US3377056A - Drying apparatus - Google Patents

Description

April 9, 1968 i. BOYE DRYING APPARATUS Filed Sept. 20, 1966 ATT YS.

United States Patent iiice 3,377,056 Patented Apr. 9, 1968 3,377,056 DRYING APPARATUS Ingar Boye, Vaxjo, Sweden, assignor to Aktiebolaget Svenska Flaktfabriken, Stockholm, Sweden Filed Sept. 20, 1966, Ser. No. 580,699

Claims priority, application Sweden, Sept. 23, 1965,

2 Claims. (Cl. 263-3) ABSTRACT OF THE DISCLSURE A dryer for drying moving webs which are supported on a rotating heated cylinder comprising a hood having a plenum chamber therein adapted to direct drying medium against the web on the cylinder, the drying medium .comprising in part air returned from the drying cylinder through a fan, and in part ue gases from burners mounted in the plenum chamber.

The present invention relates to a dryer comprising a plenum chamber having at least one fan and at least one wall with means such as nozzles or apertures for directing gaseous medium onto the material to be dried.

The invention is intended primarily for use in drying moving webs or sheets such as paper webs or sheets of wall board. Such 'drying is conventionally carried out by blowing a hot gaseous medium, usually air, against the material to be dried from a plenum chamber having a wall, either perforated or equipped with nozzles, situated near the path of travel of the material. Before contacting the material toV be dried, the air is conventionally heated in an indirect heat exchanger having steam as the source of heat. Since the drying is more 'rapid if the temperature of the drying medium is raised, a higher temperature is striven 'for because in this way the dryer may be made smaller for a driven throughput of material.

When trying to raise the temperature of the drying medium by steam, the steam pressure must be increased above the saturation pressure of water vapor and soon reaches the point, at which, for reasons related to me- I chanical strength, the heat exchanger must be made very solid, and therefore heavy and expensive. At a temperature of 200 C., the saturation pressure is about 16 atmospheres for `water vapor, and with increase in teniperature the saturation pressure rises very rapidly. It is therefore scarcely worthwhile to adopt the use of steam for heat exchange, if high temperatures are desired. Aat 16 atmospheres, a drying-medium temperature of 160 to f 170 C. can be obtained without disproportionately large heat exchangers. o

To reach higher drying-medium temperatures, i.e. 180 C. or more, attempts have been made to use flue gases as a heat source instead of steam. In such case, the heatexchanger must be made so much larger, because the iilm coefficients of heat transfer between ue gases and the heat-exchange surface are considerably less than between condensing steam and the heat-exchange surface. In addition to this, problems arise in connection with the accumulation of soot on the heat-exchange surface which is in contact with the ue gas. Because of the large space requirements of such heat-exchangers, the heat-exchanger must be separated from the rest of the dryer and connected with the latter via ducts, which unfortunately are very bulky.

To eliminate the problem of soot accumulation on heatexchange surfaces when high drying-medium temperatures are desired, the heat exchanger may advantageously be replaced by a combustion chamber connected with a mixing chamber. The heating of the drying medium is effected through direct heat exchange with flue gases, which are produced in the combustion chamber and then mixed with the drying medium in the mixing chamber, to be taken then in heat-insulated ducts to the dryer and its plenum chamber. In this way, corrosion and accumulation of soot on the surfaces of a heat exchanger is avoided.

In summary, it may be stated that conventional constructions for dryers using drying-medium temperatures in excess of C. take up much space, because the space requirements of the heat exchangers or combustion chambers require these parts to be placed apart from the plenum chamber which is adjacent the material to be dried, and must be connected with the plenum chamber by means of bulky ducts.

In contra-distinction of this, the invention provides a compact dryer for temperatures considerably in excess of 180 C. A single space, i.e. the plenum chamber, serves both as a combustion chamber and mixing chamber. The bulky ducts of conventional dryers are eliminated, and the dryer does not require more space than a corresponding steam-heated dryer intended for temperature of up to 180 C. All the parts included in the dryer can be built together as a unit, so that the installation on the users premises can be restricted to the connection of pipes and wires for fuel and electricity. Moving the dryer between different machines on the users premises is simple. A conversion of an existing steam-heated dryer for maximal drying-medium temperature of about 180 C. to considerably higher temperatures can easily be effected in a short time and at low cost, using the same fans if the dryer is made in accordance with the present invention.

The invention will now be described in greater detail with reference to the accompanying drawings, which shows apparatus for drying paper on Yankee cylinder, wherein:

FIG. 1 is a cross section axis;

FIG. 2 is an enlarged view showing the perforated wall of the plenum chamber, as encircled in FIG. 1; and

FIG. 3 is a'view similar to FIG. 2 showing nozzles in the wall of the plenum chamber.

In the drawing, a rotating cast-iron cylinder 1, a socalled Yankee cylinder, is steam heated from the inside in the drying part of a paper machine. A web 2, for example paper, is conducted over the cylinder 1 and the cylindrical mantle surface of the cylinder is partly encompassed by a hood 3 enclosing a plenum chamber 4 and a suction chamber 5. Partition walls 6 separate the plenum chambers 4 and the suction chambers 5 and mount fans 7 which suck drying medium from the suction chambers 5 and blow it into the plenum chambers 4. One wall 8 of the plenum chamber is perforated with a number of vsmall holes 9 to direct drying medium from the chamber onto the web. The fans 7 are driven by motors 10 located outside the hood 3. In order to separate solid particles from the drying medium blown bythe fans 7, iiltersll are mounted in front of the fan inlets. The drying medium directed against the web 2 is returned to the suction chamber 5 by return pipes 12, the drying medium passing across the web 2 removing moisture therefrom to dry the same.

Fresh air is admitted to the suction chambers 5 through ducts 13, the chambers 5 being at a slight under-pressure in relation to the surroundings. The fresh air is mixed with the moist drying medium coming from the return pipes 12 and the mixture is sucked into the fans 7 through the lters 11. In order to restore the balance in the amounts of drying medium circulated, some of the mixture is blown otf from the plenum chambers 4 through ducts 14.

In order to heat the drying medium flowing from the fans via the plenum chambers and the holes 9 to the paper web, burners 15 are provided in the walls of the plenum at right angles to the cylinder chambers. Fuel is mixedrwith combustion air and transported to the burners 15 through pipes 16 and the flow is controlled by valves 20. Conventional burner accouterments are provided at 21 and 22. rlhe burners 15 open into the plenum chambers 4 and the mixture of fuel and combustion air burns with an open flame. 'l he iames are enclosed by casings 17 having an inner tireproof lining. The casings protect the walls of the plenum chambers 4 from radiated heat from the flames and direct the ow of combustion gases in a proper direction for the rapid mixing with the rest of the drying medium. At the exit from the casings 17, the combustion is completed and the velocity of the combustion gases may be as high as 500 m./sec. In this way, the relationship between the capacity of the burners, calculated as ow of heat, and their size is very favorable.

The casings 17 direct the combustion gases in a cone with a wide cone angle. The mixing with the drying medium is thus efficiently accomplished. With this type of burner, over-temperatures as small as 5 C. can be obtained 40 cm. from a burner. Preferably, the burners 15 and the fans 7 are placed in relation to each other so that upon exit from the casings 17, the combustion gas collides from the side with the stream of drying medium from the adjacent fan. It is preferable to insure intersection of the paths of the flue gases and the drying medium so that the axes of symmetry of the two streams lie essentially in the same plane, and in this plane the axes form an angle of between 25 and 160 with each other. Since the gas in both streams has a high absolute speed at the point f intersection, their relative velocity will be great which favors homogeneous mixing.

The function of the filters 11 is to catch the fibers from the web which are carried by the drying medium back to the return pipe 12. If the fibers were allowed to pass through the fans, some of them will be burned or charred in the plenum chambers, and on being blown against the web, would discolor the same. The fibers must also in some way be removed from the filter, and this is done in a known manner, i.e. in machines intended for continuous operation preferably through vacuuming with a device according to the copending U.S. patent application of Wallin, Ser. No. 544,650 filed Mar. 30, 1966. In the drawing, however, only doors 18 are provided in the hood, which during operation may be opened to receive a vacuum-cleaner nozzle which sucks off the fibers. In the suction chambers there is, during operation, a slight under-pressure, so that upon opening the doors 18 cold air from outside the hood goes into the hood and prevents the hot drying medium inside the hood from flowing against the person who opens the doors. Manual vacuuming of the fibers from the filters 11 is therefore easy to carry out during operation of the dryer.

A controller 19 is provided in conjunction with the power supply to the fans 7 to actuate valve means 20 in the fuel supply line 16 whereby the burner 15 is extinguished when the fans 7 stop.

The invention is, of course, not restricted to the form of design illustrated and described, but can be varied considerably within the scope of the appended claims.

Thus, for example, the burners may operate with liquid fuels such as lighter mineral oils or solid fuel particles such as coal dust. The casing 17, which encloses the ame may be dispensed with if measures are adopted to protect the surrounding parts from over-temperature due to heat radiation. The fans may be of axial or semi-axial type. The combustion air may be mixed wih the fuel either in advance of the burner as shown, or may be mixed directly in the burner.

I claim:

1. An apparatus for drying material comprising a plenum chamber having a wall confronting the material and means in said wall for directing drying medium from said chamber onto the material, fan means operable to discharge gaseous medium into said chamber, suction means leading from said material to said fan means to cycle gaseous medium from said material through said fan and said plenum chamber and back to said material, and means to heat said cycling medium, the improvement wherein said heating means comprises a plurality of burners mounted in said plenum chamber, and means to supply combustion fuel to said burners, said burners being mounted to direct hot flue gases into said plenum chamber to mix with the cycling medium therein whereby said drying medium includes flue gases, and said fan means comprises separate fan means for each of said burners.

2. In apparatus for drying endless web material cornprising a heated rotating cylindrical surface supporting said web, a hood surrounding said surface and mounting a plenum chamber having a wall confronting the material and means in said wall for directing drying medium from said chamber onto the material, fan means mounted on said hood operable to discharge gaseous medium into said chamber, suction means mounted on said hood leading from said material to said fan means to cycle gaseous medium from said material through said fan and said plenum chamber and back to said material, at least one burner mounted in said plenum chamber to heat said cycling medium, and means to supply combusion fuel to said burner, said burner being mounted to direct hot flue gases into said plenum chamber to mix with the cycling medium therein whereby said drying medium includes ue gases.

References Cited UNITED STATES PATENTS 2,218,282 10/1940 Downs 34-122 X 2,268,986 1/1942 Hess et al 263-3 X 2,268,988 1/1942 Hess et al 34-122 X 2,297,314 9/1942 Offen 263-3 2,471,802 5/1949 Walter et al. 34-160 2,473,629 6/ 1949 Andrews 263-3 2,604,313 7/1952 Grantham 263-33 2,731,732 1/1956 Harris et al 263-3 X 2,985,210 5/1961 Magnuson 34-122 X FREDERICK L. MATTESON, I R., Primary Examiner.

A. D. HERRMANN, Assistant Examiner.

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