INDUSTRY INSIGHT
Did you know that airflow can AFFECT the readings in your kiln? by Henco Viljoen, Dry Kiln Specialist at Nyle Dry Kilns
W
hen conducting an experiment on the effect of airflow speed over the wet bulb recently, I came across some interesting findings.
EXPERIMENT: I installed three wet bulbs and one dry bulb inside an insulated drying chamber 3' deep x 7' wide x 6' high. All of the wet bulbs were set up at precisely ž" above the water level. I then installed fans to simulate airflow over two of the wet bulbs. The first wet bulb (#1) had 500fpm airflow over it, the second wet bulb (#2) had 200fpm, and the third wet bulb (#3) had zero fpm. The chamber was heated to 160oF, at which time the chamber's circulating fans and heater were turned off. The dry and wet bulb temperatures were measured and logged until the dry bulb temperature reached 100oF.
Test station wet bulb fan
Ensuring constant airflow over a wet bulb in both Fans Forward and Reverse directions.
W W W. N H L A .C O M
FINDINGS: It was interesting to see that at 160oF dry bulb, the wet bulb with zero airflow (#3) showed an average of 6oF higher than wet bulb #2 (200fpm), and 9oF higher than wet bulb #1 (500fpm). The difference decreased the lower the dry bulb dropped being 3oF for wet bulb #2 and 4.5oF
for wet bulb #1 when the dry bulb got down to 100oF. I can only conclude that the higher the temperature inside your drying chamber, the more crucial adequate airflow over the wet bulb becomes. How will this affect how the kiln runs? If the airflow is too low, a higher wet bulb reading will force vents open or more open if vents modulate. So the EMC / RH reading calculated based on perceived dry & wet bulb readings will be higher than actual . . . meaning conditions are harsher than what you think it is. In a kiln equipped with fans that reverse direction every few hours (with only one wet bulb), results are often high airflow over the wet bulb in one direction, and low airflow in the other. To put in terms of control elements: more venting in one direction than the other! The net result: uneven drying, more energy usage, longer drying times, longer conditioning times, and more drying defects. CONCLUSION: ensure that your wet bulb has at least 200-250fpm constant airflow over it at all times, or make use of more than one wet bulb.
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