14
2.6 - WOOD CHARACTERISTICS
Wood is formed mainly by cellulose and lignite. It
also contains other substances, such as resin (fir -
pine), tannin (oak - chestnut) and, obviously, a large
quantity of water.
Good quality woods are oak, ash, beech, maple and
fruit trees, except cherry; medium quality wood is:
chestnut and birch; sufficient quality wood: lime tree,
poplar and willow. Resinous trees are, normally,
second-rate quality fuels.
Wood is therefore an extremely heterogeneous
fuel, due to different essences (beech, oak, fruit,
resinous), different moisture content, and due to
different shapes and dimensions.
Boiler operation will inevitably be effected by all
these factors. Especially by the log dimensions, the
moisture content and the way wood is loaded in the
boiler.
WOOD MOISTURE
The calorific value of the different types of wood
depends on their moisture content, as illustrated in
the table. Boiler output and autonomy will diminish
as the moisture increases. The table provides the
power reduction factor based on the moisture of the
wood being used. The heat output of the NOVAIREX
boiler is calculated for wood with a 17% moisture
content.
As an example, wood which has been dried for
2 years in a sheltered area has a 25% moisture
content.
Example:
moisture of the wood being used = 25%
output = nominal output x 0.86
Correction factors for wood moisture
% OF
HUMIDITY
CALORIFIC
VALUE
kWh/kg
CORRECTION
FACTOR
15
20
25
4.50
4.18
3.87
1
0.93
0.86
DIMENSIONS
The dimensions, together with the moisture content,
contribute to determine the boiler’s output. Small
pieces (with a length, however, conforming to indi-
cations indicated hereby) are more easily flammable
and therefore have the tendency to increase the
boiler’s output and therefore to reduce its auton-
omy. Moreover, they fall with greater facility, in the
lower chamber, reducing the risk of the formation of
“bridges”. The so-called “bridge” is an empty space
in the firewood storage compartment, where unburnt
wood logs remain. In this case the bed of embers
is not fed with continuity and the refractory burner’s
slot is uncovered. In these conditions, preferential
air ducts are created with a very small flame due
to an excess of air.
CALORIFIC
VALUE
MJ/kg
16.20
15.05
13.93
Summary of Contents for NOVAIREX 35
Page 1: ...INSTALLATION AND MAINTENANCE INSTRUCTIONS NOVAIREX 35 45 55 ...
Page 55: ......
