Proceedmgs of the 7th Iniernaiumal Worhng Conference on Storei-prcduci Protectwn - Volume 2

The system for decreasing temperature by ventilation in grainsilo

Zhang Lailm", Ll Chaobm", Wang Jmshur' and Ll Zonghani

Abstract

The necessity of augmentmg the system for decreasmgtemperature by ventilation m gram silo IS mamly descnbedm the paper For decreasmg temperature, It IS moreeconomical and effective usmg ventilation than that oftransfernng gram from one silo to another Three types ofventilation system used in silo were mtroduced. Amongwhich, the tube ventilation IS the least expensive and theSImplest m operation. Tube ventilation should be used as thepnncipal type m SIlo ventilation. When design andmanagement of the ventilation system, the design of atrinlet, choose for venulation chance and distnbution ofventilation system should be noticed

Introduction

For a long time , the expenence of stonng gram m silo ISlacked because the flat storage ISmamly used in Chma. Oncegram heats up, dew and IS mfected by pests, gram must betransferred from one s110 to another, because the silo builtlacks of the necessary equipment for storage ThIS makes alot of diffrculties for the safe storage of gram, meanwhile,increases the lost In order to solve the above problems, ItIS necessary to study the storage technology of gram m s110The system for lowmg temperature by ventilation m gramsIlo was dIscussed m thIS paper

The Necessity Adding Ventilationand Lowing Temperature in Silo

Gram stored m SIlo 1Seasy to be affected by the temperatureand humIdIty The change on temperature of gram surface ISobVIOUS,on other hand, gram Itself IS a good msulator ofheat. The heat can not easIly spread. The gram IS easy toheat up when affected by bad factors, such as temperaturedIfference between the ambIent and that inSIde the sIlo, theaIr current m gram bulk enhances The damp-heat spread

1 POBox 62, Zhengzhou Gram College, Zhengzhou 450052, PR Chma

2 Kunxl Gram Depot of Kunmmg, Klll1Illing, P R Chma

makes gram which IS nearby the wall and surface dew. Ifheat and damp don't be decreased immediately, It will causeheat heavily and water will transfer If It contmues, waterwill gather on the roof and wall, tlus WIll cause heavy loss ofgramWhen gram heats up, the handlmg system of silo can be

used to recirculate gram among silos for lowmg temperatureas an emergency way. The temperature of gram WIll nse upquickly after recirculation because potential heat has notbeen ehmmated If there IS ventilation system m SIlo,ventilauon can make out immediately when stored gramheats up ThIS can remove the heat, prevent gram fromheatmg and from water transfernng, and Improve thestability for gram storage.In addition, frequent recirculatmg of gram IS not a good way

m respect of econormc aspects The cost for transfernng gramfrom one silo to another IS about Rl\t1B 0.6-0. 9Yuan/ton.Vennlation for lowmg temperature costs two third less thanrecrrculation and funugation So silo must be equipped Withventilation system for lowing temperature

The Form of Ventilation System in Silo

The mam forms of ventrlation system m silo mclude radialand vertical ventilatron The radial ventilation refers that airduct IS distributed on the wall which makes airflow cross thegram bulk m honzontal direction. The charactenstics ofdIameter ventllatlon are that the aIrflow path IS short, thesystem deSIgn IS complex and the constructlon cost IS hIgh.It can be operated only If the SIlo IS fIlled WIth gram Thevertlcal refers that aIr duct IS dlstnbuted at floor of s110The aIrflow crosses the gram layer from bottom to top (orfrom top to bottom) . The charactenstlcs of vertlcalventllatlOn are that the system deSIgn IS SImple, cost IS lowand ventIlatlon can be carned out ImmedIately after loadmgof the sIlo IS started. But the aIrflow path IS long, thereSIstance IS bIg, and It needs hIgh pressure fanAccordmg to dIfferent floor structure of sIlo, the vertical

ventIlatlOn can be dIVIded mto two forms, that IS, flat floorventilation and hopper floor ventllatIon. The former mcludesfull floor ventilation and hollow ventilatlOn and so on. Fullfloor ventilation IS one of the forms that found frequently mvertical ventIlation systems But the cost IS hIgh, and theextenslOn and use IS hmlted The later mcludes parallel duct

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Proceedmgs of the 7th International Working Conference on Stored-product Protectwn - Yclume 2

ventilation (FIgure 1) and standmg duct ventilation (FIgure2) Vertical ventilation ISWidely used in silo m Chma for ItSlow cost, easy construction and operation, and umformairflow distribution.

.1,· ·· .: · - t-

: :::' t:. ,:"'t'" .: · :~~ .. ~ • 4iI. , .. , ." , '.'

". • •• r ... • I

••• t' I.. t

.2

J. l t. . .. . . ..." I :. .':, t' : ' -'.\, .. : .. . . .' .. ""II. . ••. " . " ." .'

Fig. 1. Duct ventIlatIon system m awl SIlo1 - fan 2 - SIlo 3 - ventIlatIon duct

Fig. 2. Standmg duct ventilatIon system m awl sIlo1 - SIlo 2 - standmg duct 3 - mlet -unloadmggramexchange eqUIpment 4 - fan

MultI-functional ventilation and fumigatIon eqUIpment ofsilo belongs to vertIcal ventIlatIOn eqUIpment (Figure 3). Itcombmes the function of ventilatIon With fumigation It canbe used for ventilatIon, closed loop fumigatIon as well. It ISalso helpful to offset the gramng of gram particles caused byloadmg and unloamng. ThIS equipment can increase the

stabihty of stored gram m many ways.

· . /:: II ~:: ..•.., I..· II":: t··.' oJ 11- . t' .t •• ..

:'.:: -: II ~~'.'... . .-.• .. . -II- .· ..... . . .. ~ II"· ..'. . t' - · · . . ... :-·0" -:-,:II,·.t".· ,. . . .. ... .

3

Fig. 3. Multi- functional ventilatIon and fumIgatIon systemfor lOWing temperature m SIlo1 - fan 2 - SIlo 3 - multI-functIonal ventIlationand furrugatlOn eqUIpment

Several Problems Which Should BeNoticed During Design and Management

of Ventilation System in Silo

Design of the inlet of air duct in vertical ventilation

The desIgn of the mlet of arr duct of silo greatly affects theperformance of ventilation system. Thfferent air mlets are usedm different structllred hoppers Generally there are three landsof arr mlet. (1) For parallel duct ventilation, direct hole abovethe angle under hopper ISused as air mlet (FIgure 1). It can notbe used m concrete oo.se (2) The dIameter of the outlet of thehopper m most of the silos IS about 600 - 900mm A steelhopper with a diameter of 300mm is often added to the concretehopper for the convenience of unloading. In that case, themterval space can be used as arr mlet of ventilatIon (FIgure 2) .(3) When hole can not be opened above the angle of hopper orhopper oo.seof silo IS too small (diameter IS only 300mm), theexchange equipment of Inlet and discharge is recommended(FIgure 3). It consists of two cylmders By rotatIon of mnercylinder, unloading of gram and ventilatmg IS shifted. It hasbeen proven to be a snnple and effectIve type of air inlet.

Distribution of ventilation duct

In order to ensure effectIve ventIlation, mstributIon ofventIlatIon duct must be made according to ratIo ofdistances. When ventIlatIon by duct, the distance betweenducts where ducts collect should be notIced. Too small

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Proceedmqs of the 7th Internatumal Warkmg Conference an Stored-proiuc! Protecium - Yolume 2

distance will influence the unloading of gram and cause Jamor mconvemence of unloading. When inlet truckmg , site ofunloading gram are determmed m standmg ducts, balanceunloadmg should be ensured, or facihties in silo wtll bedestroyed, site of unloading WIll be Jammed

Determination of specific air volume

Because of the special structure of silo, ventilauon of SIlocan only be used to decrease gram temperature with smallarr flow rate, but It can not be used to dry gram. Increasmgarr volume WIll mcrease resistance of gram layer There ISsuccess m ventilation by weak air , It can ensure safety ofstored gram It has been proved by the results of electncalsimulated test (Zhang Lailm 1992)

Determination of super-facial air velocity

Duct diameter IS determmed generally by arr velocity mduct Duct apparent arr velocity IS used to collate ductdiameter m design. In order to mmmuze the systemresistance, the super-facial velocity of the duct shall be lessthan 0 . 15m/s and the perforated ratio of the air ducts shallbe as big as possible

Ventilation at appropriate time is important inventilation management, it relates to effect ofventilation

EqUIlibnum moisture theory of gram IS the base for theselectIon of appropnate ventilatIOn tIme. VentIlatIon mustaccord With the followmg temperature and humIdItycondItIon. If any condltlon can not be fixed, ventIlatIonshould be stopped.HumIdIty conditIon: Accordmg to atmospheric

temperature and humIdIty, the eqUIlibnum mOIsture contentof gram can be calculated. Compare the eqUIlIbnum mOIsturecontent WIth gram mOIsture content, If the former IS lessthan the later, ventllatlon will not mcrease the grammOIsture content, ventIlatIon can be made; If the former ISgreater, ventIlatIon will mcrease the gram mOIsturecontent, ventIlatIOn can not be made

Temperature

When average temperature of gram bulk IS 8°C greaterthan atmosphere temperature, ventIlatIOn effect IS ObVIOUS,ventIlatlon can be made.If the followmg condItIons are fIt, ventIlatIon can be

stopped: I. When the dIfference between averagetemperature of gram bulk and atmosphenc temperature ISless or equal to 4"C, contmuous ventIlatIon has no ObVIOUSeffect II. GradIent of gram bulk temperature IS less than orequal to 1°C1m. III. GradIent of mOIsture content of grambulk IS less than or equal to 0.3 % 1mThere are two forms of fan workmg, that IS, press and

suction. VentilatIon form should be adopted according to

practical condition If gram m the top heats up, ventilationby suction on top of the silo or by blowmg on the bottom ofthe SIlo should be adopted Because cold air enter grambulk from bottom of silo, gram m bottom IS first cooled,damp and hot waste gas goes away from the top directly, theeffect of lowing temperature IS good.

Operating management of ventilation

SIlo provided With good ventilatmg system guarantees thesafe storage of gram. The ventilation system IS effectiveonly If It IS performed properly. Therefore, m practice,ventilation IS determmed according to the change m theambient temperature, the gram temperature andtemperature of air m the headspace of the silo. If ventilationIS to be conducted, first the appropnate ventilation tuneshall be determmed; second, the surface of the gram bulkshall be leveled; third, all the outlet of atr shall be openedfor the easy exhausting of waste air and avoiding watercondensing on the surface of gram bulk; fourth, ventilationshall be Implemented in several steps to decrease the gramtemperature gradually, and to prevent water condensing mgram bulk; fifth, msects are found m the silo, arecircula tmg ductmg system can be added to the silo forminga closed loop system to perform fumIgatIon

Conclusions

VentIlation for lowmg temperature IS an Important way tokeep safety of stored gram. SIlo augunng ventIlation andlowmg temperature system can elImmate temperaturedlfJerence, balance gram temperature and mcrease StabIlItyof stored gram. For lOWing temperature, ventIlatIon ISmoreeconomIcal and effectlve than reCIrculatIon of gram amongsIlos Nowadays, ventIlatIon m sIlo have three forms,vertical ventIlatIon has the fIrst pnonty for ItS less cost,SImple operatIon and good effect But the resistance ofvertical ventilatIon IS great, It should be conSIdered whenchoosmg fans. When deSIgn and management of ventIlatIonsystem, the rule' safety, effectIve, economIcal' should beobE'yed. DeSIgn of aIr mlet, appropnate tlme of startmgventilatIon and the operatIon of ventilation shall be fullyconSIdered.

References

Du Guodong, SIlo ventIlatIon techmque Gram Storage,1988 (4) :52- S4

Zhang Lailm, The Importance of the deSIgn of gram storagetechnology m VIew of storage. Gram DlstnbutIonTechnology. 1997 (2): 24 - 26.Prdimmary report on the simulatIon of ventIlatIOn of grambulk. Shanxm Gram and all Technology. 1992 (1): 29-

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