US2981528A - Drying system - Google Patents

Description

April 25, 1961 c, cu 2,981,528

DRYING SYSTEM Filed Aug. 14, 1956 V IN VENTOR 7 CHARLES R- CULP i1 Mam ATTORNEY DRYING SYSTEM Charles R. Culp, Landisville, Pa., assignor to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Filed Aug. 14, 1956, Ser. No. 603,888

1 Claim. (Cl. 263-8) This invention relates to adrying system. The invention will be described in conjunction with the drying of porous fibrous insulation products such as a mineral wool board used as an acoustical insulation material.

In the manufacture of insulation products, the board is generally formed by mixing the fibers in a dilute water slurry with a suitable binder such as starch, gypsum, or the like. The slurry is deposited onto a forming wire, and the slurry is formed into a board upon removal of a portion of the water, depositing the fibers as a mat. The wet mat may be mechanically fissured to provide an attractive appearance in the finished product and to enhance its sound absorbing efficiency. The mat so formed is delivered to a drier where water is removed, and the binder may be activated if it is of the heat-convertible or activatable type. As the mat is delivered to the drier, it may contain in the order of 30 to 50 pounds of water per cubic foot. The quantity of water will vary over wide limits, depending upon the nature of the mat-forming equipment, the allowable suction or mechanical pressure applied to the .mat to express water from it, and other variable factors. With a fissured mineral wool acoustical board, the problem is not so much a matter of moisture escape from the mat, for the mat is open and porous and a myriad of paths for the free escape of moisture are presented. There is evidence, however,

showing that there is difiiculty in getting the heat to penetrate to the center of the board through the dried outer surfaces which act as thermal insulators. Drying may take 180 minutes or more for a board one inch thick and about four feet square. The board should be substantially fully dry when it leaves the drier. Only a few percent of moisture can be tolerated, normally.

One of the most expensive elements of an acoustical board-making plant, therefore, both from the capital in vestment required and the cost of product production, is the drier equipment. If the drier is inadequate, the speed of production must be slowed down to insure proper drying of the mats for subsequent cutting, sanding, and other finishing operations. Multiple stage, hot air type continuous driers with a plurality of decks for supporting the mats or boards during movement through the drier stages are customarily used, and gas or oil firing is usually employed.

With driers of this type, the heated air is directed into ,the individual drier zones and the spent heated air is generally returned to the heater for reheating and recirculation. As the board moves from stage to stage, the temperature of the heated air generally is decreased, for there is' the. danger of product degradation because of excessively high temperatures. In mineral fiber products, this problem is principally one of discoloration and charring which results from overheating.

An optimum temperature curve can be developed for drying mineral fiber acoustical boards, starting at the entrance of the boards into the heating chamber and continuing until the desired dried condition is attained at exit of the boards from the drier. This curve will depend upon a number of factors, such as the nature of the fibers, the type of binder used, the density of the product, the structure of the product, e.g. porous, fissured,

, etc., the relative humidity in the drier, the speed of move= ment of the boards through the drier, the'speed of the heated air used for drying, and other variable factors. With conventional forced air convection type driers, such ideal temperature conditions are not achieved, and as a consequence most efficient drying is not effected.

The principal object of this invention is to provide a forced air convection type drier which may be better controlled to attain a desired temperature distribution within the drier, thus to provide for more efiicient drying.

Another object of the invention is to provide a forced air convection type drier with an in-the-air-stream burner arrangement for boosting the temperature of heated air delivered by a primary heated air source, thus more eiiectively to control the air temperature within the drier.

Other objects of the invention will be apparent from consideration of the following description of an embodiment of the invention which will be described in conjunction with the attached drawing, in which:

Figure 1 is a diagrammatic view of a drier system embodying the invention, and

Figure 2 is a diagrammatic perspective view of an in the-stream burner arrangement, showing its relationship to the conveyor for the material to be dried.

For purposes of illustration, a drying unit has been chosen in which air, heated by a gas fired burner, is delivered from the center of the drying unit and is returned for reheating at the ends of the unit, for this represents an especially difiicult problem in temperature control in driers, which problem may be solved successfully by the present invention. v

In the system shown in Figure 1, two zones constituting anentrance stage of a drier for a mineral fiber acoustical board are shown. There usually will be three drying stages, an entrance stage, an intermediate stage, and an exit stage, each separately heated and each separately controlled. In drying mineral fiber acoustical board and other products which contain large amounts of water, it is possible to heat the entrance stage to a much higher temperature than the exit stage, for the board is essentially dry when it reaches the end of that stage, and discoloration and charring of the product will result if too high a temperature is maintained in that stage. In the entrance stage, air at temperatures well above the dis coloration or actual charring or ignition points of the product may be applied without danger, for the wet condition of the product and the heat absorption resulting from the conversion of the water into steam as it leaves the product prevent the product from attaining too high a temperature.

Studies have indicated that input air at a temperature of 675 F. to 700 F. may be applied to the entrance stage of the drier but should be r duced to about 425 F.

to 450 F. at the exit end of the drier. It will be clear, of course, that the board temperature will actually be much lower, being below 212 F. for most of the time, until almost completely dry, certainly in the interior of the board below the outer skin surface. Optimum drying temperatures can be determined experimentally. The present invention makes it possible to more nearly approach the ideal drying temperature conditions than is possible with conventional drying equipment.

Referring again to Figure 1, the drier stage shown is divided into two zones designated in the drawing as zone 1A and zone 1B. A partition 2 separates the two zones. Conventional gas heaters 3 and 4 are positioned to heat air for delivery to zones 1A and 113, respectively. The heaters 3 and 4 may be of conventional construction ,including gas flame burners with suitable pilot and flame controls and the usual safety devices, with a forced air delivery system and suitable return and make-up air conduits.

An open slat metal conveyor 5 is mounted to convey workpieces 6 through the heating chamber. The chamber is formed by a housing 7, suitably ported at 8 and 9 to permit unobstructed movement of the workpieces through the chamber while avoiding excess dissipation of heated air outwardly through the ports or uncontrolled intake of cold air through the ports.

In the embodiment illustrated, the heated air from-the heaters 3 and 4 is delivered in streams A from the center of each of the heaters, moves in the direction of the arrows, and is returned as streams B to the heaters for reheating and recirculation, together with make-up air supplied to the burners for proper combustion and additional make-up air, if necessary. Operating under normal conditions, the average temperature of the heated air as it leaves the heaters 3 and 4 in streams A may be in the order of 700 F., and the return air as it enters the heaters from streams B may be in the order of 400 F., assuming all of the heat is supplied by the heaters 3 and 4. From this it will be clear that the workpieces 6 as they enter zone 1A are not subjected to as high a temperature as is ideal for most eflicient drying, for air at the highest permissible temperature should be directed onto the workpiece as it enters the initial heating zone. With a center air delivery type heating unit, this is virtually impossible.

According to the present invention, there is mounted in the forward portion of zone 1A a pair of in-the-stream gas burners 10 and 11, one disposed above the upper run of conveyor 5 and one disposed below it as shown in Figure 1. Similar burners 12 and 13 are positioned in the air stream moving from the heater 3 toward the partition 2.

These are ribbon flame burners as indicated diagrammatically in Figure 2. The burners are of a length adequate to properly heat the air in the chamber in the immediate vicinity of the workpieces 6 which are moved through the drier. With workpieces about 48" x 48", the burners will be about 4 long. Each of the units 10, 11, 12, and 13 may have a capacity to deliver a B.t.u. output of 565,000 per hour, providing a total output in zone 1A of 2,260,000 B.t.u.s. Each of the units will be provided with the conventional control and safety equipment, air and gas mixers, butterfly control valves, etc. One blower may be provided to supply air for combustion for each pair of units '10 and 11 and 12 and 13. Flame failure and automatic ignition units will be provided. These have not been included in Figure 2 because they are conventional and form no part of the present invention. Any suitable source of heat having'the required high thermal output may be used and will be effective for increasing the temperature of the heated air delivered by the heaters 3 and 4. Gas fired ribbon heaters are ideally suited for the purpose. Conventional radiant heaters are not acceptable where the product must not be substanpossess the required thermal output and are otherwise not suited for use in a forced convection drying system.

It will be noted that the flames from the burners 10 and 11 will be directed to the left as viewed in Figure 1, concurrent with the direction of travel of heated air from the heater 3, as indicated by the arrows. The flames from burners 12 and 13 will be directed to the right and thus will be in the direction of the heated air flow in the portion of the drier zone in which these burners are mounted. The burners may be mounted about 4" above and below the articles 6 to be dried. The flames may be individually adjusted to obtain the desired temperature conditions in the drier.

With the arrangement shown in Figure 1, the heated air from heater 3 which may be delivered at a velocity of about 1200 feet per minute and at an initial temperature of 700 F. loses some of its heat as it moves over the relatively colder and wet workpieces 6. When this air passes through the heating zone of the in-the-stream burners 10 and 11, each having a heating capacity of about 565,000 B.t.u.s, the temperature of the air will be elevated; and thus the air delivered to the forward end of zone 1A will be hotter than otherwise would be possible without the burners 10 and 11. Similar control of heat in the portion of zone 1A near the partition 2 is effected by the in-the- stream burners 12 and 13.

The actual temperature of the air closely adjacent to the surface of a workpiece 6 moving through the drier may be continuously measured by suitable thermocouples or other heat responsive devices. While four in-thestream burners have ben shown in zone 1A in the diagrammatic view, Figure l, the number of burners used will depend on the desired heat input and control for any particular drier. It will be clear that for some services it may be desirable to install a sufiicient number of the in-the-stream burners in one or more of the heating zones to obtain all of the heating effect from these burners. In this system, a forced air circulating fan may be used to replace the heater unit (gas heater 3 or 4, for instance) for the zone or, of course, the gas heater may be shut off and the air circulating portion of the unit continued in operation. Such an arrangement will provide the ultimate in heat control, for each of the in-the-stream burners may be individually controlled; and if the zone includes a number of these burners in relatively closely spaced relationship, the heat distribution throughout the length of the zone may be relatively close ly controlled to approach the ideal drying curve for most efficient drying of the material moving through the drier.

Where close control is not so important, fewer of the in-the-stream burners may be used. This has been illustrated diagrammatically in zone 13 of Figure l where a pair of in-the-stream burners 14 and 15 are provided in the forward end, near partition 2 to increase the temperature there and elfect better temperature control, avoiding too great a drop in temperature from the end of zone 1A to the beginning of zone 1B. No further burners are provided in zone 1B.

While the invention has been described in conjunction with the drying of mineral fiber acoustical boards, the invention is of wider application. It is particularly useful in the drying of water-laden fibrous boards of an open, porous nature when dried. The drying of cellulosic fiber insulation board presents similar problems to those discussed above, and in addition care must be exercised to avoid fires in the driers resulting from uncontrolled temperature increase in the board, above its ignition temperature. The invention provides a system which is well suited for the drying of such insulation board.

I claim:

A forced convection drying system comprising a drying chamber including means defining a drying zone through which articles to be dried are -passed, an open, air pervious conveyor for carrying material to be dried tially'discoloredduring-drying. Also, such heatersdonot through said drying zone, and means for establishing a 5 temperature distribution in said drying zone extending along the path of movement of said material through said drying zone comprising a primary source of gaseous heating medium remote from said drying zone and including means for supplying heat to air at said source and means for force circulating said gaseous heating medium from said remote source through said drying zone above and below said conveyor for engagement with both sides of said material during its movement through said drying zone, a plurality of open flame convection heaters positioned within said drying chamber directly in said drying zone and adjacent to said path of movement of said material through said zone and in the stream of gaseous heating medium from said primary heater,-

with a plurality of said burners being positioned above said conveyor and a plurality of said burners being positioned below said conveyor and with the burners positioned to deliver the open flames therefrom in a direction concurrent with the direction of movement of the stream of gaseous heating medium in the drying zone for supplying heat to said circulating gaseous heating medium from said primary source at a plurality of locations within said chamber both above and below said conveyor to establish said temperature distribution in said chamber.

References Cited in the file of this patent UNITED STATES PATENTS

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