1、18.1CHAPTER 18ABSORPTION EQUIPMENT Water/Lithium Bromide Absorption Technology 18.1Ammonia/Water Absorption Equipment . 18.7Special Applications and Emerging Products. 18.9Information Sources 18.10HIS chapter surveys and summarizes the types of absorptionTequipment that are currently manufactured an
2、d/or commonlyencountered. The equipment can be broadly categorized by whetherit uses water or ammonia as refrigerant. The primary products in thewater refrigerant category are large commercial chillers, which uselithium bromide (LiBr) as absorbent. There are three primary prod-ucts in the ammonia re
3、frigerant category: (1) domestic refrigerators,(2) residential chillers, and (3) large industrial refrigeration units.This chapter focuses on hardware (i.e., cycle implementation),not on cycle thermodynamics. Cycle thermodynamic descriptionsand calculation procedures, along with a tabulation of the
4、types ofabsorption working pairs and a glossary, are presented in Chapter 2of the 2009 ASHRAE HandbookFundamentals.Absorption units have two major advantages: (1) they are acti-vated by heat, and (2) no mechanical vapor compression isrequired. They also do not use atmosphere-harming halogenatedrefri
5、gerants, and reduce summer electric peak demand. No lubri-cants, which are known to degrade heat and mass transfer, arerequired. The various equipment can be direct-fired by combus-tion of fuel, directly heated by various waste fluids, or heated bysteam or hot water (from either direct combustion or
6、 from hotwaste fluids). Figure 1 illustrates the similarities between absorp-tion and vapor compression systems.With natural gas firing, absorption chilling units level the year-round demand for natural gas. From an energy conservationperspective, the combination of a prime mover plus a waste-heat-p
7、owered absorption unit provides unparalleled overall efficiency.WATER/LITHIUM BROMIDE ABSORPTION TECHNOLOGYComponents and TerminologyAbsorption equipment using water as the refrigerant and lith-ium bromide as the absorbent is classified by the method of heatinput to the primary generator (firing met
8、hod) and whether theabsorption cycle is single- or multiple-effect.Machines using steam or hot liquids as a heat source areindirect-fired, and those using direct combustion of fossil fuels asa heat source are direct-fired. Machines using hot waste gases asa heat source are also classified as indirec
9、t-fired, but are oftenreferred to as heat recovery chillers.Solution recuperative heat exchangers, also referred to aseconomizers, are typically shell-and-tube or plate heat exchangers.They transfer heat between hot and cold absorbent solution streams,thus recycling energy. The material of construct
10、ion is mild steel orstainless steel.Condensate subcooling heat exchangers, a variation of solu-tion heat exchangers, are used on steam-fired, double-effectmachines and on some single-effect, steam-fired machines. Theseheat recovery exchangers use the condensed steam to add heat tothe solution enteri
11、ng the generator.Indirect-fired generators are usually shell-and-tube, with theabsorbent solution either flooded or sprayed outside the tubes, and theheat source (steam or hot fluid) inside the tubes. The absorbent solu-tion boils outside the tubes, and the resulting intermediate- or strong-concentr
12、ation absorbent solution flows from the generator through anoutlet pipe. The refrigerant vapor evolved passes through a vapor/liq-uid separator consisting of baffles, eliminators, and low-velocityregions and then flows to the condenser section. Ferrous materials areused for absorbent containment; co
13、pper, copper-nickel alloys, stain-less steel, or titanium are used for the tube bundle.Direct-fired generators consist of a fire-tube section, a flue-tube section, and a vapor/liquid separation section. The fire tube istypically a double-walled vessel with an inner cavity large enoughto accommodate
14、a radiant or open-flame fuel oil or natural gasburner. Dilute solution flows in the annulus between the inner andouter vessel walls and is heated by contact with the inner vesselwall. The flue tube is typically a tube or plate heat exchanger con-nected directly to the fire tube.Heated solution from
15、the fire-tube section flows on one side ofthe heat exchanger, and flue gases flow on the other side. Hot fluegases further heat the absorbent solution and cause it to boil. Fluegases leave the generator, and the partially concentrated absorbentsolution and refrigerant vapor mixture pass to a vapor/l
16、iquid sepa-rator chamber. This chamber separates the absorbent solution fromthe refrigerant vapor. Materials of construction are mild steel forthe absorbent containment parts and mild steel or stainless steel forthe flue gas heat exchanger.Secondary or second-stage generators are used only in double
17、- ormultistage machines. They are both a generator on the low-pressureside and a condenser on the high-pressure side. They are usually of theshell-and-tube type and operate similarly to indirect-fired generatorsThe preparation of this chapter is assigned to TC 8.3, Absorption and Heat-Operated Machi
18、nes.Fig. 1 Similarities Between Absorption and Vapor Compres-sion SystemsFig. 1 Similarities Between Absorption and Vapor Compression Systems18.2 2010 ASHRAE HandbookRefrigeration (SI)of single-effect machines. The heat source, which is inside the tubes, ishigh-temperature refrigerant vapor from the
19、 primary generator shell.Materials of construction are mild steel for absorbent containment andusually copper-nickel alloys or stainless steel for the tubes. Dropleteliminators are typically stainless steel.Evaporators are heat exchangers, usually shell-and-tube, overwhich liquid refrigerant is drip
20、ped or sprayed and evaporated. Liq-uid to be cooled passes through the inside of the tubes. Evaporatortube bundles are usually copper or a copper-nickel alloy. Refrigerantcontainment parts are mild steel. Mist eliminators and drain pans aretypically stainless steel.Absorbers are tube bundles over wh
21、ich strong absorbent solu-tion is sprayed or dripped in the presence of refrigerant vapor. Therefrigerant vapor is absorbed into the absorbent solution, thusreleasing heat of dilution and heat of condensation. This heat isremoved by cooling water that flows through the tubes. Weak absor-bent solutio
22、n leaves the bottom of the absorber tube bundle. Mate-rials of construction are mild steel for the absorbent containmentparts and copper or copper-nickel alloys for the tube bundle.Condensers are tube bundles located in the refrigerant vaporspace near the generator of a single-effect machine or the
23、second-stage generator of a double-effect machine. The water-cooled tubebundle condenses refrigerant from the generator onto tube surfaces.Materials of construction are mild steel, stainless steel, or othercorrosion-resistant materials for the refrigerant containment partsand copper for the tube bun
24、dle. For special waters, the condensertubes can be copper-nickel, which derates the performance of theunit.High-stage condensers are found only in double-effectmachines. This type of condenser is typically the inside of the tubesof the second-stage generator. Refrigerant vapor from the first-stage g
25、enerator condenses inside the tubes, and the resulting heatis used to concentrate absorbent solution in the shell of the second-stage generator when heated by the outside surface of the tubes.Pumps move absorbent solution and liquid refrigerant in theabsorption machine. Pumps can be configured as in
26、dividual (onemotor, one impeller, one fluid stream) or combined (one motor,multiple impellers, multiple fluid streams). The motors and pumpsare hermetic or semihermetic. Motors are cooled and bearings lubri-cated either by the fluid being pumped or by a filtered supply of liq-uid refrigerant. Impell
27、ers are typically brass, cast iron, or stainlesssteel; volutes are steel or impregnated cast iron, and bearings arebabbitt-impregnated carbon journal bearings.Refrigerant pumps (when used) recirculate liquid refrigerantfrom the refrigerant sump at the bottom of the evaporator to theevaporator tube b
28、undle in order to effectively wet the outside sur-face and enhance heat transfer.Dilute solution pumps take dilute solution from the absorbersump and pump it to the generator.Absorber spray pumps recirculate absorbent solution over theabsorber tube bundle to ensure adequate wetting of the absorber s
29、ur-faces. These pumps are not found in all equipment designs. Somedesigns use a jet eductor for inducing concentrated solution flow tothe absorber sprays. Another design uses drip distributors fed by grav-ity and the pressure difference between the generator and absorber.Purge systems are required o
30、n lithium bromide absorptionequipment to remove noncondensables (air) that leak into themachine or hydrogen (a product of corrosion) that is produced dur-ing equipment operation. Even in small amounts, noncondensablegases can reduce chilling capacity and even lead to solution crystal-lization. Purge
31、 systems for larger sizes (above 359 kW of refrigera-tion) typically consist of these components: Vapor pickup tube(s), usually located at the bottom of largeabsorber tube bundlesNoncondensable separation and storage tank(s), located in theabsorber tube bundle or external to the absorber/evaporator
32、vesselA vacuum pump or valving system using solution pump pressureto periodically remove noncondensables collected in the storagetankSome variations include jet pumps (eductors), powered bypumped absorbent solution and placed downstream of the vaporpickup tubes to increase the volume of sampled vapo
33、r, and water-cooled absorbent chambers to remove water vapor from the purgedgas stream.Because of their size, smaller units have fewer leaks, which canbe more easily detected during manufacture. As a result, small unitsmay use variations of solution drip and entrapped vapor bubblepumps plus purge ga
34、s accumulator chambers.Palladium cells, found in large direct-fired and small indirect-fired machines, continuously remove the small amount of hydro-gen gas that is produced by corrosion. These devices operate on theprinciple that thin membranes of heated palladium are permeableto hydrogen gas only.
35、Corrosion inhibitors, typically lithium chromate, lithiumnitrate, or lithium molybdate, protect machine internal parts fromthe corrosive effects of the absorbent solution in the presence of air.Each of these chemicals is used as a part of a corrosion control sys-tem. Acceptable levels of contaminant
36、s and the correct solution pHrange must be present for these inhibitors to work properly. SolutionpH is controlled by adding lithium hydroxide or hydrobromic acid.Performance additives are used in most lithium bromide equip-ment to achieve design performance. The heat and mass transfercoefficients f
37、or the simultaneous absorption of water vapor andcooling of lithium bromide solution have relatively low values thatmust be enhanced. A typical additive is one of the octyl alcohols.Single-Effect Lithium Bromide ChillersFigure 2 is a schematic of a commercially available single-effect,indirect-fired
38、 liquid chiller, showing one of several configurationsof the major components. Table 1 lists typical characteristics of thischiller. During operation, heat is supplied to tubes of the generatorin the form of a hot fluid or steam, causing dilute absorbent solutionon the outside of the tubes to boil.
39、This desorbed refrigerant vaporFig. 2 Two-Shell Lithium Bromide Cycle Water ChillerFig. 2 Two-Shell Lithium Bromide Cycle Water ChillerAbsorption Equipment 18.3(water vapor) flows through eliminators to the condenser, where itis condensed on the outside of tubes that are cooled by a flow ofwater fro
40、m a heat sink (usually a cooling tower). Both boiling andcondensing occur in a vessel that has a common vapor space at apressure of about 6 kPa.The condensed refrigerant passes through an orifice or liquid trapin the bottom of the condenser and enters the evaporator, in whichliquid refrigerant boils
41、 as it contacts the outside surface of tubes thatcontain a flow of water from the heat load. In this process, water inthe tubes cools as it releases the heat required to boil the refrigerant.Refrigerant that does not boil collects at the bottom of the evapora-tor, flows to a refrigerant pump, is pum
42、ped to a distribution systemlocated above the evaporator tube bundle, and is sprayed over theevaporator tubes again.The dilute (weak in absorbing power) absorbent solution thatenters the generator increases in concentration (percentage of sor-bent in the water) as it boils and releases water vapor.
43、The resultingstrong absorbent solution leaves the generator and flows throughone side of a solution heat exchanger, where it cools as it heats astream of weak absorbent solution passing through the other side ofthe solution heat exchanger on its way to the generator. Thisincreases the machines effic
44、iency by reducing the amount of heatfrom the primary heat source that must be added to the weak solu-tion before it begins to boil in the generator.The cooled, strong absorbent solution then flows (in somedesigns through a jet eductor or solution spray pumps) to a solutiondistribution system located
45、 above the absorber tubes and drips or issprayed over the outside surface of the absorber tubes. The absorberand evaporator share a common vapor space at a pressure of about0.7 kPa. This allows refrigerant vapor, which is evaporated in theevaporator, to be readily absorbed into the absorbent solutio
46、n flow-ing over the absorber tubes. This absorption process releases heat ofcondensation and heat of dilution, which are removed by coolingwater flowing through the absorber tubes. The resulting weak absor-bent solution flows off the absorber tubes and then to the absorbersump and solution pump. The
47、 pump and piping convey the weakabsorbent solution to the heat exchanger, where it accepts heat fromthe strong absorbent solution returning from the generator. Fromthere, the weak solution flows into the generator, thus completingthe cycle.These machines are typically fired with low-pressure steam o
48、rmedium-temperature liquids. Several manufacturers have machineswith capacities ranging from 180 to 5840 kW of refrigeration.Machines of 18 to 35 kW capacities are also available from interna-tional sources.Typical coefficients of performance (COPs) for large single-effect machines at Air Conditioni
49、ng and Refrigeration Institute(ARI) rating conditions are 0.7 to 0.8.Single-Effect Heat TransformersFigure 3 shows a schematic of a single-effect heat transformer (orType 2 heat pump). All major components are similar to the single-effect, indirect-fired liquid chiller. However, the absorber/evaporatoris located above the desorber (generator)/condenser because of thehigher pressure level of the absorber and evaporator compared to thedesorber/condenser pair, which is the opposite of a chiller.High-pressure refrigerant liquid enters the top of the evaporator,and heat released from a waste
