B-l 066 September l 967 The Use of Conditioned Air For Maintaining Quality Oi Stored Sorghum Grain TEXAS A&M UNIVERSITY E Texas Agricultural Experiment Station H. O. Kunkel, Acting Director, College Station, Texas Contents Summary .......................................................................... -- 3 Acknowledgments ................................... ...................... .- 4 Introduction .... .. 5 Requirements for Grain Quality Preservation ........ .. 6 Mold Development ................................................ -- 6 Insect Activity ......................................................... -- 6 Germination ............... .- 6 Recommendations ....... -_ _. .... -- 6 Equipment and Procedures._ i 6 Thermodynamic Considerations ................................. -- 7 Design Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -- 11 Cooling Zone Movement l ’ Grain Temperature ...................................... .; ....... "l3 Grain Moisture Content ' i 14 Heat Gain ............................................................... -.18 Design Methods for Determining up, Refrigeration Capacity ......................................... Design Method No; 1 ___________________________________ .. Design Method No. 2 ....... ........................... Design Method No.‘ 5 ................................... .. Effects of Conditioned-a'ir Storage y on Grain Quality .................................................. Insect Control ________________________________________________ __ Germination ___________________________________________________ _=i_i§ Moltt .............................. __ i’ Grain Moisture Content ______________________________ Design Procedure _____________________________________________ __ I Cooling Procedure ________________________________________ - Design Conditions __________________________________ ___ ____ Airflow Requirements __________________________________ Refrigeration Capacity ________________________________ Insulation Requirements ___________________________ ____i References _______________________________________________________________ ,_ i cry in was conducted to determine the feasi- taining the quality of stored grain by “the temperature and relative humidity of air. ‘ rtant consideration in storing sorghum i . loss in quality resulting from improper _ itions. Requirements for quality preserva- qn mold development, insect activity and are presented in this report. B ynamic considerations as they relate to controlled storage environments for bulk scussed. The initial vapor pressure of i in the grain and the partial pressure of 1 the conditioned air circulating through ss were found to be very important ically in the design of controlled en- i orage systems. it research, grain quality was maintained .ditions surrounding the grain were main- esirable temperature and relative humid- V’ or example, no loss in quality resulted aving an initial moisture content of 18.19 was conditioned with air having a dry- f ture of 45° 5F. and a relative humidity ately 70 percent for 194 days. Low- 'n with moisture contents as high as l4 _ stored without quality deterioration by _ 'oned air having a dry-bulb temperature *- a relative humidity in equilibrium with oisture grain. Factors governing the economical design of conditioned-air storage systems were studied. Results show that the time required to cool bulk-stored grain can be described by two time periods. The first period takes into account the lag time required for the lead- ing edge of the cooling zone to move out of the grain mass. The other period is determined by the depth of the zone and the rate at which it moves. Both zone movements are described mathematically in this report. Several methods were developed to determine the refrigeration capacity needed to maintain the quality of stored grain. These methods are described with a discussion of the effects of each on the equipment capacity and load requirements. In all tests conducted to date, there was always a decrease in grain moisture content at the beginning of the cooling period regardless of the entering air relative humidity. This was due to the cold entering air being at a lower dew-point temperature, or vapor pressure, than the grain at its initial temperature. It was also shown that this moisture loss can be re- established and maintained at an initial level by controlling the conditions of the interstice air. Sev- eral methods of maintaining predetermined moisture levels are reported. A solution to a typical design problem for a conditioned-air storage system, based on the findings of this research, is presented and discussed. Acknowledgments The authors wish to acknowledge the assistance of the following: H. W. Schroeder, plant pathologist, MQRD, ARS, USDA, for conducting mold studies; L. E. Clark, assistant professor, Department of Soil and Crop Science for performing germination tests; R. L. Hanna, associate professor, Department of Ento- mology, for his assistance in conducting insect studies; J. E. Roberts, farm manager, for furnishing grain for these studies; and D. R. Stipe and K. R. Beerwinkle, former instructor and instructor, respectively, Depart- ment of Agricultural Engineering, for their engineer- ing assistance. ; The Use of Conditioned Air é wnocaEss HAS BEEN MADE in recent years of aeration equipment and techniques O 'n in storage. These practices have (“for maintaining quality of low-moisture ’_ ompanied by a program of inspection fumigation for insect control. However, i developed in aerated storages that are - to elevator operators. air is used for aeration, the mois- Q» the stored grainis often reduced below if- lduring the storage period. This is due which occurs when grain is cooled with This drying results in substantial loss l‘ in turn sizeable monetary loss. ' ile, based on the amount of grain stored present time,_a 1 percent reduction in i‘ the original storage level represents ual loss of more than $2.7 million for alone. This points to a need for (equipment to reduce or, if possible, 5loss. In tests conducted in Texas (2), the weight loss occurring over a 9-10- period was due to a reduction in mois- Q- tion. i} a - o in storing grain is possible insect ts destroy. an estimated 2 percent of i - w grain annually (3). Not only do i ‘grain, but losses are also caused by For Maintaining Quality Of Stored Sorghum Grain Nat K. Person, ]r. ]. W. Sorenson, ]r. W. E. McCune Price Hobgood* heating, spoiling and reduction in grade caused by infestation. With the passage of the Pure Food, Drug, and Cosmetic Act in 1938, the Food and Drug Admin- istration (FDA) recognized that the problem of in- temal or hidden infestation must be reduced or eliminated at the point of contamination if the con- sumer was to be protected from unsanitary foodstuffs. Flour mills, bakeries and other food industries are able to clean and remove foreign matter from grain but are only partially successful in removing internal grain infestation. It is, of course, possible to control insects in stored grain by frequent inspections and fumigations. How- ever, since insect infestation is a cycling problem, repeated use of chemical fumigants has caused addi- tional problems which in some cases are more dan- gerous than contaminated grain. All fumigants are quickly lethal, or acutely toxic, to man at concen- trations effective against insects, and elaborate safety precautions must be observed at all times if they are used (4). Also, the use of bromide and cyanide fumi- gants has created residue hazards in grain which is held in storage for long periods (4). Preliminary tests have shown that the use of conditioned air may be an effective and economical ‘Respectively, assistant professor, professor, professor, and pro- fessor and head, Department of Agricultural Engineering, Texas A8cM University. method of controlling the quality of stored grain by eliminating insects, controlling moisture contents and maintaining the germination. Even though condi- tioned-air storage is nothing more than a method by which the temperature and moisture content of stored grain can be controlled in order to maintain quality, it accomplishes the same purposes as natural aeration systems without the same degree of risk. REQUIREMENTS FOR GRAIN QUALITY PRESERVATION An important consideration in storing sorghum grain is the loss in quality which may result from mold and insect activity if the grain is not stored in the proper condition. There is a definite relationship between grain temperature and moisture content and the time which grain can be held in storage before mold development and insect activity become a problem. Mold Development Christensen (5) found that moisture content, temperature and time are all intimately related to the growth of molds in stored grains. Thus, the higher the moisture and temperature, within the limits of growth of the fungi involved, the shorter the per- missible storage time. Christensen found that wheat at a moisture content of 14.5-15.0 percent can be stored safely at 68°-77° F. for a few months, but not for a year, while at the same moisture content and at a temperature of 50°-59° F. it presumably could be stored for a year without serious damage from molds. The lower limit of temperature for the growth of most storage molds is about 40° F., and the optimum temperature for growth of most of them is 80°-90° F. (6). Semeniuk (7) found that a minimum relative humidity of 80 percent in bulk bins is required for continued growth of molds. A relative humidity of 80 percent corresponds to an equilibrium moisture content of about 15 percent, wet basis, for sorghum _ grain at 70° F. Insect Activity Insect activity in stored grain can also be cor- related with time, temperature and moisture content. Sorghum grain having a moisture content of 9-10 per- cent normally will not support insect activity due to the low relative humidity of the. interstice air. On the other hand, humidities in high-moisture grain still cannot support insect activity if the temperature is not in the optimum range. Pedersen (9) reports that stored-grain insects are capable of functioning only over certain ranges of temperature. Because of their inability to maintain a constant body temperature, they have to rely on the temperature 0f their en- vironment. Grain temperature is probably the most significant factor affecting the distribution of stored- grain insects, since research has shown that these insects become inactive and eventually die of starva- tion at temperatures of 50°-60° F. (10). 6 Germination Under official grain grading standards, ’ tion tests are not required to establish sorghum grain unless stored for planting ‘ However, germination tests are good indica the condition of the stored grain. Grain. low in germination is usually subjected to lo I than high quality seed because of an incre damaged and cracked kernels‘. Germination I also an indicator as to the storage condi ' there is evidence that grain with low ge, properties is also low in quality. I Recommendations In order to maintain the quality of stored grain, the temperature and moisture cont w, grain must be controlled. The maximum: obtain a controlled grain condition befo *- quality occurs cannot be stated precisely. the time periods shown in Table 1 have pro factory when grain is stored under natural methods at various moisture contents and‘ temperature of 85°-95° F. EQUIPMENT AND PROCEDURES j The overall objectives of this research j 1. To determine the design factors, -l requirements and operating procedures for 4 ing quality of both low- and high-moisture ; I under controlled air conditions. Factors measure of quality were mold damage, insec U.S. grade and germination. 1 2. To develop methods, procedures ment for maintaining a selected or predi level of moisture in stored grain. I To accomplish the objectives of this I three test bins were used, Figure 1. Each; 6 feet in diameter. They were designat Bin 2 and Bin 3. Test Bin 1 consisted u‘ diameter bin centered inside the 6-foot bi provided a 12-inch layer of grain which a k insulator and reduced the heat flow into; section. The walls of the outer bin were: with 2 inches of insulation having a the ductivity of 0.25 Btu per (hour) (square foot inch). Both bins were installed on a perfo - TABLE 1. MAXIMUM ALLOWABLE DAYS FOR i SORGHUM GRAIN STORED AT INITIAL TEMP '1, OF 85°-95° F. BEFORE LOSS IN QUALITY RESU) INSECT AND/OR MOLD ACTIVITY A Grain moisture Maximum - M content—- cooling u percent (W.B.) days’) 12 to 14 so I 14 to 16 20 a 16 to 1s 7 j. RATORY GRAIN BINS I AIR FLOW ‘ . I f"'--—--—-1'———U- j ‘rurm AIR DUCT ,L_-_-.-. =2: E JAMPLINe PORTS—< numzn I-=== ==i am =93 .~ I ., L ‘o In .. ""1 l 3 1c: PLATE #9., _ I '=== PERFORATED FLOOR L- ILLED WATER con. P" ..* I - - --' PLENUM —— CHAMBER -_ liisk-flwi,‘ 6|_o|| ‘I em NO. I RN AIR DUCT ____6|_ 011i’ $040511: WASHER /' | EPOXY SAMPLING . a it non Ponrs—< _ ,, 3-D N6 DEVICE PERFORATED ‘f’ R07 [ '9; PLENUM _ CHAMBER ‘n: I ‘w: _ _l am NO. 2 I rm like} _ I‘ SAMPLING . Poms 6 '°"—"‘"' in 2E i me _‘ 1. L J - I x ____1__ I am NO. a __ -aectional view of experimental conditioned-air plenum chamber 6 feet in diameter and h. 2 was constructed from a standard 290- -'ty hopper-bottom bin sprayed with l inch insulation. A perforated floor was i" ting an air plenum in the hopper-shaped e bin. was constructed with a so that a 2-inch space was provided for n in order to minimize heat gain. Four "ulation having the same thermal conduc- 1 were installed on this bin. Each test at the top, and air ducts were installed closed system for recirculating conditioned SCHEMATIC OF TEST BINS AND REFRIGERATION EQUIPMENT HEAT EXCHANGE R __|mm|| 117mg 3-WAY VALVE * r T0 WATER IAIN I I rivuun WATER CHLLER ' rum 6.4.. __ uonuumue s-wAv VALVE 'BIN NO. I Figure 2. Schematic of test bins and refrigeration equipment. air. The direction of air flow was from the bottom to the top of the grain mass in each bin. Two conditioning units were constructed in order to provide conditioned air to the test bins. Bin l and Bin 3 were interchanged so that the same condition- ing unit could be used for both bins. This unit con- sisted of a chilled water coil with the necessary con- trols. The bins were arranged so that when Bin 3 was connected to the conditioning system, Bin l could be operated as a conventional aeration system using atmospheric air. A chilled-water-spray air washer was used to condition air for Bin 2. Each of the condi- tioning units was piped to a 2-ton water chiller, Figure 2. In each bin, sampling ports were installed at each foot level to allow grain samples to be taken for test purposes. Thermocouples were also located at each foot along the center axis of the bins and at various other points in the bin and circulating systems for temperature measurements. THERMODYNAMIC CONSIDERATIONS Grain temperature and moisture content can be established in bulk stored grain by controlling the conditions of the air surrounding the individual kernels. To maintain these air conditions for a period long enough to establish a predetermined grain condi- tion requires that air be circulated through the grain 7 mass. This air must be supplied with properties which will provide a potential driving force, such as enthalpy and vapor pressure, in order for heat and moisture to be transferred. In actual practice, grain temperature is of primary concern, since grain quality can be maintained over a wide range of moisture contents if the grain tem- perature is held in the 50°-60° F. range. Moisture considerations must be secondary regardless of the initial moisture content of the grain, because a vapor pressure differential will usually exist during the cooling period. This differential will result in a transfer of moisture from the grain to the air. This does not mean that the partial pressure of the water vapor in the air is not important. On the contrary, the initial vapor pressure of the moisture in the grain and the partial pressure of the vapor in the conditioned air circulating through the grain mass is very important thermodynamically in the design of conditioned-air-storage systems. In fact, within certain air dry-bulb temperature limits and grain moisture contents, grain temperatures can be controlled entirely by the specific humidity of the conditioned air enter- ing the grain mass. Consequently, the specific humid- ity as well as the dry-bulb temperature of the circulated air must be considered in any environmental storage design. Consider the effects of an improper design hu- midity on the grain temperature in an installation designed to cool the grain in small increments of temperature. The results obtained are shown in Figure 3. Grain was placed in storage at a tempera- ture of approximately 90° F., and a design dry-bulb temperature of the air entering the grain was selected at 85° F. The specific humidity of the air entering the grain was at a level to maintain the entering air relative humidity at 80 percent, while the equilibrium relative humidity of the grain was 60 percent. This caused moisture to be transferred from the air to the grain at a rate which allowed a net exchange of heat to the grain, thereby increasing its temperature. AIR FLOW RATE" 0.2 CFM PER BUSHEL ‘o0 GRAIN DEPTH FROM BOTTOM OF am 95 85 TEMPERATURE OF INTERSTICE AIR -°F O IO 2O 3O 4O 5O 6O 7O 8O AE RATION TIME - HOUR5 Figure 3. Relationship of interstice-air temperature and time when air is circulated through grain at a condition which allows moisture to be transferred from the air to the grain. AIR now ane- ~B i aus m 2° GRAIN DEPTH F é,"- 0F am q ' | | G s ‘é é g |oc Li’ 5 PLENL M m u: 9o —' m ‘ ‘&4 Fr. 3 5 2 FT. é “ so ~ * ' D 7c o no 2o so 4o AERATION TIME-HOURS | no f _o_: AIR FLOW RATE- . _ \2 FT. \ I5 u: \ ‘I El IC \ \ ° o. ' \ | FT. m; g >- m 9c \ I; -1 \ Q El ' g f, o. 1o I m _ PLENJM ‘l? so LL 8 5 - O. m 30o 1o 2o so 4o AERATION TIME-HOURS Figure 4. Dry-bulb temperatures and specific h interstice air at different depths along the center‘ mass of grain when grain is cooled by the e ' moisture. When an adsorbent such as silica gelf_ by a film of water, heat is evolved (ll). ~ liberation is called heat of wetting and is be due to attractive forces. A similar c p‘ expected to occur with grain; therefore, thej adsorbed by the grain is called the heat of ' which is the sum of the normal heat of c i plus some quantity known as heat of wet ' two individual heat terms must be used ', calculate the total change in the enthalp during a process of this type. Even thougi perature of the air entering the grain was the temperature of the grain, a sufficient i moisture was transferred to the grain for A adsorption to more than offset the decre enthalpy due to the grain-air temperature This resulted in a net increase in the en grain. The thermodynamic effects resultingj proper design specific humidity in contro ' ments required to cool grain can be bulk stored grain. If grain moisture con reduced during the storage period witho affecting the economy of the storage ope i the time required for cooling or the refri _ can be reduced. These reductions result; evaporative cooling effect obtained when _ transferred to the air from the grain. ithe cooling time and load cannot be resent. l te this further, consider the results (if cooled by the evaporation of moisture lationships of temperature and humidity ling are shown in Figure 4. In this the dry-bulb temperature of the condi- held constant at the initial grain tem- h specific humidity of the entering air {at a value that allowed moisture to be V m the grain to the air. Results show ure is evaporated from the grain, the are cooled due to the sensible and latent Q ‘zation exchange, as is the case in an T1» ling process. _ iabatic saturation process, Figure 5, f» air enters the saturator at state 1. If f, contains an adequate supply of water, fwill leave in a saturated condition. If the entering air wet-bulb temperature, ‘_ ‘ng air dry-bulb temperature will equal wet-bulb temperature. However, if the l be supplied at a sufficient rate to satu- ‘ng air, as is the case with grain, then c’ dry-bulb temperature would not equal (air wet-bulb. Also, if the water tempera- 'ally at the dry-bulb temperature (Tm) f r temperature could only approach the LT“ until sufficient moisture was evapo- A‘ this temperature. If the dry matter in f’ t the same temperature as the water dur- then the internal grain temperature iv the wet-bulb temperature during the ' 'ch moisture was removed. The tempera- y f the grain approaches the wet-bulb of the surrounding air depends upon the (moisture evaporated and the latent heat on. iv; am shown in Figure 6 represents a for cooling grain in storage. The total é; and leaving the system must be equal, or §H2=HI+HW+HK+Q (1) h} enthalpy of entering air mixture, Btu M enthalpy of leaving air mixture, Btu enthalpy of rejected moisture, Btu enthalpy of rejected heat, Btu heat gain from outside, Btu SATURATED AIR Tdz I Twl we ' WSATURATED WATER TEMPERATURE ' T“ V. Schematic of an adiabatic saturation device. \ MOISTURE Q H2 = H|+Hw+ Hk+Q Figure 6. Heat balance diagram for cooling grain in storage. The enthalpy of the moist air entering (H1) and leaving (H2) can be calculated as the sum of the indi- vidual specific enthalpies of the dry air and water vapor components of the mixture. The specific enthalpy of the dry air is given as T ha = f (C19- dT _ (2) 0 where h, = specific enthalpy of dry air, Btu per lb. T = temperature of air, °F. To = base temperature, °F. (Cp), I specific heat of dry air, Btu per lb.-°F. Assuming a base temperature (To) of 0° F. then the solution to Equation (2) becomes ha = (Growl T (s) 9 The specific enthalpy of the water vapor is given by T hwv Z (hfQT + f (Ctbw dT (4) where o hwv I specific enthalpy of water vapor at temperature T, Btu per lb. of water vapor (hrQT = latent heat of vaporization at tempera- ture T, Btu per lb. of water vapor (Cp)w I specific heat of water, Btu per lb.-°F. T I temperature of water vapor, °F. To I base temperature, °F. Integrating Equation (4) and combining with Equa- tion (3) then the specific enthalpy of the saturated mixture becomes h, or b2 I (CD), T+ [(Cp)v,. (T—T(,) + (hfg)T] w (5) If hg is the enthalpy of saturated steam at a tem- perature equal to the dry-bulb temperature of the air- vapor mixture, this equation can be expressed as h, or ha I h, + whg (6) co I specific humidity of the air, lbs. water vapor per lb. of dry air hg I specific enthalpy of water vapor at the dry- bulb temperature, Btu per lb. If the base temperature of water vapor is selected as 32° F., the values for Equation (6) can be obtained directly from steam tables or a psychrometric chart. Over any interval of time (9 + A6) then 9+A9 t H1 or H2 I f G [ha + cohg] d9 (7) 9 where G _I air flow rate, lbs. of dry air per unit of time Therefore, the total change in the air enthalpy as it passes through the grain becomes 9+A9 H2"*H1ifG[ha2 + 9 G-i-AG f G [hal "l" C01 hgl] I 9 At the present time, it is assumed that the terms Hw and HK in Equation (1) represent the total en- thalpy change of the grain. There is some question as to the validity of this assumption, and this will be discussed later. With this assumption, "the specific enthalpy of the grain can be calculated by hG I hag + wwg h, (9) where hG I specific enthalpy of moist grain, Btu per lb. dry grain hd, I specific enthalpy of dry grain, Btu per lb. 10 wwg I specific humidity of grain, lbs. wa lb. dry grain i hf I enthalpy of water in grain, Btu per Making the proper substitutions in Equation ‘ total numerical change in the grain enthalpy total depth L during any time period 6 to 9+ A‘ be determined by L L B AHG z f [Wds (Cp)dgT] dL+ f [Wws (crdwTl ‘i 0 0 ’ L L 0f[Wdg(Cp)dgT] dL+g [Wwg(Cp)wT] dL 6 where AHG I enthalpy change of grain, Btu Wag I weight of dry grain, lbs. per unit g W I weight of water in grain, lbs. o_ depth ; (Cp)dg I specific heat of dry grain, Btu per) (Cp)w I specific heat of water, Btu per lb. T I grain temperature as a function depth, °F. " w g Due to the difficulty of expressing T and ‘Q terms of L in Equation (10) an accurate n y, approximation of AHG can be obtained by the ing equation for (n) small increments of L, . “=21 AHG = 5 (was. (<1.>d.(1x>a +§ (Vvwzfh (<1. . G:1 G:1 Ii (was. (<1.>d.("r.>. 3i M...» ((1.). (n) a=1 0:1 Subscripts f and i denote final and initial M respectively. Y The normal procedures for determining 4 ternal conditions of a grain mass in a c0 environment storage are to observe the tem with thermocouples located within the mass ‘ determine the moisture content of the grain ' ically. These temperatures are usually ass p be both air and grain temperatures. If mo' being transferred from the grain to the air I ically during the cooling process, then the r couple readings may not indicate the true. t ture of the grain. This would mean that if n? temperature T in Equation (11) was assum, to the dry-bulb temperature used in Equation ( the heat balance Equation (1) would not ho ; This research indicates that this may be the I though no work has been reported which p ~ liperature is lower than the indicated air v while moisture is being transferred. in temperature is assumed to be at the temperature, it then appears that Equa- have an additional term on the right iiequation for equality. Written in terms Qthalpy then, the following may apply _ isture has been transferred from the grain he :_- hdg "l" (Ow: hf-AH final specific enthalpy of the moist grain, ptu per lb. dry grain “nal specific enthalpy of the dry grain, (final specific humidity of grain, lbs. water r lb. of dry grain "enthalpy of water in grain, Btu per lb. } eat due to moisture transfer, Btu per lb. =3 grain the AH term in Equation (l2) could result design of the system cooling capacity. Course, still under the assumption that the w grain temperatures are equal. In this _ term must be reduced to some mathe- ression for design purposes. ‘plest procedure for estimating the re- i‘ ing capacity in a system design would be the sensible grain load in the following QG I WCp AT (13) _ grain load, Btu (initial weight of grain, lbs. specific heat of grain at initial moisture content, Btu per lb.-°F. I difference in initial and final grain tem- i, peratures, °F. ' sts have shown that the calculated load in n is not sufficient to cool the grain to the SPECIFIC HEAT OF GRAIN-BTU PER POUND J FAHRENHEIT Approxi- mate grain moisture content, Sorghum percent Wheat Barley Oats Rice grain 13.0 0.39s. 13.0 0.370 ' 13.0 0.405 0.337 13.0 0.419 0.402 13.0 0.390 12.0 0.447 13.0 0.490 24.0 0.533 AIR FLOW RATE-OJ CFM PER BUSHEL GRAIN esprit FROM sorrow or em DRY-BULB TEMPERATURE OF INTERSTICE AIR -' F so O 20 4O 60 80 IOO I20 I40 I60 I80 AERATION TIME" HOURS AIR FLow RATE-O. | cm PER ausnet ' GRAIN DEPTH FROM BOTTOM OF BIN SPECIFIC HUMIDITY OF INTERSTIGE AIR GRAINS PER L8 OF DRY AIR O 2O 40- 6O 8O IOO I20 I40 I60 I80 AERATION TlME- HOURS Figure 7. Dry-bulb temperatures and speciic humidities of interstice air at different depths along the center axis of the mass of grain during the cooling period. final design temperature unless the entering air tem- perature is lower than the final design temperature of the grain. If Equation (l3) is used to calculate the load required to cool the grain to the entering air temperature under the conditions shown in Figure 7, then the final temperature of the grain would be that shown after 200 hours of operation if it is assumed that a thermocouple indicates the true grain tempera- ture. This load compares closely to the change in the air load during the cooling period when the specific heat of grain at different moisture contents is selected from Table 2. It should be noted that the grain did not reach the design temperature. The term AH could be responsible for this difference under the equal grain-air temperature assumption. DESIGN FACTORS Cooling Zone Movement When cool air is forced through a mass of grain, a cooling zone will develop and progress through the grain in the direction of air flow. This zone move- ment is illustrated by the temperature patterns in Figure 7. The thickness of the zone and the speed at which it can progress through the grain mass depends upon some function which describes the cool- ing rate of grain in relation to air velocity. ll At any instant of time, Newton's law of cooling may describe the rate of change of grain temperature for any point in the mass. This is given by $1,? = —K <14) where t = time T, = air temperature T == grain temperature K = proportionality constant The solution to Equation (l4) follows: 1 /K-%{—+ T = T, T = ce"K‘ + T, when t I 0, T I initial grain temperature, Ti, so that T, = c + T, or c = T, — T, then T = (T, — T,) e-K‘ + T, T — T, 711i Ta This is recognized as the half-response equation. 01' = e-K‘ (15) However, since it is impractical to supply the volume of air required to maintain the air tempera- AIRFLOW RATE -- CFM PER BTUSHEL I 5 IO 5O I00 COOLING TIME - HOURS 12 EQUATIONS FOR PREDICTING COOLING TIME! ture surrounding each kernel of grain at a i level, Equation (15) cannot be applied direc A must be related to airflow. For design purp effect of air flow rate on cooling time must _1 scribed by two different time periods. The first would account for the lag time required for t.) ing edge of the zone to move out of the gra The other period would be determined by the . of the zone and the rate atwhich it moves. l The time for the leading edge of the h move through the grain (TL) has been descri? Miller (18) as the time at which the air lea A grain first starts to decrease in temperature. time can be expressed as i‘ TL = 3.9 Q-°-°4 where TL = time for leading edge to move _ grain, hours Q = flow rate of air entering the grain, {i bushel ~ Equation (16) is valid over all ranges of air and conditions. The conditions of the air and grain will t, the movement of the trailing edge of the zone, such factors as evaporative cooling and heat of ’ T|_= 3.9 0"” T1-= 29 d“ Figure 8. Time req i‘ cooling zone to move .1 sorghum grain aerated-j in equilibrium with -‘ grain moisture content,‘ desired final temperat ‘ the air conditions entering the grain are at a constant dry-bulb temperature and a ‘it; u ‘dity which would be in equilibrium with grain at the initial moisture content, then gent can be closely approximated by Te = 29 Q-M“ <17) i. time for trailing edge to move through grain, hours (16) and (17) have been plotted in Figure - ience. 'me for the trailing edge of the zone to move f~ grain mass, Figure 8, would represent the g time for the mass whenever the entering j b temperature has been selected at a value A e final desired grain temperature. perature ' al temperature which the grain can attain 'ned-air storages may not be the same as i b temperature of the entering air. In most g l only approach the entering air tempera- "s is probably due to one of the following l) method used to determine the actual grain _‘= e; (2) amount of moisture transferred from ‘the grain; and (3) quantity of heat moving outside into the grain mass. if interstice-air temperatures shown in Figure 'dered to be the true grain temperature, then 3+ ce between grain and entering-air tempera- d be due to heat gain and moisture transfer. 1r portion of this difference, in this case, resulted from heat gain due to the low air- qused and the small distance from the wall f'nt of measurement. The temperatures in as compared to those in Figure 9 indicate i} creased airflow rate decreases the difference the interstice air and entering-air tempera- ii the difference in temperature, however, is TERINIG AIR ‘g SORGHUM GRAIN ;,~ RUN MOISTURE (we) f; u i- |4 % z - _ - :3 v. j» --- us °/. fl AIR ENTERING GRAIN =3 FLOW RATE = 0.194 CFM PER eu. 1e DRY-BULB =5|.§ °F 3 new POINT =45 F e \ 5 L gm“ a i l," 2O 3O 4O 5O 6O 70 8O 9O I00 AERATION TIME - HOURS Grain cooling pattern showing the relationship of i‘ and entering-air temperatures. SORGHUM GRAIN u. °|- RUN MOISTURE (we) m I00 | --_-_ 14% 3' 2 — - -— us v. u § AIR ENTERING GRAIN 9 9° FLOW RATE - 2.5 cm PER au. l- onv-auta =s|.s g new POINT = 4s 'F u, ac E % I 7c \ e \ s: ~~ a 60 \ i; \ _ i‘ {-1 5 5o é \— ENTERING m l1’ 4o - O IO 2O 3O 4O 5O AERATION TIME- HOURS A: SORGHUM GRAIN 3 RUN MOISTURE (w a) m 24° | i-- 14 1. Q 5 2 _.--- s: at n- <2oc AIR ENTERING GRAIN Q >- FLOW RATE = 2.5 cm PER su. u: ‘I DRY-BULB =5: .5 ‘F r- ° \\ oew POINT m5 ‘F Z u. I60 u. o \ Q m _l E m I20 \ Q e g u, a0 I E \ sures ms AIR o < t; 5 4o ‘§¥ o B.‘ m o 1o 2o 3o 4o 50 AERATION TIME — HOURS Figure l0. Temperature and specific humidity of interstice air in relation to aeration time when cooling a mass of grain in storage. not due to heat gain. Since the interstice-air tempera- ture of l4 percent grain approaches closer to the enter- ing-air temperature, then it must be concluded that the rate of moisture transfer has some influence. The interstice-air specific humidity leaving both the l3 and 14 percent moisture grain was lower than the specific humidity of the entering air after approxi- mately l0 hours for an airflow rate of 2.5 cfm per bushel. This indicates that moisture was being trans- ferred to the grain from the air after the initial 10- hour period. Even though it appears from Figure 10 that an equal quantity of moisture was transferred, it must be assumed that the l3 percent grain would adsorb more moisture than the 14 percent moisture grain when the specific humidities of the interstice air are the same. This being the case, the energy required to condense the water vapor onto the grain surface would cause the greater difference in tempera- tures observed for 13 percent moisture grain than the 14 percent grain. If energy is released from the grain in order to evaporate moisture, the temperature of the interstice air could be lower than the dry-bulb temperature of the entering air. This would be true if sufficient moisture could be transferred during the entire cool- ing period, as shown in Figure ll. Interstice-air tem- 13 AIR FLOW RATE —- 0J2 CFM/BU. TEMPERATURE (‘FJ l2 A 24 ' 36 48 6O 72 84 96 I08 I20 AERATION TIME-HOURS peratures given in Figure ll would remain below the entering-air temperture as long as moisture was being evaporated from the grain. Once the moisture transfer is reversed, then the interstice-air temperatures would probably rise above the entering-air tempera- ture. Grain Moisture Content The effect of any storage method or procedure on the final grain moisture content is related to the difference in the vapor pressures of the moisture in the grain and the air surrounding the grain. The flow of moisture between air and grain is always from points of high to low vapor pressure. In order to prevent any transfer of moisture during the storage period, the difference in the vapor pressures between the grain and air must be zero. In storage facilities using atmospheric air to cool grain, the air is allowed to circulate through the grain any time the dry-bulb temperature is at a desirable level without any regard to the vapor pressure. In most areas of Texas when this procedure is used, there is usually a differential in the grain and air vapor pressures which results in a loss in grain moisture content. A comparison of the moisture content of grain aerated with atmospheric air and grain aerated with conditioned-air is given in Figure 12. After a storage period of 188 days, the average moisture con- tent of grain aerated with atmospheric-air was reduced from 12.80 to 12.35 percent (wet basis). This was a moisture loss of 0.45 percentage point. Two bins of grain aerated with conditioned-air gained 0.53 and 0.26 percentage points in moisture over the same 14 Figure ll. Relationship g depth and aeration time stice-air temperature. I32 I44 I56 I68 I80 storage period. The amount of moisture loss n I on the entering air conditions and rate of airflp In order to prevent grain moisture transfer 5, the storage period, it must be realized that ; i‘ any constant level of temperature and moist _ tent can have one and only one partial pressure ated with its internal moisture. Consequently; can be only one pressure due to the water va the air which would prevent any moisture n; between the air and grain. Also the tempera the grain must be reduced to some maxim u for quality preservation. t The dry-bulb temperature of the air ente i grain would eventually be fixed at some leve the circulating air would be the heat transfer n- For a constant grain temperature, the grain n content would then be the only factor gove L vapor pressure and would be the design vapor ATMOSPHERIC All _' _ counmouzn AIR GRAIN MOISTURE CONTENT — PERCENT (‘JET BASIS) o 2o no so so loo izo mo STURNEE TIHE — DAY$ Figure 12. Comparative moisture contents of grain aera natural air and conditioned air. , I the cooling medium. The temperature of the cooling l/ ,' I,’ medium could then be used to establish the vapor l,’ I/ / ’, pressure of the air needed for moisture equilibrium ' l,’ 1/ ,’ since the dew-point temperature has the same relation- // 1/ // ship to vapor pressure as the specific humidity. / // ’/,/ The dew-point temperature for moisture equi- / // librium in conditioned-air storages must be main- /( // tained at or below the dew-point temperature pf “=6 <19. determined from the desired maximum final grain / / . temperature and the grain equilibrium relative hu- / // / midity. Any dew-point temperature above this value / / would cause the grain temperature to be higher than // the maximum temperature at the correct relative / / humidity. If the dew point of the air is below the / / desired value, the equilibrium grain moisture content / would increase since the air is still in the saturated 3Q 4O 5O 6O 70 B0 9O IOO . - . - o /_ RELATIVE numom! m state. However, if sensible heat 1S added to this air, {Sorghum grain equilibrium relative humidity curves. the dew Point and Vapor pressure can be maintained ‘ at the same level while the dry-bulb temperature is increased. This increase in dry bulb would decrease required to maintain the desired grain the relative humidity to the desired value for equi- l librium. It is then necessary only to sense and control oer to maintain an equilibrium Condition the relative humidity at some value which would allow Ming air through a grain mass, it is necea no transfer of moisture. The dry-bulb temperature Ho] the specific humidity of the entering of the air, and hence the grain temperature, would be ‘are is only one specific humidity at which maintained below the maximum allowable value when a ressures are equal. To maintain moisture the proper relanve humldlty 1s reached‘ the sPeeltle hurnldltY must he Considered The grain temperature can be maintained at the at‘ rtY ot the all’ Condition Whleh is requlred- maximum value only if the dew-point temperature l _ProPertY ls the drYbulh temperature of of the air is equal to the dew-point temperature . determined bY the neeessarY final grain corresponding to the grain moisture vapor pressure. ‘ to Prevent loss ot quality- These tWo The needed relative humidity would be reached at Will then dellne a state at Whleh onlY one the exact maximum grain temperature whenever the i‘ ‘dltl’ ean eXlst- Therefore, the neeessarY air was heated sensibly. The dry-bulb temperature P tor moisture equilibrium can then be as well as the relative humidity could then be used terms of the relative humidity at a pre- for control purposea dry-bulb temperature and can be obtained _ _ _ _ l‘, similar to the one in Figure 1g Figure l4 illustrates the control of grain moisture content by using a dew-point temperature below the _ e equilibrium relative humidity has been some economical method must be used i the humidity of the entering air at this ' ee methods were investigated in this re- .ese methods involved two possible prin- 8'“ N°'2"962 * AIR FLOW RATE-ozo ‘eat and return-air bypass. I E acal duct heaters were installed in two bins, i; a different type of conditioning unit for 8% age proper air conditions for grain moisture g; g I ese heaters were controlled by Dunmore- ER’ 1 'ty sensing elements through suitable on- 5 2 E . . . . . . g a the air conditioning units was a chilled w A, while the other system used was an air- ' unit. In both cases, the flow rate of the ° 25 5° 75 '°° '25 '5° '75 2°° 225 25° was sufficient for the air leaving STORAGE T'ME'DAYS foning unit to approach a saturated stata Figure l4. Illustration showing the relationship of grain mois- v this saturated Condition was maintained ture content to storage time when the dew-point temperature V, 3 . . was maintained below the required level for moisture equilibrium ~mt temperature of the an Could be Con’ and the relative humidity controlled to provide equilibrium at ately by controlling the temperature of the origina1 grain mgisture content, 15 needed level and controlling the relative humidity. The relative humidity of the circulated air was main- tained at approximately 80 percent for the first 210 days of storage after which time the air was controlled at the proper condition for moisture equilibrium at the original grain moisture content. The curves in Figure l4 show that the moisture content increased, after the initial loss, up to the time the control was started. After this period the moisture began to decrease and would have returned to the initial level if the proper air conditions had been maintained for sufficient time. Another procedure which can be used in con- junction with reheat is to limit the quantity of heat supplied to the air without any type of control. The amount of heat necessary for maintaining the correct air conditions for a zero vapor pressure differential can be accurately calculated if the air conditions en- tering the grain are not influenced by outside condi- tions. This is not the case in practical installations because of heat gain into the air-supply duct. The amount of heat moving into the duct varies with the outside air temperature but can be determined as some mean value. If this value is below the quantity of heat needed to establish the relative humidity, then additional heat must be supplied by the heater. This procedure was used in Bin 2 during one of the tests. The available results from this test indicate that grain moisture equilibrium can be maintained within certain limits. The relative humidity of the air which was supplied to the grain was selected at a level to maintain the moisture content at approxi- mately l3 percent instead of the initial moisture con- tent of approximately l2.5 percent. The heater volt- age was then set to maintain a mean relative humidity for moisture equilibrium at this level. The curve for Bin 2 in Figure 12 shows the results. It is anticipated that the curve would level out at approximately 13.25 percent moisture content after sufficient time but would oscillate about this moisture level due to the hourly fluctuations in heat gain. It should also be 8O 9O I O0 I l0 .016 POUNDS OF MOISTURE PER POUND OF DRY AIR Q0 _60 7O 6O 9O I00 I l0 DRY BULB TEMPERATURE'DEGREES FAHRENHEIT Figure 15. Illustration of properties of air resulting from the mixture of two air streams. 16 pointed out that these fluctuations in heat ;-I.f an oscillation in the dry-bulb temperature of _ entering the grain. This in turn causes an number of zones to move through the grain. l In the reheat system, the heat which "i. added to the air for moisture equilibrium can an excessive factor controlling the overall -p The dry-bulb temperature of Tithe saturated air I the conditioning unit must be "increased appro 10°-15° F. before it enters the grain. The ' mate load in this case for heat alone would g to 4.0 Btu per pound of dry air if all the supplied by the heater. This would result in v_ mum reheat load of about 0.825 Btu per h bushel for conditioned-air storages when air is :_ at a rate of 0.05 cfm per bushel. The quantity‘ entering the supply duct, however, could be reduce this load on the heater. I The return-air-bypass method has the ad w in most cases of reducing the power require the system as compared to the reheat type This method depends upon the psychromet i ciple of air mixtures. For example, consi mixing of two air streams having properties a‘ A and B in Figure 15. The resultant properti mixture must lie on the line AB and may be} at point C. The location of point C depen the percentage of air supplied from each ’_ . Point C would therefore lie half-way betwee- A and B if each air stream supplied 50 percen, total mix. If only one-third of the total V“ supplied from the stream at point B, then would lie one-third the distance from point mixture point will always lie closest to representing the air that forms the largest of the mixture. After the grain has been cooled in con air storages, the air leaving the grain and air entering the conditioning unit will app -_ condition of the air entering the grain. The humidity of the air leaving the grain after the’. period would be higher than the entering’ humidity if the entering air relative humi lower than the equilibrium relative humidi g grain. If the relative humidity of the en- was in equilibrium or higher than the ; librium relative humidity, the specific h; u) the air leaving the grain would then be eq lower than the entering humidity. In any i properties of the entering and leaving air two states so that a straight line drawn betw: states will pass through a region which can “ for maintaining the original grain moisture with the bypass method. ' i During one test, the relative humidity o? entering Bin 2 after a storage period of 28 g 85 percent. The corresponding average ; i perature and moisture content was 635° F. - _ percent, respectively. The equilibrium re 40' 45 5O 55 6O 65 7O 75 DRY BULB TEMPERATURE-DEGREES FAHRENHEIT under these conditions is 50.5 percent; air was losing moisture to the grain. l conditions, the specific humidity would across the conditioning unit. process after 28 days of storage is l6. Point A represents the air condi- the grain. This air was sensibly heated a result of the heat gain due to the fan heat moving into the ducts. At point B the conditioning unit where the dry- was lowered, and the dew-point was increased, point C. Point D repre- heat gain between the conditioning grain. conditions, the relative humidity of the grain must be lowered in order a relative humidity to maintain grain 'um at approximately 13.1 percent. humidity would be approximately 61 a maximum grain temperature of 55° F., point E lies on the air mixing line may be obtained through the return- by controlling the quantity of air the conditioning unit or bypass duct. be selected at a lower dry-bulb tempera- care of any heat gain in the supply ducts the relative humidity to any great E does not fall in line BC, adjusting circulating through the conditioning unit point C to move along the saturation line point. This would allow point E to and proper grain moisture could still by the return-air-bypass method. .o|4——— .013 — ID O .Ol2 -—~ .Oll llI I IIIIIIIIIIIIIIIIII] ~1 m 0 O .009 O1 O Figure 16. Psychrometric proc- ess of circulated air after 28 days of conditioned-air storage. .008 l U! O .007 .006 b O .005 I .004 .003 POUNDS OF MOISTURE PER POUND OF DRY AIR N O GRAINS OF MOISTURE PER POUND OF DRY AIR -O02 llllllillllllllllllllllIIIIIIIIII — w o o .OOI — .000 65 9O O In all tests conducted, there was always a decrease in grain moisture content at the beginning of the cooling period regardless of the entering relative humidity. This was due to the cold entering air being at a lower dew-point temperature, or vapor pressure, than the grain at its initial temperature. To illustrate, consider the effects on the grain moisture content when saturated air was supplied to Bin 1 for 104 days, Figure l7. A decrease in grain moisture content occurred during the cooling period even though the entering air was saturated. Figure l7 also points out the wide variation in moisture contents during the periods of moisture I9 / BIN NO.I—I962 K‘ ‘IR FLOW RATE ~ QZO CFM/BU. aorrou \ E t: ~ u o I kl n. / \ I . I- z u: z \ 8 I!’ - q _ - _ ~ _ _ ‘ ' ’ ‘ ‘- \\ k {Q '7 x/AVERA e \ D -* \ - +- /‘ ~ _ _-- -" . g '1 z ’ _ g I g x’ row o .' / u: ' / O 25 5O 75 IQO Ii 5 I50 I75 ZOO 2Z5 STORAGE TI ME- DAYS Figure 17. The effect of circulating saturated air on the grain moisture content at different times during the storage period. 17 transfer. An increasing gradient existed between the top and bottom up to approximately 104 days. How- ever, when the supply-air relative humidity was re- duced below the equilibrium relative humidity after 104 days, the gradients decreased. This indicates that when the entering air humidity is supplied at the equilibrium humidity the gradient will increase during the initial cooling period but decrease to an insignificant value in a reasonable period of time. The moisture loss which occurs during the cooling period is not considered a problem when grain is stored for a reasonable time. Results have shown that the average moisture content can be re-established after the grain has been cooled by controlling the relative humidity of the entering air at the proper level. H eat Gain The heat which moves into the grain due to a temperature differential between warm atmospheric air and cool grain may cause the temperature of a layer of grain next to the wall to become excessive. Any increase in grain temperature above the design temperature may result in serious damage from insect infestation. To maintain temperatures at the desired level in the outer layers of grain, some provision must be made to reduce the amount of heat transferred through the walls of a structure. One way to accomplish this is to reduce the exposed surface. This can be done by the proper selection of the storage structure. For instance, con- sider the surface area of a rectangular bin (length twice the width) compared to a cylindrical bin of equal volume, Figure 18. For the same volume and height, less surface area is exposed in a round bin than a rectangular bin with the given configuration. Surface area can further be minimized by reducing the height of a round bin. Figure 19 shows that the exposed surface area can be materially decreased as the height of the bin is decreased for any constant volume. This '4 Qfi} > $9 v-o.a's1x §Q z f‘ _ a m a’ 6;’ g // Z :1 >- U I < h! u: .. / B / E SURFACE AREA“ RECTANGLLAR BIN (EQW) x Figure 18. Relationship of the surface areas of rectangular and cylindrical bins having equal volumes and heights. 18 \ / / s/ ’ 7“ / M /Zé% W%” SURFACE AREA OF CYLINDRICAL BIN ———-@- Figure 19. Relationship of bin depth to surface area - drical bins of equal volumes. would be the same as increasing the bin dial that in selecting a round bin for conditioned- ages it appears that heat gain could be red p selecting a bin with the largest diameter or. height possible. 1 Another way of reducing the heat trans the bin is to improve the insulating quality walls with building insulation. The thic insulation required will depend on the types sulating material used, the bin construction i temperature difference between the outside an of the bin wall. Regardless of the thickness of insulati circulating air inside the grain bin must still to remove the heat which moves in. lncrea thickness will, however, reduce the rate at w heat can be transferred to the inside. Since moving through the grain must be used to u heat out, some increase in grain temperat always result. This increase in grain tempera I" exist as a temperature gradient within the ; in both the horizontal and vertical planes sifl those shown in Figure 20. The temperature j in this installation resulted from the heat ga’; one inch of epoxy foam insulation. The ai f was 0.2 cfm per bushel. ‘ Until further tests can be conducted =1 fluence of temperatures on stored-grain insec should be considered the maximum tempe effective control of insects. By limiting this ture to 60° F., the heat which the air could can be calculated from (la z Wu C1101) (TI-Ti) where q, = heat air can remove which ferred into bin from outside, Bt W, = airflow rate available for remo v transferred from outside, lbs. ~ hr. i i 11 |a"Y|e"—I—|s"-I—|e"—- i4‘ s ‘ 5s’ s?’ 1:5‘ 96°F 54° 53° 53° 10° 90°F "i- + + "' ' 1cm PER au. s3’ F ical horizontal and vertical temperature gradients f~ ss resulting from heat moving from outside into ispecific heat of air, Btu per lb. dry air-°F. y 'nal average air temperature, °F. initial air temperature, °F. assumptions must be made before Equa- be used directly. It must be assumed . heat which moves from outside the bin f- can be accurately calculated from a nperature. Also, it must be assumed t wall temperature is a true average ll temperatures at the bottom and top , and that the temperature lag within 3: would be such that the average outside ejover a 24-hour period would be appro- ign purposes. Another assumption is dry-bulb temperature of the air leaving F: can be assumed to equal the average er a distance Y, which is the horizontal top of the grain mass over which the allowed to penetrate. _ heat which the conditioned air could gen be estimated from Equation (18) by imum wall temperature and a distance 7-- the temperature gradient would be 'st. This Y distance would actually quantity of air (l/Va) which would be fremove the heat. the following example which is illus- i e 21: ; 1s ft. diameter, 20 a. high _'¢y_4000 bu. irate-OJ cfm per bu. olume of entering air—13.0 cu. ft. per ‘I air Maximum wall temperature I 60° F. T, = 80° F. T, = 55° F. Y = 9 a. Surface area (A) I 1130 sq. ft. _(_Q)(C)(5°) X Sp. Vol. T, I 57.5° F. Cp(a) I 0.24 Btu per lb. of dry air °F. 1rD2 _ 1r(D — 2Y)2 __ 4 4 W“ _ 7r D2 4 where D I bin diameter, ft. I distance which heat is allowed to move into grain mass, ft. Q I design airflow rate, cfm per bu. C I bin capacity, bu. Sp. Vol. I specific volume of entering air, cu. ft. per lb. dry air 11-[18—2(9)]2 w<18>2 _ 4 4 (19) (0.1)(400 X60) WaI 100i 4 X 13.0 I 1846 lbs. dry air per hr. q, I (1846)(0.24)(57.5—55) I 1108 Btu per hr. Y______-_-~__-_- i T, = 51.5" 55°F Y _"1 60°F '//////// / Ta =ao°|= /////IINSULATION - x" THICK fig 2a; _ .1" /'\ Ti =55°F (AIR ENl 5 £2 Z i 60°F 'ERING GRAIN) ///////////////////// Figure 21. Cross-sectional view of storage bin used to illustrate method to calculate heat gain and insulation thickness. 19 Under normal atmospheric conditions during the summer months, W, would not be large enough to prevent excessive grain temperatures without the addition of insulation. With an average outside tem- perature of T, and an inside mean wall temperature of Tm, the necessary thickness of insulation would be as follows, if the resistance of the bin wall and the inside thermal conductance is assumed to be negligible. A (T, — Tm) l X I K 20) qa f0 ( where X I insulation thickness, in. K I insulation thermal conductivity, Btu- in. per hr.-sq. ft.-°F. A I surface area, sq. ft. T, I average outside air temperature, °F. Tm I mean wall temperature, °F. q, I heat transferred into bin, Btu per hr. f I outside thermal conductance, Btu er hr.- 0 P sq. ft.-°F. The thickness of epoxy foam insulation in the above illustration would then be: 11s0(s0-57.5) 1 X I 0.17 I- --_ _ _- 1108 4 X I 4 inches It can be shown from Equations (18) and (20) that for equal volumes of grain the thickness of in- sulation to minimize heat gain can be reduced as the bin height decreases whenever Y is equal to the bin radius. If Y is less than the bin radius, this relation- ship does not hold true. As long as Y is a maximum, the expression which contains D and Y in Equation (19) is unity. However, once this expression becomes less than unity the quantity of air available to remove heat decreases faster than the exposed surface area with decreased bin heights. It appears from test data that some method in addition to insulation may have to be used to limit grain temperatures to a safe level in small bins. One proposed method would be to use a double-wall stor- age structure and circulate cold air between the walls. The outside wall could then have a practical thick- ness of insulation installed on it since the volume of air necessary to remove the heat transferred from the outside would not be limited by high fan power re- quirements as it would be inside the grain bin. Air could then be entered into the double wall at approxi- mately 55° F. and exhausted at 60° F. in order to maintain the inside wall temperatures at or below 60° F. The pounds of dry air circulating in the double wall per hour per square foot to remove this heat gain without excessive wall temperatures can be determined from Figure 22. 20 nslo on 5- rem-canvas olrrnlfl ncmsn cuvsnmo um tuvnn Al. GD 1P 2 E n‘ J / POUNDS nav AIR PER noun PER scum: root I a n ' / Z v BTU PER HOUR PER SQUARE FOOT Figure 22. Airflow rate required to limit heat uml: grain stored in a double-wall bin. i DESIGN METHODS FOR DETERMINING i REFRIGERATION CAPACITY i‘ Design Method No. I The design method most familiar to ;~Yl the use of Equation (l8), which stated: ' QG I W Cp AT This equation considers sensible heat é only and assumes that no latent and biologi , exchange takes place during the cooling i) Therefore, when this equation is used, it m assumed that there is no exchange in moist I the grain to the air and no significant load = heat of respiration during the cooling process. The following equation takes into acco moisture transferred from the grain to the the heat of respiration and was found to be I for calculating refrigeration requirements for I grain: ~ I W C t —t + h W QG I d8 Pmg) (1 1)] I: ta) w“) h W H ‘(n Wm] + R QG I grain load, Btu. I initial weight of dry grain, lbs. I specific heat of dry grain, Btu (dz) o I ( F.). _: t, I initial temperature of grain, °F. t, I final temperature of grain, °F. h, Ienthalpy of water at initial ; perature, Btu per lb. i’ h, I enthalpy of water at final grain - ture. WW0 I initial weight of water in grains’ 1 .95 W ll Wwm I final weight of water in grain, HR I heat of respiration, Btu. The refrigeration capacity required to c based on the above equation may be exp follows: _ Q6 - Q“ ' Tmovf; <22) A 'geration capacity, tons l ' load, Btu ;-- hours ue of QR in Equation (22) represents the _ on the conditioning unit. Therefore, in "ce, the time required to cool the grain the value selected for Tc but will re- r period of time. ,At present, there is no od of predicting cooling time when this od No. 2 ‘ method which simplifies load calcula- ‘a v upon the fact that the air exhausting 'n during the initial stages of cooling is u» equilibrium with the grain at its ' ture and moisture content when the is supplied at rates normally used for ‘owing the initial condition of the grain _' evolved due to respiration, the conditions _ st air can be calculated. Research has respiration heat values for sorghum grain j contents of 14.6 percent or lower and as high as 100° F. are small compared élheat load and therefore can be neglected er, for higher moisture contents, the § eat was found to be very significant due dous increase in respiration and should “w in the cooling load requirements. lating system, the exhaust air would - It conditions of the air entering the con- 't assuming no heat gain into the air duct. in condition of the air leaving the con- lit can be determined by knowing the conditions entering the grain. Thus, the erence of the air entering and leaving ing unit can easily be determined. When hich air is supplied through the grain is the capacity of the refrigeration equip- determined. m in Figure 23 illustrates the relation- enthalpy of the air entering and leaving _l ing unit to cooling time when air is through the grain. The enthalpy differ- 4), represents the load on the conditioning per pound of dry air and includes the ‘required to reduce the relative humidity _ ving the conditioner to the design level. i d be pointed out that the refrigeration 3 ined by this method is based on the oad which is the initial difference in g entering-air enthalpies. After the lead- the cooling zone passes through the grain, I of air exhausted from the grain starts hz-h‘ I LOAD ON CONDITIONING UNIT IN 'BTU PER LB. OF DRY AIR h3-hl I REHEAT LOAD REQUIRED TO REDUCE THE RELATIVE HUMIDITY OF THE AIRLEAVING THE CONDITIONING UNIT TO THE DESIGN LEVEL. AIR EXHAUSTING FROM GRAIN AND ENTERING CONDITIONING UNIT ENTHALIPY (h) —-> AIR ENTERING awn :3 _.__--|_.__ _ _ _ _ _ _._1---._ ___ 1 I Il-Am LEAVING | | conomonmc . UNIT | rm: PERIODS roa coouns I I zons TO novs mnoucn cum: rf-l-EAPING TRAILING-it EDGE EDGE 2 coouns "rm: (c) ---> Figure 23. Diagram illustrating the relationship of the enthalpy of air entering and leaving the conditioning unit to the cooling time when air is recirculated through the grain. to decrease and continues to decrease until it reaches a minimum value at the end of the cooling period. Design Method No. 3 Theoretical consideration will be given to a pro- posed method of design that may minimize the over- design problem encountered in Design Method No. 2. In this method, air is supplied at twice the rate used in Design Method No. 2, and the design load is based on one-half the initial difference in enthalpy of the exhaust air and the entering air. The total load on the conditioning unit is the same for both methods. However, as shown in Figure 8, the cooling time is not exactly inversely proportional to airflow rate, and, for this reason, the cooling time for Design Method No. 3 is slightly longer than the time required for Design Method No. 2. A comparison of these two design methods is illustrated in Figure 24. The enthalpy difference, (hf-bl), represents the load in Btu per pound of dry air when the design is based on the initial maximum DESIGN METHOD no. z BASED on HAXIHUH EnTnALPY DIFFERENCE, (hz-h‘). \1——-EXHAUST AIR FOR DESIGN nsrnoo no. 3 nnszo on ons- \\ DESIGN METHOD no. 2 HALF o? mxmun znmtrv nnrrznzncs, -h 3 . AIRFLOH RATE FOR DESIGN METHOD NO. l |s nnc: "m: an: use» ron ossucn T METHOD no. z. __ "3 ® nznm LOAD 5 E \ EXHAUST All FOR E \ DESIGN nzmoo no. 3 AIR LEAVING conm- \\ 5 | TIONING unrr run DESIGN \\\ n no. 3. ~]——— ré "- "U"’P""*-- ~ | m: LEAVING connmonmc unn’ Iron oesmn nsmoo no. 2 I I I I | I t‘ ch cc 1d t, coounc TIHE (c)——-b Figure 24. Enthalpy of recirculated air versus cooling time for design methods 2 and 3. 21 load used in Design Method No. 2. In this case, over- design would occur from time tb to td. In Design Method No. 3, the load is based on one-half the maximum enthalpy difference. This is shown in Figure 24 as (hy-hg) Btu per pound of dry air. In this method, the load on the conditioning unit is constant until time tc. After this period, the differ- ence in enthalpy of the air entering and leaving the conditioning unit would be less than the design value, resulting in a slight overdesign situation from time tc [O te. i The increase in cooling time for Design Method No. 3, (tg-td) in Figure 24, represents an increase of approximately 20 percent. For example, based on the data in Figure 8, an airflow rate of 0.10 cfm per bushel would be required if it is desired to cool grain in 145 hours based on Design Method N0. 2. The cooling time required for Design Method No. 3, using twice the airflow rate, or 0.20 cfm per bushel, would then be 174 hours. If it is desired to cool the grain in the same time period required for Design Method No. 2, an airflow rate could be selected from Figure 8 to correspond to a cooling time equal to one-half the desired total cooling time. If the cooling time is again assumed to be 145 hours, airflow rates of 0.10 and 0.25 cfm per bushel would be required for Design Methods 2 and 3, respectively. Although the cooling time is the same for both methods in this case, the total load on the conditioning unit is greater for Design Method No. 3 than it is for Design Method No. 2. Design Method No. 3 has two distinct advantages: (l) the conditioning unit is fully loaded for a greater percentage of the cooling time compared to Design Method No. 2, thus making more efficient use of the conditioning unit and (2) the time required for the leading edge of the cooling zone to pass through grain is always less for this method than it is for Design Method No. 2, thus providing more favorable condi- tions for quality preservation. The main disadvantage TABLE 3. EFFECT OF STORAGE TEMPERATURES ON RICE WEEVIL INFESTATION IN SORGHUM GRAIN to Design Method No. 3 is that higher airflow, are required which would, in turn, increase the 3, pressure requirements for the system and nec an increase in the size of the fan and motor. EFFECTS OF CONDITIONED-AIR STORAGE ON GRAIN QUALITY Insect Control Adult rice weevils in brass, cylindrical placed at various locations in Bins 1 and 2 to‘ mine the effects of conditioned-air storage fa and natural aeration systems on insect controL; cage was 1 inch in diameter and 5% inche‘ Y” Fifteen pairs of adult rice weevils and 37 ; A 14.5 percent-moisture-sorghum grain were pla} each cage. The depth of grain in each bin A feet. Cages were placed in the center of Bin l y following locations: 1, 5, 9-foot levels and at the near the grain surface. Two cages, one near thi and one at the center, were placed in Bin 2 _ following depths: 1, 5 and 9 feet. I‘ All insect cages remained in the test bins i” storage temperatures given in Table 3 for 35, Then the cages were removed, and the insects oi The weevils were removed, and the test cages at the same locations in the bins to determii reproductive ability of these insects under each ;_ condition. Cages were removed after 25 days - progeny counted. Results of the storage conditions on insect a‘ are given in Table 3. Rice weevil infestation ' appear to be a storage problem in conditi i storage structures under the temperatures of the The percent mortality of insects exposed to t wall temperatures was 86.3 percent after the F days. The insects which were located in the _ center region of the bin had a mortality of cent. The insects which were exposed to the temperatures in the natural aeration bin had tality of 61.8 percent during the same 35-day Type of storage Temperature, degrees Fahrenheit Number of insects at end of j and location First 35 days Additional 25 days First 35 days Additional c of cages Mean Max. Min. Mean Max. Min. Live Dead Live i Conventional storage Center of bin: ' 1 foot from bottom 86 89 83 78 88 66 11 19 29 5 feet from bottom 87.5 89 86 80 89 71 15 15 69 9 feet from bottom 89 91 87 82 91 93 8 21 33 Conditioned air storage Center of bin: 1 foot from bottom 51.5 54 49 51 53 49 0 29 0 5 feet from bottom 57 59 55 56 58 54 0 28 0 9 feet from bottom 60 63 57 61 60 59 1 27 0 Near bin wall: 1 foot from bottom 65.7 69.5 62 65 70 60 0 29 0 5 feet from bottom 71.5 75 68 69 73 65 3 17 0 9 feet from bottom 77.2 81.5 73 72 75 69 8 23 0 Z2 LATIONSHIP OF AERATION TIME TO MOISTURE CONTENT OF GRAIN‘ Moisture content —- Percent Grain depth — Feet 1 2 3 4 5 6 7 8 9 Average 17.45 17.65 17.75 18.50 18.30 18.15 18.30 18.70 18.90 18.19 17.35 17.70 17.90 19.10 18.80 18.45 19.10 19.50 19.85 18.64 17.23 17.18 17.25 18.20 18.05 17.80 18.40 18.75 19.05 17.99 16.92 17.14 17.29 18.00 17.75 17.45 17.75 17.85 18.30 17.61 ‘ 17.20 17.16 17.42 18.05 17.63 17.24 17.14 17.75 17.75 17.48 16.98 16.99 17.42 17.70 17.52 17.16 17.24 17.60 17.48 17.34 17.30 17.08 17.22 17.80 17.78 17.27 17.47 17.55 17.62 17.45 17.07 17.18 17.37 17.90 17.51 17.07 17.22 17.30 17.56 17.35 17.10 17.15 17.32 17.65 17.51 17.16 17.40 17.35 17.42 17.34 16.96 17.14 17.37 17.60 17.41 17.10 17.20 17.19 17.33 17.26 16.55 16.99 17.13 17.61 17.10 17.09 17.08 16.95 17.03 17.06 16.00 16.51 16.90 17.19 16.78 16.65 16.75 16.64 16.75 16.68 15.71 16.05 16.60 17.01 16.62 16.91 16.84 16.45 16.60 16.53 15.62 15.75 16.42 16.70 16.82 16.82 16.54 16.54 16.15 16.36 15.22 15.45 16.15 16.51 16.00 16.74 16.51 16.51 15.82 16.10 15.55 15.09 15.32 15.65 15.88 16.52 16.11 16.26 15.32 15.74 15.64 15.45 15.99 15.66 15.78 15.79 15.73 16.17 15.50 15.75 '7 0.12 cfm per bushel. d of the second storage period of 25 days, Ir occurred in the conditioned-air storage er, in the bin aerated with natural air, of 158.4 percent was observed based on inumber of insects which were first placed y“ Of this number, only 7.1 percent were fafter the additional 25 days. results do not necessarily compare with ta concerning the temperature limitations ’ '1s. The mean wall temperatures in the air bin were as high as 77.2° F. with a i 81.5° F. These temperatures would lport insect activity in natural aeration e high mortality and zero percent emer- conditioned-air bin may have resulted q edures used. It was not possible to place ~ - the test bins at the start of the cooling sequently, the insects were subjected to é-perature change when they were placed i The same temperature conditions existed 'nning of the test as well as during the A Additional tests need to be conducted - if this rapid change in temperature ‘ e test results. to determine the effects of conditioned- rocedures on the germinating qualities of 'c tests were {conducted on high-moisture _ can be shown that no significant loss 'on occurs in high-moisture grain, it can f? that low-moisture grain can be stored ir conditions without any germ damage. i; an initial moisture content of 18.19 per- “ditioned with air having a dry-bulb tem- perature of 45° F. for 194 days. The relative humidity of the entering air was approximately 73 percent for about 50 days after which time it gradually decreased to approximately 65 percent at the end of 194 days. The relative humidity was allowed to decrease so that the grain moisture content could be reduced during the storage period. The results of this drying effect are given in Table 4. The effects of these storage conditions on the germination properties of the grain are shown in Table 5. The average germination at the start of these tests was 77.95 percent. After a storage period of 194 days, this value was 74.32 percent or a loss of 3.63 percent. Mold Since molds will develop at a greater rate in high- moisture grain than low-moisture, mold analyses were made on the same stored grain as the germination TABLE 5. PERCENT GERMINATION OF STORED GRAIN AS RELATED TO STORAGE TIME, BIN NO. 1, 1961 Depth of Storage time — days grain, Start of feet test 8 47 85 114 194 — — — — ——Percent—————-— 1 78.0 75.0 68.0 64.5 73.0 77.0 2 77.0 79.7 76.5 75.0 75.0 72.0 3 77.7 75.0 73.5 69.0 73.0 72.0 4 76.0 76.0 59.0 75.0 76.0 76.0 5 74.5 76.7 70.0 73.0 74.0 70.0 6 84.2 79.0 76.5 69.0 83.5 74.0 7 78.7 82.2 84.5 77 .0 83.5 79.0 8 76.2 78.2 68.0 66.5 82.0 77.0 9 79.3 79.0 73.0 70.5 74.0 72.0 Average 7 7 .95 77.84 72.11 71.05 77.11 74.32 TABLE 6. MOLT) COUNTS FROM SORGHUM SEED STORED IN TEST BIN, 1961-62 Kernels from Percent kernels Depth of Aeration which molds infested with: grain, time, were isolated, Field Storage feet days percent molds molds 1 Beginning 100 100 0 2 of test 100 99 2 3 99 99 0 4 99 99 1 5 100 100 l 6 100 100 1 7 100 100 1 8 100 100 0 9 97 97 1 1 85 l00 l00 0 2 85 100 l00 0 3 85 100 l00 0 4 85 l00 99 1 5 85 l00 100 0 6 85 100 100 0 7 85 100 l00 0 8 85 l00 l00 2 9 85 100 100 4 1 194 l00 l00 1 2 194 100 100 1 3 194 l00 100 0 4 194 l00 l00 1 5 194 100 l00 2 6 194 100 l00 0 7 194 100 100 4 8 194 l00 100 0 9 194 l00 99 4 tests. Results of these tests are shown in Table 6. Qualitatively, the mold flora were the same after 194 days of storage as at the start, which is an indication that the storage conditions maintained were unfavor- able for storage mold growth. No adverse effects are anticipated from the high infestation of field fungi since field fungi do not seem to be associated with deterioration of sorghum seed during storage (2). Grain Moisture Content Results of all tests to date show that the grain moisture content can be controlled with conditioned- air storage procedures. Even though test results prove that a reduction in grain moisture is inherent during the cooling period in this type of storage, moisture can be transferred back to the grain by controlling the relative humidity of the interstice air. Although no difficulty was encountered by adding moisture to stored grain in these tests, research has indicated that there may be some limitations in this practice due to excessive bin wall pressures resulting from large in- creases in grain moisture contest (20). Since the quantity of water removed from the grain depends upon the relative humidity of the enter- ing air, it should be pointed out that this type of storage can be used to reduce the moisture content of stored grain as well as maintain the original mois- ture level. If it is desirable only to maintain the original moisture content, then the grain should re- 24 main in storage for a period of time s L; return the moisture back to the grain. It is p sible to predict this time because of the involved. The airflow rate, grain moisture f relative humidity of the entering air are l these variables. Normally the quantity of wat must be returned to the grain after the cooli ;‘ in order to re-establish the original moisture. would be approximatelyithe quantity req f increase the moisture by one-half of one r i a wet basis. i DESIGN PROCEDURE To illustrate the procedure to be use design of a conditioned-air storage system, co .2 following problem and its solution. l. Problem: To design a controlled en 'l system at College Station, Texas, for storing‘ bushels of sorghum grain. The storage facili of eight steel bins having a capacity of 36,0 ti? each. These bins are 32 feet in diameteran high. The grain will be received with -= temperature and moisture content of 95° F. percent, respectively. It is required that i mass be cooled to 55° F. with a final grain content of 14 percent. It is desired that the cooled to the final temperature in a m ' 25 days after filling. The estimated filling p bin per day. Each bin has an aeration syste of supplying air at a rate of 0.1 cfm per bi present. Y Cooling Procedure When more than one storage bin is in a system design, a cooling procedure must be ~-3_ to select the most economical number F conditioning units. This procedure must if account the number of bins, filling rate of T and the maximum time which grain can r in these bins before it should be cooled. l In the example problem, the number a required to fill the bins is 8. This would n‘ if one conditioning unit was used the n J would have to be 4 days for each bin in ord the last bin filled in 25 days. The airflow » 4-day cooling period would be approximately per bushel, Figure 8. Since this rate is in .9 <5 oi é) t; Aee latod Day! to I and Cool ' Tanki Band on a a 9 l! I6 22 23 _ 7-Day Goollnq t‘ Ported Sforaqe Period ‘Hetero Grain Ii 7 7 IZ l2 l1 l7 Cooled (Days) Figure 25. Filling and cooling procedure for eight bins and aerated with two conditioning units. . lite 0f 0.1 cfm per bushel, it would be cal to use two units. In this case the pure would be as shown in Figure 25. ons "tions on which the design must be b» temperature I 95° F. moisture content I 15% f. (based on design maximums for July at 1 College Station, Texas) lbulb I 95° F. “Bbulb = 80° F. point I 75° F. l ‘ve humidity —_— 52% py I 43.6 Btu per lb. dry air 'c volume I 14.4 cu. ft. per lb. dry air iHAIRZ i-min bulb I 95° F. i bulb = 91° F. i point I 90° F. jtive humidity I 85% A}. py I 57.1 Btu per lb. dry air Q ic volume I 14.7 cu. ft. per lb. dry air 21-’ e conditions leaving the grain are based that the air conditions leaving the mass uilibrium with the grain. l}; grain bulb = 55° F. l; bulb I 50° F. point I 45° F. tive humidity I 70% alpy = 20.2 Btu per lb. dry air 1'fic volume I 13.1 cu. ft. per lb. dry air l-bulb temperature of the air entering the lected at the same level as the desired temperature. The relative humidity of selected from Figure 13 so that it would f- with 14 percent moisture grain at i; conditioner l. bulb I 45° F. i bulb = 45° F. l point I 45° F. tive humidity; I 100% falpy I 17.62 Btu per lb. dry air 'fic volume I 12.84 cu. ft. per lb. dry air conditions assume that the air leaving the i; will be saturated. A dew-point tempera- F. was selected so that this air can be heated sensibly to 55° F. and have a relative humidity of 70 percent as required by the entering air condi- tions to the grain. Research has indicated that no supplemental heat is necessary since normal heat gain will be approximately 5°-10° F., the amount needed to maintain the entering air conditions to the grain. In case some supplemental heat is needed in a par- ticular design, heaters may be installed or some pro- vision may be made to use the heat exhausted from the condenser coils of the conditioning unit. Airflow Requirements Airflow rate I 0.1 cfm/bu. This value was selected from the available airflow in the existing aeration system and from Figure 8. Total flow I (0.1) (36000) I 3600 cfm Mass flow rate I I 16,488 lbs. dry 131 air per hr. Refrigeration Capacity If the design was based on a re-circulating air system, the change in the enthalpy of the air, Ah, as it passes through the grain mass would be equal to 57.10 — 17.62 I 39.48 Btu per pound dry air. The resulting refrigeration capacity per unit would then be (16,488) (39.48) 12,000 If the design was based on circulating outside air through the conditioning unit then Ah I 43.60 — 17.62 I 25.98 Btu per lb. dry air or I 54.2 tons (10,488) (25.98) 12,000 It will then be more economical to condition outside air until the enthalpy of the air leaving the grain mass is less than that of the outside air. This period can be determined by the time required for the lead- ing edge of the zone to move through the mass, Figure 8. After approximately 40 hours, the system can be operated as a re-circulating system. I 35.7 tons It should be noted that in this design method, no consideration need be given to the heat gain due to outside air conditions since the grain is at the same dry-bulb temperature as the outside air. By basing the design load on either the outside air conditions or the air leaving the grain mass, the refrigeration capacity would be large enough to take care of heat gain as the grain cools. The refrigeration capacity for the design problem was calculated to be 35.7 tons. However, two 35-ton units could probably be selected because some evapo- rative cooling will occur. Insulation Requirements If the maximum wall temperature is 60° F., the average outside air temperature is 85° F. and the 25 temperature gradient in the horizontal plane is allowed to extend over the full radius of the bin, the thickness of insulation could be calculated from Equations 18, 19 and 20. Using Equation 19, the quantity of air available to remove the heat transmitted from outside the bin would be as follows: 1r D2 1r(D—2Y)2 __ 4 _ 4 (Q) (C) (50) W" _" 7TD2 X Sp. v01. 4 1432? 7122-32? _ 4 _ 4 (0.1) (30000) (00) "' 11-(32)2 X 13.1 " 4 = 16,488 lbs. dry air per hr. If the final average air temperature exhausting from the grain is 57 .5° F ., then from equation 18, the heat which can be removed by the air is: (la I Wa C1101) (Tf—Ti) (16488) (.24) (575-55) = 9893 Btu per hr. Select an insulation such as epoxy or polyurethane spray-foam insulation. If the mean wall temperature is 57.5° F., then by equation 20, the thickness of this type of insulation would be: A(T,,——Tm) 1 x = K —————— — — q. f. X ._ 17 17 "‘ ' 9892 _' ' = 2.63 or 3" The solution to the example problem would be to install two 35-ton units on the first bins loaded. With an airflow rate of 0.1 cfm per bushel, these units could be moved to the other bins every 7 days until all bins were cooled. Outside air should be condi- tioned for approximately 40 hours, after which the system should be connected so that the air leaving the grain mass can be re-circulated through the condi- tioning unit. The units will have to be operated periodically’ on each bin after all of the grain is cooled in order to prevent excessive increase in grain tempera- tures due to heat gain and to reduce the grain moisture content to l4 percent. This solution appears to be the most practical at present. This would, however, depend on the cooling procedure. If only one bin is used as a conditioned- air storage system, the refrigeration capacity selected from this design method would be based on maximum conditions. This would mean that as soon as the leading edge of the zone has moved out of the grain mass, the refrigeration equipment would not be fully 26 AERATION TIME ~ HOURS SORGHUM 6R RUN M015 so 1 1 2 - - 1 - a --- 1 §=\\ AIR sursnme emu A \ 1=1.ow RATE = 0.12s DRY-BULB - 51.5‘F \ new POINT I 45‘ F 4o a \\ ‘so \. 5O 2c\ " 5111511110 A R SPECIFIC ENTHALPY OF lNTERSTlGE AIR — BTU PER LB 0F DRY AIR 0 2O 40 6O 8O I00 I20 I40 I60 I80’ AERATION TIME '- HOUR$ Figure 26. Specific enthalpy of interstice air in aeration time when cooling a mass of grain in storage. loaded. The load would gradually decrease and reach a minimum at the end of the cooling ' This relationship is shown in Figure 26 by the ence in the leaving and entering-air enthalpi i The solution of Equation (l3) would yield‘ approximation of the total load on the con unit. The normal ‘procedure would be to div total load by the cooling time in order to o average load. However, the system would loaded during the initial cooling period designed by the same amount at the end of the period. Preliminary tests indicate that this _ may be practical for reducing the initial refril unit size. Additional tests need to be cond determine the effect of this design method actual cooling time. RVEFERENCES l. Holman, L. E. Aeration of grain in commercial“. Marketing Research Report N0. 178. Transpo “ Facilities Research Division, AMS, USDA, Nov 2. Sorenson, ]. W., ]r., Kline, G. L., Redlinger, L. , port, M. G. and Aldred, W. H. Research 0n fa and storage of sorghum grain. Bulletin 885. T i cultural Experiment Station, December 1957. 3. Garner, C. F. and Ridgway, R. L. Control of Farm-stored Grain. Texas Agricultural Extensi f L-217A, Texas A8¢M University. o‘: sonsnum 0 2 RUN MOIS w 6° 1. 1 '9: E . 3' "" l m < 5o €—- \‘ AIR ENTERING GRAIN r l! >. \ \ now RATE = 0.091 f.’ n: \ DRY-BULB =s1.s'1= g a 4o ‘\\ new POINT =45°F ‘6 ° \\ g- 3 3° \\\ _.l \*k ~ < 05 ‘ H. . = 11* \ . —- F“ 2O . 5 ,2 ENTERING AIR Q (D E 1o G 111 . 31 0 2o 40 so a0 10o 12o 14o 160 1801 ti» y, W. Keith. Fumigation hazards as related to the ‘ , chemical and biological properties of fumigants. -tro1 Magazine, July 1961. 4- n, Clyde M. Deterioration of stored grains by i The Botanical Review 23: (2) 108-134, February i» B. D., Bayliss, Mary E. and Richardson, L. R. The hip of interspace relative hu_midity to growth of and heating in feed ingredients and feed mixtures. ent of Biochemistry and Nutrition, Texas A8cM v'uk, G., Anderson, T. S. and Alcock, A. W. Micro- in storage of cereal grains and their products. Ameri- Association of Cereal Chemists. Monograph series 2: l, St. Paul, Minnesota, 1954. ltural Engineers Yearbook, 1964. i J. R. Effects of temperature and moisture on A.‘ grain insects. Proceedings of Conference on Stored __» Insects and Their Control. Kansas State University. ‘ey, W. Keith and Pedersen, J. R. Physical and g» nical methods of stored-product insect control. Pro- i-tgs of Conference on Stored Grain Insects and Their o1. Kansas State University. Manhattan, Kansas. ber, 1961. eld, J. L. 'ce-Hal1, Inc. Thermal environmental engineering. p. 274. 1962. 12. 13. 14. 15. l6. 17. 18. 19. 20. Pfalzner, P. M. The specific heat or wheat. Canadian Journal of Technology. Vol. 29, pp. 261-268. 1951. Babbitt, J. D. The thermal properties of wheat in bulk. Canadian Journal of Research F. Vol. 23,pp. 388-401. 1945. Disney, R. W. The specific heat of some cereal grains. American Association of Cereal Chemists. Vol. XXXI, No. 3, May, 1954. Haswell, G. A. A note on the specific heat of rice, oats, and their products. American Association of Cereal Chemists. Vol. XXXI, No. 4, p. 341. Kelly, C. F. Methods of ventilating wheat in farm storage. United States Department of Agriculture, Circular No. 544, p. 73. 1940. Miller, C. F. Thermal conductivity and specific heat of sorghum grain. Unpublished Thesis, Department of Agri- cultural Engineering, Texas A8cM Universtiy. 1963. Miller, J. A. The effects of aeration time for various air- flow rates on the properties of the effluent air from grain aerated in storage. Unpublished Thesis, Department of Agricultural Engineering, Texas AScM University. 1965. Haile, Danel G. The effect of respiration heat of sorghum grain on the design of conditioned-air storage systems. Un- published Thesis, Department of Agricultural Engineering, Texas AScM University. 1967. Dale, A. C. and Robinson, R. N. Pressures in Deep Grain Storage Structures. Agricultural Engineering, Vol. 35, No. 8, pp. 570-573, August, 1954. 27 Texas Agricultural Experiment Station Texas A8cM University College Station, Texas 77843 H. 0. Kunkel, Acting Dircctor- Publication ‘x a £5.