LOGO Unit Operation and Process Heat Transfer Widelia Ika Putri, S.T.P., M.Sc.

March 2012  Pasteurization of milk, juice, etc.  Refrigeration of fruits and vegetables.  Meat freezing.  Thermal sterilization in canned food.  Evaporation  Distillation  Drying

March 2012 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.

March 2012 Heat transfer Radiation Convection Conduction

 Rate of transfer = driving force / resistance  Rate of heat transfer = temperature difference / heat flow resistance of medium. March 2012

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. March 2012

 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 March 2012

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 1 st Law March 2012

March 2012  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)

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). March 2012

Soal 1. Hitung kecepatan transfer panas dari bahan 3x4 m, tebal 0,2m, di satu sisi 0 o C dan di sisi lainnya 35 o C. Jika bahan tersebut adalah: a. alumunium; b. kuningan; c. besi; d. tembaga; e. karet March 2012

Sweat (1986)  k = 0.25X c X p X f X a X w  X = mass fraction  c = carbohydrate  p = protein  f = fat  a = ash  w = water March 2012

Soal 1.Hitung thermal conductivity dari bahan berikut ini: March 2012 foodWater (%) Protein (%) Fat (%) Carbohydrate (%) Apel Brokoli Tepung gandum Susu sapi Ayam panggang Ikan cod Almond

 Steady State Conduction through a Single Slab March 2012

March 2012  Steady State Conduction through a Multilayer Slab

Soal March 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 -18 o C dan sisi terluar 20 o C, hitung flux panas yang melalui dinding?

 Steady State Conduction through a Cylinder Slab March 2012

Soal Company Logo 1. Pipa baja berdiameter luar 5cm membawa uap air pada 150 o C. 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 35 o C?

March 2012 Steady State Convective Heat Transfer Natural Convection The movement is caused by heat transfer itself. Convection Forced Convection The movement is caused by factor independent of the transfer.

 A stationary layer (film) of fluid which in contact with the solid’s surface -> thickness δ  Temperature in the bulk -> T 2 ; temperature in the boundary layer -> T 1  A coefficient convective of heat transfer -> March 2012

 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. March 2012

March 2012 Natural convection

Soal 1.Papan gabus tebal 10 cm pada satu sisi bersuhu -12 o C dan sisi lain 21 o C. Berapa kecepatan transfer panas melalui 1m 2 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 -18 o C dan di bagian luar dinding 18 o C. Hitung kecepatan transfer panas melalui 1m 2 dinding? March 2012

Soal 1. Hitung kecepatan panas yang hilang dari dinding vertical suatu oven dengan konveksi alami. Suhu dinding 50 o C dan di udara ambient 20 o C. Tinggi dinding 1.2m dan lebar 3m? March 2012

March 2012 Forced convection

Soal 1. Jus jeruk, didinginkan, mengalir dengan kecepatan 4m 3 /jam melalui pipa berdiameter 0.05m. Sifat dari jus jeruk pada suhu pengolahan adalah: ρ= 1060kg/m 3 ; µ= Pa.s; c p = 3900J/kg.K; k= 0.54 W/m.K. Hitung koeffisien panas konveksi antara jus jeruk yang mengalir di dalam pipa dengan dinding pipa? March 2012

March Gandum pipilan dikeringkan dengan udara panas dalam aliran turbulent di fluidized bed dryer. Kecepatan udara 0.5m/s dengan suhu 93 o C. Pipilan gandum diasumsikan berbentuk bulat dengan diameter 1mm. Hitung koeffisien transfer panas konveksi?

Radiation March 2012  Radiation covers a vast array of phenomena that involve energy transport in the form of waves, in the wavelength range of to m.  Radiation => cooling the emitting body and heating the receiving body.  > 0 o K => all substances emit electromagnetic radiation.  Heat transfer by radiation does not require the presence of material medium.

 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. March 2012

March 2012  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.

Soal March Hitung net heat transfer dengan radiasi dari sebongkah roti di oven pada suhu 177 o C, jika emissivitasnya 0,85 dgn total luas area m 2 dan suhu 100 o C?

LOGO