I assume you refer to a tunnel type kiln, where the wet stacks go in one end, and dry stacks exit the other end, with the air flow direction blowing in at the dry end and out the wet end?
We have lots of these type of kilns in South Africa, and have successfully upgraded and improved the drying quality and drying time through automation
By Design these kilns are very forgiving for species like pine and Eucalyptus, because the RH% increases automatically form the Dry/hot end to the wet end due to moisture uptake from the timber.
These kilns can also be a major headache if you don't understand the dynamics of the kiln. As in all kilns, the air flow is one of the most important factors in the kiln. I've seen the airflow going wrong when:
1.stacks are too close to each other, and stacking lines are not aligned. This causes a Polaroid type effect. You must be able to see from the one end of the kiln, through to the other side...looking through the stacks. If the stacking is not good enough to do this, you need to leave a space between each stack of at least 8-10 inches, to allow for air flow to find its own way.
2.The plenums are too small
3.The plenums are too big. This happens when the kiln is not full. If your kiln takes eg. 8 stacks to fill the kiln,and there is only 2 stacks, they must be placed next to the plenums a the the wet and dry end. Rather leave a gap in the middle between the 2 stacks. The the kiln is not full, strange things happen to the air flow!!
On the kilns we have here, the vent is situated at the wet side of the kiln, above the door, and there is a small gap below the wet side door. When the vent is open, dry air is sucked into the vent, and hot moist air blows out underneath the door. It is important to know where these actions must take place, as it is a tell tale sign of air flow problems. We had one site where we could not achieve good results, and after a lot of investigation discovered that of the 3 fans, one fan's blades where 50mm shorter than the other 2. This created a pressure differential between the fans when the kiln was full of timber and the doors closed, resulting in almost zero air flow through the stacks. Off course, when the doors are open, they blow like hell, because of no resistance. The blow and suction action of the vents, as well as the RH% that just kept rising, even with vents open was the indicators of air flow problems.