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Author Topic: Calculating dehumidifier output and MC loss  (Read 287 times)

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Offline GeneWengert-WoodDoc

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Calculating dehumidifier output and MC loss
« on: September 19, 2019, 05:51:12 PM »
if you have a tight kiln, all the moisture that is evaporated from the lumber will be vapor in the air.  This vapor then will be condensed by the dehumidifier and run out of the kiln as liquid water.  The kiln operator often is faced with trying to figure the moisture loss per day based on the amount of water collected from the DH unit.

Here is a straightforward way to make these calculations.  First, in drying lumber, moisture is a percentage of the oven-dry weight of the lumber.  So, figure out the weight of oven-dry lumber in the kiln.  (Note that we seldom worry about the water in the stickers, although there is some indeed.). To do this calculation, follow some of the instructions in

Using the oven dry weight, ODWT, we know that 1% MC is 1% of this number.

All we have left to do is convert the volume of water we measured into pounds.  (The easiest way would be to weigh the water...too easy and we might not have an accurate scale.). Another quick way is to estimate that 1 pint of water equals one pound.  The more precise conversion is one pint equals 1.043 pounds.

Example:  We have 200 bf of 4/4 red oak sawn at 1-1/8 = 1.125 average thickness  but was scaled at 1.00 and the lumber is 82 (98) long but scaled at 80.  The first step is to correct for the lumber volume, as the basic data is for the weight of a piece 1.00" thick and the exact length, such as 80 (96).  With hardwoods, we do not have to worry about the width correction.  The correction factor, CF = (1.125 / 1.000) x (98 / 96) = 1.148.

Now, the 30% MC weight for red oak, using column C, is 3793 pounds for exactly 1000 BF, 1 thick and no over-length.  so, we need to get the oven dry weight by dividing by 1.30. Next, we need to apply the correction factor.  
ODWT = 3791 / 1.30 x 1.148 = 3348 pounds.  
This means 33.48 pounds of water is 1% MC for 1000 bf.  Then 200 bf will be 0.2 of this value, so 33.48 x 0.2 = 6.70 pounds.
Using 1.043 pounds per pint, 1% MC = 6.70 /1.043 = 6-1/2 pints.  This is just a bit over 3/4 gallon = 1% MC for  200 bf of red oak, 1-1/8 thick and 2 of overlength.  Remember that this is for a kiln with no leaks or a working exhaust fan.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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