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 40 MPa. 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 ga
4、ses 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 (adsorpt
5、ion) 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 gly
6、col 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 mor
7、e 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 prevent
8、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 atmosphere
9、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 liquefiedDrying
10、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 air
11、 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 addre
12、ssed 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, sys
13、tems 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 simu
14、l-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 e
15、quipment 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 atele
16、vated 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 HandbookHVA
17、C Systems and Equipment (SI)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 meth
18、odof dehumidification. This is advantageous when the gas is compara-tively warm, has a high moisture content, and the desired outlet dewpoint is above 5C. Frequently, refrigeration is combined with des-iccant dehumidification to obtain an extremely low dew point at min-imum cost.Liquid DesiccantsLiq
19、uid-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 v
20、apor 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 at101.325 kPa. The grap
21、h has the same general shape as a psychro-metric chart, with the relative humidity lines replaced by desiccantconcentration 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 tempera
22、ture 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 equilibriu
23、mhumidity 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 complet
24、e the 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 of68C; liquid-desiccant
25、conditioners using this solution can cool airto temperatures as low as 54C.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
26、of the 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 diff
27、erential 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 p
28、ressures.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 tempe
29、rature and moisture content of the gas to be dried,reactivation methods, etc. Factory-assembled units are available upto a capacity of about 38 m3/s. Greater capacities can be obtainedwith field-erected units.Fig. 2 Flow Diagram for Liquid-Absorbent DehumidifierFig. 3 Flow Diagram for Liquid-Absorbe
30、nt 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, whic
31、hacts 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 pa
32、cking 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 equipment
33、.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 t
34、he 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 o
35、f 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 co
36、ntrolling 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-desic
37、cant 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 while
38、 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 consis
39、ts 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 g
40、reaterheat 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 th
43、er-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 smal
44、l 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 surfac
45、e. 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 solution
46、. 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 t
47、o 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 c
