Proceedings of the 7 th International W01'k1,ngConference on Stored-product Protectwn - Yolume 2

Aspects of comparison and selection of grain dryers

LlU Fangjnr , Xiao Yuanzhuang", Hong Jmle2 and Xu Zengtacr

Abstract

A preliminary comparison on the different type of graindryers currently being used in Heilongjiang Province ofChina is made in this paper, on the bases of knowledge oftheir development. Suggestions on the selection of graindryers and fundamental technical specifications on graindryers are proposed.

Introduction

In 1997, the total grain output of China were nearly 0.5billion tons, among which total amount of moist grain(moisture content was about 30% w. b.) procured by thegram depots in the North-eastern four provinces and regionswere 32.2 million tons. Grain drying m China is the worldmost difficult one due to large amount of moist gram to bedried, poor quality of gram and great moisture removal.As a developing country and a great agricultural country,

China has paid much attention on the research and design,importation and manufacture of grain dryers. Currently,there are 527 sets grain dryers installed in over 800 gramdepots m Heilongjiang Province, total drying capacity ofwhich are 6 rrullion tons per year. Difference in drying cost,dried grain quality, specific initial capital cost and pollutionproduced m drying operation, resulted difference meconomic and social benefits to the users and thesurrounding regions. In this paper, some basic cnteria forselection of grain dryers and the recommended dryers aregiven, based on a comparison study on several dryers whichare popular in Heilongjiang Province.

A Review of Grain Dryersin North Eastern China

Most of the grain in North-east were dned by the followingdryers before 1992.

1 Heilongjiang Keda Gram Drymg Technology DevelopmentCorporation, 162 BalIng Street, Harbin 150008, P R Chma

2 Department of Gram Storage, Heilongjiang Provincial Gram Bureau,137 Nanmalu, Harbm 150010, P R Cluna

Brick structure mixed flow dryer

One type was the brick structure mixed flow dryer used inseries combination of several dryers (2, 3 or 4 dryingtowers in combination, 1 or 2 coolmg towers incombination), either directly heated or mdirectly heated.

Steam dryer

Another type was steam dryer LIke bnck structuredryers, steam dryers were also used in serial combination.Gram was heated by steam flowing inside pipes throughconduction. The cooling structure of steam dryer IS thesame as bnck structure dryerOther types of grain dryers, such as hot water dryer,

cross flow dryer, concurrent flow dryer, chain conveyerdryer, fluidized bed dryer, rotary dryer and fixed bed dryeretc. were also available m that period, but the dominatedryers were the above mentioned two dryers.After 1992, With the reform of grain mdustry, investing

m gram dryers became a hot point Nearly 100manufacturers began to produce grain dryers although manyof which were poorly equipped. Thus resulting manyproblems m using gram dryers, such as unreasonable designof process, or unable to reach designed drying capacity andmoisture removal, or worn out m less than 2 years, causedgreat losses to the users.Focusing on grain dryer model selection, in 1994, we

began to enhance the management of the procurement ofgram dryers Five aspects of innovation on gram dryerswere pointed out based on experts' evaluations, which wereas follows:1. Full steel structures should be used in dryersmanufacturmg in stead of brick structures.

2 Several dryers in senal combmation and multi-passelevation of grain should be replaced With single dryermcorporatmg preheating, drying and coohng sections.

3. Direct heating should be replaced With indirect heating,manual operated furnace should be replaced Withmechanical transmitted grate furnace.

4. Dryers should come from speciahsed factories in stead ofbeing built by the depots themselves.

5. The drymg installation should be equipped with upper anddown flow buffer bins, on-lme moisture meters andcentral monitonng systems.

6. Through several years' hard work and taking advantagesof the central government's great attention on the

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Proceedings of the 7 th International War'king Conference on Stored-product Protection - Volume 2

construction of grain distribution facilities in 1996 and1997, we enhanced the general management of theprocurement of grain dryers. New model dryers began tobe introduced into grain depots. Three dryers were

imported from France and the U. S. A. Since then, 285sets of new dryers have been built, increasing the yearlydrying capacity by 3 million tons. The major newly builtdryers are:

Fig. 1. Brick structure mixed flow dryer.

Mixed flow dryer

The former mixed flow dryers were of bricks structures,now they are fully made of steels. With the application ofnew technology, drying capacity and drying uniformity havebeen improved. The daily through-put and moisture removalof the newly built dryers are 200 - 500 tons and 10 - 15%respectively.

Concurrent flow dryer

Concurrent flow dryers normally have 3 concurrent dryingsections and 1 counter flow cooling sections. The dailythrough put and moisture removal is 100 - 500 tons and lO-15% respectively.

Cylindrical cross flow dryer

The main working parts of the dryer are two concentriccylindrical column screens. Grain flow downwards betweenthe two screens, whereas drying air crosses the grain layerto remove the moisture from grain.The dryer's structure is very simple, drying and cooling

are accomplished in one single dryer. The daily through-put

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and moisture removal are 100 - 500 tons and 10 - 15%respectively. Seasonal wind affects the drying uniformity ofthis dryer, frequent cleaning of the screens is needed. It isdifficult to adjust the drying uniformity of this dryer

Fig. 2. Steel structure mixed flow dryer.

Proceedings of the 7th International Wor'king Conference on Stored-product Protection - Vohune 2

Fig.3. Concurrent flow dryer.

Fig.4. Rectangular cross flow dryer.

Rectangular cross dryer

This cross flow dryer features a simple structure and lowinitial cost. Like cylindrical cross dryer, the rectangularcross flow dryer is not suitable for removing moisturecontent of more than 15%, due to its poor dryinguniformity.

Selection of Dryers

Currently, mixed flow dryer, concurrent flow dryer andcross flow dryer are the most popular dryers in China.Comparisons on the three types of dryers are shown in theattached table.Such comparisons are based on the basic principles and the

actual operating efficiencies of the dryers. In practice, firedanger is also a criterion for evaluating dryers' performanoe.As far as fire danger is concerned. from the most dangerousto the least dangerous are mixed flow dryer, concurrent flow

dyer and cross flow dryer.

Hot Air Furnace for Grain Dryer

On the basis of over 20 years' research and practiceexperience, co-operated with Zhengzhou Institute of GrainScience, we sucoessfully developed a coal-fired chaintransmitted grate tubular lined exchanger furnace, whichhas proved to be quite reliable.The followings are the main points to be considered in the

design and manufacturing of furnaces.1. The burning chamber and the heat exchanger shall not beconnected to each other directly, to avoid heat exchangerfrom damaging caused by irradiation shock.

2. The length of fume ducting connecting the burner andheat exchanger shall be as short as possible in order toreduce the heat loss and pressure drop.

3. The heat exchanger shall be made from vertically placedsteel tubes, fume shall flow inside the tubes, whereas airshall flow outside the tubes, for the convenience ofcleaning the filth accumulated in the interior surface ofthe tubes.

4. The former steel-brick structure of heat exchanger shallbe replaced by steel-steel structure to get a better sealingeffects.

5. Properly select and control four important operatingparameters of the heat exchanger. Those are:

• Temperature of fume at the entranoe of heat exchanger.Too low fume temperature will result in low heatexchanging efficiency; too high fume temperature willcause damage to the tubes.

· Temperature of fume leaving the heat exchanger. Thegreater this temperature is, the lower the heatexchanging efficiency will be, and if this temperature isdecreased down to the dew tempera ture of the fume, ashand sulphur it contained will cause blockage to the tubes.

· Velocity of fume. Velocity of fume shall be properlydesigned, for it is related to the heat exchangingcoefficient, the cleanness and corrosion of tubes and thepower installation of fans.

· Velocity of air. It is also related to the power installationof fans and whether the required air temperature andflow rate will be achieved.

Aspects of Requirements onthe Selection and Manufacturing

of Grain DryersThe existing national standards • Test methods of graindryers' (code of standard: GB 8876-88) and •Technicalspecifications for grain dryers' (code of standard: GB 6970-86) provide bases for the procurement and manufacturing.But these two standards are fairly rough, can Dot meet the

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Proceedings of the 7 th Internatianal Working Conference on Stored-proiuct Protection - Volume 2

requirements of the development of drying technology.Concluded from many years' experience and technicalspecifications for the procurement of grain dryers in China

Grain Distribution and Marketing Project funded by a loanprovided by the World Bank, the following technical criteriashall be carefully considered:

Fig. 5. Cylindricalcross flow dryer.

Fig. 6. Schematic structure of a chain transmitted grate coal-firedfurnace.

1. Species of grain to which the dryer is applicable;2. Maximum allowable moisture removal while quality ofgrain is maintained;

3. Drying quality, breakage ratio, discoloration, pollutionto grain, burnt kernels;

4. Specific heat consumption;5. Adaptability to fuels;6. Level of automatic control (control of grain level and 11.temperature, measurement of moisture content and 12.through-put, computerised control and process monitor); 13.

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7. Reliability (break down probability, safety);8. Adjustment of temperature and humidity, waste heatrecycling system;

9. Reasonable ground area it covered general arrangementof equipment and appearance;

10. Air pollution (altitude and intensity of smoke (fume)emission), dust and noise emission;Unit initial cost;Operating cost, fuel and electricity consumption;Ambient temperature suitable for operation;

Proceedina« of the 7 th International Worktng Conference on Stored-product Protection - Volume 2

14. Service life.Heilongjiang Provmce is in the furthest north of Chma,

ambient temperature IS low and the frost free penod IS

short Grain drying m Hetlongjiang Province features lowambient temperature, high mitral moisture content and shortavailable drying time. The general requirements on gramdryers are: advanced process design, good dried grainquality, reasonable equipment price, long service life, highdrying capacity and heat efficiency, large moisture removal,low operating cost, high level of automatisation and reliableoperation. The followings are our major cnteria forselecting gram dryers m recent years:1. MOIsture removal abihty shall be good, the dryer shall beable to dry gram from an uutial moisture content of 33%down to a safe storage moisture content m one pass.

2. Specific heat consumption shall be less than or equal to1600 -1900 kcal/kg, water, which IS 20 - 30% less thanthe existmg dryers'.

3. The dryer shall consists of drying sections and coolmgsection in one structure and be able to work in outdoorconditions (ambient temperature: - 30°C). Its expectedservice life shall be more than 15 years.

4 The expected service life of the burner and the heatexchanger constitutmg the major parts of the furnaceshall be more than 5 and 15 years respectively

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Fig. 7. A fairly perfected gram drying system flowchart

Other Major Problems

Above are some basic consideration on the technology, gramdrying is also m close relation to aspects of policy, planningand allocation etc. There are still some problems mpractice1. The allocation of grain dryers shall be in accordance Withthe pattern of gram distribution There used to be anargument on either "carrymg water' or 'carrying coal'in our provmce Whether gram shall be dned in pnmarydepots or mtermediate grain depots have been considered

5. The furnace shall be able to use bituminous coal as fuel.It shall be mechamcally fed, shall be equtpped withmechanical cham transmitted grate, mechanical cinderremover and tubular heat exchanger.

6. Items and ways of automatic control shall be:· air temperature, the vanance of air temperature shall bewithin ± lOoC;· automatic interlock control and alarm of gram levels m thebuffer section of grain dryer;

• automatic indication and alarm of air temperature andtemperature of fume entenng the heat exchanger;• automatic indicanon of grain temperature;· gram dischargmg speed could be easily adjusted manually7. the dryer shall be equipped With upper flow and downflow buffer bins, shall be loaded and unloadedmechanically.

8. the non-umfornuty of moisture content of dned grainshall be WI thm 2%, and the temperature of dried grainshall not be greater than the ambient temperature + 8°C.The above 8 cnteria have been announced to the

manufacturers and end users of grain dryers by theprovmcial gram bureau. The manufacturing andprocurement of gram dryers shall be in accordance withthese cntena.

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i t

m the scheme design of Chma Grain Distribution andMarketmg Project. Based on many years' expenence, wedecide to build a grain dryer in a grain depot only if thequantity of moist grain IS 3 - 5 times greater than its sundrying capacity.

2 Factors such as quantity of moist gram, nutial cost ofdryer, yearly operatmg cost and operatmg times shall beconsidered m determmation of the designed dryingcapacity of a grain dryer, among which operating factoris the key factor. The most appropriate operating timesare 75 days The formula to determine the designcapacity is: designed capacity = (quantity of moist grain-

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Proceedings of the 7th Iniernaiumal Worhng Conference on Stored-product Protectuni - Volurne 2

(quantity of gram dried by sun light + quantity of gramtransferred to intermediate depot) )/75

3. The relationship between existing drying capacity andrestnctmg moisture content pohcy on gramprocuremen t.

Table 1. Advantages and disadvantages of three major gram dryer

It has been quite a long time for the gram industries toaccept highly moist grams due to histone reasons. From theangle of market economy, moist gram 1Sonly a semi- finishedproduct but not a complete commodity. Thus it 1Sabnormalto accept moist grain all the time

Maximum Adaptability toEnergy

Tyep moisture different typeconsumption

removal of gram

Break downprobab1hty

Pollution to theenvironment

Initialcost

16% highnuxed flow small, withwaste heatrecycle

most of the gramspecies, for 1tSair temperaturecould be adjusted

small, forsimphcrty of thewhole mstallation

small, meet therequirement of therelated NationalStandards

concurrent 16% fairly small, air many grain fairly high, for it greater than mixed lsee thanflow temp. of each species for air has qurte a few flow, waste air mixed

section 1S temp of each fans exhausting from flowdifferent section 1S different sections dryer

different causing pollutionto the dryer andenvironment

Cross flow 12% fairly small, lmuted gram fairly high, for small, but lowwaste heat could species, for 1ts frequent blockage cylindncal crossalso be recycled uneven drying to the screens dryer 1Sworse

Usually, moisture content of maize grown m ourprovmces when it 1Smatured 1S fairly high, because of thegeological and climatic conditions of the province, themaximum can be as high as 40 % This year a policy onprocurement of maize With restriction on moisture contentwill be implemented. 25 % is the standard moisture contentfor procurement of maize. A discount m weight and pricewill be given for the seller whose maize moisture contentexceeds 25 % Such policy will encourage the farmers togrow early matunng vane ties , therefore, the normalharvested moisture content of maize will be less than 30%and the existing drying capaci ty of the gram mdustry will bemore efficiently used. We disagree to develop new dryersfor the farmers, it 1Sbetter to decrease the moisture contentof farmer's maize through improvements on vanety and fielddrying instead of building farmer' dryers But the existmgdryers of the state farm system shall still be fully used.

Suggestions on the FurtherDevelopment of Grain Dryers

A newly stable research team engaged m gram drymg

research, development and manufactunng of dryingequipment shall be built, in order to enhance the macro-control, speed up the development of drying industry andconduct the training on operation and maintenance of graindryers.Speed up the work on model selection and evaluation of

gram dryers, realise the fmahzation, standardization andsenalization of gram dryers. More detailed nationalstandards shall be worked out to unify the dryer evaluatingworks, m order to ehrnmate the gram dryers of poor qualityfrom entermg the market.Set up regulansed gram dryer manufacturing base Gram

drymg 1S a kmd of very professional, techrucal andsystematic work, expenence m research, manufactunng andoperating 1Svery essential

References

He J B. 1993, Present condrtion and development trend ofgram drying machmery m Chma Proceedings of '93International Symposmm of Gram Drymg and StorageTechnology. Be1Jmg, pp.1-18.

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