34 



HARDWOOD RECORD 



April 10, 1922 



,sm^ 



Power Logging ^4^D: 

 Limber, Handling 



.ii:^ 



UJ^S^feT^~~ " 



Some Advice on Selecting and Operating a Dry Kiln 



A short time ago a manufacturer of sash and doors planned to 

 build a new dry kiln for the purpose of drying fir, spruce and hard- 

 woods. After having put in the footings he wrote to Hakdwood 

 Record for advice on the kind of kiln to select and general methods 

 of operation. This letter was referred to "C. J. M.," Hardwood 

 Record's kiln drying expert, for reply. The reply is printed 

 below because it contains a great deal of information that should 

 interest any user of dry kilns: 



The Eeply 

 Gentlemen : 



Your letter of recent date concerning dry kiln has been referred to me 

 for reply by the Hardwood Record, and I will attempt to give the infor- 

 mation desired as fully as possible, and since your statements clearly dis- 

 close a desire to accomplish good drying of fir, spruce and hardwood, I 

 direct your particular attention to the fact that the selection and posses- 

 sion of a dry kiln most suitable for the drying of the lumber enumerated 

 will not necessarily deliver the lumber as you wish it. 



Please do not think that I am overstating the matter when I say that 

 the kiln itself is only the tool used in good drying, and that the best kiln 

 is worth about 20 per cent of the success and that the remaining 80 per 

 cent must be contributed by the operator or attendant, by an understand- 

 ing and faithful application of a few simple but important conditions neces- 

 sary to good drying, and without which good drying of such lumber as 

 you enumerate cannot take place unless It be by accident. 



There is no kiln obtainable which will do intelligent drying by Itself 

 without attention and regulation, and the knowledge necessary therefor 

 has been fuU.v developed in the last decade and may be readily obtained. 

 Most of the kilns on the market are good kilns when properly operated, and 

 would produce better results if more time and effort were devoted to 

 Instruction of those who are to operate them. 



I hold no brief for any make of kiln, including the one which you have 

 mentioned. Under certain conditions some are better suited than others, 

 but good drying can be obtained In all if the kiln is not left to itself, 

 and if the kiln provides the conditions which are hereinafter mentioned. 

 You may trust these statements to be true and based on actual experience 

 in the drying of many millions of feet of hardwood lumber of very heavy 

 dimensions in all of the different makes of kilns and with more than 

 reasonable success. 



Essentials of a Good Kiln 



The essentials of a good dry kiln for the drying of hardwood and the 

 other lumber you mention, and which you should insist upon when nego- 

 tiating the purchase of a dry kiln, are as follows : 



1. The building and the doors must be constructed air-tight, and to stay 

 thus. 



2. The heating system must be so designed and constructed as to equalize 

 the temperature of the kiln room, front and rear. In many kilns there Is a 

 drop of 10* or more towards the rear. 



3. The heating system must be sufficiently ample to produce the tempera- 

 ture of not less than 1G0° F. with outdoor atmosphere at 10° below zero, 

 and it must be so subdivided and valved that you shall be able to produce 

 and hold indefinitely any degree of temperature between 100° and 160° F., 

 assuming, of course, that the steam supply is constant. For best results 

 the steam supplied to the kiln should be of not more than 5 pounds pres- 

 sure, and may be less provided that Its supply is constant. 



4. The moisture removal, whether by ventilation or by condensation on 

 water coils, must be so arranged as to permit fullest control and adjust- 

 ment. If by gravity ventilation, then the cold air inlet and the moist air 

 outlet pressures must balance, and exits as well as Inlets must be un- 

 affected by wind pressure. 



5. The endwise piling system is by far preferable In the kiln to the 

 crosswise piling, because It will more naturally assist circulation of the 

 air In the kiln so that the lumber may dry more uniformly in all parts 

 of the kiln. 



Much can be said on the subject of doors. The best are none too good. 

 1 would advise decidedly against canvas curtains. The same should never 

 be used single and are not very desirable even if double with a 12 inch air 

 space between. A single canvas curtain on the Inside, with a Hussey door 

 on the outside, 12-inch air space between, would make good arrangement, 

 provided that the Hussey door is properly constructed and hung. 



Must Have Spray Pipes 



The kiln must also be equipped with steam spray pipes running full 

 length in the shape of an H, the supply Inlet at the center of the cross 



pipe, with thp perforations so placed as to discharge the steam horizontally 

 and not against the lumber, and so connected to the low pressure and the 

 high pressure steam that either may be used as occasion demands. The 

 supply to the spray lines to be 1%-inch pipe up to and inclusive of the 

 cross piece of the H. The side lines of the latter, running full length 

 of the kiln, should be 1 14 inch pipe each. Perforations should be not 

 smaller than Is -inch and placed on 3-inch centers on both sides of pipe 

 (6 holes to the foot of pipe). 



If you insist on the above stated conditions of the kiln equipment you 

 will have a good kiln capable of giving you satisfactory dry lumber when 

 attended by a faithful and properly informed operator who is interested 

 in bis work. 



Any kiln which will comply with the requirements mentioned will be 

 able to satisfy your requirements in tlie three kinds of lumber which you 

 mention, or in any other kinds of lumber, because it will enable you to 

 vary the conditions as required by the different lumber. 



You may dry fir and spruce together by the same treatment and with 

 perfect success, but it is not advisable to mix the thickness if time Is an 

 object. The 2-inch stock requires a milder treatment than the 1-inch 

 stock : therefore will remain in the kiln a longer period than the latter. 



If different thicknesses of these woods are in the kiln at the same time, 

 then the treatment must be for the heavier thickness, and if properly 

 applied the lighter thickness will not be dry any sooner than the heavier. 

 The lighter, therefore, loses time. 



Fir is more sensitive than the spruce and the conditions of the kiln air 

 prescribed for fir must be closely adhered to If you wish to prevent any 

 checking. 



I would not mix hardwoods Into the same kiln charge with the spruce 

 and the fir. It can be done, but as a rule it does not pay to dry that 

 way. 



Principles of Air Bemoval 



Please bear In mind that in good kiln drying you cannot remove the 

 moisture from the surface any faster than this moisture transfuses from 

 the inside of the wood ; otherwise you will caseharden the surface and 

 check your lumber. Hence the necessity for the humidity of the kiln 

 air to bold in check the too fast removal of the surface moisture. It is the 

 heat which drives the Interior moisture to the surface, and this can be 

 properly distributed in any kiln, provided that the heating system Is 

 right. 



For uniform drying the internal circulation of the moist air within 

 the kiln must be sufficient over all the boards in every pile. This is very 

 Important and cannot always be obtained If the piles are laying cross- 

 wise the kiln. Endwise piling will favor this circulation and therefore 

 uniformity in drying. In an ordinary kiln this inner circulation depends 

 on the heat and the gravity of the air, and therefore is comparatively 

 weak. In a blower kiln (as the Sturtevant) this circulation is very rapid 

 and therefore greater uniformity of effect is olitained ; but with rapid 

 circulation the air must be kept at the correct humidity, otherwise surface 

 moisture may dry off very fast, with the usual ill effect. However, In such 

 a kiln temperature is regulated automatically and the humidity very 

 easily, and both can be measured without entering the kiln. 



In the above I trust that I may have fully satisfied your questions, 

 but, if I can be of further sen'ice, please write again, and when you are 

 about ready to start your kiln building I will, if you remind me by letter, 

 forward some desirable information for your kiln operator, so written 

 that he can easily understand it. Very truly yours, C. J. M. 



Questions and Answers 



[Note: Seaders of Hardwood Record are invited to submit their yard 

 and kiln problems for answer in this column. — The Editor.] 



Question 7 — Electric Lights in Dry Kilns 

 During the war our company built a battery of twelve new kilns, 

 supposed to be the last word in kiln construction. They have water 

 sprays in them and low pressure steam sprays and high pressure 

 steam sprays and a few other things. 



Nothing would do but that the kiln operator wanted an electric 

 light at each end of each kiln on the inside, controlled by an outside 

 switch. This did not look good to me, and I told the boss so, but 



