Steam usage

[Greg McCoy]

Is there an average for pounds of steam used to dry 1000 bf of lumber?

[MichaelM]

Hi Greg,

The amount of energy to dry wood per board foot varies because the initial moisture content varies among the species and the size of a rough rough board foot varies from mill to mill.  The volume of a rough board foot also varies with lumber size, for example 1000 board foot of rough green 2x4 has about 4% more cubic feet of wood in it than 1000 bf of 2x6.

One needs to look at this as the energy to remove a pound of water.  Then 2 lbs of steam per pound of water removed from the wood becomes an approximate value.  If the hemlock goes in at 3600 lb/mbf and comes out at 2000 lb/mbf, then 1600 pounds of water was removed and you would expect about 3600 pounds of steam to be used. 

For an efficient kiln you might get as low at 1.6 lb of steam per pound of water removed.  As schedules get longer (such as for thick stock and hardwoods) the number will be higher - maybe as high as 4000 BTU (4 pounds of steam) per pound of water removed from the wood.

I would be suspicious of mill measurements do not come out between 1.6 and 2.5 lb of steam per pound of water removed.  If it is outside this range there should be an obvious reason.  Weighing loads in and out is one way to measure the water removed.  Another way is to use the lumber volume (in cubic feet), wood specific gravity, and MC in and out to estimate the weight change.  Steam flow needs to come from a flow meter at the kiln.  Mills with multiple kilns have more trouble measuring because of multiple species, sizes, and products.  For one kiln and a natural gas boiler, steam use might be estimated from  gas consumption.

If you send me the numbers (not through the forum) I can help you with the calculations.

Hope that helps,
Mike M.