DIVISION OF AGRICULTURERESEARCH & EXTENSIONUniversity of Arkansas SystemAgriculture and Natural ResourcesFSA1074Grain Drying Tools: EquilibriumMoisture Content Tables andPsychrometric ChartsIntroductionSamy SadakaAssistant Professor Extension EngineerRusty BautistaInstructor, Biological andAgricultural EngineeringGrowers may be particularlyconcerned whether they should drygrain or not. In addition, they mayquestion whether to use natural air orheated air in aerating grain in thebins. These two questions are essen tial because of the economic implica tions associated with them. This factsheet attempts to provide a strategythat could help with the determina tion of grain drying systems and airheating level. The equilibriummoisture content (EMC) tables andthe psychrometric charts will be usedas tools throughout this fact sheet.Grain contains dry matter as wellas moisture at the time of harvest. Insome cases, the grain moisturecontent (MC) is higher than thatrequired for safe short term or long term storage. As a result, freshlyharvested grain must be dried to safemoisture content levels before beingstored or marketed. The safe MC forshort term and long term storage ofdifferent crops is given in Table 1.Safe MC for short term storage orlong term storage (one year or more)may be lower than that required formarketing. The variation of airtemperature and humidity valuesduring the storage period accounts forthe differences between the long termand short term safe MC values. Itshould also be mentioned that safestorage MC in the warm temperaturezone is lower than safe short termstorage in the cool temperature zone.Grain Drying BasicsAs mentioned earlier, there areinstances when a large amount ofmoisture needs to be removed fromthe grain immediately after harvest toprevent spoilage and to maintainmilling quality. Grain drying is accom plished through moisture movementfrom the grain to the air if the vaporpressure within the kernel is higherthan the vapor pressure of the airsurrounding it. In other words, themoisture moves from the kernel to thesurrounding air and is transported tothe atmosphere. Accordingly, air isnormally used as the agent to drygrain. Large amounts of natural orTable 1. Safe moisture content for grains in long and short term storage1Grain TypeCornWarm Temperatures12%Cool Temperatures15%Rough Rice12%12%14%SorghumArkansas IsOur CampusVisit our web site at:https://www.uaex.uada.eduShort Term (30 to 60 Days)Long Term(One ag.ndsu.edu/extension aben/documents/ae905.pdfUniversity of Arkansas, United States Department of Agriculture and County Governments Cooperating15%14%14%
heated air are passed through grain to achieve thepreviously mentioned levels of grain MCs for safestorage. The properties of air [temperature andrelative humidity (RH)] and the rate of airflowpassing through the grain determine the drying rateof the grain. Therefore, the rate of drying grain iscontrolled by the volume of air moving over thekernels and the vapor pressure difference betweenthe air and the grain. Increasing air volume or airtemperature will result in a decrease in drying time.However, rapid drying, using air with extremely hightemperature, can produce stress on the kernels andform a crack or fissure in the kernel. Hence, specialattention should be paid to the temperature andrelative humidity of the drying air.The following sections will explain the previouslymentioned terms and concepts as well as provideexamples that may prove useful in the decision making process; i.e., if drying is indeed necessaryafter harvest and whether to use natural aeration orheated air.Equilibrium Moisture ContentFor a given air temperature and RH, grainmoisture will eventually achieve a state of equilib rium with the environment. This grain property iscalled equilibrium moisture content (EMC). Thus,temperature and RH properties of the drying airdetermine the grain MC level. Tables A through E,presented at the end of this fact sheet, list the EMCof various grains including corn, soybean, sweetsorghum, rough rice and wheat in equilibrium withair at various temperatures and RHs.To illustrate the use of the EMC tables, assumethat air at 75% RH and 75 F temperature is beingforced through corn kernels in the bulk. This cornwill not dry below 15.7%, according to Table A. If airunder the same RH (75%) and temperature (75 F) isbeing forced through soybean, rice and wheat for along time, these grains will eventually reach theEMC of 15.2%, 15.2% and 14.6%, respectively.Effects of Air Temperature Changeson Equilibrium Moisture Contentaffected. Increasing the air RH from 75% to 85% atthe same air temperature of 75 F increases the EMCof corn from 15.7% to 17.9%. As shown in Table A, thelowest attainable corn EMC is 7.1% at air tempera ture of 100 F and air RH of 25%. Conversely, thehighest corn EMC of 22.9% is attained at air temper ature of 35 F and air RH of 90%. It should be men tioned that drying slows and becomes more difficultwhen the drying air RH is near equilibrium.Temperatures and HumidityAs mentioned earlier, nearly all grain dryingsystems use air as a medium for removing moisturefrom the grain by evaporation. For moisture evapora tion to take place, heat energy is required. In general,it takes approximately 1,100 BTUs (British thermalunits) of heat to evaporate 1 pound of water. Thiswould be 100% efficient; however, most on farm orcommercial drying operations are considerably lessefficient. In air drying systems, heat energy issupplied by the natural heat content of the air or bysupplemental heating. The amount of moisture thatair can absorb and transport as it moves through thegrain column is dependent on its temperature andRH – along with some influences from air velocity,presence of fine materials other than grain, thedistance the air travels and the MC of the grain.As the air moves through the grain column, itabsorbs moisture and, thereby, loses some or all of itsdrying capabilities.Psychrometric Chart and Its UseA psychrometric chart presents physical andthermal properties of air in graphic form. It can bevery helpful in grain drying problems and in deter mining solutions. To determine air characteristics,two values of air characteristics should be known.Following, the intersection of these two knowncharacteristics should be obtained on the psychromet ric chart. Figure 1 shows a very simple form of thepsychrometric chart. It shows eight air characteris tics, namely (1) saturation temperature, (2) dew pointtemperature, (3) enthalpy, (4) relative humidity,(5) humidity ratio (moisture content), (6) wet bulbtemperature, (7) volume of mixture and (8) dry bulbtemperature. The dry bulb temperature, representedIn the example given above, adding 10 degrees ofsupplemental heat to the air and keeping the air RHat 75% will dry corn to approximately 15.2% MC. Inother words, increasing air temperature while main taining RH level decreases the EMC value. Thisphenomenon occurs because increasing the airtemperature increases its capacity to absorb moisture.Effects of Relative Humidity Changeson Equilibrium Moisture ContentEquilibrium moisture content of air is alsoaffected by RH, thus its drying capacity would beFigure 1. A simple form of a psychrometric chart
Figure 2. A psychrometric chartby number 8, is located along the bottom horizontalaxis. The wet bulb temperature, represented bynumber 6, is located along diagonal lines leading toscale readings at the upper, curved boundary markedas 1. The relative humidity, represented by curve 4, isrunning from left to right up through the chart. Theintersection of the vertical dry bulb line and thediagonal wet bulb line will establish a “state point”for the measured air; then the relative humidity canbe determined.It should be noticed that increasing the dry bulbtemperature while keeping the wet bulb temperaturelevel would decrease the relative humidity value. Forinstance, if the ambient dry bulb temperature is 75 Fand the wet bulb temperature is 69 F, the RH of theair would be 75% (see Figure 2). At this temperatureand RH, rice could be dried to approximately 15.2%MC (Table D). If, however, 10 degrees of heat wereadded to increase the drying air to 85 F, the RH ofthe air would decrease to approximately 54%. Thelow humidity air will dry the rice to 11.4% MC, whichis below the desired MC for safe storage. To attainthe desired dried MC of 12.5%, the additional heatadded to the air should only raise the temperature by5 degrees to bring the drying air temperature toabout 80 F with 63% RH.Suggested Airflow Rate Correspondingto Grain Moisture ContentAirflow rates for drying vary from 0.5 CFM perbushel to more than 50 CFM per bushel for commer cial or batch dryers. Most on the farm airflow ratesfor drying vary from 0.5 to 6 CFM per bushel,depending on the initial MC of the grain and theamount of heat added to the drying air. Recom mended minimum airflow rates for different moisturecontent levels are shown in Table 2.Table 2. Minimum airflow rate2Moisture Content(% wet basis)11 to 13Minimum airflow rate(CFM/bushel)0.515 to 182.013 to 1518 to 2020 to 2222 and above1.03.04.06.02http://www.uaex.uada.edu/Other Areas/publications/PDF/MP297/9 storage.pdfSuggested Heat AdditionCorresponding to Drying Airflow RateWhen airflow rates are less than 1 CFM perbushel, add little or no heat. A rough guide fortemperature increases through the heaters at variousairflow rates is as follows: For an airflow rate of 1 to 2 CFM, limit thetemperature rise to 6 F.For an airflow rate of 2 to 3 CFM, limit thetemperature rise to 12 F.For airflow rates greater than 3 CFM, a 20 Ftemperature rise is permissible. A tempera ture rise above 20 F is satisfactory for somefeed grains when drying depths are less than4 feet or stirring devices are used.In most on farm storage, the grain is subjected tomodest temperatures for a long period. There mustalways be sufficient airflow to cool the upper portionsof the bin to eliminate the possibility of mold develop ment in that area. The top layer of grains is the lastsegment of the bin to reach safe moisture level.
Estimation of the Amount of ExcessWater and Drying TimeIn estimating grain drying time, the first step isto determine the number of pounds of dry grain (atthe desired MC) produced from the wet grain (atthe initial MC). This can be determined from thefollowing formula:Final weight (initial weight) ï LQLWLDO PRLVWXUH ï GHVLUHG PRLVWXUH(1)As an example, assume that you have 2,000pounds of rice at MC of 22%, determine the numberof pounds of water needed to be removed to get theMC to 13%. Note that the weight of rice at final MCof 13% is 45 pounds per bushel.By applying the previous equation using theinitial moisture content of 22%:Final weight 2,000 ï ï Final weight 1,793 pounds 1,793/45 39.8 bushels(2)The second step is to calculate the pounds ofwater that must be removed.Pounds of water 2,000 – 1,793 207 poundsTherefore, the number of pounds of water to beremoved per bushel 207/39.8 5.19 pounds perbushel.The third step is to estimate the drying time. Thefollowing formula can be used to estimate drying time:Drying time (H) (W)(AF) (1.07) (Toutï Tin )(3)Where:H is the heat required to vaporize 1 pound of water(1,100 BTU per pound)W is the excess amount of water per bushel to beremoved (pound/bushel)AF is the airflow rate per bushel (CFM/bushel)Tin is the air temperature entering the drying bin ( F)Tout is the air temperature leaving the drying bin ( F)Thus, it would take 88.93 hours to remove5.19 pounds of water from a bushel of rice. To dry anynumber of bushels will require 88.93 hours as long asan airflow rate of 4 CFM per bushel is maintained. Itshould be mentioned that drying time estimates arequite dependent on outside air temperature andhumidity. Thus, they should be used as a rough guideunless a constant temperature decrease through therice is maintained.Estimating Air Volume Through GrainAs mentioned earlier, air is the medium thattransports moisture from the grain to the atmosphere.The volume of air moving through a column of grainis controlled by the type and size of fan and the resis tance to airflow (static pressure) it meets. This staticpressure varies depending on air velocity and thevariety, depth, presence of fine materials other thangrain and MC of the grain. Static pressure is usuallymeasured with a manometer (see illustration inFigure 3), which is a clear plastic or glass “U” tubepartially filled with colored water. A ruler is placedbetween the columns formed by the “U” tube. Oneend of the “U” tube is open to the atmosphere. Theother end is attached to a rubber hose, which isplaced over a small hole in the duct or plenum. Air ispressured through the hole in the duct, which willforce the water column in the “U” tube to move sothat there is a differential between the two levels ofwater in the tube. This difference in water levels is ameasurement of the static pressure (inches of water),thus the reference to fans being capable of moving acertain number of cubic feet of air per minute atvarious inches of static pressure. Static pressure canalso be measured using a Magnehelic differentialpressure gage. Therefore, the power of a certain typeof fan and the static pressure determine the amountof air to be delivered by that fan. Table 3 showstypical fan performance chart.open toatmosphereconnected to thesystem for whichpressure is to bemeasuredFor the previous example, assume that the airtemperature entering the rice is 95 F. The air tem perature leaving the rice at the top of the bin is 80 F.The fan is moving 4 cubic feet of air per minute perbushel through the rice. Determine the drying time.Time (1,100) (5.19) 88.93 hours(4) (1.07) (15)P1RairP1Lhh1h1Drying time can be calculated as follows:(1,100) (5.19 pounds per bushel)Drying time &)0 SHU EXVKHO ð ð ï airliquid 1PINThP2RP2L(4)l