KONSEP DASAR TERMODINAMIKA

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KONSEP DASAR TERMODINAMIKA

• AGUS HARYANTO
• FEBRUARI 2010

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THERMO vs. HEAT TRANSFER

• Thermodynamics stems from the Greek words therme
  (heat) and dynamis (power or motion), which is
  most descriptive of the early efforts to convert
  heat into power. Today thermodynamics is broadly
  interpreted to include all aspects of energy and
  energy transformations, including power
  generation, refrigeration, and relationships
  among the properties of matter.
• Heat transfers the science that deals with the
  determination of the rates of such energy
  transfer.

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THERMO vs. HEAT TRANSFER (cont)

• Thermodynamics membicarakan sistem keseimbangan
  (equilibrium), bisa digunakan untuk menaksir
  besarnya energi yang diperlukan untuk mengubah
  suatu sistem keseimbangan, tetapi tidak dapat
  dipakai untuk menaksir seberapa cepat (laju)
  perubahan itu terjadi karena selama proses sistem
  tidak berada dalam keseimbangan.
• Heat Transfer tidak hanya menerangkan bagaimana
  energi itu dihantarkan, tetapi juga menaksir laju
  penghantaran energi. Inilah yang membedakan Heat
  Transfer dengan thermodinamika.

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APLIKASI

• Tubuh manusia
• Meniup kopi panas
• Perkakas elektronik (sirip, heat sink)
• Refrigerator (AC, Kulkas)
• Mobil (siklus engine, sirip, radiator)
• Pembangkit listrik (turbin, boiler)
• Industri (penyulingan, pendinginan, pengeringan,
  dll).

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DIMENSI dan SATUAN

• Dimensi (M,L,T,?) ? homogen
• Satuan SI Units (m, s, kg, K)
• Kesalahan umum
• 1. Tidak paham
• 2. Usaha minimal, kurang latihan
• 3. Tidak terampil melakukan konversi satuan
• Trik perhitungan harus menyertakan satuan

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SECONDARY UNITS

• Secondary units can be formed by combinations of
  primary units. Example

• F m.a

• P F/A

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SISTEM vs. LINGKUNGAN

• A system is defined as a quantity of matter or a
  region in space chosen for study.
• The mass or region outside the system is called
  the surroundings.
• The real or imaginary surface
  that separates the system from
  its surroundings is called the
  boundary

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OPEN vs. CLOSSED SYSTEMS

• Closed system ( control mass) Mass cant cross
  the boundary, but energy can.
• Volume of a closed system may change.
• Special case, if no energy cross the boundary,
  that system is called an isolated system.

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CLOSSED SYSTEM
A closed system with a moving boundary.
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OPEN vs. CLOSSED SYSTEMS

• Open system ( control volume) is a properly
  selected region in space. It usually encloses a
  device that involves mass flow such as a
  compressor, turbine, or nozzle.
• Both mass and energy can cross the boundary of a
  control volume.
• The boundaries of a control volume are called a
  control surface, and they can be real or
  imaginary.

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OPEN SYSTEM
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OPEN SYSTEM
Open system ( control volume) with one inlet and
one outlet (exit) and a real boundary.
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SIFAT-SIFAT SISTEM

• Any characteristic of a system is called a
  property.
• Some familiar properties are pressure P,
  temperature T, volume V, and mass m. The list can
  be extended to include less familiar ones such as
  viscosity, thermal conductivity, modulus of
  elasticity, thermal expansion coefficient,
  electric resistivity, and even velocity and
  elevation.
• Intensive properties are those that are
  independent of the mass of a system, such as
  temperature, pressure, and density.
• Extensive properties are those whose values
  depend on the sizeor extentof the system.
• Extensive properties per unit mass are called
  specific properties (specific volume (v V/m),
  specific energy (e E/m).

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SIFAT INTENSIF vs. EKSTENSIF
TUGAS (dikumpul Senin) Sebuah apel dibelah dua.
Buatlah daftar sifat intensif dan ekstensifnya
Criterion to differentiate intensive and
extensive properties.
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SIFAT-SIFAT SISTEM PENTING

• Densitas atau massa jenis masa per satuan volume
• Volume spesifik, kebalikan dari densitas volume
  per satuan masa (m3/kg)
• Densitas relatif atau specific gravity nisbah
  densitas suatu substansi dengan densitas
  substansi standar pada suhu tertentu (biasanya
  air pada 4oC di mana ? 1000 kg/m3)

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ENERGY SISTEM TERMODINAMIKA

• BENTUK ENERGI
• 1. Energi Kinetik (KE) ?
• 2. Energi Potensial (PE) ? PE mgh
• 3. Energi dakhil atau Internal Energy (U)
• ENERGI TOTAL
• E U KE PE
• e u ke pe (per satuan massa)

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POSTULAT KEADAAN

• All properties (can be measured or calculated)
  completely describes the condition, or the state,
  of the system. At a given state, all the
  properties of a system have fixed values. If the
  value of even one property changes, the state
  will change to a different one.
• The number of properties required to fix the
  state of a system is given by the state
  postulate
• The state of a simple compressible system is
  completely specified by two independent,
  intensive properties.

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PROSES dan SIKLUS

• Any change that a system undergoes from one
  equilibrium state to another is called a process
• The series of states through which a system
  passes during a process is called the path
  (lintasan) of the process.

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MACAM-MACAM PROSES

• Proses isotermal proses pada suhu T konstan.
• Proses isobaris proses pada tekanan P konstan.
• Proses isokhoris (isometris) proses pada volume
  spesifik ? konstan.
• Proses adiabatik proses di mana tidak terjadi
  pertukaran kalor dengan lingkungan.
• Proses isentropik proses pada entropi S konstan.
  

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STEADY-FLOW PROCESS

• The terms steady and uniform are used frequently
  in engineering, and thus it is important to have
  a clear understanding of their meanings.
• The term steady implies no change with time.
• The opposite of steady is unsteady, or transient.
  
• The term uniform, however, implies no change with
  location over a specified region.

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PROSES dan SIKLUS

• A system undergoes a cycle if it returns to its
  initial state at the end of the process.

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TEKANAN

• Tekanan (P) gaya (F) per satuan luas (A).
• Satuan tekanan adalah pascal (Pa) N/m2.
• Untuk benda padat gaya per luas satuan tidak
  disebut tekanan, tetapi tegangan (stress).
• Untuk fluida diam, tekanan adalah sama ke segala
  arah.
• Tekanan di dalam fluida meningkat sesuai dengan
  kedalamannya akibat berat fluida (pengaruh
  gravitasi) sehingga fluida pada bagian bawah
  menanggung beban yang lebih besar daripada fluida
  di bagian atas.
• Tetapi tekanan tidak bervariasi pada arah
  horisontal.
• Tekanan gas di dalam tangki dapat dianggap
  seragam karena berat gas terlalu kecil dan tidak
  mengakibatkan pengaruh yang berarti.

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TEKANAN UKUR, ATM, VAKUM

• Tekanan aktual pada posisi tertentu disebut
  tekanan absolut dan diukur secara relatif
  terhadap tekanan vakum, yaitu tekanan nol mutlak.
  
• Kebanyakan pengukur tekanan dikalibrasi untuk
  membaca nol di atmosfer (tekanan atmosfer lokal).
  
• Perbedaan tekanan absolut dan tekanan atmosfer
  disebut tekanan ukur (pressure gage).
• Tekanan di bawah tekanan atmosfer disebut tekanan
  vakum (vacuum pressure) dan diukur dengan
  pengukur vakum yang menunjukkan perbedaan antara
  tekanan atmosfer dan tekanan absolut.
• Pgage Pabs Patm (untuk P gt Patm)
• Pvac Patm Pabs (untuk P lt Patm)

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TEKANAN UKUR, TEKANAN ATMOSFER, TEKANAN VAKUM
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PENGUKUR TEKANAN
MANOMETER

• PRESSURE GAGE

BAROMETER
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PRINSIP MANOMETER

• Perhatikan gambar
• Seimbang ??F 0
• P1 P2
• A P1 A Patm W
• di mana W m g ? V g ? A h g
• P1 Patm ? h g
• ?P P1 - Patm ? h g

Tekanan ukur di dalam tangki
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EXAMPLE Manometer

• A manometer is used to measure the pressure in a
  tank. The fluid used has a specific gravity of
  0.85, and the manometer column height is 55 cm,
  as shown in Figure. If the local atmospheric
  pressure is 96 kPa, determine the absolute
  pressure within the tank.

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EXAMPLE SOLUTION
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EXAMPLE MULTIFLUID MANOMETER
Water in a tank is pressurized by air, and the
pressure is measured by a multifluid manometer
(see Figure). The tank is located on a mountain
at an altitude of 1400 m where the atmospheric
pressure is 85.6 kPa. Determine the air pressure
in the tank if h1 0.1 m, h2 0.2 m, and h3
0.35 m. Take the densities of water, oil, and
mercury to be 1000 kg/m3, 850 kg/m3, and 13,600
kg/m3, respectively.
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SOLUTION
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APLIKASI MANOMETER
Measuring the pressure drop across a flow section
or a flow device by a differential manometer

• P1 ?1g(a h) - ?2gh - ?1ga P2
• P1 - P2 (?2 - ?1)gh
• Untuk ?2 gtgt ?1
• P1 - P2 ?2 g h

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BAROMETER Torricelli
Patm ? g h
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EXAMPLE3 BAROMETER

• Determine the atmospheric pressure at a location
  where the barometric reading is 740 mm Hg and the
  gravitational acceleration is g 9.81 m/s2.
  Assume the temperature of mercury to be 10oC, at
  which its density is 13,570 kg/m3.

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EXAMPLE3 SOLUTION
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TEKANAN ATMOSFER
ELEVASI (m) TEKANAN (kPa) TEKANAN (mmHg)
0 (sea level) 101.325 760.00
1000 89.88 674.15
2000 79.50 596.30
5000 54.05 405.41
10,000 26.5 198.77
20,000 5.53 41.48
Rule of thumb naik 10 m, tekanan atmosfer turun
1 mmHg
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EFEK KETINGGIAN
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TEMPERATURE

• Thermodinamika ? SUHU MUTLAK
• Satuan kelvin (K) untuk SI
• Satuan renkine (R) untuk USCS

Konversi T(K) T(oC) 273.15 T(R) T(oF)
456.67 T(oC) 1.8T(oC) 32 T(R) 1.8 T(K)
CAUTION ?T(K) ?T(oC) ?T(R) ?T(oF)
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EXAMPLE4 TEMPERATURE

• During a heating process, the temperature of a
  system rises by 10C. Express this rise in
  temperature in K, F, and R.

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PR

• Soal No 1-6C, 1-7C, 1-15C, 1-16C, 1-17C, 1-20C,
  1-21C, 1-22C, 1-23C, 1-24C, 1-29, 1-31, 1-34C,
  1-35C, 1-36C, 1-39C, 1-40, 1-42, 1-43, 1-44,
  1-45, 1-48, 1-51, 1-53, 1-55, 1-57, 1-59, 1-61,
  1-62, 1-63, 1-65, 1-66, 1-73, 1-85, 1-88, 1-101,
  1-103, 1-105, 1-106, 1-108, 1-120, 1-121,
  1-122, 1-123, 1-125.
• Kelompok THERMO
• Kelompok DYNAMICS