Teknik swirl flow pada heat exchanger adalah pengaturan geometris atau sisipan tabung untuk aliran paksa yang membuat putaran dan/atau aliran sekunder di dalam tabung. Teknik swirl yang biasa digunakan pada industri terdiri dari berbagai macam yaitu:
Twisted Tape Inserts
Dalam desain heat exchanger aliran laminar, Twisted tape dapat digunakan secara efektif untuk meningkatkan perpindahan kalor. Twisted tape adalah perangkat tambahan sederhana yang dapat dipasang menjadi tubular baru atau yang sudah ada. Peningkatan twisted tape dicapai dengan menginduksi pusaran mengalir dalam fluida sisi tabung dan meningkatkan perpindahan panas pada kerugian tekanan tambahan yang rendah. Twisted tape mencampur aliran curah dengan baik dan karenanya berkinerja lebih baik dalam aliran laminar daripada sisipan lainnya.
Tabung bergelombang dibuat dari: tembaga, paduan tembaga, baja karbon, baja tahan karat, dan titanium. Gelombang ditentukan oleh pitch gelombang, kedalaman gelombang, dan jumlah gelombang. Sebagian besar tabung bergelombang memiliki awal gelombang tunggal yang ditentukan oleh kedalamannya e dan pitch aksialnya p dan sudut heliks. Pitch gelombang aksial terkait dengan diameter internal di, sudut heliks relatif terhadap sumbu tabung, dan jumlah awal ns, dengan persamaan berikut:
p = ∏ di/ns tan β
Diameter luar di atas gelombang di bagian luar tabung sama dengan ujung datar dari tabung. Diameter dalam diambil sebagai diameter luar kurang dari dua kali ketebalan dinding tabung. Diameter internal maksimum biasanya digunakan untuk menentukan koefisien perpindahan kalor internal. Mereka memiliki fabrikasi yang lebih mudah, pengotoran terbatas, dan peningkatan perpindahan kalor yang lebih tinggi dibandingkan dengan peningkatan faktor gesekan.
Aliran di saluran bergelombang dikaitkan dengan aliran sekunder, penekanan aliran sekunder aliran dengan melawan gaya sentrifugal, dan penghancuran aliran sekunder dengan timbulnya turbulensi. Dalam pengondensasian, tabung bergelombang meningkatkan koefisien perpindahan kalor karena hanya lapisan tipis dari kondensat cair yang tersisa di puncak, sementara sebagian besar mengalir ke dasar karena tegangan permukaan dan efek gravitasi. Film tipis memberi hambatan yang sangat kecil, dan karenanya koefisien kondensasi sangat tinggi.
Doubly Enhanced (Peningkatan Ganda) Surfaces
Jika peningkatan diterapkan pada permukaan tabung dalam dan luar, peningkatan tabung menjadi dua kali lipat. Sirip dengan rusuk heliks berkontribusi pada area permukaan tambahan (sekitar 30%–40%) tetapi lebih signifikan perubahan pola aliran fluida, yaitu, mendorong pencampuran dan turbulen mengalir. Aplikasi untuk tabung yang ditingkatkan ganda adalah media pendingin, industri kimia, dan tabung kondensor dan evaporator.
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Memaksimalkan laju perpindahan kalor adalah salah satu usaha terpenting mengoptimalkan kinerja heat exchanger. Salah satu teknik yang biasa dilakukan adalah menambahkan perangkat tertentu untuk meningkatkan turbulensi aliran dan pembentukan lapisan batas aliran.
Teknik yang sering dipakai adalah penyisipan tabung (tube inserts). Teknik ini mengacu pada perangkat yang dimasukkan ke dalam tabung halus. Tube inserts relatif murah, relatif mudah untuk memasukkan dan mengeluarkan tabung untuk operasi pembersihan.
Perangkat sisipan dimasukkan ke dalam saluran aliran untuk meningkatkan fluida mengalir di dekat permukaan perpindahan kalor. Perangkat ini biasanya berbentuk streamline shape, cakram, statis mixer, dan mesh atau sikat, dan sisipan kumparan kawat (coil inserts). Model coil inserts adalah perangkat yang sering dijumpai.
Coil inserts menyediakan cara yang efisien dan murah untuk meningkatkan perpindahan kalor di dalam tabung heat exchanger. Namun, coil inserts memiliki kerugian jika ada kontak yang tidak sempurna antara koil dan tabung dinding. Untuk sisipan, parameter yang mempengaruhi peningkatan adalah ketinggian gangguan, gangguan spasi atau pitch, sudut heliks, dan bentuk disrupsi. Coil inserts meningkatkan heat transfer dalam aliran turbulen secara efisien. Kinerja aliran turbulen lebih baik daripada aliran laminar. Penyisipan tabung biasanya memiliki cincin tarik untuk pemasangan dan pelepasan untuk dibersihkan.
Perangkat hiTRAN memberikan peningkatan koefisien perpindahan panas relatif terhadap tabung biasa. Ketika fluida mengalir melalui tabung biasa, fluida yang paling dekat dengan dinding mengalami gaya gesek drag yang memiliki efek memperlambat fluida di dinding. Lapisan batas laminar dapat secara signifikan mengurangi koefisien perpindahan kalor sisi tabung dan kinerja heat exchanger. Memasukkan Elemen hiTRAN yang terdiri dari turbulator kawat unik ke dalam tabung akan mengganggu lapisan batas laminar, menciptakan geseran fluida tambahan dan pencampuran, sehingga meminimalkan efek gesekan. Perangkat hiTRAN sangat efektif meningkatkan efisiensi perpindahan kalor dalam tabung yang beroperasi pada bilangan Reynolds rendah (laminar ke transisi mengalir); meskipun peningkatan perpindahan kalor paling besar di daerah aliran laminar (sampai 20 kali), manfaat yang signifikan dapat diperoleh pada rezim aliran transisi (hingga 15 kali), dan rezim aliran turbulen (hingga 3 kali).
Elemen Matriks hiTRAN dapat dibuat dari berbagai bahan, termasuk sebagian besar grade dari stainless steel, baja karbon rendah, hastelloy, titanium, tantalum, monel, inconel, dan tembaga paduan untuk ketahanan korosi umum. Bahan yang paling umum digunakan adalah stainless steel Kelas 304 dan 316.
Dalam operasi mode pemanasan, aliran berputar memiliki efek konveksi sentrifugal yang menguntungkan, yang dapat meningkatkan koefisien perpindahan kalor, sedangkan dalam operasi mode pendinginan, aliran berputar memiliki konveksi sentrifugal yang merugikan yang bahkan dapat menurunkan koefisien perpindahan kalor konveksi.
Teknik penunjang heat transfer diatas sangat berkaitan dengan mekanika fluida. Salah satu metode yang paling umum untuk mendesain suatu sistem heat exchanger adalah menggunakan Computational Fluid Dynamics (CFD), yaitu metode menyelesaikan persamaan-persamaan mekanika fluida bahkan reaksi kimia menggunakan komputer, sehingga diperoleh hasil yang komprehensif dan detail.
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Turbulensi aliran fluida pada proses perpindahan kalor sangat penting untuk menunjang laju perpindahan kalor. Berbagai macam cara dilakukan untuk menambah turbulensi aliran pada heat exchanger. Teknik roguh surfaces adalah teknik yang sering dijumpai pada heat exchanger di industri.
Teknik rough surfaces (permukaan kasar) mengacu pada tabung dan saluran yang memiliki elemen kekasaran dalam bentuk tonjolan yang teratur seperti punggungan berulang tegak lurus terhadap arah aliran. Konfigurasi umumnya dipilih untuk menunjang turbulensi daripada untuk meningkatkan luas permukaan perpindahan kalor. Teknik ini dapat diterapkan pada salah satu permukaan heat exchanger utama atau heat exchanger bersirip (fin):
Pelat datar
Tabung melingkar
Annuli memiliki kekasaran pada permukaan luar tabung dalam
Sirip
Rough surfaces diproduksi dalam banyak konfigurasi, mulai dari kekasaran jenis butiran pasir acak untuk tonjolan diskrit. Biasanya rough surface menggunakan teknik rib yang disusun berulang seperti heliks/ulir. Teknik ini menghasilkan kinerja perpindahan kalor yang baik dalam aliran turbulen fase tunggal tanpa penurunan tekanan yang drastis. Parameter utama rib heliks adalah tinggi ridge e, pitch p, dan sudut heliks. Rasio luas internal relatif terhadap tabung polos dengan diameter yang sama sekitar 1,3 hingga 2,0. Sirip internal dapat dari berbagai bentuk penampang seperti persegi, persegi panjang, segitiga, setengah lingkaran, busur, gelombang sinus, dll. Beberapa produsen tabung sirip internal heliks terkemuka adalah Fintube LLC dan Wolverine Tube Inc.
Efek utama rib pada perpindahan kalor menyebabkan pemisahan aliran dan reattachment (pemasangan kembali) dan pola aliran di atas heliks rib. Elemen kekasaran diproduksi dengan proses machining, forming, pengecoran, atau pengelasan. Berbagai sisipan atau struktur pembungkus seperti sirip lilitan kawat tabung juga dapat memberikan tonjolan permukaan.
Pengaplikasian: Rough Surfaces biasa dipakai pada aplikasi aliran turbulen dengan fluida bernomor Prandtl tinggi. Dalam aplikasi aliran laminar, kekasaran skala kecil tidak efektif, tetapi puntiran sisipan pita dan tabung bersirip internal tampaknya menjadi teknik augmentasi yang disukai.
Peningkatan internal dengan rib kasar berulang. (a) Punggung persegi, (b) setengah lingkaran, (c) melingkar, dan (d) rincian kekasaran rib berulang. Catatan: diameter luar tabung-d atau diameter kumparan, diameter dalam tabung-d butir c harus dibaca sebagai di, ketebalan tonjolan- e, kedalaman rib hr, panjang tabung tanpa sirip L, pitch p-rib, ketebalan rib -t, ketebalan dinding dua tabung, lebar tonjolan W dan sudut heliks dari rusuk –β.). Sumber: Heat Exchanger Design Handbook 2ed (2013)
(a and b) Wire–loop wound fin tube. Sumber: Heat Exchanger Design Handbook 2ed (2013)
Perancangan rough surfaces sangat erat kaitanya dengan mekanika fluida. Salah satu metode yang paling umum untuk mendesain suatu fin heat exchanger adalah menggunakan Computational Fluid Dynamics (CFD), yaitu metode menyelesaikan persamaan-persamaan mekanika fluida bahkan reaksi kimia menggunakan komputer, sehingga diperoleh hasil yang komprehensif dan detail.
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Masalah peralatan heat exchanger seperti fouling telah ada selama bertahun-tahun. Fouling dapat menghambat kinerja peralatan, mengurangi rentang hidup komponen dan menyebabkan kegagalan. Fouling adalah masalah kompleks, memakan biaya besar, dan mempengaruhi kinerja sistem industri. Perlu adanya inovasi untuk menambah kinerja heat exchanger.
Untuk meningkatkan kinerja heat exchanger terdapat beberapa teknik peningkatan perpindahan kalor.Teknik peningkatan perpindahan kalor biasanya berfungsi meningkatkan pencampuran fluida dengan meningkatkan kecepatan aliran, ketidakstabilan atau turbulensi, dan pembatasan pertumbuhan lapisan batas dekat dengan permukaan perpindahan kalor.
Teknik yang sering dipakai oleh industri adalah treated surface. Treated surface biasanya berupa penambahan coating (lapisan) baik secara kontinu atau terputus-putus. Treated surface seperti pelapis hidrofobik dan pelapis berpori paling efektif untuk perubahan fasa tetapi tidak berlaku untuk konveksi satu fasa. Tingkat kekasaranlah yang mempengaruhi perpindahan kalor satu fasa.
Treated Surface Heat Exhanger
Dengan penambahan coating yang memiliki kekasaran, Turbulensi laju perpindahan kalor dapat meningkat. Selain itu teknik coating juga dapat mencegah terjadinya pengotoran (fouling) pada permukaan heat exchanger.
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Water is the most abundant substance in most foods. Classification of food is divided into three groups according to their water content (high, medium and low humidity). Fruits, vegetables, juices, raw meat, fish and dairy fall into the high humidity category. Breads, hard cheeses, and sausages are examples of medium-humidity foods, while the low-moisture groups are dehydrated vegetables, whole grains, powdered milk and dry soup mixes.
The importance of the function of water in food goes far beyond the quantity in the composition of the food. Water is essential for the good texture and appearance of fruits and vegetables. In such products, loss of water usually results in lower quality. In other hand, water, becomes an important requirement for the occurrence and support of chemical reactions and microbial growth, and prevents chemical spoilage of food.
It is now well known that the effect of water on food stability cannot be related solely to quantitative water content. For example, honey containing 23% water is very stable in storage while dehydrated potatoes will undergo rapid decay at half-high water content. To explain the effect of water, parameters that reflect the quantity and ‘effectiveness’ of water are needed. This parameter is water activity.
Water activity aw is defined as the ratio of the vapor pressure of the food to the vapor pressure of pure water at the same temperature.
aw = P/P0
P = partial pressure of water vapor of food at temperature T P0 = equilibrium vapor pressure of pure water at temperature T. The same type of ratio also determines the relative humidity of air, RH (usually expressed as a percentage):
RH = (P’/P0)*100%
P’ = Partial pressure of water vapor in the air If food is in equilibrium with air, then p = p’ . Therefore the water activity of the food is equal to the relative humidity of the atmosphere in equilibrium with the food. For this reason, water activity is sometimes expressed as the equilibrium relative humidity, ERH.
aw = ERH/100
Range aw
Product
above 0.95
Fresh fruit, vegetables, milk, meat, fish
0.90-0.95
semi-hard cheese, anchovies, bread
0.85-0.90
hard cheese, sausage, butter
0.80-0.85
fruit juice, jelly, wet pet food
0.70-0.80
jam, dry cheese, legumes, plums
0.50-0.70
raisins, honey, seeds
0.40-0.50
almond nut
0.20-0.40
Non-fat milk powder
<0.2
biscuits, roasted ground coffee, sugar
Typical water activity in food
The main mechanisms that contribute to the decrease in water vapor pressure in food are solvent-solute interactions, binding of water molecules to the poles of polymeric substances (e.g. polysaccharides and proteins), adsorption of water on the solid surface matrix and capillary forces. In high-moisture foods, such as fruit juices, depression can be attributed entirely to water-solute interactions. If the food is considered an ‘ideal solution’, this water vapor pressure applies Raoult’s law:
P = XwP0
Where Xw is the water content (in mole fraction) of the food. Therefore the water activity of an ideal aqueous solution is equal to the molar concentration of water Xw. The water activity of foods with high humidity (with an aw of 0.9 or higher) can be calculated quite accurately by this method.
A pressure vessel is a closed container designed to accommodate a high pressure gas or liquid fluid that is substantially different from the ambient pressure. Pressure vessels have wide applications in industries such as oil and gas, chemicals, petrochemicals, distillation towers, nuclear reactors, natural gas storage systems, and hot water storage tanks.
Various sizes and shapes of pressure vessels are manufactured for various purposes. In general, the type of shape that is often encountered is a long cylindrical model with two heads. Pressure vessels work at internal pressures that are higher or lower than air pressure. In addition, the operating temperature of these systems is also different.
A simple example of a pressure vessel is a pressure cooker on cooking utensils. Pressure cookers are made of high-pressure, heat-resistant metals such as stainless steel. A pressure cooker works to hold the hot steam pressure in the pan so that the steam pressure does not leak until the cooking ingredients soften completely due to heat pressure. Therefore, the pressure cooker lid is quite difficult to open because it uses a strong magnet so that the pressure does not leak.
Pressure vessels are designed to work to reach the pressure level required for certain applications. Pressure vessels can apply pressure either directly through valves, gauges, or indirectly through heat transfer. Potential pressure levels range from 15 psi to about 150,000 psi, while temperatures are often above 400 °C (750 °F). Pressure vessels can accommodate fluids ranging from 75 liters (20 gallons) to thousands of liters.
Shell: The main component of a pressure vessel to accommodate pressure. The shell is usually cylindrical, conical, or spherical.
Head: Head is useful for closing the shell. Heads are generally curved. The reason for the curved shape is that it is more resistant to pressure and allows the head to be light and inexpensive.
Nozzle: A cylindrical component that penetrates into the shell or head. Nozzle is used to install inlet and outlet pipes, install measuring instruments (altitude, temperature, pressure).
Support: Support is used to support all pressure vessel loads so that they stand firm.
Types of pressure vessels that are often found in industry
Process Vessel: these vessels are designed to contain and store liquids only and are used for integrated operations in petrochemical facilities, refineries, gas plants, oil and gas production facilities, and other facilities.
Autoclave: These vessels are usually cylindrical in shape because their round shape is better able to withstand high pressure safely. The autoclave is designed to accommodate items that are placed inside and then the lid is tightly closed.
High pressure vessels: the most durable vessels on the market capable of operating under the heaviest loads and providing the best resistance to corrosion, temperature and pressure. High-pressure vessels are usually made of stainless steel. These vessels are particularly suitable for use in: high speed mixers, chemical reactors and supercritical extraction systems.
Heat Exchanger: a device that transfers heat from one medium to another. Heat exchangers are most commonly used in industrial facilities such as iron and steel, petroleum, petrochemical, gas, power generation, food, pharmaceutical, leather, textile, air conditioning, ships, and marine industries. Learn more about heat exchangers >>click here!
Pressurized water tank: In a tank water well system, this tank generates water pressure by using compressed air to force it down above the water. Due to this pressure, water is forced out of the tank through pipes inside your home when the valve is opened.
Vacuum tank: The vacuum tank functions to filter air or liquid through suction, outgassing, pumping, or a combination of techniques. These tanks use pressure to prevent contamination, purification and dehydration.
Boiler: a closed pressure vessel used to heat a liquid. This heating fluid is used for cooking, power generation, central heating, water heating, and sanitation.
Selection of pressure vessel material
Materials that are often used in the design of pressure vessels are:
Carbon steel (with carbon content below 0.25%)
Manganese carbon steel (stronger than carbon steel)
Low alloy steels
High alloy steels
Austenitic stainless steel
Non-ferrous materials (aluminum, copper, nickel, and alloys)
In the design of pressure vessels, there are standards and codes that govern. The ASME Boiler and Pressure Vessel Code (ASME Code) is a well-known standard for pressure equipment and components worldwide and provides manufacturer certification and quality assurance. ASME sets standards for the design, materials, manufacture, inspection, testing, and operation of boilers and pressure vessels (including electric boilers, heating boilers, nuclear power plant components and transportation tanks). More than 100 countries use ASME standards and codes. The addition of the ASME certification mark to pressure equipment gives more confidence to business partners, users and governments.
Due to the complexity of pressure vessel design, analytical calculations are too complicated or even impossible. One of the most commonly used methods is to use existing standards, but sometimes these standards cannot cover in detail and comprehensively the design of pressure vessels. unique or custom vessel, so that computer modeling methods are used to calculate the structural parameters, or also known as Finite Element Analysis (FEA).
To prepare mechanical engineers to master various skills related to pressure vessels, we also provide training from trainers who are experts in their fields, both experience in the field and academics. Here are some of the training themes that we offer related to pressure vessels or vessels in general:
https://pttensor.com/wp-content/uploads/2020/01/pressure_vessel_code_aster-e1615547393864.jpg200289adminhttps://pttensor.com/wp-content/uploads/2024/11/tensor-logo-green-background-300x300.jpegadmin2021-07-26 08:05:062021-07-26 08:05:06pressure vessels in industry
Nowadays, along with the emergence of the issue of the energy crisis, waste accumulation, lack of water and other environmental issues related to the growth of the human population who continues to build infrastructure for homes, factories, offices and others, the concept of green building is also growing.
Green building is a building that combines environmentally friendly products, starting from natural materials, concepts to save energy and water, or using recycled materials to utilize materials that cannot be decomposed or wasted. Design considerations or products that contribute to safety, and environmental health are also an important part of green building, one of the important considerations is the design of a good Heating Ventilation and Air-conditioner (HVAC).
Since the building will basically be inhabited by humans, and humans need good air to breathe, the design of good ventilation is quite essential. There is special emphasis on Indoor Environmental Quality (IEQ) on green building which must consider humidity control, air filtration, contamination control and of course ventilation.
There are various ways to get good ventilation in a building, while in green buildings the methods that are often used are natural ventilation, energy recovery ventilation, whole-house fans, energy-saving exhaust fans, and a combination of these methods to get the sufficient amount of ventilation. Whatever method is used; a green building designer must ensure that the combination of methods or equipment supports each other.
Nowadays, green building attracts the attention of many parties, because the concept is profitable in terms of energy use and sustainable for the environment globally, as well as in terms of business and marketing for building developers. Basically, ventilation systems in buildings in general and green buildings do not have a significant difference, but in the design of green buildings we expect the use of energy to be minimal.
To achieve adequate ventilation, sometimes passive airflow is insufficient and mechanical equipment such as fans or the use of an Energy Recovery Ventilator (EVR) system is required. Some of the standards that can be applied to mechanical whole-house ventilation are to meet ASHARAE 62.2 requirements with the following details:
Whole-house mechanical ventilation system and controls installed to supply a certain rate of outside air (62.2 section 4), including ventilation limits at 62.2 section 4.5 (e.g. maximum 7.5 cfm/100 sq.ft) for “warm-humid” climates as defined by the IECC.
Air transfer (used air in other rooms/ducting) should not be used.
The air inlet is located at least 10 feet from the source of the contaminant.
Air flow must be tested first to meet the design criteria of the manufacturer: for example, to check the air mass flow rate value is in accordance with the specifications.
Then, according to ASHARAE 62.2 section 5, local exhaust ventilation that leads out of the room must be installed in each bathroom and kitchen.
Regardless of the method used for ventilation, the important thing that must be considered for green building designers is to ensure that the methods used support each other and do not reduce each other’s performance, for example installing an exhaust fan with an air flow rate which after being tested has lower flow rate than the specification, this may happen if there is intervention from other tools installed in inappropriate locations.
Or maybe the installation of air inlets from blowers or fans whose flow is not evenly distributed throughout the room: there are areas with high local air velocity but there are also areas that are not touched by air circulation at all and many more.
To overcome these problems, analytical/manual calculations are sometimes not possible due to special building plans or new building types and there has never been a previous reference, one of the best and rapidly developing alternatives currently being used is to use computer computing methods to model the air flow, or temperature distribution in the room, or also known as Computational Fluid Dynamics (CFD).
Using the CFD method, we can model unlimited room models like whatever we want and the locations of tools such as fans, exhausts, inlets, blowers and others, even this method can also model natural convection flows by taking into account the buoyancy that occurs due to differences in density due to differences in air temperature.
Moreover, if needed, we can model the exterior of the building being blown by the wind to see how much air is entering the building and air passing through the building just like that.
The pictures below are examples of a CFD analysis on a church building to see the characteristics of the air flow when passing through the building under various conditions (doors open/closed, windows open/closed, people inside/empty and so on).
simulation of air flow around the building
simulated airflow entering the building
Simulation of indoor air flow with forced convection system
Vector plot of velocity in the ventilation inlet system
The above simulations were made using openFOAM CFD software. Using the CFD software, we are able to get a more in-depth and specific insight into the designs we make.
For those of you mechanical engineers who are improving their skills in the HVAC field, we also provide solutions, namely training on topics about HVAC with trainers who are very experienced in their fields to improve your skills and competencies as a professional engineer. Here are some training topics related to HVAC topics:
The design of the hull of ships and boats is quite a challenging job because there are quite a number of variables that should be taken into account. One of the calculations that is quite important in hull design is resistance, because the magnitude of the resistance determines the effectiveness of the hull design that we make, more than that, the resistance will also affect the amount of engine power needed to move the ship. The first thing we need to understand in calculating this drag is the interaction of the hull with the propulsion system.
The ship’s propulsion system interacts with the ship’s hull and changes the flow pattern and resistance value of the ship itself. To facilitate the analysis of the drag on the hull, sometimes the propeller and hull analyzers are separated during laboratory testing, and even the rudder is sometimes separated during testing. The method that is currently developing is using a computer, namely the Computational Fluid dynamics (CFD) method. CFD method may be used for comprehensive analysis with the help of computer modeling.
Free surface wave modeling around the hull with openFOAM CFD
The calculation of the power required by the ship can be calculated using the general power equation:
POWER = FORCE * SPEED
Force is the drag or resistance of the ship and speed is the speed of the ship itself. From the above equation it can be shown that the resistance of the ship is directly proportional to the power required to move the ship.
The power required to push the ship forward is certainly less than the power generated by the thrust generated by the propeller, so the power generated by the propeller can be calculated using the following relationship:
THRUST POWER = THRUST * ADVANCE SPEED
Thrust is the force generated by the propeller that may the ship moving forward, while the advance speed is the velocity of the water flow before it reaches the propeller. On the propeller tested without the hull, this advance speed can be calculated easily based on the flow velocity. Meanwhile, in the presence of a hull, this velocity changes due to the interaction of the flow with the hull.
Thrust resulting from the propulsion test is higher than the hull resistance produced without a propeller, so calculations for additional resistance are needed with the following factors:
The propeller adds flow velocity to the rear of the hull which increases frictional resistance.
The propeller reduces the pressure on the back of the hull due to the high velocity (Bernoulli’s law).
The advance speed of the propeller is generally slower than the wake speed of the ship. The wake speed is divided into three components:
Friction wake: due to viscosity, the relative speed of the vessel is slowed at the boundary layer, resulting in flow separation and resulting in a wake.
Potential wake: Velocity at the stern is similar to that of the bow with a lower velocity at the stagnation point producing a wake flow.
Wave wake: the wave system on the ship changes locally due to the orbital velocity below the wave, the wave fragment due to the propeller also increases the wake.
Wave wake is only significant for Froude number, Fn > 0.3. Wake on the towing tank test model is larger than the full scale boundary layer, then the flow separation is also relatively large and a correction factor is needed. The wake behind the ship without the propeller is called the nominal wake, while the wake behind the propeller is called the effective wake. The flow around the propeller accelerates the flow rate by about 5-20%.
Based on the above relationships, it can be calculated the ratio of effective power to thrust power which represents the efficiency of the hull design that we make, this parameter is also called hull efficiency which is mathematically written as follows:
Then, based on the relationship between torque and rpm of the propeller, the power generated by the propeller can be calculated by the following equation:
POWER = 2.pi * RPM * TORQUE
Power at the above equation is less than the brake power, or the power generated by the propeller turning engine.
FACTORS CAUSED RESISTANCE ON THE HULL
After discussing the interaction between the hull and the propeller, then we will discuss the sources of ship resistance in general. In general, the causes of ship resistance are often separated into each category as follows:
1. Friction resistance: friction between the walls of the boat and the water causes a velocity difference between the speed on the wall of the boat and the speed of the water around the boat. The difference in velocity results in the emergence of a boundary layer that restrains the ship’s motion.
CFD simulation of frictional resistance in the hull using openFOAM
2. Viscous pressure resistance: the shape of the ship moving forward causes changes in the flow pattern around it, especially at the fore and aft. At the front there is stagnation pressure due to “stopping” the flow due to hitting the front of the ship, this pushes the ship with a force towards the rear. The wake at the rear of the ship produces an area of low pressure, this sucks the ship with a backward force. Ships with an elongated shape tend to have low resistance, but greater friction resistance, and vice versa.
3. Wave resistance: the energy in the waves is generated by the motion of the ship which produces drag. This wave is divided into two primary and secondary waves.
(1) primary wave system: The flow at the fore and aft of the ship is lower than the middle, and the highest at the stagnation pressure at the fore. The pressure difference produces a primary wave pattern whose shape depends on the velocity, but the location of the maximum, minimum and zero height of the wave is not affected by the velocity. The wave height is directly proportional to the square of the velocity.
(2) Secondary wave system: On the free surface, waves are formed and flow towards the back of the ship. The wave pattern consists of transverse and divergent waves. In deep water, the wave forms an angle of 19.5 deg , the angle is not affected by the shape of the ship. In shallow water, the angle is 90 deg and gets smaller and smaller.
The design, especially the calculation of the resistance on the ship, is very complex if it is carried out using analytical calculations (pure mathematics), so experiments are needed to calculate it more easily, for example using a towing tank. However, the testing costs and time for prototyping are relatively high; so that the current design trend for ship hulls is to use computational fluid dynamics (CFD) using computational methods.
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Fatigue is a material damage caused by repeated loading for a long time. If a metal is subjected to repeated loads (stress or strain), the metal will fracture. Damage due to repeated loads is called fatigue failures, generally this occurs after the use of the material for a long time. The damage occurs without warning, suddenly, and completely. More than 90% of the causes of mechanical failure are caused by fatigue fracture.
Phases in Fatigue Fracture:
Initiation crack
Crack propagation
Fracture failure
Fatigue fracture phases. Source: PT. Hesa Laras Brilliant
In general, the process of crack initiation occurs on the surface of the weak material or areas where there is a concentration of stress on the surface, such as scratches, notches, holes, etc., due to repeated loading. Furthermore, the beginning of these cracks develops into microcracks, the propagation or combination of these microcracks then forms macrocracks which will lead to failure. After that, the material will experience a final fracture, because the material has undergone a stress and strain cycle that results in permanent damage.
Basically, fatigue failure begins with the occurrence of cracks on the surface of the material. It proves that fatigue properties are very sensitive to surface conditions, which are influenced by several factors, including surface roughness, changes in surface properties, and surface residual stress. Therefore, the endurance limit is highly depends on the quality of the surface finish. Surface treatment may change the surface condition and residual stress on the surface. Surface treatment that produces compressive residual stress will result in increased fatigue resistance, while surface treatment that produces tensile residual stress will decrease its fatigue resistance.
At the surface of the material the highest concentration of compressive or tensile stress occurs. If the surface conditions are receiving tensile stresses, the residual compressive stress on the surface will result in a greater resultant compressive stress. The compressive stress will inhibit the initiation of crack, so that the fatigue resistance will increase, and the opposite will happen if there is residual tensile stress on the surface. The initial location of cracks in components or metals that are subjected to dynamic or cyclic loading is at the point of the region that has the minimum strength and or the point of the region experiencing the maximum stress.
Failure of components or structures can be divided into two main categories. First, quasi-static failure (failure that does not depend on time, and resistance to failure is expressed by strength). Second, time-dependent failure (resistance to failure is expressed by age or life time). Metal fatigue (fatigue fracture) is included in the time-dependent failure.
Factors Affecting Metal Fatigue
Loading
Load type: uniaxial, bending, torsion
Load pattern: periodic, random
Load amount
Load cycle frequency
Material condition (grain size, strength, solid solution reinforcement, second phase reinforcement, strain reinforcement, microstructure, surface finish), component size).
Working process (casting process, forming process, welding process, machining process, heat treatment process)
The oil and gas industry is one of the industries that demands highly engineered and varied processes. In this article, we will discuss some of the most common processes encountered in the oil and gas production process.
An Examples of the production processes in oil and gas. Source: Oil and Gas Production Handbook (2006)
Wellheads
The wellhead is above the actual oil or gas well leading to the reservoir. The wellhead can also be an injection well, used to inject water or gas back into reservoir to maintain pressure and level to maximize production. This process consists of reinforcing the wellbore with a casing, evaluating the formation pressure and temperature, and then installing the appropriate equipment to ensure an efficient flow of natural gas out of the well. Well flow is controlled by a choking device.
Manifolds/gathering
Onshore – The well flow is brought to the main production facility via a pipeline collection network and manifold system. The aim is to regulate production so that production levels increase, utilize the reservoir as well as possible, adjust the composition of the well flow (gas, oil, etc.) appropriately.
Offshore – Dry completion of the well in the central main field feeds directly to the production manifold, while the outer wellhead tower and subsea installation feed via multiphase pipelines back to the production rungs. A riser is a system that allows the piping to “go up” to the superstructure. To float a structure, this involves taking the weight of the structure and moving it. For heavy crude oil, diluents and heating may be required to reduce viscosity and facilitate flow rates.
Separation
Some wells have pure gas production which can be taken directly to gas treatment and/or compression. More often, wells consist of a combination of gas, oil and water and various contaminants must be separated and processed. The production of separators comes in various shapes and designs.
Gas Compression
The gas from the pure gas well has sufficient pressure to be supplied directly to the pipeline transport system. The gas from the separator generally loses so much pressure that it must be compressed to be transported. The turbine compressor benefits their energy by using a fraction of the compressed natural gas. The turbine functions to operate a centrifugal compressor, which contains a type of fan that compresses and pumps natural gas through pipes.
Metering, Storage and Export
Metering on oil and gas are at the measurement site which aims to measure several variables of oil and gas fluid flow in the pipeline. The quantities observed in the pipe are pressure, temperature, fluid level, flow rate.
Most industries do not allow local storage of gas, but oil is often stored before loading on ships, such as shuttle tankers that carry oil to larger tanker terminals, or directly to crude carriers. Offshore production facilities without pipelines rely on storage of crude oil in the ship’s bottom or hull, to allow a shuttle tanker to offload goods about once a week. More complex productions generally have tank warehouses that store more crude oil to deal with changes in demand, transportation delays, etc.
The processes above are closely related to fluid mechanics. One of the most common methods for designing a system of production processes in the oil and gas industry is to use Computational Fluid Dynamics (CFD), which is a method of solving fluid mechanics equations and even chemical reactions using a computer, so that comprehensive and detailed results are obtained. >> Click here to learn more about CFD!
Devold, Havard. 2006. Oil and Gas Production Handbook: An Introduction to Oil and Gas Production. Oslo: ABB ATPA Oil and Gas.
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