Control of lower explosive level LEL in a multiple zone dryer

3 Control of lower explosive level (LEL) In a multiple zone dryer 

Ur Recoverable strain energy per unit volume stored in the coating 

3 Bird, R. B., W. E. Stewart and E. N. Lightfoot. Transport Phenomena, John Wiley Sons, 1960. 

5 Caimcross, R. A., A. Linibert, L. F. Francis, and L. E. Scriven, 24th aimual meeting of Fine Particle Society, 24-28 August 1993, Chicago, IL. 

9 Cary, J. D. and E. B. Gutoff, Analyze the Drying of Aqueous Coatings Chem. Eng. Prog., 2, 73 (1991). 

If the solvent concentration of a zone exceeds certain level, then the system becomes to be in danger of explosion due to the flammability of organic solvent vapor. This level of solvent concentration is called as lower explosive level (LEL), and the dryer should be operated below the LEL prior to any constraints. Thus each zone needs sufficient airflow rate to meet these needs. However, too much airflow rate results in waste of energy to heat the excess air, and it increases the load of waste gas facilities (e.g., VOC emission control units). Therefore the airflow rate of each zone and the ratio of recycled to fresh air should be optimized. The typical airflow system for a multiple zone dryer is shown in Figiue 7.3.32. The supplying air of each zone is consisted of fresh and recycled (used or returned from the exit gas) air as shown in the figure. LEL can be calculated if we know the evaporation rate of solvent of a zone. The evaporation rate of solvent can be measiued or calculated as we explained in previous sections. Thus we can distribute the fresh air to each zone to meet the LEL safety without substantial increase in the total exhaust air. 

Evaporation rate of solvent [kg/min] measured force of peeling Fraction open area (B/Er) 

Nozzle to coating surface distance Fleat conductivity [J/m C] 

---