contoh surat memohon pekerjaan

Lot 4444,

Jalan Kampung Kong Sai,

Air Merbau ,77200 Jasin,

Melaka,Malaysia.

0136++++++

kami859@yahoo.com

CSC STEEL SDN. BHD. 180, 31 August 2010

Kawasan IndustriAyer Keroh,

Ayer Keroh, 75450 Melaka,

Malaysia.

Tel : 606-2310169

Dear Sir/Madam

I am writing in response to your advertisement in JobStreet.com inviting applications for Application Mechanical Engineer upon completion of my Bachelor’s Degree in April 2013. I believe I have the skills as well as the spirit to work for the company.

Allow me to briefly introduce myself. I am currently second-year mechanical engineering student, and will graduate from University Technology Mara in April 2013. Through the advertisement, I study that you are seeking candidates which can study and execute equipment revamp project for improvement, prepare trouble shouting report, cases analyzes and preventives improvement.

With strong qualities in work that i have, i have a keen interest and would pursue my careers in engineering. Enclosure resumes giving you details of my personal particulars.

I am also competent in the use of computers software like Microsoft Office, Adobe Photoshop, Solid Work, Matlab and C++ programming. I speak and write well in Bahasa Melayu, English and basic Japanese.

I would be grateful if you can arrange an interview so that i can explain my qualifications further. You can contact me at any time. I look forward to hearing from you. Thank you in advance for your time.

Thanks you for your consideration.

Yours sincerely,

Signature

AHMAD BIN MOHAMAD

jangan lelaki biasa

Jangan lelaki biasa, Nabi Muhammad pun terasa sunyi tanpa wanita

Tanpa wanita,

hati, fikiran dan perasaan mereka akan resah

Masih mencari,

walaupun sudah ada segala-galanya

Apa lagi yang tiada di syurga?

Namun Nabi Adam A.S tetap merindui Siti Hawa

Kepada wanitalah lelaki memanggil ibu, isteri dan puteri

Dijadikan mereka dari tulang rusuk yang bengkok 

Untuk diluruskan lelaki

Tetapi andai lelaki sendiri yang tidak lurus

Tidak mampu untuk meluruskan mereka

Manakan mungkin kayu yang bengkok

Mampu menghasilkan bayang-bayang yang lurus

Luruskanlah wanita dengan cara yang ditunjuk oleh Allah

Kerana mereka dicipta sebegitu rupa oleh Allah

Didiklah mereka dengan panduan dariNya

Jangan jinakkan mereka dengan harta

Nanti mereka semakin liar

Jangan hiburkan mereka dengan kecantikan

Nanti mereka menderita

Kenalkan mereka dengan Allah

Zat yang kekal

Kerana disitulah kuncinya

Akal senipis rambutnya, tebalkan dengan ilmu

Hati serapuh kaca, kuatkan dengan iman

Perasaan selembut sutera, hiasilah dengan akhlak

Suburkanlah kerana dari situlah nanti

Akan nampak nilaian dan keadilan Tuhan

Akan terhibur dan bahagialah hati mereka

Walau tidak dinobatkan ratu cantik dunia

Walau tidak dipuja sebagai si puteri

Bisikkan ke telinga mereka

Bahawa kelembutan itu bukan satu kelemahan

Bukan pula satu beban dari Tuhan

Sebaliknya disitulah kasih sayang Tuhan

Kerana

Rahim wanita yang lembut itulah

Yang mengandungkan lelaki

Wanita yang lupa hakikat kejadiannya

Pasti tidak terhibur dan tidak menghiburkan

Tanpa ilmu, iman dan akhlak

Wanita bukan sahaja tidak boleh diluruskan

Bahkan mereka pula yang membengkokkan

Itulah hakikat dan akibat apabila wanita tidak kenal Tuhan

Mereka tidak akan mengenal diri sendiri

Apalagi mahu mengenal lelaki

Apabila wanita derhaka

Dunia lelaki huru-hara

Bila tulang rusuk patah

Rosaklah jantung, dan hati yang dilindunginya

Wahai lelaki

Jangan pula hanya mengharapkan ketaatan

Tetapi binalah kepimpinan

Sebelum memimpin wanita menuju Allah

Pimpinlah diri sendiri dahulu kepada-Nya

Jinakkan diri dengan Allah

Nescaya jinaklah segala-galanya di bawah pimpinan kita

Sejahat mana pun lelaki

pasti mengharapkan

Wanita solehah bakal menyinari kehidupannya

Namun janganlah mengharapkan isteri sepertI Siti Fatimah

Kalau peribadi sendiri belum lagi seperti Saidina Ali

what is robust design?

DEFINITION

Robust design method also called Taguchi Method pioneered by Dr. Genichi,greatly improves productivity by consciously considering the noise factors and the cost of failure to ensure customer satisfaction.it is defined as a design of a product so that its functionality varies minimally despite of disturbing factor influences. Robust Design also proven development philosophy focused on achieving target reliability. Approaching this aggressive goal requires that Robust Design principles be an early and integral part of the development cycle.

OBJECTIVE

The objective is to make the end-product immune to factors that could adversely affect performance. Robust Design requires that the following four factors be considered in the design process: signal, response, noise, and control.

ADVANTAGES

• Robustness reduces variation in parts by reducing the effects of uncontrollable variation
• Robust design lower quality parts or parts with higher tolerances can be used and a quality product can still be made
• Saves the company money
• More appeal to the customer
•  Effectively deal with the noise

DISADVANTAGES

• The problem becomes large quickly
• Using orthogonal arrays, it assumes the noise factors are independent, which may be helpful in setting up the experiment, but is not necessarily a good assumption

jangan kahwini wanita yang 6

Pesanan Rasulullah SAW untuk kaum Lelaki, 

"Jangan engkau kahwini wanita yang enam, jangan yang ananah, yang mananah, dan yang hananah dan jangan engkau kahwini yang hadaqah, yang basaqah dan yang syadaqah". 

Wanita Ananah
Wanita yang banyak mengomen itu dan ini. Apa yang diberi atau dilakukan suami untuk rumah tangganya semua tidak kena dan tidak berpuas hati.


Wanita Mananah
Wanita sebegini yang menidakkan usaha dan jasa suami sebaliknya mendabik dada dialah yang banyak berkorban untuk membangunkan rumah tangga. Dia suka mengungkit-ungkit apa yang dilakukan untuk kebaikan rumah tangga. Biasanya wanita ini bekerja atau berkedudukan tinggi dan bergaji besar.


Wanita Hananah
Menyatakan kasih sayangnya kepada suaminya yang lain yang dikahwininya sebelum ini atau kepada anaknya dari suami yang lain dan wanita ini berangan-angan mendapatkan suami yang lebih baik dari suami yang sedia ada. Dalam kata lain wanita sebegini tidak bersyukur dengan jodohnya itu. Wanita sebegini yang mengkufuri nikmat perkahwinan. Dia juga merendahkan kebolehan dan kemampuan suaminya.


Wanita Hadaqah
Melemparkan pandangan dan matanya pada tiap sesuatu lalu menyatakan keinginannya utk memiliki barang itu dan memaksa suaminya untuk membelinya. Selain itu, wanita ini suka mengikut nafsunya. Wanita sebigini memeningkan kepala lelaki. Dia tengok apa saja dia mahu. Dia suka membandingkan dirinya dengan diri orang lain. Suka menunjuk-nujuk. Wanita inilah yang menjadikan suami dulu handsome sekarang sudah botak. :)

Wanita Basaqah

Ada 2 makna:

Pertama,wanita sebegini yang suka bersolek dan menghiaskan diri. Dia menghias diri bukan untuk sumainya tetapi untuk ditunjuk-tujukkan kepada dunia. Suka melawa. Wangnya dihabiskan untuk membeli make-up, kasut dan barang kemas. Wanita begini juga suka dipuji-puji. Kalau dia kebetulan menjadi isteri orang ternama dan menjadi pula ketua dalam kumpulan itu, orang lain tidak boleh mengatasi dirinya.

Kedua, dia marah ketika makan dan tidak mahu makan kecuali sendirian dan diasingkannya bahagiannya.

Wanita Syadaqah

Banyak cakap, tidak menentu lagi bising. Kebecokan itu juga menyebabkan segala kerja yang dibuatnya tidak menjadi, hanya tukang sibuk dan komen saja.

p/s: dicatat oleh Imam Al-Ghazalli...

lab report: force convection

OBJECTIVE

To demonstrate the effect and the use of finned surface and pinned surface to improve the heat transfer in forced convection.

INTRODUCTION

Convection is the movement of molecules within fluids (i.e. liquids, gases). It cannot take place in solids, since either bulk current flows or significant diffusion can take place in solids. Convection is one of the major modes of heat transfer and mass transfer.

Forced convection is a mechanism, or type of heat transport in which fluid motion is generated by an external source (like a pump, fan, suction device, etc.). It should be considered as one of the main methods of useful heat transfer as significant amounts of heat energy can be transported very efficiently and this mechanism is found very commonly in everyday life, including central heating, air conditioning, steam turbines and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchangers, pipe flow, and flow over a plate at a different temperature than the stream (the case of a shuttle wing during re-entry, for example). However, in any forced convection situation, some amount of natural convection is always present whenever there are g-forces present (i.e., unless the system is in free fall). When the natural convection is not negligible, such flows are typically referred to as mixed convection.

The removal of excessive heat from system components is essential to avoid damaging effects of burning or overheating. Therefore, the enhancement of heat transfer is an important subject of thermal engineering. Extended surfaces (fins) are frequently use in heat exchanging devices for the purpose of improve the heat transfer between a primary surface and the surrounding fluid. Various types of heat exchanger fins ranging from relatively simple shapes, such as rectangular, square, cylindrical, annular, tapered or pin fins, to a combination of different geometries, have been used. The study of improving heat transfer performance is referred to as heat transfer augmentation, enhancement or intensification.

The heat transfer augmentation is very important subject in industrial heat exchangers and other thermal application. Extended surfaces, which are popularly known as fins, are extensively used in air-cooled automobile engines and in air-cooled aircraft engines. Fins are also used for the cooling of computer processors, and other electronic devices. In various applications heat from the fins is dissipated by natural as well as forced convection and radiation. Fins are used as arrays in all the applications.

Thus, our experiment is to demonstrate the use of a fin (extended surface) to improve the heat transfer in forced convection. More about this was explained details in this report.

THEORY

Heat transfer from an object can be improve by increasing the surface area in contact with the air by adding fins or pins normal to the surface. This can be seen in Newton’s Law of Cooling that states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings, which defines the convection heat transfer rate.

The constant of proportionality h is termed the convection heat-transfer coefficient. The heat transfer coefficient h is a function of the fluid flow, so, it is influenced by the surface geometry, the fluid motion in the boundary layer and the fluid properties as well. The effect of the surfaces can be demonstrated by comparing finned and unfinned surfaces with a flat plate under the same conditions of power and flow.

A heated surface dissipates heat to the surrounding fluid primarily through a process called convection. Heat is also dissipated by conduction and radiation, however these effects are not considered in this experiment. Air in contact with the hot surface is heated by the surface and rises due to reduction in density. The heated air is replaced by cooler air, which is in turn heated by the surface, and rises. This process is called free convection.

Convection heat transfer from an object can be improved by increasing the surface area in contact with the air. In practical it may be difficult to increase the size of the body to suit. In these circumstances the surface area in contact with the air may be increased by adding fins or pins normal to the surface. These features are called extended surfaces. A typical example is the use of fins on the cylinder and head on an air-cooled petrol engine. The effect of extended surfaces can be demonstrated by comparing finned and pinned surfaces with a flat under the same conditions of power input and airflow.

Forced convection is a mechanism, or type of heat transport in which fluid motion is generated by an external source (like a pump, fan, and suction device,). It should be considered as one of the main methods of useful heat transfer as significant amounts of heat energy can be transported very efficiently and this mechanism is found very commonly in everyday life, including central heating, air conditioning, steam turbines and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchangers, pipe flow, and flow over a plate at a different temperature than the stream. However, in any forced convection situation, some amount of natural convection is always present whenever there are g-forces present (unless the system is in free fall). When the natural convection is not negligible, such flows are typically referred to as mixed convection.

EQUIPMENTS

The surfaces are shown in the figure below. The finned surface consists of 9 fins that are each 0.1 m high and 0.068 m wide. The pinned surface consists of 17 pins that each have a diameter of 0.013 m and are 0.068 m long.

1. Bench top unit with holder
2. Sensors for measuring temperature and flow velocity
3. Air duct
4. "cylinder" heating element
5. Temperature sensor
6. Measuring glands
7. Fan
8. "finned" heating element
9. "flat plate" heating element
10. Display and control unit
11. Handheld sensor to measure airflow velocity

REFERENCES

1. T.D. Eastop, A. McConkey, Applied Thermodynamics For Engineering Technologists 5^th Edition, Pearson Prentice Hall, 1993.
2. Yunus A.Cengel, Heat and Mass Transfer, Third Editions (SI Units) Mc Graw Hill.
3. Yunus A. Cengel, Michael A. Boles, Thermodynamics: An Engineering Approach 5^th Edition, Mc Graw Hill, 2006.
4. www.en.wikipedia.org

maksiat umpama dadah!

Apa yang anda faham tentang maksud maksiat? Kita sering mengata orang ini buat maksiat, orang itu menggalakkan maksiat, tetapi adakah kita tahu apa maksud maksiat itu sendiri? Mengikut wikipedia, maksiat itu bererti melakukan perkara yang dilarang, tercela dalam syariat Islam yang mana pelakunya akan mendapat dosa dengan apa yang dilakukan.

Manusia sering lupa, dosa maksiat boleh menyebabkan kita melakukan dosa yang lain. Seharusnya, kita hendaklah bertaubat dengan segera jika kita sedar akan keterlanjuran diri kita yang melakukan maksiat. Mohonlah keampunan daripada Allah SWT yang Maha Besar, yang sentiasa membuka pintu taubat untuk hamba-hambaNya.

Maksiat ini umpama dadah. Jika kita tidak merawat maksiat, ia akan membuka satu lagi lorong kemaksiatan.

Maksiat akan menghalang seseorang itu dari menerima hidayah dari Allah SWT. Pelaku maksiat akan merasa tidak aman dan tenteram di sepanjang kehidupannya. Hati pelaku maksiat sentiasa dalam keadaan gelisah, seiring dengan dosa-dosa yang dilakukan.

Jadi, marilah sama-sama kita hindari diri kita dari melakukan maksiat. Kuatkan iman kita, tingkat amal ibadat kita kepada Allah SWT. Semoga kita sentiasa berada dalam kalangan orang-orang yang beriman.

InsyaAllah..

p/s: kalau nak elak perkara maksiat, perlu jauhi perkara yang mengundang maksiat..

doa sewaktu sujud dan keistimewaannya

Sewaktu bersujud, kita berada amat hampir dengan Allah SWT. Katakanlah apa sahaja di dalam hati hajat kita sebagaimana yang dilakukan oleh Rasulullah SAW yang biasa memanjangkan sujudnya dengan memperbanyakkan zikir dan doa didalamnya. Sabda Rasulullah SAW;

"Suasana yang paling hampir antara seseorang hamba dengan Tuhannya ialah di kala ia bersujud kerana itu hendaklah kamu memperbanyakkan doa di dalamnya."

Banyak doa yang diamalkan oleh Rasulullah SAW untuk kita ikuti. Salah satunya yang paling baik untuk diamalkan ialah doa yang dibaca sewaktu sujud akhir dalam solat.

"Ya Allah, tambahkanlah bagiku rezeki yang banyak lagi halal, iman yang benar, ilmu yang bermanfaat, kesihatan yang elok, kecerdikan yang tinggi, hati yang bersih dan kejayaan yang besar."

Semoga dengan mengamalkan doa itu kita akan mendapat manfaat atau sekurang-kurangnya menjadi cita-cita kita dalam mencari keredhaan Allah di dunia mahupun di akhirat.

Banyak kelebihan memanjangkan sujud dan memperbanyakkan doa di dalamnya.

Rasulullah SAW biasa berbuat begitu sehingga pernah para sahabat hairan kerana lamanya baginda bersujud.

Keistimewaan umatnya yang bersujud telah disebut oleh baginda dalam sabdanya yang bermaksud;

"Tiadalah ada seorang umatku melainkan aku yang akan mengenalinya di hari kiamat"

Mendengarkan itu, para sahabat pun bertanya;

"Bagaimanakah engkau dapat mengenali mereka dalam khalayak ramai wahai Rasulullah?"

Jawab baginda;

"Tidakkah engkau melihat seandainya sekumpulan unta dimasuki oleh seekor kuda yang amat hitam, sedang di dalamnya pula terdapat sekor kuda putih bersih, maka adakah engkau tidak dapat mengenalinya?"

Sahabat Menjawab;

"Bahkan !"

Rasulullah SAW menyambung;

"Kerana sesungguhnya pada hari itu (kiamat) muka umatku akan putih (berseri-seri) disebabkan mereka bersujud (di dunia), segala anggota mereka (terutama anggota wuduk) putih berseri-seri oleh cahaya wuduk!!!"

KEISTIMEWAANNYA

Ahli neraka juga mendapat keselamatan kerana bekas sujudnya. Rasulullah SAW bersabda yang bermaksud;

"Apabila Allah hendak melimpahkan Rahmat (kebaikan) kepada ahli-ahli neraka yang Dia kehendaki, Dia pun memerintahkan malaikat supaya mengeluarkan orang-orang yang menyembah Allah, Lalu mereka dikeluarkan dan mereka dikenali dengan kesan-kesan sujud (di dahi mereka), di mana Allah SWT menegah neraka memakan (menghapuskan) bekas-bekas sujud itu, lalu mereka pun keluar dari neraka, maka setiap tubuh anak Adam akan dimakan api neraka selain bekas sujud"

 Begitulah Allah memuliakan hambaNya yang bersujud. Orang yang bersujud mendapat keistimewaannya apatah lagi di dalam sujud itu kita berdoa. Sudah tentu mendapat perhatian yang sewajarnya daripada Allah SWT. Semoga kita akan menjadi hamba yang benar-benar mendapat rahmat dariNya.

p/s: lamakan waktu bersujud boleh mengubati sakit migrain..

dialog antara manusia, malaikat dan syaitan

Manusia: Eh, dah subuh dah?
Malaikat: Bangunlah wahai anak Adam. Tunaikan solat subuh mu
Syaitan: Alahhhhhh, sekejaplah, mengantuk ni... Awal lagi ni.. zzZZz

Manusia: Nak makan, laparlah..
Malaikat: Wahai Anak Adam, mulakanlah dengan Bismillah....
Syaitan: Ahh, tak payahlah... dah lapar ni! Makan saja.. Sedapnya...

Manusia: Hari ni nak pakai apa ye?
Malaikat: Wahai anak Adam, pakailah pakaian yang menutup aurat.
Syaitan: Ehh, panaslah, tak ada style langsung, nampak kampung!

Manusia: Alamak, dah lewat!
Malaikat: Bersegeralah wahai manusia , nanti terlewat ke pejabat.
Syaitan: Ahhh? Awal lagi? Mmm.. Aaahh.. Suruh si X punchkanlah!

Manusia: Azan sudah kedengaran.. ..
Malaikat: Wahai anak Adam, bersegeralah menunaikan kewajipan
Syaitan: Baru pukul berapa.. Rilex lah.. Awal lagi ni.

Manusia: Eh, eh.... Tak boleh tengok ni, berdosa..
Malaikat: Alihkanlah pandanganmu, sesungguhnya Allah Maha Melihat dan Maha Mengetahui!
Syaitan: Perggghh... Best tu... Rugi ooo kalau tak tengok ni.

Manusia: Saudaraku sedang melakukan dosa
Malaikat: Wahai anak Adam, cegahla ia..
Syaitan: Apa kau sibuk? Jangan jaga tepi kain orang, lantak dia lah

Manusia: Elok kalau aku sampaikan nasihat ini kepada orang lain
Malaikat: Wahai anak Adam, nasihat-menasihati lah sesama kamu..
Syaitan: Sendiri pikirlah, semua dah besar, buat apa susah-susah... Pandai-pandai lah

Manusia: Aku telah berdosa..
Malaikat: Wahai anak Adam, bertaubatlah kamu, sesungguhnya Allah Maha Pengampun
Syaitan: Tangguhlah dulu, lain kali boleh bertaubat... Muda lagi kan.. Lagipun hidup hanya sekali.. Rugilah..

Manusia: Kalau pergi, mesti seronok!
Malaikat: Wahai anak Adam, kakimu ingin melangkah ke jalan yang dimurkai Allah, berpalinglah dari jalan itu.
Syaitan: Jangan bimbang, tiada sesiapa yang tahu... Jomlahhh!

Manusia: Uuhhh.. Letihnya harini.. Tak solat lagi ni
Malaikat: Wahai anak Adam, taatilah Allah dan RasulNya, kebahagiaan di dunia dan akhirat untukmu. Solat

               itu wajib bagimu.
Syaitan: Hey anak Adam, ikutilah aku, kebahagiaan di dunia (sahaja) dan kebahagiaan diakhirat untukmu nanti ada juga (jangan mimpilah!)

Sekarang anda mempunyai 3 pilihan:

1. Anda: Patut ke aku biarkan article ini tetap di sini saja??
2. Malaikat: Ingatkan pada kawan yang anda kenal.. Sebarkanlah!
3. Syaitan: Tak payahlah sibuk-sibuk.. Biar je di sini.. Bukan ada orang nak baca pon..

p/s: feel free to copy..:)

lelaki yang ditarik wanita masuk ke neraka

Di akhirat nanti ada 4 golongan lelaki yg akan ditarik masuk ke neraka oleh wanita. Lelaki itu adalah mereka yg tidak memberikan hak kpd wanita dan tidak menjaga amanah itu. Mereka ialah:

Ayahnya

Apabila seseorang yg bergelar ayah tidak mempedulikan anak2 perempuannya didunia. Dia tidak memberikan segala keperluan agama seperti mengajar solat,mengaji dan sebagainya Dia membiarkan anak2 perempuannya tidak menutup aurat. Tidak cukup kalau dgn hanya memberi kemewahan dunia sahaja. Maka dia akan ditarik ke neraka oleh anaknya.

Suaminya

Apabila sang suami tidak mempedulikan tindak tanduk isterinya. Bergaul! bebas di pejabat, memperhiaskan diri bukan utk suami tapi utk pandangan kaum lelaki yg bukan mahram. Apabila suami mendiam diri walaupun seorang yg alim dimana solatnya tidak pernah bertangguh, saumnya tidak tinggal, maka dia akan turut ditarik oleh isterinya bersama-sama ke dlm neraka.

Abang-abangnya

Apabila ayahnya sudah tiada,tanggungjawab menjaga maruah wanita jatuh ke bahu abang-abangnya dan saudara lelakinya. Jikalau mereka hanya mementingkan keluarganya sahaja dan adiknya dibiar melencong dari ajaran Islam,tunggulah tarikan adiknya di akhirat kelak.

Anak-anak lelakinya

Apabila seorang anak tidak menasihati seorang ibu perihal kelakuan yg haram disisi Islam. bila ibu membuat kemungkaran mengumpat, memfitnah, mengata dan sebagainya...maka anak itu akan disoal dan dipertanggungjawabkan di akhirat kelak....dan nantikan tarikan ibunya ke neraka.

15 perkara yang dilakukan akan mengundang bala

Daripada Ali bin Abi Thalib ra, Rasulullah SAW bersabda:

"Apabila umatku telah membuat 15 perkara, maka BALA pasti akan turun kepada mereka, iaitu:

1.  Harta negara hanya beredar dalam kalangan orang tertentu sahaja 
2. Amanah dijadikan suatu sumber keuntungan 
3. Zakat dijadikan hutang
4. Suami menurut kehendak isteri (dalam perkara bertentangan syariat)
5. Anak derhaka terhadap ibunya
6. Akan tetapi (anak tadi) sangat baik terhadap kawan-kawannya
7. Ia (anak) suka menjauhkan diri daripada ayahnya
8. Suara sudah ditinggikan di dalam masjid
9. Yang menjadi ketua sesuatu kaum adalah orang yang paling hina di antara mereka
10. Seseorang dimuliakan kerana ditakuti kejahatannya
11. Arak sudah diminum secara berleluasa
12. Kain sutera banyak dipakai (oleh kaum lelaki)
13. Para artis disanjung-sanjung
14. Muzik banyak dimainkan/didendang kan
15. Generasi akhir umat ini akan melaknat generasi pertama (para sahabat terdahulu). Maka pada ketika itu hendaklah mereka menanti angin merah atau GEMPA BUMI ataupun mereka akan dirubah menjadi makhluk lain 

(Hadis Riwayat At-Tirmidzi)

lab report: heat exchanger

OBJECTIVE

The main objective for this experiment is to demonstrate the effect of the flow rate variation on the performance characteristics of a counter-flow and parallel flow concentric tube heat exchanger.

Specific objectives for this experiment include:

• Learning how the operation of concentric tube heat exchanger. 
• Developing a set of experiments to obtain statistically significant trends for the overall heat transfer coefficient and the inside heat transfer coefficient as a function of water velocity.
• Observing the difference between parallel-flow and counter flow operation of the heat exchanger.

INTRODUCTION

Heat exchanger is a device built for efficient heat transfer from one medium to another. A solid wall may separate the media, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. 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).The main purpose of heat exchanger is to remove the heat from the hot fluid and transfer it into the cold fluid. There are 3 types of heat exchanger, parallel flow, counter flow, and cross flow. However, in this experiment, we only consider the counter-flow heat exchanger and parallel flow. Counter flow exists when the two fluids flow in opposite directions. Each of the fluids enters the heat exchanger at opposite ends. Because the cooler fluid exits the counter flow heat exchanger at the end where the hot fluid enters the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid. Parallel flow exists when two fluids flow in parallel directions. Each of the fluids enters the heat exchanger at parallel end.

The variables that affect the performance of a heat exchanger are the fluids’ physical properties, the fluids’ mass flow rates, the inlet temperature of the fluids, the physical properties of the heat exchanger materials, the configuration and area of the heat transfer surfaces, the extent of scale or deposits on the heat transfer surfaces, and the ambient conditions. The comparison between counter-flow and parallel flow also can be determined through this experiment which is explained more in discussion part in this report.

THEORETICAL BACKGROUND

One fluid (hot) convectively transfers heat to the tube wall where conduction takes place across the tube to the opposite wall. The heat is then convectively transferred to the second fluid. Because this process takes place over the entire length of the exchanger, the temperature of the fluids as they flow through the exchanger is not generally constant, but varies over the entire length. The rate of heat transfer varies along the length of the exchanger tubes because its value depends upon the temperature difference between the hot and the cold fluid at the point being viewed.

The way that a heat exchanger works is hot water and cold water entering the exchanger, where the process of cold water gaining some heat and the hot water losing some takes place, before they both exit the exchanger. What is actually happening is, the hot water is heating either the inside or the outside of the tubes in the exchanger, depending on where it is flowing, by what is known as convection.

Then the heat is conducted through the tubes to the other side, either the outside or the inside, where it is then converted back into the cold water raising its temperature. Convection is a mode of heat transfer that involves motion of some fluid that either absorbs heat from a source or gives heat to some surrounding. Conduction is a mode of heat transfer in which the heat is moving through a stationary object or fluid. For a heat exchanger that flows parallel or counter current then the coefficient of heat transfer is called the overall coefficient of heat transfer. It is calculated using the log mean temperature difference, which is found two different ways, depending on whether the flow is parallel or counter.

A heat exchanger is a device by which thermal energy is transferred from one fluid to another. The types of heat exchangers to be tested in this experiment are called single-pass, parallel-flow and counter-flow concentric tube heat exchangers. In a parallel-flow heat exchanger, the working fluids flow in the same direction. In the counter flow exchanger, the fluids flow in parallel but opposite directions.

HISTORY

The primary advantage of a concentric configuration, as opposed to a plate or shell and tube heat exchanger, is the simplicity of their design. As such, the insides of both surfaces are easy to clean and maintain, making it ideal for fluids that cause fouling. Additionally, their robust build means that they can withstand high pressure operations. They also produce turbulent conditions at low flow rates, increasing the heat transfer coefficient, and hence the rate of heat transfer. There are significant disadvantages however, the two most noticeable being their high cost in proportion to heat transfer area; and the impractical lengths required for high heat duties. They also suffer from comparatively high heat losses via their large, outer shells.

The simplest form is composed of straight sections of tubing encased within the outer shell, however alternatives such as corrugated or curved tubing conserve space while maximising heat transfer area per unit volume. They can be arranged in series or in parallel depending on the heating requirements. Typically constructed from stainless steel, spacers are inserted to retain concentricity, while the tubes are sealed with O-rings, packing, or welded depending on the operating pressures.

While both co and counter configurations are possible, the countercurrent method is more common. The preference is to pass the hot fluid through the inner tube to reduce heat losses, while the annulus is reserved for the high viscosity stream to limit the pressure drop. Beyond double stream heat exchangers, designs involving triple (or more) streams are common; alternating between hot and cool streams, thus heating/cooling the product from both sides.

EXPERIMENTAL PROCEDURE

1. This experiment was started by setting the machine for parallel-flow.
2. The hot water inlet temperature was setted 60oC with decade switch.
3. The cold water volumetric flow rate ( ... ) was set to run at a constant 2,000 cm3/min.
4. Initially, the hot water volumetric flow rate ( ... ) was set to 1,000 cm3/min.
5. This step was repeated for the volumetric flow rate of 2,000, 3,000, and 4,000 cm3/min.
6. Six temperature reading was recorded in the result table.
7. Sufficient time (approximately to be 1 to 4 minutes) is allowed, in order to achieve steady conditions.
8. Step 2 to 7 is repeated for counter-flow heat exchanger operation. 

REFERENCE

1. Heat and Mass Transfer (A Practical Approach) – 3rd Edition, Yunus A. Cengel, McGraw Hill (2006) 
2. http://en.wikipedia.org/wiki/Concentric_tube_heat_exchanger
3. http://www.concentrictubeheatexchanger.com/

Kebakaran di Kolej Jati Uitm Shah Alam

Mungkin ada yang penah dengar cerita tentang kebakaran di kolej jati. Agak mengejutkan juga api yang marak nampak besar gak laa. Alhamdulilah, kebakaran tersebut tak melibatkan kemalangan nyawa manusia. Kat sini aku ade attachkan video tentang kebakaran tersebut.

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