20 



HARDWOOD RECORD 



March 25, 1921 



Electrically Driven Sawmills 



'By Allan E. Hall, Milwaukee, Wis. 



(.Continued from March to Issue) 



[f; 



Fig. No. 5. 



-Overcut Slab Slasher in the mill of D. K. Jtflris 

 driven by an induction motor. 



18 Sawmill boil- 

 ers are either ordi- 

 nary horizontal 

 tubular or of water- 

 tube types. When 

 burning sawdust a 

 large furnace is re- 

 quired, which is 

 generally of Dutch- 

 oven design. A fuel- 

 storage house is 

 built close to boil- 

 ers, with the fuel 

 conveyors arranged 

 either to discharge 

 sawdust from the 

 mill direct to the 

 furnaces or to carry 

 it on to the storage 

 house and return it 

 when needed. When 

 the belted mill and 

 non-condensing Cor- 

 liss engine are replaced by the condensing steam turbine and motor- 

 driven mill the steam consumption of the prime movers will be cut 

 about one-half, which allows a reduction of boiler capacity to be 

 considered in comparing the cost of the entire jilant. The boiler 

 capacity needed for various auxiliary steam cylinders used in the 

 mill will of course remain the same as before. 



19 The belted sawmill, except in plants too small to be of 

 practical interest, is generally driven by a simple non-condensing 

 Corliss engine. It has not been found worth while to install com- 

 pound or condensing engines when fuel was valueless, simply for 

 the reduction in boiler plant. When motor driving of a proposed 

 plant is in contemplation, the prime mover may be a simple or 

 compound, condensing or non-condensing, steam engine direct- 

 connected to the generator, or else a steam turbo-generator. Fuel 

 cost of course makes the gas or oil engine out of the question and 

 plant cost eliminates hydroelectric power except in rare cases. 



20 It has been said in an earlier paragraph that the standard 

 sawmill plant includes a planing mill. This should be placed 200 

 feet or more from the nearest roofed point of the sawmill proper 

 (to meet insurance rules), which means 300 to 600 feet from the 

 main power house. Motor driving of new planing mills, except 

 when very small, has become almost universal, so that this is gen- 

 erally assumed at the start. If we belt-drive the sawmill, we must 

 then have one prime mover for it, and a steam-electric plant for 

 the planing mill. It is obvious that there should be only one boiler 

 plant and one engine room, containing both prime movers. But if 

 the sawmill as well as planing mill is electrically driven, the two 

 prime movers may be combined into one large enough to drive both 

 mills, which is economical in first cost and is often done. 



21 For the same reasons which have brought it into favor in 

 other industries, the condensing steam turbo-generator has become 

 almost universal in motor-driven sawmills; and compound con- 

 densing engines are not now running in any mill with which the 

 writer is acquainted. When deciding on the method of power 

 transmission, the choice for large mills is commonly made between 

 (a) a sawmill belted from a simple Corliss engine and a planing 

 mill motor-driven by a steam turbo-generator, and (b) both mills 

 motor-driven by a turbo-generator. The comparison of costs which 

 follows is on this basis for the larger mill, though for the smaller 



Jeffris, Louisiana, 



one both mills are 

 belted from engines. 

 22 The power 

 factor of the gen- 

 i erator in this class 

 of service will be 

 low, averaging 

 about 70 per cent. 

 In one case, when 

 first installed, the 

 power factor was 64 

 per cent, and a syn- 

 chronous condenser 

 was installed in the 

 power house, doing 

 no mechanical work 

 blit raising the 

 power factor to 80 

 per cent. In gen- 

 eral, no auxiliary 

 equipment for pow- 

 er-factor correction 

 I is used, but a gen- 

 erator sufficiently large to do the work at low power factor is pur- 

 chased. It is to be remembered that we are always assuming fuel 

 to be of no value, so that small differences in steam consumption 

 are not material. 



23 In a shaft-driven mill the weight and cost of the lineshaft 

 and various countershafts are considerable. The power-receiving 

 section of the lineshaft is about four inches in diameter in a single 

 band mill, and eight inches to ten inches in diameter in the largest 

 mills, and the shaft may be 300 feet long. In large mills counter- 

 shafts will be required for the log jack, log cut-off saw, log canter, 

 band mill, gang mill, resaws, edgers (this requires a right-angle 

 drive, usually through mortise gears), slasher, trimmer, hog, lum- 

 ber sorter, timber trim saw and lath mill. When the individual 

 motor drive is installed, all these shafts with their attachments are 

 not required; and the difference in the total of sawmill machinery 

 is almost entirely in these omissions. All the cutting machines, 

 conveyors and transfer and roll drives will be little changed. The 

 amount of this saving is given below in three cases, the figures 

 covering all sawmill machinery proper, installed, but no power- 

 house equipment. 



Mechanical Drive 

 Weight, Lb. Price 



Mill No. 1 445,296 $112,345 



Mill No. 2 701,500 173,560 



Mill No. 3 763,150 189,079 



Electrical Drive 

 Weight, Lb. Price 

 402,749 $104,410 

 620,000 153,698 

 674,150 166,789 



24 In driving sawmills by motors the main driving belt is saved 

 and most of the cutting machines can be directly connected to 

 motors. The small auxiliary machiner_y can usually be grouped so 

 as to drive from three to five units with one motor, through a small 

 countershaft and belts. This reduces the number of small motors, 

 but increases the belting. The proper balance between motor cost 

 and belting cost is a matter of judgment for the owner or engineer. 

 It is not practical to design a sawmill entirely without belts, but 

 the belting may be reduced to 25 per cent (or less) of that needed 

 for a shaft-driven mill; and all angle and quarter-twist drives may 

 be taken out. For the three mills given above, the belting esti- 

 mate follows: 



