mechanical design of heat exchanger.
To calculate shell thickness against design pressure.
To check the thickness of tube against design pressure.
To calculate the channel thickness that is of both front and rear ends.
Flange calculations.
Tube sheet calculations.
To design and calculate the thickness and dia of baffles.
To calculate number of tie-rods and dia of tie-rods.
To calculate the number of spacers, dia and length of spacer.
To calculate the size and thickness of partition plates.
Location of Impingement plate.
To design and calculate size and thickness of inlet and outlet nozzles.
To select and calculate the size of gasket.
Calculation of Hard-Ware items that is bolts, Nuts, washer etc.
Material selection.
MECHANICAL DESIGN CODES Introduction
Pressure vessel codes, which also cover the mechanical design of shell and tube exchangers, fulfill various functions; in several countries a national code is legally enforced, and compliance with the code is mandatory for items supplied to that country, whether built there or imported. Table 6.1 shows the status of the principal codes covering exchanger design. Where no code is shown for a country, the table shows the codes generally accepted. The requirements of U.S., U.K., and German codes, as they affect shell-and-tube exchangers, are listed. The codes aim to achieve safe construction and give rules for design and fabrication, which are based on experience with conventional plant. However, when the components differ from conventional design or are outside the range of the code rules, or when more certain assurance of safety is required, it may be necessary to justify the design using alternative codes or a stress analysis or component testing. These alternatives must be agreeable to the purchaser and the inspecting authority. Codes also provide a useful tool for the education of junior engineers or those entering the vessel and exchanger field, and this aspect should not be overlooked when new codes or rules are being formulated. As well as giving rules for design and fabrication, most codes are specific as to acceptable materials, but usually the range of materials is wide enough to allow the designer a choice. Once the designer has made a choice of material, the design code gives an allowable design stress that is used to dimension the pressurized components, and the code specifies any special fabrication requirements for that material, such as heat treatment, for example. Although codes do list acceptable materials, they also permit other materials.
ASME VIII
The listed materials are taken from specifications of the ASTM. In order to use other materials for ASME-coded vessels, special application must be made to, and approval secured from, the ASME Code Committee.
Table 0.1 Principal pressure vessel codes
BS 5500 Materials other than those listed in the code may be used by agreement between purchaser and manufacturer provided that they are covered by a written specification as comprehensive as the BS specification for the equivalent material and that, the design stresses are determined in a manner consistent to BS. A. D. Merkblatter The A. D. Merkblatter W series of specifications lists acceptable materials, but others may be authorized with the agreement of the inspecting authority. In the latter case the W specifications give requirements that must be satisfied. ASME Boiler and Pressure Vessel Code Section VIII, Div. 1 This code gives minimum requirements for the design, fabrication, inspections and certification of vessels with design pressures between 1.03 bar g (15 psig) and 206 bar (3000 psig). Where the design needs to be justified by a full stress analysis, Div. 2 of this code
should be used. New editions of the code are usually issued every 3 year, but interim revisions are made twice yearly in the form of addenda. The ASME code references used refer to the 1977 edition.
TEMA: (Standards of Tubular Exchanger Manufacturers Association)
These standards serve to supplement and define the ASME Pressure Vessel Code for all shell-and-tube exchanger applications (double-pipe exchangers are not included). Although TEMA is linked specifically to ASME VIII, it is a useful standard that can be used to supplement other national codes. Recommendations for construction are given in three classes, the class being specified by the purchaser. The design rules for each class are the same, the only difference being in dimensions and details of construction. Class R is for the generally severe requirements of petroleum and related processing applications. Class C is for the generally moderate requirements of commercial and general process service. Class B is for chemical process service. The numbering system is common to all classes, and the TEMA references used refer to class R, C, or B of the 1978 edition. This edition carries for the first time a section entitled "Recommended Good Practice" relating to aspects not covered in the main sections of the standard, particularly the requirements for exchangers with shell diameters from 1524 to 2540 mm (60 to 100 in). The numbering system used is the same as in the three main sections of TEMA, and an * is used throughout to denote that there is an additional recommended good practice.
BS 5500: The British Standard Specification for Unfired Fusion-Welded Pressure Vessels
This recently introduced code (41 replaces BS 1500 and 351515 and is intended to unify the U.K. requirements for all pressure vessels. A major departure for this code is that the purchaser now specifies the construction category (BS 5500 3.4), which then defines the amount of non-destructive testing and restricts the permitted materials. The degree of testing is not now reflected by the use of design stress-reduction factors. The code references used in Sec. 6.3 refer to the third issue of BS 5500.
A.D. Merkblatter: German Pressure Vessel Regulations
These regulations are in the form of data sheets covering different aspects of vessel design and construction, and are produced by a group of associations. Revisions are made from time to time to keep up with advances in the knowledge. Some aspects of vessel and exchanger design are not covered, and the method is agreed upon by the purchaser, inspecting authority, and designer. The code references used refer to the 1977 edition of A. D. Merkblatter.
International codes
The International Standards Organization (ISO) has been endeavoring for some time to write an international code for pressure vessels. Various working groups have written draft sections of the code, and these were submitted in late 1973 to form a proposed draft standard DIS 2694, this draft was considered by the subscribing countries to ISO, but when votes were cast in 1974 many major countries voted against its acceptance. The draft was then sent back to the organizing secretariat and working groups with comments, but there has been no action to date. In Europe, the EEC has published a Council Directive of July 27, 1976, giving a general framework to common provisions for vessels and methods of inspecting them. The intention is that separate directives for different categories of vessels will lay down the technical requirements for design and methods of inspection, and thus will achieve free movement of vessels within the EEC and avoid multiple inspections. European standards will be organized by CEN (Comite European de Normalisation), and a draft for "simple" pressure vessels has been written. Unfired pressure vessels are being considered by CEN working group 54, but the EEC Directive is awaited.
TEMA DESIGNATIONS FOR SHELL-AND-TUBE HEAT EXCHANGERS
DESIGN AND FABRICATION OF ONE SHELL TWO PASS HEAT EXCHANGER
3.1 How to design a shell and tube heat exchanger
3.1.1 size
3.2 effective design shell and tube heat exchanger
BAFFLES: Baffles must generally be employed on the shell-side to support the tubes, to maintain the tube spacing, and to direct the shell-side fluid across or along the tube bundle in a specified manner. There are a number of different types of baffles and these may be installed in different ways to provide the flow pattern required for a given application Classification of Baffles
Baffles are either normal or parallel to the tubes. Accordingly, baffles may be classified as transverse or longitudinal. The transverse baffles direct the shell-side fluid into the tube bundle at approximately right angles to the tubes, and increase the turbulence of the shell fluid. Every shell and tube exchanger has transverse baffles except the X and K shells, which have only support plates. The longitudinal baffles are used to control the direction of the shell side flow. For example, F, G , and H shells have longitudinal baffles. In the F shell, an overall counter flow is achieved.
Transverse Baffles Transverse baffles are of two types: (1) Plate baffles: Three types of plate baffles are (1) Segmental, (2) Disk and doughnut (3) Orifice baffles (2) Rod baffles. . Segmental
Baffles
The segmental baffle is a circular disk (with baffle holes) having a segment removed. Predominantly, a large number of shell and tube exchangers employ segmental baffles. This cutting is denoted as the baffle cut and it is commonly expressed as a percentage of the shell inside diameter. Here the percent baffle cut is the height, H, given as a percentage of the shell inside diameter, D,. The segmental baffle is also referred to as a single segmental baffle. The heat transfer and pressure drop of cross flow bundles are greatly affected by the Baffle cut. The baffle cuts vary from 20 to 49% with the most common being 20-25%, and the optimum baffle cut is generally 20%, as it affords the highest heat transfer for a given Pressure drop. Baffle cuts smaller than 20% can result in high pressure drop. As the baffle cut increases beyond 20%, the flow pattern deviates more and more from cross flow and can result in stagnant regions or areas with lower flow velocities; both of these reduce the thermal effectiveness of the bundle
Baffle Spacing. The practical range of single-segmental baffle spacing is to 1 shell diameter , though optimum could be 40-50% . TEMA Table RCB-4.52 [3] provides maximum baffle spacing for various tube outer diameters, tube materials, and the corresponding maximum allowable temperature limit. The baffles are generally spaced between the nozzles. The inlet and outlet baffle spacings are in general larger than the “central” baffle spacing to accommodate the nozzles, since the nozzle dimensions frequently require that the nozzle should be located far enough from the tube sheets.
