Heuristics
“Rules of Thumb”
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Turton: Chapter 11
Use of Experience-Based Principles to:
•
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•
•
•
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Check new process designs
Provide equipment size and performance estimates
Help with troubleshooting problems with existing systems
Verify computer simulations and results
Provide initial estimates for simulator inputs
Use as a starting place for initial designs
Developing preliminary process layouts
Turton indicates that heuristics:
• Don’t guarantee a solution
• May contradict other heuristics
• Can reduce the time to solve problems
• Depends on the context in which it is used
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Example of Heuristic Usage
• What is the density of water in SI units? In English units?
• What does a gallon of milk weigh? What about 4 gallons of milk?
• What does a cubic meter of air weigh?
• The ratio of
L
G
is approximately ____?
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Creation of Heuristics / Shortcuts
• Turton suggests a 3 step process:
• Predict
• Authenticate/Analyze
• Reevaluate/Rethink:
Example: suppose you calculated a turbulent heat transfer coefficient inside
a 1.5” pipe as:
5250
W
O
2 o , for water at 21 C and a velocity of 1.83 m/s.
m C
Now, you need to estimate a heat transfer coefficient when the water temperature
is 93OC and a velocity of 3.05 m/s.
Solution: assume still turbulent flow and that the viscosity and density of water
may change significantly over this temperature range. So find appropriate
equation for calculation of Nusselt number.
hD
 0.023 Re 0.8 Pr 0.4
k
 
hD
 0.027 Re 0.8 Pr 0.333  
k
 s 
0.14
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Creation of Heuristics / Shortcuts
 
hD
 0.027 Re 0.8 Pr 0.333  
k
 s 
0.14
Let subscript 1 be old, 2 be new T
Property
21 deg. C
93 deg. C
Ratio (93o/21o)
𝝆 [𝒌𝒈/𝒎𝟑]
997.4
963.2
0.966
𝒌 [𝑾/𝒎𝒐𝑪]
0.604
0.678
1.12
𝑪𝒑 [𝒌𝑱/𝒌𝒈𝒐𝑪]
4.19
4.20
1.00
𝝁 [𝒌𝒈/𝒎/s]
9.8 x 10-4
3.06 x 10-4
0.312
0.8
h2  h1
k 2 D1 0.027  D2 u2  2 1   Cp2  2 k1 

 

k1 D2 0.027  D1 u1 1  2   Cp1 1 k 2 
k 
h2  h1  2 
 k1 
0.667
 D1 
 
 D2 
0.2
0.8
 u2  2   Cp2 

 

u

Cp
 1 1  1
0.333
0.333
 2 
 
 1 
 1 
 
 2 
0.14
0.3267
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Creation of Heuristics / Shortcuts
Property
21 deg. C
93 deg. C
Ratio (93o/21o)
𝝆 [𝒌𝒈/𝒎𝟑]
997.4
963.2
0.966
𝒌 [𝑾/𝒎𝒐𝑪]
0.604
0.678
1.12
𝑪𝒑 [𝒌𝑱/𝒌𝒈𝒐𝑪]
4.19
4.20
1.00
𝝁 [𝒌𝒈/𝒎/s]
9.8 x 10-4
3.06 x 10-4
0.312
k 
h2  h1  2 
 k1 
0.667
 D1 
 
 D2 
h2  52501.12 
0.667
0.2
1
0.8
 u2  2   Cp2 

 

 u1 1   Cp1 
0.2
 3.05 


 1.83 
0.8
0.333
 1 
 
 2 
0.966 1.00
0.8
h2  5250 1.0785 1 1.5048 0.9727 
0.3267
0.333
1
 1 


 0.312 
1.463
0.3267
h2  12,125.7
W
m 2 oC
So for same diameter pipe; with water flowing; ignoring effects of T on k , Cp, and  :
hT   336.69
u 0.8

0.3267
 
W
m
2 o
C
with : u [ ms ] and  [ mkgs ]
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Chapter 11 Heuristics
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Example
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Example
Estimate size of
flash drum V-102
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Heuristics
“Rules of Thumb”
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Example
Estimate size of heat
exchanger E-105
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In Class
Exercise
Estimate size of
reboiler E-106
Given:
• mps (medium pressure steam) = 10 barg
• T-101 operates at 3 bar
• From simulation, reboiler duty = 2,512,500 W
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Heuristics??
Homework: Problem 11.1 Turton
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