RH%, control/manipulate

Relative humidity, control/manipulate the variables in the kiln.

The relative humidity is generally adjusted through 2 methods: Vent/heat or dehumidifier.

Dehumidification systems can be installed on both large and small kiln. These systems do seem to have their strongest market in the smaller kiln market for hardwood, as they do not require a boiler system. The major drawback of dehumidification systems is the high cost of operation (even though they are suppose to be more energy efficient). The higher operation cost is because the complete system is electrically operated. As heating cost with electricity is much higher then gas, oil, or wood waste, the dehumidification kiln proves uneconomical in the final analysis.

The dehumidifier system is somewhat similar to an air conditioning unit. The units have a cold coil and a hot coil. In a dehumidification system, the humid air in the kiln is pasted over the cold coil whereby humidity is condensed and run off. The excess heat created is channeled to the hot coil where the same air is past through and some energy recovered. This system works best in high humidity climate.

The vent/heat kiln are the work horse of the lumber industry. These units are not as affordable in smaller kiln size below 45 m3 (20,000 bf) because a external heat system is required to generate the hot water or steam.

The vent/heat system relies on the fact that when a cubic foot of air is heated, the relative humidity drops. By using this principle in a vented kiln, the humid warm air in the kiln is vented out and replaced with cold air. This new cold air is then heated and the relative humidity is then lowered. One of the biggest advantages of the vent/heat system is that it is relatively inexpensive to make the kiln more powerful than a dehumidifier when it comes to potential water removal. For venting and exhaust systems there are the standard roof (or top of wall) vents. This has been the common method of venting kilns for decades. Recently, more and more kilns are manufactured with power venting to increase capacity. Furthermore, they also save energy by removing moisture from the wettest area in the kiln (instead of the driest as with standard roof vents). In addition, some manufacturers (including Kiln-direct) offer heat recovery system which reuses the warm humid exhaust to heat the incoming air. These heat recovery units can make a heat/vent system nearly as energy efficient as a dehumidifier, but will operate at much lower cost by using an less expensive heat source than electricity.