Monday, March 8, 2010

Penentuan Koefisien perpindahan Kalor Menyeluruh: Alat Penukar Kalor

by Ali Hasimi Pane


Gambar 1


Perhatikan gambar diatas, dimana fluida panas diasumsikan mengalir dalam pipa.

Panas dialirkan secara konveksi dari fluida panas kedinding pipa, kemudian panas dikonduksikan melalui dinding pipa dan dikonveksikan kembali dari dinding pipa kefluida dingin pada sisi luar pipa. Perpindahan panas ini terjadi dikarenakan adanya perbedaan temperatur antara aliran fluida panas dan fluida dingin. Maka untuk menentukan laju perpindahan panas yang terjadi perlu ditentukan harga koefisien perpindahan panas menyeluruh (U):




Dimana A adalah luas permukaan pipa, berdasarkan atas diameter sisi luar atau sisi dalam pipa. Dalam prakteknya, dameter luar sebenarnya selalu digunakan


Sementara atau LMTD adalah perbedaan temperatur rata – rata antara aliran fluida (panas dan dingin). Dalam hubungannya dengan koefisien perpindahan kalor menyeluruh disebut sebagai perbedaan temperatur rata – rata logaritma, dapat ditentukan:
▪ Untuk aliran searah


▪ Untuk aliran berlawanan


Jika diasumsikan,
adalah hambatan energi thermal, yang mana dari mekanisme perpinda-han pada gambar 1, dapat ditentukan:


Dimana hi dan ho dan adalah koefisien perpindahan panas konveksi untuk aliran pada sisi dalam dan sisi luar pipa. Sementara adalah luas permukaan sisi dalam pipa.


Jika persamaan 6 kita kalikan dengan Ao , maka:


Persamaan 8 tersebut berlaku untuk alat penukar kalor dalam kondisi baru atau tidak terjadi faktor pengotoran pada pipa. Jika terjadi faktor pengotoran maka koefisien perpindahan panas menyeluruh dapat ditentukan:


Dimana
adalah tahanan energy thermal faktor pengotoran pada sisi dalam dan luar pipa.




Referensi

R.W. Serth, “Process Heat Transfer Copyright”, UK: Elsevier Ltd, 2007.
Dr. Eduardo Cao, “Heat Transfer in Process Engineering” , New York: The McGraw-Hill Companies, Inc, 2010.
Adrian Bejan & Allan D. Kraus, “Heat Transfer Handbook”, New Jersey: John Wiley & Sons, Inc, 2003.
T. Kuppan, “Heat Exchanger Design Handbook”, New York: Marcel Dekker, Inc, 2000.

Friday, November 13, 2009

Dasar - dasar Sistem Pembangkit Tenaga

Abstrak

Sistem pembangkit tenaga adalah merupakan sumber utama penghasil energi baik untuk kebutuhan industri maupun kebutuhan publik lainnya. Dimana sistem ini kebanyakan menggunakan bahan bakar fosil baik itu berbahan bakar gas, cair maupun padat. Efisiensi sistem menjadi perhatian utama untuk sistem ini karena berhubungan dengan performance dan konsumsi bahan bakar. Maka untuk mencapai efisiensi siklus yang dikehendaki dilakukan beberapa modifikasi terhadap sistem dengan menambahkan beberapa peralatan selain peralatan utama atau dengan siklus kombinasi (siklus gas dan siklus uap). Melalui artikel ini akan dikenalkan siklus – siklus dasar dari sistem, siklus modifikasi dan kombinasi, kemudian pada proses selanjutnya akan dibahas dengan analisa – analisa dasar dengan kasus – kasus dan data – data yang valid.

Keyword: Sistem pembangkit tenaga, efisiensi, performance, siklus modifikasi, siklus kombinasi, analisa siklus.



Gambar 1. Skematik Sistem Uap

Pada artikel sebelumnya telah dibahas tentang peranan penting peralatan/mesin utility untuk berlangsungnya proses produksi diindustri. Pada artikel berikut ini akan diulas tentang bagian dari sistem utility yaitu sistem pembangkit tenaga (power plant system). Beberapa contoh kategori sistem pembangkit tenaga, seperti:

• Sistem pembangkit tenaga uap
• Sistem pembangkit tenaga gas
• Sistem pembangkit tenaga diesel
• Sistem pembangkit tenaga nuklir
• Sistem pembangkit tenaga air
• Sistem pembangkit tenaga panas bumi
• Sistem pembangkit tenaga matahari
• Sistem pembangkit tenaga angin dan gelombang laut

Dalam artikel ini tidak akan dibahas semua kategori sistem pembangkit tenaga tersebut, dalam kesempatan kali ini hanya akan dibahas tentang sistem pembangkit tenaga uap, dengan fokus pada analisis dasar yang berhubungan erat dengan ilmu thermodinamika untuk mengetahui performance sistem yaitu efisiensi thermal sistem.

Siklus Rankine adalah bagian dari ilmu thermodinamika yang menjadi referensi dasar untuk mengenal, memahami dan menganalisa sistem pembangkit tenaga uap. Maka tulisan ini akan mengulas tentang siklus Rankine dari siklus Rankine ideal hingga siklus Rankine yang mengalami modifikasi untuk mendapatkan efisensi siklus yang lebih baik, sehingga ekonomis dalam penggunaan bahan bakar. Oleh karena itu, sangat diharapkan bahwa pengertian dari siklus Rankine ini harus benar – benar dipahami oleh semua sarjana teknik mesin secara umum.

1. Siklus Rankine Saturasi


Gambar 2. Diagram Alir dan Diagram T – s Siklus Rankine Saturasi

Analisis performancenya berdasarkan thermodinamika:


 2. Siklus Rankine dengan Superheater


Gambar 2. Diagram Alir dan Diagram T – s Siklus Rankine Superheat

Analisis performancenya berdasarkan thermodinamika:


3. Siklus Rankine dengan SUperheater dan Reheater


Gambar 3. Diagram Alir dan Diagram T – s Siklus Rankine dengan Superheater dan Reheater

Analisis performancenya berdasarkan thermodinamika:



Dasar – dasar analisa yang ditinjau dari analisa thermodinamika ini adalah secara khusus har mampu dipahami oleh semua sarjana teknik mesin karena ini adalah merupakan skill/studi dasar yang sangat umum dan juga bisa dipahami oleh jurusan/bidang keahlian lainnya. Dari tulisan yang disajikan tersebut merupakan pandangan umum dan harus didalami dengan melihat dan memahami studi – studi kasus yang khusus membahas hal tersebut, fungsi dengan memahami ini akan dirasakan dan sangat membantu serta menjadi nilai tambah bagi sarjana teknik mesin itu sendiri ketika akan terjun ke bidang profesional seperti industri yang menangani energi ataupun sistem pembangkit tenaga untuk komersial atau kebutuhan industri itu sendiri.

Pada pembahasan selanjutnya akan diulas Siklus Rankine dengan Pemanas Air Umpan (Feedwater Heating). Dimana alat pemanas umpan adalah merupakan alat untuk mengurangi ketakmampubalikan ekonomisator tetapi tidak bisa menghapusnya sama sekali. Pemanasan air umpan meli-puti ekspansi adiabatik normal didalam turbin. Pemanas air umpan dapat dikategorikan:

• Pemanas air umpan terbuka atau kontak langsung
• Pemanas air umpan jenis terteutup dengan kurasan berjenjang mundur
• Pemanas air umpan jenis tertutup dengan kurasan dipompa maju

Pemanas air ulang ini sendiri adalah dimaksudkan juga untuk meningkatkan efisiensi siklus pembangkit daya. Akan dibahas pada tulisan artikel berikutnya. By alpconsultant.




 













Thursday, November 5, 2009

Idealisasi Equipment Industry

Abstrak

Industri – industri yang bergerak dalam bidang produksi baik itu untuk menghasilkan bahan baku ataupun produk jadi, dimana didalam area industri tersebut akan terdapat berbagai macam peralatan/mesin untuk menjalankan siklus produksi, peralatan/mesin yang dimaksud seperti peralatan/mesin utility, peralatan/mesin produksi dan peralatan/mesin pendukung lainnya. Dimana peralatan/mesin ini secara fungsional saling keterkaitan satu sama lainnya, akan tetapi yang memegang peranan utama adalah peralatan/mesin utility bisa seperti penghasil energi sementara peralatan/mesin proses produksi dan lainnya sebagai pengguna energi. Untuk mengenal atau memahami sistem dan menganalisa performance untuk mengetahui efisiensi dan keefektifan sistem dalam penggunaan energi, maka seyogyanya harus mengetahui pengidealisasi sistem, teori- teori dasar, terapan dan studi pendukung lainnya yang dianggap penting ketika akan menganalisa suatu sistem. Dengan memahami dan menguasai metode – metode penganalisaan sistem yang dilandasi pengetahuan yang tepat maka akan sangat membantu dalam mengefisiensikan penggunaan energi dan menekan biaya produksi suatu industri.


Keyword: Industri,peralatan/mesin, utility, proses produksi, performance, efisinesnsi, idealisasi




 Gambar 1. Peran posisi peralatan/mesin utility terhadap sistem proses produksi

Berbicara tentang industri atau pabrikasi, maka pertanyaannya apa sumber energi yang digunakan untuk menggerakkan semua peralatan industri hingga menghasilkan bahan baku ataupun produk jadi, peralatan yang dimaksud seperti mesin utility, mesin produksi dan peralatan pendukung lainnya. Kebanyakan industri baik yang bergerak dalam produksi makanan, produksi hasil pertanian, pertambangan batu bara, minyak, industri baja dan lainnya, mendayagunakan kemampuan akan mendirikan suplai energi untuk kebutuhan sendiri yaitu dengan membangun sistem power plant, seperti sistem uap (ketel uap,turbin uap), sistem power plant gas (turbin gas), mesin pembakaran dalam dan bisa gabungan antara sistem uap dan sistem gas (sistem power plant combinasi)



Gambar 2. Sistem power plant uap menggunakan bahan bakar batu bara (coal)

Sistem power plant yang digunakan untuk keberlangsungan proses produksi disebuah industri adalah sistem uap, sistem gas dan mesin pembakaran dalam seperti mesin Diesel. Sistem uap (gambar 2) sangat fami-liar diaplikasikan diindustri karena dapat dimanfaatkan untuk beberapa fungsi seperti:
  • Menghasilkan energi listrik untuk mengge-rakkan/menjalankan semua mesin baik utility maupun mesin proses produksi.
  • Untuk menghasilkan panas yaitu bisa dengan menggunakan langsung uap saturasi yang dihasilkan, ataupun uap kondensat dari buangan turbin untuk digunakan dalam proses produksi sebelum dibuang kelingkungan.
Dalam kesempatan ini, hanya akan ditekankan bahwa besarnya peranan penting terhadap skill dasar yang harus dimiliki oleh para tamatan sarjana teknik mesin. Secara umum pandangan masyarakat umum dan industri terhadap tamatan sarjana mesin tidak dipandang hanya mengetahui masalah energi, konstruksi maupun material, karena secara domestik ketersedian lapangan pekerjaan tidak cukup memadai jika berdasarkan tamatan teknik mesin yang memiliki keahlian pada konsentrasi studi khusus. Jadi setiap sarjana teknik mesin seyogyanya harus memiliki prinsip – prinsip pengetahuan dasar dan keahlian dasar yang bersinkroninasi terhadap jurusan teknik mesin tersebut.

Kembali kepada topik pembahasan utama, dimana peralatan/mesin industri yaitu mesin utility dan mesin produksi dan peralatan pendukung lainnya. Mesin – mesin utility tersebut merupakan sistem utama dalam lingkungan industri yang mensupport sehingga sistem proses produksi berjalan, dapat dilihat seperti gambar (1).

Oleh karena itu, dimana mesin utility memegang peranan utama dalam industri, maka skill dan penguasaan pengetahuan dasar yang harus dimiliki adalah kemampuan dalam mengalisis performance dan troubleshooting dan juga perencanaan pemeliharaan sistem – sistem industri, dengan maksud tercapainya suatu efisiensi dan ekonomis-nya operasional industri secara keseluruhan.

Contoh beberapa model pengetahuan dasar yang harus diketahui dan dikembangkan dalam beberapa sistem industri:

1. Diagram Alir dan Siklus Sistem Uap



Efisiensi Siklus

Dimana:
Hl     = Panas total uap pada tekanan masuk
H2   = Panas total uap pada tekanan kondensor
Hw2 = Panas total air pada tekanan kondensor


2. Diagram Alir dan Siklus Sistem Refrigerasi 

Formula untuk menganalisa performance sistem:


Dimana:
QL = Panas yang dilepas oleh kondensor
Wnet,in = Kerja yang dilakukan oleh kom-presor




3. Diagram Alir dan Siklus Sistem Turbin Gas

Formula untuk menganalisa performance sistem:








atau








Dimana:
Wnet = Kerja netto turbin
Qin = Panas yang masuk secara isobar
Qout = Panas yang keluar secara isobar
rp = Rasio tekanan
k = Konstanta panas spesifik

4. Sistem Heat Exchanger


Formula untuk menganalisa keefektifannya:

Dimana:

Qactual = Cc (Tc, out – Tc, in) = Ch, in (Th, in – Th,out)
Cc = mc . cpc
Ch = mh . cph
Qmax = Cmin (Th,in – Tc,in)
Cmin = Harga terkecil dari Cc atau Ch

Dan banyak lagi teori pengantar pengidealisasian sistem – sistem industri dalam hal untuk memahami metode analisa dasar performance sistem. Dalam hal ini seorang tamatan sarjana teknik mesin berperan penting untuk devisi ini.

Oleh karena itu, pemahaman teori – teori dasar yang berhubungan dengan sistem industri secara garis besar harus diketahui dan dikuasai oleh semua sarjana teknik mesin, walaupun pada saat masa mengikuti perkuliahan mendalami bagian – bagian materi spesialisasi dari jurusan teknik mesin, karena seyogya pihak industri tidak melihat secara eksplisit atau kekhususan spesialisasi studi yang diikuti tetapi melihat secara umum/keseluruhan dari jurusan teknik mesin tersebut.

Kesimpulan, materi ini adalah bahagian dari tawaran studi pembahasan yang kami kelola (Advance of Learning Program) dan banyak tawaran – tawaran studi lainnya yang kami khususkan berhubungan dengan sistem – sistem industri. By alpconsultant.

Skill, pengetahuan dan wawasan berkembang apabila diasah dan dikaji ulang secara berkesinambungan dan akan membuat orang tersebut memiliki karakter dan kepribadian sesuai dengan apa yang dipelajari

Referensi:
  1. 1.Zoran K. Morvay and Dušan D. Gvozdenac “Applied Industrial Energy and Environmental Management”, © 2008 JohnWiley & Sons Ltd, United Kingdom
  2. A.K. Raja, Amit Prakash Srivastava, Manish Dwivedi “Power Plant Engineering” © 2006 New Age International (P) Ltd., Publishers
  3. G.F. Hundy – A.R. Trott – T.C. Welch “Refrigeration and Conditioning” Fourth Edition
  4. Yunus A. Cengel dan Michael A. Boles, “Thermodynamics (An Engineering Approach)”, Fifth Edition McGraw-Hill
  5. Yunus A. Cengel,” The Basic Principles of Heat Transfer”, McGraw-Hill





Thursday, October 22, 2009

Motor Oils - Fuel Economy vs. Wear

By Blaine Ballentine, Central Petroleum Company


Conventional wisdom states that engine oils that increase fuel economy allow less friction and prolong engine life. The purpose of this article is to challenge conventional wisdom, particularly concerning modern (GF-3 ILSAC/API Starburst) engine oils.

Fuel Economy: Does Anyone Really Care?
First, we should face the fact that the American consumer does not appear to care too much about fuel economy. The No. 1 selling passenger vehicle is the Ford F-Series Pickup. Five of the top 10 best-selling vehicles are trucks, and trucks outsell cars. Some of the trucks are called sport-utility vehicles, otherwise known as SUVs, because their owners don’t want to admit they are trucks. The mass (size, weight) of these vehicles is not conducive to great fuel economy.

Additionally, consider how most vehicles are driven. Anyone accelerating slowly or driving at the speed limit to conserve energy is a danger to himself and other drivers who are in a much bigger hurry.

Auto manufacturers, on the other hand, are concerned about fuel economy. The manufacturer faces big fines if the fleet of cars it produces falls short of the Corporate Average Fuel Economy (CAFE) requirements imposed upon them by the federal government.

 Figure 1. Bearing Wear

The March to Thinner Oils

Thinner oils are being used these days for three reasons: They save fuel in test engines, the viscosity rules have changed, and manufacturers are recommending thinner grades.

The Sequence VI-B is the test used to evaluate fuel economy for the GF-3 specification. The VI-B test engine is fitted with a roller cam where the old Sequence VI test used a slider cam. The old Sequence VI test responded well to friction modifiers, but the Sequence VI-B responds to thinner oils.

The test oil’s fuel efficiency is compared to the fuel efficiency of a reference oil in the Sequence VI-B test. To pass, the test oil must improve fuel economy one to two percent, depending on viscosity grade. SAE 5W-20 must produce higher relative fuel efficiency than SAE 5W-30.

It is interesting to note that the reference oil is fully PAO synthetic SAE 5W-30. To qualify for the GF-3 Starburst, ordinary mineral oils had to beat the fuel economy of the full synthetic reference oil. (It seems there is more to fuel economy than a magic base oil.)

Another factor in fuel economy is temporary polymer shear. These polymers are additives known as viscosity index improvers (or modifiers). Polymers are plastics dissolved in oil to provide multiviscosity characteristics. Just as some plastics are tougher, more brittle or more heat-resistant than others, different polymers have different characteristics.

Polymers are huge molecules with many branches. As they are heated, they uncoil and spread out. The branches entangle with those of other polymer molecules and trap and control many tiny oil molecules. Therefore, a relatively small amount of polymer can have a huge effect on oil viscosity.

As oil is forced between a bearing and journal, many polymers have a tendency to align with each other, somewhat like nesting spoons. When this happens, viscosity drops. Then when the oil progresses through the bearing, the polymer molecules entangle again and viscosity returns to normal. This phenomenon is referred to as temporary shear.

Because the Sequence VI-B test responds to reductions in viscosity, oil formulators rely on polymer shear to pass the test. A shear stable polymer makes passing the GF-3 fuel economy test much more challenging.

New rules defining the cold-flow requirements of SAE viscosity grades (SAE J300) became effective in June 2001. The auto manufacturers were afraid that modern injection systems might allow the engine to start at temperatures lower than the oil could flow into the oil pump. Consequently, the new rules had a thinning effect on oil.

The auto manufacturers now recommend thinner oils for their vehicles than in the past. Years ago, SAE 10W-40 was the most commonly recommended viscosity grade, later migrating to SAE 10W-30. SAE 5W-30 is most popular now, but Ford and Honda recommend SAE 5W-20. It is likely that more widespread adoption of SAE 5W-20 and other thin oils may occur to help comply with CAFE requirements.

Because of the change in cold-flow requirements and the fuel economy test pushing formulators toward the bottom of the viscosity grade, today’s SAE 10W-30 oils are more like yesterday’s (GF-1 spec) SAE 5W-30 oils. On top of that, there is a trend toward auto manufacturers recommending thinner grades. This seems ridiculous. SUVs and trucks, with their inherently less-efficient four-wheel drive and brick-wall aerodynamics, need powerful, gas-guzzling engines to move their mass around in a hurry. In response, auto manufacturers recommend using thin oils to save fuel. Incredible!

Viscosity and Wear
Thinner oils have less drag, and therefore less friction and wear. Right? Perhaps in the test engine or engines that experience normal operation. But somewhat thicker oils may offer more protection for more severe operations such as driving through mountains, pulling a boat, dusty conditions, short trips, high rpm, overloading, overheating and overcooling.

Any abrasive particles equal to or larger than the oil film thickness will cause wear. Filters are necessary to keep contaminants small. The other side of the equation is oil film thickness. Thicker oil films can accommodate larger contaminants.

Temperature has a big effect on viscosity and film thickness. As a point of reference, one SAE grade increase in viscosity is necessary to overcome the influence of a 20°F increase in engine temperature. At a given reference point, there is approximately a 20°F. difference between viscosity grades SAE 30, 40 and 50. SAE 20 is somewhat closer to 30 than the other jumps, because SAE 30 must be 30°F higher than SAE 20 to be roughly the equivalent viscosity.

In other words, an SAE 20 at 190°F is about the same kinematic viscosity as an SAE 30 at 220°F, which is about the same viscosity as an SAE 40 at 240°F. This approximation works well in the 190°F to 260°F temperature range. One might be surprised at the slight amount of difference between straight viscosity vs. multiviscosity oils with the same back number (for example, SAE 30, SAE 5W-30, and SAE 10W-30).

If an SAE 50 oil at 260°F is as thin as an SAE 20 oil at 190°F, imagine how thin the oil film becomes when you are using an SAE 5W-20 and your engine overheats. When an engine overheats, the oil film becomes dangerously thin and can rupture.

Ford is bumping up against its CAFE requirements and recommends SAE 5W-20 oil for most of its engines in the United States. It claims SAE 5W-20 is optimal for fuel efficiency and wear.

To determine if SAE 5W-20 oils provide the same level of protection as SAE 5W-30 oils, Dagenham Motors in England, one of the largest Ford dealers in Europe, was consulted. SAE 5W-30 is required for warranty purposes in England, and SAE 5W-20 is not even available. If SAE 5W-20 were better for both fuel economy and wear, why would Ford not recommend it for its same engines in Europe?

Antiwear Property Changes
Another change that occurred in passenger car motor oils with GF-2 and GF-3 is a more stringent limit on phosphorus, which is part of the zinc phosphate (ZDDP) antiwear additive. The auto manufacturers are concerned that phosphorus will deposit on surfaces of the catalytic converter and shorten its life.

This is a complicated issue, and the deposits depend on the specific ZDDP chemistry and the finished oil formulation. The industry was unsuccessful in designing an engine test for an oil’s catalytic converter deposit forming tendencies. Therefore, the auto manufacturers set an arbitrary limit for motor oil of 0.1 percent phosphorus.

Antiwear additives are important in the absence of a hydrodynamic film, such as in the valve train. The antiwear additives are activated by frictional heat, which causes them to react with the hot surface and form a chemical barrier to wear.

The mechanism by which phosphorus deposits form on catalytic converter surfaces is not fully understood. It does not correlate directly with oil volatility or oil consumption. On the other hand, if engine wear causes oil consumption to increase, the risk of forming phosphorus deposits in the converter would increase dramatically. It seems that preventing wear and oil consumption should be a priority.

In the past, oil formulators could make a premium product by simply adding more ZDDP. A similar move today would result in an oil formulation that would not support new car warranties.

Short-term Thinking
As wear increases, the efficiency of an engine declines. Valve train wear slightly changes valve timing and movement. Ring and liner wear affect compression. The wear hurts fuel efficiency and power output by an imperceptible amount at first, but then the difference in fuel economy between an SAE 10W-30 and SAE 5W-20 is hardly noticeable. Efficiency continues to decline as wear progresses. Perhaps optimizing wear protection is the way to reduce fuel consumption over the life of the engine.

Certainly engines that have experienced significant ring and liner wear benefit from thicker oils. Thicker oil use results in compression increases, performance improvements and reduced oil consumption.

High-mileage oils are a relatively new category of passenger car motor oils. These products typically contain more detergent/ dispersant and antiwear additives than new car oils. They typically contain a seal swell agent and are available in thicker viscosity grades than most new cars recommend. “High mileage” seems to be defined by “as soon as your car is out of warranty.”

Figure 2. Ring Wear


What To Use
Although thinner oils with less antiwear additive outperform more robust products in the 96-hour fuel economy test, it is not clear that such products save fuel over the useful life of the engine.

Every fluid is a compromise. Oils recommended by the auto manufacturers seem to compromise protection from wear under severe conditions to gain fuel economy and catalyst durability. It is important to recognize that to use a product that offers more protection from wear will most likely compromise your warranty. Thicker oils also compromise cold temperature flow, which may be of concern depending upon climate and season.

The best protection against wear is probably a product that is a little thicker (such as SAE 10W-30 or 15W-40) and has more antiwear additives than the oils that support the warranty. The best oil for your vehicle depends on your driving habits, the age of your engine and the climate you drive in, but it is not necessarily the type of oil specified in the owner’s manual or stamped on the dipstick.

Reference
http://www.machinerylubrication.com/article_detail.asp?articleid=518

Hydraulic drive system

A hydraulic or hydrostatic drive system or hydraulic power transmission is a drive or transmission system that uses hydraulic fluid under pressure to drive machinery. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the kinetic energy of the flow.

Such a system basically consists of three parts. The generator (e.g. a hydraulic pump, driven by an electric motor, a combustion engine or a windmill); valves, filters, piping etc. (to guide and control the system); the motor (e.g. a hydraulic motor or hydraulic cylinder) to drive the machinery.

Principle of a hydraulic drive
Principle of hydraulic drive systemPascal's law is the basis of hydraulic drive systems. As the pressure in the system is the same, the force that the fluid gives to the surroundings is therefore equal to pressure x area. In such a way, a small piston feels a small force and a large piston feels a large force.

The same counts for a hydraulic pump with a small swept volume, that asks for a small torque, combined with a hydraulic motor with a large sweptvolume, that gives a large torque.

In such a way a transmission with a certain ratio can be built.

Most hydraulic drive systems make use of hydraulic cylinders. Here the same principle is used- a small torque can be transmitted in to a large force.

By throttling the fluid between generator part and motor part, or by using hydraulic pumps and/or motors with adjustable swept volume, the ratio of the transmission can be changed easily. In case throttling is used, the efficiency of the transmission is limited; in case adjustable pumps and motors are used, the efficiency however is very large. In fact, up to around 1980, a hydraulic drive system had hardly any competition from other adjustable (electric) drive systems.

Nowadays electric drive systems using electric servo-motors can be controlled in an excellent way and can easily compete with rotating hydraulic drive systems. Hydraulic cylinders are in fact without competition for linear (high) forces. For these cylinders anyway hydraulic systems will remain of interest and if such a system is available, it is easy and logical to use this system also for the rotating drives of the cooling systems.

Hydraulic cylinder

Hydraulic cylinders (also called linear hydraulic motors) are mechanical actuators that are used to give a linear force through a linear stroke. A hydraulic cylinder is without doubt the best known hydraulic component. Hydraulic cylinders are able to give pushing and pulling forces of millions of metric tons, with only a simple hydraulic system. Very simple hydraulic cylinders are used in presses; here the cylinder consists out of a volume in a piece of iron with a plunger pushed in it and sealed with a cover. By pumping hydraulic fluid in the volume, the plunger is pushed out with a force of plunger-area * pressure.

More sophisticated cylinders have a body with end cover, a piston-rod with piston and a cylinder-head. At one side the bottom is for instance connected to a single clevis, whereas at the other side, the piston rod also is foreseen with a single clevis. The cylinder shell normally has hydraulic connections at both sides. A connection at bottom side and one at cylinder head side. If oil is pushed under the piston, the piston-rod is pushed out and oil that was between the piston and the cylinder head is pushed back to the oil-tank again.

The pushing or pulling force of a hydraulic cylinder is:

F = Ab * pb - Ah * ph
F = Pushing Force in N
Ab = (π/4) * (Bottom-diameter)^2 [in m2]
Ah = (π/4) * ((Bottom-diameter)^2-(Piston-rod-diameter)^2)) [in m2]
pb = pressure at bottom side in [N/m2]
ph = pressure at cylinder head side in [N/m2]

Apart from miniature cylinders, in general, the smallest cylinder diameter is 32 mm and the smallest piston rod diameter is 16 mm.

Simple hydraulic cylinders have a maximum working pressure of about 70 bar, the next step is 140 bar, 210 bar, 320/350 bar and further, the cylinders are in general custom build. The stroke of a hydraulic cylinder is limited by the manufacturing process. The majority of hydraulic cylinders have a stroke between 0,3 and 5 metres, whereas 12-15 metre stroke is also possible, but for this length only a limited number of suppliers are on the market.

In case the retracted length of the cylinder is too long for the cylinder to be build in the structure. In this case telescopic cylinders can be used. One has to realize that for simple pushing applications telescopic cylinders might be available easily; for higher forces and/or double acting cylinders, they must be designed especially and are very expensive. If hydraulic cylinders are only used for pushing and the piston rod is brought in again by other means, one can also use plunger cylinders. Plunger cylinders have no sealing over the piston, or the piston does not exist. This means that only one oil connection is necessary. In general the diameter of the plunger is rather large compared with a normal piston cylinder, because this large area is needed.

Whereas a hydraulic motor will always leak oil, a hydraulic cylinder does not have a leakage over the piston nor over the cylinder head sealing, so that there is no need for a mechanical brake.

Hydraulic motor

The hydraulic motor is the rotary counterpart of the hydraulic cylinder.
Conceptually, a hydraulic motor should be interchangeable with hydraulic pump, because it performs the opposite function -- much as the conceptual DC electric motor is interchangeable with a DC electrical generator. However, most hydraulic pumps cannot be used as hydraulic motors because they cannot be backdriven. Also, a hydraulic motor is usually designed for the working pressure at both sides of the motor. Another difference is that a motor can be reversed by a reversing valve. Another factor affecting the operation of hydraulic motors is fluid flow rate. Pressure in a hydraulic system is like the voltage in an electical system and fluid flow rate is the equivalent of current. Pressure provides the force and flow rate the speed. The size of the pump decides the flow rate not just the pressure.

Hydraulic valves
These valves are usually very heavy duty to stand up to high pressures. Some special valves can control the direction of the flow of fluid and act as a control unit for a system.

Open and closed systems


Principle circuit diagram for open loop and closed loop system.A open system is one where the hydraulic fluid is returned into a large unpressurised tank at the end of a cycle through the system. In contrast a closed system is where the hydraulic fluid stays in one closed pressurised loop without returning to a main tank after each cycle. See open and closed systems.









Reference:
http://en.wikipedia.org/wiki/Hydraulic_system

Tuesday, October 20, 2009

Understanding Filter Efficiency and Beta Ratios

Jeremy Wright, Noria Corporation

Filter ratings are an often misunderstood area of contamination control. On several recent occasions, I have witnessed someone describing a filter by its nominal rating. A nominal rating is an arbitrary micrometer value given to the filter by the manufacturer. These ratings have little to no value. Tests have shown that particles as large as 200 microns will pass through a nominally rated 10-micron filter. If someone tries to sell you a filter based on an "excellent" nominal rating of five microns, run away.

Absolute Rating
Another common rating for filters is the absolute rating. An absolute rating gives the size of the largest particle that will pass through the filter or screen. Essentially, this is the size of the largest opening in the filter although no standardized test method to determine its value exists. Still, absolute ratings are better for representing the effectiveness of a filter over nominal ratings.

Figure 1

Beta Rating
The best and most commonly used rating in industry is the beta rating. The beta rating comes from the Multipass Method for Evaluating Filtration Performance of a Fine Filter Element (ISO 16889:1999).


 Table 1. Effect of Filtration Ratio (Beta Ratio) on Downstream Fluid Cleanliness

To test a filter, particle counters accurately measure the size and quantity of upstream particles per known volume of fluid, as well as the size and quantity of particles downstream of the filter. The ratio is defined as the particle count upstream divided by the particle count downstream at the rated particle size. Using the beta ratio, a five-micron filter with a beta 10 rating, will have on average 10 particles larger than five microns upstream of the filter for every one particle five microns or greater downstream.

The efficiency of the filter can be calculated directly from the beta ratio because the percent capture efficiency is ((beta-1)/beta) x 100. A filter with a beta of 10 at five microns is thus said to be 90 percent efficient at removing particles five microns and larger.

Caution must be exercised when using beta ratios to compare filters because they do not take into account actual operating conditions such as flow surges and changes in temperature.

A filter's beta ratio also does not give any indication of its dirt-holding capacity, the total amount of contaminant that can be trapped by the filter throughout its life, nor does it account for its stability or performance over time.

Nevertheless, beta ratios are an effective way of gauging the expected performance of a filter.

I hope this new knowledge of filter efficiency ratings enables you to make a more informed purchase the next time you buy a filter.

reference:
http://www.machinerylubrication.com/article_detail.asp?articleid=1289