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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