pemampat

PENGENALAN
Pemampat merupakan sebuah alat mekanikal yang digunakan untuk menghasilkan udara yang bertekanan tinggi mengikut kepeluan yang dikehendaki. Pemampat merupakan sebuah alat atau mesin yang penting di dalam sebuah kilang, industri, prasarana, woksyop, mahupun di dalam kenderaan dan peralatan rumah yang kita gunakan seharian. Pemampat boleh ditemui dalam pelbagai jenis dan saiz. Sesebuah pemampat mempunyai beberapa perbezaan dari segi binaan dan juga kecekapan pemampat tersebut. Di dalam tugasan ini, jenis-jenis pemampat akan diterangkan secara lebih terperinci.

OBJEKTIF
Melalui tugasan, pelajar akan dapat :-
Mengenali jenis-jenis pemampat
Mempelajari binaan
Menghuraikan prinsip kerja pemampat mengikut jenisnya
Membandingkan kebaikan dan keburukan untuk setiap jenis pemampat
Membuat rujukan untuk pemampat

PEMAMPAT (COMPRESSOR)
Pemampat adalah sejenis alat mekanikal yang berfungsi untuk meningkatkan tekanan udara dengan menurunkan isipadu udara tersebut bagi setiap kuantiti. Secara umumnya, pemampat dan pam adalah sama, iaitu meningkatkan tekanan bendalir untuk menghasilkan suatu gerakan. Namun, secara khususnya, pemampat hanya boleh memampatkan udara sahaja manakala pam hanya boleh digunakan untuk cecair sahaja.

Jenis-Jenis Pemampat
Pemampat terdiri dari beberapa jenis utama. Carta di bawah menunjukkan jenis-jenis pemampat.
Carta 1 : Jenis-jenis pemampat
Seperti di dalam carta di atas, pemampat boleh dikelaskan kepada dua jenis iaitu, pemampat anjakan positif (Positive Displacement Compressor) dan juga pemampat dinamik(Dynamic Compressor). Di bawah jenis anjakan positif, ianya terbahagi lagi kepada dua iaitu jenis salingan(reciprocating) dan ‘rotary’(putaran). Di bawah jenis pemampat dinamik, biasanya ianya terbahagi kepada dua lagi iaitu aliran jejari(centrifugal) dan aliran paksi(axial).

Pemampat Anjakan Positif
Pemampat Salingan(Reciprocating)
Pemampat ‘reciprocating’ atau juga dikenali sebagai pemampat piston kerana ia menggunakan piston untuk melakukan kerja pemampatan. Pergerakan dalam sistem pemampat ini dipacu oleh ‘crankshaft’ yang menggunakan motor elektrik, enjin stim ataupun enjin pembakaran dalam.
Pemampat ini menggunakan injap spring bebanan automatik (automatic spring loaded valve) dalam setiap silinder untuk mengawal buka-tutup injap masukan (inlet valve) dan injap lepasan (discharge valve).

1.1 Binaan
Crankshaft – Berfungsi untuk menggerakkan piston
Connecting rod – Menghubungkan crankshaft dengan crosshead
Crosshead – Berfunsi dalam menghubungkan crankshaft untuk menukarkan
pergerakan berputar kepada pergerakan salingan. Ianya terletak
di antara connecting rod dan piston.
Distance piece – Terlerak di antara penggerak utama (prime mover) dan
Pemampat.
Piston rod – Menghubungkan piston kepada crosshead
Piston – Berfungsi untuk memampatkan gas di dalam silinder
Piston ring – seal antara piston dan dinding pemampat
Injap masukan – membenarkan udara masuk ke dalam silinder
Injap hantaran – Melepaskan gas dari silinder selepas dimampatkan
Head – Dipasang pada hujung pemampat, sebagai penutup.
Clearance pocket – Membolehkan operator mengubah kapasiti pemampat
1.2 Prinsip Operasi

Rajah 1.1 - Binaan asas pemampat piston
Graf 1 - tekanan melawan isipadu
Pada bahagian 1, tekanan di dalam silinder adalah lebih redah daripada tekanan atmosfera. Dengan itu, injap masukan akan terbuka dan membenarkan udara mengalir memasuki ke dalam silinder dan injap hantaran akan terbuka
Pada bahagian 2, piston akan mula bergerak ke atas dan memulakan proses pemampatan. Pergerakan ini dinamakan ‘up stroke’. Pada masa yang sama, injap masukan akan tertutup. Udara akan yang terperangkap di dalam silinder akan termampat.
Pada bahagian 3, apabila tekanan udara yang dimampatkan oleh piston telah mencapai tahap yang telah disetkan, injap hantaran akan terbuka. Piston akan bergerak hingga ‘top dead center’ (TDC).
Pada bahagian 4, piston akan mula turun semula ke bawah. Injap masukan akan terbuka dan proses sedutan udara ke dalam silinder akan berulang dan injap hantaran akan terbuka.

1.3 Pemampat Salingan Satu Peringkat (Single Acting)
Pemampat salingan satu peringkat adalah pemampat yang menggunakan satu piston ataupun satu bahagian pada satu piston untuk melakukan satu kitar kerja pemampatan yang lengkap. Ini bermakna, udara akan masuk ke dalam silinder apabila injap masukan terbuka. Selepas itu, piston akan bergerak ke atas untuk malakukan kerja pemampatan sabil injap masukan akan tutup. Apabila mencapai satu tahap tekanan yang telah disetkan, injap hantaran akan terbuka dan piston terus bergerak ke atas. Selepas itu, injap hantaran akan tertutup dan piston akan bergerak semula ke bawah. Kitar akan bermula semula.
Rajah 1.2 – Pemampat salingan satu peringkat

1.4 Pemampat Salingan Dua Peringkat (Double Acting)
Pemampat salingan dua peringkat adalah pemampat yang menggunakan dua piston ataupun satu piston dengan menggunakan kedua-dua bahagian piston tersebut untuk melakukan kerja pemampatan udara. Bagi satu piston, semasa ia bergerak ke hadapan (forward strokes), kerja pemampatan akan dilakukan. Dan apabila patah semula (backward stroke), ia juga akan melakukan kerja pemampatan.
Bagi yang menggunakan dua piston, udara akan disedut masuk ke dalam silinder. Piston pertama akan memampatkan udara tersebut. Apabila udara tersebut telah dimampatkan pada piston pertama, suhunya akan meningkat. Dengan itu, ianya disejukkan dengan melalukan pada bahagian penyejuk. Selepas itu, ianya akan disalurkan kepada [iston kedua untuk melakukan kerja pemampatan sekali bagi meningkatkan lagi tekanan udara tersebut sebelum dikeluarkan.

Rajah 1.3 - Pemampat salingan dua peringkat (satu piston)

Rajah 1.4 - Pemampat salingan dua peringkat(dua piston)

Rajah 1.5 - Pemampat salingan yang menggunakan tiga piston

1.5 Pemampat Salingan Gegendang (diaphragm) / Pemampat Salingan ‘Membrane’
Pemampat jenis menggunakan hampir sama dengan pemampat salingan yang lain, cuma ia tidak menggunakan piston, tetapi ia menggunakan gegendang. Kerja pemampatan dilakukan oleh ‘flexible membrane’. ‘Flexible membrane’ ini digerakkan dengan mekanisma ‘crankshaft’ dan rod yang dijana menggunakan motor elektrik.

Pemampat gegendang biasanya digunakan untuk memampatkan gas hydrogen dan gas asli.
Rajah 1.6 - Pemampat gegendang ‘three stages’ untuk memampatkan sehingga 41 Mpa
1.6

Kebaikan Pemampat Salingan
Rekabentuk yang mudah
Kos dalaman rendah
Mudah dipasang
Model ‘two stage’ mempu menjana kecekapan yang tinggi
Tidak memerlukan minyak yang berterusan
Menghasilkan kuasa kuda (hp) yang besar
1.

7 Keburukan Pemampat Salingan
Kos penyelenggaraan yang tinggi
Banyak alat ganti
Mempunyai masalah getaran (vibration)
Tidak beroperasi sepenuhnya pada satu masa.
1.8 Kegunaan
Biasanya digunakan unutk memampatkan gas hydrogen dan gas asli. Ianya boleh didapati dalam pelbagai peringkat kuasa kuda yang dijanakan dari 5 hp sehingga 30 hp. Terdapat juga model yang mempunyai kuasa kuda terbesar iaitu 1000 hp.
2. Pemampat Putaran (Rotary)
Pemampat jenis putaran beroperasi seperti pam jenis putaran. Sebuah rotor berbentuk silinder terletak di dalam penutup silinder ( cylindrical casing). Beberapa buah piring bergerak berjejari dipasang pada rotor yang berfungsi untuk menetapkan penghubung dengan dindingnya. Daya yang selari dijana daripada putaran rotor yang akan menjadi lebih besar dan kecil.
2.1 Pemampat Putaran Jenis Skru (Screw Type)
Pemampat jenis ini terdiri daripada dua unit rotor, iaitu yang berbentuk heliks (skru) yang akan berpusing dan mengecilkan isipadu udara yang disalurkan melaluinya. Pemampat ini menggunakan minyak semasa udara melalui rotor. Ianya bertujuan sebagai medium penyejuk, pelinciran, dan juga penghadang kebocoran (seal).
Rajah 1.7 – Pemampat Jenis skru
Binaan

Rajah 1.8 – Binaan dan bahagian-bahagian dalam pemampat

2.1.2 Prinsip Operasi
Apabila bekalan kuasa dihidupkan, rotor pertama dan rotor kedua akan berputar. Dengan itu, ia mewujudkan satu kawasan yang kandungan tekanan udaranya rendah dari tekanan atmosfera. Dengan itu, udara akan mesuk ke dalam.
Semasa melalui rotor, udara akan dosemburkan dengan sejenis minyak yang bertujuan untuk menurunkan suhu yang terhasil dari mampatan, sebagai penghadang kebocoran (seal) dan juga sebagai medium pelinciran.
Rajah 1.9 – Aliran udara semasa operasi
Apabila mencapai satu tahap tekanan yang dihasilkan, udara seterusnya akan memasuki sistem pemisah minyak (oil separator system). Ianya akan memisahkan minyak dari udara termampat. Biasanya pemampat putaran ini dipanggil jenis lembab (wet type). Ianya tidak mempunyai ‘timing gear’ seperti jenis kering (dry type).
Rajah 1.10 – Mekanisme dalam pemampat
2.1.3 Kebaikan Pemampat
Binaan yang mudah
Kos penyelenggaraan yang rendah
Mudah dipasang
Pemampat ‘two-stage’
Banyak digunakan di kawasan Loji
2.1.4 Keburukan Pemampat
Mempunyai jangka hayat yang rendah
Kelajuan putaran sangat laju
Oil injected design have oil carryover
Menyebabkan kawasan persekitaran yang kotor
2.2 Pemampat Putaran Jenis ‘Sliding-Vane’
Pemampat putaran jenis ini terbahagi lagi kepada dua, iaitu jenis lembab (oil flooded) dan kenis kering(oil lubricated). Ianya terdiri daripada sebuah silinder, sebuah slot rotor, dan beberapa bilah ‘vanes’ yang dipasang pada slot rotor. Bilah ‘vane’ bebas untuk menyisip ke dalam dan keluar (slide in and out) pada slot kerana terdapat suatu jarak di antara rotor dengan dinding silinder.
Saiz pemampat ‘sliding vane’ adalah lebih kecil daripada saiz pemampar salingan (piston) yang mempunyai perbadingan dari aspek hantaran udara dan keupayaan aliran. Walau bagaimanapun, kecekapan operasi pemampat ini adalah lebih baik berbanding pemampat dinamik.
2.2.1 Binaan
Rajah 1.11 – Binaan dan Bahagian pada pemampat
Prinsip Operasi
Bagi jenis lembab (oil flooded), apabila udara dimasukkan ke dalam silinder, minyak akan disemburkan. Jenis ini menggunakan kuantiti minyak yang lebih banyak kerana minyak tersebut untuk menjadi pelincir bagi pemampat, menyejukkan udara yang dimampatkan, dan menjadi penghadang kebocoran (seal) untuk poket udara (air pockets) yang terletak di antara bilah ‘vanes’ dan dinding silinder.
Bagi jenis kering (oil lubricated), kuantiti minyak yang digunakan adalah lebih sedikit. Ini adalah kerana minyak tidak digunakan untuk menyejukkan udara yang termampat. Udara termampat akan disejukkan dengan ‘water jackets’ dan ‘aftercoolers’
Pemampat putaran ‘sliding vane’ jenis kering menggunakan karbon atau bilah Telfon untuk menakung minyak pelincir. Ianya berfungsi untuk melindungi silinder dari mengalami kehausan akibat pergeselan antara besi dengan besi.
Kedua-dua jenis bagi pemampat ‘sliding vane’ ini memerlukan sistem pengasing minyak (oil separator system) untuk mengasingkan minyak dari udara. Bagi jenis lembab (oil flooded), pengasing minyak mestilah mempunyai kapasiti yang tinggi termasuk sebuah penukar haba untuk menyejukkan minyak sebelum dialirkan semula ke dalam pemampat.
Kebaikan Pemampat
Binaan yang mudah
Mudah untuk dipasang
Kos pertengahan rendah
Kos penyelenggaraan yang rendah
Jangka hayat pemampat yang lama
Bebas dari pencemaran alam persekitaran
Keburukan Pemampat
Binaan suntikan minyak mempunyai masalah minyak berlebihan
Hanya terdapat dalam single-stage
Kecekepan yang rendah
Pemampat Putaran Jenis Cincin Cecair(Liquid-Ring)
Pemampat ini juga dikenali dengan pemampat piston cecair dan berbentuk hampie dengan pemampat ‘sliding vane’. Ianya beroperasi hampir sama dengan jenis ‘vane’, ianya boleh menyedut udara ke dalam silinder tanpa merosakkan bilah pemampat. Kecekapan pemampat ini adalah yang paling rendah.
Binaan
Penutup (Casing)
Batang Pemacu (off-center drive shaft)
Rotor dengan bilah tetap
Cecair (biasanya air)
Rajah 1.12 – Binaan dan bahagian dalam pemampat
2.3.2 Prinsip Operasi
Bilah rotor akan membawa cecair di sekeliling penutup. Semasa rotor berputar, cecair tersebut akan bertindak sebagai daya yang selari yang mengalir di sekeliling penutup.
Disebabkan rotor dan penutup selari, cecair akan bertindak sebagai ruang mampatan bahagian dalam.
Apabila rotor melepasi bahagian dalam, udara akan mengalir masuk ke dalam pemampat dan akan menambahkan saiz cecair pada ruang mampatan tadi.
Saiz cecair mampatan itu akan mengecil apabila menghapiri ruang keluar pada pemampat dan udara akan dikeluarkan apabila ruang semakin mengecil.
Air akan masuk penutup untuk menyejukkan ruang penutup dan sekitarnya.
Di dalam udara yang termampat, terdapat air yang terkandung di dalam udara. Dengan itu, pengasing kelembaban digunakan untuk menyingkirkan air di dalam udara hantaran.
2.3.3 Kebaikan Pemampat
Udara termampat yang dihasilkan bebas dati minyak dan kotoran (debu)
Baik untuk peralatan yang menggunakan udara yang bersih
2.3.4 Keburukan Pemampat
Binaan yang rumit
Kos mahal
Memerlukan penjagaan yang rapi
2.3.5 Kegunaan
Hasil udara mampatan yang dihantar melalui pemampat ini lebih bersih kerana tidak melibatkan penggunaan minyak dan juga tidak terdedah kepada habuk. Oleh itu, udara mampat yang dihasilkan sesuai untuk peralatan yang mementingkan kebersihan.
Pemampat Dinamik (Dynamic Compressor)
Pemampat meningkatkan tekanan sesuatu kuantiti udara dengan menambahkan halaju, tenaga keupayaan pergerakan pada udara tersebut. Kemudian, tenaga tersebut ditukarkan kepada tenaga apabila kadar alir udara menjadi rendah yang dikawal oleh suatu alat.
1 Pemampat Aliran Jejari (Centrifugal Compressor)
Rajah 1.13- Pemampat aliran jejari
1.1 Binaan
Impeller (rotor)
Diffuser (Stator)
Casing (Penutup)
Diaphragm (Gegendang)
Balancing Piston
Seal
Rajah 1.14 - Stator dan rotor pemampat
Rajah 1.15 – Rajah diffuser
Rajah 1.16 – Binaan pemampat aliran jejari
1.2 Prinsip Operasi
Rajah 1.17 – Aliran udara semasa operasi
Udara akan mengalir masuk ke dalam penutup di bahagian masukan (inlet).
Udara akan memasuki ruang impeller yang sedang berpusing dengan kelajuan yang tinggi akan meningkatkan halaju udara yang melaluinya.
Rajah 1.18 Mekanisme pergerakan udara dalam fan
Udara tersebut akan melalui diffuser (stator) yang akan menukarkan halaju udara yang keluar dari impeller menjadi udara bertekanan tinggi dengan merendahkan halaju udara tersebut.
1.3 Kebaikan Pemampat Aliran Jejari
Menghasilkan udara bertekanan tinggi
Efficiency over wide rotational speed range
Binaan yang mudah
Kurang berat
Tidak memerlukan kuasa yang tinggi untuk permulaan
1.4 Keburukan Pemampat Aliran Jejari
Memerlukan kawasan angin masuk yang besar
Kos tinggi
Sistem kawalan yang sukar
Memerlukan penjagaan yang rapi
2 Pemampat Aliran Paksi (Axial Flow)
Pemampat jenis ini menggunakan bilah-bilah seperti kipas angin yang bertindak sebagai rotor. Di hilir pada setiap bilah tersebut terdapat stator yang akan membantu manghalakan aliran udara kepada rotor yang berikutnya.
Rajah 1.19 Pemampat aliran paksi
2.1 Binaan
Rajah 1.20 – Binaan pemampat
2.2 Prinsip Operasi
Apabila pemampat sedang beroperasi, bilah-bilah rotor akan menolak udara di dalam pemampat keluar melalui bahagian hantaran. Disebabkan tekanan atmosfera di luar pemampat lebih tinggi, udara akan engalir masuk ke dalam pemampat melalui ruang masukan.
Udara yang memasuki pemampat akan ditingkatkan kelajuan alirannya apabila melalui rotor yang sedang berputar dengan kelajuan yang tinggi. Apabila halaju udara yang malalui bilah rotir dalam keadaan laju, tekanan ram yang mengalirkan udara turut meningkat.
Rajah 1.21 – Aliran udara dalam pemampat
Berbanding dengan pemampat aliran jejari, udara yang bekelajuan tinggi tadi akan ditukarkan menjadi udara yang bertekanan tinggi dengan merendahkan kelajuannya apabial melalui diffuser. Bagi pemampat jenis ini, udara tadi akan melalui stator tetap yang berada di setiap hilir bilah rotor. Dengan itu, halaju aliran udara tadi akan menjadi rendah dan seterusnya akan meningkatkan tekanan udara tersebut.
2.3 Kebaikan Pemampat Aliran Paksi (Axial Flow)
Kecekapan puncak tinggi
Aliran terus malalui kecekapan ram yang tinggi
‘Small frontal area forgiven airflow’
2.4 Keburukan Pemampat Aliran Paksi
Binaan yang sukar
Kos tinggi
Keupayaan bergatung kepada berat (relatively high weight)
Memerlukan kuasa yang tinggi semasa hendak memulakan operasi.
KESIMPULAN
Daripada tugasan yang telah dibuat, pelajar dapat mengenali dan mengklasifikasikan jenis-jenis pemampat yang terdapat pada hari yang biasanya digunakan di dalam industri. Selain itu, pelajar dapat mempelajari prinsip operasi untuk setiap jenis pemampat.

RUJUKAN
http://en.wikipedia.org/wiki/Diaphragm_compressor
Petronas Gas – Training Module (Mechanical) : Basic Compressor
www.globalsecurity.org/military/library/policy/army/fm/1-506/ch3.htm
www.sundyne.com/ind/details/0,10494,CLI1_DIV92_ETI7609,00.html
www.tpub.com/content/doe/h1018v2/css/h1018v2_88.htm
www.tpub.com/content/fc/14104/css/14104_91.htm
http://www.daveycompressor.com/

LAMPIRAN RUJUKAN
Gas compressor
From Wikipedia, the free encyclopedia
Jump to: navigation, search
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Compression of a gas naturally increases its temperature.
Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, so the main action of a pump is to transport liquids.
Contents
[hide]
1 Types of compressors
1.1 Centrifugal compressors
1.2 Diagonal or mixed-flow compressors
1.3 Axial-flow compressors
1.4 Reciprocating compressors
1.5 Rotary screw compressors
1.6 Scroll compressors
1.7 Diaphragm compressors
2 Temperature
3 Staged compression
4 Prime movers
5 Applications
6 See also
7 References
[edit] Types of compressors
As shown above, there are many different types of gas compressors. The two primary categories are:
Positive displacement compressors with two sub-categories:
Reciprocating
Rotary
Dynamic compressors also with two sub-categories:
Centrifugal
Axial
The more important types in each of the four sub-categories are discussed below.
[edit] Centrifugal compressors
Main article: Centrifugal compressor
Figure 1: A single stage centrifugal compressor
Centrifugal compressors use a vaned rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa).
Many large snow-making operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines.
[edit] Diagonal or mixed-flow compressors
Main article: Diagonal or mixed-flow compressor
Diagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent centrifugal compressor.
[edit] Axial-flow compressors
Main article: Axial-flow compressor
An animation of an axial compressor.
Axial-flow compressors use a series of fan-like rotating rotor blades to progressively compress the gasflow. Stationary stator vanes, located downstream of each rotor, redirect the flow onto the next set of rotor blades. The area of the gas passage diminishes through the compressor to maintain a roughly constant axial Mach number. Axial-flow compressors are normally used in high flow applications, such as medium to large gas turbine engines. They are almost always multi-staged. Beyond about 4:1 design pressure ratio, variable geometry is often used to improve operation.
[edit] Reciprocating compressors
A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders.
Main article: Reciprocating compressor
Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors up to 1000 hp are still commonly found in large industrial applications, but their numbers are declining as they are replaced by various other types of compressors. Discharge pressures can range from low pressure to very high pressure (>5000 psi or 35 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units.[1]
[edit] Rotary screw compressors
Diagram of a rotary screw compressor
Main article: Rotary screw compressor
Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 3 hp (2.24 kW) to over 500 hp (375 kW) and from low pressure to very high pressure (>1200 psi or 8.3 MPa). They are commonly seen with roadside repair crews powering air-tools. This type is also used for many automobile engine superchargers because it is easily matched to the induction capacity of a piston engine.
[edit] Scroll compressors
Main article: Scroll compressor
Mechanism of a scroll pump
A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involute, archimedean spiral, or hybrid curves. They operate more smoothly, quietly, and reliably than other types of compressors.
Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid between the scrolls.
[edit] Diaphragm compressors
Main article: Diaphragm compressor
A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in touch with the gas being compressed.
Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as well as in a number of other applications.
Reciprocating compressor
From Wikipedia, the free encyclopedia
Jump to: navigation, search
A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders.
A reciprocating compressor is a compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.[1] [2]
The intake gas enters the suction manifold, then flows into the compression cylinder where it gets compressed by a piston driven in a reciprocating motion via a crankshaft, and is then discharged. We can categorize reciprocating compressors into many types and for many applications. Primarily, it is used in a great many industries, including oil refineries, gas pipelines, chemical plants, natural gas processing plants and refrigeration plants. One specialty application is the blowing of plastic bottles made of Polyethylene Terephthalate (PET).
Diaphragm compressor
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A diaphragm compressor is a variant of the classic reciprocating compressor with backup and piston rings and rod seal. The compression of gas occurs by means of a flexible membrane, instead of an intake element. The back and forth moving membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in touch with pumped gas. For this reason this construction is the best suited for pumping toxic and explosive gases. The membrane has to be reliable enough to take the strain of pumped gas. It must also have adequate chemical properties and sufficient temperature resistance.
A diaphragm compressor is the same as a membrane compressor.
Reciprocating or Piston compressors are the most common machines available on the market. They are positive displacement compressors and can be found in ranges from fractional to very high horsepowers. Positive displacement air compressors work by filling an air chamber with air and then reducing the chamber’s volume (Reciprocating, Rotary Screw and Rotary Sliding Vane are all positive displacement compressors). Reciprocating compressors work in a very similar manner as does as internal combustion engine but basically in a reverse process. They have cylinders, pistons, crankshafts, valves and housing blocks.
Rotary Screw Compressors work on the principle of air filling the void between two helical mated screws and their housing. As the two helical screws are turned, the volume is reduced resulting in an increase of air pressure. Most rotary screw compressors inject oil into the bearing and compression area. The reasons are for cooling, lubrication and creating a seal between screws and the housing wall to reduce internal leakage. After the compression cycle, the oil and air must be separated before the air can be used by the air system.
Rotary Sliding Vane Compressors like Reciprocating and Rotary Screw compressors are positive displacement compressors. The compressor pump consists primarily of a rotor, stator, and 8 blades. The slotted rotor is eccentrically arranged within the stator providing a crescent shaped swept area between the intake and exhaust ports. As the rotor turns a single revolution, compression is achieved as the volume goes from a maximum at the intake ports to a minimum at the exhaust port. The vanes are forced outward from within the rotor slots and held against the stator wall by rotational acceleration. Oil is injected into the air intake and along the stator walls to cool the air, lubricate the bearings and vanes, and provide a seal between the vanes and the stator wall. After the compression cycle, the oil and air must be separated before the air can be transferred to the air system.
Centrifugal Compressors are not positive displacement compressors like the Reciprocating, Screw or Vane Compressors. They use very high speed spinning impellers (up to 60,000 rpm) to accelerate the air then diffuser to decelerate the air. This process, called dynamic compression, uses velocity to cause an increase in pressure. In most Centrifugal compressors, there are several of these impeller/diffuser combinations. Typically, these machines have intercoolers between each stage to cool the air as well as remove 100% of the condensate to avoid impeller damage due to erosion.
RECIPROCATING COMPRESSORS
Principles of Operation
Piston type compressor is similar to internal combustion engine, has one or more cylinder. Each cylinder has a piston plus rings, connecting rod connect to a crankshaft. The cylinder head are fitted with inlet (suction) and exhaust (discharge) valves.
Figure 1: Reciprocating Compressor Strokes
Air compressor is normally driven by external power through the crankshaft, either by Electric motor, steam engine or internal combustion engine. The Prime movers can be connected to the compressor directly by V-belts or couplings. Piston type compressor’s are the most economically used in oil & gas industries for the pressure required good used for speed below 1000 rpm.
Each operating cycle of a piston compressor is made up of 2 strokes; 1 suction & 1 discharge. The movement of the piston toward its fully extended position is called the “up stroke.” The piston’s movement in the opposite direction is called the ‘down stroke.” When operating, the piston moves back and forth very rapidly, alternating upstrokes and down strokes.
Figure 2: P-v Diagram
Refer P-v diagram of piston strokes. During the later half of the suction stroke, see stage (1), shows the inlet valve opens and allow air to flow into the cylinder while piston is moving down (suction or induction stroke). Then the piston moves up and at the beginning of the compression stroke, see stage (2), the inlet valve closes and trapped gas or air is compressed.
When gas was compressed to some preset value, the discharge valve open, see stage (3) and the piston continues to top dead center till fully discharge all gas or air. Now piston begins it downward movement to repeat the suction motion again. See stage (4) and continues doing similar stroke again and again. During this sequence both pressure and /or volume vary. For example in stage (1), the pressure is constant but volume increases and in stage (2) pressure increases while volume decreases. In fact a formal relationship between pressure, volume & temperature.
Formula shows that: , where;
P = Pressure (Psi, absolute)
V = Volume
T = Temperature (K or R, absolute)
Sub number 1 = initial conditions
Sub number 2 = final conditions
If the initial condition describe the state of the gas/air at inlet, and final conditions describe the state of the gas/air at outlet:-
P2 > P1 = the pressure is increased
V2 < v1 =" the"> T1 = the temperature increased
If the compressor is a double acting type exactly the same sequence of events takes place as for a single acting compressor. The sequence takes place on both sides of the piston. This means that when the piston is on its backward stroke gas is being drawn into the cylinder on one side of the piston. At the same time, gas is being compressed and discharged on the other side of the piston. On the forward stroke of the piston the same thing happens, but the other way around. The gas previously drawn into the cylinder is now being compressed. At the same time gas is being drawn into the cylinder on the other side of the piston.
The piston rod passes through one end of the compressor cylinder. This end of the cylinder is called the drive end. Anything which takes place on this side of the piston is said to take place on the drive side. The other end of the cylinder is called the head end. Anything which takes place on this side of the piston is said to take place on the head side. Because gas is compressed on both sides of the piston, the strokes of the piston are only called the forward stroke and the backward stroke.
ROTARY COMPRESSORS
Rotary compressors are positive-displacement machines. Works similar like rotary pumps. A cylindrical rotor is located eccentrically in a cylindrical casing. Slotted into this rotor are a number of radially moving plates which are forced to maintain contact with the casing wall. The centrifugal forces created by the rotation of the rotor will become alternatively larger and small thus providing a pumping action. When air/gas is drawn into compartments, the compression effect occurs and the pushes air/gas out to the discharge port.
Straight-lobe type
Screw type
Sliding-vane type
Liquid ring piston type
Advantages are:
No valves
Lighter in weight,
Does not exhibit any shaking force and will not vibrate much with presence of liquid.
Features that Rotary compressors have:
Impart energy to the gas being compressed by way of an input shaft moving a single or multiple rotating element
Perform compression in an intermittent mode
Do not use inlet and outlet valves
Most important features is lack of valves used which gave a lighter in weight, does not exhibit the shaking forces (no vibration). So simplicity in construction on the physical design. Each model have different rotor design, size and operating range due to its unique structure.
Principles of Operation
Screw type compressor
Screw compressor is also referred as “SRM compressor”. The capacity range is approx. 500 to 35,000 cfm (1235 m3 per min) at about 45 psi discharge pressure. Supercharge or multi-stage application pressure of 250 psi is attainable.
Helical compressors a.k.a. screw compressor, most common in the rotary group. The larger screw compressor can now range to 40,000 cfm (1412 m3/min) and almost close to the centrifugal compressor’s range. Compression is achieved by the intermeshing of the male and female rotor.
Power is applied by the male rotor and as it mesh with the female rotor, a void (empty space) is created and gas is taken in at the inlet port. While the air is passing through the rotors, it is being sprayed with oil to remove the heat of compression, seal and, of course, lubricate.
As the intermesh move on, it takes along the gas from inlet to the outlet port. The volume of the gas is progressively reduced, increasing the pressure. Once compressed to pressure, the air enters the reservoir where most of the oil is removed through an oil separator system. This type of rotary air compressor in this orientation is called a "wet" type. The wet type screw compressor do not have timing gear as in the dry type.
In order to control pressure with a rotary type compressor, the inlet valve is modulated to control at a given set point. If the system pressure is low, the valve will open and allow air into the compressor. If the system pressure is high, the valve will close down and admit less air into the compressor.
Rotary screw compressors that do not have oil sprayed onto the screws (rotors) are called 'dry" types. Since oil is not used to seal the rotors where compression takes place, there is some 1 air loss or leakage. There is no oil to cool this 'dry" type compressor, so water jackets and aftercoolers are often used to reduce the temperature of the compressor and the discharge air. The 'dry" type screw compressor will also use conventional moisture separators to trap the resulting condensation.
Sliding-Vane compressor
Sliding vane compressors are also positive displacement compressors. Two types are oil flooded (wet) and oil lubricated (dry). A sliding vane compressor has a cylinder, a slotted rotor, and several vanes that fit into the rotor slots. The vanes are free to slide in and out of the slots as the distance between the rotor and cylinder wall decreases and increases. Its widely used as a vacuum pump as well as compressor with capacity range from 50 to 6000 cfm (212 lit./min) and pressure range from 50 psi to 400 psi.(booster service)
Vane compressors are smaller than piston compressors having comparable discharge and flow capabilities. However, their operating efficiency is also slightly lower. At 100 psi they deliver slightly less than 4 cfm per hp, compared to approximately 5 cfm per hp for piston compressors. Their delivery in cfm per hp is higher than that of dynamic compressors. Most vane compressors operate at motor speeds of 1200 to 1800 rpm and require pressurized lubrication for the bearings and other rotating parts.
The liquid-ring compressor
The liquid ring compressor, sometimes called liquid piston compressor, is a special type of rotary compressor, although it looks like a vane compressor. The main components include a casing, an off-center drive shaft, a rotor with fixed blades, and a fluid, usually water.
Performs similar to vane type but this design besides doing vacuum services; it can also suck liquid without damaging the compressor blade. This is the least efficient of all the rotary types. Available capacities range from 10,000 cfm at 15 psi to 300 cfm at 100 psi.
During operation, the rotor blades carry the liquid around inside of the casing. As the rotor turns, the liquid, reacting to centrifugal force, flows the contours of the casing. Because the rotor and casing are not centered, the liquid forms a flexible interior compression chamber. As the rotor blades pass the inlet port, air is drawn into the compressor by the increasing size of the fluid-formed chamber. The chamber size begins to decrease near the compressor outlet, and the air is discharged where the chamber is smallest.
Since incoming water continuously cools the casing of a liquid ring compressor, no additional cooling is needed. Typical discharge air temperatures are kept close to that of the incoming liquid. Liquid flow is regulated by a valve. For most uses, moisture separators are used to remove liquid from the discharged air.
Liquid ring compressors have capacities of up to 5000 cfm at 75 psi in single-stage models. The same models can produce up to 10,000 cfm, but at lower pressure ranges, typically about 14 psi. Higher pressures, above 75 psi, can also be obtained, but at a reduction in capacity. Because they deliver oil-free and dust-free air, these compressors have been found to be very efficient for instrument air. The cooling effect of the liquid on the air being compressed improves efficiency. Any contaminated air entering the fluid chamber deposits the contaminants on the surface of the liquid. The liquid remains sealed in the compressor, but can be drained and flushed.
The casing of a liquid ring compressor is partially filled with a constant flow of water. The water flows from a storage tank or a portable water system. After passing through the compressor, it is pumped back to the storage tank or discharged to a drain. In operation, the liquid is spun outward by rotor blades, forming a liquid ring around the inside of the casing. The inner surface of the liquid ring extends beyond the rotor blade tips, forming separate air pockets between each pair of blades. Since the rotor is mounted off-center in the casing, the air pockets are fairly large near the inlet poet and relatively small near the discharge as shown in figure.
Figure 3: Left - Straight-lobe type; Right - Liquid piston type
CENTRIFUGAL COMPRESSORS
A.k.a. “RADIAL COMPRESSOR”. It is widely used compressor and probably the 2nd after the reciprocating compressor in usage in the process industries. A simple centrifugal compressor has only one major moving part inside the casing –an impeller.
An impeller consists of radial or backward leaning blades and a front and rear shroud. As impeller rotates, gas is moved between blades from area near shaft and radially outward to discharge into stationary section called diffuser. Energy converts from high velocity to pressure. These compressors range in volume size from 1000 to 150,000 cfm and 10 to 10,000 psi.
Principles of Operation
Fluid (air or gas) is drawn in at the center of the casing , accelerated through the passages of a rotating impeller at high speed and then passed through fixed diffusers which convert velocity energy to increased pressure. Flow capacity is proportional to speed & pressure rise is proportional to speed squared. Additional pressure is built up as the air slows down in the volute, the gradually widening discharge pipe.
AXIAL FLOW COMPRESSOR
They are mainly used for applications where the head required is low and the flow large. Usage:
Mainly as compressors for gas turbines.
In the steel industry as blast furnace blowers and
In the chemical industry for large nitric acid plants.
Principles of Operation
Axial flow compressors operate on a dynamic principle similar to that of centrifugal compressors. The inlet air is accelerated by rotating blades and is slowed down in a controlled fashion, raising its pressure at the outlet. However, in axial flow compressors, the direction of air flow is in line with the shaft, rather than perpendicular to it.
An axial flow compressor has a rotor with a series of rotor blades and a tapered cylindrical casing with fixed stator blades. When the casing is in place, each set of rotor blades alternates with a set of stator blades. The spinning rotor blades act like high-speed fans to push air down the length of the casing, while the nonmoving stator blades provide resistance to the air flow, thus increasing the air pressure.
When the compressor is operating, the rotor blades push the air in the casing out through the discharge port. This creates a low pressure area inside the casing. The atmospheric pressure outside the casing is higher, so outside air flows in through the compressor’s inlet. This air is accelerated rapidly down the length of the casing by the fanlike rotor blades. The path of air flow is illustrated in Figure 26.
As in centrifugal compressors, the high-speed air flow gains pressure when it is forced to give up some of its velocity. In axial flow compressors, however, the air flow is not slowed down in a diffuser and a volute. Instead, the nonmoving stator blades are shaped like a volute. This causes air to slow down. Since rotor blades and stator blades alternate down the length of the casing, the air is accelerated and slowed down several times before it leaves the compressor. Pressure is increased each time the air flow meets a set of stators.
Reciprocating Air Compressors
Reciprocating air compressors are positive displacement machines, meaning that they increase the pressure of the air by reducing its volume. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by a piston within a cylinder as the compressing and displacing element.
Single-stage and two-stage reciprocating compressors are commercially available.
Single-stage compressors are generally used for pressures in the range of 70 psig to 100 psig.
Two-stage compressors are generally used for higher pressures in the range of 100 psig to 250 psig.
Note that
1 HP ~ 4 CFM at 100 psi
and that 1 to 50 HP are typically for reciprocating units. Compressors 100 hp and above are typically Rotary Screw or Centrifugal Compressors.
The reciprocating air compressor is single acting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting.
Load reduction is achieved by unloading individual cylinders. Typically, this is accomplished by throttling the suction pressure to the cylinder or bypassing air either within or outside the compressor. Capacity control is achieved by varying speed in engine-driven units through fuel flow control.
Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations and provide a wide range of pressure and capacity selections.
Rotary Screw Compressors
Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances.
Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package.
Rotary screw air compressors are easy to maintain and operate. Capacity control for these compressors is accomplished by variable speed and variable compressor displacement. For the latter control technique, a slide valve is positioned in the casing. As the compressor capacity is reduced, the slide valve opens, bypassing a portion of the compressed air back to the suction. Advantages of the rotary screw compressor include smooth, pulse-free air output in a compact size with high output volume over a long life.
The oil free rotary screw air compressor utilizes specially designed air ends to compress air without oil in the compression chamber yielding true oil free air. Oil free rotary screw air compressors are available air cooled and water cooled and provide the same flexibility as oil flooded rotaries when oil free air is required.