Particle Deposition in Industrial Ductwork

 

Background: 

 

Industrial Ventilation: A Manual of Recommended Practice presents extensive guidelines for the selection and integration of various ventilation system components, including local exhaust hoods, exhaust system ductwork, air cleaning devices, and fans.  These guidelines provide the ventilation engineer with methods to estimate airflow rates and velocities necessary to protect the worker in industrial settings; however, methods for predicting particulate deposition in exhaust system ductwork are absent from this manual and are not available in a comprehensive format elsewhere. 

 

Solid particles and liquid droplets are removed from exhaust systems predominantly by gravitational settling, centrifugal forces, and inertial forces.  The relative importance of each force and, hence, the importance of deposition for each particle size is a function of ductwork geometry (length, size, number of elbows, tees, etc.), airflow parameters, and particulate properties, including size distribution and particle density.  In addition, partitioning between droplet and vapor phases of semi-volatile liquids such as in metalworking fluid mists is affected by the properties of the carrier gas, which may include temperature and vapor composition. 

 

If ductwork leads to a particulate collector, deposition may decrease loading and, subsequently, increase collector lifetime.  Alternatively, some situations may benefit from minimizing particulate loss to ductwork walls such as in the transport of costly reagents or products in pharmaceutical applications.  In either situation, an a priori knowledge of the magnitude of particle deposition versus particle size would allow optimization of a ductwork system during the design phase.  Extensive literature exists for the prediction of particle losses in small-diameter sampling tubes (1-2 cm in diameter) at relatively low Reynolds numbers (Re < 20,000).  Unfortunately, very little work has been published on the deposition of particles in large-scale, industrial ductwork where Reynolds numbers commonly reach 200,000.

 

Objectives: 

 

This work has three objectives:

  1. to evaluate and modify, as necessary, existing theory to predict particulate losses in industrial ductwork,
  2. to unify particle deposition theory so it can be utilized effectively to predict particle losses during ductwork design, and
  3. to accommodate the design efforts accomplished in objective #2 to metalworking environments by incorporating evaporation and condensation phenomena.

The work will involve development of theory, calibration of the theory in pilot-scale lab experiments, and confirmation of the theory through experiments conducted at an industrial plant. 

 

Products: 

 

This work will result in at least three manuscripts that will be submitted for publication.  Possible titles include: 

  1. “Scalability of Existing Theory for the Prediction of Particle Deposition in Industrial Exhaust Systems”,
  2. “A General Predictive Model for Particulate Deposition in Industrial Exhaust Systems”, and
  3. “The Importance of Evaporation and Condensation Phenomena in Industrial Ventilation Exhaust Systems”.

Together, this work should be sufficient to allow a new chapter or subsection in Industrial Ventilation: A Manual of Recommended Practice entitled “Estimation of Particulate Removal in Exhaust Ductwork”.