22 



HARDWOOD RECORD 



May 10, 1922 



^rPONVER CLOGGING 



BER IJANDLING; 



Steaming Gum Lumber Ahead of Yard Drying 



By C. J. M. 



A soutliern manufacturer of gum dimension stocks requests in- 

 formation concerning the steaming of this material. His desire is 

 to pile the material into a steam box as fast as produced, close the 

 box at the end of the day and steam during the night, removing 

 the material in the following morning for air drying. 



The information which he seeks deals principally with the con- 

 struction features of the steam box, method and volume of steam 

 supply and the pressure of the latter. As the questions are reflected 

 in the following paragraphs, it is unnecessary to state the same 

 here verbatim. 



One of his questions, however, is based on an erroneous assump- 

 tion, which requires explanation, to wit: ''How long a steaming, 

 at what pressure, will it take 'to drive the sap' out of the mate- 

 rial?" 



Proper Steaming Improves Gum 



The writer lias had considerable experience witli steaming of 

 gum lumber, and as a result of his observations has reached the 

 conclusion that steaming will benefit gum lumber if properly done, 

 and that it will likewise reduce the drying time of the lumber to 

 some extent, not by causing a faster transfusion of the moisture 

 from center to surface, and hence, a faster rate of drying, but by 

 an immediate forced mechanical removal of a certain limited 

 amount of the free water in the cells, from center to surface, and 

 therefore a prompt but limited reduction of the moisture content 

 of the gum wood, after which the drying proceeds no faster than if 

 there had been no steaming. This, of course, applies only to gum 

 containing free moisture, above fibre saturation, and this action 

 of free water expulsion, is ascribed by Professor Tiemann as due 

 to the expansion of the air within the inner cells of the gum 

 wood. The writer fully concurs in that opinion, adding that the 

 expanding air is probably assisted by the expanding water in the 

 wood. 



Aside from this forced removal of a very limited amount of 

 free moisture there is no perceptible change in the "sap." This 

 cannot be driven out by any amount of steaming, at any pressure. 

 A small amount of leaching probably takes place near the surface, 

 due to the action of the condensed steam, which in a small part is 

 entering the wood before the latter has become thoroughly heated 

 tlirough. This water is later expelled, together with a portion of 

 the free water already contained in the wood. Ther? are probably 

 also some chemical changes, due to the Iieat, which, however, could 

 not affect tlie drying one way or another. 



Benefits of Steaming Enumerated 



Tlie benefits actually derived from the steaming of green gum 

 lumber are: 



(a) Equalization of the moisture content which tends to more 

 uniformity in drying. 



(b) Expulsion of a limited amount of the free moisture con- 

 tained in the wood and a consequent and proportional 

 reduction of the drying time after the steaming. 



(c) Removal of all strains, internal and external, since the steam 

 makes the wood plastic and it will dry in the position in 

 which it is supported, assuming a permanent set without 

 strains. 



(d) Improvement in coloring, especially of the sapwood which 

 darkens considerably in steaming. 



What Steam Pressure to Use 



The writer has observed the steaming of gum lumber of various 

 tliicknesses in steel cylinders under pressure of 2u pounds steam 

 (Kraetzer process). Steam was turned on and permitted to rise to 

 20-pounds pressure in about 20 minutes, when it was shut off, tlie 

 lumber allowed to remain about two hours longer in the quickly 

 diminishing steam pressure, and then removed to the yard. The 

 maximum temperature to which the lumber had been exposed at 

 20-pounds pressure was about 259 degrees F. 



Careful examination of many piles of gum lumber and of heavier 

 dimension which had been subjected to the same treatment dis- 

 closed a very satisfactory condition. 



On the other hand, the writer has also obtained very excellent 

 results from steaming with high pressure steam, both saturated 

 and superheated, at atmospheric pressure only, in ordinary steam- 

 ing rooms and kilns, such steaming having been thoroughly tried 

 out on vast cjuantities of gum lumber dried under the writer's 

 supervision. 



Temperature, Not Pressure, Does the Work 



Temperatures were used, ranging from loU degrees F. to 230 

 degrees F. (superheated steam), and many carefully conducted 

 tests were made, in result of which the writer has concluded: 



1st, that the temperature attained by the wood in the steam- 

 saturated air controls the result. 



2nd, that exposure to the steam must be of sufticient duration 

 to heat the lumber clear through to the center, but that over 

 exposure is neither desirable nor beneficial. 



3rd, that the higher the temperature attained in the steam box 

 or the kiln, the shorter should be the period of exposure. 



4th, that if the steaming is to be done in steam boxes or kilns 

 (if ordinary construction, the temperature must be limited to about 

 180 degrees, to prevent unreasonable damage to the structures. 



5tli, that practical results can be obtained only with "live 

 steam" of sufficiently high jiressure to provide a sufficient and 

 rapid supply of heat. 



For the best results the steam should be well distributed into 

 the kiln or steam box through a set of perforated pipes, arranged 

 in the shape of an H, of which the two long pipes would run the 

 full length of the room, perforated on their under side with %" 

 holes on two-inch centers, to discharge the steam against the floor. 

 These two pipes would be 1%" pipe size and cross-connected at 

 their center by a 2" pipe into which should be fed the steam 

 through a 2" supply line, which latter should have a control valve 

 immediately outside of the steam box or kiln. 



In the case of our correspondent who proposes to construct 

 steam boxes 25' long, the combined areas of the %" holes on 2" 

 spacing would just about equal the area of a 2" pipe, and the 2" 

 pipe would deliver the steam to the room (7'x7'x25') as fast as 

 desired, and without danger of building up any appreciable 

 pressure within this room. 



Night Steaming Is Feasible 



As stated, the steaming is to be done during the night, the inteu- 

 tiua being to bring the steam pressure to 80 pounds, and then 

 gradually let the boiler empty its steam into the steam box, with 

 out further firing. Such a proceeding is entirely feasible, i»o- 

 vided that the boiler is of sufficient capacity to hold the amount of 



