Document 8VyBe2qO8wDm7drO5V9Gvjowy

'LAINTD'F'S EXHIBIT CERTAIN-TEED PRODUCTS CORPORATION Ardnoro Hr. 0, F. Grieve January 9, 1956 Paoli. Laboratory AGENDA 1956 GSPSDtl pLant MANAGERS MEETING C. G. Shuttleuorth - ojs cc: Mr. P. E. Fischer, V. P. ** Ardmore Mr. G. A. Hoggatt - Paoli Lab.______ Mr. LI. J Zelras - Acne Mr. H. F. Debo - Akron Mr. K. W. Broun - Blue Rapids Mr. Myron Read - Fort Dodge Mr. A. H. TenElshof - Grand Rapids Hr. E. L. Film - Sigurd Mr. J. W. Hart - Piyor Mr. H. II. Powers - Akron Hr. P. W. Codwise - Thorold Mr. H. C. Johnson - Paoli Eng. Mr. VI. G. Will - Paoli Mr. II. C. Puckett - Fort Dodge Mr, M. F. Fink - Ardmore In compliance with the requests in the third and fifth paragraphs of your letter of December 12, 1955, I have prepared a paper on assigned topic No. 17 and a copy of it is attached to each copy of this letter. * > * SGP 0021452 BOARD A1ID LATH DRYING The subject of drying gypsum vallboard and lath products is a rather controversial one and not yet entirely understood* However* there is certain data available to us from actual performance of our own dryers and from the study made of the Grand Rapids dryer in 1951* by Foster D. Snell* Herewith is a summary of data from our five plants showing operating speeds* dryer temperatures, and drying time in minutes for the three major board and lath products* HBgERATURES 3Y MOTION, ENTERING AIR PLANT MATERIAL 1 2 3 1* 5 6 DRYING TIME IN IHNUTES Acme 3/8 lath 1*65 1*60 1*75 355 370 225 Fort Dodge 3/8 lath 1*60 hho 380 270 21*0 Akron 3/8 lath 300 295 Grand Rapids 3/8 lath 300 318 305 279 Sigurd 3/8 lath 295 295 260 60o5 51 56 58 60 Acme Fort Dodge Akron Grand Rapids Sigurd 3/8 bd0 3/8 bdo 3/8 bdo 3/8 bdo 3/8 bdo 500 500 1*90 1*00 370 190 2i5o 1*30 360 250 200 300 295 300 318 278 281* 295 305 260 56 59 60 65 6? Acme 1/2 bdo Fort Dodge 1/2 bdo Akron 1/2 bdo Grand Rapids l/2 bd0 Sigurd 1/2 bdo 1*85 510 1*85 1*15 ii50 210 1*60 10*0 380 270 21*0 300 295 300 318 278 281* 300 305 260 73 71* 80 89 80 SGP 0021453 2" - Notes Production rates of lath at Acme and Sigurd is said to be governed byfacilities at take off rather than by ability of the kiln to dry the lath* Ibis shows that the gas fired kilns are drying at faster rates than the steam kilns, no doubt due to the higlier temperatures available during the con stant drying rate period* Study of the penetrometer test reports shows that end and edge calcination are not markedly different at the five plants* Of the steam plants, Akron dries all three products as fast as or faster than at either / Grand Rapids or Sigurd in spite of the fact that it has lfjQulbo steam available as against 200 lbs0 at the other two* The reason for the good performance is not altogether clear but nay be partially due to the somewhat lower evaporation normally obtained at Akron* In general, the Grand Rapids dryer is the least efficient of ary which led to the study made by Foster D,, Snell in 195k* This was a rather thorough study which amassed a large volume of data, some of which appears to be con tradictory, but some of the more pertinent conclusions from the pilot plant study are as follows: 1* The characteristic drying rate curve for homogeneous materials is divided into three parts* The first is a rising curve as the product is warmed up to its maxdiip.ua temperature* The second part is a flat plateau of constant drying rate versus time* The third is the falling rate period from the critical moisture content to dry, the drying rate versus tins sloping down ward as the product becomes more dry. 2* At temperatures of 258 to 26U F, there is no significant change in rate of drying during the constant rate period which can be attributed to the moisture content of the drying air* SGP 0021454 -3" 3 The type of cover paper on the product has a significant effect upon the constant drying rate* Papers which transmit moisture from the core to the surface faster have a faster drying rate. Mo information as to wiiich characteristics of the paper are critical was determined* At high drying temperatures* the rate of water transfer through the cover paper is less than the rate at which it can be carried away by the air. The paper resistance then becomes the limiting factor during much of the drying period. lie The rate of drying at and adjacent to the ends and edges is greater than in the centers of the board and laths So The critical moisture content at which the falling rate begins is approximately 2$% based on the dry treight* At Grand Rapids, this coincides roughly with the fourth section 6 Since the air leaving the kiln will have a high heat content, econoqjr dictates the recirculation of a large portion of the moisture laden air The water vapor obtained from the board or lath continually increases the vdIuqs of the humid air and therefore a portion must be continuously exhausted to maintain a balance in the kiln 7 The pick up capacity of the air above 260 F increases with air humidity, the degree of the air recirculation is therefore limited only ty the overall heat econoiry of the dryer 0 The rate of actual moisture pick up of the air is governed by the following factors during the constant rate period a The internal resistance of the core This is least important b The resistance of the paper covers Co The resistance of the air film at the paper inter-face This SGP 0021455 resistance is a function of the air velocity, the lower the velocity the greater the resistance# Although it has been shown that tbs resistance of the paper covering is higher than that of the core, it is evident that the air film has been the major controlling factor# Study of the operation of Grand Rapids kiln led to the following con clusions: I# The kiln is operating at maximum efficiency for the air flows now avail able and air velocities are low# 2 The controlling factor in the drying rate in at least the first three sections is the rate of moisture pick up by the air stream* This suggests an increase in air velocity in these three sections* 3* Any significant increase in kiln capacity for drying 3/8 lath, would require additional heat, that isj more boiler capacity* Note: The full capacity of the large boiler is used only on 3/8 lath* A small boiler is available on a 3tand by basis* A recent study of drying times at Grand Rapids against those at Akron and Sigurd indicates that if the former kiln could be made as efficient as the latter two, then Grand Rapids could make about 8,000,000 sq,, ft* more board and lath per year than they are now doing* A further study i now underway to determine if such a program is advisable* The contemplated changes are larger fans and ducts in the first three sections so as to increase the air velocity in the iHln and thereby the rate of pick up by the air* Air velocities in the first two sections according to Snell are about 1,000 to 1,20 ft* per minute, whereas, in the third section it is only about 300 to 700 fte per minute o If these figures are correct, it would appear to me that the third section would be the one to SGP 0021456 chance first since the low air velocity in this section causes a low drying rate. Before making a final decision on what to do at Grand Rapids, it seems to me that we ought to recheck the air velocities in the first three sections to be certain they are correct and then obtain the same data from Sigurd and Akron if it is not already availableo With these data at hand, the proper sise fans and ducts could be installed to assure optimum air velocities 0 The natter of the use of oil fired kilns is outside of try experience but we know that U.S.G, uses such kilns in nary of their plants with good results The design of the combustion chambers would no doubt have to be different and other precautionary measures taken such as neutralising the sulfurous acid produced by the combustion of the sulphur in the oils It seems to na that aqy kilns uo build in the future should be direct fired and whether gas or oil is used for fuel would be largely a matter of economics Based on the Snell reports, our kilns should possibly be operated as follows i 1. During the constant rate drying period a. Use as high temperatures as possible commensurate with other factors, in order to obtain rapid drying. / / b Recirculate as much air as possible in order to - / 1, Keep temperatures high 20 Assure rapid heat transfer to board 39 Keep heat losses downs Notes Amount of air recirculation, especially in first section, cannot be faapt so high as to cause starch spots on the board or blow-off of the paper 2. During the falling drying rate period, which at Grand Rapids is in SGP 0021457 about li3.6g of the length of the kiln*, it should be operated as follows* 1. Keep humidity as high as possible to pronote rapid high transfer and also keep the calcining temperature of the core as high as possible to reduce end and edge calcination 2 Gradually lower the tecperature to as near 200 F as possible to decrease rate of end and edge calcination*, SGP 0021458