OKKY PP

Heat Transfer

UISI For internal use only

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EM52013 Heat Transfer

Department of Engineering Management

UNIVERSITAS INTERNASIONAL SEMEN INDONESIA

OKKY PUTRI PRASTUTI, S.T, M.T

Course Details

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Course title : Heat Transfer Time slot : Monday, 12.35-15.20 Credit : 3 sks Instructor : Okky Putri Prastuti, S.T, M.T Telephone : +6281332546469 E-mail : okky.prastuti@uisi.ac.id Course books : 1. Incropera, Frank P. Dewitt, David P, 1996. Intorduction to Heat Transfer. 3rd Edition,

John Willey & Sons. 2. Geankoplis,C.J., Transport Processes and Separation Process Principles, Prentice Hall

International Inc., 4th Ed, 2003

TOPICS

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1. The concept of heat transfer (conduction, convection, radiation) 2. Analysis conduction 3. Forced Convection and Natural Convection 4. Heat exchangers 5. The concept of radiation and mass transfer

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Outline of Learning

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Week Subjects

1 Thermodynamic and Heat Transfer

2-3 Conduction Heat Transfer

4-5 Conduction and convection system

6 Steady state conduction for multi dimensional

7 Convenction Heat Transfer

8 UTS

9 External forced convection

10 Internal forced convection

11 Natural convection heat tansfer

12 Boiling and condensation

13-14 Heat exchanger

15 Radiation Heat Transfer

16 UAS

Assessment

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Tugas : 15 % Kuis 1 : 15 % UTS : 20 % Kuis 2 : 15 % UAS : 30 % Keaktifan : 5 %

Introduction of Thermodynamics

Heat Transfer t

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Definition

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Thermodynamics : The science of energy

The name thermodynamics stems from the Greek words therme

(heat) and dynamics (motion)

Thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables such as temperature, volume, and pressure.

THERMO : HEAT and TEMPERATURE DYNAMICS : MOTION

Example

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The production of chemicals, polymers, pharmaceuticals and other biological materials, and oil and gas processing, all involve chemical or biochemical reaction that produce a mixture of reaction product. (e.g : production of tert-butanol) 1. These must be separated from the mixture and purified to result in product of

societal, commercial, or medicinal value. 2. These is the area where thermodynamics plays a central role in process eng. 3. Separation processes, e.g. distillation are designed based on information from

thermodynamics such as vapor-liquid equilibrium data.

Example (cont.)

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Vapor-Liquid

Equilibrium

(VLE) data

Dimensions and Units

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- Dimension is recognize through our sensory perceptions and not

definable without the definition of arbitrary scales of measure,

divided into specific units of size.

The units have been set by international

agreement, and are

codified as the International System of Units (SI).

Dimensions and Units (cont) t

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Dimensions and Units in HYSYS v3.2

American Engineering Units (Field)

SI units

Measurements of Amount and Size

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Three measures of amount or size are in common use: Mass, m ; Number of moles, n ; Total volume, Vt

Mass, m divided by the molar mass M (molecular weight) to yield number of moles;

,M

mn Mnm

Total volume, divided by the mass or number of moles of the system to yield specific or molar volume.

Specific volume:

Molar Volume:

mVV t m

VV

t

or

nVV t n

VV

t

or

Measurements of Amount and Size (cont)

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Properties in HYSYS

Force

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21

c

2

c

s f kg m kg .80665 9 g

ms 9.80665 x kg 1 x g

1 kgf 1

* Note : The kilogram force is equivalent to 9.80665 N

Temperature

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All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water: the ice point and the steam point.

Ice point: A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure (0C or 32F).

Steam point: A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure (100C or 212F).

Celsius scale: in SI unit system Fahrenheit scale: in English unit system Thermodynamic temperature scale: A temperature scale that is independent of the properties of any substance. Kelvin scale (SI) Rankine scale (E) A temperature scale nearly identical to the Kelvin scale is the ideal-gas temperature scale. The temperatures on

this scale are measured using a constant-volume gas thermometer.

Temperature (cont.)

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Pressure

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Pressure: A normal force exerted by a fluid per unit area

ghA

gAh

A

mg

A

FP

A

mg

A

FP

Pressure (cont.)

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Absolute pressure: The actual pressure at a given position. It is measured relative to absolute vacuum (i.e., absolute zero pressure).

Gage pressure: The difference between the absolute pressure and the local atmospheric pressure. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and so they indicate gage pressure.

Vacuum pressures: Pressures below atmospheric pressure

Energy

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Work (W)

Heat (Q)

Kinetic Energy (Ek)

Potential Energy (Ep)

Internal Energy (U)

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Work (W)

FdldW

Kerja diikuti oleh perubahan volume dari fluida (ex. Compression of a gas

by a piston)

2

1

2

1

V

V

V

V

t

t

PdVW

PdVW

A

VPAddW

t

t Total volume gives total work

Work per unit mass or mole

W+ Tanda negatif menunjukkan bahwa kompresi fluida

didifinisikan sebagai kerja positif

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Heat (Q)

heatQ

-Heat moves from object with higher T to object with lower T

-Temperature difference is driving force of heat flow

W & Q are path variables

Path variables only have meaning when exchange between a

system/substance and surroundings

A substance does not contain a certain amount of work or heat

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2

2

1muEK

We will focus on 5 forms of energy

Potential Energy(Ep)

mgzEp

Internal Energy(U)

forcesular intermolec karenaenergy internal U

Question:

what is the relationship between the various forms of energy?

Kinetic Energy(Ek)

Hukum Pertama Termodinamika

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Panas yang diberikan pada suatu sistem (Q) sama dengan perubahan energi dalam (U) dan kerja yang dilakukan (W).

Hukum termodinamika I secara matematis dirumuskan:

di mana: Q = + panas masuk ke sistem

- panas keluar dari sistem

U = energi internal sistem

W = + usaha dilakukan oleh sistem

- usaha dilakukan pada sistem

WQU

Hukum I

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Jumlah energi total konstan

Ek, Ep,

U

Q W

-E(lingkungan)

E(sistem)

E(sistem) + E(lingkungan) = 0

0 WQEpEkUsistem

WQEEkU P

+

Formulasi matematis HK I

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Eksoterm Proses pelepasan energi atau transfer kalor dari sistem ke lingkungan.

Endoterm Proses penyerapan

energi atau transfer kalor dari lingkungan ke

sistem.

Ditandai dengan kenaikan temperatur sistem saat

reaksi berlangsung. Contoh: reaksi pembakaran

Ditandai dengan penurunan temperatur sistem.

Contoh: proses fotosintesis

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proses-proses termodinamika

Proses Isobarik (1)

o Tekanan konstan

Proses Isotermis (2)

o Temperatur kontan

Proses Adiabatik (3)

o Tidak ada kalor yang hilang

Proses Isokorik (4)

o Volume konstan

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proses adiabatik Tidak terjadi transfer panas selama proses, sehingga Q = 0 Proses adiabatik dapat terjadi pada sistem yang terisolasi atau

sistem yang mempunyai proses sangat cepat.

U = Q W = 0

o Untuk sistem yang mengalami kompresi secara adiabatik, maka W negatif (kerja dilakukan pada sistem) sehingga U positif.

o Untuk sistem yang mengalami ekspansi secara adiabatik, maka

W positif (kerja dilakukan oleh sistem) sehingga U negatif.

U = - W

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proses isotermal Temperatur sistem konstan selama proses berlangsung.

Untuk gas ideal, jika T = 0 (temperatur gas konstan), maka U = 0.

Sehingga:

A

B

0TRnU2

3 WQ

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proses isobarik

Tidak terjadi perubahan tekanan pada sistem selama proses.

Umumnya terjadi pada sistem yang mempunyai kontak langsung dengan tekanan atmosfer bumi.

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proses isokhorik Tidak terjadi perubahan volume selama proses.

Proses ini juga disebut volume konstan, isometrik, isovolumik.

Proses ini terjadi pada sistem yang tertutup rapat dan kuat.

Ketika volume sistem tidak berubah, maka tidak ada kerja yang dilakukan, sehingga: W = 0 U = Q

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kapasitas kalor (heat capacity)

Kapasitas kalor (Q) adalah jumlah kalor yang diperlukan untuk menaikkan temperatur suatu zat 1oC.

Q = mcT di mana c adalah kalor spesifik (J/kg.K).

Untuk gas ideal:

Kapasitas kalor pada volume konstan

CV = 3/2 R

Kapasitas kalor pada tekanan konstan

CP = 5/2 R

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soal

Sebuah pemanas air menggunakan listrik sebagai sumbernya digunakan untuk memanaskan 3 kg air pada 80oC. Usaha yang diberikan filamen pemanas 25 kJ sementara panas yang terbuang karena konduksi sebesar 15 kkal. Berapa perubahan energi internal sistem dan temperatur akhir ?

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jawaban

Panas terbuang 15 kkal = 62,7 kJ

Q = U + W

-62,7 kJ = U -25 kJ U = -37,7 kJ

T = 76,9oC

CkgCxkgkJ

kJT o

o01,3

3/18,4

7,37

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MESIN KALOR

H

C

H

CH

H

CHQ

Q1

Q

QQ

Q

WQQW

di mana = efisiensi mesin kalor

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MESIN PENDINGIN

1W

Q

W

WQ

W

QCOPQQW CCCCH

di mana COP = Coefficient of Performance mesin pendingin

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SIKLUS KARNOT

Kurva A (12): ekspansi isotermal pada TH (kerja dilakukan oleh gas)

Kurva B (23): ekspansi adiabatik (kerja dilakukan oleh gas)

Kurva C (34): kompresi isotermal pada TC (kerja dilakukan pada gas)

Kurva D (41): kompresi adiabatik (kerja dilakukan pada gas)

Heat Transfer

Heat Transfer t

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Proses Perpindahan

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Membahas dasar-dasar proses perpindahan momentum, panas dan massa yang terdapat dalam industri dan kehidupan sehari-hari. Konsep-konsep dasar akan dijelaskan melalui aplikasinya dalam penyelesaian soal-soal.

MEKANISME PROSES PERPINDAHAN

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RESISTANCE

FORCEDRIVINGRATE

kA

dxRESISTANCE

TdFORCEDRIVING

)/( ADdx

dCJ

A

AA

dx

dCD

A

J AA

x

A

)/(kAdx

dTq

dx

dV

A

F yxy

dx

dTk

A

q

x

RATE LAW GENERAL RATE LAW

HEAT TRANSFER :

MASS TRANSFER

MOMENTUM TRANSFER

V

Diam

FOURIER LAW

FICKS LAW

NEWTON

LAW OF

VISCOSITY

MEKANISME PROSES PERPINDAHAN (cont)

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BENTUK ANALOG PERSAMAAN FLUKS SATU DIMENSI

xx

x

xA

q

x

A

A

J

xy

Cp

k

x

x

CpT

x

CA

x

Vy

DA

Fluks Diffusivity Gradien konsentrasi

Properti

Umum

Panas

Massa

Momentum

Heat Transport

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Perpindahan panas terjadi di berbagai proses (Operasi) misalnya :

Distilasi

Pembakaran bahan bakar

Penguapan & Pengeringan

Pemanasan & Pendinginan

Perpindahan panas terjadi karena adanya beda suhu (dari suhu tinggi ke suhu rendah)

Mekanisme Perpindahan Panas

Panas bisa berpindah dengan mekanisme : Konduksi

Konveksi dan Turbulensi

Radiasi

Heat Transport (cont)

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Konduksi

Panas berpindah dengan transfer energy gerak molekul-molekul yang berdekatan.

Perpindahan panas secara konduksi bisa terjadi dalam solid, liquid, gas.

Dalam gas, molekul-molekul yang panas yang mempunyai energy gerak, menularkan energynya ke molekul-molekul yang

berdampingan.

Konduksi panas dapat juga ditransfer oleh elektron bebas (misalnya dalam logam).

Heat Transport (cont)

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Konveksi

Perpindahan panas oleh bulk transport dan percampuran elemen-elemen makroskopis bagian-bagian yang lebih panas dengan bagian yang lebih dingin,

atau bisa juga pertukaran panas antara permukaan solid dan fluida.

Perlu dibedakan : Konveksi paksa Konveksi natural

Contoh : Kehilangan panas dari radiator mobil dimana udara disirkulasikan dengan

kipas.

Mendinginkan kopi dengan meniup.

Heat Transport (cont)

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Radiasi

Perpindahan panas oleh gelombang elektron maknit. Radiasi tak perlu

medium.

Contoh: -Transfer Panas dari matahari ke bumi.

-Memanaskan (memasak) makanan di dalam oven.

HUKUM KEKEKALAN

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Laju panas masuk + Laju generasi panas = Laju panas keluar + Laju akumulasi panas

dx

dTk

A

q

x

FOURIER LAW

In = qx|x Out = qx|x + x

x X + x x

| + . = |+ +

(.

FOURIER LAW

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dx

dTk

A

q

x

Dimana : qx : laju transfer panas pada arah x (W)/(Btu/h)

A : luas penampang (m2)/(ft2)

T : suhu (K)/(0F)

x : jarak (m)/(ft)

k : konduktifitas panas (W/m.K)/(Btu/h)

1 btu/h.ft.0F = 4.1365 x 10-3 cal/s.cm.0C 1 btu/h.ft.0F = 1.73073 W/m.K 1 btu/h.ft2 = 3.1546 W/m2

1 btu/h = 0.29307 W

=

2

1

2

1

=

2 1 (1 2)

PROBLEMS

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1. Calculate the heat loss per m2 of surface area for an insulating wall composed of 25.4 mm thick fiber insulating board, where the inside temperature is 352.7 K and the outside temperature is 297.1 K.

2. Calculate the heat loss per m2 of surface area for a temporary insulating wall of a food cold storage room where the outside temperature is 299 K and the inside temperature is 276.5 K. The wall composed of 25.4 mm of corkboard having a k of 0.0433 W/m.k

3. In determining thermal conductivity of insulating material, the temperatures were measured on both sides of a flat slab of 25 mm of the material and were 318.4 and 303.2 K. The heat flux was measured as 35.1 W/m2. Calculate the thermal conductivity in btu/h.ft.F and W/m.K.

THANK YOU

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