1 heuristics “rules of thumb”. 2 turton: chapter 11 check new process designs provide equipment...
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Heuristics
“Rules of Thumb”
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Turton: Chapter 11
• 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
Use of Experience-Based Principles to:
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 is approximately ____?G
L
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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: , for water at 21OC and a velocity of 1.83 m/s.Cm
Wo2
5250
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.
4.08.0 PrRe023.0k
hD14.0
333.08.0 PrRe027.0
sk
hD
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Creation of Heuristics / Shortcuts
Let subscript 1 be old, 2 be new T
14.0
333.08.0 PrRe027.0
sk
hD
Property 21 deg. C 93 deg. C Ratio (93o/21o)
] 997.4 963.2 0.966
] 0.604 0.678 1.12
] 4.19 4.20 1.00
/s] 9.8 x 10-4 3.06 x 10-4 0.312
14.0
1
2
333.0
2
1
1
2
1
2
8.0
2
1
1
2
1
2
1
2
2
1
1
212 027.0
027.0
k
k
Cp
Cp
u
u
D
D
D
D
k
khh
3267.0
2
1
333.0
1
2
8.0
1
2
1
2
2.0
2
1
667.0
1
212
Cp
Cp
u
u
D
D
k
khh
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Creation of Heuristics / Shortcuts
So for same diameter pipe; with water flowing; ignoring effects of T on
3267.0
333.08.08.0
2.0667.02 312.0
100.1966.0
83.1
05.3112.15250
h
463.119727.05048.110785.152502 h Cm
Wh
o22 7.125,12
:,, andCpk
Cm
Wo
uTh 23267.0
8.0
69.336
][][: smkg
sm anduwith
Property 21 deg. C 93 deg. C Ratio (93o/21o)
] 997.4 963.2 0.966
] 0.604 0.678 1.12
] 4.19 4.20 1.00
/s] 9.8 x 10-4 3.06 x 10-4 0.312
3267.0
2
1
333.0
1
2
8.0
1
2
1
2
2.0
2
1
667.0
1
212
Cp
Cp
u
u
D
D
k
khh
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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