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Udara sebagai Refrigeran, Mungkinkah?

Rangkuman Diskusi Mailing List Migas Indonesia – Mei 2003

Pertanyaan : Robikhun

Membaca salah satu artikel di majalah OTOMOTIF terbaru ada hal yang menarik
yakni gagasan mahasiswa ITS untuk membuat AC yang dipasang pada sepeda
motor. Berbeda dengan AC mobil yang menggunakan siklus refrigerasi yang
sudah dikenal dalam ilmu termodinamika, AC motor hanya menggunakan
sebagian siklus. Untuk lebih jelasnya lihat artikel berikut. <<AC Motor2.ZIP>>
Nah pertanyaannya, bagaimana penjelasan secara termodinamika mengenai
fenomena ini? Saya meragukan konsep pemisah udara bertemperatur rendah
dan udara bertemperatur tinggi. Bukankah energi akan berpindah dari temperatur
tinggi ke temperatur rendah?

Artikel AC Motor2.ZIP (402 KB) bisa di-download

Tanggapan 1 : Mual Sihombing

Sesuai dengan definisinya, refrigeran adalah fluida kerja pada sistem refrigerasi, pengkondisian udara, dan heat pump. Refrigeran berfungsi untuk menyerap kalor dari satu area dan membuangnya ke area lain. Jadi, sepanjang suatu zat dapat berfungsi sesuai dengan definisi di atas maka zat tersebut secara teori dapat digunakan sebagai refrigeran.

Namun selain itu ada sejumlah besar sifat fisis dan kimiawi yang perlu dipertimbangkan dalam pemilihan refrigeran, misalnya : kestabilan kimia, titik didih, titik beku, titik kritis, kalor laten penguapan, apakah zat tersebut mudah terbakar, beracun atau tidak, kesesuaian dengan komponen mesin, duit, dsb.

Udara dapat dijadikan sebagai refrigeran (untuk lebih jelasnya Bapak dapat baca di ASHRAE standar 34) dan hal ini sudah lama dipakai pada pendinginan pesawat udara.

Berbeda dengan siklus refrigerasi mekanik kompresi uap yang menggunakan
kompresor, evaporator, kondenser, dan katup ekspansi, maka sistem refrigerasi dengan refrigeran udara (gas) terdiri dari empat komponen yaitu 2 buah penukar kalor, turbin, dan kompresor. Prinsip kerja sistem refrigerasi gas pada pesawat udara – yang menggunakan siklus terbuka – dapat dijelaskan sebagai berikut :
Udara luar dikompres oleh kompresor sehingga tekanan dan temperaturnya naik.Udara dengan tekanan dan temperatur tinggi ini kemudian didinginkan oleh udara sekeliling sehingga tekanan dan temperaturnya turun sebelum kemudian diekspansikan oleh turbin. Udara yang diekspansikan di dalam turbin akan mengalami penurunan temperatur sehingga menjadi dingin. Udara dingin inilah yang didistribusikan ke kabin-kabin.

Sistem refrigerasi gas memiliki COP (Coefficient of Performances) yang lebih rendah dari pada sistem refrigerasi mekanik kompresi uap, namun juga memiliki sejumlah keuntungan: lebih sederhana, ringan, sehingga sesuai untuk pesawat udara.

Silahkan menambahkan.

Tanggapan 2 : Muchlis N

Sepertinya didalam ACT terjadi proses seperti pada gas to gas heat exchanger. Secara termodinamis hal ini bisa terjadi.

- Jadi walaupun hanya ada 1 inlet gas, saluran outletnya ada 2. Artinya di dalam alat tersebut ada split aliran menjadi 2 didalam tabung ACT yang konstruksinya dirahasiakan itu..
- Salah satu aliran tetap dibiarkan bertekanan tinggi, dan satu aliran yang lain diturunkan tekanannya ke atmosfir (expansi).
- Aliran yang bertekanan lebih rendah akan memiliki temperatur yang lebih rendah sehingga mampu mendinginkan aliran yang bertekanan tinggi (penurunan tekanan mengakibatkan penurunan suhu). Akibatnya ada kalor yang terbuang dari aliran bertekanan tinggi.
- Kemudian aliran bertekanan tinggi yang sudah didinginkan ini kemudian dialirkan ke dalam jaket, tekanannya turun menjadi atmosferis sehingga menjadi lebih dingin lagi.

Tetapi karena keterbatasan kemampuan refrigeran, maka temperatur dinginnya hanya sekitar 24 oC.

Apakah kita rela mengorbankan tarikan motor hanya untuk menurunkan suhu jaket kita menjadi 24 oC. Mendingan gak usah pake jaket aja, kan dingin juga……..kecuali pas kena macet

Tanggapan 3 : Ary Retmono

Saya rada nggak mudheng dg penjelasannya Bang Mual:

Udara di luar pesawat (apalagi kalau sdh di cruising altitude) ‘kan bisa mencapai minus 43degC (walau terbang di atas Gurun Sahara saat”summer”), lha kenapa ya, koq nggak langsung dimanfaatkan untukmendinginkan udara di dalam cabin, ketimbang harus dikompres-kompresdulu?
Tanggapan 4 : Haryo

Mas Ary,
Kalo temperaute 43 C langsung di tiupkan ke kabin, apa gak jadi daging beku
semua penumpangnya?

Waktu kuliah dulu saya sempat dikenalkan istilah “bleed air” yang dimanfaatkan untuk pendinginan udara di kabin.

Kalau tidak salah ingat, bleed air adalah udara yang “dibocorkan” dari hasil
kompresi mesin pesawat. Udara yang “hangat” ini yang kemudian dihembuskan
ke kabin.

Kalo menurut saya, yang awam tentang system refrigerasi, system seperti di
atas saja sudah cukup. Apa perlu “accessories” lain?

Mohon penjelasan dari yang familiar dengan system refrigerasi, khususnya
untuk pesawat.
Tanggapan 5 : Harsono

Ikutan menanggapi ya mas Ary.
Saya jadi ingat pernah belajar Cabin Pressure Control System, pada dasarnya
pengaturan kondisi kabin itu dikondisikan pada human comfort. Ya sebisa
mungkin tekanan dalam cabin 1 atm, suhunya pada suhu ruang normal dan
tentunya komposisi O2 dalam cabin juga mencukupi untuk penumpang. Sementara
semakin tinggi altitudenya tekanan udara semakin rendah, O2 semakin tipis
dan suhu makin rendah. Jadi selain ada pengaturan suhu ya tentunya ada
pengaturan tekanan dan penambahan O2 selanjutnya perbedaan tekanan di luar
dan didalam cabin akan menentukan kekuatan material si cabin itu.
Mohon maaf menyimpang dari tema refrigran.

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