Oleh: Ribut Sugiharto
PENDAHULUAN
455
Temperature plays an important role in maintaining the quality of stored food products. Lowering the temperature retards the rates of reactions that cause quality deterioration. It is generally agreed that the reaction rate is reduced by half by lowering the temperature by 10°C.
In earlier days, a lower temperature was obtained by the use of ice. Ice was allowed to melt in an insulated chamber that contained food prod- ucts ( Fig. 6.1 ). During melting, ice requires latent heat (333.2 kJ/kg) to be converted from the solid phase to liquid water. This heat was extracted from the product that was kept next to ice in an insulated chamber.
Today, the cooling process is achieved by the use of a mechanical refrigeration system. Refrigeration systems allow transfer of heat from the cooling chamber to a location where the heat can easily be dis- carded. The transfer of heat is accomplished by using a refrigerant, which like water changes state—from liquid to vapor. Unlike water, a refrigerant has a much lower boiling point. For example, ammonia, a commonly used refrigerant in industrial plants, has a boiling point of !33.3°C. This is a much lower temperature compared with 100°C, the boiling point of water at atmospheric pressure. Similar to water, ammonia needs latent heat to change its phase from liquid to gas at its boiling point. The boiling point of a refrigerant can be varied by changing the pressure. Thus, to increase the boiling point of ammo- nia to 0°C, its pressure must be raised to 430.43 kPa.
A very simple refrigeration system that utilizes a refrigerant is shown in Figure 6.2 . The only drawback in this illustration is the onetime use of the refrigerant. Because refrigerants are expensive, they must
6 Refrigeration
Chapter
All icons in this chapter refer to the author’s web site, which is independently owned and operated. Academic Press is not responsible for the content or operation of the author’s web site. Please direct your web site comments and questions to the author: Professor R. Paul Singh, Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, USA. Email: rps@rpaulsingh.com.
Ice
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Ribut Sugiharto
Evaporator Condenser
Jenis Media Penyerap Panas 1. Air (Es)
• Es mencair memerlukan panas latent (333,2 kJ/kg). • Panas diserap dari lingkungannya.
2. Refrigenant • Mechanical refrigeration system. • mempunyai titik didih sangat rendah (<15oC) dan
suhu kondensasi tinggi (>30oC). • Contoh: freon (12, 22, 30, 134a), amonia.
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SYARAT REFRIGERANT a. Panas latent penguapan besar. b. Tekanan kondesasi rendah. c. Suhu pembekuan dibawah suhu penguapan. d. Suhu kritis tinggi, lebih tinggi suhu kamar. e. Tidak beracun. f. Tidak mudah terbakar. g. Tidak menyebabkan korosi. h. Secara kimia stabil. i. Mudah dideteksi bila terjadi kebocoran. j. Murah. k. Tidak merusak lingkungan.
ASHRAE Standard No. 34-1978
458 CHAPTER 6 Refrigeration
A number of refrigerants used in commercial practice are halocar- bons, although their use is being severely curtailed as described later in this section. Refrigerant-12, also called Freon 12, is a dichlorodi- fl uoromethane. Its latent heat of vaporization is low compared with ammonia (R-717); therefore, considerably more weight of the refrig- erant must be circulated to achieve the same refrigeration capacity.
Table 6.1 Comparison between Commonly Used Refrigerants (Performance Based on !15°C Evaporator Temperature and 30°C Condenser Temperature)
Chemical formula
CHClF2) HFC 134a (CH2FCF3)
Boiling point (°C) at 101.3 kPa !29.8 !40.8 !26.16 !33.3
Evaporator pressure at !15°C (kPa) 182.7 296.4 164.0 236.5
Condensing pressure at 30°C (kPa) 744.6 1203.0 770.1 1166.5
Freezing point (°C) at 101.3 kPa !157.8 !160.0 !96.6 !77.8
Critical temperature (°C) 112.2 96.1 101.1 132.8
Critical pressure (kPa) 4115.7 4936.1 4060 11423.4
Compressor discharge temperature (°C)
37.8 55.0 43 98.9
Latent heat of vaporization at !15°C (kJ/kg)
161.7 217.7 209.5 1314.2
Refrigerant circulated/ton refrigeration (kg/s), ideal
2.8 " 10!2 2.1 " 10!2 2.38 " 10!2 0.31 " 10!2
Compressor displacement/ton refrigeration (m3/s)
Stability (toxic decomposition products)
No
JENIS REFRIGERANT
KOMPONEN SISTEM REFRIGERASI
EVAPORATOR Fungsi: • Tempat refrigerant berubah fase dari cair menjadi uap/
gas. • Memerlukan panas latent dan menyerap panas dari
lingkungannya. Jenis: 1. Direct expansion,
• Refrigerant menguap pada koil, dan koil menyerap panas langung dari lingkungannya.
2. Indirect expansion, • Refrigerant mendinginkan medium (air, brine, glycol),
yang kemudian dipompakan ke bahan yang didinginkan.
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EVAPORATOR
EVAPORATOR
COMPRESSOR
COMPRESSOR
CONDENSER Fungsi: • Transfer panas dari
refrigerant ke sekelilingnya (udara, air).
• Mengubah fase uap refrigerant menjadi fase cair.
Jenis: 1. Berpendingin udara. 2. Berpendingin air. 3. Evaporasi (udara + air)
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CONDENSER 467
Air-cooled condensers can be either tube-and-fi n type or plate type, as shown in Figure 6.10 . Fins on tubes allow a large heat transfer area in a compact case. The plate condensers have no fi ns, so they require considerably larger surface areas. However, they are cheaper to con- struct and require little maintenance. Both these types of condensers can be found in household refrigerators.
Air-cooled condensers can also employ artifi cial movement of air by using a fan. The fan helps in obtaining higher convective heat-transfer coeffi cients at the surface of the condenser.
Water
Water
Water
Water
Water
Hot gas
Liquid refrigerant
Figure 6.9 An open shell-and-tube condenser and double-pipe condenser. (Courtesy of Carrier Co.)
Figure 6.10 A plate and tube-and-fi n condenser. (Courtesy of Carrier Co.)
6.2 Components of a Refrigeration System
Jenis berpendingin air: 1. Open shell and tube. 2. Double pipe. 3. Plate. 4. Tube and Fin.
Jenis berpendingin udara: 1. Plate. 2. Tube and Fin
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CONDENSER
EXPANSION VALVE Fungsi: • Menurunkan tekanan refrigerant • Menentukan laju alir (jumlah) refrigerant cair masuk ke
dalam evaporator.
468 CHAPTER 6 Refrigeration
In evaporative condensers, a circulating water pump draws water from a pan at the base of the condenser and sprays the water onto the coils. In addition, a large amount of air is drawn over the condenser coils. Evaporation of water requires latent heat, which is extracted from the refrigerant. Figure 6.11 shows an evaporative condenser. These units can be quite large.
6.2.4 Expansion Valve An expansion valve is essentially a metering device that controls the fl ow of liquid refrigerant to an evaporator. The valve can be operated either manually or by sensing pressure or temperature at another desired location in the refrigeration system.
The common type of metering devices used in the refrigeration sys- tem include (1) manually operated expansion valve, (2) automatic low-side fl oat valve, (3) automatic high-side fl oat valve, (4) automatic expansion valve, and (5) thermostatic expansion valve.
A simple, manually operated expansion valve is shown in Figure 6.12 . The valve, manually adjusted, allows a desired amount of fl ow of refrigerant from the high-pressure liquid side to the low-pressure gas/ liquid side. The refrigerant cools as it passes through the valve. The heat given up by the liquid refrigerant is absorbed to convert some of
Moist air exit
Hot gas inlet Air inlet
Figure 6.11 An evaporative condenser. (From Jennings, 1970. Copyright © 1939, 1944, 1949, 1956 , 1958 , 1970 by Harper and Row, Publishers, Inc. Reprinted with permission of the publisher.)
Low-pressure gas and liquid
High-pressure liquid
Figure 6.12 A manually operated expansion valve. (Courtesy of Carrier Co.)
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470 CHAPTER 6 Refrigeration
fl oat consequently rises and opens the orifi ce, allowing the refrigerant to fl ow to the evaporator.
The automatic expansion valve maintains a constant pressure in the evaporator. As shown in Figure 6.15 , an increase in evaporator pres- sure causes the diaphragm to rise against the spring pressure, which results in the valve closing. The valve opens when the evaporator pressure decreases. This valve is used in applications that require a constant refrigeration load and constant evaporator temperature—for example, in a household refrigerator.
Thermal expansion valves contain a thermostatic bulb clamped to the side of the suction pipe to the compressor ( Fig. 6.16 ). The thermo- static bulb senses the temperature of the superheated gas leaving the evaporator. The relatively high temperature of the thermostatic bulb causes the fl uid in the bulb (usually the same refrigerant) to increase in pressure. The increased pressure is transmitted via the thermo- static tube to the bellows and the diaphragm chamber. The valve consequently opens to allow more liquid refrigerant to fl ow through. Thermostatic valves are the most widely used of all metering devices in the refrigeration industry.
6.3 PRESSURE–ENTHALPY CHARTS Both pressure and enthalpy of the refrigerant change as the refrigerant is conveyed through various components of a refrigeration system. In both the evaporator and the condenser, the enthalpy of the refriger- ant changes and the pressure remains constant. During the compres- sion step, work is done by the compressor, resulting in an increase in the enthalpy of the refrigerant along with an increase in pressure. The expansion valve is a constant-enthalpy process that allows the liquid refrigerant under high pressure to pass at a controlled rate into the low-pressure section of the refrigeration system.
Charts or diagrams have been used extensively in the literature to present thermodynamic properties of refrigerants. These charts are particularly useful during the early, conceptual stages of a refrigera- tion system design. Looking at a chart, we can easily comprehend a standard process, as well as any deviations from the standard. Most commonly used charts depict enthalpy and pressure values on the x and y axes, respectively. Another type of chart involves entropy and temperature values plotted along x and y axes, respectively. The entire
Bulb pressure
Figure 6.16 A thermostatic expansion valve. (Courtesy of Carrier Co.)
Spring pressure
Evaporator pressure
Figure 6.15 An automatic expansion valve. (Courtesy of Carrier Co.)
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470 CHAPTER 6 Refrigeration
fl oat consequently rises and opens the orifi ce, allowing the refrigerant to fl ow to the evaporator.
The automatic expansion valve maintains a constant pressure in the evaporator. As shown in Figure 6.15 , an increase in evaporator pres- sure causes the diaphragm to rise against the spring pressure, which results in the valve closing. The valve opens when the evaporator pressure decreases. This valve is used in applications that require a constant refrigeration load and constant evaporator temperature—for example, in a household refrigerator.
Thermal expansion valves contain a thermostatic bulb clamped to the side of the suction pipe to the compressor ( Fig. 6.16 ). The thermo- static bulb senses the temperature of the superheated gas leaving the evaporator. The relatively high temperature of the thermostatic bulb causes the fl uid in the bulb (usually the same refrigerant) to increase in pressure. The increased pressure is transmitted via the thermo- static tube to the bellows and the diaphragm chamber. The valve consequently opens to allow more liquid refrigerant to fl ow through. Thermostatic valves are the most widely used of all metering devices in the refrigeration industry.
6.3 PRESSURE–ENTHALPY CHARTS Both pressure and enthalpy of the refrigerant change as the refrigerant is conveyed through various components of a refrigeration system. In both the evaporator and the condenser, the enthalpy of the refriger- ant changes and the pressure remains constant. During the compres- sion step, work is done by the compressor, resulting in an increase in the enthalpy of the refrigerant along with an increase in pressure. The expansion valve is a constant-enthalpy process that allows the liquid refrigerant under high pressure to pass at a controlled rate into the low-pressure section of the refrigeration system.
Charts or diagrams have been used extensively in the literature to present thermodynamic properties of refrigerants. These charts are particularly useful during the early, conceptual stages of a refrigera- tion system design. Looking at a chart, we can easily comprehend a standard process, as well as any deviations from the standard. Most commonly used charts depict enthalpy and pressure values on the x and y axes, respectively. Another type of chart involves entropy and temperature values plotted along x and y axes, respectively. The entire
Bulb pressure
Figure 6.16 A thermostatic expansion valve. (Courtesy of Carrier Co.)
Spring pressure
Evaporator pressure
Figure 6.15 An automatic expansion valve. (Courtesy of Carrier Co.)
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KOMPONEN SISTEM REFRIGERASI