Wood Drying Kiln Fact Sheet
Drying Bell Curves Improve for Sawmills Converted to Venturis
The extent to which sawmills can translate this drying bell curve improvement into faster charge times depends on a number of factors discussed in this paper. Typical charge time improvements range between 5% and 10%.
Why Kiln Drying Bell Curves Improve with Venturis
Orifices of Venturis Optimally Sized
eSteamG has developed a proprietary sizing algorithm for use in sizing venturis for steam coil air heaters. Properly sized venturi orifices in wood drying kilns will perform optimally for the full range of the drying charge. This sizing expertise has been gained by the combined experience of 15 years in the sawmill industry with over 34 sawmill conversions between 2003 and 2011. The optimization algorithm went through an additional refinement through a three year experience with Pulp Dryer optimization.
Venturis operate with continuous flow instead of with the batch processing (pulsing) action of mechanical steam traps. Steam in the coils gives up its heat continuously and condenses. Because venturis are continuously open there is always a path for this condensate to flow. Condensate flow through the coils is optimal with this arrangement.
With a pulsing trap and the start – stop – start action, the condensate flow is more sporadic. With conventional trap you have to reaccelerate the condensate in the coil each time the trap opens. Condensate in the coil reduces the surface area of pipe available for heat transfer directly with the steam vapor. The condensate in front of the conventional trap is typically 10 to 15 degrees F cooler than with the venturi.
With the venturi it follows that the back end of the coil is hotter and you get improved heat transfer on the back end of the coil. This results in a more even heat transfer throughout the coil.
Optimal Heat Transfer
The continuous flow action of venturis optimizes the velocity of the condensate in the systems where used. eSteamG also recommends converting the distribution system to venturis, thus delivering drier saturated steam to the coils of the kiln. This combination of less condensate in the steam and less condensate insulating the walls of the coils results in improved heat transfer.
A third factor in improving performance with some systems is individually trapping coils. A common practice with kiln manufacturers is ganging coils. The most probable reason for gang trapping coils is an engineering trade-off. That is, in the past you had to evaluate the cost and downtime of repairing conventional traps versus the gains in heat transfer associated with Individually trapping the coils. With venturis, you remove the failure rate problem associated with pulsing traps and take advantage of the improved heat transfer.
Two or more ganged coils may be identical or nearly identical, but loads are almost never perfectly balanced. It doesn’t take much of a difference in heat transfer to generate a difference in load and pressure at the outlet of the trap. With even a subtle difference in loads on a set of ganged coils, the small differences in pressure are significant enough to cause a back flow into the coil with the lowest pressure. This can result in fluctuations in the ganged coils that interfere with optimal heat transfer, cause fluctuating cold spots, and damage the coils over time because of the flooding. Every coil should be individually trapped, and loads should be as balanced as possible. Individually trapping coils with permanent venturis improves heat transfer in the coils and extends the life of coils.
Venturis provide optimal heat transfer in the coils, which allow the kilns to reach their targets with faster startups.
More Consistent Drying
Once all of the coils in a kiln are converted and optimally sized, the kiln performs with more consistent drying front to back and top to bottom. There will be fewer hot and cold spots which tend to elongate the bell curve. Instead you will see a tighter bell curve of wets to dries.