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Attrition Case Study

Problem

After a routine preventive maintenance, a pellet coating process was producing noticeably more fines.  Analysis of the process showed that the exhaust air temperature was 3°C higher and pressure drop increased by 300 Pa.  Determine the root cause for this deviation?

Problem Categories

Hydrodynamics, thermodynamics, troubleshooting, and PCTSWurster.
Solution

Since the pressure drop and exhaust temperature changed, this problem is related to both thermodynamics and hydrodynamics.  Atomizing air will have a negligible effect on the thermodynamics or pressure drop, so it is not likely that atomizing air caused this deviation.  Equipment setup will impact the pressure drop, but not the exhaust temperature.  The most likely variable that caused this deviation is the process air flow rate.  To solve this problem, analyze the thermodynamics and hydrodynamics of the process before this deviation occurred.
Analysis of the Process prior to Deviation
For the analysis, the input variables needed are shown in the simulation windows below in the sunken boxes.  Heat loss for this system is 2.0 kW and the total momentum into the system is 11.7 N.

Analysis of the Atypical Process
Using the heat loss calculated above and the atypical exhaust temperature of 48°C, the thermodynamics analysis below shows the actual process air flow for the atypical process was 300 SCFM higher.  The impact of this change in the air flow rate is shown in the hydrodynamics analysis below using the new pressure drop. 

Conclusions
The analysis shows that the process air had to change by 300 SCFM to effect a 3°C rise in exhaust temperature and 300 Pa rise in pressure drop.  This change resulted in momentum increase from 11.7 to 13.5 N (16% increase), which likely explains the higher attrition rate observed in manufacturing.