Same goal, different route
Drying aims to bring the water inside the timber down to a target level without cracking the surface or leaving internal stress. The difficulty is getting the rate at which water travels from the core to the surface to keep up with the rate it evaporates from the surface.
A conventional kiln manages this with temperature and humidity control. A vacuum kiln lowers the pressure so water evaporates at a lower temperature, which makes the gap between core and surface easier to manage.
Where the difference shows
At thin sections the water has a short path; both methods work without trouble and the choice usually comes down to batch size.
As sections thicken, the moisture gradient between core and surface grows. At that point the tighter control of vacuum drying lowers the risk of surface checking and internal stress.
What the choice is based on in practice
The kiln type is set by the technical office, looking at the material and the target. The headings considered are:
- Section thickness — vacuum comes forward as it increases
- Species and starting moisture content
- Target moisture and how tight the tolerance is
- Batch size — conventional is economical for large batches
- The value of the material and the cost of getting it wrong
Steaming is a separate operation
Steaming is not an alternative to drying but a complement to it. It is applied on request to even out colour and relieve internal stress.
On visible-surface work where colour uniformity matters, steaming makes any later colouring noticeably easier.