1. CBE 465
4/7/2019
Heuristics
“Rules of Thumb”

2. Use of Experience-Based Principles to:
Turton: Chapter 11
CBE 465
4/7/2019
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
CBE 465
4/7/2019
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
CBE 465
4/7/2019
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.
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
CBE 465
4/7/2019
Let subscript 1 be old, 2 be new T
Property
21 deg. C
93 deg. C
Ratio (93/21)
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
4/7/2019
Property
21 deg. C
93 deg. C
Ratio (93/21)
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
CBE 465
4/7/2019

8. 4/7/2019
CBE 465

9. 4/7/2019
CBE 465

10. 4/7/2019
CBE 465

11. 4/7/2019
CBE 465

12. 4/7/2019
CBE 465

13. Example
CBE 465
4/7/2019

14. Estimate size of flash drum V-102
Example
CBE 465
4/7/2019
Estimate size of flash drum V-102

15. 4/7/2019
CBE 465

16. 4/7/2019
CBE 465

17. CBE 465
4/7/2019
Heuristics
“Rules of Thumb”