1. CBE 465
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Heuristics
“Rules of Thumb”

2. Use of Experience-Based Principles to:
Turton: Chapter 11
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Use of Experience-Based Principles to:
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

3. Example of Heuristic Usage
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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 ____?

4. Creation of Heuristics / Shortcuts
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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.
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.

5. Creation of Heuristics / Shortcuts
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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

6. Creation of Heuristics / Shortcuts
CBE 465
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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
So for same diameter pipe; with water flowing; ignoring effects of T on

7. Chapter 11 Heuristics
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13. Example
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14. Estimate size of flash drum V-102
Example
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Estimate size of flash drum V-102

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Heuristics
“Rules of Thumb”

18. Estimate size of heat exchanger E-105
Example
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Estimate size of heat exchanger E-105

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21. Estimate size of reboiler E-106
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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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23. Homework: Problem 11.1 Turton
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Heuristics??
Homework: Problem 11.1 Turton