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Unit Operation and ProcessHeat Transfer

Widelia Ika Putri, S.T.P., M.Sc.

March 2012

wideliaikaputri@ub.ac.id

Pasteurization of milk, juice, etc.Refrigeration of fruits and

vegetables.Meat freezing.Thermal sterilization in canned

food.EvaporationDistillationDrying

March 2012

wideliaikaputri@ub.ac.id

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

wideliaikaputri@ub.ac.id

Heat transfer

Radiation

Convection

Conduction

March 2012

Rate of transfer = driving force / resistance

Rate of heat transfer = temperature difference / heat flow resistance of medium.

wideliaikaputri@ub.ac.id

March 2012

Ohm’s LawThe 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.

wideliaikaputri@ub.ac.id

March 2012

q = dQ/dt = the rate of heat transferF = driving forceR = resistance of the medium to heat

transferk = 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

wideliaikaputri@ub.ac.id

March 2012

Steady State Conductive Heat TransferSteady state => all the properties

that define the ‘state’ of the system (temperature, pressure, chemical composition, etc.) remain constant with time.

Fourier’s 1st Law

wideliaikaputri@ub.ac.id

March 2012

wideliaikaputri@ub.ac.id

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)

March 2012

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).

wideliaikaputri@ub.ac.id

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

wideliaikaputri@ub.ac.id

March 2012

Sweat (1986)k = 0.25Xc

+ 0.155Xp + 0.16Xf + 0.135Xa + 0.58Xw

X = mass fraction c = carbohydratep = proteinf = fata = ashw = water

wideliaikaputri@ub.ac.id

March 2012

Soal 1. Hitung thermal conductivity dari

bahan berikut ini:

wideliaikaputri@ub.ac.id

March 2012

food Water (%)

Protein (%)

Fat (%)

Carbohydrate (%)

Apel 84.1 0.3 0.4 14.9Brokoli 89.9 3.3 0.2 5.5Tepung gandum

12.0 9.2 1.0 73.8

Susu sapi 87.0 3.5 3.9 4.9Ayam panggang

66.0 20.2 12.6 0

Ikan cod 82.6 16.5 0.4 0Almond 4.7 18.6 54.1 19.6

March 2012

Steady State Conduction through a Single Slab

wideliaikaputri@ub.ac.id

March 2012

wideliaikaputri@ub.ac.id

Steady State Conduction through a Multilayer Slab

March 2012

Soal wideliaikaputri@ub.ac.id

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?

March 2012

Steady State Conduction through a Cylinder Slab

wideliaikaputri@ub.ac.id

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Soal www.themegallery.com

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?

March 2012

wideliaikaputri@ub.ac.id

Steady State Convective Heat Transfer

Natural ConvectionThe movement is caused by heat transfer itself.

Convection

Forced ConvectionThe movement is caused by factor independent of the transfer.

March 2012

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 ->

wideliaikaputri@ub.ac.id

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.

wideliaikaputri@ub.ac.id

March 2012

wideliaikaputri@ub.ac.id

Natural convection

March 2012

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?

wideliaikaputri@ub.ac.id

March 2012

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?

wideliaikaputri@ub.ac.id

March 2012

wideliaikaputri@ub.ac.id

Forced convection

March 2012

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?

wideliaikaputri@ub.ac.id

March 2012

wideliaikaputri@ub.ac.id

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?

March 2012

Radiation wideliaikaputri@ub.ac.id

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.

wideliaikaputri@ub.ac.id

March 2012

wideliaikaputri@ub.ac.id

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.

March 2012

Soal wideliaikaputri@ub.ac.id

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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