ASHRAE HVAC APPLICATIONS SI CH 25-2015 DRYING AND STORING SELECTED FARM CROPS.pdf

1、25.1CHAPTER 25DRYING AND STORING SELECTED FARM CROPSDRYING . 25.2Drying Equipment and Practices. 25.2Shallow-Layer Drying 25.3Deep -Bed Drying 25.4DRYING SPECIFIC CROPS 25.7Soybeans 25.7Hay. 25.7Cotton. 25.8Peanuts . 25.8Rice. 25.9STORAGE PROBLEMS AND PRACTICES . 25.9Moisture Migration 25.9Grain Aer

2、ation . 25.9SEED STORAGE 25.11ONTROL of moisture content and temperature during storageC is critical to preserving the nutritional and economic value offarm crops as they move from the field to the market. Fungi (mold)and insects feed on poorly stored crops and reduce crop quality.Relative humidity

3、and temperature affect mold and insect growth,which is reduced to a minimum if the crop is kept cooler than 10Cand if the relative humidity of the air in equilibrium with the storedcrop is less than 60%.Mold growth and spoilage are a function of elapsed storage time,temperature, and moisture content

4、 above critical values. Approxi-mate allowable storage life for cereal grains is shown in Table 1. Forexample, corn at 16C and 20% wet basis (w.b.) moisture has a stor-age life of about 25 days. If it is dried to 18% w.b. after 12 days, halfof its storage life has elapsed. Thus, the remaining storag

5、e life at16C and 18% w.b. moisture content is 25 days, not 50 days.Insects thrive in stored grain if the moisture content and temper-ature are not properly controlled. At low moisture contents and tem-peratures below 10C, insects remain dormant or die.Most farm crops must be dried to, and maintained

6、 at, a suitablemoisture content. For most grains, a suitable moisture content is inthe range of 12 to 15% w.b., depending on the specific crop, storagetemperature, and length of storage. Oilseeds such as peanuts, sun-flower seeds, and flaxseeds must be dried to a moisture content of 8to 9% w.b. Grai

7、n stored for more than a year, grain that is damaged,and seed stock should be dried to a lower moisture content. Moisturelevels above these critical values lead to the growth of fungi, whichmay produce toxic compounds such as aflatoxin.The maximum yield of dry matter can be obtained by harvestingwhe

8、n the corn has dried in the field to an average moisture content of26% w.b. However, for quality-conscious markets, the minimumdamage occurs when corn is harvested at 21 to 22% w.b. Wheat canbe harvested when it has dried to 20% w.b., but harvesting at thesemoisture contents requires expensive mecha

9、nical drying. Althoughfield drying requires less expense than operating drying equipment,total cost may be greater because field losses generally increase asthe moisture content decreases.The price of grain to be sold through commercial market channelsis based on a specified moisture content, with p

10、rice discounts formoisture levels above the specified amount. These discounts com-pensate for the mass of excess water, cover the cost of water removal,and control the supply of wet grain delivered to market. Grain driedto below the base moisture content set by the market (15.0% w.b. forcorn, 13.0%

11、w.b. for soybeans, and 13.5% w.b. for wheat) is not gen-erally sold at a premium; thus, the seller loses the opportunity to sellwater for the price of grain.Grain QuantityThe bushel is a volume measure (0.03524 m3) that is the commonmeasure used for marketing grain in the United States, while theton

12、ne (Mg) is the more common international measure. The legalmass for the bushel in the United States is set at 25.40 kg for corn and27.22 kg for wheat. The densities of some crops are listed in Table 2.The percent of mass lost due to water removed may be calculatedby the following equation:Moisture s

13、hrink, % = 100whereMo= original or initial moisture content, wet basisMf= final moisture content, wet basisApplying the formula to drying a crop from 25% to 15%,Moisture shrink = 100 = 11.76%In this case, the moisture shrink is 11.76%, or an average 1.176%mass reduction for each percentage point of

14、moisture reduction. Themoisture shrink varies depending on the final moisture content. Forexample, the average shrink per point of moisture when drying from20% to 10% is 1.111.EconomicsProducers generally have the choice of drying their grain on thefarm before delivering it to market, or delivering

15、wet grain with aprice discount for excess moisture. The expense of drying on the farmThe preparation of this chapter is assigned to TC 2.2, Plant and AnimalEnvironment.Table 1 Approximate Allowable Storage Time (Days) for Cereal GrainsMoistureContent, % w.b.aTemperature, C1 4 10 16 22 2714 * * * * 2

16、00 14015 * * * 240 125 7016 * * 230 120 70 4017 * 280 130 75 45 2018 * 200 90 50 30 1519 * 140 70 35 20 1020 *90502514 722 190 60 30 15 8 324 130 40 15 10 6 26 90351285228 7 0074230 60255531Based on composite of 0.5% maximum dry matter loss calculated on the basis ofUSDA research; Transactions of AS

17、AE 333-337, 1972; and “Unheated Air Drying,”Manitoba Agriculture Agdex 732-1, rev. 1986.aGrain moisture content calculated as percent wet basis: (mass of water in a givenamount of wet grain mass of the wet grain) 100.*Approximate allowable storage time exceeds 300 days.MoMf100 Mf-25 15100 15-25.2 20

18、15 ASHRAE HandbookHVAC Applications (SI)includes both fixed and variable costs. Once a dryer is purchased, thecosts of depreciation, interest, taxes, and repairs are fixed and mini-mally affected by volume of crops dried. The costs of labor, fuel, andelectricity vary directly with the volume dried.

19、Total drying costsvary widely, depending on the volume dried, the drying equipment,and fuel and equipment prices. Energy consumption depends primar-ily on dryer type. Generally, the faster the drying speed, the greaterthe energy consumption (Table 3).1. DRYING1.1 DRYING EQUIPMENT AND PRACTICESContem

20、porary crop-drying equipment depends on mass andenergy transfer between the drying air and the product to be dried.The drying rate is a function of the initial temperature and moisturecontent of the crop, the air-circulation rate, the entering condition ofthe circulated air, the length of flow path

21、through the products, andthe time elapsed since the beginning of the drying operation. Out-door air is frequently heated before it is circulated through the prod-uct. Heating increases the rate of heat transfer to the product,increases its temperature, and increases the vapor pressure of theproduct

22、moisture. For more information on crop responses to drying,see Chapter 11 of the 2005 ASHRAE HandbookFundamentals.Most crop-drying equipment consists of (1) a fan to move theair through the product, (2) a controlled heater to increase theambient air temperature to the desired level, and (3) a contai

23、nerto distribute the drying air uniformly through the product. Theexhaust air is vented to the atmosphere. Where climate and otherfactors are favorable, unheated air is used for drying, and theheater is omitted.FansThe fan selected for a given drying application should meet thesame requirements impo

24、rtant in any air-moving application. It mustdeliver the desired amount of air against the static resistance of theproduct in the bin or column, the resistance of the delivery system,and the resistance of the air inlet and outlet.Foreign material in the grain can significantly change the re-quired ai

25、r pressure in the following ways:Foreign particles larger than the grain (straw, plant parts, andlarger seeds) reduce airflow resistance. The airflow rate may beincreased by 60% or more.Foreign particles smaller than the grain (broken grain, dust, andsmall seeds) increase the airflow resistance. The

26、 effect may bedramatic, decreasing the airflow rate by 50% or more.The method used to fill the dryer or the agitation or stirring of thegrain after it is placed in the dryer can increase pressure require-ments by up to 100%. In some grain, high moisture causes lesspressure drop than does low moistur

27、e.Vaneaxial fans are normally recommended when static pres-sures are less than 0.75 kPa. Backward-curved centrifugal fans arecommonly recommended when static pressures are higher than1.0 kPa. Low-speed centrifugal fans operating at 1750 rpm per-form well up to about 1.75 kPa, and high-speed centrifu

28、gal fansoperating at about 3500 rpm have the ability to develop static pres-sure up to about 2.5 kPa. The in-line centrifugal fan consists of acentrifugal fan impeller mounted in the housing of an axial flowfan. A bell-shaped inlet funnels the air into the impeller. The in-line centrifugal fan opera

29、tes at about 3450 rpm and has the abilityto develop pressures up to 2.5 kPa on 6 kW or larger fans.After functional considerations are made, the initial cost of thedryer fan should be taken into account. Drying equipment has a lowpercentage of annual use in many applications, so the cost of dryerown

30、ership per unit of material dried is sometimes greater than theenergy cost of operation. The same considerations apply to othercomponents of the dryer.HeatersMost crop dryer heaters are fueled by either natural gas, liquefiedpetroleum gas, or fuel oil, though some electric heaters are used.Dryers us

31、ing coal, biomass (e.g., corn cobs, stubble, or wood), andsolar energy have also been built.Fuel combustion in crop dryers is similar to combustion indomestic and industrial furnaces. Heat is transferred to the dryingair either indirectly, by means of a heat exchanger, or directly, bycombining the c

32、ombustion gases with the drying air. Direct combus-tion heating is generally limited to natural gas or liquefied petro-leum (LP) gas heaters. Most grain dryers use direct combustion.Indirect heating is sometimes used in drying products such as haybecause of its greater fire hazard.ControlsIn additio

33、n to the usual temperature controls for drying air, allheated air units must have safety controls similar to those found onspace-heating equipment. These safety controls shut off the fuel incase of flame failure and stop the burner in case of overheating orTable 2 Calculated Densities of Grains and

34、Seeds Based on U.S. Department of Agriculture DataBulk Density, kg/m3Alfalfa 768Barley 614Beans, dry 768Bluegrass 180 to 384Canola 643 to 770Clover 768CornEar, husked 448Shelled 717Cottonseed 410Oats 410Peanuts, unshelledVirginia type 218Runner, Southeastern 269Spanish 317Rice, rough 576Rye 717Sorgh

35、um 640Soybeans 768Sudan grass 768SunflowerNonoil 307Oilseed 410Wheat 768Table 3 Estimated Corn Drying Energy RequirementDryer Type kJ/kg of Water RemovedUnheated air 2300 to 2800Low temperature 2800 to 3500Batch-in-bin, continuous-flow in-bin 3500 to 4700High temperatureAir recirculating 4200 to 510

36、0Without air recirculating 4700 to 7000Combination drying, dryeration 3300 to 4200Note: Includes all energy requirements for fans and heat.Drying and Storing Selected Farm Crops 25.3excessive drying air temperatures. All controls should be set up tooperate the machinery safely in the event of power

37、failure.1.2 SHALLOW-LAYER DRYINGBatch DryersThe batch dryer cycles through the loading, drying, cooling, andunloading of the grain. Fans force hot air through columns (typically300 mm wide) or layers (600 to 1500 mm thick) of grain. Dryingtime depends on the type of grain and the amount of moisture

38、to beremoved. Some dryers circulate and mix the grain to prevent signif-icant moisture content gradients from forming across the column. Acirculation rate that is too fast or a poor selection of handling equip-ment may cause undue damage and loss of market quality. Batch dry-ers are suitable for far

39、m operations and are often portable.Continuous-Flow DryersThis type of self-contained dryer passes a continuous stream ofgrain through the drying chamber. Some dryers use a second cham-ber to cool the hot, dry grain before storage. Handling and storageequipment must be available at all times to move

40、 grain to and fromthe dryers. These dryers have cross-flow, concurrent flow, or coun-terflow designs.Cross-Flow Dryers. A cross-flow dryer is a column dryer thatmoves air perpendicular to the grain movement. These dryerscommonly consist of two or more vertical columns surrounding thedrying and cooli

41、ng air plenums. The columns range in thicknessfrom 200 to 400 mm. Airflow rates range from 0.7 to 2.7 m3/s percubic metre of grain. The thermal efficiency of the drying processincreases as column width increases and decreases as airflow rateincreases. However, moisture uniformity and drying capacity

42、increase as airflow rate increases and as column width decreases.Dryers are designed to obtain a desirable balance of airflow rate andcolumn width for the expected moisture content levels and dryingair temperatures. Performance is evaluated in terms of drying capac-ity, thermal efficiency, and dried

43、 product moisture uniformity.As with the batch dryer, a moisture gradient forms across the col-umn because the grain nearest the inside of the column is exposed tothe driest air during the complete cycle. Several methods minimizethe problem of uneven drying.One method uses turnflow devices that spli

44、t the grain stream andmove the inside half of the column to the outside and the outside halfto the inside. Although effective, turnflow devices tend to plug if thegrain is trashy. Under these conditions, a scalper/cleaner should beused to clean the grain before it enters the dryer.Another method is

45、to divide the drying chamber into sections andduct the hot air so that its direction through the grain is reversed inalternate sections. This method produces about the same effect asthe turnflow method.A third method is to divide the drying chamber into sections andreduce the drying air temperature

46、in each section consecutively.This method is the least effective.Rack-Type Dryers. In this special type of cross-flow dryer,grain flows over alternating rows of heated air supply ducts and airexhaust ducts (Figure 1). This action mixes the grain and alternatesexposure to relatively hot drying air an

47、d air cooled by previous con-tact with the grain, promoting moisture uniformity and equal expo-sure of the product to the drying air.Concurrent-Flow Dryers. In the concurrent-flow dryer, grainand drying air move in the same direction in the drying chamber.The drying chamber is coupled to a counterfl

48、ow cooling section.Thus, the hottest air is in contact with the wettest grain, allowingthe use of higher drying air temperatures (up to 230C). Rapidevaporative cooling in the wettest grain prevents the grain tem-perature from reaching excessive levels. Because higher dryingair temperatures are used,

49、 the energy efficiency is better than thatobtained with a conventional cross-flow dryer. In the coolingsection, the coolest air initially contacts the coolest grain. Thecombination of drying and cooling chambers results in lowerthermal stresses in the grain kernels during drying and coolingand, thus, a higher-quality product.Counterflow Dryers. The grain and drying air move in oppositedirections in the drying chamber of this dryer. Counterflow is com-mon for in-bin dryers. Drying air enters from the bottom of the binand exits fro