Forest Products Notes on...
It is potentially more energy efficient to use a dehumidifier to dry wood since this system uses a form of “heat pump” to recover heat energy which would otherwise be lost. When using a dehumidifier to dry wood this recovered heat may or may not be supplemented by another, more conventional heating system: but in total the energy input needed to dry wood using a dehumidifier is less than used in conventional Heat and Vent Kiln Drying systems.
• is convenient if there is no other way of generating heat economically.
• could operate at up to 80°C (but most operate at <50°C).
• can also be used to conserve heat in conventional heat and vent kilns (by removing heat from exhausted warm, wet air).
• 1) Warm, moist air which has passed through the timber heats up a liquid refrigerant, which evaporates. The condensation of the water also heats the refrigerant. Cool, dry air is produced.
• 2) The refrigerant is then compressed (and this also warms it).
• 3) The warmed refrigerant now warms the cool, dry air entering the kiln to produced warm, dry air which is used to remove water from wood. Refrigerant gas cools to become a liquid again.
• 4) The pressure of the refrigerant is reduced and then it is re-
• Saving in energy costs (but see below).
• Better quality wood may be produced. Some people believe better quality timber is produced when using dehumidifiers to dry wood as the wood is dried more slowly (i.e. less stresses are generated).
• A “low-
2” x 6” wooden studs
6mm polyvinyl vapour barrier
exterior grade phenolic bonded plywood
insulation (most important if operating at “higher” temps.)
•It is possible to mix species and thicknesses of timber since using a dehumidifier to dry wood is slow and reduced stresses are produced in the material.
•Less pollution is produced, since vapour from the kiln is condensed (however the condensed liquid must be disposed of carefully).
•Less supervision of the equipment is required.
•Electricity is the only energy service required.
But there are also disadvantages of using dehumidifiers to dry wood...
• The cool, dry operation of these systems (20-
• These dehumidification systems use expensive electricity.
• In association with the above, the insulation of the kiln must be good.
• Venting may be necessary (to avoid overheating) – this will necessitate increases in energy use to make good heat losses.
• In the initial stages of drying wood using dehumidification the costs are less
than for conventional heat and vent type systems, but when reaching moisture contents
<15% -
• These systems operate at cooler temperatures, so longer drying times are required than for Heat and Vent Kiln Drying systems.
• Using a dehumidifier to dry wood has the disadvantage that conditioning cannot be used to rectify case hardening (but one can buy humidification equipment if needed).
• The low temperatures these systems operate at can’t sterilise timber (>65°C is needed).
The NEXT PAGE of the notes also describes an alternative method of drying timber, one which is both very rapid and especially useful for optimal drying of wood species having low permeability (which are notoriously difficult to dry without suffering damage). This technology uses a High Temperature Timber Drying process...
The PREVIOUS PAGE is the most popular page of our notes, the subject being the Advantages and Disadvantages of Air Seasoning of Timber compared to Kiln Drying...