Welcome to the course in Heat Transfer (MMV031) – L1 Martin Andersson & Zan Wu

Agenda

• Organisation

• Introduction to Heat Transfer

• Heat Exchangers

Course improvement compared to last year

• 2015: Exercises where students are expected to solve problems themselves or in small groups are added. Instead the amount of tutorial sessions (where the teacher solved problems) are decreased.

• 2014: 3 DLs for home assignments was implemented (instead of 1)

Contents of the course

• Heat Conduction • Convection • Thermal Radiation • Condensation • Evaporation, Boiling • Heat Exchangers

Organisation

• Lectures • Tutorials • Exercises

• Mandatory home assignments

• Exam (mandatory)

Organisation

• Examiner: Assistant Professor Martin Andersson

• Teachers: Martin Andersson and Zan Wu (offices at 5th floor M-building)

• Course administrator: Elna Andersson (office at 5th floor M-building)

Organisation

• Course literature: Introduction to Heat Transfer, Sundén B., WIT Press

• Examination: June 4th @ 14-19 (Sparta A+B) – Exam is 50 p + 5 p if all home assignments are delivered in

time

– 40 % theoretical part + 60 % problem solving part

– Grade 3 requires 22 p (min 5p on theoretical part)

– Grade 4 requires 33 p (min 5p on theoretical part)

– Grade 5 requires 44 p (min 5p on theoretical part)

Guest lectures and Study Visit

• Alfa Laval (study visit) – World leading company in plate heat exchangers

• Guest lecture(s):

– John Weisend, ESS – Magnus Genrup, LTH (guru in turbines)

Introduction

• Heat is energy passing a system boundary due to a temperature difference

• Heat is a form of energy in transition.

• Heat conduction • Heat convection (natural or forced) • Thermal radiation

Introduction

Introduction

Heat conduction

T1T2

T1 > T2

q

Thickness b

λ

q = λ(T1-T2)/b

Heat conduction

Solids

Carbon Steel λ = 15- 50 W/mK Polymers, λ = 0.1-0.5 W/mK Liquids

Water λ = 0.6 W/mK Oil λ = 0.15 W/mK Gases

Air λ = 0.025 W/mK H2 (hydrogen) λ = 0.2 W/mK

Thermal conductivity (examples)

Convection

q = α(TS-T∞) = h(TS-T∞)

Tsq

U∞ T∞ Ts > T∞

How to determine α or h

• Depends on: – Flow velociy – Fluid (gas or liquid) – Geometry – sometimes on temperature – Forced convection, Natural convection, Mixed convection

• Nu = αL/λf = function (Re=UL/ν, Pr=µcp/λf , geometry) or • Nu = αL/λf = function (Gr=gβ∆ΤL3/ν2, Pr=µcp/λf , geometry) or • Nu = αL/λf = function (Re, Gr, Pr, geometry)

Thermal Radiation

Qnet = A1F12εeff (T14-T2

4)

q1

q2 T2

T1

Introduction to heat exchangers (ch 15)

What is a Heat Exchanger?

A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more fluids, between a solid surface and a fluid,

or between solid particulates and a fluid,

at different temperatures

and in thermal contact.

Classification of heat exchangers

• Transfer process • Number of fluids • Degree of surface contact • Design features • Flow arrangements • Heat transfer mechanisms

Classification of heat exchangers

Fig. 1 Heat transfer surface area density spectrum of exchanger surfaces ( Shah, 1981).

Fig. 2 Fluidized-bed heat exchanger.

Fig. 3 (a) Shell-and- tube exchanger with one shell pass and one tube pass; (b) shell-and- tube exchanger with one shell pass and two tube passes.

Fig. 4 Standard shell types and front- and rear-end head types (From TEMA, 1999).

Fig. 5 Gasketed plate-and-frame heat exchanger.

Fig. 6 Plates showing gaskets around the ports (Shah and Focke, 1988).

Fig. 7 Section of a welded plate heat exchanger.

Fig. 9 Spiral plate heat exchanger with both fluids in spiral counter flow.

Fig. 10 (a) Lamella heat exchanger; (b) cross section of a lamella heat exchanger, (c) lamellas

Fig. 11 Printed-circuit cross flow exchanger

Fig. 12 Corrugated fin geometries for plate-fin heat exchangers: (a) plain triangular fin; (b) plain rectangular fin; (c) wavy fin; (d) offset strip fin; (e) multilouver fin; (f) perforated fin.

Fig. 13 (a) Individually finned tubes; (b) flat (continuous) fins on an array of tubes.

Fig. 14 Individually fin tubes.

Fig. 15 Heat wheel or a rotary regenerator made from a polyester film.

Classification according to transfer process

Indirect contact type Direct contact type

Direct transfer Storage Fluidized bed Immiscible fluids

Gas-liquid Liquid-vapour

Single-phase Multiphase

Classification according to number of fluids

Two-fluid Three-fluid N-fluid (N > 3)

Classification according to surface compactness

Gas-to-liquid Liquid-to-liquid and phase-change

Compact β≥ 700 m2/m3

Non-compact β < 700 m2/m3

Compact β ≥ 400 m2/m3

Non-compact β < 400 m2/m3

Classification according to design or type

Tubular Plate-type Extended surface Regenerative

PHE Spiral Plate coil Printed circuit

Gasketed Welded Brazed

Double-pipe Shell-and-tube Spiral tube Pipe coils

Cross-flow to tubes

Parallel flow to tubes

Plate-fin Tube-fin

Ordinary Separating wall

Heat-pipe wall

Rotary Fixed-matrix Rotating hoods

Classification according to flow arrangements

Single-pass Multipass

Counter flow Parallel flow Cross flow Split flow Divided flow

Extended surface

Cross- Counter flow

Cross- parallel flow

Compound flow

Shell-and-tube Plate

Parallel counter flow m-shell passes n-tube passes

split-flow Divided-flow

Fluid 1 m passes Fluid 2 n passes

Classification according to heat transfer mechanisms

Single-phase convection on both sides

Single-phase convection on one side, Two-phase convection on other side

Two-phase convection on both sides

Combined convection and radiative heat transfer

Classification according to process function

Condensers Liquid-to-vapor phase-change exchangers

Heaters Coolers Chillers

Convective heat transfer

vägg

Fluid1

Fluid2

Overall heat transfer coefficient

mm1 t

TRtUAQ ∆⋅=∆⋅=

Expression for overall thermal resistance

oóoFvlw

w

iiFii

1111

oAAA

bAA

TRα

+α

+λ

+α

+α

=

Values of the heat transfer coefficient W/m2K

• Air atmospheric pressure 5-75 • Air pressurized 100 - 400 • Water, liquid 500-20 000 • Organic liquids 50 000 • Boiling 2 500 -100 000 • Condensation 3 000-100 000

Correlations for the heat transfer coefficient

• Nu = hL/k = function (flow velocity, physical properties, geometry) = function (Re, Pr, geometry)

General research needs

• How to achieve more compact heat exchangers

• High thermal efficiency

• Balance between enhanced heat transfer and accompanied pressure drop

• Material issues especially for high temperature applications

• Manufacturing methodology

• Fouling

• Non-steady operation

Fouling factors - Försmutsningsfaktorer

Tabell 15-I. FörsmutsningsfaktorerStrömmande medium F/1 α [m2K/W]

Destillerat vatten4101 −×

Sjövatten ( K 325<T ) 4101 −×Sjövatten ( K 325>T ) 4102 −×Matarvatten till ångpannor 4102 −×Bränsleolja 4109 −×Industriluft 4105.3 −×

Counter current heat exchanger

t

A

dth

dtc

dA

∆t

th,in

tc,ut

th,ut

tc,in

∆tb

∆ta

)(utin hhh ttCQ −= )(

inut ccc ttCQ −=

ch ttt −=∆ ch)( dtdttd −=∆

hph )( cmC = , cpc )( cmC =

Counter current Hex

−⋅=∆

hc

11)(CC

Qdtd

−∆=∆

hc

11)(CC

tdAUtd

−=

∆∆

hc

11)(CC

dAUttd

ccphhp )()( dtcmdtcmtdAUQd −=−=∆⋅=

Counter current Hex

Expression for overall thermal resistance

Parallel flow Hex,Co-Current Hex

)(

)(ln

)()(

utut

inin

ututinin

ch

ch

chchm

tt

tttttt

t

−

−−−−

=∆

a

b

abm

lntt

ttt

∆

∆∆−∆

=∆

t

A

dth

dtc

dA

∆t

th,in

tc,in

th,ut

tc,ut

∆tb∆ta

Arbitrary Hex

LMTDFUAQ ⋅⋅=

F korrektionsfaktor som beror av två parametrar P och R;

F correction factor depending on two parameters P and R

inin

inut

ch

cc

tt

ttP

−

−=

hp

cp

)(

)(

cm

cmR

=

R kan också skrivas; R can also be written

inut

utin

cc

hh

tt

ttR

−

−=

F vs P och/and R; Shell-and-tube heat exchanger; one shell pass, two tube passes

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.5

0.6

0.7

0.8

0.9

1.0

P

Kor

rekt

ions

fakt

or, F

R =

6.0

4.0

3.0

2.0 1.5

1.0

0.8

0.6

0.4

0.2

0.1

tc,in

tc,ut

th,ut

th,in

inin

inut

ch

cc

tt

ttP

−

−=

inut

utin

cc

hh

tt

ttR

−

−=