rtant feature of this type of dry kiln,
and the most essential one in the drying of hardwoods.

It is not generally understood that the length of a kiln has any
effect upon the quantity of material that may be put through it, but
it is a fact nevertheless that long kilns are much more effective, and
produce a better quality of stock in less time than kilns of shorter
length.

Experience has proven that a kiln from 80 to 125 feet in length will
produce the best results, and it should be the practice, where
possible, to keep them within these figures. The reason for this is
that in a long kiln there is a greater drop in temperature between the
discharge end and the green or receiving end of the kiln.

It is very essential that the conditions in the receiving end of the
kiln, as far as the temperature and humidity are concerned, must go
hand in hand.

It has also been found that in a long kiln the desired conditions may
be obtained with higher temperatures than with a shorter kiln;
consequently higher temperatures may be carried in the discharge end
of the kiln, thereby securing greater rapidity in drying. It is not
unusual to find that a temperature of 200 degrees Fahrenheit is
carried in the discharge end of a long dry kiln with safety, without
in any way injuring the quality of the material, although, it would be
better not to exceed 180 degrees in the discharge end, and about 120
degrees in the receiving or green end in order to be on the safe side.

Operation of the "Moist-air" Dry Kiln

To obtain the best results these kilns should be kept in continuous
operation when once started, that is, they should be operated
continuously day and night. When not in operation at night or on
Sundays, and the kiln is used to season green stock direct from the
saw, the large doors at both ends of the kiln should be opened wide,
or the material to be dried will "sap stain."

[Illustration: Fig. 34. Vertical Pipe Heating Apparatus for
Dry Kilns; may be used in Connection with either Live or
Exhaust Steam for High or Low Temperatures.]

It is highly important that the operator attending any drying
apparatus keep a minute and accurate record of the condition of the
material as it is placed into the drying room, and its final condition
when taken out.

Records of the temperature and humidity should be taken frequently and
at stated periods for the different thicknesses and species of
material, i