Unit Operation and Process Heat Transfer

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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 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
  increases 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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Ohms 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)
  gt q/A

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Steady State Conductive Heat Transfer

• Steady state gt all the properties that define
  the state of the system (temperature, pressure,
  chemical composition, etc.) remain constant with
  time.
• Fouriers 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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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

• 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

• 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.C)
• 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 5cm membawa uap air pada
  150oC. Pipa disekat dengan lapisan penyekat
  silinder ketebalan 3cm (k0.03W/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 The movement is caused by heat
transfer itself.
Forced Convection 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 solids surface -gt thickness d
• Temperature in the bulk -gt T2 temperature in the
  boundary layer -gt T1
• A coefficient convective of heat transfer -gt

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• h -gt depends on the properties of the fluid
  (specific heat, viscosity, density, and thermal
  conductivity), the turbulence (average velocity),
  and the geometry of the system -gt these
  parameters are dimensionless group used in heat
  transfer.

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vertical
Natural convection
sphere
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Soal

• Papan gabus tebal 10 cm pada satu sisi bersuhu
  -12oC dan sisi lain 21oC. Berapa kecepatan
  transfer panas melalui 1m2 dinding?
• Sebuah cold storage memiliki dinding yang
  tersusun dari 11 cm batu bata pd bag. terluar,
  kmdn 7,5 cm beton, dan 10 cm gabus. 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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turbulent
Forced convection
viscous
sphere
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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
  ? 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 gt cooling the emitting body and
  heating the receiving body.
• gt 0oK gt all substances emit electromagnetic
  radiation.
• Heat transfer by radiation does not require the
  presence of material medium.

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• Black body gt 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 (e) gt 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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Heat Exchanger

• Device for the exchange of heat between two
  fluids separated by partition.
• The partition is a heat conducting solid wall,
  usually made of metal.
• Extensively used in the food industry for heating
  (e.g. pasteurizer), cooling (chilled water
  generator), and heat induced phase change
  (freezing, evaporation).

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Flow Pattern
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