Gulli Project

A Minor Project Report on

DESIGN OF SHELL AND TUBE HEAT EXCHANGER

Department of Chemical Engineering

Submitted To: Mr. S.K. Sharma Associate Professor DCRUST,Murthal Submitted By: Gurpreet Singh Tanwar 11001005016 Chemical Engg. 4th Year

SUPERVISOR’S DECLARATION I hereby declare that I have checked this project report and in my opinion, this project is adequate in terms of scope and quality for the award of the degree of Bachelor of Chemiical Engineering

Signature :……………………… Name of Supervisor : Mr. SK Sharma Position : Asst. Professor Date : 21 Nov.2014

ACKNOWLEDGEMENT

I am grateful and would like to express my warmest gratitude to my supervisor Mr. SK Sharma for his guidance, his support, continuous encouragement and the confidence he has shown in me over the years in making this project possible. His constant encouragement in my dissertation work that helped to look forward to the future with enthusiasm and confidence in my abilities. I would like also to express my sincere thanks to Chemical Engineering Department who helped me in many ways and made my project possible.

ABSTRACT This project is mainly focusing on designing one type of a heat exchanger which is shell and tube heat exchanger.Heat exchangers are most used equipments used in any chemical industry. When it comes to the manufacturing industry, heat exchangers are used both for cooling and heating. Heat exchangers in large scale industrial processes are usually custom made to suit the process, depending on the type of fluid used, the phase, temperature, pressure, chemical composition and other thermodynamic properties. According to problem statement I have to calculate the no. of tubes per passes, No. of passes,Length of tube. In the problem statement water is used as a hot fluid as well as a cold fluid.The inlet and outlet temperatures of both shell and tube side fluids are given. The maximum length of pipe is given. We have to calculate the required length of tube.It should be less than the maximum length.

CONTENTS 1. INTRODUCTION 1.1 Project Background 1.2 Project Objectives 2. HEAT EXCHANGER 2.1 Introduction 2.2 Uses and Application of Heat Exchanger 2.3 Type of Heat Exchangers 3. Flow Arrangement 4. Thermal Considerations Design 4.1 Shell 4.2 Tubes 4.3 Baffles 5. Fluid side Designing 6. LMTD and Correction factor 7 .Problems in Heat Exchanger 8. Problem Statement

INTRODUCTION

1.1 PROJECT BACKGROUND Heat exchanger is a device built for efficient heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, and natural gas processing. One common example of a heat exchanger is the radiator in a car, in which a hot engine-cooling fluid, like antifreeze, transfers heat to air flowing through the radiator. To design a heat exchanger, many criteria have to be taken before making any decision. The important parameters of heat exchangers are collected and put a major consideration on it. In this project, we have to calculate the no. of tubes per passes, No. of passes, Length of tube.. We have to calculate the required length of tube. It should be less than the maximum length.

1.2 PROJECT OBJECTIVES The study of shell and tube heat exchanger and at the end to consider a problem in which I have to find i)To find the number of tubes per pass ii) To find the number of tubes iii) To find the length of tube which is less than the maximum length provided.

HEAT EXCHANGER

2.1 Introduction Heat exchanger is one of devices that is convenient in industrial and household application. These include power production, chemical industries, food industries, electronics, environmental engineering, manufacturing industry, and many others. It comes in many types and function according to its uses. So what exactly heat exchanger is? Heat exchanger is a device that is used to transfer thermal energy between two or more fluids, between a solid surface and a fluid at different temperatures and in thermal contact. There are usually no external heat and work interactions. In most heat exchangers, heat transfer between fluids takes place through a separating wall or into and out of a wall in a transient manner.

2.2 Uses and Applications of Heat Exchanger Heat exchangers are used to transfer heat from one media to another. It is most commonly used in space heating such as in the home, refrigeration, power plants and even in air conditioning. It is also used in the radiator in a car using an antifreeze engine cooling fluid. Heat exchangers are classified according to their flow arrangements where there are the parallel flow, and the counter flow. Aside from this, heat exchangers also have different types depending on their purpose and how that heat is exchanged. When it comes to the manufacturing industry, heat exchangers are used both for cooling and heating. Heat exchangers in large scale industrial processes are usually custom made to suit the process, depending on the type of fluid used, the phase, temperature, pressure, chemical composition and other thermodynamic properties. Heat exchangers mostly can be found in industries which produce a heat stream. In this case, heat exchangers usually circulate the output heat to put it as input by heating a different stream in the process. The fact that it really saves a lot of money because when the output heat no longer needed then it can be recycled rather than to come from an external source as heat is basically recycled. When used in industries and in the home, it can serve to lower energy costs as it helps recover wasted heat and recycle it for heating in another process. Typically, most heat exchangers use fluid to store heat and heat transfer can take the form of either absorption or dissipation. For instance, heat exchangers are used as oil coolers, transmission and engine coolers, boiler coolers, waste water heat recovery, condensers and evaporators in refrigeration syatems.In residential homes, heat exchangers are used for floor heating, pool heating, snow and ice melting, domestic water heater, central, solar and geothermal heating. Of course, heat exchangers have different designs which depend on the purpose it is intended

for. Brazed heat exchangers, a collection of plates which are brazed together, are used for hydronic systems like swimming pools, floor heating, snow and ice melting. The shell and coil heat exchanger design is best for areas with limited spaces as it can be installed vertically. Of course, for the highly industrial process, the shell and tube heat exchanger is the perfect solution.

2.3 Types of Heat Exchanger In industries, there are lots of heat exchanger that can be seen. The types of heat exchanger can be classified in three major constructions which are tubular type, plate type and extended surface type.

2.3.1 Tubular Heat Exchangers The tubular types are consists of circular tubes. One fluid flows inside the tubes and the other flows on the outside of the tubes. The parameters of the heat exchanger can be changed like the tube diameter, the number of pitch, tube arrangement, number of tubes and length of the tube can be manipulate. The common type of heat exchangers lie under this categories are double-pipe type, shell-and-tube type and spiral tube type. The tubular heat exchangers can be designed for high pressure relative to the environment and high pressure difference between the fluids. These exchangers are used for liquid-toliquid and liquid-to-vapor phase. But when the operating temperature or pressure is very high or fouling on one fluid side, it will used gas-to-liquid and gas-togas heat transfer applications.

2.3.1.1 Double-Pipe Heat Exchanger A double-pipe heat exchanger consists of smaller and larger diameter pipe where the smaller pipe fitted concentrically into the larger one in purpose to give direction to the flow from one section to another. One set of these tubes includes the fluid that has to be cooled or heated. The second fluid runs over the tubes being cooled or heated in order to provide heat or absorb the heat. A set of tubes is the tube bundle and it can be made up of several types of tubes such as longitudinally plain, longitudinally finned, and more. If the application requires an almost constant wall temperature, the fluids may flow in a parallel direction. It's easy to clean and convenient to disassemble and assemble. The double-pipe heat exchanger is one of the simplest. Usually, it is used for small capacity applications because it is so expensive on a cost per unit area basis.

2.3.1.2 Shell-and-Tube Heat Exchanger This exchanger is built of a bundle of round tubes mounted in a large cylindrical shell with the tube axis parallel to the shell to transfer the heat between the two fluids. The fluid flows inside the tubes and other fluid flows across and along the tubes. But for baffled shell-andtube heat exchanger the shell side stream flows across between pairs of baffles and then flows parallel to the tubes as it flows from one baffle compartment to the next. This kind of exchanger consists of tubes, shells, front-end head, rear-end head, baffles and tubesheets. The different type of shell-and-tube heat exchangers depends on different application. Usually in chemical industry and process application, it is used as oil-coolers, power condensers, preheaters in power plants and also steam generators in nuclear power plants. The most common types of shell-and-tube heat exchanger are fixed tubesheet design, Utube design and floating-head type. Cleaning this heat exchanger is easy. Instead of easily cleaning, it is also low in cost. But among all tube bundle types, the U-tube is the least expensive because it only needs one tube sheet. Technically, because of its construction in U shape, the cleaning is hardly done in the sharp bend. An even number of tube passes only can be achieved.

2.3.1.3 Spiral-Tube Heat Exchanger A spiral heat exchanger is a helical or coiled tube configuration. It consists of spirally wound coils placed in a shell or designed as co-axial condensers and co-axial evaporators that are used in refrigeration systems. The heat transfer coefficient is higher in a spiral tube than in a straight tube. Since the cleaning is impossible, the spiral tubes are suitable for thermal expansion and clean fluids. The biggest advantage of the spiral heat exchanger is its efficient use of space.

2.3.2 Plate Heat Exchangers A second type of heat exchanger is a plate heat exchanger. It has many thin plates that are slightly apart and have very large surface areas and fluid flow passages that are good for heat transfer. This can be a more effective heat exchanger than the tube or shell heat exchanger due to advances in brazing and gasket technology that have made this plate exchanger more practical. Large heat exchangers are called plate and frame heat exchangers and there allow for periodic disassembly, cleaning, and inspection. There are several types of permanently bonded plate heat exchangers like dip brazed and vacuum brazed plate varieties, and they are often used in refrigeration. These heat exchangers can further be classified as gasketed plate, spiral plate and lamella.

2.3.3 Extended Surface Heat Exchanger Extended surfaces have fins attached to the primary surface on one side of a twofluid or a multifluid heat exchanger. Fins can be of a variety of geometry—plain, wavy or interrupted—and can be attached to the inside, outside or to both sides of circular, flat or oval tubes, or parting sheets. Pins are primarily used to increase the surface area (when the heat transfer coefficient on that fluid side is relatively low) and consequently to increase the total rate of heat transfer. In addition, enhanced fin geometries also increase the heat transfer coefficient compared to that for a plain fin. Fins may also be used on the high heat transfer coefficient fluid side in a heat exchanger primarily for structural strength (for example, for high pressure water

flow through a flat tube) or to provide a thorough mixing of a highly-viscous liquid (such as for laminar oil flow in a flat or a round tube). Fins are attached to the primary surface by brazing, soldering, welding, adhesive bonding or mechanical expansion, or extruded or integrally connected to tubes.

3.FlOW ARRANGEMENT Co-current (Parallel) flow- As the name suggests, the flow of the hot and the cold fluid is taking place in the same direction in this case. As the graph shows, the temperature difference between the hot and the cold fluid keeps on decreasing from one end to the other. Counter current flow- In this setup, the hot fluid enters from one end of the exchanger and the cold from the opposite end. This results in nearly constant temperature difference between the hot and the cold fluid. This is a significant aspect and makes counter current exchangers preferable over co-current exchangers. We will discuss this point later when we talk about LMTD. Crossed flow-The cold and the hot fluid flow axis is at an angle to each other and hence, the fluids cross each other in this arrangement. The most common type of crossed flow exchanges has the angle between axes as 90 degrees.

4.THERMAL CONSIDERATIONS DESIGN

4.1 Shell Shell is the container for the shell fluid and the tube bundle is placed inside the shell. 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. 4.2 Tube Tube OD of ¾ 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 thickness is expressed in terms of true outside diameter (OD). 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.

4.2.1 Selection of tube material To be able to transfer heat well, the tube material should have good thermal conductivity. Because heat is transferred from a hot to a cold side through the tubes, there is a temperature difference through the width of the tubes. Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresses occur during operation. This is in addition to any stress from high pressures from the fluids themselves. The tube material also should be compatible with both the shell and tube side fluids for long periods under the operating conditions (temperatures, pressures, pH, etc.) to minimize deterioration such as corrosion. All of these requirements call for careful selection of strong, thermally-conductive, corrosion-resistant, high quality tube materials, typically metals, including copper alloy, stainless steel, carbon steel, non-ferrous copper alloy, Inconel, nickel, Hastelloy and titanium. Fluoropolymers such as Perfluoroalkoxy alkane (PFA) and Fluorinated_ethylene_propylene (FEP) are also used to produce the tubing material due to their high resistance to extreme temperatures. Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid cross-contamination and possibly loss of pressure. 4.2.2 Tube pitch, tube-layout and tube-count Tube pitch is the shortest centre to centre distance between the adjacent tubes. The tubes are generally placed in square or triangular patterns (pitch) as shown in the figure below.

The number of tubes that can be accommodated in a given shell ID is called tube count. The tube count depends on the factors like shell ID, OD of tube, tube pitch, tube layout, number of tube passes, type of heat exchanger and design pressure.

4.2.3 Tube passes The number of passes is chosen to get the required tube side fluid velocity to obtain greater heat transfer co-efficient and also to reduce scale formation. The tube passes vary from 1 to 16. The tube passes of 1, 2 and 4 are common in application. The partition built into exchanger head known as partition plate (also called pass partition) is used to direct the tube side flow. 4.2.4 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 recommended standards (IS:4503 or TEMA) should be followed to select the minimum tube sheet thickness. 4.3 Baffles Baffles are used to increase the fluid velocity by diverting the flow across the tube bundle to obtain higher transfer co-efficient. The distance between adjacent baffles is called bafflespacing. 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 co-efficient by inducing higher turbulence. The pressure drop is more with closer baffle spacing. The various types of baffles are shown in the figure. 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. Baffle cuts from 15 to 45% are normally used. A baffle cut of 20 to 25% provide a good heat-transfer with the reasonable pressure drop. The % cut for segmental baffle refers to the cut away height from its diameter. Following figure also shows two other types of baffles

5. Fluid Side Designing Next comes your first design decision: Which fluid goes on the shellside and which on the tubeside There is no straightforward answer, but some considerations are summarized here: • Corrosive fluids are best kept to the tubeside. Since the tubeside has less metal than the shellside, this will minimize the use of expensive metals that may be needed to withstand the fluids’ corrosive properties. • Fluids at extreme pressures and temperatures are preferably kept to the tubeside, because they are likely to require a greater metal thickness, or more expensive materials of construction. The tubes, being smaller in diameter than the shell, withstand higher pressures. • Fluids that need to be kept at a high velocity, such as water or propylene glycol for cooling, should be kept on the tubeside. • Dirty fluids, or streams that are otherwise likely to cause fouling, should go on the tubeside. This is because the tubes are easier to clean than the shell. For instance, it is often possible to clean the tubes by water jetting, having simply opened the head of the exchanger, without needing to remove the tube bundle. The shell and the outside of the tube bundle, on the other hand, are harder to clean mechanically, and chemical cleaning is often the only option. • The shellside offers a larger cross-section for vapor flow, and hence lower pressure drops. Process vapors to be condensed are therefore normally placed on the shellside, though the tubeside is generally used for condensing steam. • The baffles on the shellside help to ensure good mixing, which reduces the effects of laminar flow and therefore tends to increase heat-transfer coefficients. Hence you will get better heat transfer if viscous fluids are kept on the shellside. • Twisted tubes, static mixers or tube inserts increase turbulence and thus heat-transfer coefficients on the tubeside by reducing the effects of laminar flow. Because these are usually proprietary technologies, however, your ability to check the vendor’s performance claims may be limited. If you think you would benefit from one of these technologies, work closely with the vendor and be sure to evaluate all the options.

6. LMTD and Correction Factor LMTD: The logarithmic mean temperature difference (also known as log mean temperature difference or simply by its initialism LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. The larger the LMTD, the more heat is transferred. The use of the LMTD arises straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties. LMTD=(dT1-dT2)/(ln(dT1)/dT2) For parallel flow: dT1=Th1-Tc1 dT2=Th2-Tc2 For Counter flow: dT1=Th1-Tc2 dT2=Th2-Tc1 where, Th1: Hot fluid inlet temperature Th2: Hot fluid outlet temperature Tc1: Cold fluid inlet temperature Tc2: Cold fluid outlet temperature

Correction Factor: If a heat exchanger other than double pipe type is used the heat transfer is calculated by using a corrcetion factorapplied to LMTD for a counterflow arrangement problem with the same hot and cold fluid temperature.

7.Problems in Shell and Tube Heat Exchanger 1.Fouling 2.Tube Vibrations 3.Leakage 4,Dead Zones

Fouling Fouling is the accumulation of unwanted material on solid surfaces to the detriment of function. The fouling material can consist of either living organismsor a non-living substance (inorganic or organic). Fouling is usually distinguished from other surface-growth phenomena in that it occurs on a surface of a component, system or plant performing a defined and useful function, and that the fouling process impedes or interferes with this function. Some types of fouling are: a.Corrosion fouling b.Chemical Reaction fouling c. Biofueling Tube Vibrations Vibration of tubes in heat exchangers is an important limiting factor in heat exchanger operation. The vibration is caused by nonstationary fluid dynamic processes occurring in the flow. These are turbulent pressure pulsations (in turbulent flow), vortex initiation and separation from tubes in crossflow.This problem can cause damage to heat exchanger. Leakage Sometimes the fluid of the tube side can leak to shell side or vice versa. It may cause huge production loss. Leaks may develop at the tube to tube sheet joints of fixed tube sheet exchangers because differential thermal expansion between the tubes and the shell causes overstressing of the rolled joints. Or, thermal cycling caused by frequent shutdowns or batch operation of the process may cause the tubes to loosen in the tube holes. Floating heads or U-bend exchangers would be considered first for this type of service. If a fixed tube sheet unit is required, an expansion joint will be specified. An exchanger that will be thermally cycled two or three times a day will require superior mechanical construction such as the strength welding of tubes to the tube sheet, complete inspection of the shell and channel welds during fabrication. Welding the tubes to the tube sheets does not guarantee that a leak will not occur as sometimes weld failure due to porosity in the welds or just one poorly welded tube out of the hundreds of welds can cause a leakage. The use of double tube sheets to minimise the chances of leakage between the tube side and shell side can be a good solution to the problem.

Dead Zones Existing shell and tube heat exchangers suffer from the fact that they must typically use baffles to maintain the required heat transfer. This, however, results in “dead zones” within the heat exchanger where flow is minimal or even non existent. These dead zones generally lead to excessive fouling. Other types of heat exchangers may or may not employ baffles. If they do, the same increased fouling problem exists. Further, in heat exchangers fitted with baffles, for example, the cross flow implementation results in the additional problem of potential damage to tubes as a result of flow induced vibration. In the case of such damage, processes must often be interrupted or shut down in order to perform costly and time consuming repairs to the device.

Problem Statement (1-Shell and 2-Tube Pass Heat Exchanger) Water at the rate of 3.8kg/s is heated from 38 to 55 0C in a shell and tube Heat Exchanger.On the shell side one pass is used with wster as the heating fluid , 1.9kg/s entering the exchanger at 93 0C.The overall heat transfer coefficient is 1419 W/m2 0C. and the average water velocity in the 1.9 cm diameter tubes is 0.366 m/s. Because of space limitations the tube length must not be longer than 2.5 m.Calculate (a) The no. of tube passes. (b) No. of tubes per pass. (c) The tubes length

Assumptions: 1.Steady state conditions exist. 2.Heat loss to the surroundings is negligible. 3.Kinetic and potential energy are negligible.

We first assume one tube pass. According to problem: Tc1=38 0C Tc2=55 0C m2=3.8 kg/s m1=1.8 kg/s Th1=93 0C Th2= ? U=1419 W/m2 0C Dt=1.9 cm =0.019 m Vav=0.366 m/s c1=c2=4.18 KJ/Kg K L