US2266489A - Gypsum board, drying method - Google Patents

Description

Dec. 16, 1941. T. B. CIAMP GYPsUM BOARB, DRYING METHOD Filed July 22, '1958 3 sheets-sheet 1 H IT) l1N I/ENT'OR TH/oMAs n CAMP j Dc. 16, 1941. T B CAMP 2,266,489

GYPSUM BOARD, DRYING METHOD .Filed July 22, 1938 3 Sheets-Sheet 2 '75 @CWM /NVENTOR .THOMAS P. CAMP BY M2 i? ATTORNEY Det. 16, 1941-.

Filed July 22, 1938 T. B. CAMP GYPSUM BOARD, DRYING METHOD 3 Sheets-#Sheet 5 [N VEN TOR THOMAS F. CHMP A T'TORNEY Patented Dec. 16, 1941 2,266,489 GYPSUM BOARD, DRYTNG Ms'rnon Thomas P. Camp, Arlington Heights, lll., assignor to United States Gypsum Company, Chicago, Ill., a corporation of Illinois Application July 22, 1938, Serial No. 220,766

1 Claim.

'I'his invention relates to building boards and their drying methods, and has reference more specifically to those suitable for the drying of sheet materials, such as gypsum building boards.

In the manufacture of gypsum wall boards, it is customary to deposit a plastic core of gypsum composition between two paper cover sheets, the top or exposed cover sheet being of manilla paper and calendered to present an attractive, smooth, semi-glossy surface. This board is made in a continuous strip and after setting on a conveyor belt, it is cut to the desired length and passed continuously through a tunnel drier where most of the uncombined moisture is removed. In direct heat tunnel driers commonly used, heat is applied to the board by a rapidly moving stream of hot, gaseous products of combustion which carry a high percentage of moisture. In

indirectly heated kilns, the heated air also conthe kiln and fresh air admitted, to prevent the hot gases reaching the moisture saturation point. The board entering the kiln is-cool relative to the gases in the kiln which have a, temperature of 230250 F. and consist of steam diluted with permanent gases. When the boards enter the wet end of the drying kiln, the steam in the kiln condenses on the cover sheets of the board and causes them to become wet. As the water of condensation is hot, it quickly soaks into the paper cover sheets of the boards. As the boards progressA further into the kiln, the boards attain kiln temperature and the moisture in the paper cover sheets is again given off as steam. The effect of the liquid water in the calendered paper cover sheet, is to cause the finished, calendered surface of the cover sheet to become rough and lose its gloss, so that the benets of 'calendering are largely lost.

An object of the invention therefore, is to provide a preheating method in a board drying kiln so that the boards are preheated in hot, dry air to a temperature above 212 F., or above the dew point of the gases in the wet end of the kiln,` prior to contacting with said gases.

Another object of the invention is to provide a preheating method for boards or other sheet material, in which the material is preheated by the application of radiant heat in the presence of substantially dry air; also to improve drying kilns and methods in other respects hereinafter specified and claimed.

Reference is to be had to the accompanying Figs. 3, 4 and 5 are sectional elevations I through the drying kiln taken on lines 3-3, 4-4 and 5-5, respectively, of Fig. 1.

In the practice of my invention, a tunnel drying kiln I0 is formed by a floor IIj, side walls I2, and a roof I3. A series of roller conveyors I4 are arranged in several decks in the kiln to support a plurality of streams of boards I5. Each of the rollers I 4 is supported on a shaft I6 drivenby a sprocket wheel Il and a drive chain not shown. Removable doors I3 are provided within the side walls I2 to provide longitudinal air circulation chambers I3 and 20 at each side of the kiln. Gratings 2| connect the chambers I9 and 20 with subterranean ducts 22 which lead to a main circulating blower 23 driven by a motor 24, so that the main body of heated gases is kept rapidly circulating about the boards I5y as they move throughy the drying kiln. A discharge duct 25 leads into the side of the drying kiln from the blower 23, suitable deflectors 26 being provided in said duct'25 for the control of the air flow. A suction duct 21 leads to the blower 23 from a furnace 23 into which extend a plurality of tubular retorts 23. An oil burner 30, or other suitable fuel burning device, extends into each of the retorts 23 to heat the same and the surrounding gases entering the blower 23. The hot gaseous products of combustion enter the main gas stream in the kiln from the retorts 23. A damper 3| is provided in the duct 21' to by-pass any smoke formed in starting the furnace 28, into a stack. A motor driven blower 32 draws gases through duct 33 from the drying kiln through the kiln roof I3, and deliversv said gases through duct 34 and branch ducts 35 to each retort to aid in the combustion of the fuel. A motor driven blower 36 supplies air for the atomization of the fuel in burners 30.

The salient feature of my invention comprises means provided at the inlet end of the kiln for preheating the incoming boards in substantially dry air 'so as to prevent condensation of moisture on the surface of said boards'. To accom-- plish this, flat U-shaped heater units or ducts 33 are provided, one between each pair of conveyor roller sections I4 so that one heater section 38 lies between each pair of board streams I5. Heated'gases are passed through a by-pass the heater units which serve to preheat said boards and prevent condensation on the board surfaces as they enter -the main part of the kiln. As the inlet end of the kiln is under less than atmospheric pressure, `some leakage of air into the kiln takes place as the boards enter the kiln. This fresh leakage air passes about the heater units 38 and boards I5 and is heated by heater units 38. As this fresh leakage air has a low relative humidity when heated, it picks up any moisture driven olf from said boards by said heater units 38 and thus prevents condensation of moisture on the surface of the boards until said boards are preheated to a temperature above 212 F. After passing the heater units 38, this leakage air is drawn downwardly through gratings 45, through duct 46 and into a motor driven blower 41 to a stack 48 which discharges this leakage air into the atmosphere.

In operation, the boards I5 pass on roller conveyors I4 through the drying kiln Ill in a series of superimposed decks. The main current of drying gases pass through heating furnace 28, blower 23, duct 25, kiln I0, gratings 2I, and duct 22 back to furnace 28. The furnace 28 is heated by retorts 29 into which are directed fuel burners 38, the products of combustion entering the main gas stream in the kiln. Auxiliary air for combustion is supplied to retorts 29 through blower 32, and ducts 34 and 35. Flat preheater units 38 extend between the decks of boards at the inlet end of the kiln and receive hot gases internally from duct 25 through duct 40, said gases passing out the open legs 43 of the heater units 38 into the main gas stream of the kiln. Leakage air entering the kiln around the entering boards, is heated by passage around the heater units 38 and is withdrawn from the kiln through gratings 45, duct 46 and blower 4l, being discharged to a stack 48.

The invention has two principal advantages, (l) it prevents staining and roughening of the board surface by condensation of the water in the kiln gases on the board surfaces, and (2) a higher percentage of water vapor can be carried in the kiln gases without spoiling the board surface, thus affording marked economies in drying costs.

I would state in conclusion that while the illustrated example constitutes a practical embodiment of my invention, I do not wish to limit myself precisely to these details, since manifestly the same may be considerably varied without departing from the spirit of the invention as dened in the appended claim.

Having thus described my invention, I claim as new and desire to secure by Letters Patent:

The method of drying building boards having a gypsum composition core and a paper sheet, which comprises passing said boards successively through a preheating zone containing high temperature gases containing a low percentage of water vapor, and then through a drying zone containing high temperature gases containing a high percentage of water vapor.

THOMAS P. CAlVIP.

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