Microwaves for the control of wood-destroying insects

Mikrowellen zur Holzinsektenbekämpfung-
– Anwendungen im WFM Detmold
[Microwaves for the control of wood-destroying insects
- Applications in the WFM Detmold]
Fritz Kohler
Kohler Automation, Butzbach-Griedel
Zusammenfassung
Sichere Bekämpfungsmaßnahmen von Holzs-chädlingen
ohne Umweltbelastungen sind gefragt. Im
Rahmen des DBU-Förderprojektes wurde das Mikrowellenverfahren
auf Einflüsse von Abmessun-gen,
Holzart (Dichte), Holzfeuchte, Querschnitt, Alter,
Schimmel- und Insektenbefall, Metallein-schlüsse
(Nägel, Beschläge), Umgebungstempera-tur, Anordnung
der Geräte (oben, unten, seitlich) und daraus
resultierende Temperatur-verläufe wirksam getestet.
Zunächst auf dem Prüfstand und in der Folge auch
erfolgreich in praktischen Feldeinsätzen, incl. Weiterentwicklung
der Gerätekonstruktion.
Mit den Weiterentwicklungen kann das Mikrowellenverfahren
sicher angewandt werden und ist gut
geeignet zur Bekämpfung von lokalem Schimmelund
Insektenbefall an Balken und Brettern wie z. B.
Dielen, Riegeln, Schwellen, Sparren, Ständern. Die
Behandlung gefasster oder geformter Holzobjekte ist
grundsätzlich auch möglich, bedarf im einzelnen
jedoch noch genauerer Erprobung möglicher weiterer
Risiken in Hinsicht auf Temperaturbe-ständigkeit
von Anstrichen oder Verteilung der Feldstärke bei
sehr variablen Formen, z. B. bei Schnitzereien.
Hierzu sind weitere Detailunter-suchungen sinnvoll.
Summary
In the scope of the DBU-funded project, the microwave
procedure was tested in many experimental
series on the test stand for the influence of the type
of wood, wood moisture, cross-section, age, mould
and insect infestation, type of heating (from above /
below / the side) and the resulting temperature
courses, and metal inclusions (nails, fittings). This
was followed by successful field use with additional
data, which are discussed in the lecture, including
further development of the device design: The microwave
procedure can be applied safely and is very
appropriate for controlling local mould and insect
infestation on beams and boards, such as floorboards,
locking bolts, thresholds, rafters, and stands.
The risks and the usage restrictions are pointed out
and discussed:
The wood moisture is always so high that the wood
absorbs microwaves and is thus warmed. For this
reason it is not possible to only heat up the insects.
The lethal temperature can only be reliably achieved
by heating up the entire piece of wood.
Due to the wavelength (12.2 cm in vacuum) and the
wave form (nearly sinusoidal), the temperature increase
cannot be linear. However, as a result of the
convection connected with it, an acceptable temperature
distribution can be achieved. The highest
temperatures always occurred inside the wood, but in
an area near the surface, the first 3 cm (down to 4/λ).
Therefore, measures necessary to avoid excessively
high temperatures can be required here as a function
of the treatment time. By means of intermittent or
regulated operation of the device, disturbing condensation
and overheating of metal inclusions can be
avoided.Moreover, “hot spots” (irreproducible focusing
of the microwaves) have to be considered.
These could be restricted by an appropriate distribution
of the field strength in the antenna design (slot
antenna).
The critical and lethal temperatures, which are to be
ensured, are realised not in the core, as described in
DIN 68800, but in the corners of the distant areas.
Depending on the installation position and the dimensions
of the pieces of wood that are to be treated
it is more appropriate to work with several devices
simultaneously.
Since the energy transformation takes place in the
wood’s interior and the lethality is also a function of
the time, a thermal insulation that is transparent to
microwaves is required. Hard Styrofoam and glass
wool have proven very appropriate for this. The

devices must be exactly oriented for an optimal
treatment period. As a consequence, an intensive
user training is required, with increasing experience
substantial shorter treatment periods are required.
Even if the device manufacturers have fulfilled all
safety regulations, the entire safety chain can only be
ensured by the user.
The heat-generating field strength of microwaves
decreases according to the square of the distance.
Microwave devices are optimised by means of construction
measures such that the devices can be positioned
closer to the interior corners and edges, the
areas which are to be treated, in difficult installation
locations (e.g. rafters on roof beams). In addition,
adaptation to beam thickness can be made with
swivelling reflectors. In this manner scattering losses
are avoided to a great degree.
Treatment costs are also dealt with. They are basically
a function of the dimensions, type of wood
(density), wood moisture, device arrangement
(heating from above / from below / from the side)
and the ambient temperature. But the installation site
and position (e.g. in a church tower) can influence
the costs substantially. However, it can basically be
stated that a microwave control measure is normally
more reasonable than restoration. Furthermore,
judging from our experience, both 50€/m and
500€/m can be seriously calculated and economical.
Costs for the treatment of surfaces (floor-boards,
parquet flooring) are of the same order of magnitude
(from 50€/m²).