2. heat transfer
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LOGO
Unit Operation and ProcessHeat Transfer
Widelia Ika Putri, S.T.P., M.Sc.
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Pasteurization of milk, juice, etc.Refrigeration of fruits andvegetables.Meat freezing.Thermal sterilization in cannedfood.EvaporationDistillationDrying
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Heat transfer A dynamic process in which heat istransferred spontaneously from one bodyto another a lower temperature body.The rate of heat transfer depends uponthe differences between the bodies, thegreater the difference in temperature, thegreater the rate of heat transfer.Temperature difference between thesource of heat and the receiver of heat is
therefore the dr iv ing fo rce in heattransfer. An increase in the temperature differenceincreases the driving force and therefore
increasing the rate of heat transfer.
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Ohms Law
The rate of transport (the quantitytransported per unit time) isproportional to the driving force andinversely proportional to the
resistance of the medium to thetransport.
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q = dQ/dt = the rate of heat transferF = driving forceR = resistance of the medium to heattransfer
k = conductance of the medium toheat transfer
Heat flux (J) = the rate of transportproportional to the area A available tothe transport (the rate of transportper unit area) => q/A
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Q = heat transferred (J)T = temperature (K)t = time (s)k = thermal conductivity of themedium (J/s.m.K)(W/m.K)z = distance in the direction of thetransport (m)
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Soal
1. Hitung kecepatan transfer panas daribahan 3x4 m, tebal 0,2m, di satu sisi0 o C dan di sisi lainnya 35 o C. Jikabahan tersebut adalah: a. alumunium;
b. kuningan; c. besi; d. tembaga; e.karet
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Sweat (1986)
k = 0.25X c + 0.155X p + 0.16X f +0.135X a + 0.58X wX = mass fractionc = carbohydrate
p = proteinf = fata = ash
w = water
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Soal
1. Hitung thermal conductivity daribahan berikut ini:
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food Water(%)
Protein(%)
Fat(%)
Carbohydrate(%)
Apel 84.1 0.3 0.4 14.9Brokoli 89.9 3.3 0.2 5.5
Tepunggandum
12.0 9.2 1.0 73.8
Susu sapi 87.0 3.5 3.9 4.9
Ayampanggang
66.0 20.2 12.6 0
Ikan cod 82.6 16.5 0.4 0
Almond 4.7 18.6 54.1 19.6
d l k b d
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Steady State Conduction through aSingle Slab
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id li ik i@ b id
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Steady State Conduction through aMultilayer Slab
id li ik t i@ b id
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1. Dinding dari ruang penyimpantersusun dari 3 lapisan, dari yangterdalam:
Stainless steel, tebal 2mm Thermal insulation, tebal 80mm (k =
0.03 J/s.m.K) Beton, tebal 150mm
Bila sisi terdalam adalah -18o
C dansisi terluar 20 o C, hitung flux panasyang melalui dinding?
ideliaikap tri@ b ac id
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Steady State Conduction through aCylinder Slab
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www themegallery com
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Soalwww.themegallery.com
Company Logo
1. Pipa baja berdiameter luar 5cmmembawa uap air pada 150 o C. Pipadisekat dengan lapisan penyekatsilinder ketebalan 3cm
(k=0.03W/mK). Hitung kecepatanpanas hilang per meter panjang pipa, jika suhu lapisan terluar adalah 35 o C?
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wideliaikaputri@ub ac id
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A stationary layer (film) of fluid whichin contact with the solids surface ->thickness Temperature in the bulk -> T 2 ;
temperature in the boundary layer ->T1 A coefficient convective of heattransfer ->
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wideliaikaputri@ub ac id
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h -> depends on the properties of thefluid (specific heat, viscosity, density,and thermal conductivity), theturbulence (average velocity), and
the geometry of the system -> theseparameters are dimensionless groupused in heat transfer.
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Soal
1. Papan gabus tebal 10 cm pada satusisi bersuhu -12 o C dan sisi lain 21 o C.Berapa kecepatan transfer panasmelalui 1m 2 dinding?
2. Sebuah cold storage memilikidinding yang tersusun dari: 11 cmbatu bata pd bag. terluar, kmdn 7,5cm beton, dan 10 cm beton. Suhu
rata2 di bag. dalam -18 o C dan dibagian luar dinding 18 o C. Hitungkecepatan transfer panas melalui1m 2 dinding?
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Soal
1. Hitung kecepatan panas yang hilangdari dinding vertical suatu ovendengan konveksi alami. Suhu dinding50 o C dan di udara ambient 20 o C.
Tinggi dinding 1.2m dan lebar 3m?
p @
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p @
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Forcedconvection
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Soal
1. Jus jeruk, didinginkan, mengalirdengan kecepatan 4m 3 /jam melaluipipa berdiameter 0.05m. Sifat dari jus
jeruk pada suhu pengolahan adalah: = 1060kg/m 3 ; = 3.10 -3 Pa.s; c
p=
3900J/kg.K; k= 0.54 W/m.K. Hitungkoeffisien panas konveksi antara jus
jeruk yang mengalir di dalam pipa
dengan dinding pipa?
p @
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2. Gandum pipilan dikeringkan denganudara panas dalam aliran turbulent difluidized bed dryer. Kecepatan udara0.5m/s dengan suhu 93 o C. Pipilangandum diasumsikan berbentuk bulatdengan diameter 1mm. Hitungkoeffisien transfer panas konveksi?
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Black body => a body that absorb theincident radiation totally; theabsorbtivity of black body is unity andits reflectivity and transmissivity areboth zero .
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Emmisivity ( ) => the ratio of emissive power of a real body to thatof a black body at the sametemperature; always less than unity.
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Soal
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1. Hitung net heat transfer denganradiasi dari sebongkah roti di ovenpada suhu 177 o C, jika emissivitasnya0,85 dgn total luas area 0.0645 m 2 dan suhu 100 o C?
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LOGO