During the drying cycle the vents (or DH-unit) are used to remove moisture from the kiln environment thereby allowing more moisture to be evaporated from the wood. We are always trying to keep a certain relative humidity (or wet bulb depression) to prevent the lumber from drying too fast. On some occasions (especially on slow drying species) you are not able to keep the relative humidity sufficiently high even though there is no venting. In this case we are beginning to have a high drying pressure on the wood and the chance for degrade is significantly increased. This leaves us with two options: Add humidity to the air through steam or water spray or reduce the fan operation either by slowing the fan speed or interval operated fans. Kiln-direct suggests you take a closer look at interval operated fans for this purpose.
The other reason to add humidity to the air is for stress relieving or conditioning. This is done at the end of the drying process to relieve the stresses in the wood that has been created during the drying process. During conditioning we are actually adding moisture into the surface of the wood.
There are two ways to add humidity to the air:
Steam spray,
This is the most common method and is a very well proven one. It seems to be more efficient and provide shorter conditioning cycles. However, the shorter conditioning cycle is probably due to the fact that most steam systems are capable of adding more moisture to the air in a shorter period of time. The steam system is believed to use more energy than cold water spray. And it is not always to achieve the right depression due to the known phenomenon of the wet bulb chasing the dry bulb. This comes from the fact that when steam is added, heat is also added and will often push the chamber temperature higher.
Water spray,
this is a relatively new method and consists of high pressure water being pushed through very fine spray nozzles, which creates a very fine mist or fog. This mist is so fine that it immediately evaporates completely and thereby raises the relative humidity in the kiln. Opposed to the steam spray which added heat to the kiln, the mist actually uses heat, which allows you to control the kiln temperature by adding heat to make up for heat consumption from the evaporation of the mist. This misting method is just beginning to become more prevalent in kilns.
Learn more about the design behind the Humidi-Mist system.
