Welcome to the course in Heat Transfer (MMV031) – L1...Values of the heat transfer coefficient W/m...
Transcript of Welcome to the course in Heat Transfer (MMV031) – L1...Values of the heat transfer coefficient W/m...
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
−
−=