1、24.1CHAPTER 24DESICCANT DEHUMIDIFICATION AND PRESSURE-DRYING EQUIPMENTMethods of Dehumidification. 24.1DESICCANT DEHUMIDIFICATION . 24.2Liquid-Desiccant Equipment . 24.3Solid-Sorption Equipment 24.4Rotary Solid-Desiccant Dehumidifiers 24.4Equipment Operating Recommendations. 24.7Applications for Atm
2、ospheric-Pressure Dehumidification 24.8DESICCANT DRYING AT ELEVATED PRESSURE . 24.10Equipment 24.10Applications . 24.11EHUMIDIFICATION is the removal of water vapor from air,Dgases, or other fluids. There is no pressure limitation in thisdefinition, and sorption dehumidification equipment has beende
3、signed and operated successfully for system pressures rangingfrom subatmospheric to as high as 6000 psi. In common practice,dehumidification usually refers to equipment operating at essen-tially atmospheric pressures and built to standards similar to othertypes of air-handling equipment. For drying
4、gases under pressure, orliquids, the term dryer or dehydrator is normally used.This chapter mainly covers equipment and systems that dehu-midify air rather than those that dry other gases or liquids. Both liq-uid and solid desiccants are used; they either adsorb water on thedesiccants surface (adsor
5、ption) or chemically combine with water(absorption).Nonregenerative equipment uses hygroscopic salts such as cal-cium chloride, urea, or sodium chloride. Regenerative systems usu-ally use a form of silica or alumina gel; activated alumina; molecularsieves; or lithium chloride, calcium chloride, or g
6、lycol solution. Inregenerative equipment, the water removal mechanism is reversible.The choice of desiccant depends on installation requirements, equip-ment design, and chemical compatibility with the gas to be treated orimpurities in the gas. Chapter 32 of the 2009 ASHRAE HandbookFundamentals has m
7、ore information on desiccant materials and howthey operate.Some applications of desiccant dehumidification includeKeeping buildings and HVAC systems dry to prevent mold growthLowering relative humidity to facilitate manufacturing and han-dling of hygroscopic materialsLowering the dew point to preven
8、t condensation on productsmanufactured in low-temperature processesProviding protective atmospheres for heat treatment of metalsControlling humidity in warehouses and caves used for storagePreserving ships, aircraft, and industrial equipment that wouldotherwise deteriorateMaintaining a dry atmospher
9、e in a closed space or container, suchas the cargo hold of a ship or numerous static applicationsEliminating condensation and subsequent corrosionDrying air to speed drying of heat-sensitive products, such ascandy, seeds, and photographic filmDrying natural gasDrying gases that are be liquefiedDryin
10、g instrument and plant airDrying process and industrial gasesDehydration of liquidsFrost-free cooling for low-temperature process areas such asbrewery fermenting, aging, filtering, and storage cellars; blastfreezers; and refrigerated warehousesFrost-free dehumidification for processes that require a
11、ir at a sub-freezing dew-point humidityThis chapter covers (1) the types of dehumidification equipmentfor liquid and solid desiccants, including high-pressure equipment;(2) performance curves; (3) variables of operation; and (4) some typ-ical applications. Using desiccants to dry refrigerants is add
12、ressed inChapter 7 of the 2010 ASHRAE HandbookRefrigeration.METHODS OF DEHUMIDIFICATIONAir may be dehumidified by (1) cooling it or increasing its pres-sure, reducing its capacity to hold moisture, or (2) removing mois-ture by attracting the water vapor with a liquid or solid desiccant.Frequently, s
13、ystems use a combination of these methods to maxi-mize operating efficiency and minimize installed cost.Figure 1 illustrates three methods to dehumidify with desiccantmaterials or equipment. Air in the condition at Point A is dehumid-ified and cooled to Point B. In a liquid-desiccant unit, air is si
14、mul-taneously cooled and dehumidified directly from Point A to Point B.In a solid-desiccant unit, this process can be completed by precool-ing and dehumidifying from Point A to Point C, then desiccatingfrom Point C to Point E, and finally cooling to Point B. It could alsobe done with solid-desiccant
15、 equipment by dehumidifying fromPoint A to Point D and then cooling from Point D to Point B.CompressionCompressing air reduces its capacity to hold moisture. The result-ing condensation reduces the airs moisture content in absoluteterms, but produces a saturated condition: 100% relative humidity ate
16、levated pressure. In atmospheric-pressure applications, this methodis too expensive, but is worthwhile in pressure systems such asThe preparation of this chapter is assigned to TC 8.12, Desiccant Dehumid-ification Equipment and Components. Fig. 1 Methods of Dehumidification24.2 2012 ASHRAE HandbookH
17、VAC Systems and Equipment instrument air. Other dehumidification equipment, such as coolers ordesiccant dehumidifiers, often follows the compressor to avoid prob-lems associated with high relative humidity in compressed-air lines.CoolingRefrigerating air below its dew point is the most common method
18、of dehumidification. This is advantageous when the gas is compara-tively warm, has a high moisture content, and the desired outlet dewpoint is above 40F. Frequently, refrigeration is combined with des-iccant dehumidification to obtain an extremely low dew point at min-imum cost.Liquid DesiccantsLiqu
19、id-desiccant conditioners (absorbers) contact the air with aliquid desiccant, such as lithium chloride or glycol solution (Figures2 and 3). The water vapor pressure of the solution is a function of itstemperature and concentration. Higher concentrations and lowertemperatures result in lower water va
20、por pressures.A simple way to show this relationship is to graph the humidityratio of air in equilibrium with a liquid desiccant as a function of itsconcentration and temperature. Figure 4 presents this relationshipfor lithium chloride/water solutions in equilibrium with air at14.7 psi. The graph ha
21、s the same general shape as a psychrometricchart, with the relative humidity lines replaced by desiccant concen-tration lines.Liquid-desiccant conditioners typically have high contact effi-ciency, so air leaves the conditioner at a temperature and humidityratio very close to the entering temperature
22、 and equilibrium humidityratio of the desiccant. When the conditioner is dehumidifying, mois-ture absorbed from the conditioned airstream dilutes the desiccantsolution. The diluted solution is reconcentrated in the regenerator,where it is heated to elevate its water vapor pressure and equilibriumhum
23、idity ratio. A second airstream, usually outside air, contacts theheated solution in the regenerator; water evaporates from the desic-cant solution into the air, and the solution is reconcentrated. Desic-cant solution is continuously recirculated between the conditionerand regenerator to complete th
24、e cycle.Liquid desiccants are typically a very effective antifreeze. As aresult, liquid-desiccant conditioners can continuously deliver air atsubfreezing temperatures without frosting or freezing problems.Lithium chloride/water solution, for example, has a eutectic point of90F; liquid-desiccant cond
25、itioners using this solution can cool airto temperatures as low as 65F.Solid SorptionSolid sorption passes air through a bed of granular desiccant orthrough a structured packing impregnated with desiccant. Humid airpasses through the desiccant, which when active has a vapor pres-sure below that of t
26、he humid air. This vapor pressure differentialdrives water vapor from the air onto the desiccant. After becomingloaded with moisture, the desiccant is reactivated (dried out) byheating, which raises the vapor pressure of the material above that ofthe surrounding air. With the vapor pressure differen
27、tial reversed,water vapor moves from the desiccant to a second airstream calledthe reactivation air, which carries moisture away from the equip-ment.DESICCANT DEHUMIDIFICATIONBoth liquid and solid desiccants may be used in equipment de-signed for drying air and gases at atmospheric or elevated press
28、ures.Regardless of pressure levels, basic principles remain the same, andonly the desiccant towers or chambers require special design con-sideration.Desiccant capacity and actual dew-point performance depend onthe specific equipment used, characteristics of the various desic-cants, initial temperatu
29、re and moisture content of the gas to be dried,reactivation methods, etc. Factory-assembled units are available upto a capacity of about 80,000 cfm. Greater capacities can be ob-tained with field-erected units.Fig. 2 Flow Diagram for Liquid-Absorbent DehumidifierFig. 3 Flow Diagram for Liquid-Absorb
30、ent Unit with Extended Surface Air Contact MediumFig. 4 Lithium Chloride EquilibriumDesiccant Dehumidification and Pressure-Drying Equipment 24.3LIQUID-DESICCANT EQUIPMENTLiquid-desiccant dehumidifiers are shown in Figures 2 and 3. InFigure 2, liquid desiccant is distributed onto a cooling coil, whi
31、chacts as both a contact surface and a means of removing heat releasedwhen the desiccant absorbs moisture from the air. In Figure 3, liquiddesiccant is distributed onto an extended heat and mass transfer sur-face (a packing material similar to that used in cooling towers andchemical reactors). The p
32、acking provides a great deal of surface forair to contact the liquid desiccant, and the heat of absorption isremoved from the liquid by a heat exchanger outside the airstream.Air can be passed through the contact surface vertically or horizon-tally to suit the best arrangement of air system equipmen
33、t.Depending on the air and desiccant solution inlet conditions, aircan be simultaneously cooled and dehumidified, heated and dehu-midified, heated and humidified, or cooled and humidified. When theenthalpy of the air is to be increased in the conditioner unit, heat mustbe added either by preheating
34、the air before it enters the conditioneror by heating the desiccant solution with a second heat exchanger.When the air is to be humidified, makeup water is automaticallyadded to the desiccant solution to keep it at the desired concentration.Moisture is absorbed from or desorbed into the air because
35、of thedifference in water vapor pressure between the air and the desiccantsolution. For a given solution temperature, a higher solution concen-tration results in a lower water vapor pressure. For a given solutionconcentration, a lower solution temperature results in a lower watervapor pressure. By c
36、ontrolling the temperature and concentration ofthe desiccant solution, the conditioner unit can deliver air at a pre-cisely controlled temperature and humidity regardless of inlet airconditions. The unit dehumidifies the air during humid weather andhumidifies it during dry weather. Thus, liquid-desi
37、ccant conditionerscan accurately control humidity without face-and-bypass dampers orafter-humidifiers. System performance can easily be changed as pro-cess drying requirements change by altering temperature, concentra-tion, or both to meet the new requirements. Solution strength can bemonitored whil
38、e in operation and can easily be adjusted to compen-sate for aging. In most cases, the solution retains its effectiveness forthe life of the equipment, assuming that proper filtration is main-tained.Heat RemovalWhen a liquid desiccant absorbs moisture, heat is generated.This heat of absorption consi
39、sts of the latent heat of condensation ofwater vapor at the desiccant temperature and the heat of solution(heat of mixing) of the condensed water and the desiccant. The heatof mixing is a function of the equilibrium relative humidity of thedesiccant: a lower equilibrium relative humidity produces a
40、greaterheat of mixing.The total heat that must be absorbed by the desiccant solutionconsists of the (1) heat of absorption, (2) sensible heat associatedwith reducing the dry-bulb temperature of the air, and (3) residualheat carried to the conditioner by the warm, concentrated desiccantreturning from
41、 the regenerator unit. This total heat is removed bycooling the desiccant solution in the conditioner heat exchanger(Figure 3). Any coolant can be used, including cooling tower water,groundwater, seawater, chilled water or brine, and direct-expansionor flooded refrigerants.Regenerator residual heat,
42、 generally called regenerator heatdumpback, can be substantially reduced by using a liquid-to-liquidheat exchanger to precool the warm, concentrated desiccant trans-ferred to the conditioner using the cool, dilute desiccant transferredfrom the conditioner to the regenerator. This also improves the t
43、her-mal efficiency of the system, typically reducing coolant and heatinput by 10 to 15%.RegenerationWhen the conditioner is dehumidifying, water is automaticallyremoved from the liquid desiccant to maintain the desiccant at theproper concentration. Removal takes place in a separate regenerator.A sma
44、ll sidestream of the desiccant solution, typically 8% or less ofthe flow to the conditioner packing, is transferred to the regeneratorunit. In the regenerator, a separate pump continuously circulates thedesiccant solution through a heat exchanger and distributes it overthe packed bed contactor surfa
45、ce. The heat exchanger heats the des-iccant solution with low-pressure steam or hot water so that its watervapor pressure is substantially higher than that of the outside air.Outside air is passed through the packing, and water evaporates intoit from the desiccant solution, concentrating the solutio
46、n. The hot,moist air from the regenerator is discharged to the outdoors. A side-stream of concentrated solution is transferred to the conditioner toreplace the sidestream of weak solution transferred from the condi-tioner and completes the cycle.The regenerators water removal capacity is controlled
47、to matchthe moisture load handled by the conditioner. This is accomplishedby regulating heat flow to the regenerator heat exchanger to main-tain a constant desiccant solution concentration. This is most com-monly done by maintaining a constant solution level in the systemwith a level controller, but
48、 specific-gravity or boiling-point control-lers are used under some circumstances. Regenerator heat input isregulated to match the instantaneous water removal requirements,so no heat input is required if there is no moisture load on the con-ditioner. When the conditioner is used to humidify the air,
49、 the regen-erator fan and desiccant solution pump are typically stopped to saveenergy.Because the conditioner and regenerator are separate units, theycan be in different locations and connected by piping. This can sub-stantially lower ductwork cost and required mechanical space.Commonly, a single regenerator services several conditioner units(Figure 5). In the simplest control arrangement, concentrated desic-cant solution is metered to each conditioner at a fixed rate. Thereturn flow of weak solution from each conditioner is regulated tomaintain a constant operating level in the
