maximizing productivity, maximizing recovery and improving process monitoring
Goals of silviculture
meets the goals and objectives of the landowner, provides for timely availability of many forest resources, produces predictable harvests over the long term, balances biological/ecological and economic concerns to ensure renewability of resources, provides for regeneration, effectively uses growing space and site productivity, considers forest health issures
Even aged stands have
one or two age classes with a well-developed canopy with a regular top at a uniform height
Uneven aged stands have
at least three well-represented and well-defined age classes, differing in height, age, and diameter.
Proper felling, bucking and handling of operations in the forest will
increase profits and decrease value lost at the sawmill.
Goals of forest operations
production of straight logs, grouping and placement of defects through good bucking practices, bucking logs to correct lengths, clean felling cuts, good log-handling procedures.
Proper bucking can reduce
sweep, crook and taper loss
75% of a sawmill's cost is
over 24 inches in diamer, butt log, wood in quality zone is straight grain with no defects, has small straight taper, heart center is limited to 40% of log diameter.
wood supply considerations
availability, suitability, accessibility, harvesting and transportation
if the volume is adequate to supply a viable manufacturing operation, determine availability at an affordable rpice over planned life of sawmill
are stand valuable, what are the characteristics of the stands and the trees
close to the mill? is terrain difficult? too much rain or snow? environmental consideration? incorrect surveys?
log volume determination
accurate estimation of yield of lumber, tallied by species, diameter, taper, height
doyle, scribner, international
(1 inch thickness by 12 inches wide by 1 feet long)/12 = 1 bdft
(lumber tally - log scale)/log scale
LRF is better than overrun because
it is not based on arbitrary log rules and easier to work with when estimating lumber recovery fluctuations due to equipment changes or wood supply
log yard scaling and tallying
necessary for financial statements
beginning inventory + delivery - ending inventory
log yard functions
recei0ve, tally, store, sort and deliver logs to the log deck. to maintain an adequate inventory of suitable raw material in the yard and deliver it to the mill in feed
general considerations for log yards
land should be leveled, shouldn't be too dry in case of checking, fire hazards, environmental consideration of log leeching in log ponds, roads very ease of access, distance to the mill.
advantages of a log pond
no fire hazards, smooth and flat surface permitting low cost conveying, transportation, no sprinkling costs
logs being lost when floods happen, extractions being leaked into the environment, marine borers attack your logs
Why do we debark logs
bark is detrimental to pulp, prolong life of sawmill cutting tools, reduce mill cleanup and maintenance time, reduce safety and fire hazards, enable the sawyer the and scanner to see and evaluate a better breakdown.
revolving inner ring and stationary outer ring
consists of small, short cylindrical cutterhead on a pivoted arm
high pressure water
rotatating in drum for pulp may cause damage
why dry wood
transportation cost decreases, stability increases, strength increases, biodegradation decreases, gluing and finishing quality increases, easiness of impregnation with liquids increases
low cost, short time, good quality
high cost, long times, bad time
3 variables of timber drying
temperature, relative humidity, velocity
Parts of a kiln
controller dry bulb, controller wet bulb, insulated door, exterior wall, steam spray line, fans, heater coil, insulation, reflective ceiling coating, fan motor, hinged baffle, control room, climate control unit and record, steam boiler for humidification
Different types of insulation
wood = short service life, degradation ( thermal, chemical, fungal), brick = thermal expansion results in cracks, cement = low thermal insulation, cracks, aluminum and stainless steel are the best
Insulation must be dry for it to work
touch the walls of the kiln to check if it's hot
Check for gaps in bolt connections and the foundation
foundation and bolt connections on kiln structure is common place of degradation.
advantages = narrow MC, minimized defects, economical , continuous supply of lumber, constant heat load, use of heat exchangers / disadvantages = longer drying times, difficulty in changing drying conditions, market rigidity, not proper for changing fiber quality
advantages = reduction of heating coils, automated energy management, lower power consumption, better drying quality, less noise, shorter drying time, lower spraying because molecules transport both heat and humidity
thermal energy is needed by the water
molecules for activation and diffusion, conversion to liquid phase and evaporation.
when there is no transfer of heat between a thermodynamic system and its surroundings
combustion gases into the kiln directly
heat from hot pipes via air to wood
burn fuel into convert water to steam into kiln
fuel for the boiler
oil, natural gas, wood residue
super-heated steam produced at high pressure is
super-heated (150 psi, 365 degrees Farenheit), normal steam is (15 psi, 250 degrees Farenheit)
When operating with super-heated steam a
steam engineer needs to be present which is very costly
Boiler treatment is necessary
there are impurities in the water like suspended solids, dissolved solids, dissolved gases.
steam will flow under pressure differential from boiler to
heat exchangers in the kiln
iron or copper pipes with steel or aluminum fins to deal with corrosion problems. distance between fins in correlation to air flow controls convective heat transfer
required to catch the steam turned liquid coming out from the heat exchangers
direct fired kilns mix hot gases produced by
burning gas, oil, sawdust or other fuel and circulate them directly through the lumber piles => no heat exchangers
difficult to control temperature and impossible to control
H in the kiln
Impossible to condition
lumber at the end of drying cycle for internal stress release
Good for high
temperature drying of commodity lumber
air circulation systems are the
air circulation is the energy
(heat) distribution system in the kiln
temperature drop across lumber load and
fans should be reversible every 6-8 hours in order maintain drying ballance
fans should have variable
RPM. Air speeds should slow down after hitting below fiber saturation point
If line shaft then
the motor will be visible outside of kiln
HP ~ u^3
if you want to double u then increase HP by 8
better air flow characteristics
better thermal energy distribution, reduced final MC variation between lumber pieces, less drag, less energy to the fans
low pressure side = wet air
high pressure side = dry air
top and bottom baffles force
air to go through the lumber channels
the plenum area
minimum distance from wall to edge of load in order to maintain uniform airflow entering the load material, as distance decreases, air velocity increases and uniform flow diminishes
air flaps used to
to direct air flow patterns and heat distribution throughout the lumber layers which reduces the vertical MC variation between lumber.
dry kiln vibration problems
fans are a major cause of vibrations, issues = not balanced misalignment mechanical looseness, bearing failure, vibrations can result in premature failure due to high wear of fans and in catastrophic events such as flying blades or flying shafts. Need a maintenance schedule.
The objective of drying process is to maintain fixed
T, H, and u conditions in the kiln with time.
Opening vents decreases
Wet steam or cold atomized water spray system
thermal energy recovery system
fresh air coming in is heated by exhaust air coming out. Relieves stress on boilers.
Objectives in lumber preparations
minimize degrade, obtain desired final MC, maximize lumber throughput, poor stacking practices = higher degradation, good stacking practices = dries faster, less warping
stickers = thin and narrow pieces of rectangular cross-section that creates the channels between lumber layers for air flow. Helps form lumber packages.
stickers are made from
clear, straight grained woods, plastics or aluminum or mild steel, must be flush with ends of the boards
manual stacking ok for
small operations but expensive and time
vertical gap between 2 packs of lumber
recommended kiln cart dimensions
4 feet long, 6 inch side channels on truck to protect the wheels from forklift damage, and boxed beam bunk supports on 2 foot centers
stickers must be
aligned columns in a package.
timbers in a stack should be
same thickness for the stickers should be able to lie flat on
Crossouts are the
kiln blocks put between packages
vertical air gaps in baby squares will
slightly reduce drying time and produce a better M profile within timbers
vertical loading used when
in high temp fast drying and when lumber exhibits high slope of grain or contains too much juvenile or reaction wood that could cause shape deformation
a set of dry bulb and wet bulb temperature as a function of time or average moisture content. MC based are the best schedules, but not reliable need to monitor MC with technology
start with mild conditions and slowly go to more severe conditions done by raising temperature and lowering relative humidity to produce steady decrease in MC
steps of a kiln schedule
warm-up period (low T, high H) heat up kiln and lumber, drying step 1 (T increases, H decreases) dry lumber to FSP, drying step 2 (T increases, H decreases or both stay constant) remove bound water while moving M toward target, equalizing M between boards (hardwoods), conditioning (high T, high H) stress relief, cooling period - heat off, vents closed, fans on
In some cases with high T drying of softwoods
, even the conditioning step is skipped
multi-zone drying - having variable temperature between zones
electronic temperature sensors
most common temperature sensor is metal RTD which responds to heat by increasing its resistance to electrical current, the thermistor type is when resistance decreases as temperature increases, thermocouple type generates a small voltage proportional to the temperature at the location where dissimilar metals are joined.
electronic humidity sensors
wet bulb humidity sensors are most commonly used and oldest, keep cloth clean from deposits and rust main, maintain adequate water flow, air should be directed over the wick, the wick covers RTD, humidity electronic sensors are designed to eliminate wet bulbs with problems related to waste water from wet bulb trough, freezing, corrosion or sediment build up, European system where a current is ran across a wafer of cellulose or wood measuring current or resistance which is used to calculate EMC. The condition of the wafer is similar to the condition on the surface of lumber
Measuring MC in the lumber yard
using a measuring device with pins which needs to plunged along the grain, needs to species specific, needs a temperature set and reads a MC vs handheld capacitance meters
Sensors are required in kilns to obtain
Temp sensors placed at 2 ends of lumber pile to measure temperature drop across load which temp in minus temp out.
non-destructive evaluation - acoustic emissions measures the sound waves created during cracking of the cell-walls in drying to control the drying process.
Pages linking here (main versions and versions by same user)