Ahmad Zaki Mubarok Maret 2012 Departemen Ilmu dan Teknologi Pangan Universitas Brawijaya Sub topik: Prinsip Umum Deskripsi Sistem Heat (Panas) Sifat Saturated dan Superheated Steam Soal-soal Beberapa proses yang berkaitan dg panas 1
ENERGI BALANCE? PRINSIP UMUM Kesetimbangan energi dalam sistem mengikuti hukum termodinamika pertama: hukum konservasi energi. Persamaan dasar kesetimbangan energi: Energi masuk = Energi keluar + Akumulasi Jika dalam sistem steady state, akumulasi = 0. 2
APA ITU TERMODINAMIKA? Termodinamika adalah cabang dari ilmu pengetahuan yang mempelajari tentang transformasi energi dari satu bentuk ke bentuk lainnya. Termodinamika Ilmu yang berkaitan dengan perubahan bentuk atau lokasi energi dan dapat dikaitkan juga dengan dinamika energi. Termodinamika dari suatu proses: dapat dilihat dari transformasi energi yang terjadi sebagai hasil dari suatu proses DESKRIPSI SYSTEM Terdiri dari materi yang memiliki batas tertentu dan didefinisikan dalam konteks wilayah tertentu Batas bisa nyata atau imaginasi Wilayah yang berada di luar batas dianggap sebagai lingkungan Contoh sistem: sebuah pabrik atau bagian-bagian dari pabrik Dua tipe umum sistem: Sistem terbuka Sistem tertutup 3
SISTEM TERBUKA Terdapat transfer materi antar sistem dengan lingkungan Terdapat transfer energi SISTEM TERTUTUP Tidak ada transfer materi Terdapat transfer energi antara sistem dengan lingkungan 4
SISTEM TERTUTUP DENGAN BATAS BERGERAK ISOLATED SYSTEM The fact that, in reality, a thermos is not perfect in keeping things warm/cold illustrates the difficulty in creating an truly isolated system. The only truly isolated system is the universe as a whole 5
DESKRIPSI SYSTEM Kondisi Steady State: Massa sistem tidak berubah Perubahan energi panas dalam sistem tidak dipengaruhi oleh waktu Kondisi Unsteady State: Massa sistem dapat tidak berubah Perubahan energi panas dalam sistem dipengaruhi oleh waktu BENTUK TRANSFER ENERGI 1. PANAS / HEAT (Q): energi panas yang ditransfer antara dua benda karena adanya perbedaan temperatur. 2. KERJA / WORK (W): transfer energi yang disebabkan oleh selain perbedaan temperatur, misalnya perbedaan tegangan, torque, atau gaya. 6
HEAT (PANAS) Sensible heat is defined as the energy transferred between two bodies at different temperatures, or the energy present in a body by virtue of its temperature. Latent heat is the energy associated with phase transitions, heat of fusion, from solid to liquid, and heat of vaporization, from liquid to vapor. ENTHALPY Enthalpy is the heat content of a unit mass of steam or water at the indicated temperature and pressure. Panas Jenis (Cp) adalah jumlah panas yang menyertai perubahan unit suhu per unit massa. 7
Properties of Saturated and Superheated Steam Steam and water are the two most used heat transfer mediums in food processing. The steam tables that list the properties of steam are a very useful reference when determining heat exchange involving a food product and steam or water. Properties of Saturated and Superheated Steam At temperatures above the freezing point, water can exist in either of the following forms: 1. Saturated Liquid Liquid water in equilibrium with its vapor. The total pressure above the liquid must be equal to or be higher than the vapor pressure. If the total pressure above the liquid exceeds the vapor pressure, some other gas is present in the atmosphere above the liquid. If the total pressure above a liquid equals the vapor pressure, the liquid is at the boiling point. 8
2. Vapor-Liquid Mixtures Steam with less than 100% quality. Temperature and pressure correspond to the boiling point; therefore, water could exist either as saturated liquid or saturated vapor. Addition of heat will not change temperature and pressure until all saturated liquid is converted to vapor. Removing heat from the system will also not change temperature and pressure until all vapor is converted to liquid. Steam Quality: The percentage of a vapor-liquid mixture that is in the form of saturated vapor. 3. Saturated Vapor This is also known as saturated steam and is vapor at the boiling temperature of the liquid. Lowering the temperature of saturated steam at constant pressure by a small increment will cause vapor to condense to liquid. The phase change is accompanied by a release of heat. If heat is removed from the system, temperature and pressure will remain constant until all vapor is converted to liquid. Adding heat to the system will change either temperature or pressure or both. 9
4. Superheated Steam Water vapor at a temperature higher than the boiling point. Addition of heat to superheated steam could increase the temperature at constant pressure or change both the pressure and temperature at constant volume. Removing heat will allow the temperature to drop to the boiling temperature where the temperature will remain constant until all the vapor has condensed. Saturated Steam Table The temperature and absolute pressure correspond to the boiling point, or the temperature and pressure under which steam can be saturated. The absolute pressure at a given temperature is also the vapor pressure. The entries under saturated vapor give the properties of steam at the boiling point. The entries under evaporation are changes due to the phase transformation and are the difference between the properties of saturated vapor and saturated liquid. 10
Properties of steam having less than 100% quality If steam is not 100% vapor, the properties can be determined on the basis of the individual properties of the component. If x = % quality h = xhg + (1 x)hf Example: Calculate the enthalpy of steam at 120 o C having 80% quality. From the saturated steam tables, at 120 o C, saturated steam or water has the following properties: hf = 503,72 kj/kg hg = 2706,07 kj/kg h = 2706,07 (0,8) + 503,72 (0,2) = 2164,86 + 100,75 = 2265,62 kj/kg Note that only the temperature of steam is given in this problem. If either the temperature or pressure is given, but not both, steam is at the boiling point. 11
Contoh soal 1: Hitung jumlah air yang dibutuhkan heat exchanger untuk mendinginkan pasta tomat sebanyak 100 kg/jam dari suhu 90 o C menjadi 20 o C. Suhu air masuk 20 o C. Peningkatan suhu air untuk mendinginkan pasta tidak boleh melebihi 10 o C. Panas jenis pasta tomat sebesar 2846 J/kg.K dan panas jenis air sebesar 4187 J/kg.K. Pasta tomat 100 kg/jam Cp = 2846 J/kg.K 90 o C Air Air T2 = T1 + 10 o C Contoh soal 2: Jika 1 kg air pada tekanan 198,5 kpa dan suhu 110 o C, kemudian tekanan diturunkan menjadi 47,36 kpa. Hitung: a. Temperatur akhir setelah tekanan diturunkan. b. Steam quality setelah tekanan diturunkan. 12
Contoh soal 3: Hitung jumlah uap air pada 120 o C yang harus disuplai untuk memanaskan tomat sebanyak 100 kg dari 15 o C menjadi 80 o C dengan sistem injeksi secara langsung. Panas jenis tomat = 3200 J/kg.K. Uap air 120 o C kondensasi Air 80 o C Tomat 15 o C Cp = 3200 J/kg.K Tomat 80 o C Campuran Tomat + Air (80 o C) Referensi: Fundamentas of Food Process Engineering; RT Toledo Introduction to Food Engineering; RP Singh, DR Heldman 13
Energy is expensive... Thank You 14