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MESIN KONVERSI ENERGISemester GasalTA 2011/2012
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PTOPIK: Sistem Pendingin
• Menjelaskan komponen-komponen utama mesin
pendingin-pemanas dan cara kerjanya.
• Memahami prinsip kerja siklus refrifgerasi
• Melakukan perhitungan dan perancangan siklus
pendingin-pemanas.
• Mengetahui sifat-sifat refrigerant yang baik
• Menghitung beban pendinginan
• Menghitung parameter prestasi (COP)
Tujuan
Mahasiswa mampu :
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PPengertian penting
Refrigeration : Perpindahan kalor dari media
bertemperatur rendah ke media bertemperatur
lebih tinggi.
Refrigerators : Mesin yang menghasilkan refrigeration
Refrigeration cycles : siklus yang digunakan dalam
menghasilkan refrigeration.
Refrigerants : Fluida kerja yang digunakan dalam
refrigerators.
Heat pumps : Refrigerators yang digunakan untuk
pemanasan
1 Ton of Refrigeration = Kalor yang diambil dari 1 ton
(2,000 lb) air yang bersuhu 32 F sehingga
menjadi es pada 32 F selama 24 jam
1 Ton = 12,000 Btu/h = 3.517 kW
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PHukum II Termodinamika
Kalor selalu mengalir dari medium bertemperatur
tinggi ke medium bertemperatur rendah
Kalor hanya mengalir jika ada perbedaan temperatur
Tujuan : Mengambil kalor dari medium bertemperatur
rendah dan memberikannya ke medium yang
bertemperatur lebih tinggi
Apakah pernyataan ini memenuhi Hukum II
Termodinamika?
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PRefrigerator dan Pompa Kalor (heat pump)
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PRefrigerator dan heat pump
Refrigerator dan heat pumps pada dasarnya
merupakan peralatan yang sama.
Refrigerator dan heat pumps berbeda hanya pada
tujuannya saja.
Tujuan dari refrigerator adalah mengambil
kalor (QL) dari medium bersuhu rendah
(mempertahankan ruang pendingin tetap
dingin)
Tujuan dari heat pump adalah mensuplai
kalor (QH) ke medium bersuhu tinggi
(mempertahankan ruang pemanas tetap
panas)
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Konstruksi DasarMesin Pendingin
• A : Kompresor
• B : Fan Motor
• C : Pipa Ekspansi
• D : Kondenser
• E : Evaporator
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1. KompresorAlat untuk memampatkan gas refrigerant (pendingin) yang masuk supaya
dapat mencair di Kondensor.
2. Kondenser
Melepaskan panas yang diambil Refrigerant di Evaporator dan mencairkannya.
3. Pipa Kapiler/Ekpansi
Pipa Kapiler yang berdiameter kecil, berfungsi menurunkan tekanan aliran .
Dengan turunnya tekanan memungkinkan Refrigerant untuk menguap.
4. Evaporator
Evaporator adalah media penguapan bagi cairan Refrigerant dan selama menguap,
Refrigerant menyerap panas dari udara disekitarnya.
Catatan :
4 ( empat ) komponen diatas saling berhubungan satu sama lain menjadi satu siklus di mana refrigerant bersirkulasi,Selama sirkulasi berlangsung refrigerant diuapkan secara berulang-ulang untuk
proses pendinginan.
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Aliran Refrigerant dalam Siklus
Gas Tekanan Tinggi 240 Psi
Temperatur Tinggi 90 oC
Cam
pura
n
Cair 50 oC, 240 PsiCair 5 oC, 70 Psi
Cam
pura
n
Gas Tekanan Rendah 70 Psi
Temperatur Rendah 10 oC
Kondenser
Kompresor
Kapiler
Fan Fan
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PCOP : Refrigerator and heat pump
Unjuk kerja (prestasi) refrigerator dan heat pump
dinyatakan dalam coefficient of performance (COP),
yang didefinisikan sebagai:
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PReverse Carnot Cycle = Carnot Heat Pump
T-s Diagram
T
s
P2
P1
1
23QH
4
QL
COLD medium at TL
QL
WARM medium at TH
QH
Win
Condenser
Evaporator
Compressor
Turbine
3 2
41
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PSiklus carnot dibalik (The Reversed Carnot Cycle)
Refrigerator atau heat pump yang bekerja
berdasarkan siklus Carnot yang dibalik (reversed
Carnot cycle) disebut refrigerator Carnot atau
Pompa Kalor Carnot (a Carnot heat pump)
COP –nya adalah :
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The Ideal Vapor Compression Refrigeration Cycle
• In an ideal vapor compression
refrigeration cycle, the refrigerant enters
the compressor as a saturated vapor and
is cooled to the saturated liquid state in
the condenser. It is then throttled to the
evaporator pressure and vaporizes as it
absorbs heat from refrigerated space.
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The ideal vapor compression refrigeration cycle consists of
following four processes.
• 1 – 2: Isentropic compression in a compressor.
• 2 – 3: Constant pressure heat rejected in condenser.
• 3 – 4: Throttling in an expansion device (same enthalpy
remains constant)
• 4 – 1: Constant pressure heat absorption in an
evaporator.
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P • In a household refrigerator, the freezer
compartment where heat is absorbed by the
refrigerant serves as the evaporator. The coils
behind the refrigerator, where heat is dissipated
to the kitchen air serve as the condenser.
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P • The area under the process curve on a T-s
diagram represents the heat transfer.
• Another diagram frequently used in the analysis
of vapor-compression refrigeration cycle is P-h
diagram.
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Actual Vapor-Compression Refrigeration Cycles
• There are many irreversibilities that occurs in various
components. Two common sources of irreversibilites
are fluid friction (causes pressure drop) and heat
transfer to or from surrounding.
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Cascade Refrigeration Systems
• For applications that require large temperature and pressure
ranges, refrigeration is performed in stages(2 or more).
• Large pressure range means poor compressor performance.
• Performing refrigeration in stages is achieved by Cascade
Refrigeration Cycles (that is more than a refrigeration cycle
operating in series).
• Cascading improves the COP of a refrigeration system.
• The refrigerant in both cycles could be the same or different.
• Using the following figure ,write expressions for mass flow rates
ratio and COP?
• See Example 10.3
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Cascade Refrigeration Systems
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PMultistage Compression Refrigeration Systems
• The heat exchanger in Cascade Refrigeration System can be
replaced by a mixing chamber if the refrigerant in the two cycles is
the same.
• Such system is called Multistage Compression Refrigeration
System.
• Liquid refrigerant (exit of condenser) expands to the mixing (flash)
chamber pressure where part of it vaporizes ( see Fig.)
• The saturated vapor mixes with the superheated vapor (point 3)
from the exit of the low pressure compressor.
• Hence, two-stage compression with inter-cooling.
• Multistage Compression decreases the work of the compressor
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Multistage Compression Refrigeration Systems
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Gas Refrigeration Cycles
• Gas Refrigeration Cycle is reversed Brayton cycle (see Fig.).
• Note, the expansion process is performed in a turbine rather than a throttling valve as in vapor compression refrigeration systems (Why?).
• The heat transfer processes donot take place at constant temperatures. Hence, it differs from Carnot Cycle.
• Hence, Gas Refrigeration Cycle do have lower COPs relative to vapor–compression refrigeration cycles. Illustrate by a T-s diag.?
• Gas Refrigeration Cycles involve simple lighter components (Aircraft cooling) and can incorporate regeneration (suitable for liquidation of gases)
• Multistage Compression decreases the work of the compressor
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Gas Refrigeration Cycles
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Gas Refrigeration Cycle with Regeneration
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Absorption Refrigeration Systems
• Refrigeration in which there is a source of inexpensive
thermal energy at a temperature of 100 to 200OC is
absorption refrigeration
• The refrigerant is absorbed by a transport medium and
compressed in liquid form.
• The most widely used absorption refrigeration system
is the ammonia – water system where ammonia serves
as the refrigerant and water as the transport medium
• Other absorption refrigeration systems include water-
Lithium bromide where water serves as a refrigerant
(limited applications-Why?).
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Absorption Refrigeration Systems
• The basic principles can be discussed by the Ammonia
absorption refrigeration cycle shown in Fig.
• ARS are: complex, occupy more space and less
efficient (hence, expensive compared to vapor
compression systems).
• In ARS liquid is compressed instead of vapor, thus the
work input is very small compared to vapor
compression systems.
• Write an expression for the COP of an ARS?
• Derive an expression for the maximum COP of an absorption
refrigeration system and comment?
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Absorption chillers
• Absorption chillers are air-conditioning systems based
on absorption refrigeration.
• Absorption chillers cooling capacity decreases sharply with
decrease in source temperature.
• The COP is affected less by decrease in source temperature.
• Read more about absorption chillers.
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PSyarat-syarat refrigeran adalah
• Tidak beracun
• Tidak dapat terbakar atau meledak sendiri bila bercampur
dengan udara, pelumas dan sebagainya
• Tidak menyebabkan korosi terhadap logam yang dipakai pada
sistem refrigerasi.
• Bila terjadi kebocoran mudah dicari
• Mempunyai titik didih dan tekanan kondensasi yang rendah
• Mempunyai susunan kimia yang stabil, tidak terurai setiap kali
dimampatkan, dikondensasikan dan dievaporasikan
• Perbedaan antara tekanan evaporasi dan tekanan kondensasi
harus sekecil mungkin
• Mempunyai panas latent evaporasi yang besar, agar panas yang
diserap evaporator besar jumlahnya dengan bahan pendingin
sedikit.
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PKODE WARNA REFRIGERAN
Bahan pendingin diidentifikasi dengan nomor-nomor
dibelakang huruf R (yang berarti refrigeran)
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PPENGGUNAAN REFRIGERAN
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PPEMAKAIAN REFRIGERAN