Eur.J.Wood Prod.
DOI10.1007/s00107-010-0496-9
7s管理培训B R I E F O R I G I NA L S K U R Z O R I G I NA L I A Microwave drying of bamboo B.E.Prasad·Krishna K.Pandey
Received:20May2010
©Springer-Verlag2010
Abstract The results on microwave drying of a bamboo species,Dendrocalamus stocksii,are prented in this pa-per.Bamboo specimens were dried in a continuous belt mi-crowave dryer at varying microwave power ranging from 400to1600Watt for different time periods,and moisture loss was determined.Microwave drying rates were com-pared with kiln drying rates.The result showed that good quality of drying of D.stocksii could be achieved using a conveyor belt microwave dryer.The time of drying is sig-nificantly reduced in microwave drying from veral days in conventional kiln drying to couple of hours in microwave drying.
Mikrowellentrocknung von Bambus
1Introduction
Bamboo is a very fast growing plant with high strength and can be ud as a structural material.It posss very high moisture content and needs immediate drying in or-der to avoid biological decay.The susceptibility to biologi-cal degradation and deformation owing to excessive shrink-age necessitates quick drying of bamboos.The drying is en-ergy intensive and a time consuming process involving ma-jor share of expens during timber processing.
The conventional drying methods require long drying times in order to obtain high quality material free of degra-dation.There are different methods of wood drying;the B.E.Prasad·K.K.Pandey( )
Institute of Wood Science and Technology,18th Cross Malleswaram,Bangalore560003,India
e-mail:kkpandey@icfre
K.K.Pandey
e-mail: most common one is air and kiln drying.Less common drying methods for industrial processing bad on high-frequency electromagneticfields are microwave drying (MW),a combination of vacuum and MW or radio fre-quency(RF)and vacuum.The major probl
ems of the con-ventional kiln drying methods are high energy consump-tion and low drying efficiency.Microwave,as an alternative, can dramatically decrea drying times in comparison to conventional kiln drying(Antti1995;Leiker and Adamska 2004).
Apart from accelerating drying,microwave drying in-creas permeability and facilitates treatment of refractory wood species with prervatives and also helps relaxing growth and drying stress in timber(Torgovnikov and Vin-den2009;Vinden et al.2010).
In this work,the drying behavior of a bamboo species, Dendrocalamus stocksii,was studied using a microwave dryer.Microwave drying of bamboo species has not yet been investigated.Bamboo specimens were dried in a continu-ous belt microwave dryer at varying microwave power rang-ing from400to1600Watt for different time periods,and moisture loss was determined.Microwave drying rates were compared with kiln drying rates.
2Material and methods
D.stockssi(average length∼6m;average diameter∼25 mm),were collected from the Karnataka Forest Department, Bangalore.Kiln drying was performed by stacking100bam-boo culms in10rows(length∼1.5m)at40°C in a dehumid-ifier dryer(Bry-Air,Bangalore India).Six reference speci-
mens were ud to estimate average moisture loss/moisture content.The dried pieces were visually checked for drying defects(cracking,splitting,etc.)after
exposure.
Eur.J.Wood Prod.
Fig.1Profile of moisture loss (a)and moisture content(b)of D.stocksii for kiln drying(at 40°C)
Abb.1(a)Feuchteverlust und (b)Feuchtegehalt von D. stocksii in Abhängigkeit der Dauer der technischen Trocknung
(40°C)
Microwave heating was carried out using a locally fabri-
cated microwave continuous belt drier.The microwave cav-
ity was of rectangular shape(chamber dimension of193cm ×32cm)provided with an exhaust fan on the top.The MW power was generated by means of two air-cooled mag-
netrons(frequency2.4GHz,capacity1.2kW).Bamboo
culms were placed on the conveyor belt moving to and fro
at a conveyor speed of50mm/s.The microwave radia-
tions were generated perpendicular to the length of the bam-
翻毛皮boo specimen.The six replicates of bamboo(sample size ∼1.5m long and average diameter of25mm)were expod to microwave drier for different time periods at different mi-
crowave power of400,800and1600Watt,and moisture loss
was determined.
3Results and discussion
The average moisture loss(ML)was determined:ML(%)=
100(Wi−W f)/Wi;where Wi is the initial weight of the sam-
ple prior to exposure,and W f is the weight of samples after
drying.The moisture content(MC)of the bamboo speci-
mens was calculated:MC(%)=100(W i−W o)/W o;where W o is oven dry weight of sample.
The profiles of moisture loss and moisture content for
从军行二首
kiln drying of D.stocksii is shown in Fig.1.Time required
for drying of bamboo specimens to achieve up to∼10%MC
was about16days for kiln drying.
高二下学期班主任工作计划The bamboo specimens were expod to microwave pow-
ers ranging from400to1600Watt in a microwave cham-
ber equipped with a conveyor belt.Continuous exposure to
microwave radiation,particularly at higher MW power,re-
sulted in excessive heating and thermal decomposition of bamboo.Therefore,the culms were expod to microwave dryer continuously up to20minutes and then allowed to cool down for20minutes.The cycle was continued until constant weight was obtained.
The profile of moisture loss and moisture content of D. stockii during microwave drying is shown in Fig.2.It can be en that rate of moisture loss was very high in the ini-tial stages of high moisture content.Water evaporates faster during the early drying period when moisture content is very high.The drying time required to achieve about10%MC was∼120min at MW power of1600W.
Microwave input power was found to have a significant effect on the drying rate and drying quality of bamboo.The rate of moisture loss incread rapidly with increa in MW power.The drying rate was determined using moisture loss and/or moisture content(MC)data.The moisture content decreas w
ith time.Rate constant of moisture loss(k)was calculated byfitting moisture content data infirst order ex-ponential function(1),
MC=A0+A1e−kt(1) The values of rate constant for moisture loss during MW drying were found to be0.0015,0.008and0.02min−1at 400,800and1600W,respectively.
Comparison of Figs.1and2shows a vast reduction in drying times in MW drying.The drying time is reduced from days in kiln drying to minutes in MW drying.Visual comparison of drying degradations(splitting and cracking and other degradations)of microwave dried bamboo speci-mens with kiln dried specimens was carried out,and it was obrved that MW drying has no adver effect on dry-ing quality.In microwave drying,material is expod to
农村科技Eur.J.Wood Prod.
Fig.2Profile of moisture loss (ML)and moisture content (MC)of D.stocksii at
Microwave input power of 800Watt and 1600Watt.(a )and (b )are moisture loss at 1600W and 800W,respectively;(c )and (d )are moisture content at
800W and 1600W,respectively Abb.2Feuchteverlust (ML)und Feuchtegehalt
(MC)von D.stocksii bei einer Mikrowellenleistung von 800Watt und 1600Watt in Abhängigkeit der
Trocknungsdauer.(a )und (b )Feuchteverlust bei 1600bzw.800Watt.(c )und
(d )Feuchtegehalt bei 800Watt bzw.1600
Watt
electromagnetic field with high frequency microwave radi-ation.The dipole molecule such as water begins to turn at the same frequency of electromagnetic field.Agitating the molecules with high frequency (MW frequencies 300MHz to 300GHz)in this way increas the temperature of the material.
Bamboo species though very uful for variety of end us,have significant problems related to prervative treat-ment as compared to timber species.Microwave drying technology can address this problem as has been obrved for timbers (Torgovnikov and Vinden 2009;Vinden et al.2010).
4Conclusion
鼻窦手术
The results showed that a good quality of rapid drying of D.stocksii could be achieved using a conveyor belt mi-crowave dryer.In order to avoid charring and thermal de-composition of bamboo material,continuous MW exposure
should be avoided.Drying rate increas rapidly with in-crea in MW input power.The time of drying in microwave is drastically reduced from veral days in kiln drying to couple of hours in MW drying.
Acknowledgements This rearch was supported by NBM New Delhi.Authors express their sincere thanks to the Director,IWST for his encouragement and support.
References
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of Radiata pine railway sleepers for prervative treatment.Eur J Wood Prod.doi:10.1007/s00107-010-0428-8
血糯米的功效与作用>不同意英语
