Power Venting a Kiln

A discussion of the ideal location for powered vents for a large drying kiln. May 14, 2006

I'm going to be installing 4 power vents on a 50,000 bf package kiln. I'm going to install them over the existing vent holes (I have 5 in the front and 5 in the back). Should I put 2 in the front and 2 in the back or put them all on one side? Also, if the fans inside the kiln are blowing toward the side that has the power vent, should the power vent suck air in when the vents open?

Forum Responses
(Sawing and Drying Forum)
From contributor B:
I assume the vents are in the roof? It is always best to exhaust wet, cooler air (air that has just exited the load) before it goes back through the heating coils, otherwise you are heating air that you then discharge (a portion of).

In a typical overhead fan package kiln, it doesn't naturally work that way, because you are pressurizing the side of the kiln you are pushing air toward, and those vents naturally become the exhaust. On the opposite side of the fan, you have created suction, and those vents become intake vents.

I have hooked up power vents in different ways, but always balanced the number on each side of the fan wall. Once they are installed, you can experiment with them by reversing the vent fan direction and watching the wet bulb control to see if it is helping you out or not.

I would suggest you wire them up so they work opposite of the kiln fans. That is, the vents on the suction side pull air out of the kiln, and those on the pressure side pull air in. When the circulating fans reverse direction, the vent fans should also reverse direction. Again, watch the kiln controller chart or computer screen and you should be able to see an improvement over what you have now. Crystal clear, huh? Guaranteed someone else will say the opposite!

P.S. We have one customer who, because of the kiln layout, always runs the power vent fans in the same direction. At least he's right half the time!

From contributor D:
I agree with contributor B. We've tried various ways and the way he suggests works really well. Just bear in mind that the intake fan will be cold and collect a lot of condensation when the vents close.

From Gene Wengert, forum technical advisor:
I am interested - why you are installing power vents? Is this for softwoods or a hardwood species that dries so fast that the present vents cannot vent fast enough? Can you tell us what species and at what MC it enters the kiln?

From the original questioner:
We dry green 4/4 and 5/4 hard maple. We installed them on our small kilns that were built sometime in the 50's for air dried lumber. We don't usually keep lumber around long enough to air dry, so we had to do something to help vent the moisture better while drying green lumber. They helped a lot with the first step of the schedule and also shortened the schedule by a day or two. Our 50,000' bf kilns do not have variable speed fans and our green mc on average is 65%. So we were hoping that the power vents would help us on those kilns, too. That average mc of 65% is for the winter months. Which is a good part of the year usually in the UP.

From Gene Wengert, forum technical advisor:
Indeed, the situation you describe is ideal for power vents. Additional thought: If you can get a grant from the power company or government, you might also think about using a heat exchanger in conjunction with the power vents to reheat the incoming air. You (or anyone in the same situation) should also have someone look over the kiln to make sure that heating is adequate to accommodate the faster drying and also that the internal circulation fans are appropriate. (Analogy: It is important to make sure that you do not put a supercharger on a car engine that needs new rings and valves.)