2 ' N L N -108*11/2012
NGHIEN curu TINH T O A N
HE SO TRAO DOI NHIET TRONG BUONG LLTA, LO HOI DOT TRAU LOP SOI CAI TIEN
RESEARCH AND CALCULATION OF HEAT TRANSFER COEFFICIENTS IN THE RiCE-HUSK FIRED IMPROVEMENTS FLUIDIZED BED BOILER FURNACE
KS. HOANG TUAN ANH, TS. NGUYEN VAN TUYEN - Dai hoc Bieh Khoa Thanh phi Hd Ohi Mmh Bai bao trinh bay nghien cO'u tinh toin he so trao doi nhiet trong buong lu'a Id hvi dot trau ldp sdi cai tien. Ket qui tinh toin dwgv dinh gii thong qua thiec nghiem kiem cho'ng, ket hgp vdn cic kit qui cua mgt so tic gia khac tu- dd dua ra nhu-ng ket luin lien quan. Mgt so so lieu chu yeu thu nh$n duw: nhw sau: He so trao doi nhiet buong dot la 119,2W/m'K, toe do soi toi thieu (van toe sol bgt nho nhat) Umf la 0,2m/s vi toe dp gio cap U li 0,45m/s, nhiet dp trung binh buong dot la 813°C va nhiet dg trung binh Idp soi la 856°C. Moi quan he giO'a he so trao doi nhiet buong dot (87,7 ^ 119,2W/m'K) v&i tai Id hori (3.000 -^ S.OOOkg/h), nhiet dp buong dot (631 -f 813°C) va toe dp gid cap (0,27 ^ 0,45m/s) la quan he tuyen tinh.
1. DAT V A N fit
VPi tong lyang vo trau lai Vifit Nam khoang 8 trieu tan (Sp Ndng nghiep vi Phat tnin ndng thdn, 2010) thi h^ng nam nguon sinh khfii nay cfi th^ tao r a x ^ p x i 102xl0^GJ nSng l y p n g , d6i vo'i Bong bSng sfing eyu long (DBSCL) eon so nay t y a n g yng khoang 4 triPu tan trau (S6 lieu kinh ti xa hdi 13 tinh. thinh pho DBSCL - Cue Thdng Ke Cin Tha, Tdng cgc Thong ki. 2009) va 52x10^GJ nang lypng.
CPe Ifi hai cfing nghipp PPt triiu cfing s u i t nho va trung binh (<30 tan hai/giP) d y p c du nhap vao Viet Nam tu" nSm 2005, ehu yeu theo 2 phyang thyc; i) trinh diPn cong nghe, v^ ii) hap dong chia khfia trao tay giCpa nha cung cPp thi§t bj n y p c ngoai va ehu dau t y trong nyPc. Jit nam 2007 eac ca s a trong nyfi'c cung da bSt d^u chfi tao 16 hai dot trau Tuy nhifin thac t^ vin hanh, bao dyfi'ng va sya eh&a ePe IP hai nSy trong nhyng nam qua cho thay efin t6n tai nhieu r^o can ky thuat de cfi the li'ng dung cfing nghp dot trPu hieu qua tai Viet Nam
Y t y a n g thiet ke Id hoi cdng nghiep ddt triu trong budng ddt l&p sdi bgt cai tiin la s y dung chinh tro nhieu Ii6u trau lam vat lieu lo'p SOL Bufing dfit lap sfii eai tien khfing s y dung van lieu tra thi> hai, khfing dat d^n 6ng hap thu nhiet trong lap soi Dieu n^y mang y nghT quan trpng trong thiet ke vP vpn hanh VPi y t y a n g thiet ki nay, co the yng dung khai thac hoan chinh 16 hai cfing nghifip cfing suat den 30 tan hai/giP san x u l t hai nyfi'c b§o hoa vpi
^p lyc den 25 barg.
II. D 6 | TU'O'NG NGHIEN CLFU
Dac tinh nhifin \\iu t r i u trong bang 1. Thpc nghi0m cho thay nhien lieu t r i u eP the b i t chdy t?i nhi^t dp 340°C. Nhifin li^u t r i u cfi t h l chSy on djnh khi d p \&\ lap sfii tai nhifit dfi lap 400°C, tho^t chit boc nhanh, nhyng chay coc 1^ khfi khSn (Chen G u a n y i e t a l . , 1998)
Bang 1. B§c tinh nhien Hju triu Nhiet tri theo m i u lam vi
Nhi?t tn i h ^ j j L H V Nhiet tri cao HHV
c "ar"
12 915,9 14 106,3
kJ/kg kJ/kg Thanh phSn h6a hoc (% theo khfii lu'o-ng) theo m i u ISm viec "ar"
Dfi am Dp tro Carbon Hydrogen Nitrogen Chlorine Sulfur Oxygen Tons
10.94 18.05 34.58 4.23 0,46 0.00 0,05 31,69 100,00
% %
% %
% %
% %
D|c tinh c6n^ ngh^ (% theo khfii luo-nal
%
D<ilim Dfi tro Chat bfic G^cbon CO (Jinh Tonq
10,94 18,05 56,57 14,44 100
% %
% %
Dac t i n h tro
%
Dp ket dinh cua tro Nhiet dfi nfinq chay
Thlp/trung binh 975.89
"c
[Nguon: Thw vi$n nhiSn Ii0u Cdng ty Thennoflow, l\/ly, 2012\
Thfing so thiet ke buong l y a d y p c ehi rfi trong hinh 1 v^ bang 2.
N L N ' 1 0 8 - 1 1 / 2 0 1 2 ' 3
Hinh 1 Sa do bo tn diem do va thong so thiet ke buong dot Bing 2 Thdng sfi thiSt ki budng ddt TT
'
2 3 4 5 6 7 6 9 10 11 12
Dai liFOTig Nhiet tri thap lam v i | c cua nhien lieu Hieu suat cua 16 hci Ap suat lam viec Nhiet d$ hCTi baoh6a Nhiet apnu'O'c c i p San luvng ho'i dinh mi>c Su^t tieu hao nhifin lieu Tieu hao nhi6n li$utlnh toan
The tich buong dot Dien tich m^t cat ngang buong li>a Dien tich be mSl nhgn nhiet
Dien tich m | t l h o S n g g h i Ki hieu Q'l.
1
t..
tnc D B Bti V
H Fg„,
D o n
kJ/kg
%
kG/c m2
"C
"C kg/h kg/h kg/s m^
m ' n,' m^
Gia t n 12 915 90
84,57 10 184 30 5 000 1 215,98 1 197,74 12,5 3,21
30,9 1,86 III. K £ T Q U A TiNH TOAN (bang 3)
3.1. He sfi trao doi nhiet
Hi so trao doi nhiet vi tai Ifi hai t y hinh 2 moi quan he giO'a hfi so trao doi nhiet bufing dot 87,7 -5-119,2 W W K va tai Ifi hai 3.000 ^ 5.000 kg/h 1^ quan hfi tuyen tinh.
Hi so trao ddi nhiit vi nhiet dd budng ddt t y hinh 3 m6i quan hp giO-a hfi s6 trao doi nhifit 87,7 -H 119,2W/mV vi nhiPt dp buong dot 631 - 8 1 3 ° C I S q u a n he tuyen tinh
Hi s6 trao ddi nhiit vi tic dg gid cip tO hinh 4 moi quan h$ giOa hp so trao d6i nhipt buong dot 87,7 ^ 119,2W/m^K vp toe dP giP d p 0,27 -r 0,45 m/s la quan h§ t u y i n tinh
Hinh 2. Quan h0 giua h$ so trao doi r'-'et va tai Id hoi
J -U-'
^
Hinh 3. Quan h0 giira h$ s6 trao doi nhiet va n buong ddt
1
f.io
1 1
s ™
1
"
-'"-T -j^/n
\V_^_ u
" "
Hmh 4 Quan hi giOs he so trao doi nhi$t va toe dp gid cap
4 ' N L N -108*11/2012
S^no 3. /Wof sfi kit Qua td chuan
TT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 25 16 17 18 19 20 21 22 23
" "24 25 26 27
Dai lipo-ng Nhi&ia$ trung binh Id-p sfli Kh6i luQ'ng neng ciJa trau Du'cmg klnh cua hattrSu Otfong kinh hat tro
t o e a$ SOI toi thi4u (v^n toe soi bpt nho nhat) Toe dp s6i dang pittftng (slugging) nho n h i t T 6 C d$ bat dSu chuyen tif soi bpt sang soi r6i T 6 C ao k^t thOc-chuyen ti> s6i bpt sang soi r6i Tflc dp gi6 c i p
0 ^ rong ciia 16'p hat c/ ch4 do bat d i u soi Chifeu cao \irp hat tro
Chi^u cao ldm vipc lop soi Van toe tang bpt trung binh He so truyen nhiet tir pha bpt He so truyen nhiet tCf pha nhu tu'ong He so toa nhiel d6i lu'u cua hat H^ s6 toa nhipt d6i lu'u cua khi Hp so toa nhl^t bi>c xa trong lo'p soi He s6 toa nhiet trong 16'p sfli Nhipt do trung binh buong lOa Toe do khoi trong bufing lira
Chieu cao n g i t v$n c h u y i n - Transport Disengaging Height He so toa nhipt d6i lu'u \ir san p h i m chay d§n bk mat ong smh hoi trong bu6ng d6t
He so trao d6i nhi^t buong dot Nhipt d$ khdi ra khoi bufing iCra Nhi$t d$ trung binh buong li>a Nhipt lu'p'ng truyen cho d^n smh h c i
K i h i e u
Bts PB d p
U K
U
Eml
H,
U b ht he ha,„.
hconv '""hraa ho 9BI
"TDH"
hco„y K
'GB7'"
Q ^
q trinh tinh toin
Ban vj
"C k g / m '
m m m m m/s m/s m/s m/s m/s m m m/s W/m^K WIm'K W/m^K W/m^K W/m^K W/m^K
"C m/s
W/m=K W/m^K
°c "'"
kJ/kg Gia trj
855.5 120,0 3,25 1,08^
0,2 0,5 2,6 4,0 0.45 0,45 0,3 0.5 1.1 65,7 17.1 24,1 0,0068 97,1 121,2 813 1,9 1,8 78,3 119,2 656
" sTs
"6.956,4
Ghi c h i j
•11=0.81. d,ph=4Sd»ff=3,2
Stewart va Davidson, 1967 HorJo va Monshita, 1988 Ar<10*, Mono vii Monshita (1988) He so hinh dgng hgt 0=0,81
Glicksnnan, 1984
George and Grace, 1979
3.2. Nhiet dp buong dot vi tii Id hci Ket qua tinh to^n d y p c trinh bay trong hinh sau:
1
1
1—,o
l-^.m
SDN
MOO GSM
3 1 5 0 7<«
JSOO 75»
3 7 5 0 SO*
4 0 0 0 SBH
«50 SDK
J 5 0 0 5S«
4 7 5 0 100%
5 0 0 0
™
m.1
Hinh 5 Ket qua tinh toan nhi$t do buong dot va tai Id hai TO hinh 5 ta thciy quan he tuyen tinh gi&a nhifit dp buong dot 631 -r 813°C v^ tai Ifi hai 3.000 - 5.000 kg/h.
IV. NHAN XET K ^ T QUA TJNH T O A N 4 . 1 . €)di t y p ' n g 16 h a i t h i f e n g h i e m Da t i l n hanh thyc nghipm kiem chyng tren m§u IP hai eai ti4n. Do lyu l y a n g hai va nhiet dp buong dot, so sSnh gia trj do vPi g\i tn tinh to^n tai dieu kien van hSnh t y a n g Crng vdi thiet ke, x^c dinh sat
so & danh gia dfi chinh xac eua chyang trinh tinh to^n trao doi nhiet trong buPng lya lopi Ifi hai nSy.
4.2. S a d f i b d t r i d i i m d o
S a dp bfi tri diem do d y p c trinh bay trong hinh 1 4.3. P h y a n g t i e n d o
Do nhiPt dfi: C§p nhifit (thermocouple) lopi K, dai nhiet dfi do tCf 0°C tPi 1 2 6 0 X .
Do lyu l y a n g hai: Dong ho do lyu lu-png hai DN80, dai do 315 -^ 7.809kg/h, sai so ± 1 % g\i tri dpc, Pp suat 78kg/cm^ (Max.), nhifet dfi l^m vi$c -40 ~ +280°C
4.4. KPt qua d o
Ket qua do nhipt dp buong d6t vi l y u lypng hei d y a c trinh bSy trong hinh 6. T y hinh 6 ta nh$n th^y m6i quan hp tuyen tinh g i y a tai Ifi hai 3.000-^5,000 kg/h va nhipt dp bu6ng dfit 615-4-814 °C CSc diem c y c tieu trong khoang thai gian do y n g vPi mu'c huy dong cfing suat IP hai khoang 3 - 4 tan hai/ gifi' vSo thPi d i l m giao ca cua nh^ may.
4.5. So s a n h gia trj d o v a i gia t r i t i n h t o ^ n
^ Gi^ trj do d y p c so s^nh vPi s6 lipu tinh todn t?i dieu kipn vpn hSnh t y a n g yng vPi t h i l t ke nhy sau.
NLN'108-11/2012* 5
^ r^-'^vy^^v^
Hinh 6 Ket quii thin: nghi$m kiem chiing nhi^l 66 budng dot va tai Id hoi Bang 4 So sanh kit qua tinh toan vai gia tri do
Tk\ Id h(yi
60%
cdng s u i t
80%
cdng su^t
100%
cong s u i t
Nhiet dO b u o n g / C ) LLPU lu'p'ng hoi (kq/h) Nhiet dd buong (°C) Lifu lu-png hoi (kq/h) Nhiet do budng ("C^
LU'U lu'g'ng hoi (kq/h)
T r u n g binh gid
trj d o
616
3 028,65
735
4 011.35
804
4 926,395 GiS tri
t i n h t o a n
631
3 000
730
4 000
813
5 000 Sai s d giu'a
gia trj do va gia tri t i n h toan
15
28,65
11,35
9
73,61 Do c h i n h
xac
2,37%
0,95%
0,68%
0,28%
1,1%
1,47%
Ket qua tinh toan phij hap vPi gia tri do vPi sai so nhiet dfi bufing dot 0,68 ^ 2,37% v^ lyu lyang hai 0,28 ^1,47% tai dieu kien vpn hanh tyang yng vPi thiet ke, ti> dfi ta nhSn x6t ket qua tinh loan la tin cay.
4.6. So sanh k^t qua t i n h toan vo'i kPt qua ciJa mot so tac gia da c o n g bd
So sanh ket qua tinh toan vPi mfit sfi kfit qua da cfing bo cua c^c tcic gia khae dyac chi ro treng bang 5.
Ket qua nghidn eyu neu trong bai bao n^y phu hpp vPi Ccic ket qua cua mfit so tac gia da cong bo Sai khcic giya ket qua tinh toSn vai cac ket qua d§
cfing bo \i do:
- CPc k i t qua cua mfit so tPe gia dS cong bo dya trfin cSc nghifin eyu a cac mfi hinh thyc nghiem kh^c nhau do viy ton tai sai khac khi dem so sanh vPi mfi hinh nPy.
- Hi so trao doi nhipt trong Ifi hai lap sfii nfii chung v^ dac bifit Ifi hai lap soi eai t i l n chiu anh hyfi'ng cua tfic dp gifi cap...
T y nhOng so sPnh ddnh gici trfin ta thay phyang phcip tinh toan nfiu trPn IS tin epy vi Sp dung d y a c
Cic k i t qua cong bfi trong bai bao nPy cfi the dung IPm c a so" de tnen khai tinh toPn thp* k l che tpo, loi y u hfia thiet k l n h i m t h y a n g mpi hoa "; hai dot trau ldp sfii cai t i l n
Sang 5 So sanh kit qua tinh toan voi mot s6 ket qua da cdng bo
P Sathitruangsak, 2009
M Rozainee et all, 2008
Pham Hoang Lirong, 2006 L, Armesto et all, 2002 Sheng Huang et all, 2001 Chen Guanyi e t a l l , 1998 W Pnns, 1997 F J o h n s o n e t a l l , 1987 Ketquacua bao cao, 2012
He s 6 t r a o d d i nhiet b u d n g d d t , W/m2K
7 3 - 1 1 0
100 119,2
Tdc d o g i d c i p , m/s
0,4-0,8 3,3 U,n(
(0,66 VOI U,r<=0,2) 0,67-1,7 1,2 0,18 1-2 0,5-0,8
0,45 Nhiet d o t r u n g binh lo'p SOI, °C
750-850 840 700
800-900
856 V. K ^ T LUAN
Bai bao da trinh bay k i t aua tinh toSn trao doi nhipt trong bufing lt>a Ifi hai dot t r i u 16'p sfii cai tien, so sanh danh giS giya k i t qua tinh toSn vi thyc nghifim k i l m chang k i t hap vpi sfi li$u mfit so tac gia da cfing bfi. Anh h y a n g cua cPc thfing s6 t h i l t k l vi van hanh buong dot 16'p sfii cai tien den qua trinh trao doi nhi^t cung d y a c dya ra thao luSn.
Mfit so k i t qua chu y l u ti> nghifin CCPU nSy nhy sau"
- Tai tai dinh m y c (100% cfing suat) cua IP hai.
he sfi trao doi nhifit buong dfit IP 119,2 W/m^K, toe dfi sfii toi thieu (vSn toe sfii bpt nho nhat) Umt IS 0,2m/s va tfic dp gio d p U IS 0,45 m/s. Nhi^t dp trung binh bufing dot ia 813°C vS nhiet dO trung binh lap sfii IS 856°C
6* NLN-108 Ml/2012
- Moi quan hp giO-a he so trao doi nhiet buong dot 87,7 - 119,2 W/m^K vai tai Ifi hai 3.000^5 000 kg/h, nhiet dp bufing dot 631 ^ 813°C va tfic dfi gio cSp 0,27 -f 0,45 m/s la quan hp t u y i n tinh.
K i t qua tinh toan phu hap vPi giS tn do tai d i l u kiSn van hanh t y a n g i>ng vfi-i t h i l t k l vS so lieu mot II so tac gia da cfing bo. Do vSy co the xem IS tin eay
vS Sp dung vao thyc t l d y p c . ABSTRACT
This paper presents research on calculation of heat transfer coefficient in the rice-husk fired Improvement Fluidized Bed Boiler Furnace - IFBBF. Calculation results were compered with expenmental data. Some main results received are as follows: freeboard heat transfer coefficient is 119.2 W/rr?K, minimum fluidization velocity is 0.2 m/s and fluidization velocity is 0.45 m/s, fluidized bed temperature is 856 °C and freeboard temperature is 813
°C The relations between heat transfer coefficient and unit load, freeboard temperature and bed velocity are analyzed The linear relations could be accepted for the IFBBF design regarding to heat transfer coefficient range 87 7-r119-2W/m^K. unit load 3000^000kg/h, freeboard temperature 631 SI3 °C and bed velocity 0.27^.45m/s.
TAI LIEU THAM KHAO
[1]. P Sathitruangsaka, T. Madhiyanonb, S Soponronnarita (2009) Ricehusk co-finng with coal in a short-combustion- chamberfluidized-bedcombustor (SFBC), Fuel. 88(8), 1394-1402.
[2] M Rozainee, S.P, Ngo, A A Salema K.G Tan, M, Anffin, Z N Zainura (2008) Effect of fluidising vetocity on the combustion of rice husk in a bench-scale fluidised bed combuslor for the produclion of amorphous nee husk ash Bioresource Technology 99(4), 703-71
(3] L Amiesto, A. Bahillo, K Veijonen, A. Cabanillas and J Otero (2002) Combustion behaviour of nee husk in a bubbling fluidised bed.
Bbmass and Bioenergy, 23(3), 171-179
[4] Sheng Huang, Shan Jing, Jinfu Wang, Zhanwen Wang, Yong Jin (2001) Silica while obtained from ncehusk in a fluidized bed Powder Tec/Jno/ogy. 117(3), 232-238 5) F Johnson, B A
[5] Andersson, B Leckner (1987) Heat flow measurements in fluidized bed boilers. Proceedings of a!" International Conference on FBC - Boston, (1), 592-597
[6] W Pnns (1987) Fluidized bed combustion of a single carbon particle. PhD dissertation. University Twente, Enschede (the Netherlands]
[7] Chen Guanyi, Fang Mengxiang, Luo Zhongyang, Yu Chunjiang, Li Xuantian, Ni Mmgjiang, Cen Kefa (1998) The Study on Combustion Charactenstics of Rice Husk Fired Fluidized Bed Boiler Journal of combustion science and technology
[8] T Madhiyanon, P Sathitruangsak and S Soponronnant (2010) Combustion charactenstics of nce-husk in a short-combustion- chamber fluidized-bed combustor (SFBC). Applied Themial Engineering. 30(4), 347-353
[9] Prabir Basu (2006) Combustion and Gasification in Fluidized Beds Taylor & Francis Group, LLC.
[10] Simeon N Oka, EJ. Anthony (2004) Fluidized Bed Combustion. Marcel Dekker, Inc Newyork - Basel
111]. Guangxi Vue, Hai Zhang, Changsui Zhao, Zhongyang Luo (2009) Proceedings of the 20" International Conference on Fluidized Bed Combustion Tsinghua University Press, Beijing and Sprmger-Verlag Berlin Heidelberg
[13] Phgm HoSng Lu'ong, Do V3n ThSng (2006), "Nghien cim 1/^ img dung cdng ngh$ ldp sdi vA Idp sdi tuan hoin & Viet nam", B^o cSo liln d« d^ An 10/HO-UTCN 2005-2006 OHBKHN
Phan bien: PGS. I S T r y a n g Duy NghTa
TAN DUNG NHIET THAI TUT DAN NGUNG MAY DIEU HOA KHONG KHI CONG SUAT NHO DUN NUdC NONG
DE TIET KIEM NANG LUONG
B U I N G Q C H U N G , DH Bach khoa- Dfi Quoc gia Tp. Ho Chi Wnh Viec sO dung miy dieu hda khong khi vi miy nwov ndng da tr& nen pho bien trong phan l&n cac hp gia dinh o* cic thinh pho lan cua nudfc ta hien nay. Tuy nhiin cac thiet bj nay dang dwac sir dung mpt each dpc lip dong thdi tiiu ton mpt luv'ng dien nang hang thing ding ke trong cac hp gia dinh. Bai bio niy dua ra mpt hwang tich hgp hai he thong may dieu hda khong khi vi may nied'c ndng noi tren de tiet kiem ning lieg-ng bang each tin dung nhiet thai to- cac dan ngwng miy dieu hda khong khi di dun n u o c ndng. Mgt md hinh thuv nghigm thu hoi nhiet thai tO' dan ngung cua miy dieu hoa khong khi lo^i hai phan rdri (split type air conditioning) su' dung thiit bj trao doi nhigt dang ong xoan da dwo'c hoin thanh. Bang vigc do dac cic thong so hoat dpng cua md hinh da chd'ng minh tinh kha thi cOa mo hinh si> dung thiet bj trao doi nhiet ong xoan cung nhw vigc thu hoi nhiet thai khong anh hwong den hoat dpng binh thwdng cua miy dieu hoa khdng khi.
I. 016^1 THIEU
Nhifit thai tCr t h i l t bi ngyng ty cua hp thong lanh d y a c xem la nguon nhiet thai cP nhiet dp thap Cfi nhieu p h y a n g Sn de thu hoi nhipt thai cua hi thong lanh, tuy nhien ph6 b i l n thyfi-ng IS s y dyng nguon nhiet nSy d l dun nfing n y P c phyc vu sinh hoat hoSc gia nhiet s a bfi cho eSc qua trinh cong nghipp [1].
Boi vPi vipc s y dung mSy d i l u hfia khfing khi hi0n nay, c h y a eP mfit nghiPn cO'u cy t h l v l lupng dipn nang tieu thy hSng thSng trong eSc hp gia dinh cP s y dyng mSy dieu hda. Tuy nhien neu dya vao cSc so lipu thong ki, cong suat difin ti&u thg eho hfi t h i n g dieu hfia trong cac cfing trinh nhy eao I c , vSn phPng, khach san chiem tir 4 5 % d i n 55% tong lyang dien tifeu thu [2]. NgoSi ra mSy nyfi'c nfing cCing d y a c xem IS mfit trong nh&ng t h i l t bj tifeu thti diPn IPn so vPi cSc thiet bi difin gia dung khSe [3]
O i n nay dS co nhieu nghi6n cu-u de giam lypng dipn nang tieu thy cua mSy dieu hfia khfing khi vS mSy n y p c nfing. D l i vPi mSy dieu hfia IS thay doi nhiSt dp cSi dSt ban ngSy, ban dem, thay doi lu'u lypng khfing khi theo phy tai t h y c t l [4] hoSc si>
dung bam nhipt d l t i l t kiem nSng l y a n g [5]. Vi$c su' dyng ong nhiet dao dong de thu hoi nhipt thai t y thilt