2. Heat Transfer

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Unit Operation and Process
Heat Transfer
Widelia Ika Putri, S.T.P., M.Sc.
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Pasteurization of milk, juice, etc.
Refrigeration of fruits and
vegetables.
Meat freezing.
Thermal sterilization in canned
food.
Evaporation
Distillation
Drying
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Heat transfer
 A dynamic process in which heat is
transferred spontaneously from one body
to another a lower temperature body.
 The rate of heat transfer depends upon
the differences between the bodies, the
greater the difference in temperature, the
greater the rate of heat transfer.
 Temperature difference between the
source of heat and the receiver of heat is
therefore the driving force in heat
transfer.
 An increase in the temperature difference
increases the driving force and therefore
increasing the rate of heat transfer.
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Heat transfer
Conduction
Convection
Radiation
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Rate of transfer = driving force /
resistance
Rate of heat transfer = temperature
difference / heat flow resistance of
medium.
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Ohm’s Law
The rate of transport (the quantity
transported per unit time) is
proportional to the driving force and
inversely proportional to the
resistance of the medium to the
transport.
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q = dQ/dt = the rate of heat transfer
F = driving force
R = resistance of the medium to heat
transfer
k = conductance of the medium to
heat transfer
 Heat flux (J) = the rate of transport
proportional to the area A available to
the transport (the rate of transport
per unit area) => q/A
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Steady State Conductive Heat Transfer
Steady state => all the properties
that define the ‘state’ of the system
(temperature, pressure, chemical
composition, etc.) remain constant
with time.
Fourier’s 1st Law
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Q = heat transferred (J)
T = temperature (K)
t = time (s)
k = thermal conductivity of the
medium (J/s.m.K)(W/m.K)
z = distance in the direction of the
transport (m)
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Thermal conductivity
• A property of the material (the
material’s ability to transfer heat).
Thermal diffusivity
• The ratio of thermal conductivity to the
“volumetric heat capacity” of material
(the ratio of the material’s ability to
transfer heat to its capacity to store
heat).
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Soal
1. Hitung kecepatan transfer panas dari
bahan 3x4 m, tebal 0,2m, di satu sisi
0oC dan di sisi lainnya 35oC. Jika
bahan tersebut adalah: a. alumunium;
b. kuningan; c. besi; d. tembaga; e.
karet
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Sweat (1986)
k = 0.25Xc + 0.155Xp + 0.16Xf +
0.135Xa + 0.58Xw
X = mass fraction
c = carbohydrate
p = protein
f = fat
a = ash
w = water
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Soal
1. Hitung thermal conductivity dari
bahan berikut ini:
food
Water
(%)
Protein
(%)
Fat
(%)
Carbohydrate
(%)
Apel
84.1
0.3
0.4
14.9
Brokoli
89.9
3.3
0.2
5.5
Tepung
gandum
12.0
9.2
1.0
73.8
Susu sapi
87.0
3.5
3.9
4.9
Ayam
panggang
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
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Steady State Conduction through a
Single Slab
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Steady State Conduction through a
Multilayer Slab
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Soal
1. Dinding dari ruang penyimpan
tersusun dari 3 lapisan, dari yang
terdalam:
• Stainless steel, tebal 2mm
• Thermal insulation, tebal 80mm (k =
0.03 J/s.m.K)
• Beton, tebal 150mm
Bila sisi terdalam adalah -18oC dan
sisi terluar 20oC, hitung flux panas
yang melalui dinding?
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Steady State Conduction through a
Cylinder Slab
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Soal
1. Pipa baja berdiameter luar 5cm
membawa uap air pada 150oC. Pipa
disekat dengan lapisan penyekat
silinder ketebalan 3cm
(k=0.03W/mK). Hitung kecepatan
panas hilang per meter panjang pipa,
jika suhu lapisan terluar adalah 35oC?
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Steady State Convective Heat Transfer
Convection
Natural
Convection
Forced
Convection
The movement
is caused by
heat transfer
itself.
The movement
is caused by
factor
independent of
the transfer.
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A stationary layer (film) of fluid which
in contact with the solid’s surface ->
thickness δ
Temperature in the bulk -> T2;
temperature in the boundary layer ->
T1
A coefficient convective of heat
transfer ->
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h -> depends on the properties of the
fluid (specific heat, viscosity, density,
and thermal conductivity), the
turbulence (average velocity), and
the geometry of the system -> these
parameters are dimensionless group
used in heat transfer.
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Natural
convection
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Soal
1. Papan gabus tebal 10 cm pada satu
sisi bersuhu -12oC dan sisi lain 21oC.
Berapa kecepatan transfer panas
melalui 1m2 dinding?
2. Sebuah cold storage memiliki
dinding yang tersusun dari: 11 cm
batu bata pd bag. terluar, kmdn 7,5
cm beton, dan 10 cm beton. Suhu
rata2 di bag. dalam -18oC dan di
bagian luar dinding 18oC. Hitung
kecepatan transfer panas melalui
1m2 dinding?
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Soal
1. Hitung kecepatan panas yang hilang
dari dinding vertical suatu oven
dengan konveksi alami. Suhu dinding
50oC dan di udara ambient 20oC.
Tinggi dinding 1.2m dan lebar 3m?
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Forced
convection
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Soal
1. Jus jeruk, didinginkan, mengalir
dengan kecepatan 4m3/jam melalui
pipa berdiameter 0.05m. Sifat dari jus
jeruk pada suhu pengolahan adalah:
ρ= 1060kg/m3; µ= 3.10-3Pa.s; cp=
3900J/kg.K; k= 0.54 W/m.K. Hitung
koeffisien panas konveksi antara jus
jeruk yang mengalir di dalam pipa
dengan dinding pipa?
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2. Gandum pipilan dikeringkan dengan
udara panas dalam aliran turbulent di
fluidized bed dryer. Kecepatan udara
0.5m/s dengan suhu 93oC. Pipilan
gandum diasumsikan berbentuk bulat
dengan diameter 1mm. Hitung
koeffisien transfer panas konveksi?
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Radiation
Radiation covers a vast array of
phenomena that involve energy
transport in the form of waves, in the
wavelength range of 10-7 to 10-4 m.
Radiation => cooling the emitting
body and heating the receiving body.
> 0oK => all substances emit
electromagnetic radiation.
Heat transfer by radiation does not
require the presence of material
medium.
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Black body => a body that absorb the
incident radiation totally; the
absorbtivity of black body is unity and
its reflectivity and transmissivity are
both zero.
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Emmisivity (ε) => the ratio of
emissive power of a real body to that
of a black body at the same
temperature; always less than unity.
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Soal
1. Hitung net heat transfer dengan
radiasi dari sebongkah roti di oven
pada suhu 177oC, jika emissivitasnya
0,85 dgn total luas area 0.0645 m2
dan suhu 100oC?
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