Reefer Kiln Help Needed!

[JHEPP08]

Thanks guys. I'll be drying lumber not firewood .
So I need to keep temp real constant? I can't just stoke it up an get the heat up over 100?

[Ga_Boy]

To dry lumber in a kiln you have to follow a schedule.  The schedule gives the temperature and relative humidity that you have to maintain.  You have to maintain the temperature and relative humidity to keep the lumber from loosing moisture to quickly.  You are limited to 1 to 2 % moisture loose per day.  If you exceed this you risk damaging your lumber to the point where it is not useable.

As the lumber dries, you increase the temperatrue and decrease the relative humidity level in the kiln.  Changes are made to the temperature and relative humidity based on the moisture content of the lumber you are drying.  As the mositure content in the lumber drops the kiln schedule tells you what to rise the temperature to and to what value you lower the relative humidity to.

When lumber is dried in a kiln, it dries from the outside to the interior of the lumber.  There are two types of water in the cell structure of a wood.  These types of water are loose and bound.  Loose water is just that, it is not attached to the cell structure of the wood.  The loose water is the first water to leave the wood cell during the drying process.  Bound water is chemically bound to the cellular structure of a molecule of wood.  To remove bound water from wood requires energy in the form of heat.  If the bound water is removed too quickly the structure of the wood is damaged; we call this checking.  Checking is visually seen in the form of cracks in the wood.

Now, remember that wood dries from the outside to the interior of the lumber.  By drying in this manner it creates a moisture gradient. If the moisture gradient becomes to great the interior of the lumber will not dry and this creates stress and renders the lumber unusable.  This is called case hardening.  Case hardening presents it's self as interior stress causing the lumber to bend when you cut it.

You may have seen case hardening or stress when you have ripped lumber on a table saw and seen the cut piece either bends in to the blade or away from the blade.

I'll post a picture of the kiln schedule for White Oak.  This will give you an idea of how lumber is dried.

See the pictures below.  Let me know if you have questions.





Mark

[Ga_Boy]

Kiln Schedule Cover page.



White Oak Kiln Schedule

[Ga_Boy]

More information on how the atmosphere of the kiln is controlled.

When drying wood, the heat source raises the kiln charge temperature up to the initial temperature (usually 110 degrees F).  Depending on the starting temperature of the lumber when it enters the kiln, bringing it up to the starting temperature takes about 24 hours.  As the lumber heats up it start giving off moisture, which raises the relative humidity. 

The kiln schedule for the type of lumber you are drying will give you the temperature and relative humidity initial presets.  You program the initial presets in to your kiln controller and it monitors the kiln environment.  When heat is needed, the controller sends a signal to your heat source to add heat.  When the temperature is too high, the controller sends a signal to the exaust fan to dump the excess heat.  When the relative humidity is too low the controller sends a signal to the misting system to add humidity.  When the relative humidity is to high, the controller sends a signal to the exaust fan to dump the excess humidity.  When the exaust fan drums excess heat or humidity fresh air is drawn in to the kiln through a fresh air intake louver. 

The controller monitors the kiln environment and makes adjustments as necessary to maintain the programmed set points which are stated in the drying schedule.   As the moisture content in the lumber decreases, you raise the temperature and lower the relative humidity values programmed in your controller.  Changing the temperature and relative humidity values is easy, most controllers use the Partlow 1160 series control module.  There is an up value button and down value button on the Parlow control module.  To increase the programmed value you hold the up value button until you reach the desired value and press the down value button to decrease programmed value.

From my experience drying lumber is 90% science and 10% art.  After you have ran a few lumber charges through your kiln you will get a feel for how to get the best performance from your configuration (this is the 10% art).  As long as you follow the kiln schedules for the species and thickens of lumber you are drying, you basically monitor the controller and check your kiln samples each day to determine moisture content. 

I will discuss how to determine and check kiln sample moisture content in a day or so. 

In the mean time if you have specific questions, let us know. 



Mark

[GeneWengert-WoodDoc]

This is an excerpt from a talk I gave at the Great Lakes Kiln Drying Association two weeks ago.  Note the improved white oak schedule at the end.  It is a brief discussion, so questions are welcome.

One note that might help with the previous posting is that "loose" water is usually called free water.  It takes about 1000 Btu to evaporate a pound of free water and maybe 30 more for bound water.  During drying, both free and bound water are being evaporated at the same time.  Also, the moisture gradient will be the largest when drying first begins...the first five days about...and this is when surface checks are created.  Be very careful in the initial drying, and then you will not have surface checks or honeycomb.

Regarding casehardening, it is virtually impossible to prevent casehardening.  Casehardening is more accurately called residual drying stress, as it has nothing to do with hardness.  In air drying, the high humidity every morning relieves the stress.  Similarly for a solar kiln without auxiliary heat.  If not relieved, then we need to steam the lumber briefly at the end of the kiln cycle...or use vapor spray.

Here is the talk--
Basically, there are two main areas that must be examined:
   - - environmental conditions that inhibit or slow drying
   - - lumber characteristics that inhibit or lengthen drying



ENVIRONMENTAL CONDITIONS

There are three environmental variables that control the rate and quality of drying.  They are
      *  temperature
      *  relative humidity (RH), sometimes expressed as wet-bulb depression or EMC
      *  velocity of the air flowing past the lumber

The drying rate is controlled by precise control and manipulation of the three critical variables.  The key is the word "precise."  Often, for best quality, temperatures must be controlled to within ½ degree F; humidities within 1% RH, and velocities within 50 fpm.

Temperature
As the temperature in a dryer increases, the drying rate increases.  Further, hot air holds more moisture than cooler air, so drying is usually more uniform throughout the dryer.  Hotter temperatures, however, do mean that the wood is weaker; therefore, hotter temperatures mean more risk of checks and splits, as well as more risk of warp.  On the other hand, when stickering is perfect, the hotter wood will be held flat by the sticker more easily than cooler wood (that is, the stickers bend the wood flat).  Conversely, poor stickering means more warp.  Hotter temperatures usually mean lower RHs and a greater risk of over-drying, which aggravates warp and machining problems.

Relative Humidity
Lower RHs mean faster drying, more uniform drying throughout the pile, flatter lumber, and brighter lumber.  However, lower RHs can mean a higher risk of checks and splits and more risk of over-drying and subsequently excessive warp and machining problems.

The relative humidity is an expression of the amount of moisture in air compared to the maximum amount that it can hold at the same temperature.  Relative humidity is important in drying--at any given moisture content, the drying rate of the wood can be expressed as

      Rate = coefficient x (100 - RH)

What this expression means that if initially the RH is 80%, but then is lowered to 60%, the lumber will dry twice as fast.  That is, at 80%, the rate is [coefficient x 20]; at 60%, the rate is [coefficient x 40].  The coefficient in this equation is a constant at a given MC, temperature, and velocity.  As the wood dries, the coefficient gets smaller (that is, drying rate slows); as the temperature increases the coefficient increases; and as the velocity increases, the coefficient increases.  The coefficient also is dependent on species and thickness of the lumber.

Relative humidity is not measured directly, but is determined by measuring the wet-bulb temperature and then the RH value is looked-up in an RH table of wet-bulb and dry-bulb temperatures.  (Wet-bulb thermometer is a regular thermometer with a wet muslin wick on the thermometer; the wick is kept wet with distilled water.  Brisk air velocity across the wick produces cooling, compared to the dry-bulb temperature.)  This RH table was initially formulated over 75 years ago.

Because of the occasional problem with supplying water to the wet-bulb thermometer, in supplying adequate air flow, and in keeping the wick clean, some drying control systems use a cellulose wafer as the humidity sensor.  The cellulose gains and losses water in response to changes in RH.  The electrical resistance of the wafer is related to RH or, more commonly, to EMC (equilibrium moisture content) of the air.  EMC can be related directly to shrinkage, which in turn is closely related to stresses, which in turn is related to several types of degrade, including checks, splits, and some warp.  Hence, EMC can be a useful expression for indicating the moisture content of the air in a dryer.

As might be expected, the RH (and the EMC) of air increases as the air moves through the load of lumber.  (In contrast, the wet-bulb temperature is uniform throughout the pile.  Although this may seem strange, it is indeed true, in an air path that does not go across heating coils or does not have outside air (hot or cold) introduced in the airstream.)  Because the RH increases as the air moves through the pile of drying lumber, the drying rate slows through the pile.  Likewise, as the EMC increases through the pile, the risk of stress related degrade falls.  Generally, the RH or EMC of the entering air is measured and controlled, as the entering air conditions are the most severe.  However, we are beginning to see some operating scenarios using the change in RH or EMC through the load as a control variable as well.

Air Velocity
When lumber is quite wet, the major resistance to drying is how fast the air can scrub the water molecules off the surface and how fast heat can be supplied to evaporate moisture.  (The process is controlled by the boundary layer.)  As a result, changes in velocity result in direct changes in drying rate--higher velocities mean faster drying.

However, as the lumber becomes quite dry (under 20% MC), the major resistance to drying is the speed with which the molecules can move from the interior of the wood to the surface.  Hence, the velocity of the air through the pile (or the boundary layer) has very little effect on drying.

At intermediate MCs, the boundary layer and internal diffusion will be of nearly equal magnitude, so both are very influential.

Higher velocities also mean more uniform drying throughout the pile.

An important concept in air flow, is the volume of air going into the load is equal to the amount coming out.  There cannot be a build up of air within the pile.  Therefore, air velocity is usually measured on the exit side of the pile, as it is very easy to do so, but it is hard to measure velocity accurately on the entering air side.  One problem that must be considered, however, in measuring air flow in this manner is that there is a tendency for the air to move upward as the air moves horizontally through the piles.  Hence the exit air side may show higher velocities near the roof than near the floor, but this pattern may not exist on the entering air side of the load.  Nevertheless, this upward trend does exist and does indicate the need for horizontal baffles in the space between adjacent packs, especially in track kilns when drying green lumber.  (Recall green lumber is more influenced by velocity than is partly dried.)


Effects of temperature, relative humidity, and velocity on drying
______________________________________________________________
Role of Temperature.  When the temperature rises,

a) lumber dries faster, which in turn means brighter, flatter lumber with more risk of checking
   b) lumber dries more uniformly throughout the kiln
   c) lumber develops darker (usually browner) colors
   d) lumber is weaker and therefore lumber is more prone to checking and splitting
e) lumber is weaker and therefore lumber is more prone to warping unless stacking is precise
   f) insects and fungi are less active above 100 F and are killed above 130 degrees F


Role of Relative Humidity.  When the humidity drops,
a) lumber dries faster, which in turn means brighter, flatter lumber with more risk of checking
   b) lumber dries more uniformly throughout the kiln
   c) lumber develops lighter colors
   d) lumber is stronger and therefore less prone to warping
   e) lumber is stronger and therefore less prone to checking and splitting


Role of Velocity.  When the velocity is increased,
a) above 40% MC, lumber dries faster, which in turn means brighter, flatter lumber with more risk of checking
   b) below 20% MC, velocity has very little effect on the drying rate
   c) lumber dries more uniformly throughout the kiln
______________________________________________________________


LUMBER CHARACTERISTICS

Rate.  The following wood characteristics and features affect the rate of drying:
Slower drying         Faster Drying
--------------------------------------------------------------------------
Heartwood            Sapwood

Quartersawn         Flatsawn
Thicker            Thinner
Face or edge grain      End grain
Flat grain            Knots

High density wood       Low density wood
Bacterially infected      Not infected
—————————————————————————

Heartwood / Sapwood.  The effect can vary from just a few percent slower drying for heartwood to over 30% slower with a wood like white oak.

Quartersawn / Flatsawn.   The ray cells help moisture move in the radial direction, so flatsawn will often dry faster.   The difference for many species of hardwoods is that heartwood will dry 15% more slowly; for softwoods, the difference is less.

Thick / Thin.  As a general rule, doubling the lumber's thickness increases the drying time by 2.5 times.  As a rough approximation, each 1/16" thicker (1-3/167" versus 1-1/8") means 9% longer drying time.

Bacterially Infected.  When a tree is heavily infected with the wetwood bacteria, there will be an increased initial MC.  For example, red oak will be 100% MC instead of 75% MC.  Further, bacterially infected wood dries more slowly.  So, there is more water and the slower drying with heavily infected wood.

Moisture Content.  The following characteristics affects the length of drying; that is, two very important factors in drying time are the incoming MC of the lumber and the final required MC.  In almost all cases, it is the few wettest pieces of lumber that determine the drying time, not the average MC.  That is, the kilns runs until the wettest pieces are dry enough.  Implied in this discussion is the selection of the wettest piece and then the accurate measurement of the wettest piece's MC.

Example:  A load of 4/4 oak that has been air-dried is put into the 50 MBF capacity kiln.  The wettest lumber (from the lower layers of the bottom pack of lumber, more recently stacked for air drying, and located in the center of the air drying yard) is 35.0% MC.  The drying rate is 2.5% MC loss per day.  The total estimated drying time for this load is 14.0 days.  However, with better air drying,  If the wettest piece of lumber were only 30% , the drying time could be reduced by two days.  This time savings is worth about $50 per MBF or a savings of $2500 for the kiln load.  The annual production from the kiln would increase from 26 loads a year to 30 loads a year or 200 MBF more.


PRACTICAL STEPS AND PROCEDURES

Kiln Schedule
Here is a kiln schedule for 4/4 and 5/4 white oak producing the highest quality, but the extra time  required, the schedule could be considered  too long.  The basic question is "Is production or quality is more important."

In general, and probably in almost all cases, accelerating the kiln schedule is not wise, as the  risk of quality loss increases.  However, as the original kiln schedules were developed with some conservativeness, a little tweaking is possible, mainly in the humidity level    and when the humidity is lowered.


--------------------------------------4/4 + 5/4 WHITE OAK -----------------------------------
Sample MC      Dry-Bulb   Wet-Bulb     EMC         
       (%)              (F)      (F)    (%)      
-------------------------------------------------------------------------------------------------------
   Over 40%         105     101       17.5            
   40% - 35%         110     105       16.2            
   35% - 30%         110     102       13.3              
   30% - 25%         120     106       10.0            
   25% - 20%         130      95      5.0            
   20% - 15%         140     105          5.0            
   15% -  end         160     125          5.0         

  Equalization       NONE
  Conditioning (stress relief) as required       
————————————————————————————————————

[WDH]

Drying 8/4 quartersawn white oak is Slowwwwwwwwww.

[5quarter]

WDH...Yes, but you'll make it up in sales.  ;)

[WDH]

That quartersawn 8/4 stuff just got much more valuable.