Beberapa peneliti mengenai kesehatan tidur di Inggris mencari tahu apa kaitan posisi tidur seseorang dengan kepribadian kita. Ternyata kaitannya cukup erat. Berikut adalah 6 posisi tidur yang paling umum dari sekitar 1.000 responden dan kepribadian mereka.
Fetal. Posisi ini menempati posisi tertinggi dengan 41 persen responden mengatakan, mereka menggunakan posisi ini saat tidur. Orang yang tidur dengan posisi ini tidur dengan meringkuk, seperti bayi yang berada di dalam rahim. Umumnya tidur di salah satu sisi tubuh, kedua kaki menekuk dekat dengan perut. Kedua tangan menekuk di depan tubuh. Kepribadian orang yang tidur dengan posisi ini umumnya terlihat tangguh di luar, tapi sebenarnya sensitif di dalamnya, juga pemalu. Tipe ini juga mudah dan cepat relaks, tidak terlalu lama memusingkan masalah.
Log atau seperti balok kayu. Sebanyak 15 persen responden penelitian ini mengatakan tidur dengan posisi ini. Layaknya balok kayu, posisi tubuh tipe log umumnya tidur dengan menumpu pada salah satu sisi tubuh, kaki dan tangan lurus, tidak tertekuk. Orang yang tipe tidurnya seperti ini ditengarai memiliki kepribadian yang santai dan mudah bersosialisasi. Amat mudah percaya kepada orang asing. Namun, sayangnya kadang mudah tertipu oleh janji orang lain.
Yearner atau perindu. Selayaknya orang yang ingin memeluk, tipe yearner tidur dengan menumpu salah satu sisi tubuh, kaki lurus tak tertekuk, namun tangan seperti ingin menggapai. Lengan tergeletak lurus, seperti ingin menggapai. Tipe seperti ini biasanya memiliki kepribadian yang terbuka, namun tetap memiliki sikap sinis dan mudah curiga. Tipe ini juga lamban dalam mencapai keputusan dan sulit untuk mengubah pendirian dan pendapatnya.
Prajurit. Posisi ini menempati posisi keempat. Seperti prajurit yang selalu berada dalam keadaan siap siaga, posisi prajurit merupakan posisi orang yang tidur dengan keadaan rapi. Tubuh dalam keadaan telentang, kedua kaki dalam keadaan lurus, masing-masing lengan berada di sisi tubuh, juga lurus. Biasanya, orang yang tidur dengan tubuh ini adalah orang yang pendiam dan pemalu, tidak menyukai keributan. Tipe ini juga memiliki standar penilaian yang tinggi.
Freefaller. Sebanyak 7 persen responden penelitian ini mengatakan tidur dengan posisi ini. Biasanya, orang dengan tipe posisi tidur ini cenderung ceroboh, tergapah-gopoh, juga sensitif, namun memiliki kepercayaan diri yang cukup tinggi. Sayangnya, sulit untuk menerima kritik.
Bintang laut. Untuk mereka yang tidur dengan posisi kedua lengan tertekuk dan beristirahat di sisi kepala, mereka ditengarai memiliki kepribadian yang sangat sosial. Biasanya, mereka yang tidur dengan posisi yang dimiliki 5 persen para responden ini memiliki kepribadian yang mudah berteman. Siap untuk mendengarkan orang lain dan mudah menawarkan bantuan. Tipe ini pun memiliki sifat yang rendah hati, karenanya tak terlalu menyukai menjadi pusat perhatian.
Namun, para peneliti ini juga mengingatkan, seperti apa pun posisi tidur Anda, yang terpenting adalah bahwa Anda mendapatkan cukup waktu tidur, juga tidur dengan nyenyak. Kekurangan tidur dan masalah tidur lain bisa menimbulkan masalah pada kesehatan.
Sabtu, 03 April 2010
pajak untuk mobil dengan emisi tinggi
Indonesia mungkin patut meniru Inggris. Demi menekan tingkat polusi udara yang ditimbulkan dari emisi gas buang kendaraan bermotor, pemerintah dinegara tersebut memberlakukan pajak baru.
Mulai 1 April 2010, pemerintahan Inggris memberlakukan pajak tambahan baru, vehicle excise duty (VED), bagi mereka yang ingin memberi kendaraan. Setiap pembeli diharuskan memilih kendaraan dengan emisi gas buang yang rendah.
Mobil yang menghasilkan gas buang karbondioksida (CO2) lebih kecil dari 130 gr/km akan dibebaskan pajak VED di tahun pertama. Pajak akan dimulai dari yang terendah 110 pound (sekitar Rp 1,5 juta), untuk gas buang 131-140 gr/km akan dikenakan secara berjenjang hingga yang tertinggi mencapai 950 pound (Rp 13,09 juta) khusus untuk kendaraan yang emisinya lebih besar dari 255 g/km.
Pajak VED harus dibayarkan pada saat menerima mobil dari diler dan seterusnya setiap tahun sekali, saat pengurusan pajak kendaraan tahunan. Mobil yang bisa merasakan manfaat dari pajak ini adalah yang memillki emisi gas buang CO2 antara 121-130 gr/km karena hanya akan menanggung beban pajak maksimal 120 pound (Rp1,65 juta).
Namun Chief of the Society of Motor Manufacturers and Traders Inggris menyayangkan mengenai kebijakan ini. Menurutnya peraturan tersebut belum cukup jelas.
"Kami kecewa pemerintah tidak menunda pengenalan tarif VED baru yang lebih tinggi ini. Pajak yang berkaitan dengan kelestarian lingkungan harus lebih jelas dan konsisten sehingga pengendara bisa yakin mereka juga kana mendapatkan keuntungan dari keputusan ini," tegasnya seperti dilnasir Autoevolution, Sabtu (3/4/2010).
Sementara itu data penjualan mobil baru di Inggris mencatat, kendaraan dengan kategori emisi yang rendah baru mencapai 7,2 persen dari total peredaran mobil baru di negara tersebut.
feedwater heater
A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler.[1][2][3] Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system.[4] This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle. Many of the locomotive systems are ACFI type.
In a steam power plant (usually modeled as a modified Rankine cycle), feedwater heaters allow the feedwater to be brought up to the saturation temperature very gradually. This minimizes the inevitable irreversibilities associated with heat transfer to the working fluid (water). See the article on the Second Law of Thermodynamics for a further discussion of such irreversibilities.
Contents
Cycle discussion and explanation
It should be noted that the energy used to heat the feedwater is usually derived from steam extracted between the stages of the steam turbine. Therefore, the steam that would be used to perform expansion work in the turbine (and therefore generate power) is not utilized for that purpose. The percentage of the total cycle steam mass flow used for the feedwater heater is termed the extraction fraction[4] and must be carefully optimized for maximum power plant thermal efficiency since increasing this fraction causes a decrease in turbine power output.
Feedwater heaters can also be open and closed heat exchangers. An open feedwater heater is merely a direct-contact heat exchanger in which extracted steam is allowed to mix with the feedwater. This kind of heater will normally require a feed pump at both the feed inlet and outlet since the pressure in the heater is between the boiler pressure and the condenser pressure. A deaerator is a special case of the open feedwater heater which is specifically designed to remove non-condensable gases from the feedwater.
Closed feedwater heaters are typically shell and tube heat exchangers where the feedwater passes throughout the tubes and is heated by turbine extraction steam. These do not require separate pumps before and after the heater to boost the feedwater to the pressure of the extracted steam as with an open heater. However, the extracted steam (which is most likely almost fully condensed after heating the feedwater) must then be throttled to the condenser pressure, an isenthalpic process that results in some entropy gain with a slight penalty on overall cycle efficiency.
Many power plants incorporate a number of feedwater heaters and may use both open and closed components.
Feedwater heaters are used in both fossil- and nuclear-fueled power plants. Smaller versions have also been installed on steam locomotives, portable engines and stationary engines. An economiser serves a similar purpose to a feedwater heater, but is technically different. Instead of using actual cycle steam for heating, it uses the lowest-temperature flue gas from the furnace (and therefore does not apply to nuclear plants) to heat the water before it enters the boiler proper. This allows for the heat transfer between the furnace and the feedwater to occur across a smaller average temperature gradient (for the steam generator as a whole). System efficiency is therefore further increased when viewed with respect to actual energy content of the fuel.
siklus PLTU
Sebuah pembangkit listrik jika dilihat dari bahan baku untuk memproduksinya, maka Pembangkit Listrik Tenaga Uap bisa dikatakan pembangkit yang berbahan baku Air. Kenapa tidak UAP? Uap disini hanya sebagai tenaga pemutar turbin, sementara untuk menghasilkan uap dalam jumlah tertentu diperlukan air. Menariknya didalam PLTU terdapat proses yang terus menerus berlangsung dan berulang-ulang. Prosesnya antara air menjadi uap kemudian uap kembali menjadi air dan seterusnya. Proses inilah yang dimaksud dengan Siklus PLTU.
Air yang digunakan dalam siklus PLTU ini disebut Air Demin(Demineralized), yakni air yang mempunyai kadar conductivity (kemampuan untuk menghantarkan listrik) sebesar 0.2 us (mikro siemen). Sebagai perbandingan air mineral yang kita minum sehari-hari mempunyai kadar conductivity sekitar 100 – 200 us. Untuk mendapatkan air demin ini, setiap unit PLTU biasanya dilengkapi denganDesalination Plant dan Demineralization Plant yang berfungsi untuk memproduksi air demin ini.
Secara sederhana bagaimana siklus PLTU itu bisa dilihat ketika proses memasak air. Mula-mula air ditampung dalam tempat memasak dan kemudian diberi panas dari sumbu api yang menyala dibawahnya. Akibat pembakaran menimbulkan air terus mengalami kenaikan suhu sampai pada batas titik didihnya. Karena pembakaran terus berlanjut maka air yang dimasak melampaui titik didihnya sampai timbul uap panas. Uap ini lah yang digunakan untuk memutar turbin dan generator yang nantinya akan menghasilkan energi listrik.
Secara sederhana, siklus PLTU digambarkan sebagai berikut :
Siklus PLTU
1. Pertama-tama air demin ini berada disebuah tempat bernama Hotwell.
2. Dari Hotwell, air mengalir menuju Condensate Pump untuk kemudian dipompakan menuju LP Heater (Low Pressure Heater) yang pungsinya untuk menghangatkan tahap pertama. Lokasi hotwell dan condensate pump terletak di lantai paling dasar dari pembangkit atau biasa disebut Ground Floor. Selanjutnya air mengalir masuk keDeaerator.
3. Di dearator air akan mengalami proses pelepasan ion-ion mineral yang masih tersisa di air dan tidak diperlukan seperti Oksigen dan lainnya. Bisa pula dikatakan deaerator memiliki pungsi untuk menghilangkan buble/balon yang biasa terdapat pada permukaan air. Agar proses pelepasan ini berlangsung sempurna, suhu air harus memenuhi suhu yang disyaratkan. Oleh karena itulah selama perjalanan menuju Dearator, air mengalamai beberapa proses pemanasan oleh peralatan yang disebut LP Heater. Letak dearator berada di lantai atas (tetapi bukan yang paling atas). Sebagai ilustrasi di PLTU Muara Karang unit 4, dearator terletak di lantai 5 dari 7 lantai yang ada.
4. Dari dearator, air turun kembali ke Ground Floor. Sesampainya di Ground Floor, air langsung dipompakan oleh Boiler Feed Pump/BFP(Pompa air pengisi) menuju Boiler atau tempat “memasak” air. Bisa dibayangkan Boiler ini seperti drum, tetapi drum berukuran raksasa. Air yang dipompakan ini adalah air yang bertekanan tinggi, karena itu syarat agar uap yang dihasilkan juga bertekanan tinggi. Karena itulah konstruksi PLTU membuat dearator berada di lantai atas dan BFP berada di lantai dasar. Karena dengan meluncurnya air dari ketinggian membuat air menjadi bertekanan tinggi.
5. Sebelum masuk ke Boiler untuk “direbus”, lagi-lagi air mengalami beberapa proses pemanasan di HP Heater (High Pressure Heater). Setelah itu barulah air masuk boiler yang letaknya berada dilantai atas.
6. Didalam Boiler inilah terjadi proses memasak air untuk menghasilkan uap. Proses ini memerlukan api yang pada umumnya menggunakan batubara sebagai bahan dasar pembakaran dengan dibantu oleh udara dari FD Fan (Force Draft Fan) dan pelumas yang berasal dariFuel Oil tank.
7. Bahan bakar dipompakan kedalam boiler melalui Fuel oil Pump. Bahan bakar PLTU bermacam-macam. Ada yang menggunakan minyak, minyak dan gas atau istilahnya dual firing dan batubara.
8. Sedangkan udara diproduksi oleh Force Draft Fan (FD Fan). FD Fan mengambil udara luar untuk membantu proses pembakaran di boiler. Dalam perjalananya menuju boiler, udara tersebut dinaikkan suhunya oleh air heater (pemanas udara) agar proses pembakaran bisa terjadi di boiler.
9. Kembali ke siklus air. Setelah terjadi pembakaran, air mulai berubah wujud menjadi uap. Namun uap hasil pembakaran ini belum layak untuk memutar turbin, karena masih berupa uap jenuh atau uap yang masih mengandung kadar air. Kadar air ini berbahaya bagi turbin, karena dengan putaran hingga 3000 rpm, setitik air sanggup untuk membuat sudu-sudu turbin menjadi terkikis.
10. Untuk menghilangkan kadar air itu, uap jenuh tersebut di keringkan di super heater sehingga uap yang dihasilkan menjadi uap kering. Uap kering ini yang digunakan untuk memutar turbin.
11. Ketika Turbin berhasil berputar berputar maka secara otomastis generator akan berputar, karena antara turbin dan generator berada pada satu poros. Generator inilah yang menghasilkan energi listrik.
12. Pada generator terdapat medan magnet raksasa. Perputaran generator menghasilkan beda potensial pada magnet tersebut. Beda potensial inilah cikal bakal energi listrik.
13. Energi listrik itu dikirimkan ke trafo untuk dirubah tegangannya dan kemudian disalurkan melalui saluran transmisi PLN.
14. Uap kering yang digunakan untuk memutar turbin akan turun kembali ke lantai dasar. Uap tersebut mengalami proses kondensasi didalam kondensor sehingga pada akhirnya berubah wujud kembali menjadi air dan masuk kedalam hotwell.
Siklus PLTU ini adalah siklus tertutup (close cycle) yang idealnya tidak memerlukan lagi air jika memang kondisinya sudah mencukupi. Tetapi kenyataannya masih diperlukan banyak air penambah setiap hari. Hal ini mengindikasikan banyak sekali kebocoran di pipa-pipa saluran air maupun uap di dalam sebuah PLTU.
Untuk menjaga siklus tetap berjalan, maka untuk menutupi kekurangan air dalam siklus akibat kebocoran, hotwell selalu ditambah air sesuai kebutuhannya dari air yang berasal dari demineralized tank.
Jumat, 02 April 2010
condensor
n systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant. Condensers are typically heat exchangers which have various designs and come in many sizes ranging from rather small (hand-held) to very large industrial-scale units used in plant processes. For example, a refrigerator uses a condenser to get rid of heat extracted from the interior of the unit to the outside air. Condensers are used in air conditioning, industrial chemical processes such as distillation, steam power plants and other heat-exchange systems. Use of cooling water or surrounding air as the coolant is common in many condensers.
Example types of condensers
A surface condenser is an example of such a heat-exchange system. It is a shell and tube heat exchanger installed at the outlet of every steam turbine in thermal power stations. Commonly, the cooling water flows through the tube side and the steam enters the shell side where the condensation occurs on the outside of the heat transfer tubes. The condensate drips down and collects at the bottom, often in a built-in pan called a hotwell. The shell side often operates at a vacuum or partial vacuum, often produced by attached air ejectors.
In chemistry, a condenser is the apparatus which cools hot vapors, causing them to condense into a liquid. See "Condenser (laboratory)" for laboratory-scale condensers, as opposed to industrial-scale condensers. Examples include the Liebig condenser, Graham condenser, and Allihn condenser. This is not to be confused with a condensation reaction which links two fragments into a single molecule by an addition reaction and an elimination reaction.
In laboratory distillation, reflux, and rotary evaporators, several types of condensers are commonly used. The Liebig condenser is simply a straight tube within a cooling water jacket, and is the simplest (and relatively least expensive) form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense. Being more complex shapes to manufacture, these latter types are also more expensive to purchase. These three types of condensers are laboratory glassware items since they are typically made of glass. Commercially available condensers usually are fitted with ground glass joints and come in standard lengths of 100, 200, and 400 mm. Air-cooled condensers are unjacketed, while water-cooled condensers contain a jacket for the water.
Larger condensers are also used in industrial-scale distillation processes to cool distilled vapor into liquid distillate. Commonly, the coolant flows through the tube side and distilled vapor through the shell side with distillate collecting at or flowing out the bottom.
A condenser unit used in central air conditioning systems typically has a heat exchanger section to cool down and condense incoming refrigerant vapor into liquid, a compressor to raise the pressure of the refrigerant and move it along, and a fan for blowing outside air through the heat exchanger section to cool the refrigerant inside. A typical configuration of such a condenser unit is as follows: The heat exchanger section wraps around the sides of the unit with the compressor inside. In this heat exchanger section, the refrigerant goes through multiple tube passes, which are surrounded by heat transfer fins through which cooling air can move from outside to inside the unit. There is a motorized fan inside the condenser unit near the top, which is covered by some grating to keep any objects from accidentally falling inside on the fan. The fan is used to blow the outside cooling air in through the heat exchange section at the sides and out the top through the grating. These condenser units are located on the outside of the building they are trying to cool, with tubing between the unit and building, one for vapor refrigerant entering and another for liquid refrigerant leaving the unit. Of course, an electric power supply is needed for the compressor and fan inside the unit.
Direct contact condenser
In this type of condenser, vapors are poured into the liquid directly. The vapors lose their latent heat of vaporization; hence, vapors transfer their heat into liquid and the liquid becomes hot. In this type of condensation, the vapor and liquid are of same type of substance.
Example types of condensers
A surface condenser is an example of such a heat-exchange system. It is a shell and tube heat exchanger installed at the outlet of every steam turbine in thermal power stations. Commonly, the cooling water flows through the tube side and the steam enters the shell side where the condensation occurs on the outside of the heat transfer tubes. The condensate drips down and collects at the bottom, often in a built-in pan called a hotwell. The shell side often operates at a vacuum or partial vacuum, often produced by attached air ejectors.
In chemistry, a condenser is the apparatus which cools hot vapors, causing them to condense into a liquid. See "Condenser (laboratory)" for laboratory-scale condensers, as opposed to industrial-scale condensers. Examples include the Liebig condenser, Graham condenser, and Allihn condenser. This is not to be confused with a condensation reaction which links two fragments into a single molecule by an addition reaction and an elimination reaction.
In laboratory distillation, reflux, and rotary evaporators, several types of condensers are commonly used. The Liebig condenser is simply a straight tube within a cooling water jacket, and is the simplest (and relatively least expensive) form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense. Being more complex shapes to manufacture, these latter types are also more expensive to purchase. These three types of condensers are laboratory glassware items since they are typically made of glass. Commercially available condensers usually are fitted with ground glass joints and come in standard lengths of 100, 200, and 400 mm. Air-cooled condensers are unjacketed, while water-cooled condensers contain a jacket for the water.
Larger condensers are also used in industrial-scale distillation processes to cool distilled vapor into liquid distillate. Commonly, the coolant flows through the tube side and distilled vapor through the shell side with distillate collecting at or flowing out the bottom.
A condenser unit used in central air conditioning systems typically has a heat exchanger section to cool down and condense incoming refrigerant vapor into liquid, a compressor to raise the pressure of the refrigerant and move it along, and a fan for blowing outside air through the heat exchanger section to cool the refrigerant inside. A typical configuration of such a condenser unit is as follows: The heat exchanger section wraps around the sides of the unit with the compressor inside. In this heat exchanger section, the refrigerant goes through multiple tube passes, which are surrounded by heat transfer fins through which cooling air can move from outside to inside the unit. There is a motorized fan inside the condenser unit near the top, which is covered by some grating to keep any objects from accidentally falling inside on the fan. The fan is used to blow the outside cooling air in through the heat exchange section at the sides and out the top through the grating. These condenser units are located on the outside of the building they are trying to cool, with tubing between the unit and building, one for vapor refrigerant entering and another for liquid refrigerant leaving the unit. Of course, an electric power supply is needed for the compressor and fan inside the unit.
Direct contact condenser
In this type of condenser, vapors are poured into the liquid directly. The vapors lose their latent heat of vaporization; hence, vapors transfer their heat into liquid and the liquid becomes hot. In this type of condensation, the vapor and liquid are of same type of substance.
heat exchanger
A heat exchanger is a device built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact.[1] 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 the heat source, being a hot engine-cooling fluid, water, transfers heat to air flowing through the radiator (i.e. the heat transfer medium).
Heat exchangers may be classified according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is most efficient, in that it can transfer the most heat from the heat (transfer) medium. See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.
For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.
The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the log mean temperature difference (LMTD). Sometimes direct knowledge of the LMTD is not available and the NTU method is used.
Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and Tube heat exchangers are typically used for high pressure applications (with pressures greater than 30 bar and temperatures greater than 260°C).[2] This is because the shell and tube heat exchangers are robust due to their shape.
There are several thermal design features that are to be taken into account when designing the tubes in the shell and tube heat exchangers. These include:
Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.
Tube thickness: The thickness of the wall of the tubes is usually determined to ensure:
There is enough room for corrosion
That flow-induced vibration has resistance
Axial strength
Availability of spare parts
Hoop strength (to withstand internal tube pressure)
Buckling strength (to withstand overpressure in the shell)
Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including the space available at the site where it is going to be used and the need to ensure that there are tubes available in lengths that are twice the required length (so that the tubes can be withdrawn and replaced). Also, it has to be remembered that long, thin tubes are difficult to take out and replace.
Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter which leads to a more expensive heat exchanger.
Tube corrugation: this type of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance.
Tube Layout: refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30°), rotated triangular (60°), square (90°) and rotated square (45°). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular.
Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the fluid to flow upward and downwards between the tube bundle. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle which consists of two concentric baffles, the outer wider baffle looks like a donut, whilst the inner baffle is shaped as a disk. This type of baffle forces the fluid to pass around each side of the disk then through the donut baffle generating a different type of fluid flow.
Conceptual diagram of a plate and frame heat exchanger.
A single plate heat exchanger
Plate heat exchanger
Another type of heat exchanger is the plate heat exchanger. One is composed of multiple, thin, slightly-separated plates that have very large surface areas and fluid flow passages for heat transfer. This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger. Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasketed type to allow periodic disassembly, cleaning, and inspection. There are many types of permanently-bonded plate heat exchangers, such as dip-brazed and vacuum-brazed plate varieties, and they are often specified for closed-loop applications such as refrigeration. Plate heat exchangers also differ in the types of plates that are used, and in the configurations of those plates. Some plates may be stamped with "chevron" or other patterns, where others may have machined fins and/or grooves.
Adiabatic wheel heat exchanger
A fourth type of heat exchanger uses an intermediate fluid or solid store to hold heat, which is then moved to the other side of the heat exchanger to be released. Two examples of this are adiabatic wheels, which consist of a large wheel with fine threads rotating through the hot and cold fluids, and fluid heat exchangers.
Plate fin heat exchanger
This type of heat exchanger uses "sandwiched" passages containing fins to increase the effectivity of the unit. The designs include crossflow and counterflow coupled with various fin configurations such as straight fins, offset fins and wavy fins.
Plate and fin heat exchangers are usually made of aluminium alloys which provide higher heat transfer efficiency. The material enables the system to operate at a lower temperature and reduce the weight of the equipment. Plate and fin heat exchangers are mostly used for low temperature services such as natural gas, helium and oxygenliquefaction plants, air separation plants and transport industries such as motor and aircraft engines.
Advantages of plate and fin heat exchangers:
High heat transfer efficiency especially in gas treatment
Larger heat transfer area
Approximately 5 times lighter in weight than that of shell and tube heat exchanger
Able to withstand high pressure
Disadvantages of plate and fin heat exchangers:
Might cause clogging as the pathways are very narrow
Difficult to clean the pathways
boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications
Materials
The pressure vessel in a boiler is usually made of steel (or alloy steel), or historically of wrought iron. Stainless steel is virtually prohibited (by the ASME Boiler Code) for use in wetted parts of modern boilers, but is used often in superheater sections that will not be exposed to liquid boiler water. In live steam models, copper or brass is often used because it is more easily fabricated in smaller size boilers. Historically, copper was often used for fireboxes (particularly for steam locomotives), because of its better formability and higher thermal conductivity; however, in more recent times, the high price of copper often makes this an uneconomic choice and cheaper substitutes (such as steel) are used instead.
For much of the Victorian "age of steam", the only material used for boilermaking was the highest grade of wrought iron, with assembly by rivetting. This iron was often obtained from specialist ironworks, such as at Cleator Moor (UK), noted for the high quality of their rolled plate and its suitability for high-reliability use in critical applications, such as high-pressure boilers. In the 20th century, design practice instead moved towards the use of steel, which is stronger and cheaper, with welded construction, which is quicker and requires less labour.
Cast iron may be used for the heating vessel of domestic water heaters. Although such heaters are usually termed "boilers", their purpose is usually to produce hot water, not steam, and so they run at low pressure and try to avoid actual boiling. The brittleness of cast iron makes it impractical for high pressure steam boilers.
Fuel
The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Electric steam boilers use resistance- or immersion-type heating elements. Nuclear fission is also used as a heat source for generating steam. Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbines.
Configurations
Boilers can be classified into the following configurations:
"Pot boiler" or "Haycock boiler": a primitive "kettle" where a fire heats a partially-filled water container from below. 18th century Haycock boilers generally produced and stored large volumes of very low-pressure steam, often hardly above that of the atmosphere. These could burn wood or most often, coal. Efficiency was very low.
Fire-tube boiler. Here, water partially fills a boiler barrel with a small volume left above to accommodate the steam (steam space). This is the type of boiler used in nearly all steam locomotives. The heat source is inside a furnace or firebox that has to be kept permanently surrounded by the water in order to maintain the temperature of the heating surface just below boiling point. The furnace can be situated at one end of a fire-tube which lengthens the path of the hot gases, thus augmenting the heating surface which can be further increased by making the gases reverse direction through a second parallel tube or a bundle of multiple tubes (two-pass or return flue boiler); alternatively the gases may be taken along the sides and then beneath the boiler through flues (3-pass boiler). In the case of a locomotive-type boiler, a boiler barrel extends from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increase the heating surface compared to a single tube and further improve heat transfer. Fire-tube boilers usually have a comparatively low rate of steam production, but high steam storage capacity. Fire-tube boilers mostly burn solid fuels, but are readily adaptable to those of the liquid or gas variety.
Water-tube boiler. In this type,the water tubes are arranged inside a furnace in a number of possible configurations: often the water tubes connect large drums, the lower ones containing water and the upper ones, steam and water; in other cases, such as a monotube boiler, water is circulated by a pump through a succession of coils. This type generally gives high steam production rates, but less storage capacity than the above. Water tube boilers can be designed to exploit any heat source and are generally preferred in high pressure applications since the high pressure water/steam is contained within small diameter pipes which can withstand the pressure with a thinner wall.
Boiler for steam locomotive[3]
Flash boiler.A specialized type of water-tube boiler.
Fire-tube boiler with Water-tube firebox. Sometimes the two above types have been combined in the following manner: the firebox contains an assembly of water tubes, called thermic syphons. The gases then pass through a conventional firetube boiler. Water-tube fireboxes were installed in many Hungarian locomotives, but have met with little success in other countries.
Sectional boiler. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are assembled on site to create the finished boiler.
water turbine
water turbine is a rotary engine that takes energy from moving water.
Water turbines were developed in the nineteenth century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. They harness a clean and renewable energy source.
Theory of operation
Flowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts through a distance (force acting through a distance is the definition of work). In this way, energy is transferred from the water flow to the turbine
Water turbines are divided into two groups; reaction turbines and impulse turbines.
The precise shape of water turbine blades is a function of the supply pressure of water, and the type of impeller selected.
Reaction turbines
Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow.
Newton's third law describes the transfer of energy for reaction turbines.
Most water turbines in use are reaction turbines and are used in low (<30m/98ft) and medium (30-300m/98-984ft)head applications. In reaction turbine pressure drop occurs in both fixed and moving blades.
Impulse turbines
Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which change the direction of the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy.
Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.
Newton's second law describes the transfer of energy for impulse turbines.
Impulse turbines are most often used in very high (>300m/984ft) head applications .
Power
The power available in a stream of water is;
where:
P = power (J/s or watts)
η = turbine efficiency
ρ = density of water (kg/m³)
g = acceleration of gravity (9.81 m/s²)
h = head (m). For still water, this is the difference in height between the inlet and outlet surfaces. Moving water has an additional component added to account for the kinetic energy of the flow. The total head equals the pressure head plus velocity head.
= flow rate (m³/s)
Pumped storage
Some water turbines are designed for pumped storage hydroelectricity. They can reverse flow and operate as a pump to fill a high reservoir during off-peak electrical hours, and then revert to a turbine for power generation during peak electrical demand. This type of turbine is usually a Deriaz or Francis in design.
Efficiency
Large modern water turbines operate at mechanical efficiencies greater than 90% (not to be confused with thermodynamic efficiency).
gas turbine
A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. (Gas turbine may also refer to just the turbine element.)
Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the cumbustor; combustion of the fuel increases the temperature and therefore the pressure of the fluid in the fixed volume space. The products of the combustion (which are created as a result of the chemical reactions) are also created within the fixed volume and increase the pressure and interal energy of the gas even more. As these pressures are however less that the pressure leaving the compressor section of the engine, the combustion product mass is forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine also reduces the temperature and pressure of the exhaust gas.
Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks.
Theory of operation
Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.
In practice, friction and turbulence cause:
non-isentropic compression: for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.
non-isentropic expansion: although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.
pressure losses in the air intake, combustor and exhaust: reduces the expansion available to provide useful work.
Brayton cycle
As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. And combined heat and power (co-generation) uses waste heat for hot water production.
Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternative-rotor assembly (see image above), not counting the fuel system. However, the required precision manufacturing for components and temperature resistant alloys necessary for high efficiency often make the construction of a simple turbine more complicated than piston engines.
More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.
As a general rule, the smaller the engine the higher the rotation rate of the shaft(s) needs to be to maintain top speed. Turbine blade top speed determines the maximum pressure that can be gained,this produces the maximum power possible independent of the size of the engine. Jet engines operate around 10,000 rpm and micro turbines around 100,000 rpm.
Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. These bearings are being surpassed by foil bearings, which have been successfully used in micro turbines and auxiliary power units.
Types of gas turbines
Aeroderivatives and jet engines
Diagram of a gas turbine jet engine
Airbreathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets, whereas those that generate most of their thrust from the action of a ducted fan are often called turbofans or (rarely) fan-jets.
Gas turbines are also used in many liquid propellant rockets, the gas turbines are used to power a turbopump to permit the use of lightweight, low pressure tanks, which saves considerable dry mass.
Aeroderivatives are also used in electrical power generation due to their ability to startup, shut down, and handle load changes more quickly than industrial machines. They are also used in the marine industry to reduce weight. The GE LM2500 and LM6000 are two common models of this type of machine.
Amateur gas turbines
Increasing numbers of gas turbines are being used or even constructed by amateurs.
In its most straightforward form, these are commercial turbines acquired through military surplus or scrapyard sales, then operated for display as part of the hobby of engine collecting.[2][3] In its most extreme form, amateurs have even rebuilt engines beyond professional repair and then used them to compete for the Land Speed Record.
The simplest form of self-constructed gas turbine employs an automotive turbocharger as the core component. A combustion chamber is fabricated and plumbed between the compressor and turbine sections.[4]
More sophisticated turbojets are also built, where their thrust and light weight are sufficient to power large model aircraft.[5] The Schreckling design[5] constructs the entire engine from raw materials, including the fabrication of a centrifugal compressor wheel from plywood, epoxy and wrapped carbon fibre strands.
Like many technology based hobbies, they tend to give rise to manufacturing businesses over time. Several small companies now manufacture small turbines and parts for the amateur. Most turbojet-powered model aircraft are now using these commercial and semi-commercial microturbines, rather than a Schreckling-like home-build.[6]
steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884.[1]
It has almost completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.
The first device that may be classified as a reaction steam turbine was little more than a toy, the classic Aeolipile, described in the 1st century by Hero of Alexandria in Roman Egypt.[2][3][4] More than a thousand years later, in 1551, Taqi al-Din in Ottoman Egypt described a steam turbine with the practical application of rotating a spit. Steam turbines were also described by the Italian Giovanni Branca (1629) and John Wilkins in England (1648).[5] The devices described by al-Din and Wilkins are today known as steam jacks.
Parsons turbine from the Polish destroyer ORP Wicher.
The modern steam turbine was invented in 1884 by the Englishman Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW of electricity.[6] The invention of Parson's steam turbine made cheap and plentiful electricity possible and revolutionised marine transport and naval warfare.[7] His patent was licensed and the turbine scaled-up shortly after by an American, George Westinghouse. The Parson's turbine also turned out to be easy to scale up. Parsons had the satisfaction of seeing his invention adopted for all major world power stations, and the size of generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity. Within Parson's lifetime the generating capacity of a unit was scaled up by about 10,000 times,[8] and the total output from turbo-generators constructed by his firm C. A. Parsons and Company and by their licensees, for land purposes alone, had exceeded thirty million horse-power.[6]
A number of other variations of turbines have been developed that work effectively with steam. The de Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed before running it against a turbine blade. Hence the (impulse) turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with any pressure of steam, but is considerably less efficient.
The Brown-Curtis turbine which had been originally developed and patented by the U.S. company International Curtis Marine Turbine Company was developed in the 1900s in conjunction with John Brown & Company. It was used in John Brown's merchant ships and warships, including liners and Royal Navy warships.
For the types of steam turbine, including the Rateau multistage see Leander Project.
Types
Steam turbines are made in a variety of sizes ranging from small <1 hp (<0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines.
[edit]Steam Supply and Exhaust Conditions
These types include condensing, noncondensing, reheat, extraction and induction.
Noncondensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are available.
Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.
Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.
Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled.
Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.
Casing or Shaft Arrangements
These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications.
Principle of Operation and Design
An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.
Turbine efficiency
Schematic diagram outlining the difference between an impulse and a reaction turbine
To maximize turbine efficiency the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.
Impulse turbines
An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.
As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss".
Reaction turbines
In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.
Operation and Maintenance
When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10 to 15 RPM to slowly warm the turbine.
Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. It is, however, essential that the turbine be turned with dry steam. If water gets into the steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.
Speed regulation
The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control.[9] Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials. During normal operation in synchronization with the electricity net powerplants are governed with a five percent droop speed control . This means the full load speed is 100% and the no load speed is 105%. This is required for the stable operation of the net without hunting and dropouts of powerplants. Normally the changes in speed are minor . Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor. Generally this is a basic system requirement for all powerplants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication. [10]
Direct drive
Electrical power stations use large steam turbines driving electric generators to produce most (about 80%) of the world's electricity. Most of these centralised stations are of two types, fossil fuel power plants and nuclear power plants, but some countries are using concentrating solar power (CSP) to create the steam. Steam turbines can also be used directly to drive large centrifugal pumps, such as feedwater pumps at a thermal power plant.
It has been proposed[11] that, given sufficient solar energy, silicon might be refined for use as a coal replacement for this type of engine.
The turbines used for electric power generation are most often directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. In installations with high steam output, as may be found in nuclear power stations, the generator sets may be arranged to operate at half these speeds, but with four-pole generators.[12]
Marine propulsion
Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. A steam turbine is only efficient when operating in the thousands of RPM, while the most effective propeller designs are for speeds less than 100 RPM. Therefore precise (thus expensive) reduction gears are generally used, although several ships, such as Turbinia, had direct drive from the steam turbine to the propeller shafts. The purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power. However, diesel engines are capable of higher efficiencies: steam turbine cycle efficiencies have yet to break 50%, yet diesel engines routinely exceed 50%, especially in marine applications.[13][14] [15] [16]
Nuclear-powered ships and submarines use a nuclear reactor to create steam and either use a steam turbine directly for main propulsion, with generators providing auxiliary power, or else employ turbo-electric propulsion, where the steam drives a turbine-generator set with propulsion provided by electric motors. Nuclear power is often chosen where diesel power would be impractical (as in submarine applications) or the logistics of refuelling pose significant problems (for example, icebreakers). It has been estimated that the reactor fuel for the Royal Navy's Vanguard class submarine is sufficient to last 40 circumnavigations of the globe – potentially sufficient for the vessel's entire service life.
[edit]Locomotives
Steam turbine locomotive
A steam turbine locomotive engine is a steam locomotive driven by a steam turbine.
The main advantage of a steam turbine locomotive is better balance and reduced hammer blow on the track. However, a disadvantage is that its output power is less flexible and so turbine locomotives were best suited for long-haul operations at a constant output power.[17]
The first steam turbine locomotive was built in 1908 for the Officine Meccaniche Miani Silvestri Grodona Comi in Milan. For the German National Railroad company Krupp built in 1924 the steam turbine locomotive T18 001, which was operational in 1929.
It has almost completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.
The first device that may be classified as a reaction steam turbine was little more than a toy, the classic Aeolipile, described in the 1st century by Hero of Alexandria in Roman Egypt.[2][3][4] More than a thousand years later, in 1551, Taqi al-Din in Ottoman Egypt described a steam turbine with the practical application of rotating a spit. Steam turbines were also described by the Italian Giovanni Branca (1629) and John Wilkins in England (1648).[5] The devices described by al-Din and Wilkins are today known as steam jacks.
Parsons turbine from the Polish destroyer ORP Wicher.
The modern steam turbine was invented in 1884 by the Englishman Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW of electricity.[6] The invention of Parson's steam turbine made cheap and plentiful electricity possible and revolutionised marine transport and naval warfare.[7] His patent was licensed and the turbine scaled-up shortly after by an American, George Westinghouse. The Parson's turbine also turned out to be easy to scale up. Parsons had the satisfaction of seeing his invention adopted for all major world power stations, and the size of generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity. Within Parson's lifetime the generating capacity of a unit was scaled up by about 10,000 times,[8] and the total output from turbo-generators constructed by his firm C. A. Parsons and Company and by their licensees, for land purposes alone, had exceeded thirty million horse-power.[6]
A number of other variations of turbines have been developed that work effectively with steam. The de Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed before running it against a turbine blade. Hence the (impulse) turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with any pressure of steam, but is considerably less efficient.
The Brown-Curtis turbine which had been originally developed and patented by the U.S. company International Curtis Marine Turbine Company was developed in the 1900s in conjunction with John Brown & Company. It was used in John Brown's merchant ships and warships, including liners and Royal Navy warships.
For the types of steam turbine, including the Rateau multistage see Leander Project.
Types
Steam turbines are made in a variety of sizes ranging from small <1 hp (<0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines.
[edit]Steam Supply and Exhaust Conditions
These types include condensing, noncondensing, reheat, extraction and induction.
Noncondensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are available.
Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.
Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.
Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled.
Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.
Casing or Shaft Arrangements
These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications.
Principle of Operation and Design
An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.
Turbine efficiency
Schematic diagram outlining the difference between an impulse and a reaction turbine
To maximize turbine efficiency the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.
Impulse turbines
An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.
As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss".
Reaction turbines
In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.
Operation and Maintenance
When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10 to 15 RPM to slowly warm the turbine.
Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. It is, however, essential that the turbine be turned with dry steam. If water gets into the steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.
Speed regulation
The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control.[9] Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials. During normal operation in synchronization with the electricity net powerplants are governed with a five percent droop speed control . This means the full load speed is 100% and the no load speed is 105%. This is required for the stable operation of the net without hunting and dropouts of powerplants. Normally the changes in speed are minor . Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor. Generally this is a basic system requirement for all powerplants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication. [10]
Direct drive
Electrical power stations use large steam turbines driving electric generators to produce most (about 80%) of the world's electricity. Most of these centralised stations are of two types, fossil fuel power plants and nuclear power plants, but some countries are using concentrating solar power (CSP) to create the steam. Steam turbines can also be used directly to drive large centrifugal pumps, such as feedwater pumps at a thermal power plant.
It has been proposed[11] that, given sufficient solar energy, silicon might be refined for use as a coal replacement for this type of engine.
The turbines used for electric power generation are most often directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. In installations with high steam output, as may be found in nuclear power stations, the generator sets may be arranged to operate at half these speeds, but with four-pole generators.[12]
Marine propulsion
Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. A steam turbine is only efficient when operating in the thousands of RPM, while the most effective propeller designs are for speeds less than 100 RPM. Therefore precise (thus expensive) reduction gears are generally used, although several ships, such as Turbinia, had direct drive from the steam turbine to the propeller shafts. The purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power. However, diesel engines are capable of higher efficiencies: steam turbine cycle efficiencies have yet to break 50%, yet diesel engines routinely exceed 50%, especially in marine applications.[13][14] [15] [16]
Nuclear-powered ships and submarines use a nuclear reactor to create steam and either use a steam turbine directly for main propulsion, with generators providing auxiliary power, or else employ turbo-electric propulsion, where the steam drives a turbine-generator set with propulsion provided by electric motors. Nuclear power is often chosen where diesel power would be impractical (as in submarine applications) or the logistics of refuelling pose significant problems (for example, icebreakers). It has been estimated that the reactor fuel for the Royal Navy's Vanguard class submarine is sufficient to last 40 circumnavigations of the globe – potentially sufficient for the vessel's entire service life.
[edit]Locomotives
Steam turbine locomotive
A steam turbine locomotive engine is a steam locomotive driven by a steam turbine.
The main advantage of a steam turbine locomotive is better balance and reduced hammer blow on the track. However, a disadvantage is that its output power is less flexible and so turbine locomotives were best suited for long-haul operations at a constant output power.[17]
The first steam turbine locomotive was built in 1908 for the Officine Meccaniche Miani Silvestri Grodona Comi in Milan. For the German National Railroad company Krupp built in 1924 the steam turbine locomotive T18 001, which was operational in 1929.
Langganan:
Postingan (Atom)
