US2136166A - Drying apparatus - Google Patents

Description

Nov. 8, 1938.

W. A. DARRAH DRYING APPARATUS Original Filed Oct. 15, 1933 2 Sheets-Sheet l ILI mW www W. A. DARRAH DRYING APPARATUS Nov. 8, 1938.

Original Filed Oct. 15, 1933 2 Sheets-Sheet 2 hun. WF.. SW 0H in -Hm 1- Patented Nov. 8, 1938 UNITED STATES PATENT OFFICE DRYING APPARATUS Original application October 15, 1930, Serial No. 488,852. Divided and this application April 25,

1935, Serial No. 18,133

' 12 Claims.

This application is a division of my application Serial No. 488,852. iiled October 15, i930, now matured into Patent No. 2,000,663, issued on May 7, 1935, which said application was in part a division of application Serial No. 361,920. illed May 10, 1929, for Process of drying materials and apparatus therefor.

The present| invention relates to equipment and appurtenances for the drying of materials and is particularly applicable to such objects as boards, sheets, plates and similar flattened shapes in which the ratio of surface to volume is very great. Some of the objects of this invention are to provide economical. simple and eilicient means for drying such articles as wallboard, insulation board, veneer, paper, cloth and similar materials without damage. Another object oi this invention is to carry on drying, economically, rapidly and without discoloration oi' the material being dried.

This invention provides simple, continuous and automatic means for accomplishing the above results. Other objects o! this invention will be apparent from a perusal oi the drawings, specilication and claims attached hereto.

Referring to the drawings:

Figure 1 shows a side elevation partly in section of one form of my device;

Figure 2 shows a vertical section of one form of my device; and

Figure 3 shows a pian view partly in section of one form of my device.

Referring to the drawings, I indicates a housing or container through which the material to be dried is conveyed. 2. 3 and 4 indicate several of a series of conveying rolls for moving ma terial through my device. In place of the rolls shown, I may use any desired form of conveyor mechanism such as a series oi' chains. or other commonly known mechanism. The rolls 2, 3 and 4 indicated may each be driven by a sprocket as indicated at 5, over which a continuous length o! chain 6 is passed, serving to keep the above rolls in continuous movement at uniform rate and in the same direction. These constructional details form no fundamental part of my invention and are merely illustrated to make clear one form of mechanism which I have found convenient to use.

Housing i is preferably insulated with heat retarding material 1, formed in a shell or cover around housing I. Reference numeral 8 indicates a board about to enter my device and 9 represents aboard leaving my device, the direction of travel being indicated by the arrow on the board. A heating means is shown diagrammatically at I0 connected to a fan or gas moving device II. The heater may consist oi an enclosed shell of steel or other material properly insulated and arranged to burn a desired i'uel such 6 as gas, oil, coke, coal, etc. I prefer to use gas or oil, although in many cases powdered coal or other iuels may be employed to advantage by reason of the lower cost. The exact details of constructionmi my heater are not of particular importance in this case, although I prefer an arrangement in which the fuel is supplied by means of a burner I2. Burner I2 mixes the fuel with air and delivers it to a combustion chamber I3. The products oi combustion leave chamber I3 by means of an outlet Il and enter mixing chamber I5 between fan and combustion chamber. The ian, which may be oi' any commercial type, is driven by a motor or equivalent indicated by I6 and serves to maintain a substantially continuous circulation of drying gases and products of combustion through the system. The gases leaving fan II pass through discharge duct Il and enter drier housing I, where they are distributed by al series of nozzle ducts I8; I9, 20 and 2l arranged substantially symmetrically. These distributing ducts serve to direct the flow of hot gases toward each end of the device, as shown by the arrows. The hot gases travel over the material being dried, preferably both above and below it, although if desired, on one side only. The circulating gases return through openings 22 and 23 to return ducts 2l and 25 which lead to duct 2B, where they are mixed with additional products of combustion and pass through the equipment again. Ii desired, vent stack 2l and 2B may be placed on the equipment preferably near the ends and provided with dampers i'or exhausting a portion of the moist gases.

In certain cases where the permissible temperatures are low, I may exhaust the gases entirely at the ends of the equipment instead of returning them to be reheated. In other cases, I may pass the circulating and drying gases in one direction only, these being details subject to practical operating conditions. 1

A tank or container 34 for liquid is arranged above the entrance portion of the drier and connected with a discharge duct 35 controlled by a valve 38 in such a manner as continuously to deliver liquid to the rotating distributor 31 which rubs or brushes the surface of the article 8 being dried. A similar brush or roll 38 supplied with 55 liquid from a reservoir 39 serves to apply liquid to the underside of the board.

This equipment is particularly applicable to those installations in which the drying process is carried to substantial completion within the drier and in which the operating temperatures exceed 212 F. In these cases the discoloring eifect of sulphur and other acids is particularly marked. This equipment is also of great importance in those cases in which the products of combustion are recirculated either all or in part, being reheated and delivered back to the drier.

Instead of applying the neutralizing liquid from tank 34 onto the sheets or boards being treated, I may neutralize the effect a! the acid by adding directly to the circulating gases a proper material for absorbing or combining with the acid gases. This result may be accomplished by spraying into the discharge of circulating fan ii or introducing into the intake l5 of said fan a finely powdered material such as soda ash, lime, etc. I prefer to use slaked lime, although quicklime will accomplish a similar result but apparently slightly less effectively. I have found that ammonia is particularly eiective, and I may also spray a solution of caustic soda, soda ash, ammonia, etc.

The material should be so added as to become distributed fairly uniformly through the circulating gases prior to the time that they come into Contact. with the Amaterial being treated.

According to tests which I have made, the addition of the neutralizing material to the gases does not result in wholly neutralizing the sulphur in the gases. It appears, however, that the materials added collect in part upon the sheets which are passing through the drier and in this manner serve to neutralize the acid fumes in the circulating gases at the time that the fumes approach the sheets. I have made tests of the gases taken from the system when the neutralizing eiect on the sheets was quite complete and have found that the gases still give ample evidence of a large acid content. In the case of adding a solid material in quantity, such as hydrated lime, soda ash, etc., a certain amount will collect and leave the drier with each sheet. This is somewhat troublesome in some cases, and in order to overcome this dlfilculty I have provided a series of Scrapers or brushes 39 to remove the excess of material. I may also add to one of the rolls on the discharge end of the drier, as for example 40, a cushion or scrubber made, for example, from astrong fabric. This will serve to remove the excess of neutralizing material from the under side of the sheet.

4| indicates a tank or container for holding the alkaline material, as for example liquefied ammonia. 42 represents a similar tank. Tanks 4I and 42 are connected by ducts 43 and 44 respectively to the air ducts entering the drier I. As an alternative arrangement, duct 26 is shown leading from tank 42 vto the intake of fan H.

Ducts 43 and 44 are provided with control valves 45 and 46,respectively. These valves make it possible to control the relative amounts of neutralizing material delivered into the ducts. Oil burner I2 is provided with a control valve 48 which may be automatically or manually operated as required. 'Valve 48 is mechanically interlocked with valves 45 and 46, so that the opening or closing of valve 4B, thus delivering more or less fuel to heater i0, will automatically deliver more or less neutralizing agent to the drier I. This mechanical interlocking is indicated diagrammatically by the cables 50 and 5i, although obviously any similar mechanical expedient may be employed. The lines indicating the dotted cable 52 and 53 show a familiar method of control in the alternative case in which the neutralizing material is added into the intake i5 of fan Il. Obviously, in most cases, eitherone or the other alternative would be employed, as it is unnecessary to use the two.

In Figure 2 I have shown an alternative arrangement in that hopper 54 delivers a supply of powdered material to the circulating gases. Hopper 54 may be controlled automatically as to volume if desired. Hopper 54 indicates a simple method of adding finely divided solids to the circulating gases, accomplishing substantially the same result as in the case of adding atomized liquids or gaseous ammonia.

In most cases I prefer to use gaseous ammonia, asI have found that very much smaller amounts are required to accomplish proper neutralization. Furthermore, the products which result from the neutralization when ammonia is used consist essentially of ammonium sulphate and sulphite. These substances appear under ordinary conditions as very fine powders whose quantity is so small as to be substantially unimportant. In many instances, therefore, when gaseous arnmonia is used to neutralize the acid conditions, it is unnecessary to remove the resultant solid, and in most cases a small air blast directed onto the sheet will readily remove all traces of this substance. By way of example, I have found that when drying gypsum Wallboard with this system, and burning in the neighborhood of from forty to sixty gallons of fuel oil per hour, it is advisable to use between fifteen and twenty pounds oi slaked lime per hour in case the oil contains around 1/2 of 1% of sulphur.

The same result may be obtained by using between one-half pound and one pound of liquid ammonia (gasied). While the liquid ammonia costs inherently much more than lime, the ease of handling it and applying it, the smaller amount of residue and the elimination of the necessity for cleaning, usually make the ammonia a more economical material.

Owing to the inherent expense per pound for ammonia, it ls advisable to apply only the amount required to neutralize the acid conditions resulting from the fuel. In some cases, the use of a large excess of ammonia causes a difticulty, in interfering with the sizing of the sheet or board, influencing color, etc. It is therefore highly desirable, for reasons of economy and also in order to obtain the desired quality of product, to proportion roughly the amount of neutralizing material added to the amount of sulphur delivered. Under any given set of conditions, this would mean that the amount of neutralizing ma.- terial should be varied roughly with the amount of fuel delivered to the heater, since with a constant grade of oil the amount of sulphur will vary with the amount of fuel burned.

I therefore have found it to great advantage to provide automatic means as shown for varying the amount of neutralizing material with the amount of oil.

It is to be understood that the ratio should be held substantially constant under any set of conditions but that a diil'erent ratio is required for diiferent sets of conditions. For example, if the ratio proves to be correct when one pound of ammonia iaadded to the circulating gases for each sixty gallons of oil burned,

' combustion are mixed with assuming the oil to contain one-half of 1% oi sulphur, then if the oil should contain 1% of sulphur the amount of ammonia required would le at least twice as great, or two pounds per each sixty gallons burned. In either case, the ratio should be held constant when burning varying amounts of oil having the same sulphur content.

I have further found that the amount of neutralizing material required is greater when operating at higher temperatures then when operating at lower temperatures, independent of the amount of sulphur contained in the oil or the amount of oil burned. As a theory, I account for this difference as being due to the much more rapid action of the sulphur acids at the higher temperatures.

Referring now to the operation of the equipment and process which I have invented, it should be understood that to dry satisfactorily boards and similar materials they should be subjected to a stream of warm gases. mum temperature to which the boards may be subjected varies of course with the nature of the material from which they are formed and various other factors. In the case of gypsum board, temperatures ranging from 300 to 400 F are not unusual, while in the case of fiber board, temperatures as high as F. are frequently permissible. It will be noted that at these temperatures the gases which convey the heat and remove the moisture are well above the dew point regardless of the amount of moisture carriedlby them.

I have found that drying may be accomplished satisfactorily when operating at the temperatures mentioned above with little regard to the amount of water carried by the circulating gases. In other words, an atmosphere consisting almost entirely of water vapor would form a very effective drying medium under these conditions. By returning the ilow of circulating gases and reheating them, I am able to reduce materially the fuel requirements for a given installation. Further, by adding the products of combustion directly to the circulating gases, I avoid the sc-called stack loss due to exhausting these products of combustion, which has hitherto been common practice. In general, therefore, I prefer to obtain my drying continuous stream composed principally of air and water vapor but containing also a few per cent of carbon dioxide.

It will be obvious that since the products of the circulating gases and caused to pass continually in contact with the surface of the material being dried, any substance within the products of combustion which may have a tendency to attack or combine with the material being dried is likely to cause damage.

It is, of course, well known that ordinary fuel oil contains appreciable amounts of sulphur and in some cases sulphuric acid as well as other materials such as chlorides, etc. Under the conditioris existing in the type of equipment here disclosed, any sulphur present will be oxidized to sulphur dioxide, which in contact with the moisture in the air and in the board will tend to form both sulphurous and sulphuric acid. Any sulphates present in the oil due to the neutralibation of the acids used in refining, or due to the natural impurities, will tend to form sulphur trioxide when the oil is burned, which of course,

500 to 600"y in contact with the moisture present, will normally form sulphuric acid.

I am emphasizing the effect of sulphur and using it to describe my invention because it is a common and marked condition and serves clearly to illustrate my invention. I do not, however, wish to be confined to means for neutralizing the effect of sulphur only, as under ,some conditions other materials may be equally troublesome.

I have found that when traces of sulphuric acid are present in the circulating gases, a portion of the acid is absorbed by the moisture on the surface of the material being dried, with the result that the surface of the material becomes decidedly acid.

As the drying progresses further, I have found that the water at the surface of the board is yevaporated while the sulphuric acid produced is evaporated to a very much lesser extent. This, of course, results in the formation of a fairly concentrated acid on the surface of the material being dried. If the material being dried contains large quantities of organic material such as cellulose, starch, dextrine or other carbohydrates, the addtionof heat will rapidly cause discoloration, which is usually objectionable, particularly in the case of drying wallboard, insulating board, gypsum board, etc.

I have found that a concentration of acid as little as three parts in one hundred will cause a dry sheet of paper, similar to that used in the case of gypsum wallboard, to become jet black when the temperature is raised to around 400 F. The same paper without the acid will withstand a temperature of 400 F. for a limited time `without serious change of color.

`When it is considered that most wallboards are used as building materials for finishing the interior of buildings an therefore, are subjected to rigid purchasing specifications as to color, uniformity, etc., it will be appreciated that the effect of acid discoloration may be so serious as to render the product unsalable.

In order to overcome this difficulty, I have `developed the equipment and process' here disfound that by adding to the surface of the board or sheet before it enters the drier a solution designed `to neutralize the effect of the acid without discoloring the surface of the board or sheet, it is possible to dry rapidly at high temperatures with direct products of combustion under conditions which would otherwise be impossible.

A wide range of materials may be employed, depending upon the conditions which must be met.

In the case of phuric acid on gypsum wallboard, I have found that by washing the surface in the manner disclosed inthe drawings with a solution of calcium hydrate, calcium carbonate, caustic soda, sodium silicate, sodium carbonate, borax, soap solution, etc., the desired result may be obtained. It should be understood that it is not necessary to add all of the materials specified above, as any one will be effective if added in the proper portions. the other hand, in some cases I have found that the addition of s'everal materials mixed together in solution will give desirable results, as for example by the prevention of the formation of crystals in the surface of the board, etc. f

It will be evident from the above that the essential feature is to add a material which will combine with and neutralize the acid condition, thus closed.

neutralizing the effect of sulmaintaining a concentration of acid so low that it will not discolor the organic materials present at the maximum temperatures that the board or sheet is subjected to during the drying process.

It is, of course, desirable to apply the material to both top and bottom surfaces of the board, and

any time prior to drying. 'I'hus the material may be added in the manufacture of the board or sheet or, in the case of the gypsum wallboard, in the manufacture of the paper covering which is on the board.

I have found in commercial practice that it is entirely impossible to produce a commercially clean or salable board with certain grades o! fuel, unless the surface of the board is previously treated in the manner here outlined:

Certain materials are more satisfactory than others as neutralizers. In the case of a gypsum wallboard a solution of lime in water is quite satisfactory, as the net result of the reaction is to produce calcium sulphate which is chemically similar to gypsum and which as produced in this process is a fine White powder not directly noticeable on the surface of the board.

Solutions of caustic soda when passed through a drier under the conditions here. speciied irequently form sodium carbonates which are in turn converted into sulphates by the action of the acid. Under some conditions. large amounts of sodium sulphate will form visible needles or crystals which are objectionable.

'I'he addition of small amounts of sodium silicate or other gelatinous or colloidal materials will frequently prevent the formation of noticeable crystals on the surface of the board or sheet, and I consider as one of the decided advantages of my invention the use of a mixture of colloidal materials with the neutralizing compound, so that the resultant product does not form -large or unsightly or otherwise objectionable crystals. Traces of soap solution will serve the same purpose under certain conditions, as will also the addition of small amounts of commercial borax.

It should be understood that there are many possible modiiications of my invention without departing from the spirit of this disclosure.

While I have referred primarily to fuel oil as a source of heat and also the source of discoloration, it should be understood that other fuels such as gas, coal or coke will frequently give similar My invention,A therefore, should not be confined to devices burning oil only.

While I prefer to practice my invention by returning the gases for further recirculation, as this method gives greater economy, it should be understood that in general the same conditions exist and the same results are obtained in case a stream of hot gases containing products of combustion is circulated through the drier and then discharged without employing the recirculating principle.

Obviously, many forms of driers may be employed, such as chain conveyors. oscillating mechanism, cars, etc., or the process may be employed in the so-called batch be particularly understood that I l these materials, and those skilled in the art will readily select additional materials which will be effective. In general, any compound which actively combines with the acid formed will be eflecculties here described.

In order to give a specific statement of conditions encountered, I wish to state that I have found that fuel oil containing 4% acid formed be determined in each individual case.

Having now fully described my invention. what I claim as new and wish to secure by Letters Patent is as follow 1. An apparatus for drying materials which consists of a housing, means for passing material neutralization is uniform.

3. An apparatus for drying materials which consists of a conveyor, a housing, a source of hot products of combustion, a device for circulating said products of combustion through' said hous- 4. An apparatus for drying material which consists of ahousing, means for passing material to be dried through said housing, a combustion chamber, a device for circulating gases in contact with said material, a duct connecting said combustion chamber with said circulating device, a duct connecting said housing with said circulating device, and automatically operable means for applying an acid-neutralizing substance directly to the surface of said material before being dried.

5. An apparatus for drying materials which consists of a housing, a support for said material being dried, a source tion, a devicefor circulating said products of combustion through Said housing in contact with said material, and means for neutralizing said products of combustion prior to circulation through said housing.

6. An apparatus for drying materials, consisting of a housing, a support within said housing for the material being dried, a combustion chamber, a burner device delivering fuel to said oombustion chamber, a control device for controlling the amount of fuel delivered, a container for acidneutralizing substance, means connecting said container for said acid-neutralizing substance with said drying apparatus, a device for controlling the amount of said neutralizing substance delivered, and means interlocking said fuel control device with the device controlling the delivery of said acid-neutralizing substance.

7. In a drier for articles consisting of or having a surface of cellulosic material the improvements which comprise a drier housing, means for passing combustion gases into contact with the material therein, means for conveying the articles through the housing, means for applying a neutralizing material to the surface of the articles prior to their exit from said housing, and means for removing some of said material from the surface of the articles after they are dry.

8. In a drier for articles consisting of or having a surface of cellulosic material the improvements which comprise a drier housing, means for passing combustion gases thereinto, means for conveying the articles through the housing, means of hot products of combus-4 for applying a neutralizing material to the surface of the articles prior to their admission into the drier, and means for removing some of said material from the surface of the articles after they are dry.

9. In a drier for flat boards of building material the improvements which comprise means for conveying the boards into, through and out of the drier, means for applying an acid-neutralizing material to the surface of the boards before they encounter heated combustion gases which are circulating through the drier, and means for brushing neutralized and excess neutralizing material from the boards just prior to their discharge from the drier.

l0. In a drier for articles consisting of or having a surface of cellulosic material, the improvement which comprises a drier housing, means for passing the articles therethrough, means for producing combustion gases, means for circulating said gases uniformly over the surface of said material, and means for constantly injecting into said circulating gases a sufficient flow of neutralizing agent to maintain neutralization of the acidic substances in said housing.

11. In a drier for articles consisting of or having a surface of cellulosic material the improvement which comprises a drier housing, means for passing the articles therethrough, a variable burner for producing combustion gases, means for circulating said gases uniformly over the .surface of said material, and means variable in accordance with the variation of said burner for mixing with said gases a material capable of neutralizing acids contained in the gases.

12. In a drier for articles consisting of or having a surface of cellulosic material the improvement which comprises a drier housing, means for passing the articles therethrough, a burner for producing combustion gases, means for circulating said gases in contact with said material, means for injecting into said gases an agent capable of neutralizing the acidic substances of said gases, and means for varying the injection of said neutralizing agent relative to the amount of acidic substances in said housing.

WILLIAM A. DARRAH.

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