Transcript No Slide Title

Topic 8
Heat Exchangers
FOOD1360/1577
Principles of Food Engineering
Robert Driscoll
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Introduction
• Ref: S&H 4.4
• Workhorses of the food industry.
• Main job: heating and cooling products.
• Ideal: continuous steady state.
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Concept
• Example: countercurrent heating:
out
in
Heat
Hot fluid
Cold product
in
out
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Example
www.yourdictionary.com/ahd/h/h0112300.html
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Looking at components:
•
HEATING OR COOLING FLOW:
Input energy:
Sensible ?
Latent ?
Output energy:
Sensible ?
Dryness ?
Transfer energy:
Heating or cooling ?
Resistances
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Example 1: Sensible
• Flow is 25 kg/s, cp=3.85 kJ/kg.C
Output: T=50oC
Input: T=80oC
 Q

Transfer energy  loss in heat  Q
in
out
 c pTin  m
 c pTout  25  3850  80 - 50 
m
 2.9 MW
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Example 2: Steam
• Use steam tables
• For hS, saturated vapour. For hC, saturated liquid
Condensate out
(hC, TC)
Steam in
(hS, TS)
TC=85oC
TS=120oC
 Q

Transfer energy  loss in heat  Q
in
out
 hin  m
 hout  25  2706 - 347 
m
 59 MW
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Looking at components:
•
PRODUCT FLOW:
Transferred energy:
Cooling or heating ?
Output energy:
Sensible ?
Dryness ?
Input energy
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Looking at components:
•
TRANSFERRED ENERGY:
 Q
 Q

Q
in
out
 UATavg
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Overall Heat Transfer Coefficient
• Define U as overall htc:
Q  UATavg
• Q is heat.
• ∆Tavg is average ∆T between the two fluids.
• A is contact area.
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Resistances to heat flow
• What opposes heat flow?
• Hot fluid to surface – convection
• Through surface – conduction
• Surface to product - convection
Hot fluid
Surface
Cold fluid
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From heat transfer notes
Resistance terms
• Hot fluid to surface:
• Through surface:
• Surface to product:
Thus:
1
R1 
h1 A1
1
R3 
h2 A2
dx
R2 
kA
1
1
dx
1

 
UA h1 A1 kA h2 A2
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Practise
• Data:
• Hot fluid: h1=1800 W/m2.K
• Cold fluid: h2=120 W/m2.K
• Surface: dx=0.13 mm, k=210 W/m.K
• Surface area: A=3.7 m2
SOLUTION
1
1
0.00013
1



UA 1800  3.7 210  3.7 120  3.7
 0.00015  0  0.0023
U  112 W/m2K
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Methods for finding U
• Empirical: run water, measure T.
• Manufacturer: test data.
• Calculate: use HT theory.
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Estimating ∆Tavg
• Where to measure ∆T?
• Inlet?
• Outlet?
• Halfway?
• Best method (for no phase changes):
Use ∆Tlm
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Log Mean Temperature
Difference (LMTD)
T1
T2
• For a heat exchanger,
T1  T2   T3  T4 
Tlm 
ln T1  T2  T3  T4 
• Tlm means log mean
temperature difference
T3
T4
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Example:
• Find LMTD for water at 16oC cooling milk at
68oC in a countercurrent HE, if the exit
temperatures are 25oC for the water and
33oC for the milk.
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Diagram
Milk out
33oC
Milk in
68oC
Hot fluid
Cold product
Water in
16oC
Water out
25oC
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Solving
T1  T2   T3  T4 
Tlm 
ln T1  T2  T3  T4 
33  16  68  25

ln 33  16 68  25
o
 28.0 C
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Bulk and wall temperatures
• Bulk temperature Tb is the average
•
temperature over a given flow cross-section.
Wall temperature Tw is temperature near a
wall or boundary.
• Often we assume Tb.
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The Main Energy Balance
Q  UATlm
Q i  m cPTi
Q o  m cPTo
Q  UATlm  m cP To  Ti 
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Example calculation
• A parallel flow tubular heat exchanger is used
•
to chill water (flowing at 1.05kg/s) from 32oC
to 3oC using a brine solution at -8oC. The
brine exits at 10oC from the chiller.
If the heat exchange area is 1m2, determine:
• The heat transfer rate
• The overall heat transfer coefficient
Hint: check for cross-overs by
making a quick plot of all T’s.
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Plot
• Is this possible?
32
Cross-over:
physically
impossible
WATER
10
T
BRINE
3
-8
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Try again !
WATER
32
10
3
BRINE
-8
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Now get correct Tlm
dT1   dT2 
Tlm 
ln dT1 dT2 
32  10  3  8

ln 32  10 3  8
 15.9o C
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Calculations
• First calculate log mean temperature
difference (Ans = 15.9oC)
q  UATHE  m
 c p Tw
U (1)(15.9)  1.05  4,180  (32  3)
U  8.0 kW /m2K
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Second example
• After running the plant for a year, the water
•
chills to only 70C. The brine and water flow
rate are unchanged. What is the change in
the overall heat transfer coefficient of the heat
exchanger?
(solution left as exercise: Ans = 6.0 kW/m2.K)
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Convective Heat Transfer in
Boiling Liquids
•
•
•
Heated surface to boiling liquid: convection.
Heat flux varies with temperature difference.
Two types of boiling:
•
•
Pool boiling (evaporation at air surface, T<5oC).
Nucleate boiling (evaporation at heating surface).
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Graph
q/A
T
5
50
500
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Summary
∆T<5oC
Natural convection
>5oC
Bubbles enhance convection
Vaporisation near heating surface, forms bubbles
Bubbles condense before reaching liquid-vapour interface
5-55oC
Vapour bubbles formed may reach liquid
vapour interface
Best agitation, but bubbles start to block surface.
> 55oC
Vapour bubbles blanket heat transfer surface.
>300oC
Radiation starts
Modelling h (for water)
• For nucleate boiling conditions (up to 20oC)
h50(T )2.5
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The Two KPI
• Key Performance Indicators (KPI) of heat
exchangers:
• Overall heat transfer coefficient U
• Heat exchange effectiveness
• Unit thermal efficiency for a heat exchanger: the
average heat transfer per unit length.
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U
• Would like to maximise U:
• More product through equipment, OR
• Smaller equipment
• So promote turbulence, fast flow.
• U is affected by flow conditions on both sides
of the H.E. surface.
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Effectiveness
• How uniformly does it heat?
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Unit Thermal Efficiency
• Average heat transfer per unit length.
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Area/Volume ratio
Liquid heats fastest if A/V is high.
• Cylinder:
A  2rL
V  r L
A V  2 r
2
• So decrease radius to get better A/V.
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Radius and Pressure
We know that:
• Laminar: f=16/Re
P

 u 2  8
Re
L
• Turbulent: f=0.0791/Re1/4
P

 u 2  0.04 0.25
Re
L
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Example: water at 20oC
Mass flow vs. Pressure (0.2 kg/s)
1.00
Laminar
Turbulent
Re=2100
Re=10000
Pressure drop
0.80
0.60
0.40
0.20
0.00
0
10
20
30
40
Radius, mm
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60
70
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Comparing the two:
Since Re  R, we have a problem:
• Best HT – decrease radius.
• Reduce pressure drop – increase radius.
• Also, narrow tubes foul easily and are hard to
clean.
Design is a compromise.
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Types of Heat Exchangers
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Main Types
• Jacketted pans
• Parallel plate
• Shell and tube, double tube
• Tube and fin
• Scraped surface
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1) Jacketted pans or kettles
• heating/cooling liquid
in thin jacket around
stirred food vessel.
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2) Plate HE
• racks of rectangular plates
• gaskets between plates
• pressure packed
• food / heating fluid alternate between plates
• high efficiency, due to large surface to volume
•
•
ratios
plates are patterned to increase turbulence
require careful cleaning
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Pics
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Heating in a plate heat
exchanger
•
•
Orange – heating medium
Blue
– heated product
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Peclet Number
• Define Peclet number:
Pe 
2ducp 
k
Sensible Heat Gain

AxialConduction
• If Pe<100, axial conduction significant
• No single equation for h
• complex configurations
• manufacturer’s equations: Nu = f(Re, Pr)
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3) Shell and tube exchangers
• Uses:
• heating/cooling liquids
• condensing vapours
• Many designs:
• double tube
• triple tube
• multi-tube
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Double tube
• Solve using equation for flow in pipe
• Same equations for both CON and
•
•
•
COUNTER current flow
Use LMTD in calculations
Use same equations as for shell and tube
(unless boiling)
To increase HT, rippled pipe walls.
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Diagram for Double Tube
product
heating
medium
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Example
• Used for cooling wine.
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Diagram for Triple Tube
product
heating
medium
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Multi-tube
• Empty shell with a large number of pipes or
•
•
tubes (typically 100).
Used in evaporators (calandria)
Multipass: use baffles to direct heating fluid
across the tubes several times.
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Shell&Tube
P+V
S
steam chest
Also called a
calandria.
tubes
C
F
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SECTION
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Shell and tube heat exchanger
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Shell and tube heat exchanger
www.secshellandtube.com
http://en.wikipedia.org/wiki/Shell_and_tube_heat_exchanger
http://en.wikipedia.org/wiki/Shell_and_tube_heat_exchanger
Diagram for Multipass
steam
inlet
outlet
condensate
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4) Tube and fin HE
• used for HT with gases
• maximising HT area
• may rely on natural convection
• fans may be used (forced convection)
• like the back of a fridge!
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5) Scraped-surface HE
• scraper blades on central rotor of a jacketted
•
•
•
•
•
cylindrical closed vessel
boundary layer constantly remixed into bulk
heat transfer fluid in outer jacket
used for viscous products (which would
otherwise burn-on)
can handle freezing (but not hardening)
high viscosity  low h
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Cross section
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Diagram of
SSHE
• Commercial
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Applications of SSHE
• Cooling
• Ice cream
• Margarine
• Butter
• Heating
• Starch gelatinisation
• Pasteurisation of thick mixtures
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Other methods
• Direct steam injection
• Steam injected directly into product
• Large T, small M effect.
• Many others, e.g. ohmic heating.
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Applications of Heat
Exchangers
All heat transfer processes use heat
exchangers. Following are some
examples.
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1) Pasteurisation
• A mild heat treatment used for short-term
•
•
•
extension of shelf-life
E.g. beer and milk.
Uses plate HE, spray HE, water baths
Controlled cooling required as product exits
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2) Sterilisation
• More intense heat treatment
• Destroys enzymes and bacteria
• Purpose: longer shelf life
• UHT: systems of SSHE and holding tubes to
give precise heating, lethality and cooling,
before aseptic packaging.
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3) Cooking
• Many cooking processes are performed in
•
•
jacketted kettles.
Steam is used to maintain process
temperature.
Heat is lost by evaporation and activation
energies of the cooking reactions.
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4) Evaporation
• Evaporators are used to concentrate liquids
• Shell and tube heat exchangers (calandrias)
•
•
are mainly used (sometimes plate HE)
Steam is the heat exchange fluid.
Use mixers at high viscosities
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5) Refrigeration
• The mechanical refrigeration cycle uses two
•
HE as it pumps heat from a cool room to the
environment.
Both units are tube and fin heat exchangers,
with fans
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Flow configurations
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Direction of Flows
• concurrent (or co-current) mode
• countercurrent mode
• steam approximates crosscurrent
HEATING
FLUID
PRODUCT
COUNTERCURRENT
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CONCURRENT
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Concurrent
• Large dT at inlet
• Hottest fluid meets coldest liquid
• Lower product exit T
• Lower thermal efficiency
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Countercurrent
• More uniform dT
• Higher exit T
• Better thermal efficiency
• Steam: closest to cross flow.
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Construction materials
• Need to avoid corrosion, contamination
• Must be easy to clean
• Long life
• So, stainless steel (many forms)
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Maintenance
• Not only the food side can be fouled.
• Water hardness can cause scale from Ca and
•
•
Mg carbonates and oxides.
Bacteria and algae can cause bio-fouling.
Heat exchangers need regular cleaning.
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Questions
• Would HT be better with LAMINAR or
•
•
TURBULENT flow?
How would fouling (burn-on) affect HT?
Why are PP HE used with milk?
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Problem
• Find exit temperature of water entering a
double-tube HE at 16oC if U=1800W/m2.K,
flowrate is 9.8 kg/s, and dTlm is 56oC. The
inside pipe area is 17.4 m2.
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Solution
• Heat transferred = heat gain by water:
Q  UAT  1800  17.4  56  1754 kW
 m c p Tout  Tin 
 Tout  16  1754 9.8  4.18  58.8 C
o
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END OF TOPIC
Do textbook examples and problems!
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