Dapat dikatakan bahwa dalam mempelajari Refrigerasi dan AC merupakan terapan dan teori perpindahan Panas dan Thermodinamika. thermodinamika dan Perpindahan Panas dikembangkan dari serangkaian konsep yang dikembangkan di dunia Fisika dan Matematika. Suatu benda pasti memiliki sifat Thermodinamika seperti : suhu, tekanan, massa dan juga energi. Oleh karenanya pembahasannya masalah pada bagian ini akan lebih baik apabila dimulai dari pembahasan aspek-aspek dari kedua masalah tersebut. Kita akan coba kemukakan beberapa prinsip dasar yang penting untuk perhitungan yang digunakan dalam rancangan dan pemganalisaan sistem Air Cinditioning.
Satuan yang biasa digunakan dalam bidang Air Conditioner adalah British sistem ( The Yard Pound Sistem ), Metric Sistem dan Sistem International (SI). Ketiga satuan tersebut bisa dikonversikan sehingga mempermudah dalam pemakaian satuan.
TEKANAN
1. Massa
Massa adalah berat yang terkandung dalam suatu benda yang biasanya memakai satuan gram dan kilogram, 1 gr adalah satu centimeter kubik (cm3) air dengan temperatur 0 0C kali massa jenisnya disebut 1 gr .
2. Gaya
Gaya didefinisikan sebagai dorongan atau tarikan. Suatu yang dapat membuat benda yang diam bergerak dan benda yang bergerak berhenti atau membelok arah gerakannya
Persamaan gaya : F = A x P
F = gaya dalam Newton (N) atau kgf
A = luas dalam mm2 atau cm2
P = tekanan dalam (Pa) atau kg/cm2
Berat : gaya yang bekerja pada benda itu karena gaya tarik bumi (gravitasi). Berat sebenarnya
adalah gaya, jadi untuk kata berat sering digunakan gaya berat. Gambar 1-2
Satuan gaya dalam Conventional Metric System adalah Kilogram Force (kgf), S.I Metric System
dalam Newton (N) dan British System dalam Pound Force (lbf).
Kilogram Force (kgf) : gaya yang memberikan percepatan sebesar 9,807 m/s2 kepada benda
yang massanya 1 kg. Gambar 1-3a
Newton (N) : gaya yang memberikan percepatan sebesar 1m/s2 kepada benda yang massanya
1 kg . Gambar 1-3b
Hubungan antara kgf, Newton dan lbf ditunjukan pada tabel 1-2 di bawah ini :
3.Tekanan
Tekanan adalah gaya yang bekerja per satuan luas. Dapat juga dikatakan bahwa tekanan sebagai ukuran intensitas gaya pada suatu satuan luas. Pada benda padat keseluruhan berat benda itu menekan permukaan yang terkena kontak langsung, pada benda cair bukan hanya tekanan kearah bawah tetapi juga kebagian sisi wadah dan untuk benda gas tekanan menekan kesegala arah.
Gambar 1-4 rumus tekanan :
P =F dibagi A
dengan : P = Tekanan
F = Total gaya
A = Luas area
4. Hukum Pascal
Pada pertengahan abad 17, seorang ahli Matematika dan ilmuwan Prancis yang bernama Blaise Pascal mengadakan percobaan dengan menggunakan air dan tekanan. Hasil percobaan tersebut dikenal dengan hukum Pascal.
Hukum Pascal : tekanan yang dihasilkan oleh cairan akan di pindahkan kesegala arah dengan sama rata.Pada gambar 1-5 piston menekan cairan, tekanan menyebar kesegala arah pada tiap-tiap wadah akan sama besarnya tanpa memperdulikan bentuknya.
5. Tekanan Atmosfir
Tekanan Atmosfir : udara mempunyai berat karena ditarik oleh gaya tarik bumi. Berat ini menyebabkan suatu tekanan yang menuju kesegala arah dan disebut tekanan atmosfir. Makin tinggi dari permukaan bumi lapisan udara makin tipis. Hal ini disebabkan karena gaya tarik bumi makin tinggi makin berkurang.Pengukuran tekanan uap air ini menggunakan Barometer air raksa. Besarnya tekanan 1 atmosfir di permukaan air laut kira-kira sebesar 1,033 kgf (14,7 psi, 1,03 kg/cm2, 760 mmHg, 76 cmHg, 29,92 inHg), Gambar 1-6. Besarnya tekanan itu tidak tetap, tetapi bervariasi tergantung pada daerah yang diukur dari permukaan air laut, perubahan temperatur, jumlah kandungan uap air dan udara.
Barometer pada gambar 1-7 dipakai pipa kaca sepanjang 36 inch, salah satu ujungnya tertutup kemudian sebuah piringan dan sejumlah cairan mercury. Isilah pipa kaca tersebut dengan cairan mercury sampai penuh, kemudian tutup dengan ibu jari, balikkan dan celupkan ke dalam piringan yang berisi mercury (Hg) kemudian ibu jari lepaskan pelan-pelan . Ternyata mercury di dalam pipa kaca akan menurun, sehingga ada rongga hampa udara di ujung pipa yang tertutup. Tekanan atmosfir menekan permukaan mercury di dalam pipa kaca itu setingggi 760 mm (29,92 in) dan biasa disebut tekanan atmosfir. (gambar 1-7)
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