The Dehumidification Handbook
The Dehumidification HandbookThird EditionISBN 0-9717887-0-7Copyright 1989, 2002 and 2019 Munters Corporation
The Dehumidification HandbookThird EditionCopyright 1989, 2002 and 2019 Munters CorporationReuse Of This MaterialFor book reviews and personal educational and professional use, readersare encouraged to excerpt or photocopy and distribute any usefulportion of this handbook, provided that the source of the material isappropriately referenced and the Munters copyright is acknowledged.For electronic distribution or if the material will be included in anyprinted publication, Munters Corporation must provide writtenauthorization in advance of publication. For those uses, please contactMunters Corporation Marketing Department by mail at 79 MonroeStreet, Amesbury, MA 01913, or by email at dhinfo@munters.com.DisclaimerAlthough great care has been taken in the compilation and publicationof this book, no warranties either expressed or implied are given inconnection with the material. Neither the contributors nor the publishertake any responsibility whatsoever for any claims arising from its use.The entire risk of the use of this information is assumed by the user.However, to improve future editions, the publisher welcomes any and allcomments, corrections or suggestions from readers.
CONTRIBUTORSWe would like to express our gratitude to Lew G. Harriman III as editor of the first twoeditions of this book and for guiding this work with interest. Also, we would like to thankthe following original contributors and 1989 Editorial Advisory Board members for theirwork; Enno Abel, Nick Baranov, Bruce Bonner, Steven Brickley, Luiz Felipe de Carvalho,Douglas Kosar, Hansi Kruger, Ralph Lahmon, Milton Meckler, Mohamed Moledina, TerryPenny, Ing. Eugenio E. Sanchez, Daniel Schroeder, James Staunton, Steven Toth, KennethWelter, Sumner Weisman, Roland Wimmerstedt, and Jose Zulueta.In addition, we’d like to acknowledge and give special thanks to Steve Brickley, Jim Judgeand Lindsay Judge for their content work and contributions to this (Third) edition of thebook. We also extend special thanks to Doug Des Champs for the cover design and booklayout of this (Third) edition and T.P.S Inc. for the printing and binding.Munters Corporation
TABLE OF CONTENTS1. INTRODUCTION . 102. PSYCHROMETRICS . 12Dry Bulb Temperature Degrees Fahrenheit . 15Relative humidity - Percent of saturation . 15Humidity Ratio - Grains of water vapor . 16Vapor pressure - Inches of mercury . 17Dew point temperature - Degrees Fahrenheit . 18Enthalpy - Btu’s per pound of air . 18Wet-bulb temperature - Degrees Fahrenheit . 193. METHODS OF DEHUMIDIFICATION . 22Cooling-based Dehumidification.24Desiccant Dehumidifiers .29Liquid spray-tower. 32Solid packed tower . 34Rotating horizontal bed . 36Multiple vertical bed . 38Rotating Honeycombe . 39Comparing desiccant dehumidifiers . 40Choosing between desiccant and cooling dehumidifiers. 444. APPLICATIONS . 46Corrosion Prevention .48Military storage . 49Electronics protection. 49Power plant layup. 49Lithium battery production . 49Condensation Prevention .50Ice rinks . 51Water treatment plants . 51Surface preparation & coating . 51Injection molding . 51
4. APPLICATIONS (CONTINUED) . 46Mold/Fungus Prevention.52Archival storage . 53Seed storage . 53Cargo protection . 53Breweries . 53Moisture Regain Prevention .54Candy packaging . 55Semiconductor and pharmaceutical clean rooms . 55Safety glass laminating. 55Composite manufacturing . 55Product Drying .56Investment castings . 57Plastic resin drying . 57Candy coating. 57Fish drying . 57Dry Cooling .58Supermarkets . 59Hotels and motels . 59Sick buildings . 59Advanced HVAC systems . 595. MOISTURE LOAD CALCULATIONS. 60Selecting Design Conditions .62Moisture Load Sources .64Periodic vs. continuous loads . 78Fresh air moisture load . 78Sample moisture load calculations .81Warehouse dehumidification . 82Glass lamination room . 86
6. DESICCANT DEHUMIDIFIER PERFORMANCE . 92Operating Variables .941. Process inlet moisture . 962. Process inlet temperature . 963. Air velocity through the process side . 974. Air temperature entering reactivation . 985. Moisture of air entering reactivation . 996. Velocity of air through reactivation . 1007. Amount of desiccant presented to the airstream . 1028. Desiccant sorption and desorption characteristics . 1037. SYSTEM DESIGN . 106Passive storage - Museum Example .109Step One — Define the purpose of the project . 109Step Two — Establishing control levels and tolerances . 110Step Three — Calculate heat and moisture loads . 110Step Four — Size the components to remove the loads . 111Step Five — Select the control system . 112Passive Storage - Military Example .114Active Storage - Refrigerated Warehouse Example .119Commercial HVAC - Supermarket Example.129Industrial HVAC - Pharmaceutical Tableting .139Product Drying - Candy Coating Example .1508. OPTIMIZING MIXED SYSTEMS . 162Example Case Description .164System 1 – Dry the make-up air only . 164System 2 – Pre-cool the make-up air and dry the blend with a desiccant unit . 166System 3 – Pre-cool the blended air before it enters the desiccant unit . 167System 4 - Eliminate all pre-cooling and remove all moisture with desiccants . 168Comparing Alternatives.170
9. HUMIDITY & MOISTURE INSTRUMENTATION. 174Duty Cycle & Operating Environment .177Instrument Functions .178Repeatability vs. Accuracy .180Relative Humidity Sensors .180Mechanical expansion hygrometer . 180Electronic expansion hygrometer . 181Electronic capacitance sensor. 182Electronic resistive sensor . 182Psychrometric instruments. 183Absolute Humidity Sensors .185Gravimetric train . 185Condensation hygrometers. 185Aluminum oxide sensors . 186Salt equilibrium sensors . 187Electrolytic hygrometers . 188Material Moisture Content Sensors .188Coulombic Karl Fischer titration . 188Infrared absorption . 189Equilibrium moisture detectors . 189Resistance moisture sensors . 191Microwave absorption. 192Radio frequency (capacitance) sensors . 192General Observations .194Sensor placement . 194Measuring moisture below 10% relative humidity . 194Environmental chambers. 195
10. MINIMIZING COSTS & MAXIMIZING BENEFITS . 196Identifying & Quantifying Economic Benefits .198Operational cost reduction . 198Reducing cost of capital investments. 201Improved profits through improved quality . 203Improving operational responsiveness . 204Minimizing costs .205Minimizing first cost. 205Minimizing operating cost . 208Summary .213Worksheets .215APPENDIX . 216Weather Design Data .217Filter Selection Guide .221Steam Data .222Conversion Factors .223Dew Points at Altitudes and High Pressure .225HoneyCombe Dehumidifier Performance Curves .226Photo Credits .227Moisture Load Calculation Sheets .228Psychrometric Chart . Fold-out Sheet
1INTRODUCTION1INTRODUCTION2PSYCHROMETRICS3METHODS OF DEHUMIDIFICATION4APPLICATIONS5MOISTURE LOAD CALCULATIONS6DESICCANT DEHUMIDIFIER PERFORMANCE7SYSTEM DESIGN8OPTIMIZING MIXED SYSTEMS9HUMIDITY AND MOISTURE INSTRUMENTATION10AMINIMIZING COST & MAXIMIZING BENEFITSAPPENDIX
INTRODUCTIONThis handbook explains how and why to dehumidify air. It iswritten for the engineer who has a basic understanding ofbuilding heating and cooling systems, or who operates abuilding or process which is influenced by atmospheric humidity.The text assumes the reader is interested in the technology, andhas a need to apply it to gain some economic benefit. The text alsoassumes the reader is interested in specific examples as well as thetheory of dehumidification. These assumptions guide the necessarycompromises any book must make between accuracy and clarity,and between details which illustrate a specific case and those whichillustrate general, abstract principles.Part of the information collected here comes from other technicalreferences which deal incidentally with dehumidification indiscussions of related topics. The American Society of Heating, AirConditioning and Refrigerating Engineers (ASHRAE) has providedhelpful information through its Handbook series. In addition, manyindustries have provided data which can assist the dehumidificationengineer working in other fields.The primary information source is the collected experience ofhundreds of engineers who deal with dehumidification and humiditycontrol on a daily basis. These individuals have been very generouswith their time and hard won understanding of dehumidificationtechnology. Every day they must balance the opposing pressuresof optimum theoretical system performance against compellinglimitations of available information, time and budget. The systemdesign examples are fictitious, but every element contained in themhas occurred in the field.In the last few years, dehumidification technology has emergedfrom its industrial heritage to take an expanding role in commercialand institutional building heating and cooling systems. The entirefield is changing rapidly. It has been a difficult decision to limit theinformation here to what is well understood by the contributors and inwidespread use as of the publication date. Many new applications andnew equipment designs will become available in the next few years.Undoubtedly some applications are missing from this handbookbecause the contributors may not be fully aware of them or choosenot to describe them at this time. For these omissions we mustapologize. The contributors and the Editor hope the reader will be kindenough to inform the publisher of his or her own special knowledgeand experiences, so that future editions of this handbook can beincreasingly useful.11
2PSYCHROMETRICSPsychrometric Chart & VariablesDry Bulb TemperatureRelative HumiditySpecific HumidityVapor PressureDew point TemperatureEnthalpyWet-bulb TemperatureAdditional References
PSYCHROMETRICSThe practice of humidity control assumes a familiarity with theproperties and behavior of moist air. This is the science ofpsychrometrics. While it is not necessary to have a deepunderstanding of the thermodynamics of moist air, it is useful tounderstand the terms used in the trade, and understand why particularconcepts are essential in designing systems.Psychrometric terms and equations are described in detail in theHandbook of Fundamentals published by the American Society ofHeating, Air Conditioning and Refrigerating Engineers (ASHRAE). Whenan engineer needs to understand all aspects of psychrometrics, theASHRAE Handbook of Fundamentals provides an excellent reference.This chapter is not concerned with complete precision for alltemperatures and pressures of air and water mixtures. We assume thedesigner is working at sea level air pressures, and is concerned withair-water mixtures at temperatures between -20 and 100 F. Theinformation in the chapter describes the basic terms in a simple way,and shows how charts and graphs can be used to understand the overallpattern of air-moisture dynamics.13
CHAPTER TWOEarly in the twentieth century, a German engineer named RichardMollier invented a graphic method of displaying the properties of various mixtures of air and water vapor. This device has different names indifferent counties — the i-x diagram, Mollier diagram or psychrometricchart — but the names all refer to similar technical graphic displays.At first, the chart can be rather daunting, because it displays so muchinformation in a small space. However, once the basic informationelements are understood, the chart becomes an essential reference toolwhen designing temperature and humidity control systems.This chapter describes each of the properties of moist air in turn, andthen shows how these can be found quickly by using a psychrometricchart. The chart is useful both for the information it contains, and therelationships it shows between air at different conditions. It not onlyshows the “trees” in the psychrometric jungle, but shows the whole“forest” as well, allowing an engineer to gain a sense of how easy ordifficult it might be to change the air from one condition to another.14FIGURE 2.1The Mollier diagram, or psychrometric chart,provides a comprehensive overview of thethermodynamic properties of air-watermixtures. If any two properties of the airmixture are known, the chart allows anengineer to quickly determine all its otherproperties.The relationships shown in the psychrometricchart change with total air pressure. Whenworking with air at elevations above 2500ft. — or with compressed air — the engineermust consult different charts for accuratedata.
PSYCHROMETRICSTo help make the abstract properties of air more concrete, we willimagine a container filled with a pound of air. The air has beentaken from a sea-level building kept at typical comfort conditions— 70 degrees Fahrenheit and 50% relative humidity. Knowing thesetwo properties of the air, we can determine all others by using apsychrometric chart. But we will begin by defining what we know.Dry Bulb Temperature Degrees FahrenheitFIGURE 2.2FIGURE 2.3The air temperature is 70 degrees. Whenpeople refer to the temperature of the air,they are commonly referring to its dry bulbtemperature — what can be read from astandard thermometer that has no water onits surface. This is also called the “sensible”temperature of the air — the heat which canbe sensed by a dry thermometer. (The “wetbulb” temperature is measured by a wet thermometer, as described laterin this chapter.)On a psychrometric chart, the dry bulb temperature of the air isdisplayed at the bottom, increasing from left to right.Relative humidity Percent of saturationFIGURE 2.4FIGURE 2.5In our example, the air is at 50% relativehumidity. Essentially, relative humidityexpresses the moisture content of air as apercent of what it can hold when the air issaturated.1 Like the name suggests, it is not ameasure of the absolute amount of moisturein the air; it measures the moisture containedin the air relative to the maximum value atthe dry bulb temperature of the air sample. Since that maximumincreases with temperature, the term relative humidity has caused muchconfusion. When people refer to relative humidity, it is important todefine the dry bulb temperature of the air they are referring to.1This definition is accurate in concept, but strictly speaking, relative humidityis the ratio the actual water vapor partial pressure in moist air at the dew pointpressure and temperature to the reference saturation water vapor partial pressureat the same temperature and barometric pressure. The difference between thisdefinition and the one above is normally significant only outside of the humancomfort range.15
CHAPTER TWOThis is not to say that relative humidity is not useful. Quite the contrary.Most materials absorb moisture in proportion to the relative humidityof the surrounding air. Appropriately, many humidity control systemsrespond to sensors which measure relative rather than humidity ratio.However, in designing a system to control air moisture, it is importantto define both the relative humidity and its concurrent dry bulbtemperature range.On the psychrometric chart, relative humidity is displayed as a series ofcurves, increasing from the bottom of the chart to the “saturation curve”which forms the left boundary. The saturation curve represents 100%relative humidity.Humidity Ratio Grains of water vapor per pound of airTo define the amount of moisture inthe air, we use its weight comparedto the weight of dry air. This is likecounting the water molecules andadding their weight together. Theweight is measured in grains, andthere are 7000 grains in a pound. Inour example, when the air is at 70 Fand 50% rh, its specific humidity is55 grains of water per pound of dryair. In other words, in our sample, there is one pound of dry air and 55grains water vapor, or 7055 grains of total weight.FIGURE 2.6The psychrometric chart allows us to determine the humidity ratio ofour air sample. By starting at the 70 F, 50% point on the chart, we canread the humidity ratio by tracing a horizontal line to the chart’s rightedge, where the scale indicates the weight of the moisture in grains perpound of dry air.16FIGURE 2.7
PSYCHROMETRICSOther psychrometric charts will display the humidity ratio as a smalldecimal fraction representing the mass of water divided by the mass ofair. It amounts to the same thing. If you multiply the decimal fraction by7000, you can convert the ratio to grains of water per pound of air, sincethere are 7000 grains in a pound. Multiply the ratio by 1000, and thevalue is expressed in grams per kilogram. In all cases, the vertical scaleat the right edge of the chart indicates the amount of moisture in the airsample.Vapor pressure Inches of mercuryLike all gas molecules, each water molecule exerts pressure on thesurrounding environment. The amount of vapor pressure at a certainmoisture content is the sum of the pressures of all the water molecules.FIGURE 2.8FIGURE 2.9The unit of measure is inches ofmercury — in other words, howhigh the water vapor can lift acolumn of mercury due only toits own partial pressure. In thisexample, the water vapor lifts acolumn of mercury 0.37 incheshigh.In conventional heating and cooling, engineers often measure airpressure in inches of water. Realizing that mercury weighs 13.6 times asmuch as water, it becomes apparent that a large amount of water vaporexerts a considerable force. The pressure difference is enough to peelpaint off of wooden siding in the winter. There is more moisture insidethe wood than outside, so the pressure difference forces the paint off thesurface. As explained further in Chapter 3, desiccant dehumidifiers usedifferences in vapor pressure to attract water molecules out of the airand onto the desiccant surface.The vapor pressure scale to the right of the chart increases linearly, justlike humidity ratio.17
CHAPTER TWODew point temperature Degrees fahrenheitIf moist air is cooled, it cannot hold the same amount of moisture. Atsome point, the moisture will condense out of the air onto any nearbysurface. This point depends on the amount of moisture in the air, and iscalled the dew point temperature. The higher the amount of moisture inthe air, the higher the dew point temperature.FIGURE 2.10In our example, the air has a moisture contentof 55 grains. This amount of moisture willcondense if the air temperature is droppedto 50 F. For instance, if a cold can of beer istaken from the refrigerator at a temperatureof 50 F and placed in our air sample, the cansurface will cool the air from 70 to 50 , andmoisture in the air next to the can will beginto condense.FIGURE 2.11In other words, our air sample is “saturated” when cooled to 50 F — ithas reached a condition of 100% relative humidity.The left edge of the psychrometric chart is sometimes called thesaturation curve. If you draw a line horizontally to the left from the 70 ,50% rh condition, it will intersect the edge — the saturation curve — ata temperature of 50 . This particular chart also repeats the dew point ona vertical bar to the right of the chart.Cooling-b
Douglas Kosar, Hansi Kruger, Ralph Lahmon, Milton Meckler, Mohamed Moledina, Terry Penny, Ing. Eugenio E. Sanchez, Daniel Schroeder, James Staunton, Steven Toth, Kenneth Welter, Sumner Weisman, Roland Wimmerstedt, and Jose Zulueta. In addition, we’d like to acknowl
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