Forest Products Notes on...

Using a Dehumidifier to Dry Wood

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.

Using a dehumidifier to dry wood:

• 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).

Principles of using dehumidifier systems to dry wood

• 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-exposed to the warm, moist air (i.e. back to step 1 above).

Attractions of using a dehumidifier to dry wood

• 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-technology” approach is feasible (in contrast to expense of “heat and vent” type equipment). A simple, cheap kiln can be constructed using a cheap dehumidifier using:

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...

Disadvantages of Using a Dehumidifier to Dry Wood

• The cool, dry operation of these systems (20-40°C, 30-50% RH) may be “too dry” for green timber, which might sustain drying defects (or “degrade”). This is made worse if the air is (too) warm and dry (this can arise due to too efficient heat recovery a in dehumidifier timber drying system - and also from the heat generated by circulation fans and motors). It is important therefore that an appropriate control system is used with sufficient ventilation when using a dehumidifier to dry wood.

• 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% - 20% there is less water to condense, so less heat is recovered - so more heat is needed from “other” sources (and electricity is expensive).

• 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...

Image of High Temperature Timber Drying Page

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...

Image of Advantages and Disadvantages of Air Seasoning of Timber compared to Kiln Drying Page

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