ASHRAE HVAC APPLICATIONS IP 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. 25.8Cotton. 25.8Peanuts . 25.9Rice. 25.9STORAGE PROBLEMS AND PRACTICES . 25.9Moisture Migration 25.9Grain Ae

2、ration . 25.10SEED STORAGE 25.12ONTROL 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 humidit

3、y and temperature affect mold and insect growth,which is reduced to a minimum if the crop is kept cooler than 50Fand 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 conte

4、nt above critical values. Approxi-mate allowable storage life for cereal grains is shown in Table 1. Forexample, corn at 60F 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 stor

5、age life at60F 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 50F, insects remain dormant or die.Most farm crops must be dried to, and maintain

6、ed 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. Gr

7、ain 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 harvestingw

8、hen 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 mec

9、hanical 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

10、 price discounts formoisture levels above the specified amount. These discounts com-pensate for the weight of excess water, cover the cost of waterremoval, and control the supply of wet grain delivered to market.Grain dried to below the base moisture content set by the market(15.0% w.b. for corn, 13

11、.0% w.b. for soybeans, and 13.5% w.b. forwheat) is not generally sold at a premium; thus, the seller loses theopportunity to sell water for the price of grain.Grain QuantityThe bushel is the common measure used for marketing grain inthe United States. Most dryers are rated in bushels per hour for as

12、pecified moisture content reduction. The use of the bushel as a mea-sure causes considerable confusion. A bushel is a volume measureequal to 1.244 ft3. The bushel is used to estimate the holding capacityof bins, dryers, and other containers.For buying and selling grain, for reporting production and

13、con-sumption data, and for most other uses, the bushel weight is used.For example, the legal weight of a bushel is 56 lb for corn and 60 lbfor wheat. When grain is marketed, bushels are computed as the loadweight divided by the bushel weight. So, 56,000 lb of corn (regard-less of moisture content) i

14、s 1000 bushels. Rice, grain sorghum, andsunflower are more commonly traded on the basis of the hundred-weight (100 lb), a measure that does not connote volume. The rela-tionship between bushel by volume and market bushel is the bulkdensity (listed for some crops in Table 2). For some crops, the mar-

15、ket has defined a test weight parameter, lb/bu. Test weight is essen-tially the bulk density, with bushels and cubic feet related by thedefinition of 1 bushel = 1.244 cubic feet.The terms wet bushel and dry bushel sometimes refer to themass of grain before and after drying. For example, 56,000 lb of

16、 25%moisture corn may be referred to as 1000 wet bushels or simply 1000bushels. When the corn is dried to 15.5% moisture content (m.c.),only 49,704 lb or 49,704/56 = 888 bushels remain. Thus, a dryerrated on the basis of wet bushels (25% m.c.) shows a capacity 12.6%higher than if rated on the basis

17、of dry bushels (15.5% m.c.).The percent of weight lost due to water removed may be calcu-lated by the following equation:The 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.aTempe

18、rature, F30 40 50 60 70 8014 * * * * 200 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 6 226 90351285228 70301074230 60255531Based on composite of 0.5% maximum dry matter loss calculated on the

19、basis ofUSDA research; Transactions of ASAE 333-337, 1972; and “Unheated Air Drying,”Manitoba Agriculture Agdex 732-1, rev. 1986.aGrain moisture content calculated as percent wet basis: (weight of water in a givenamount of wet grain weight of the wet grain) 100.*Approximate allowable storage time ex

20、ceeds 300 days.25.2 2015 ASHRAE HandbookHVAC ApplicationsMoisture shrink, % = 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

21、%, or an average 1.176%weight reduction for each percentage point of moisture reduction.The moisture shrink varies depending on the final moisture content.For example, the average shrink per point of moisture when dryingfrom 20% to 10% is 1.111.EconomicsProducers generally have the choice of drying

22、their grain on thefarm before delivering it to market, or delivering wet grain with aprice discount for excess moisture. The expense of drying on the farmincludes both fixed and variable costs. Once a dryer is purchased, thecosts of depreciation, interest, taxes, and repairs are fixed and mini-mally

23、 affected by volume of crops dried. The costs of labor, fuel, andelectricity vary directly with the volume dried. 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 fast

24、er the drying speed, the greaterthe energy consumption (Table 3).1. DRYING1.1 DRYING EQUIPMENT AND PRACTICESContemporary 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 moistureco

25、ntent of the crop, the air-circulation rate, the entering condition ofthe circulated air, the length of flow path 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 t

26、he rate of heat transfer to the product,increases its temperature, and increases the vapor pressure of theproduct 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

27、the product, (2) a controlled heater to increase theambient air temperature to the desired level, and (3) a containerto 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,

28、and theheater is omitted.FansThe fan selected for a given drying application should meet thesame requirements important 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

29、the resistance of the air inlet and outlet.Foreign material in the grain can significantly change the re-quired air 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.

30、Foreign particles smaller than the grain (broken grain, dust, andsmall seeds) increase the airflow resistance. The 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

31、 pressure require-ments by up to 100%. In some grain, high moisture causes lesspressure drop than does low moisture.Vaneaxial fans are normally recommended when static pres-sures are less than 3 in. of water. Backward-curved centrifugalfans are commonly recommended when static pressures are higherth

32、an 4 in. of water. Low-speed centrifugal fans operating at 1750rpm perform well up to about 7 in. of water, and high-speed cen-trifugal fans operating at about 3500 rpm have the ability todevelop static pressure up to about 10 in. of water. The in-line cen-trifugal fan consists of a centrifugal fan

33、impeller mounted in thehousing of an axial flow fan. A bell-shaped inlet funnels the airinto the impeller. The in-line centrifugal fan operates at about3450 rpm and has the ability to develop pressures up to 10 in. ofwater with 7.5 hp or larger fans.After functional considerations are made, the init

34、ial cost of thedryer fan should be taken into account. Drying equipment has a lowpercentage of annual use in many applications, so the cost of dryerownership per unit of material dried is sometimes greater than theenergy cost of operation. The same considerations apply to othercomponents of the drye

35、r.Table 2 Calculated Densities of Grains and SeedsBased on U.S. Department of Agriculture DataBulk Density, lb/ft3Alfalfa 48.0Barley 38.4Beans, dry 48.0Bluegrass 11.2 to 24.0Canola 40.2 to 48.2Clover 48.0Corn*Ear, husked 28.0Shelled 44.8Cottonseed 25.6Oats 25.6Peanuts, unshelledVirginia type 13.6Run

36、ner, Southeastern 16.8Spanish 19.8Rice, rough 36.0Rye 44.8Sorghum 40.0Soybeans 48.0Sudan grass 32.0SunflowerNonoil 19.3Oilseed 25.7Wheat 48.0*70 lb of husked ears of corn yield 1 bushel, or 56 lb of shelled corn. 70 lb of ears ofcorn occupy 2 volume bushels (2.5 ft3).MoMf100 Mf-25 15100 15-Table 3 E

37、stimated Corn Drying Energy RequirementDryer Type Btu/lb of Water RemovedUnheated air 1000 to 1200Low temperature 1200 to 1500Batch-in-bin, continuous-flow in-bin 1500 to 2000High temperatureAir recirculating 1800 to 2200Without air recirculating 2000 to 3000Combination drying, dryeration 1400 to 18

38、00Note: Includes all energy requirements for fans and heat.Drying and Storing Selected Farm Crops 25.3HeatersMost crop dryer heaters are fueled by either natural gas, liquefiedpetroleum gas, or fuel oil, though some electric heaters are used.Dryers using coal, biomass (e.g., corn cobs, stubble, or w

39、ood), 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 combustion gases with the drying air. Direct combu

40、s-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 addition to the usual temperature controls for drying ai

41、r, 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 orexcessive drying air temperatures. All controls should be set up tooperate the machinery saf

42、ely in the event of power 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 (typically12 in. wide) or layers (2 to 5 ft thick) of grain. Drying time dependson the type of grain and th

43、e amount of moisture to be removed. Somedryers circulate and mix the grain to prevent significant moisturecontent gradients from forming across the column. A circulation ratethat is too fast or a poor selection of handling equipment may causeundue damage and loss of market quality. Batch dryers are

44、suitablefor farm 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 al

45、l times to move 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 thed

46、rying and cooling air plenums. The columns range in thicknessfrom 8 to 16 in. Airflow rates range from 40 to 160 cfm per cubicfoot of grain. The thermal efficiency of the drying process increasesas column width increases and decreases as airflow rate increases.However, moisture uniformity and drying

47、 capacity increase as air-flow rate increases and as column width decreases. Dryers aredesigned to obtain a desirable balance of airflow rate and columnwidth for the expected moisture content levels and drying air tem-peratures. Performance is evaluated in terms of drying capacity,thermal efficiency

48、, and dried 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 device

49、s that split 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 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 divi