Ebook Handbook of air conditioning and refrigeration: Part 2

CHAPTER 14 REFRIGERATION ABSORPTION 14.1 ABSORPTION SYSTEMS 14.1 Type of Absorption System 14.1 Historical Development 14.2 Cost Analysis 14.2 Applications 14.3 14.2 PROPERTIES OF AQUEOUS LITHIUM-BROMIDE SOLUTION 14.3 Mass Balance in Solution 14.3 Vapor Pressure 14.3 Equilibrium Chart 14.4 Enthalpy-Concentration Diagram 14.5 14.3 DOUBLE-EFFECT, DIRECT-FIRED ABSORPTION CHILLERS 14.6 System Description 14.6 Air Purge Unit 14.8 Series Flow, Parallel Flow, and Reverse Parallel Flow 14.8 Flow of Solution and Refrigerant 14.9 14.4 PERFORMANCE OF DOUBLE-EFFECT, DIRECT-FIRED ABSORPTION CHILLER 14.11 Mass Flow Rate of Refrigerant and Solution 14.11 Thermal Analysis 14.12 Coefficient of Performance 14.14 14.5 ABSORPTION CHILLER CONTROLS 14.16 Capacity Control and Part-Load Operation 14.16 Crystallization and Controls 14.17 Cooling Water Temperature Control 14.17 SYSTEMS: Safety and Interlocking Controls 14.18 Monitoring and Diagnostics 14.18 14.6 OPERATING CHARACTERISTICS AND DESIGN CONSIDERATIONS 14.18 Difference between Absorption and Centrifugal Chillers 14.18 Evaporating Temperature 14.19 Cooling Water Entering Temperature 14.19 Heat Removed from Absorber and Condenser 14.19 Condensing Temperature 14.19 Corrosion Control 14.20 Rated Condition of Absorption Chiller 14.20 Minimum Performance 14.20 14.7 ABSORPTION CHILLER-HEATERS 14.20 Heating Cycle 14.21 Actual Performance 14.22 14.8 ABSORPTION HEAT PUMPS 14.22 Functions of Absorption Heat Pump 14.22 Case Study: Series-Connected Absorption Heat Pump 14.22 14.9 ABSORPTION HEAT TRANSFORMER 14.24 System Description 14.24 Operating Characteristics 14.24 Coefficient of Performance 14.26 REFERENCES 14.26 14.1 ABSORPTION SYSTEMS Type of Absorption System Absorption systems use heat energy to produce refrigeration or heating and sometimes to elevate the temperature of the waste heat. Aqueous lithium bromide (LiBr) is often used to absorb the re-frigerant, the water vapor, and provides a higher coefficient of performance. Current absorption systems can be divided into the following categories: Absorption chillers use heat energy to provide refrigeration. Absorption chiller-heaters use heat energy to provide cooling or heating. 14.1 14.2 CHAPTER FOURTEEN Absorption heat pump extracts heat energy from the evaporator through the absorber, adds to the heat input in the generator, and releases them both to the hot water in the condenser for heating. Absorption heat transformers elevate the temperature of the waste heat to a value higher than any other input fluid stream supplied to the absorption heat transformer. Historical Development In the 1950s and 1960s, both centrifugal chillers driven by electric motors and absorption chillers using steam as heat input to provide summer cooling were widely used in central refrigeration plants. Steam was widely used because excess steam was available in summer in many central plants that used steam to provide winter heating, and because energy costs were of little concern. After the energy crisis in 1973, the price of natural gas and oil used to fuel steam boilers drasti-cally increased. The earliest single-stage, indirect-fired steam absorption chillers had a coefficient of performance (COP) of only 0.6 to 0.7. They required more energy and could not compete with electric centrifugal chillers. Many absorption chillers were replaced by centrifugal chillers in the late 1970s and 1980s. Because of the high investment required to build new power plants, electric utility companies added high-demand charges and raised cost-per-unit charges during peak usage periods. In recent years, double-effect, direct-fired absorption chillers have been developed in both Japan and the United States with a COP approximately equal to 1. Cost Analysis A cost analysis is often required. Aumann (1996) recommended that in addition to the chiller itself, the energy and the initial costs of the auxiliaries, such as condenser pumps, cooling tower, and tower fans, be included in the cost analysis because of the higher heat rejection in the absorption chiller. The auxiliary energy costs can be 30 percent higher for absorption chillers than for electric. Sun (1991) compared two designs for a project with a 1200-ton (4220-kW) design refrigeration load. One design is three 400-ton (1406-kW) electric centrifugal chillers, and the other is two electric centrifugal chillers plus one absorption chiller. The cost of gas for this project is $0.46 per therm in summer, from April through November, and $0.50 per therm in winter. One therm is equal to 100,000 Btu/h (29,300 W) of heat energy input. The cost of electricity is $0.126 per kWh during on-peak periods and $0.0433 per kW/h for the off-peak periods. The on-peak period is the 8-h weekday daytime determined by the electric utility. The additional electricity demand charge is $3.60 per kW. The centrifugal chiller uses 0.7 kW/ton (5.02 COP), and the additional energy use for the auxil-iaries for the centrifugal chiller is about 0.1 kW/ton higher than that for the absorption chiller. The double-effect, direct-fired absorption chiller uses 0.12 therm/ton. The comparison cost ( in dollars) of these two alternatives for the third 400-ton (1406-kW) chiller operated 8 h/day, 5 days/wk, 8 months for summer and 4 months for winter is as follows: Initial cost Chiller Cooling tower Demand Energy Electricity Summer Winter Absorption chiller 200,000 53,000 2126 7200 3598 Centrifugal chiller 80,000 48,000 11,491 36,700 18,337 REFRIGERATION SYSTEMS: ABSORPTION 14.3 Gas Summer Winter Total energy cost before tax Total energy cost after tax 22, 969 12,474 $48,417 $54,469 $66,528 $74,844 The absorption chiller has an annual energy cost saving of $20,375 and a simple payback period of about 6 years. Applications According to the Air Conditioning, Heating and Refrigeration News, April 14, 1997, the shipments of new absorption chillers from the manufacturers in the United States in 1996 numbered 579, whereas the shipments of centrifugal and screw chillers in 1996 numbered about 9200. Absorption chillers had a share of about 5 percent of the new large chiller market in the United States in the mid-1900s. Absorption chillers have the advantage of using gas and are therefore not affected by the high electric demand charge and high unit rate at on-peak hours. Most absorption chillers use water as refrigerant, and its ozone depletion potential is zero. Absorption chillers are advantageous to com-bine with electric chillers so that electric chillers will undertake the base load and the absorption chillers handle the load at on-peak hours. Although absorption chillers have a higher initial cost than centrifugal compressors, in many locations in the United States where the cost ratio of electric-ity to natural gas is favorable, installation of absorption chiller for use during on-peak hours or even normal operating hours is sometimes economically beneficial. Absorption chillers may have a sim-ple payback of several years. 14.2 PROPERTIES OF AQUEOUS LITHIUM-BROMIDE SOLUTION Mass Balance in Solution The composition of a solution is generally expressed by the mass fraction of its components. In a solution containing lithium bromide (LiBr) and water, X is used to indicate the mass fraction of lithium bromide, i.e., X 5 ml 1l mw (14.1) where ml 5 mass of lithium bromide in solution, lb (kg) mw 5 mass of water in solution, lb (kg) The mass fraction of water in solution is 1 2 X. If the mass fraction of LiBr in a solution is ex-pressed as a percentage, it is known as the concentration of LiBr. Vapor Pressure In general, the total pressure of a solution is equal to the sum of the vapor pressures of the solute and of the solvent. However, in the case of a lithium bromide–water solution, the vapor pressure of pure LiBr can be ignored because its value is much smaller than that of water. 14.4 CHAPTER FOURTEEN When LiBr is dissolved in water, the boiling point of the solution at a given pressure is raised. However, if the temperature of the solution remains constant, the dissolved LiBr reduces the vapor pressure of the solution. When a solution is saturated, equilibrium is established. The number of molecules across the interface from liquid to vapor per unit time is equal to the number of molecules from vapor into liquid. If the number of liquid molecules per unit volume is reduced due to the presence of a solute then the number of vapor molecules per unit volume is also reduced. Consequently, the vapor pres-sure of the solution is decreased. Equilibrium Chart The properties of an aqueous lithium bromide solution, including vapor pressure, temperature, and the mass fraction at equilibrium, may be illustrated on an equilibrium chart based on the Dühring plot, as shown in Fig. 14.1. The ordinate of the equilibrium chart is the saturated vapor pressure of water in log-scale millimeters of mercury absolute (mm Hg abs) and the corresponding saturated temperature (°F). The scale is plotted on an inclined line. The abscissa of the chart is the temperature of the solu-tion (°F). Mass fraction or concentration lines are inclined lines and are not parallel to each other. At the bottom of the concentration lines, there is a crystallization line or saturation line. If the temperature of a solution of constant mass fraction of LiBr drops below this line—or if the mass fraction of LiBr of a solution of constant temperature is higher than the saturated condition—the part of LiBr salt exceeding the saturated condition tends to form solid crystals. FIGURE 14.1 Equilibrium chart for aqueous lithium-bromide (LiBr) solution. (Source: Carrier Corporation. Reprinted with permission.) REFRIGERATION SYSTEMS: ABSORPTION 14.5 Enthalpy-Concentration Diagram When water is mixed with anhydrous lithium bromide at the same temperature to form a solution adiabatically, there is a significant increase in the temperature of the solution. If the mixing process is to be an isothermal process, i.e., if the temperature of the process is to be kept constant, then heat must be removed from the solution. Such a heat transfer per unit mass of solution is called the inte-gral heat of the solution Dhi, or heat of absorption, in Btu/lb (kJ/kg). Based on the common rule of thermodynamics, Dhi is negative. FIGURE 14.2 Enthalpy-concentration diagram for aqueous LiBr solution. (Source: ASHRAE Handbook 1989, Fundamentals, Reprinted with permission.) ... - tailieumienphi.vn