US2202143A - Dry kiln and in the art of kiln drying - Google Patents

Description

May 28, 1940. J, F, OB

DRY KILN AND IN THE ART OF KILN DRYING 7 Sheets-Sheet 1 m m hw Filed June 3, 1935 R INVENTOR.

y 1940. J. F. COBB DRY K ILN AND IN THE ART OF KILN DRYING Filed Jime s, 19:55 7 Sheets-S het 2 Hm rlhn W INVm May 28, 1940. F, B 2,202,143

DRY KILN AND IN THE ART OF KILN DRYING Filed June 5, 1955 7 Sheet-Sheet a Q 0 Q I\ O a r F! p Ill .5533:

. I V Q INVENTOR.

May 28, 1940. J COBB 2,202,143

DRY KILN AND IN THE ART OF KILN DRYING Filed June 3, 1935 .7 SheetsSheet 4 INVENTOR.

May 28, 1940. J. F. COBB 1m KILN AND I: THE ART OF KILN DRYING 7 Sheets-Sheet 5 Filed June 3, 1935 '7 Sheets-Sheet 6 Filed June 3, 1935 May 28, 1940.

MN I Nu m m V m May 28, 1940. J, F, OBB 2,202,143

DRY KILN AND IN THE ART OF KILN DRYING Filed June 5, 1935 7 Sheets-Sheet 7 Patented May 28, 1940 DRY KILN AND in "ins and er nun parme James Forrest Cobb, Portland, Greg. Application June 3, 1935, Serial Ne. 24,655

10 Claims.

This invention relates to cross circulating drying kilns of the character of that shown in my co-pending application, Serial No, 492,026, filed October 29, 1930, of which this is a continuation in part and it has for its object to provide a kiln presenting marked features of advantage and improvement over the kilns oi the prior art and their methods of control and operation as will be more particularly pointed out in the detailed de- -scription which follows and succinctly defined in the appended claims.

In the accompanying drawings where my invention is illustrated in preferred form of embodiment:

Figure I is a longitudinal vertical section of one form of embodiment o! my invention showing my heating system" and-air circulating system within the kiln, with the fan ducts removed.

Figure II is a plan view of Figure I, but showing the fan ducts in place and heating systems within the kiln.

Figure III is a transverse vertical section at IIIIII of Figure I showing the fan ducts and heating systems in place.

Figure IV is a transverse vertical section at the line IV-HV of Figure I.

Figure V illustrates a. modified form of Ian ducts having baiiies located within the fan ducts instead of on the corners of the fan duct wall.

Figure VI is a plan view of a modified form of embodiment of my invention furnished with right and left hand fans opposed in pairs, and showing my control system diagrammatically.

Figure VII is a longitudinal vertical section at VII-VII Figure VI showing a modified form of my kiln illustrating a heating system at each end and fans located above the stock-loads with fan ducts removed.

Figure VIII'is a transverse longitudinal section of a part of my kiln illustrating a modified form of heating system in operative relationship to the fans, with fan ducts removed.

Figure IX is a view of another modification of my'invention in respect to the heating system in relation to the fans with fan ducts removed.

Figure X isa view' of my kiln illustrating another modified form of heating system with fan ducts removed.

Figure XI illustrates diagrammatically a preferred form of embodiment of my heating systems.

Figure XII is a plan view 01' a -modified form of my invention showing fans above the stock-loads and in one corner of the kiln and a modified form of heating system.

Figure XIII is a vertical longitudinal section to it.

(ill. 34-46) taken through the tan ducts illustrated in Figure ml with the heating system thereof superimposed thereon, in dotted lines and diagrammati-' caily illustrating my control system.

Figure XIV illustrates a transverse vertical section XIV2QV of Figures m and m.

Figure XV illustrates a transverse vertical sec= tion at XV-XV oi Figure VI and VH.

Figure XVI is a detailed illustration of a dia= phragm valve detached. Figure XVII is a crosssection of one of my spray nipples illustrated in Figures I, m and IV. Figure XVIII is a plan viewoi nay-min illustrating a modified form 02 air circulation system with larger capacity fans at the ends thereoi'.

Figure m is a longitudinal vertical section along the lines HX-W 01' Figure III Figure m is a cross section along thelines XX-XX of Figures m and m. 1

Referring to the numerals on the drawings, in each figure thereof where they appear, i' indicates opposite side walls of the M111, 2 the floor, 3 the roof, and i the end walls defining a closed chamber and constituting the outside walls of the kiln. Said wallsmay be of any suitable and preferred material and construction, and are combined in any manner preferred so as to confine within the closed chamber or m the gaseous circulatory medium with the heat that is imparted The end walls, 4, or at least one of them. are preferably made to include some form of door by which material to be dried may be conveniently introduced into the kiln and withdrawn from it.

Extending lengthwise through the kiln are tracks .or rails i which are supported on the ground as in Figure XV or at suitable intervals within the kiln at desired elevation above the fioor 2 as by cross-beams 6. The tracks are designed to support and carry wheeled trucks I. which may be of any usual form preferred, and which are united in pairs, preferably by crossbeams or bunks 8 on which the lumber or other material to be dried is piled in the usual manner.

Suspended from the cross-beams 6 or extended above the beams 31 are suitable hangers or sup ports which are preferred as means of support for the pipe. Separating the heating pipes are the pipe spacers which hold the pipes separated so as to permit a free air flow across them.

The heating systems preferably consist of return pipe systems, as shown for example in Figures I, II, VI and VII, the legs of each 01' which are joined in units at top and bottom by the headers l3 and It and/or i4 and it, which aresupplied with steam from a source of supply not illustrated, as by means of supply pipes I! which are controllable as by automatic diaphragm valves ll or 29 or hand controlvalves.

Each of the heating systems A and B consist of return pipes having two legs, one above the other, the upper one 2| leading from the top or supply header l3, and the lower one 22 leading into the bottom or drain header IS vertically the kiln by L's 21 and a nipple 28. The L's 21 and the nipples 28 or other suitable connections are used to permit the outer pipes 2| and 22 of the heating system A to lie in substantial parallelism and uniform proximity on the outside of the pipes 2| and 22 in the inner system B on both legs of each system.

The heating systems, whether one or more, are located on one or more sides of the kiln as illustrated, or at suitable intervals, in case of a multiple track kiln which is contemplated by my invention. The heating systems are installed so that steam condensation will drain by gravity, as for example, from the top headers J3 and Il to the bottom headers l5 and I6, through the return fittings at the opposite end of the heating system, designated return joint end for convenience.

It is well known in the art that uniform distribution of heat and humidity throughout the length of the kiln is essential in a charge kiln and that one of the functions of circulation is to distribute uniformly heat and humidity conditions throughout the length of the kiln. I accomplish this by the arrangement and control of my fan, heating and humidifying systems.

In the past, heating systems in internal fan kilns have not been designed with special consideration of the rapid loss in heating capacity of the steam as it travels within the pipes from one end of the heating system to the other. Consequently, the end of the kiln at which the steam enters has been the hottest end, which is an objectionabie condition.

The use of temperature and humidity controls having bulbs, one of which is indicated at 15 in Figure I, are well known and commonly used in kilns. The bulbs are usually located at the same end of the kiln at which the steam enters. The heating system aggravates the tendency for a higher temperature at the end where the steam enters by raising the temperature which automatically cuts off the steam supply to the heating system when the temperature at the bulb reaches the setpoint. The fast circulation produced by the fan system, in rapidly recirculating the kiln atmosphere across the pipes, quickly conveys the heat from the said ,pipes to the temperature control bulb soon after the steam is turned on. In many cases, when very little steam is required to maintain the set temperature, the automatic control instrument frequently turns the steam on and then off the heating system before the steam can pass to the opposite end of the heating system from which it enters. Consequently, the upper end of the kiln or that portion adjacent to the top header end of the heatiglg system ishotter than other portions of the I overcome this objection by using a heating system of single or multiple units each having a single return,so that the bottomheaders l5 and/ or l6 are located just below and at the same portion of the kiln as the top or supply headers l3 and I4, and by providing suflicient slope in the pipes to permit water of condensation to drain by gravity from the top to the bottom headers. This provides a space between the upper and lower legs of the heating system or systems at the steam feed or upper end of the kiln and provides very little space ,between the two legs of each system at the opposite or lower end of the kiln.

In this manner the heating system is related to adjacent fans so that a greater volume of air is circulated across one leg at the header end' of the heating system than across the other leg, and air is circulated across both the legs substantially equally at the return joint end of saidsystern. By circulating less air across the header ends of both the legs taken collectively than across the retum joint ends of said legs, I am able to effect more uniform distribution of heat in the kiln.

By positioning the fans in overhead fan kilns (see Figures VII and XIX) so that their longitudinal axes are above the level of the top leg of the heating system, I am able to circulate substantially all the air across the bottom leg of the heating system its full -length, and a portion only of the air across the supply header end of the top leg of the heating system, because of the vertical spacing of the headers. At the lower 'or return legs of the heating system because the pipes at this end are located close together in the path of the circulating atmosphere. In this way I am able to effect more uniform heat distribution in an overhead fan kiln.

By dividing the heating system into multiple units, two units on each side as illustrated in Figures I and II, and by reducing the amount of heating surface within the kiln to a practical minimum, the steam when turned on will pass fromend to end of each or either system before. the temperature within the kiln is raised to the set point, controlled by the thermostatic temperature control bulbs, aforesaid.

Two heating systems may be divided longitudinally with one heating system 64 at one end of the kiln and another heating system 66 at the other end of the kiln as illustrated in Figures VII, XIII and XIX. The same relationship described above between the heating system and adjacent fans applies to each.

For producing kiln atmospheric circulation, I prefer to use substantially such means as are commonly known as the internal fan cross circulation system. In such a system a series of reversible rotary fans 3| are mounted, preferably,

. wall l constituting in effect a closure on that I side for the compartment to which it is appropriated, the side of the compartment opposite each solid well being open.

The solid walls di when employed are, disposed,

respectively, in succession on alternate-sides ofthe fan so as to divide the series of compartments into units or fan ducts of which each alternate one is open on one side and closed on the other side. By the disposition just described of the several walls 39, so, and as, the result is in their assemblage, to impart to the wall so constituted throughout its entire length what is designated as a zigzag wall or effect.

The angle at which the walls 39 meet the walls ii, as shown in Figures II, IV, and V1, is an obtuse angle on the fan side, but it is obvious that the angle may be'varied, if desired, as in some instances it may be (see for example Figure II). Each of the walls last named is planted on the bottom'fi of the kiln, or if overhead fans are used, in contact with air-tight closure means assembled, on their upper side, with the ceiling or root 3. The fanducts are completed by a floor M which is laid in contact with air tight closure on the top of the walls 39, 60, and M, when the fans are below the loads, and are attached to the bottoms of said walls in like manher when the fans are above the stock-loads.

I prefer to use a series of right hand fans at one end of the kiln and'a series of leftv hand fans at the opposite end, the zigzag wall being reversed in direction at a medial portion of the kiln as shown in Figure II to provide cross circulation from one side to the other the full length of the kiln, in which case in the intermediate compartment where right and left hand fans face each other, two fans will blow against each other in counter-acting currents in one di-' rection of circulation and withdraw atmosphere from the same compartment in the reverse direction of circulation as shown in Figure II.

The employment of right and left hand fans opposed in pairs distributed the full length of the kiln so that each pair of fans may discharge counteracting currents one against the other in either direction of rotation illustrated in Figure VI, affords a feasible method of circulation, and can be used to embody'my invention.

In any embodiment of my invention, I may provide means for elfecting more circulation of air in the end sections of the kiln which would be otherwiseits coolest sections. This may be accomplished by the use of simplebafiies as illustrated in Figures II,III, V, XII, and XIII or using larger capacity fans in the end section of the kiln as illustrated in Figures XII and XIX.

Referring to Figure III, I' show baiiies, and 45, the baflies 44 being relatively narrower than the bafiles 45 because the latter must be prepotent and to reverse the circulatory currents and direct them towards the ends of the kiln when the fan circulation is directed against them. On reversal of circ ulation, the baffles 44 would be brought into efiectlve use. They need be relatively narrower than the baiiies 55 because the air from the fans being in such case-directly driven towards the ends of the-kiln, their function is accomplished by directing the air partially 1 transversely and, at the same time, allowing a certain amount of directional flow towards the ends, due to the direct action of the fans.

Upon reversal of the fans the transverse air circulation of the kiln is reversed and the longitudinal circuit of air circulation at each end is also reversed. Each-circuit of longitudinal air circulation at each end is also reversed.- Each circuit of longitudinal air circulation moves from the center portion of the kiln towards one of the two ends in one direction of fan rotation, and upon reversal of fan rotation each longitudnal circuit is reversed and moves toward one end from the middle portion of the kiln on the opposite side of the stock loads from theside on which it moved, before the fans were reversed.

It will be understood that the bafiles ,M and d5 are preferably set with permanent flexure so that they may be made by adjustment to direct as much additional movement of atmosphere towards the ends of the kiln as may be effective to prevent those ends from becoming too low in temperature and too high in humidity. On the contrary, it is made possible upon occasion, to reduce the temperature and raise the humidity at the ends of the kiln by reducing the air flow toward the ends of the kiln or by reducing the amount of circulation at the ends of the kiln.

Referring to Figure II, it will be noted that the bellies 44 are disposed in series at opposite ends on one side of the kiln, and the baiiles 45 are disposed in series at opposite ends on the other sldeof the kiln. 1

In Figures XII, H11 and XIV, the baflles 44 and 65 are shown as located on adjacent sides on1y of the fans, inasmuch as the fans are lofiexure to. guide and distribute the impinging circulating medium within the kiln.

It is desired to set thesewing baiiles so that the air of circulation from the fans 3! will be mainly transverse in course but will'havej a slight directional flow towards the respective .-,ends of the kiln where I prefer to direct additional heated air from the fans 3!, to replace cooler air at the doors where it has lost tional air flow sets up longitudinal recirculation toward the doors from the medial portion on one side thereof and toward the part of the kiln remote from the doors on the opposite side of the kiln.

It is to be understood that-the fans may be located above as .well as below the stock-loads of material to be dried. 'In'this case, the relative disposition of the heating system to the heat. This direcfans already described above, is maintained. In some cases the fans may be located in one corner of the kiln, as shown in Figure XIV. In that case, the heating system may be in two multiple units of pipes instead of three or four as illustrated in other figures.

Their primary function being to direct the air from the fans to the normally cooler parts of the kiln, they may be located at the corners of the ducts in Figure '11 or midwise of the duct, as in Figure V, or they may be omitted all together when right and left hand fans are used in series atopposite end of the kiln.

In my kiln, I prefer to provide fresh air conduits 46 and 41 having openings 49 and 50 respectively at each fan for the intake of outside or atmospheric air into the kiln. In the case of overhead fans, ventilators may be substituted for said conduits, and will perform the double purpose of fresh air intake and of exhaust from the kiln.

The fresh air conduits 46 and 41 are provided, respectively, with fresh air doors 52 and 53, which may be opened at will to let fresh air into the respective conduits 46 and 41. The

air conduits may have openings 49 and 50 provided with covers 56. Said covers are preferably sliding doors and can be used, respectively, to

cover all or part of the openings 49 and 50 so that the fresh air intake at each fan is adjustable in such manner as to provide a desired medium of fresh air to each fan. Preferably only, one fresh air conduit 46 or 41 will be used at a time. The one on the suction side of the fans will be lused and the door 52 or 53 of the opposite side will be closed, depending upon the directioniof circulation of the fans, considering that the fans are reversible. I alsoprovide ventilators 51 in the roof of the kiln which may be opened at will to ventilate excess moisture from' the kiln.

There are several modifications of my heating system illustrated in Figures VIII, IX, and X,

which give the same relationship of effective radiating surface to air circulation as that shown in Figure I. In these illustrations, part of the air circulated is permitted to bypass part of the heating pipes of the heating system at the end of the system at which the steam enters. At the lower end or return leg end of the system, a substantially greater volume of the air is passed in operative communication with the pipes of both legs of the heating system.

The amount of air bypassing the heating units varies in difierent cross sections of the kiln; but the closer to the point of feeding steam, the relatively greater the amount of air that is bypassed; and the closer to the return leg ends of the heating system, the greater the volume of .kiln atmosphere that is circulated across the pipes. This relationship of volume of air passing across the pipes to the heating emciency of the pipes is maintained. The numerals of Figure XI are applicable to the heating systems of Figures VII, VIII, IX, X, and XIX. In Figures VIII, IX, X,

and XI the heating systems illustrated are on ures II, VI, and XII.

A particular feature of my kiln is the method of control of temperature and humidity therein illustrated diagrammatically in Figures VI, VIII, XII, and XIII. I preferably employ any well known instrument for the control of temperature and/or humidity for example, that described in Patent No. 1,624,887 issued to E. H. Bristol, April 12, 1927. The wet control bulb 12 having a water box 15 and wick 18 and the dry control bulb 13, both at the same end and same side of the kiln are a part of individual or separate controllers, and control the wet and dry bulb temperatures at that end by actuating diaphragm valves 11 and 16 on the spray and heating systems 58 and 64, respectively. The dry control bulb 83 at the opposite end of the kiln controls the dry bulb temperature at that end by actuating the diaphragm valve 86, controlling the heating system 66 at that end of the kiln.

I have discovered that the wet bulb temperature, in an enclosed dry kiln having substantially the same amount of fresh air intake and ventilation outlet at the two ends is substantially constant. Thus, by controlling the dry bulb temperature at each of the two ends, I am able to efiect temperature and humidity control at the two ends and substantially throughout-the kiln.

I preferably'provide dual dry bulbs on each of the dry bulb tube systems at each end of the kiln with one bulb thereof on each side of the stockload at each end of the kiln to give control of the air entering the said stock-load in either direction of transverse air circulation. Two dry bulbs, for example, 83 and 83A are connected by capillary tubes 14 or 84 and are connected by a capillary 1| or 8| for example, to one actuating element such as a helical coil IS in the instrument of Patent No. 1,624,887. When dual dry bulbs are used, the bulb in the hottest air actuates the helical coil of the instrument through effecting the higher vapor pressure therein.

The control system in my kiln is particularly suitable for drying stock which varies in initial moisture content. Diflerent loads of material v vary in initial moisture content, and because of this variationthere is difficulty in maintaining uniform drying conditions throughout the length of the kiln. Stock having higher moisture content requires more heat to maintain a drying temperature under given conditions of circulation than does stock having a lower moisture content. My control system makes it possible to maintain uniform drying conditions of humidity and temperature throughout the kiln by furnishing additional heat when and where needed regardless of the variation of moisture content of the stock in the kiln. Although moisture content of the stock does or does not vary at the two ends of my kiln and the amount of moisture evaporated from the stock does or -does not vary at the two ends of the kiln, I am able to effect control of the temperature and humidity of the entering air at the two ends of the kiln by using a separate heating system and a dry bulb control thereof at each end of the kiln and a humidifying means therein and a wet bulb control thereof at one end.

2,202,148 This principle applies to my kiln operated progressively as well as charge system. For example, in a progressive kiln, when green lumber is put into the kiln every day at the green end uniformly distributed circulation, a control of,

the dry bulb temperature at each end, and a wet bulb control at one end, to give independent control of the temperature and humidity at each end ofthe kiln.

Reference is made to my co-pending appiica-'.

In Figures XVIII, XIX, and XX, I illustrate a different form of air circulating system with fans turned at right angles to the longitudinal axis of the kiln. The fans 90 are driven in any suitable above. The effective relationship of the air cirfollows; The kiln, being charged with material manner, for example, by individual motors 9|, which are reversible in rotation and which circulate air across heating systems 84 and 66 on one side of the fans and heating system 2l-22 extending the full length of the kiln described culation to the heating systems is exactly the same as when other designs of air circulating systems are used. The fans 90 are preferably of the same size and rotated at the same speed and give uniform air fiow transversely within the kiln. The end fa'ns 90A and 90B are preferably of larger capacity to provide more circulation at the ends of the kiln.

The heating systems 64 and 66 also may have extended surface radiation atthe'ends of the kilnto provide a greater heating effect to counteract heat losses through the ends of the kiln.

The operation of mykiln is substantially as to be dried and the doors, if more than one, closed, the fans are set in motion and t e an matic temperature and humidity-controls, wellknown in the art, are set for a given schedule of temperature and humidity at each end of the kiln. Steam is supplied to one of the heating systems to give sufilcient radiating surface to permit a rapid flow of steam from the top to the bottom header. The rapid transverse or cross circulation of air cooled by drying contact with the stock to be dried and then passing across the pipes in transverse circuits condenses a considerhowever, by passing a greater amount of the circulating medium across all the pipes of one leg and a greater amount of air across the other leg at the return bend end thereof than across the header end thereof, whereby a greater uniformity of the heating of the kiln results.

The baifies when used are set so as to deflect the. greater amount of circulation towards the ends of the kiln or regions which are naturally relatively cooler and higher in humidity, due to heat losses as through the doors. By adjusting the baflies in the manner already indicated, a

midity. By the control of For example a drop Figure II)-.

can be obtained. A similar adjustment of .the extending latdirection of the spray nipples 60 teraliy from the spray pipe 68, having a cap 62 and perforated holes 6i as illustrated inFigure XVII, produces a uniform distribution of huthe supply of steam to the heating coils at the two ends, the adiustment of the directional flow of the circulating medium, and the adjustment of the direction of and the supply ofsteam to the spray nipples, I

be usedat one end, may be used at the such case, it would pensating factors to increase the radiation at the lower end of the kiln. This may be accomplished by using greater surface radiation, such as additional or fin pipe at the'lower end of the heating system to compensate for the less effectiveness of the heating system at that region (see. Compensation could also be made for less radiation at any point in the kiln by use of larger fans or of fans having a greater angle of blade which would permit them to discharge more air across the stock loads and the heating system at the end sections of the kiln or other sections where the capacity of the drying atmosphere may be less (see Figure XIX). In fact, extended by amplified surface radiation, the use of larger fans, or fans having a greater angle of blades may be used at any points in the kiln or to increase the drying effect of the circulating atmosphere in lieu of the arrangement described above. 1

The principle of my invention contemplating means for compensating for greater heat and humidity losses at the ends of a kiln, for example, may be employed in ki ns having distributed fans operatively mounted on transverse shafts as well as on longitudinal shafts (see Figure XX). In any case, greater heating surface at the ends of the kiln, adjustment of humidity spray discharge pipes to supply drier steamat the ends of the kiln and wetter steam in the intermediate section, or the use of larger fans or fans having a greater angle of blade to circuate a greater volume of air at the ends of the kiln, may be used to accomplish the desired result. I prefer to employ any or all ofthese means operating individually or simultaneously to uniform drying conditions within the kiln.

When fans on transverse shafts are used, I prefer to use right hand fans atone end and left hand fans at the oppositeend of the kiln installed so that the efiective direction of discharge of right hand fans will be toward the end of the kiln at which they are located, and the effective be necessary to use com-- f provide more direction of the left hand fans will be toward their end of the kiln. In this manner greater heating effect is attained at the two ends of the kiln, or whatever cooler regions thereof may exist towards which they may be respectively made to discharge.

It is understood that this invention is not limited to the exact design and form illustrated and described. Modifications of the structures and methods herein described and equivalents are self-evident to those skilled in the art; therefore,

more uniform distribution of heat and humidity it is to be understood that the invention includes within its scope whatever changes fairly come within either the terms or the spirit of the appended claims.

Having described my invention, what I claim 1. In a dry kiln, means for efiecting reversible cross circulation therein, heating means therein at one end, heating means therein at the other end, humidifying means in said kiln, a dry bulb controller having thermostatic bulbs at one'end for controlling the heat supplied to the heating means at that end, a dry bulb controller having a thermostatic bulb at the other end for controlling the heat supplied to said heating means at that end, a single wet bulb control in said kiln for the actuation of said humidifying means, whereby the temperature and humidity are controlled at each end of said kiln, and means for controlling the temperature of the kiln atmosphere before it enters the load comprising dual dry bulbs on one of said dry bulb controllers, one of said dual bulbs being in the path of circulation as atmosphere moves froma heating means to a load in each direction of air circulation.

2. In a cross circulation dry kiln having stock loads therein, a heating system at one end, a heating system at the other end, a humidifying system in said kiln, a dry bulb controller having a thermostatic bulb at one end, for controlling the heat supplied to the heating system at that end, a dry'bulb controller having a thermostatic bulb at the other end of said kiln for controlling the heat supplied to said heating system at that end,

. a dual dry bulb for each of said dry bulb controllers, each on the opposite side of adjacent loads from the said dry bulbs, a wet bulb control having a thermostatic wet bulb in said kiln for actuating the humidifying system, whereby the temperature and humidity of entering air are controlled at each end of said kiln.

3. In an overhead fan dry kiln having uniformly distributed ventilation therein, the combination with, a series of reversible fans operatively mounted and distributed in substantial alignment, substantially the full length of the kiln, for effecting cross circulation across stock loads therein, two return coil heating systems comprising top andbottom headers, and top and bottom legs connected respectively thereto, said headers being vertically spaced apart at one end of said heating system, said legs converging, and being connected by a return joint at its opposite end, whereby said headers may be supplied with steam and drained respectively, one of said r heating systems being at one end of said kiln and the other being at the opposite end, each of said heating systems being so related to the adjacent fans that a greater volume of air is circulated across one leg than across the otherleg, and more air is circulated across the return joint end than across the header ends collectively of said legs, a dry bulb controller for each of said heating systems, each controller being provided with dual dry bulbs, one bulb of each controller being located in the path of circulation as atmosphere moves from a heating system to a stock load in one direction of circulation, and

the other bulb being located in the path of circulation as atmosphere moves from said heating system to a stock load in the other direction of circulation, thereby controlling the temperature of the kiln atmosphere before it enters the load at each end of the kiln in each direction of air circulation.

4. In a dry kiln having fans distributed therein heat supplied to said heating means at the other end, a single wet bulb controller having a thermostatic wet bulb in said kiln for the actuation of said humidifying means, whereby the temperature and humidity are controlled at each end of said kiln, and means for controlling the temperature of the kiln atmosphere before it enters the load comprising dual dry bulbs on one of said dry bulb controllers, one of said dual bulbs being in the path of circulation as atmosphere moves from a heating means to a load in both directions of air circulation.

5. In a dry kiln of the reversible cross circulation type, for a stock load therein, air circulation means, atmosphere heating means and control means for said'heating means, said control means being characterized by dual bulb apparatus, one bulb of which is located in the path of air circulation on each side of the heating means, one of said bulbs being in the path of circulation of kilnatmosphere as it moves from theheating means to pass across an adjacent stock load, and the other bulb being in the path of circulation of atmosphere as it moves from an adjacent load to pass across the heating means, said bulbs being jointly connected to the operative control means, whereby the actuation of said control means is automatically taken over by the bulb in the position having the highest temperature. 4

6. In a dry kiln having a seriesof reversible fans operatively mounted and distributed in substantial alignment the full length of the kiln, the combination of two return coil heating systems comprising top and bottom headers, and top and bottom legs connected respectively thereto, said headers of each system being vertically spaced apart at one 0nd thereof, said legs of each system converging and being connected by a return bend at its opposite end, said headers being supplied with steam and drained respectively, one of said heating systems being at one end of said kiln and the other being at the opposite end thereof, each of said heating systems being so related to the adjacent fans that a greater volume of air is circulated across one leg than across the other leg thereof, and more air is circulated across the return bend end than across the header end of the leg across which a lesser volume of air is circulated, a dry bulb controller having thermostatic bulbs in the path of air circulation at one end of said kiln for controlling the heat supplied to the heating system at that end, a dry bulb controller having a 'thermostatic bulb at the other end of said kiln for controlling the heat supplied to said heating system at that end, humidifying means in said kiln, a single wet bulb controller having the thermostat located at one selected position in the kiln whereby, temperature and humidity are controlled throughout the kiln, and means for controlling the temperature of the kiln atmosphere before it enters the load'comprising dual dry bulbs on one of said dry bulb controllers, one

of said dual bulbs being in the path of circula- 7 tion as atmosphere moves from a heating system to a load at one end of the kiln in both directions of air circulation.

r '7. The method of controlling the temperatur and humidity of atmosphere before it enters stock loads distributed throughout a kiln,which consists of circulating atmosphere transversely across said stock loads therein, of reversing the circulation at time intervals of heating the atmosphere at each end of the kiln before it circulates across the stock loads, of determining the dry bulb temperature of the atmosphere immediately before it enters the stock loads in either direction of circulation, 'of controlling'the heat added to the atmosphere in response to variations in the dry bulb temperature and in humidifying the atmosphere in response to variations in the wet bulb temperature-at but one end of said kiln, whereby more uniformly dried stock is obtained.

8. In a cross circulation dry-kiln having stock loads therein, a heating means at one end, a heating means at the other end, a humidifying spray in said kiln, means for reversing the direction of air circulation in the kiln, a dry bulb controller for controlling the heat supplied to the heating means at the one end, a dry bulb controller for controlling the heat supplied to the heating means at the other end, dual dry bulb apparatus for each of said-dry bulb controllers, each bulb of which being located in an atmospheric space between a heating coil and a stock load, one of said dual bulbs for ,each controller being in the path of circulation of kiln atmosphere which moves from the heating means across an adjacent stock load, and the other bulb for each controller being in the path of circula-' tion of the atmosphere which moves from an adjacent load across heating means, a wet bulb control having a thermostatic wet bulb in said kiln for actuating the humidifying spray, where by the temperature and humidity of the kiln atmosphere before it enters, a stock load are controlled at each end oi said kiln.

atively connected to each of said controllers,

.cally taken over by the bulb in the position hav- 9. In a dry'kiln having thereinstock varying in moisture content, a substantially uniformly distributed transverse reversible air circulating means therein, a steam spray extending substantially the full length of the kiln for supplying I heat and moisture the full length of. the kiln, a wet bulb temperature control having a thermostatic bulb in said kiln for said spray, an individual heating means at each end of the kiln, and a dry bulb temperature controller at each- 10 end of the kiln for the individual heating means, dual dry bulb apparatus for each of said dry bulb controllers one bulb of which being located on the entering air side of a stock load between the heating means and the load, and the-other bulb of which being located between the stock load and the heating means on the leaving air side of the load, said dual dry bulbs being operwhereby actuation of said controller is automatiing the highest temperature.

10. In a dry kiln of the reversible cross circulation type, air circulation means, atmosphere heating means of the return coil type located and 25 steam fed at one end of the kiln, a separate atmosphere heating means of the return coil type with a supply header located and steam fed at the other end of the kiln, and control means for each of said heating means characterized by dual 80 bulb apparatus so disposed with respect to the heating means that one bulb is on one side .of the load and the other bulb is on the opposite side and each is in the path of circulation after the circulated atmosphere has passed across the I heating means in either direction of atmosphere circulation, the said bulbs being operatively connected to the control means, whereby actuation of each of said controllers is automatically taken over by the one of its bulbs in the position having 40 the highest temperature.

JAMES FORREST coma.

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