DUCT - spiru.cgahr.ksu.edu
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1. then we can readily show that the air velocity in the vertical direction Vyertical m s 18 Vuertical 1 X 1078 x Pg Hecea Qs 2 and the time for the air to traverse pass through the height of the store tar sec is 1 x 10 t 3 air Pa Q Consequently when we define the treatment rate using a specific air flow rate this automat ically ensures that e the vertical velocity Vyertical is proportional to the seed height equation 2 e all geometric factors floor area and height cancel out leaving the time for the azr to traverse the height of the store tair dependent only on the specific air flow rate Q and the bulk seed density pg equation 3 and as pg is roughly constant e the air traverse time tair 1s determined by the specific air flow rate Note that there is a difference between the air traverse time and the time for a cooling front to traverse the seed The traverse time for a cooling front is much greater than the air traverse tame because it takes a large volume of air to cool a little seed The cooling front traverse time depends on a number of factors including the air traverse time the seed type the seed moisture content and the initial seed temperature As shown in Part 2 of the SGRL Aeration Manual Wilson in press conservative design temperature and moisture conditions can be chosen that allow selection of a design specific air flow rate value Qs design that ensures an acceptable2. maximum aeration cooling time teooting under worst case conditions For example for aeration of wheat in cooler parts of Australia we aim for a maximum cooling time teooting of 5 weeks and recommend Qs design Values of 0 95 s tonne for horizontal stores and 0 6 s tonne for vertical stores Recirculating Fumigation Systems Atr Change Time H For recirculating fumigation sys tems program DUCT specifies the fumigation treatment rate by the number of hours taken to recirculate the air in the store H hours air change Flow Through Fumigation Systems These systems vent low fumigant concentrations through an unsealed seed store to atmosphere During this process fumigant concentrations are cost effectively maintained at a sufficient concentration over a time that kills all insect phases The program automatically calculates the appropriate treatment rate for you when flow through fu migation systems are selected Recommended Values of Duct Flow Uniformity T The ratio of the height to the width of a store is important because it affects the uniformity of flow through the seed bulk 1836 The flow tends to be more uniform in tall narrow stores which we call vertical stores than it is in wide squat stores which we call horizontal stores To decide whether a store is vertical or horizontal we suggest use of the following rule A vertical store has a mean grain depth Hgrain greater than or equal to twice the square
3. root of the total floor area of the store Astore If the store doesn t satisfy this condition it is a horizontal store Algebraically the store is vertical if A grain gt 2y Agtore 4 and the store 1s a horizontal if grain lt 2v Astore 5 Because the flow tends to be more uniform in vertical stores than it is in horizontal stores the duct flow uniformity 7 needed to achieve good flow uniformity in the seed bulk depends upon the seed store type For e horizontal seed stores you need high 7 values for example r 0 9 to assure uniform seed treatment and for e vertical seed stores you can use lower r values for example 0 5 lt r lt 0 9 You can have lower values of for vertical stores because away from the duct the flow redistributes and becomes more uniform Consequently low flow regions at one end of the duct affect only seed in the region of the duct Flow redistribution usually occurs within a height about equal to the width of the store provided the seed treatment rate is determined by a specific air flow rate Q then the vertical velocity is proportional to the height of the store equation 2 As vertical stores have a greater height than horizontal stores the vertical velocity in such stores is also higher Consequently even if there is a lower flow rate region at one end of the duct the vertical velocity in that region is still high enough to ensure satisfactory treatment Clearly i
4. 5th Int Working Conf on Stored Products Protection Bordeaux France September 1990 DUCT A PC Program for Sizing and Evaluating Grain Store Ducts S G WILSON Stored Grain Research Laboratory SGRL CSIRO Division of Entomology PO Box 56 Highett 3190 AUSTRALIA Abstract In the past seed store ducts have usually been designed by use of velocity limiting rules and guess work rather than by predictive methods To make a predictive method accessible to designers we have developed program DUCT It is an easily used IBM Personal Computer PC or PC compatible program that predicts velocity and pressure distributions and determines duct flow uniformity and fan pressure load for seed store ducts Program DUCT can also find the smallest duct size that achieves a specified flow uniformity or fan pressure load This enables designers to readily optimize their duct design by finding the smallest duct size that achieves an acceptable flow uniformity or fan pressure load This paper describes the features of program DUCT and considers some of the factors influencing selection of the program s input values Introduction Previous Work Computer models capable of predicting seed store duct performance see for example Shove and Hukill 1963 Marchant and Nellist 1977 Burrell et al 1982 have existed for some time However these predictive models have not been made widely available to duct designers Duct de signers have theref
5. am Program DUCT e enables design of aeration and fumigation ducts in vertical and horizontal seed stores e of any floor plan and any duct lay out and applies to packed or unpacked seed and e to suction and blowing ducts The program employs stable numerical methods that enable its calculations to take only a few minutes on a basic PC The program includes an on line help facility that explains how to select its input values The program s User Manual expands on the material included in this paper and explains how to design duct layout The program also plots graphs to enhance the user s understanding of its results These graphs show the variation of air pressure and longitudinal and face velocities along the duct The graphs and all text output from the program can be saved and printed For blowing ducts in aeration systems the program also calculates the temperature rise through the fan Tables 1 to 5 included in the program s User Manual Wilson in press and in the programs on line help give values ebablingthe program to be used for a witle variety of grains oilseeds and legumes and for common seed store and duct materials The program applies to seven common duct geometries these being e round ducts e half round ducts e above floor rectangular ducts e in floor rectangular ducts e split in floor rectangular ducts e shedder plate ducts and e corner ducts Duct Flow Uniformity and Fan Pressure Loa
6. d Two factors that are very important to the design of ducts are 1 the flow uniformity along the duct and 2 the amount of pressure that the fan needs to generate in order to force the air along the duct and through the seed bulk We call this pressure the fan pressure load 1 Duct Flow Unsformity r As air flows along a duct some of it must also pass through perforations in the duct face to aerate or fumigate the seed bulk The flow rate through the duct face is called the face velocity u m s Ideally to ensure that seed at one end of the duct does not receive more aeration of fumiga tion treatment than seed at the other end the face velocity should be the same at any point 1834 along the duct This ideal of complete flow uniformity cannot be achieved unless the duct is very large and expensive so in practice we usually strike a compromise by accepting a reduced flow uniformity in return for a smaller less expensive duct Program DUCT calculates the duct flow uniformity 7 that is dimensionless as r amin 1 Umaz where the subscripts maz and min indicate the largest and smallest u values respectively that occur along the duct Velocity limiting methods of duct design do not allow you to determine the duct flow uni formity and this is precisely why they can cause problems in design of long ducts Fan Pressure Load F The fan pressure load arises from two sources a The pressure drop cause
7. d by the height of the seed that the air passes through This pressure drop is unavoidable and is the minimum possible fan pressure load Fmin b The pressure drop caused by constriction of the flow field around the duct This value is effected by the size of the duct Program DUCT takes into account both of these pressure drops and allows you to use a search mode explained below to optimize the pressure drop caused by flow constriction Evaluation and Search Modes l There are two situations commonly faced by duct designers For existing ducts they need to evaluate whether those ducts are suitable for a particular application When designing new ducts they need to be able to find the optimum szze needed to achieve satisfactory duct performance To meet these needs program DUCT can be used in two modes 1 Evaluation mode finds the duct flow uniformity 7 and fan pressure load F Pa that occur for a specified duct cross sectional area A m You can use evaluation mode to decide if existing ducts are adequate or would be useful in new circumstances Search mode determines the smallest duct area A needed to attain a specified 7 value or a specified F value As above floor duct costs increase at somewhere between the square and the cube of the duct width even small reductions in duct size can give considerable cost savings You can use search mode to design new ducts After the search program DUCT allows you t
8. d packing causes closure of the spaces between the seeds This restricts the flow of air through the seed increasing the pressure load on the fan and making it more difficult to treat the seed Packing of seed occurs over time because of compression caused by the weight of the seed above the duct Vibration for example from passing trains or unbalanced fan motors also increases packing Program DUCT allows the designer either to ignore seed packing or to specify the of packing Allowance for up to 30 packing is recommended where seed is to be stored for lengthy periods How to Optimize Duct Design If a duct is unnecessarily large it will perform well but it will be expensive If a duct is too small its performance will not be satisfactory the fan pressure load will be high or there will be a non uniform flow along the duct causing over treatment of seed at the fan end and an insufficient seed treatment at the other end A duct designer s most challenging task is to optimize the duct size to find the smallest duct that will give satisfactory flow uniformity and fan pressure load To allow you to easily find the optimum duct size program DUCT has a search mode that can perform two types of search e it can find the duct size that is duct cross sectional area that gives a specified value of flow uniformity 7 and e it can find the duct size that gives a specified value of fan pressure load F We will refer to these two search
9. e that sometimes has to be paid for the convenience and better flow distribution achieved by vertical stores 2 It is not possible to reduce F below Fmin 1839 a If Fmin gt 1500 Pa the best you can do is to reduce F to a value somewhat greater say 100 Pa greater than Fan but still above 1500 Pa b If Fmin lt 1500 Pa we suggest that you try to reduce F to about 1500 Pa It may also be possible to reduce F below 1500 Pa If you do you should judge whether the further reduction is worthwhile by balancing duct cost against fan capital and running costs 3 F is influenced by the duct geometry and width The fan pressure load is determined by both the seed depth and by flow constriction around the duct If we make the duct larger and wider this will lessen the flow constriction around the duct and the fan pressure load will therefore be reduced Sometimes cost effective reductions in F can be achieved by making the duct only a lit tle larger and hence wider or by changing the duct geometry Some geometries have a greater width for a given duct cross sectional area For example switching from an in floor rectangular duct to a split in floor rectangular duct increases the width for the same cross sectional area Also for rectangular ducts you can obtain a greater width by decreasing the height to width ratio of the duct However you may also find that in obtaining reductions in F the duct becomes unaccept ably
10. es as the t search and F search respectively Recommended Search Procedure search mode must be used correctly to avoid problems We recommend that you adopt the following procedure 1838 e Perform a 7 search first thereby obtaining the duct size needed to achieve the specified T value The r search will also give values for the fan pressure load occurring with that T value Fy and the minimum possible fan pressure load Fmin e You only need to perform an F search if F is too high greater than 1500 Pa If Fy is less than 1500 Pa your design problem is solved you have already obtained the smallest duct size needed to give the specified flow uniformity with an acceptable fan pressure load You do not need to do an F search If F is greater than 1500 Pa you may be able to obtain a useful reduction in fan pressure load by changing the duct geometry or making the duct marginally larger To find out if this is possible use the program s F search e For the F search keep the F values that you specify between F and Fmin Reducing F from F towards Fmin will make the duct progressively larger Consequently you need to balance duct cost which is determined largely by duct size against fan capital and running costs which are determined by F You will probably need to perform a number of F searches to allow you to strike this balance Effect of Fan Pressure Load F The fan pressure load F e determines both the fan run
11. f the vertical store only just satisfies equation 4 then r should be closer to 0 9 than to 0 5 As the height increases beyond 2 Agtore 80 T can be reduced toward 0 5 T values in Evaluation Mode In evaluation mode the program calculates the flow uniformity T achieved by a duct of specified size Although you can make your own judgements as to whether the r value is acceptable the program will also advise you as to whether the 7 value satisfies the following recommended ranges Table 1 Satisfactory 7 Ranges when Evaluating Duct Performance Store Type Height Range Satisfactory 7 Range according to Store Geometry Horizontal Heed lt 2V Astore 0 87 lt 7 lt 0 98 Wide Vertical ZV Astore lt Heed lt dy Astore 2 Faia lt T lt 0 93 slore Narrow Vertical Heed gt 4 Astore 0 5 lt 7 lt 0 93 1837 T values in Search Mode In Search mode the program searches to find the duct area that achieves a specified flow uniformity r The r value must therefore be specified before the program can begin its search You can specify your own T value or e have the program automatically specify a 7 value according to the following Table 2 Search 7 Values according to Store Geometry Store Type Height Range T Value used in the Search Horizontal Heed lt 2V Astore 0 9 Wide Vertical 2VAno lt Heed lt 4VAmare 02 Fasil Narrow Vertical Heed gt 4V Astore 0 5 How to Allow for Seed Packing See
12. large You must therefore exercise your own judgement as to what is reasonable you need to balance factors such as the temperature rise through the fan and duct capital cost against fan capital and running costs Usually to keep the duct size and cost reasonable you will have to accept some flow constriction around the duct The best fan pressure load you can achieve will therefore be somewhat greater say around 100 Pa greater than the minimum fan pressure load Fyin Note that reducing F by making the duct larger gives a more uniform duct flow 4 Don t perform the F search first Normally you should vary r values between 0 5 and 0 9 as recommended in the section above titled Selection of Input Values Recommended Values of Duct Flow Uniformity 7 Usually this 0 5 to 0 9 range of 7 values will not cause problems in the program However F values vary widely and it is not possible to prescribe a safe limited range of F values as we can for the r values Consequently if you don t use a 7 search first to obtain F and Fmin and then during the F search keep F between these values you can easily choose e An impossible F value that is below Fmin the program will detect this and stop or e An value greater than Fp that is so high that it cannot be obtained without the program making the duct very small This is likely to cause an extremely non uniform duct flow leading to numerical problems tn the program the program shou
13. ld detect the onset of these problems warn you of the cause and then stop So starting off by defining a r value is safe but starting off defining an F value when you don t know what F values are reasonable can easily lead to problems Conclusions Program DUCT provides a number of unique and useful features Firstly it makes a predictive method for evaluating duct performance available to anyone having access to an IBM Personal Computer PC or PC compatible Unlike design programs that use velocity limiting rules pro gram DUCT predicts duct performance by solving the momentum equation governing flow along seed store ducts It also evaluates duct performance by calculating the fan pressure load F and a new performance index the flow uniformity 7 that is defined in this paper 1840 Secondly program DUCT has two modes of operation an evaluatzon mode that predicts the performance of a specified duct and a search mode in which the program searches to find the duct size needed for a particular application This unique feature allows users to specify the desired duct performance by selecting a value of duct flow uniformity 7 or fan pressure load F Then the program finds the smallest duct size that achieves that performance Designers can therefore use search mode to size ducts Thirdly having regard to needs for versatility program DUCT can be applied to any duct lay out and store geometry and to seven common duct ge
14. ning cost and e the temperature rise that occurs in the air passing through the fan We should therefore ensure that F is not unnecessarily high Further for blowing systems the temperature rise through the fan beneficzally lowers the relative humidity of the air flowing into the seed thereby reducing the likelihood of the inlet air wetting the seed e bul it reduces the capacity of the aeration air to cool the seed To balance these factors we recommend that you try to keep F below 1500 Pa We have obtained the value of 1500 Pa from our own attempts to balance duct size against fan running cost You may wish to use a different value We have chosen the value of 1500 Pa because e pressure loads below 1500 Pa can readily be met by common efficient centrifugal fans and e it gives a temperature rise of less than 2 C A 2 C temperature rise provides a useful reduction in relative humidity of the inlet air with an acceptable loss of cooling capacity Factors Influencing an F search In performing an F search you should keep the following in mind l Fmin 18 sometimes greater than 1500 Pa The minimum fan pressure load Fmin occurs when the duct is very large Then the pres sure load associated with flow constriction around the duct is minimized and the pressure load is almost totally that due to the depth of the seed In tall vertical aerated stores the large seed depth makes Fmin greater than 1500 Pa This is the pric
15. o interactively choose from duct sizes you have available a size nearest to that found by the program s search The program will then evaluate the performance of the chosen duct size Below in the section titled How to Optimize Duct Design we explain how to use search mode 3 As each new variable is introduced its dimensions that is its units are also given Where a variable is dimensionless this is indicated by a hyphen Selection of Input Values Treatment Rate of ar or fumigant We use the term treatment rate to refer to the flow rate of aeration air or fumigant through the seed Program DUCT allows for three different types of treatment aeration recirculating fumigation and flow through fumigation For each type of treatment there is a different way of specifying the treatment rate Aeration Systems Specific Air Flow Rate Q As seed stores become taller it is necessary to increase the upward air velocity to ensure that the time for the azr to traverse the height of the store tair 8 is maintained at about the same value This adjustment can be made automatically regardless of store geometry if the treatment rate is specified using a specific air flow rate Q 2 s tonne Q is called a specific flow rate because it is the flow rate per mass of seed If we take the bulk density of the seed as Pq kg m the mean height in the store as H eea m and the floor area of the store served by the duct as Aserved M
16. ometries This paper examines some of the factors that influence your selection of the program s input values including choice of treatment rate in both aeration and fumigation systems including for aer ation systems the relationship between air velocity through the seed and specific air flow rate a method for deciding whether a store is of a vertical or horizontal type on the basis of the ratio of mean seed height H eeg to the floor area of the store Astore choice of flow uniformity 7 values according to store geometry in evaluation mode by recommending a satisfactory range of 7 values and x in search mode by specifying a 7 value for use in the search and allowance for seed packing and e explains how to use the program s search mode to optimize duct design Acknowledgements The author is grateful to the Australian Wheat Board and the Australian Bulk Grain Handling Authorities for financially sponsoring this work References Bridges T C et al 1988 An aeration duct design model for flat grain stores Trans A S A E 31 4 1283 L288 Brook R 1979 Aeration systems for dry gram AEIS 391 Agricultural Engmeering Depart ment Mich State Univ Burrell N J et al 1982 Air distribution from ventilation ducts under grain J agric Engng Res 27 4 337 354 Greenwood K 1988 Redesigning structures to hold surplus grain Agric Engng 69 7 16 18 Hellevang K J 1984 C
17. ore based their designs on velocity limiting rules Shove Brook 1979 Navaro and Calderon 1982 Hellevang 1984 These rules attempt to maintain uniform duct flow by keeping duct longitudinal and face velocities below set limits For common geometries such as rectangular or circular stores a number of computer programs have recently become avail able Bridges et al 1988 Greenwood 1988 Prive 1989 that automate the use of these rules Consequently seed store ducts continue to be designed using velocity limiting rules and without predicting the velocity and pressure drop distributions along the duct Problems can arise from this approach particularly for long ducts Ams The main aims of this work are 1 to make widely availabletto duct designers an easily used IBM Personal Computer PC or PC compatible computer program program DUCT that allows them to predict the air velocity and pressure distributions occurring along seed store ducts and 2 to set up program DUCT to function as a design tool that allows designers to easily find the smallest and cheapest duct size needed to attain a specified duct flow uniformity or fan 1 Program DUCT is available for purchase from SGRL 7IBM is a trade mark of International Business Machine Corp 1833 pressure load Conventional predictive methods only evaluate the performance of a duct of already known duct size and must be used iteratively to optimize duct size Features of the Progr
18. rop storage management AE 791 Agricultural Engineering Depart ment North Dakota State Univ Marchant J A and Nellist M E 1977 Air Pressure and Flow in Ducted Crop Drying Systems J agric Engng Res 22 303 310 Navarro S and Calderon M 1982 Aeration of Grain in Subtropical Climates Food and Agricultural Organization of the United Nations Rome Prive S A 1989 Technical profile Grates for storage aeration World Grain 89 3 18 19 Shove G C USA A r Flow Analysis of Grain Ventilation Ducts PhD thesis Iowa State University Ames Lowa Shove G C and Hukill W V 1963 Predicting pressure gradients in perforated grain ventilation ducts Trans A S A E 6 2 115 122 Wilson S G in press Aeration Manual Part 2 Engineering Design Stored Grain Research Laboratory Publication CSIRO Division of Entomology Wilson G in press User Manual DUCT A PC program for designing grain store ducts Stored Grain Research Laboratory Publicateon CSIRO Division of Entomology 1841
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