kevchan's version from 2015-12-16 00:28


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Lumber is created tomaximize wood strength, maximize recovery, optimize wood utilization, surface quality
Surface quality meansmaximize wood-glue strength, maximize coating durability, utilize lower strength wood, optimize wood utilization
Maximizing value involves the balance ofmaximizing productivity, maximizing recovery and improving process monitoring


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Goals of silviculturemeets 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


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Even aged stands haveone or two age classes with a well-developed canopy with a regular top at a uniform height
Uneven aged stands haveat least three well-represented and well-defined age classes, differing in height, age, and diameter.
Different types of even-aged standsclear-cut, seed tree, shelterwood, coppice, patch cut, retention


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Proper felling, bucking and handling of operations in the forest willincrease profits and decrease value lost at the sawmill.
Goals of forest operationsproduction 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 reducesweep, crook and taper loss
75% of a sawmill's cost isits logs
Perfect logover 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.


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wood supply considerationsavailability, suitability, accessibility, harvesting and transportation
availabilityif the volume is adequate to supply a viable manufacturing operation, determine availability at an affordable rpice over planned life of sawmill
suitabilityare stand valuable, what are the characteristics of the stands and the trees
accessibilityclose to the mill? is terrain difficult? too much rain or snow? environmental consideration? incorrect surveys?


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log volume determinationaccurate estimation of yield of lumber, tallied by species, diameter, taper, height
log rulesdoyle, scribner, international


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board foot(1 inch thickness by 12 inches wide by 1 feet long)/12 = 1 bdft


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overrun(lumber tally - log scale)/log scale
LRF is better than overrun becauseit is not based on arbitrary log rules and easier to work with when estimating lumber recovery fluctuations due to equipment changes or wood supply


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log yard scaling and tallyingnecessary for financial statements
usagebeginning inventory + delivery - ending inventory
log yard functionsrecei0ve, 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


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general considerations for log yardsland 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.


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advantages of a log pondno fire hazards, smooth and flat surface permitting low cost conveying, transportation, no sprinkling costs
disadvantageslogs being lost when floods happen, extractions being leaked into the environment, marine borers attack your logs


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Why do we debark logsbark 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.


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ring debarkerlog rotates
mechanical ringrevolving inner ring and stationary outer ring
rosserheadconsists of small, short cylindrical cutterhead on a pivoted arm
hydraulichigh pressure water
drumrotatating in drum for pulp may cause damage


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why dry woodtransportation cost decreases, stability increases, strength increases, biodegradation decreases, gluing and finishing quality increases, easiness of impregnation with liquids increases
Drying dreamslow cost, short time, good quality
Drying nightmarehigh cost, long times, bad time
3 variables of timber dryingtemperature, relative humidity, velocity
Parts of a kilncontroller 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 insulationwood = 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 worktouch the walls of the kiln to check if it's hot
Check for gaps in bolt connections and the foundationfoundation and bolt connections on kiln structure is common place of degradation.
Progressive kilnsadvantages = 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
Compartment kilns 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 watermolecules for activation and diffusion, conversion to liquid phase and evaporation.
adiabaticwhen there is no transfer of heat between a thermodynamic system and its surroundings
direct firedcombustion gases into the kiln directly
indirect firedheat from hot pipes via air to wood
steam systemburn fuel into convert water to steam into kiln
fuel for the boileroil, natural gas, wood residue
super-heated steam produced at high pressure issuper-heated (150 psi, 365 degrees Farenheit), normal steam is (15 psi, 250 degrees Farenheit)
When operating with super-heated steam asteam engineer needs to be present which is very costly
Boiler treatment is necessarythere are impurities in the water like suspended solids, dissolved solids, dissolved gases.
steam will flow under pressure differential from boiler toheat exchangers in the kiln
heat exchangersiron 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
steam trapsrequired to catch the steam turned liquid coming out from the heat exchangers
direct fired kilns mix hot gases produced byburning gas, oil, sawdust or other fuel and circulate them directly through the lumber piles => no heat exchangers
difficult to control temperature and impossible to controlH in the kiln
Impossible to conditionlumber at the end of drying cycle for internal stress release
Good for hightemperature drying of commodity lumber
air circulation systems are the
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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 variableRPM. Air speeds should slow down after hitting below fiber saturation point
If line shaft thenthe motor will be visible outside of kiln
HP ~ u^3if you want to double u then increase HP by 8
better air flow characteristicsbetter thermal energy distribution, reduced final MC variation between lumber pieces, less drag, less energy to the fans
low pressure side = wet airhigh pressure side = dry air
top and bottom baffles forceair to go through the lumber channels
the plenum areaminimum 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 toto direct air flow patterns and heat distribution throughout the lumber layers which reduces the vertical MC variation between lumber.
dry kiln vibration problemsfans 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 decreasesH
Wet steam or cold atomized water spray systemincreases H
thermal energy recovery systemfresh air coming in is heated by exhaust air coming out. Relieves stress on boilers.
Objectives in lumber preparationsminimize 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.
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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 forsmall operations but expensive and time
chimneyvertical gap between 2 packs of lumber
recommended kiln cart dimensions4 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 bealigned columns in a package.
timbers in a stack should besame thickness for the stickers should be able to lie flat on
Crossouts are thekiln blocks put between packages
vertical air gaps in baby squares willslightly reduce drying time and produce a better M profile within timbers
vertical loading used whenin 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
kiln schedulea 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
types of schedulesstep type, ramp type, ramp/step type, continuously rising type, oscillating type
best strategystart 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 schedulewarm-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
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electronic temperature sensorsmost 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 sensorswet 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 yardusing 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 obtainambient conditions
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.