Unit Operation and Process Heat Transfer

Unit Operation and Process

Heat Transfer



Pasteurization of milk, juice, etc.



Refrigeration of fruits and

vegetables.



Meat freezing.



Thermal sterilization in canned

food.



Evaporation



Distillation



Drying

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

Heat transfer

Radiation Convection Conduction

Rate of transfer = driving force /

resistance

Rate of heat transfer = temperature

difference / heat flow resistance of medium.

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.

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

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.

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

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).

Soal

1. Hitung kecepatan transfer panas dari bahan 3x4 m, tebal 0,2m, di satu sisi 0o_{C dan di sisi lainnya 35}o_{C. Jika}

bahan tersebut adalah: a. alumunium; b. kuningan; c. besi; d. tembaga; e.

Sweat (1986)

k = 0.25X_c + 0.155X_p + 0.16X_f + 0.135X_a + 0.58X_w X = mass fraction c = carbohydrate p = protein f = fat a = ash w = water

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

Steady State Conduction through a

Steady State Conduction through a

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.C)

• Beton, tebal 150mm

Bila sisi terdalam adalah -18o_{C dan} sisi terluar 20o_{C, hitung flux panas} yang melalui dinding?

Steady State Conduction through a

Soal

1. Pipa baja berdiameter 5cm

membawa uap air pada 150o_{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 35o_C?

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 -> T₂;

temperature in the boundary layer -> T₁

A coefficient convective of heat

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.

Natural convection sphere

Soal

1. Papan gabus tebal 10 cm pada satu sisi bersuhu -12o_{C dan sisi lain 21}o_C. 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 gabus. Suhu rata2 di bag. dalam -18o_{C dan di} bagian luar dinding 18o_{C. Hitung} kecepatan transfer panas melalui 1m2 _dinding?

Soal

1. Hitung kecepatan panas yang hilang dari dinding vertical suatu oven

dengan konveksi alami. Suhu dinding 50o_{C dan di udara ambient 20}o_C.

Forced convection turbulent

viscous sphere

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}-3_{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

2. Gandum pipilan dikeringkan dengan udara panas dalam aliran turbulent di fluidized bed dryer. Kecepatan udara 0.5m/s dengan suhu 93o_{C. Pipilan}

gandum diasumsikan berbentuk bulat dengan diameter 1mm. Hitung

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.

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.

Emmisivity (ε) => the ratio of

emissive power of a real body to that of a black body at the same

Soal

1. Hitung net heat transfer dengan radiasi dari sebongkah roti di oven

pada suhu 177o_{C, jika emissivitasnya} 0,85 dgn total luas area 0.0645 m2

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).

Flow Pattern

Parallel flow 1 Countercurrent flow 2 Cross flow 3

Jacketed Pan

Kumparan pemanas

Plate Heat Exchanger