Drying case hardening

Although recycling of moist air does reduce the drying time because of the increased linear velocity, an equivalent amount of fresh air is much more effective because of its lower humidity. The points in favor of moist air recycle, however, are saving in fuel when the fresh air is much colder than 170°F and possible avoidance of case hardening or other undesirable phenomena resulting from contact with very dry air. 

Although direct contactive heat transfer can provide high temperatures and neat concentrations and at the same time be smml in size, its use may not always be preferable because of undesired side effects such as drying, contamination, case hardening, shrinkage, off color, and dusting. 

Another phenomenon that may affect the critical moisture content is known as case-hardening. In this instance, the surface of the material is dried so rapidly that a layer of dry, non-porous material forms. This over-dried surface acts as a barrier to moisture diffusion, since diffusivity decreases with moisture concentration. This may occur in vacuum drying, which will be discussed later in the chapter. To reduce the risk of case-hardening, the relative humidity of the drying gas may be increased to assist in maintaining a higher surface EMC until the internal moisture has diffused to the surface. 

Case hardening is one condition that the kiln operator prefers to live with and cure rather than prevent. It is not economic to dry timber too slowly. In practice the aim is to dry lumber as fast as one dare without getting excessive checking and to... 

The eonditioning treatment can be overdone. If the wood is steamed for too long and the surfaee fibres re-adsorb too much water this will induce an excessive compression set in the surface fibres. At the end of drying, when the surface fibres return to the natural equilibrium moisture content they will want to shrink more than normal wood (because of the excessive compression set), thus producing tension at the surface and compression in the interior. The lumber has been reverse case hardened. This condition cannot be treated. 

Case hardening only becomes obvious when wood is recut or when more material is dressed off one face than the other. When case hardened boards are ripped the two halves crook or bow and pinch the saw. The distortion is towards the saw cut because the surface zones of the board are in compression while the centre of the board is under tension. An edge groove, for tongue and grooving, will pinch too. Similarly a board that is machined more heavily on one face will bow towards that face. Kiln-dried lumber ought to be dimensionally stable, and such distortion on processing is undesirable. 

Case Hardening. Break up feed material and increase air flow or agitation to speed up drying process. 

Metallurgy Bright annealing of steel, de-tinning of scrap metal, extraction of metals from ores, dry reducing gas, case hardening... 

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