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1.1. REFRIGERATION BASICS1.1.1. Vapor Compression Refrigeration CycleThe term refrigeration, as part of a building HVAC system, generally refers to avapor compression system wherein a chemical substance alternately changesfrom liquid to gas (evaporating, thereby absorbing heat and providing a coolingeffect) and from gas to liquid (condensing, thereby releasing heat). This cycleactually consists of four steps:1. Compression: Low-pressure refrigerant gas is compressed, thus raisingits pressure by expending mechanical energy. There is a correspondingincrease in temperature along with the increased pressure.2. Condensation: The high-pressure, high-temperature gas is cooled byoutdoor air or water that serves as a heat sink and condenses to aliquid form at high pressure.3. Expansion: The high-pressure liquid flows through an orifice in theexpansion valve, thus reducing the pressure. A small portion of theliquid flashes to gas due to the pressure reduction.4. Evaporation: The low-pressure liquid absorbs heat from indoor air orwater and evaporates to a gas or vapor form. The low-pressure vaporflows to the compressor and the process repeats.
As shown in Figure 1.1, the vapor compression refrigeration system consists offour components that perform the four steps of the refrigeration cycle. Thecompressor raises the pressure of the initially low-pressure refrigerant gas. Thecondenser is a heat exchanger that cools the high-pressure gas so that it changesphase to liquid. The expansion valve controls the pressure ratio, and thus flowrate, between the high- and low-pressure regions of the system. The evaporator isa heat exchanger that heats the low-pressure liquid, causing it to change phasefrom liquid to vapor (gas).Thermodynamically, the most common representation of the basicrefrigeration cycle is made utilizing a pressureenthalpy chart as shown inFigure 1.2. For each refrigerant, the phase-change line represents the conditionsof pressure and total heat content (enthalpy) at which it changes from liquid to gasand vice versa. Thus each of the steps of the vapor compression cycle can easilybe plotted to demonstrate the actual thermodynamic processes at work, as shownin Figure 1.3.Point 1 represents the conditions entering the compressor. Compression ofthe gas raises its pressure from P1 to P2. Thus the work that is done by thecompressor adds heat to the refrigerant, raising its temperature and slightlyincreasing its heat content. Point 2 represents the condition of the refrigerant
leaving the compressor and entering the condenser. In the condenser, the gas iscooled, reducing its enthalpy from h2 to h3.Point 3 to point 4 represents the pressure reduction that occurs in theexpansion process. Due to a small percentage of the liquid evaporating because ofthe pressure reduction, the temperature and enthalpy of the remaining liquid is alsoreduced slightly. Point 4, then, represents the condition entering the evaporator.Point 4 to point 1 represents the heat gain by the liquid, increasing its enthalpyfrom h4 to h1, completed by the phase change from liquid to gas at point 1.
For any refrigerant whose properties are known, a pressure-enthalpy chartcan be constructed and the performance of a vapor compression cycle analyzedby establishing the high and low pressures for the system. (Note that Figure 1.3represents an ideal cycle and in actual practice there are various departuresdictated by second-law inefficiencies.)
1.1.2. RefrigerantsAny substance that absorbs heat may be termed a refrigerant. Secondaryrefrigerants, such as water or brine, absorb heat but do not undergo a phasechange in the process. Primary refrigerants, then, are those substances thatpossess the chemical, physical, and thermodynamic properties that permit theirefficient use in the typical vapor compression cycle.In the vapor compression cycle, a refrigerant must satisfy several (andsometimes conflicting) requirements:1. The refrigerant must be chemically stable in both the liquid and vaporstate.2. Refrigerants for HVAC applications must be nonflammable and havelow toxicity.3. Finally, the thermodynamic properties of the refrigerant must meet thetemperature and pressure ranges required for the application.Early refrigerants, developed in the 1920s and 1930s, used in HVAC applicationswere predominately chemical compounds made up of chlorofluorocarbons(CFCs) such as R-11, R-12, and R-503. While stable and efficient in the range oftemperatures and pressures required for HVAC use, any escaped refrigerant gaswas found to be long-lived in the atmosphere. In the lower atmosphere, the CFCmolecules absorb infrared radiation and, thus, contribute to atmosphericwarming. Once in the upper atmosphere, the CFC molecule breaks down torelease chlorine that destroys ozone and, consequently, damages the atmosphericozone layer that protects the earth from excess UV radiation.The manufacture of CFC refrigerants in the United States and most otherindustrialized nations was eliminated by international agreement in 1996. Whilethere is still refrigeration equipment in use utilizing CFC refrigerants, no newequipment using these refrigerants is now available in the United States.Researchers found that by modifying the chemical compound of CFCs bysubstituting a hydrogen atom for one or more of the chlorine or fluorine atomsresulted in a significant reduction in the life of the molecule and, thus, reduced thenegative environmental impact it may have. These new compounds, calledhydrochlorofluorocarbons (HCFCs), are currently used in HVAC refrigerationsystems as R-22 and R-123.While HCFCs have reduced the potential environmental damage byrefrigerants released into the atmosphere, the potential for damage has not been totally eliminated. Again, under international agreement, this class of refrigerantsis slated for phaseout for new equipment installations by 20102020, withtotal halt to manufacturing and importing mandated by 2030, as summarized inTable 1.1.A third class of refrigerants, called hydrofluorocarbons (HFCs), are not regulated by international treaty and are considered, at least for the interim, to be the most environmentally benign compounds and are now widely used in HVAC refrigeration systems.HVAC refrigeration equipment is currently undergoing transition in the use of refrigerants. R-22 has been the workhorse for positive displacement compressor systems in HVAC applications. The leading replacements for R-22 in new equipment are R-410A and, to a lesser extent, R-407C, both of which are blends of HFC compounds.R-134A, an HFC refrigerant, is utilized in positive-pressure compressor systems. At least one manufacturer continues to offer negative-pressure centrifugal compressor water chillers using R-123 (an HCFC), but it is anticipated that, by 2010, the manufacture of new negative pressure chillers usingHCFCs will be terminated. These same manufacturers already market a positivepressure centrifugal compressor systems using R-134A (though one manufacturer currently limits sales to outside of the United States since many countries, particularly in Europe, have accelerated the phaseout of HCFCs).
Based on the average 2025 year life for a water chiller (see Chap. 8) and the HCFC refrigerant phaseout schedule summarized in Table 1.1, owners should avoid purchasing any new chiller using R-22. After 2005, owners should avoid purchasing new chillers using R-123.ASHRAE Standard 34-1989 classifies refrigerants according to their toxicity (A nontoxic and B evidence of toxicity identified) and flammability(1 no flame progation, 2 low flammability, and 3 high flammability).Thus, all refrigerants fall within one of the six safety groups, A1, A2, A3, B1, B2, or B3. For HVAC refrigeration systems, only A1 refrigerants should be considered. Table 1.2 lists the safety group classifications for refrigerants commonly used in HVAC applications.
1.2. CHILLED WATER FOR HVAC APPLICATIONSThe basic vapor compression cycle, when applied directly to the job of building cooling, is referred to as a direct-expansion (DX) refrigeration system. This reference comes from the fact that the building indoor air that is to be cooled passes directly over the refrigerant evaporator without a secondary refrigerant being utilized. While these cooling systems are widely use in residential, commercial, and industrial applications, they have application, capacity, and performance limitations that reduce their use in larger, more complex HVAC applications. For these applications, the use of chilled water systems is dictated.Typical applications for chilled water systems include large buildings (offices, laboratories, etc.) or multibuilding campuses where it is desirable to provide cooling from a central facility.As shown in Figure 1.4, the typical water-cooled HVAC system has three heat transfer loops:Loop 1 Cold air is distributed by one or more air-handling units to the spaces within the building. Sensible heat gains, including heat from temperature-driven transmission through the building envelope; direct solar radiation through windows; infiltration; and internal heat from people, lights, and equipment, are absorbed by the cold air, raising its temperature. Latent heat gains, moisture added to the space by air infiltration, people, and equipment, is also absorbed by the cold air, raising its specific humidity. The resulting space temperature and humidity condition is an exact balance between the sensible and latent heat gains and capability of the entering cold air to absorb those heat gains.
Loop 2 The distributed air is returned to the air handling unit, mixed with the required quantity of outdoor air for ventilation, and then directed over the cooling coil where chilled water is used to extract heat from the air, reducing both its temperature and moisture content so it can be distributed once again to the space.As the chilled water passes through the cooling