US3191250A

US3191250A - High speed drying apparatus for refractory shell molds

Info

Publication number: US3191250A
Application number: US365547A
Authority: US
Inventors: Jr Edward J Mellen; Fasselle Robert J De; John M Webb
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-04-16
Filing date: 1964-04-16
Publication date: 1965-06-29
Anticipated expiration: 1982-06-29

Description

June 29,1 5 E4. MELLEN, JR, Em. 3 1 1,250

HIGH SPEED DRYING APPARATUS FOR REFRACTORY SHELL MOLDS Filed April 16, 1964 6 Sheets-Sheet 1 v a a INVENTORS Edward JlMelleu Jr- Bert .J; deFaJs elle 3,191,250 HIGH SPEED DRYING APPARATUS FOR REFRACTORY SHELL MOLDS Filed April 16, 196 4 June 29, 19 5 E. J. MELLEN, JR., ETAL.

6 Sheets-Sheet 2 INVENTORS Edward J.Me llen Jr. Robert deFwsselle HIGH SPEED DRYING APPARATUS FOR REFRACTORY SHELL MOLDS Filed April 16, 1964 June 1965 E. J. MELLEN, JR, ETAL 6 Sheets-Sheet 3 INVENTORS Edward J1 Mel leu Jr.

Robert J. deFusselle John M Webb June 1965 E. J. MELLEN, JR., ETAL 3,

HIGH SPEED DRYING APPARATUS FOR REFRACTORY SHELL MOLDS 6 Sheets-Sheet 4 Filed April 16, 1964 INVENTORS Edward JIMellerv Jr.

Robert I. deFasselle John M. Webb J1me 1965 E. .1. MELLEN, JR, ETAL 3,191,250

HIGH SPEED DRYING APPARATUS FOR REFRACTORY SHELL MOLDS 6 Sheets-Sheet 5 Filed April 16, 1964 INVENTORS dward J2 Mellen Jr. Robert J. deFusselZe John MT ebb v w ww United States Patent 3,191,250 HIGH SPEED DRYKNG APPARATUS FOR REFRACTORY SHELL MQLDS Edward I. Mellen, Jr., 2123 Lee Road, East Cleveland, @hio; Robert J. de Fasselle, Gates Mills, Ohio; and John M. Webb, Chagrin Falls, Uhio; said De Fasselie and said Webb assignors to said Mellen Filed Apr. 16, 1964, Ser. No. 365,547 13 Claims. (Cl. 2287) This application is a continuation-in-part of application Serial No. 15,690, filed March 17, 1960, now abandoned, and a continuation-in-part of application Serial No. 742,- 554, filed June 17, 1958, now Patent No. 2,932,864, dated April 19, 1960.

This invention relates to a high speed drying apparatus for rapid production of thin-walled or shell molds of refractory material for precision casting of metal articles. These articles can range in size from a few ounces to several hundred pounds. Molds are formed by the dipping of a destructible pattern in a refractory slurry, drying the refractory materials on the patterns, repeating the process to build up a suitable thickness of mold wall, and then destroying or removing the pattern material and firing the mold.

Shell molds made for use in the well-known lost-wax process of investment casting are formed by repeatedly dipping the destructible pattern in ceramic slurry and allowing each coat to dry after each dipping operation to build up a thin shell comprising many layers of ceramic material. The destructible pattern is formed to close tolerances as an accurate reproduction of the desired metal part. The first ceramic layer applied to the pattern dries easily but is easily damaged by expansion or contraction of the wax pattern. After the first coat is dried and the pattern is again placed in the dip bath, the liquid penetrates the first coat, whereby the drying of each successive layer becomes more and more ditficult. The proper conditions for drying susccessive coats are thus subject to continual change. Cracking of the shell molds is a very serious problem, particularly when wax patterns are employed, because of the very high coeflicient of expansion of the wax.

Prior to applicants invention, many hours were required to form a satisfactory shell mold on a wax pattern. In a typical prior art process, the time for drying one coat was several hours because it was thought that the drying must be carried out at room temperature. Before applicants invention, it was believed necessary to avoid rapid drying of each layer of ceramic material applied to the wax pattern because of the tendency of the shell molds to crack, the tendency of different layers to separate, and also the tendency of the binder to migrate to the surface by capillary attraction when the water was quickly evaporated. The established prior art procedure was to dry slowly at room temperature in order to prevent expansion or contraction of the wax and to avoid low relative humidities (i.e., below 50%). In fact, such precautions are still followed today when drying at room temperature as pointed out on page 21 of Foundry Trade Journal, July 4, 1963, wherein the following statement appears:

Experience with shell drying in still air has shown that at least a 50% relative humidity is required and with these conditions individual coat drying times of 3 to 6 hours between dips are not uncommon.

Because of the necessity of producing high quality shell molds which are accurately formed and which have the ability to withstand the heat shock when filled with molten metal, those skilled in the art, prior to this invention, considered rapid drying highly undesirable and usually disastrous because of the sunsatisfactory shell molds which resulted.

Prior to the present invention, the lost-wax process was slow and expensive and was generally considered inferior to the frozen mercury process. The present invention departs radically from the teachings of the prior art and involves increasing the rate of production of shell molds by several hundred percent without sacrifice of high quality. The invention involves a novel basic concept wherein the wax patterns are dipped in liquid ceramic material and then passed through a long narrow tunnel which carries heated air through the tunnel at high velocity to obtain rapid drying. Means are provided for controlling the wet-bulb and dry-bulb temperatures and the relative humidity to obtain rapid drying of the ceramic layer or the shell mold surrounding the wax pattern without objectionable variation of temperature of the wax pattern by maintaining the wet-bulb temperature substantially the same as the temperature of the wax pattern. The high velocity air does not pick up suificient moisture to cause any substantial change in the temperature of the air as it moves from one end of the tunnel to the other, and there is no substantial expansion or contraction of the wax pat tern. Because the dry-bulb temperature is changed at different drying tunnels, it is possible to employ one endless conveyor to carry all of the patterns through all of the drying tunnels at the same velocity. Because the wetbulb temperature at opposite ends of each tunnel is substantially the same, it is possible to move the air in several parallel tunnels in one direction only toward an enclosed work zone containing the ceramic dip tanks and the dusting apparatus even though the single conveyor must reverse the direction of movement of the shell molds from one drying tunnel to the next.

The apparatus of the present invention permits rapid production of uniform thin walled molds with uniform permeability utilizing destructible pattern materials such as conventional waxes which are solid at normal room temperatures. Another feature of the invention is the ability to provide dimensional accuracy.

One object of the present invention is to provide an apparatus for building up and drying a plurality of refractory layers on a destructible pattern to make a mold suitable for precision casting within as short a time as three hours or even less.

Another object is to provide an apparatus for making a shell mold for precision casting which is satisfactory and practical in every way and gives tolerances and other physical characteristics which are at least equal to those obtained by the frozen-mercury casting process.

Still other objects, uses and advantages of the present invention will become apparent to those skilled in the art from the following description and claims and from the drawings in which:

FIGURE 1 is a plan view of an apparatus for applying wet refractory coatings around a pattern and drying the same in accordance with the present invention;

FIGURE 2 is a fragmentary elevational view taken along the lines indicated at 22 in FIGURE 1 and showing the dipping enclosure and drying apparatus of FIG- URE 1 with parts broken away and shown in section;

FIGURE 3 is a fragmentary sectional view taken along the lines indicated at 3-3 in FIGURE 2 with parts broken away;

FIGURE 4 is a fragmentary sectional view on an enlarged scale of the exhaust damper system of the drying apparatus of FIGURE 1;

FIGURE 5 is a fragmentary sectional view;

FIGURE 6 is a fragmentary view on an enlarged scale showing a ceramic mold formed over a pattern and suspended from a continuous conveyor which carries the molds in and out of drying tunnels;

FIGURE 7 is a front elevational view with parts broken away of a dusting apparatus used with thdapparatus of the present invention;

FIGURE 8 is a fragmentary side elevational view of the dusting apparatus of FIGURE 7;

FIGURE 9 is a fragmentary rear elevationalgview of the dusting apparatus of FIGURE 7;

FIGURE 10 is a perspective view of an alternate form of a drying apparatus; 7

FIGURE 11 is a fragmentary elevational view of the drying apparatus of FIGURE 10 with parts broken away; FIGURE 12 is a'fragmentary side view of the drying apparatus of FIGURE 10;

FIGURE 13 is a fragmentary sectional view similar to FIGURE 3 and taken generally at the line 13-713 of FIGURE showing a modified form of the apparatus of this invention; 1

FIGURE 14 is a fragmentary top plan view of the.

apparatus of FIGURE 13 with parts omitted and broken away and shown in section; and p I I FIGURE 15 is a fragmentary vertical sectional view taken substantially at the line 15-45 of FIGURE 13.

. Referring more particularly to the-drawings in which like parts are identified by the same numerals throughout the several views, FIGURES 1 to 9 show one form of apparatus for performing the process described in apparts plicants U.S. Patent:No. 2,932,864 and described in their application Serial No. 15,690, filed March 17, 1960. In

performingthis process the temperature of the wax or tained at any point from the wet bulb temperature up-* ward, say generally about 85 to 110 F. using a Wet bulb of 75 F. The dry bulb temperature used is dependent upon the number of wet layers to bedried, the nature The wet" leg 16.

of the ceramics and pattern materials and the. configuraw tion of the mold.

A plan view of apparatus for practicing the present in-" vention is shown in FIGURE 1. This consists of a drying apparatus A which comprises a seriesof connected

tunnels

5, 6, 7., 8, 9, 10 and'll, which are U shaped in horizontal.cross-section, and a series of dip tanksfwhich are located adjacent the entrance of each of the drying tunnels within working enclosure B which also houses dusting apparatuses by means of which wet molds are sanded or dusted with relatively fineparticles of refractory material. Enclosure B; also serves as a soaking pit V in which partially processed ceramic molds may be stored temporarily between passes through the drying tunnels conveyor hook 58 to suspend the pattern from the conveyor. The pattern maybe referred to as a cluster,'each 'cluster containing a multiplicity of parts such as turbine blades and buckets. The shell mold is formed by dipping the patternin a slurry of ceramic material to deposit a thin layer, such as layer 39, over the, pattern. A series of ceramic layers deposited in this way forms the shell mold. I a v The molds are hung on a continuous conveyor chain 12 which transports them in and out of the drying tunnels. Before following the path of a mold. in and out of the drying tunnels of the drying apparatus A; the 1 means of transporting the molds'will be discussed in fur- As best seen in FIGURE 6, the conveyor ther detail. 7 means comprises a hollow rolled metal tubing 52' with a V tunnels.

. 4 longitudinalslot 53 in" the bottom and a set of generally parallel spaced rollers 54 which ride on the inside of the hollow tube adjacent the slot. A stem 55 is rigidly connected to a shaft 56 upon whichrthe-rollers are rotat ably mounted and extends down through the. slot to swivel joint 57. Hook 58 connects to swivel joint 57 and into eye 41 molded into the pattern 33. The swivel 57 is preferably provided with a rubber roller 59 which rolls alongon a rail or'shoe 60 which ismounted on one side 'of channel space 61. This rotates the pattern clusters as they are conveyed through the tunnel. Other suitable means may be employed to rotate the pattern clusters and insure even drying as they pass through the drying The pattern is first brought to the control temperature, which ordinarily is room temperature at about F. by being kept in the same room as the drying apparatus or by being kept in a separate room or. by other suitable means. Oncethe patern has reached the control temperature, it is" dipped in the first dip tank 18. The clip tank contains a ceramic slurryand deposits a wetlayer of ceramic on the pattern. 7

The path of-a wet patternthrough. drying apparatus A starts as a clusteris suspended on one of the hooks 53 of the conveyor. means. The pattern with the wet coating 'thereon is transportedby the conveyor into incoming leg 15 of drying tunnel 5 through the header or central por tion 17, and thereafter out of the tunnelthrough outgoing The cluster or mold travels throughfthe. remainder of drying apparatus A by going through drying tunnels 6, 7,'$, 9, 10 and 11. After leaving outgoing leg 16 of tunnel 5 and before entering tunnel 6,-the dried mold is dipped in dip tank 19. Dipping is preferably done manually by a worker, who may be stationed at the tunnel exit as shown in FIGURE 3. j Theworker unhooks the pattern from hook 58, dips it, dusts it it called for, and places it back on'the hook. In a similar manner, the mold is dipped and dried as each successive ceramic coat is built up. Thus, the moldpasses by dip tanks 19 through 24 inclusive during its travel in and out of the tunnels 7 through 10.. The mold emerges from tunnel 11 with an adiabatically dried coating comprising seven ceramic layers.

Inaccordance with the present invention, the incoming and outgoing legs of each drying tunnel are contoured and shaped tomaintain the passage of an air stream at high velocity past the wet molds. For this reason, the

mold and other air flow restricting objects, such as the hook 58,'takes up a substantial portion and often /3 or more oi the cross-sectional area of the contoured tunnel.

For example, FIGURE 6 shows a section of incoming leg 25 oftunnel 10 having

side walls

66 and 67. FIG- URE 6 is drawn to scale to illustrate the small size of the tunneh.

The cross-sectional outline of each of the. legs of the other tunnel are preferably the same as leg 25 of tunnel 10 for the best drying results. The shape of the tunnel legs can be varied accordingto the size and shape of the molds being dried 'so. that the outer surfaces of the wet molds are, subjected to a rapidly moving air stream which provides'a high filmcoefiicient as previously de scribed. V t a FIGURE 3 shows the first dip tank lfiat which a man stands and manually removes each pattern from the conveyor, dips it into the dip. tankand returns it to the conveyor.. After the first dip, the wet ceramic coating 39 I next'is dusted with ceramic particles.

:the dusting drum to prevent the dusty. air fromescaping into enclosure .B.' The wet ceramic "coatings are dusted by merely holding and rotating byhand a mold cluster inside the drum through an open end 45; The mold hook rests on notch 37 of support 36. The drum is rotated by a pair of driving rollers 46 and a shower of fine refractory particles is provided by this rotary action in conjunction with a plurality of buckets 47 which are attached along the inner circumference of the drum. The sand is continuously in motion, being scooped from bottom of the drum by the rotating buckets and in turn released from the buckets by the force of gravity when the buckets reach the top position.

Fines are carried into the fan 44 from the drum by means of conduit 43. The fines are transported out by the fan 44 by means of conduit 49 and into a header conduit 50 to a cyclone 51 where the tines are removed from the air stream and collected. As seen in FIGURES 7 and 8, a support 36 is provided to more easily manipulate the mold or cluster while it is being sanded.

As seen in FIGURES 1 and 3, the controlled quality air delivered to the drying tunnel which has incoming leg and outgoing leg 26, is supplied by air conditioning unit 68. The blower 69 of each air conditioning unit has a conduit 70 connected to its suction side and conduit 71 connected on its positive side. Blower 69 also supplies air to tunnel 1 1 which has incoming leg 28 and outgoing leg 29 as best seen in FIGURE 3. Located in conduit 71 are heating means comprising a hot water heating coil 72 having a plurality of metal fins 73, the coil being heated by hot water which enters the coil through water inlet 74 and leaves the coil through water drain pipe 75. Also located in conduit 71 is a means of supplying water to the air stream comprising an atomizing steam nozzle 7 6 which delivers steam into the heated air stream.

After the air has been properly treated by heating and humidifying operations the air is blown into header 27 which houses the U-shaped portions of both tunnels It) and 11. The stream of air coming into header 27 divides, part going down tunnel 19 and part down tunnel 11. The air stream entering tunnel lit, also divides, part going down leg 25 past wet molds and another part going past wet molds down outgoing leg 26 of tunnel 10. In a similar manner, tunnels 6 and 7, are supplied with rapidly moving conditioned air from header or plenum chamber 21 and

tunnels

8 and 9 are supplied from header 23. Tunnel 5 is supplied from a single header 17.

Returning again to the path of the mold in drying tunnel 5, it is noted the ceramic coating forming the mold becomes gradually drier as it continues through incoming leg 15, around 17 and down outgoing leg 16. The time required for travel of the mold in the outgoing leg 16 is important since the ceramic coating might become overstressed if the mold is allowed to remain in the drying tunnel too long after becoming dry. The lengths of the tunnels and the speed of the conveyor must be properly preselected when designing the equipment. In production, preferably about 50 to 80% of water is removed from the molds in leg 15 and the molds are preferably completely dry just before leaving tunnel 5 whereupon they are immediately dipped into a second dip tank 19 located just outside of drying tunnel 6. Thus a second layer of ceramic material is applied over the pattern, the mold again suspended from the conveyor and carried in and out of tunnel 6 where the first and second layers are dried. The process steps of applying sucessive layers of ceramic material over the pattern and adiabatic drying are repeated in the drying tunnels '7, 8, 9, 1t and 11. While it is not always necessary to dust after each dip, the procedure of dusting each wet layer after it is dipped is usually preferred. When the wet ceramic layers are dusted after each dip, the mold is built up faster, its permeability is more easily controlled and its strength increased.

Referring to FIGURE 3, it can be seen that one end of suction conduit 79 is located near the exit and entrances of the tunnel so that room air from enclosure B used for diluting the moisture-laden air coming down the drying 6 tunnel does not pass over the molds during their adiabatic drying step. Referring to FIGURE 1, to further describe the path of the molds through drying apparatus A, the built-up shell molds leave drying tunnel 11 and are carried outside the drying apparatus A and enclosure B on the conveyor chain which is moved by driving means 78.

The dried molds are removed from the conveyor chain after being conducted past driving means .73 and control panel 79 which may be used to house temperature and humidity controllers. The diluting air from the outside atmosphere preferably enters the drying chamber B from openings provided in the dryer for ingress and egress of the molds or by leakage around the doors 2 leading to the room B. The amount of air entering the system from the outside is controlled by venting some of the recycling air to the atmosphere. It can be seen that whatever amount of air is vented only an equal amount of diluting air will enter the system.

In accordance with the above considerations, we have found that an excellent drying apparatus can be made by locating the suction side of each blower 69 near the entrance and exit openings of the dryer. The discharge of each blower 69 is directed through a conduit over the air conditioning means and enters the drying tunnels at the vertex of the Us such as the header 17 of drying tunnel 5. The exhaust is taken oif said conduit preferably between the fan and the conditioning means so that energy is not expended in conditioning exhaust air. The sampling elements of the control means which modulate each of the heating and humidifying mechanisms are located at a point in the lower conduit 71 sufficiently removed from the conditioning means to assure complete mixing of air at this control point.

At the point of entry of the air from the tunnel into the suction side of each blower 69 a plenum 82 is provided with a baflle 83 and a water pan 83a to collect finely dispersed particles of ceramic material which may be picked up from the mold and prevent their recirculation. The exhaust of this apparatus enters the dip dust enclosure B from duct 94 to provide humidity as required for the processes taking place in this area. A cooling coil 92 more fully described hereinafter, may be provided to prevent excessive temperatures in this area. The quantity of exhaust air which is sent to this area and the amount it is cooled is controlled from elements located within the drying apparatus enclosure B.

Referring to FIGURE 3 to illustrate the above comments, means or" venting the moisture laden air from air conditioning unit 68 is shown comprising an exhaust stack and a damper 81. As shown in the accompanying drawings,

air conditioning units

84, 85 and 86 which are identical to the unit 68 and feed air into

tunnels

5, 6, 7, 8 and 9 respectively, may be vented in a manner similar to air conditioning unit 68. The

units

84, 85 and 86 are vented through dryer exhaust stacks 87, 88 and 89 respectively, which in turn along with dryer stack 80 of air conditioning unit 68, are headered into a main stack 90 by means of two horizontal flues 91.

As best shown in FIGURES 3 and 4, the moisture laden air exhausted through stack 80 may be used to air condition and humidity room B. A relatively large con duit 93 joins the main stack 90 at the juncture of main stack 90 and exhaust stack 80. Humidity laden exhaust air from the air conditioning units is diverted to enclosure B through condit 93 and duct 94 by means of main stack damper 95 and damper 96 which control the flow of moisture-laden air into enclosure B through conduit 93 and duct 94. The dampers, as shown in FIG- URE 3, are connected by linkage 97 which holds the dampers in a fixed position relative to each other. The dampers are opened and closed when the position of the linkage is changed by air controller 98. As seen in FIGURE 3, the

damper plates

95 and 96 are partly open. As seen in FIGURE 4, when the main stack damper 95 is closed the damper 96 is full open.

As previously indicated, when damper 96 .is "open,

humidified air flows through conduit 935 and into duct 94 where it passes over a cooling coil 92 to reduce the air temperature to about room temperature before I reaching enclosure B. I From. duct'93' the make-up air passes through enclosure B and through plenums $2 into the

air conditioning units

68, 84, 85 and 86. a

In the present invention, the wet ceramic coating is dried'rapidly yet' safely by moving air of controlled quality and velocity past the shell'mold coatings which are suspended from a moving chain in a drying tunnel for efficient processing. This is accomplished byselection and maintenance of a certain wet bulb' temperature for the drying air, as previously indicated, and by drying with gradually decreasing relative humidity asthe moldlayers are built up. 'For example, when drying wet ceramic coatings over a .waxpattern and using air with a wet bulb temperature of 75 F. the prime coat and the second coat may be quickly yet safely dried by air having a relative humidity of 45-55%. T he third and fourth coats maybe advantageously dried with a relative humidity of 35-45 percent, the fifth and sixth coats with afrelative humidity of '25-30 percent and the last coat, with a relative humidity of l 25 percent. Thus, when maintaining a wet=bulb temperature of 106, .107 'used to house clusters 108 of wetmolds during 105 and 106 are upper drying tunnel-s while 107 is a lower drying tunnel. Thev mold clusters 108 do not circulate through the drying compartments, remaining stationary once inthe compartments. While'the treated-air stream 75 F. in all of the tunnels '5 to 11,.the dry-bulb .tem- I perature must increase from the first tunnel to the; last. The dry-bulbternperature maintained by the unit 34 a used with molds of reasonable large, surfaces and small" mass withoutdanger of breakage or. blow-off of ceramic particles. Molds of greater mass might blow-off at substantially higher velocities, there having been -established no definite upper limit for this.

A prime consideration is, of course, to contour on a fixed or adjustable basis the tunnel cross-sections so that a minimum area is always provided and maximurn velocity willibe obtained for the least quantity of air moved.

The specific drying units referred to herein have had chambers with cross-sectional areas in the range of 3 to 4 sq. feet and the quantities of air have ranged from 10,000 to 15,000 cubic feet per minute per chamber although air volume as low as 4,000 cubic feet per mlne ute could be used.

to obtain the benefits of the present invention. In the present process, one method of .making a shell mold, of

In any eventa minimumvelocity of at least 800l,000- feet per minute should be used passes rapidly by the clusters.

The air conditioning unit 110-h-as a fan-11= 1, a steam nozzle 1 12' and a hot water heatingcoil 113. The conditioned air is delivered by fan 111 into afeed header 114 from where it blows down the 3 drying compartments into return header 115 where it is returned once again to fan in tunnel 109; Louvres .116 and 117 may be adjusted 'to exhaust off a suitable amountv of wet air and admit fresh air intothe units. ve'niently placed in the drying'compartments by hanging The mold clusters 108 are conthem from a long slidable hanger or'a T-shaped rail 118. FIGURE 12' shows howsmolds may be suspended from rail1=18 of compartment 106 by means of hooks 119. As noted in detail in FIGURElZ, the rail118 isT-shaped in crosssection andmay be pulled out as illustrated in dotdash lines in FIGURE 12'so that the mold clusters 108 can be suspended from the rail conveniently by a worker. A rail 118 is located in each of

channel spaces

105a, 106a, :and 107a which are dead air spaces over

compartments

105, 106 and .107 respectively. A rail 118 used in channel space 106a over compartment 106 rides on a pair of front rollers 121 which are rotatably mounted on U-shaped I housing Y123 and has a pair of rear rollers 122 which'are rotatably mounted t-oithe rail 118' itself.

' URES 1 to 9 except for a change in the construction of the ducts adjacent plenum 82 and a change in the apparatus controlling the humidity of the air passing through duct 94.. The

elements

2, 10, 11, 12, 13, 24, 25, 26, 27, 2s, 29, 35, 44,49, 50,58, 64, 65, 68, 69,, 70, '71, 72, 76, 80, 81, 82, 83, 83a, 89, 90, 91,93, 94, 95, .96, 97,98 and B shown in FIGURES 13 to .15 are the same as those used in the apparatus of FIGURES 1 to 9 and a further detailed description is, therefore, unnecessary. The path of the good strength and resistance to thermal shock, is to'apply the first layer so that it is of a thickness of .005 inch to a I .020 inch and while it is still wet it is advantageous to dust the prime layer with fine refractory particles' using sufiicient force to embed the particles therein. A. method of dusting or sanding is used that provides a denseuni-i form cloud of fine particles that strike the wet coating.

with substantial impact force. The; force is, of course, not so great that it breaks or knocks oft 'thewet prime layer from'the pattern. The above process is repeateduntil a multiplicityof layers is obtained, the thickness of thelayers generally being about from about .005 inch to .02 inch although'in some'layers the thickness may reach about 0.2 inch particularly whenonly 2'or 3 layers are used. 1

A simple low volume form of a drying apparatus is. shownin FIGURES 10 to 12 which can be used forbatchtype drying of ceramic mold clusters. The generally rectangular shaped dryer C has three compartments 105:,

conveyor is'shown by the dot-dash line 2 in FIGURE 14. Theimproved drying apparatus A is superior to the drying apparatus A previously described, particularly bethe .

air conditioning units

63, 84, 85 and 86 of the apparatus A are adapted to regulate the flow of water or steam to the nozzle 76 so as to maintain a predetermined wetbulb temperature of F. or so in each of the headers 17, 21,.23 and 27, and the heatingcoils 72.. are arranged 'tomaintain constant dry-bulb temperatures whichare different for each of said headers as previously described.

, -airfrecirculating in the apparatus A,'it is unnecessary to remove water from the air before it reaches the heating coil-s 72. The suction created vby each blower 69 draws the drier air from enclosure Bjinto each plenum-82 of the .apparatus A so that the specific humidity of the air entering each heating c'oi1 72 is less than that required in the humidity which is too high, not only because of the high humidity of the outside air entering enclosure B but also because some moisture-laden air in the top portion of the drying tunnels to 1 1 enters the room B due to its inability to overcome its momentum and turn downwardly into the plenums 82.

In order to minimize flow of air through the tunnels directly into the dipping and dusting room B, the portions of the tunnels above the plenums 82 have been constructed as shown in FIGURES 14 and 15 to increase the ability of the air in each of the legs of the tunnels 5 to 11 to slow down and turn downwardly into the low-pressure zone at 82. Only one chamber 145 is illustrated for apparatus A because the others are constructed in a similar manner, as will be apparent to one skilled in the art from this disclosure.

The entrance portions 142 of the drying tunnels which extend from the room B to each plenum 82 of the apparatus A are provided with a length at least about equal to their width, and said length is several times the length of the corresponding portions of the apparatus A to reduce the tendency of the rapidly moving air in the tunnels to move into room B. The

legs

25, 26, 28 and 29, for example, are interconnected by connecting portions 143 to provide a large generally rectangular chamber 145 above the associated plenum 82., the sides of said chamber being closed by the side walls 144 which project laterally from the

tunnels

25 and 29 substantially as shown in FIGURES 14 and 15. Because the chamber 145 extends the full width and length of the plenum 82, there is no obstruction to downward flow toward duct portion 70. As shown in FIGURE 14 the chamber 145 preferably has a width greater than the width of each leg of the tunnels and a length several times its Width.

The low pressure created in the portion 70 and the chamber 145 by the blower 69 draws the air from the room B and from the

tunnel legs

25, 26, 28 and 29 into the plenum 82 as indicated by the arrows in FIGURE 13. Because of the large cross-sectional area of the chamber 145 as compared to said tunnel legs, the air decelerates rapidly upon entering said chamber and can be drawn downwardly into the low pressure chamber before its momentum carries it straight into room B. A person standing with his face near duct portions 142 of the apparatus A will not be disturbed by dust or moisture laden air blowing straight in from the drying tunnels as is sometimes the case with the apparatus A. Also, there is more efiicient removal of the dust from room B.

The amount of outside air drawn into the dipping and dusting room B around the doors 2, the entrance and exit openings for the conveyor chain, and other openings into said room will, of course, be equal to the total amount of air exhausted from the drying apparatus through main stack 90 and

conduits

49 and 50. If this outside air does not have a high moisture content, the

automatic control of the amount of air exhausted through the stack 90 by the controller 98 and its wetbulb sensing element at 13 is sufiicient to maintain the proper wet-bulb temperature in room B, and the apparatus A will function efiectively to maintain the proper humidity in each of the drying tunnels.

The apparatus A has been modified to permit the equipment to function properly even if the outside air has a very high temperature and high moisture content. In the apparatus A the

dampers

95 and 96 may be manually set in any desired position, and the air contrailer 98 may be disconnected from its sensing element and operated manually or omitted. If the dampers and their controls are omitted from the apparatus A, the cooling coil 92 is unnecessary and may be omitted. As shown in FIGURE 13, the duct 93 of the drying apparatus A has a partition 131 and a refrigeration coil 132 rigidly mounted therein whereby the air passing through the duct flows through the refrigeration coil and/ or through the by-pass conduit 135. The amount of air passing on either side of the wall 131 is controlled by one by-pass damper 136 and a series of cool air dampers 137 which are mechanically interconnected and operated simultaneously by the piston of a double-acting control cylinder 138, the by-pass damper opening when the other dampers close and vice versa. The flow of fluid to the opposite ends of the fluid motor 138 is controlled by a conventional valve control 139 which is operably connected to the-Wet-bulb sensing element 13.

If the wet-bulb temperature in the enclosure B goes above a predetermined amount, the sensing element 13 causes the control 139 to actuate the motor 138 in a direction to close or partially close the by-pass damper 136 and open dampers 137. The recirculating moisture-laden air entering duct 93 then passes over the cooling coils of coil 132 where the moisture therein is condensed and removed from the system before such air reaches the reheat coil 140 at the entrance to the duct 94. When the wet-bulb temperature in room B goes above said predetermined amount, the motor 138 closes the dampers 137 so most or all of the air in duct 93 passes through the by-pass 135. If such by-pass is relatively small, it is preferable to provide a blower 63 to assist the blowers 69 moving air out of duct 93. The blower 63 is preferably mounted in duct 94 near the opening 14 leading to enclosure B.

The reheat coil 140 is constructed to maintain a predetermined dry-bulb temperature in the room B and will maintain such temperature regardless of the amount of cooling effected by the refrigeration coil 132. The coil 132 functions like the coil 92 to cool the air and to condense and remove some of the moisture but can have a greater cooling capacity because of the reheating at 140. The refrigerant liquid which enters the coil at 133 and leaves at 134 can be maintained at any desired constant low temperature to provide the needed cooling capacity.

The equipment described above provides means for maintaining a predetermined wet-bulb and predetermined dry-bulb temperature in the dipping and dusting room B and permits very accurate control of the wet-bulb and dry-bulb temperatures in the drying tunnels by maintaining the specific or absolute humidity in the room B below the absolute humidity of the air in the drying tunnels. In a typical installation, when the wet-bulb temperature in the drying tunnels of the apparatus A is maintained at around 75 F., the dry-bulb temperature in the legs of the tunnel 5 is preferably maintained by the air conditioning means at about 85 F. and the dry-bulb temperature in the room B cannot exceed that in said tunnel 5. The automatic controls automatically maintain dry-bulb temperature in the room B at about 75 F. to 85 F. and the wet-bulb temperature at about 65 to 70 F. The absolute humidity of the air in the room B may, for example, be maintained at or below grains per pound of air in a typical installation where the wax patterns are maintained at a tempearture of around 75 F.

The conditions maintained in room B depend on several factors and will vary in different installations depending on the relative amount of air exhausted per unit of time, the amount recirculated through duct 93, and other things. The apparatus A and enclosure B may, for example, be constructed so that 1000 cubic feet per minute (c.f.m.) of air passes out main stack 90, 1000 c.f.m. of dust-laden air passes out

conduits

49 and 50 to cyclone 51, 2000 c.f.m. of outside air infiltrate around the doors 2 and other openings to room B, 6000 c.f.m. pass through duct 93, 7000 c.f.m. pass from ,the' room B to the plenums at 82, and 7000,c.f.m. pass through the ducts'91. This example is merely by .Way of illustration.

The above description is'by way of illustration, rather than limitation, and it will be understood that; variations and modifications of the specific apparatus shown and described herein may be made without departing from v the spirit of the invention. r

Having described our invention, we claim: 1. An apparatus for rapidly forming and drying layers of refractory material over destructible patterns to form thin-walled shell molds comprising: a series of narrow tubular drying tunnels, each having an incoming leg and 7 an outgoing leg arranged side by side, thetwo legs of each said tunnel having the same length and being: open at one end of said apparatus, enclosure means at one end. of said apparatus defining a dip-dust work room and re-" ceiving the open endsof. all of said legs, an endless'overr, head conveyor extending through all of said tunnels to connect all the legs of the tunnels in series and to carry the coated patterns through all of said legs at the same speed from the first through the last tunnel, said -conmovably suspending the coated patterns from said conveying means and for continually rotating the suspended patterns as they move longitudinally through the legs of the tunnels, a series of refractory coating dip tanks dis-' posed in said enclosure means for coating the patterns with ceramic slurry, a series of ceramic dustingimeans disposed in said enclosure means for applying dry ceramic particles to the outer surfaces of the coated patterns after they have been coated with the wet slurry, each dusting means having means for continually imparting motion to said ceramicparticles, one of said dip tanks and one of a nel, dip it in the slurry of a 'dip'tank, and then move the wet pattern to the associated dusting apparatus to apply a coating of ceramic particles before the coated, pattern is placed on the conveyor for entry into the next drying tunnel, means for. regulating the humidity of the air in said enclosure means, and'a series of air, conditioning means for regulating the humidity. and dry-bulb temperature of the airsin-said drying tunnels to maintain a predetermined wet-bulb temperature and a predetermined "drybulb temperature'in each tunnel.

' 3'. An apparatusas defined inclaim 2 wherein each of said air conditioning means has a, low-pressure plenum below said tunnels and near but spaced from said dip-dustroom and a header Vat-the end of the tunnels remote from said room, said plenum being connected to said header by a duct below the tunnels which contains the blower, an airrheateryand a water-'injectionnozzle, each header supplying conditioned air to lat-least two legs of the tunnels which'return the moisture-laden air to the associated plenum, and means for continually exhausting air from said last-named duct to atmosphere including "an exhaust duct connected -to said last-named duct.

' 4. An' apparatus as defined :in claim 3'wherein each header and the' plenum associated therewith are connected to four legs of the drying tunnels and recirculate said dusting means being positioned near each tunnel,

means associated with the firsttunnel for regulating the relative humidity of the air in the first tunnel, means associated with the last tunnel for regulating the relative humidity of the air in that tunnel and means associated with the intermediate tunnels of the series between the first and last tunnels for regulating the relative humidity of the air in theintermediate tunnels, said meansfor controlling the relative humidity in said tunnels also serving as means for maintainingthe wet-bulb temperature'in all of the tunnels substantially constant, whereby changes in the-temperatures of the patterns are minimized.)

an outgoing leg arranged side'by side, the legs of said tunnels being open at one end of said apparatus, enclosure means at said one end defining an elongated dip-dust room andreceiving the open ends of said legs, an endless conveyor extending through all of said tunnels to connect 0 the legs" of the tunnels in series and to carry'thecoated 2. An apparatus for rapidly forming and drying layers;

of refractory material over wax patterns toform thinwalled shell molds comprising; a series of narrow drying tunnels, each tunnel having an incoming legxa'nd an; outgoing leg arranged sideby side, the legs of said tunnels being open at one end of said apparatus, enclosure means at said one end defining an elongated dip-dust room and. a

receiving the open ends of said legs, endless overheadconveyor means extending through all of said tunnels to connect the legs of the tunnels in series and to carry the' coated patterns through all of said legs at the same speed a into and out of said room, said conveyor being installedto move the patterns in vone direction through the incoming legs and'in the opposite direction through the'outgoing le'gs', each leg having small cross-sectional area to facilitate rapid movement of air. over the pattern, means for removably suspending the patterns from said convey- 7 ing means and for continually rotating the suspended patterns as they move through the le'gs of the tunnels, blower:

means for forcing air through both legs of each tunnel toward said dip-dust'room and ,over the patterns-therein at a velocity of at least, 800 feet per minute and for creating a low pressure zone todivert such air away from said room and todraw additionalair from said ro'ojm'toward said blower means, a dip tankv and a (lasting meansin;

patterns through all of said legs at the same speed into and out of said room, said conveyor being installed to move the patterns in one direction through the, incoming legs and in the opposite direction through the outgoing legs, each leg having a small cross-sectional area to facilitate rapid movement of air over the patterns, means for removably suspending the patterns from said vconvey-ing means,;nieans-for forcing air through both legs of each tunnel towardjsaid dip-dust room at a velocity of at least 800 feet per minute including a lower duct under the tunnel legs having a low-pressure plenum at one end near said dip-dust room and a headerat its opposite end in communication with the tunnel legs at the ends thereof remote from said room, air conditioning means in said 'd'uct responsive to the temperature and moisture content of the air for controlling the temperature and humidity of the air enteringsaid header to maintain the wet-bulb first-named dUct eXhaus'tductmeans for exhausting a said dip-dust room for returning a portion of the air'from portion of the air from said exhaust duct to atmosphere, a return duct connected between ;said exhaust duct and said exhaust duct to said room, means associated with saidreturn duct for controlling the air flowing from said a the drying tunnels, means for admitting outside air of lower moisture content into said room in an amount equal to the amount of air. exhausted to atmosphere,

blower'rneans .in said lower duct for recirculating a por-.

'tion of the air in a closed path through said exhaust duct and said return duct to said room and from said room through the open ends of the tunnel legs to said plenum and back to said exhaust duct and for recirculating another portion of the air from said header through the tunnel legs back through said plenum to the inlet end of the blower, and a dip tank and a ceramic dusting device in said room associated with each of the drying tunnels, each dusting device containing ceramic particles and having means for continually imparting motion to said particles, whereby a large number of shell molds can be mass produced in said room by dipping the patterns in ceramic slurry, dusting the patterns at the dusting apparatus and placing the patterns on the conveyor for drying in said tunnels.

6. Apparatus as defined in claim where-in a series of said dip tanks and a series of said dusting devices are provided in said dip-dust room, and suction means are provided to exhaust dust-laden air from each of said dusting devices to atmosphere, one of said dusting devices being provided near the open ends of each tunnel.

7. Apparatus as defined in claim 5 wherein the portions of the tunnel legs over said plenum chamber are constructed to provide a wide unobstructed chamber extending to the top of said legs and extending laterally from the outer side of one tunnel leg to the outer side. of another tunnel leg to decelerate and facilitate downward flow of the rapidly moving air from the tunnel legs to the plenum chamber.

8. Apparatus as defined in claim 5 wherein the plenum chamber is provided with dust removing means comprising a bafiie and a water pan, said bafile directing the air toward the water in said pan.

9. Apparatus as defined in claim 5 wherein the air controlling means for said return duct includes refrigerator means for condensing moisture from the air in said return duct to maintain a predetermined absolute humidity in said dip-dust room and heater means for regulating the dry-bulb temperature of the air in said room.

10. An apparatus for forming and drying layers of refractory material over destructible patterns to form thinwalled shell molds comprising (1) a plurality of drying tunnels, each having an elongated incoming leg and an elongated outgoing leg, said legs being arranged side-byside, (2) each of said legs having a small vertical crosssectional area for passage of said patterns to facilitate rapid flow of air, (3) the ends of said legs at one end of said apparatus being open, (4) overhead conveyor means for said coated patterns disposed in each of said legs and passing through the same to connect the legs in series, (5 means for removably suspending said coated patterns from said conveying means, (6) air conditioning means for forcing air having predetermined wet-bulb and dry-bulb temperatures into said tunnels, (7) means for delivering at least a portion of said air from said legs to said air conditioning means, (8) enclosure means surrounding the open ends of said legs and the portions of said conveying means passing into the open ends of said legs, (9) collecting means for collecting a portion of the air in said air conditioning means and delivering it to said enclosure means, (10) a series of refractory coating dip tanks disposed in said enclosure means and positioned near the open ends of said elongated incoming legs, and (11) a plurality of mechanical ceramic dusting apparatuses disposed in said enclosure means and positioned near the open ends of said legs.

11. An apparatus for forming and drying layers of refractory material over destructible patterns to form thin walled shell molds comprising (1) a plurality of drying tunnels, each having an elongated incoming leg and an elongated outgoing leg, (2) each of said legs having a small vertical cross-sectional area not in excess of 4 square feet to facilitate rapid flow of air, (3) the ends of each of said legs at one end of said apparatus being open, (4) conveying means for said coated patterns disposed in each of said legs and tunnels and passing through the same to connect the tunnel legs in series, (5) means for removably suspending said coated patterns from said conveying means, (6) air conditioning means for forcing air having predetermined wet-bulb and dry-bulb temperatures into said tunnels, (7) means for delivering at least a portion of said air from some of said legs to said air conditioning means, (8) enclosure means surrounding the open ends of said legs and the portions of said conveying means passing into the open ends of said legs, (9) collecting means for collecting a portion of the air in said air conditioning means and delivering it to said enclosure means, (10) a series of refractory coating dip tanks disposed in said enclosure means and positioned near the open ends of said elongated incoming legs, and (11) dusting means disposed in said enclosure means for applying dry ceramic particles to the outer surfaces of the wet coated patterns, said dusting means being positioned near the open ends of said legs.

12. An apparatus for rapidly forming and drying layers of refractory material over destructible patterns to form thin Walled shell molds comprising: at least seven narrow drying tunnels, each having an incoming leg and an outgoing leg arranged side by side, the legs of said tunnels having substantially the same length and being open at one end of said apparatus, enclosure means at one end of said apparatus defining a dip-dust room and receiving the open ends of all of said legs, each of said legs having a small area not in excess of 4 square feet to facilitate rapid movement of air, an endless conveyor extending through all of said tunnels to connect all the legs of the tunnels in series and to carry the coated patterns through all of said legs at the same speed, said conveyor being installed to move the patterns in one direction through the incoming legs and in the opposite direction through the outgoing legs, means for removably suspending the coated patterns from said conveying means and for continually rotating the suspended patterns as they move longitudinally through the legs of the tunnels, blower means forcing air in the same direction through both legs of each tunnel and over the patterns at a velocity of at least 800 feet per minute, a series of refractory coating dip tanks disposed in said enclosure means near the open ends of said incoming legs to permit wetting of the patterns with a ceramic slurry, a series of rotary dusting means disposed in said enclosure means and positioned adjacent the open ends of said legs for applying dry ceramic particles to the outer surfaces of the coated patterns after they have been Wet with the slurry in the dip tanks, said dusting means having means for continually imparting motion to said particles, means for removing dust-laden air from said room, means responsive to the moisture content of the air for maintaining the wet-bulb temperature of the air in all of the tunnels at a substantially constant value of about 70 to F., means for maintaining a substantially constant dry-bulb temperature in the first tunnel, means for maintaining a materially higher dry-bulb temperature in the last tunnel, and means for maintaining a substantially constant dry-bulb temperature in an intermediate tunnel higher than the drybulb temperature of the first tunnel but less than the dry-bulb temperature of the last tunnel.

13. An apparatus for mass producing ceramic shell molds on wax patterns comprising a series of narrow drying tunnels, each having an incoming leg and an outgoing leg arranged side by side and being open at its opposite ends, enclosure means forming a closed dip-dust room that is elongated in a direction transverse to the tunnel legs, a series of dip tanks in said room, each containing a ceramic slurry, a series of ceramic dusting apparatuses in said room, each dusting apparatus containing ceramic particles and having means for continually imparting motion to said particles, one dip tank and one dusting apparatus being located near each drying tunnel, an endless overhead conveyor extending through all of said tunnels to connect all of the tunnel le'gs iniseries,

room and located under said dryingtunn'els, each of said I ducts being closed at one end by a hea er that is connected to the adjacent tunnel legs and having a plenum at its opposite end near said dip-dust room, blower means "moisturewontent for regulating the temperature, and

in each duct'for forcing air to the header and through the tunnel legs and for recirculating such air from said tunnel legs through the plenum and said" duct, the tunnel legs discharging the air downwardly into each plenum and admitting additional air to said plenum from saiddip-dust room, each of said blower meansdrawing saidladditional air from said room into its duct and forcing an equal amount of air out of said duct through an; exhaust duct,

' the exhaust ducts for both ofsaid air conditioning ducts being connected to a main exhaust duct, which exhausts a minor portion of the exhaust air to atmosphere, and also being connected to a common return duct, whichcarries the remainder of the exhaust air ha-cl; to said dip-dust room, openings for admitting fresh outside air to said room in an amount equal to that exhausted to atmosphere,

means in said return duct for automatically controlling I the air recirculated into said room to regulatethe humidity v in said room, and air conditioning means in ea-ch'of said first-named ducts responsive to the air temperature and MICHAEL V BRINDISI, Primary Examiner.

humidity of the air entering the associated header to main tain a predetermined :Wet-bulb and a predetermined drybulb temperature, the air conditioning means for the different drying tunnels beingcconstructed to maintain the Wet-bulb temperaturebetweenVO" and 80 F. and at about the same value,

References Cited by the Examiner UNIT ED STAT-ES, PATENTS- -1',5s0;957 4/26 Chaffe'et al. 118-66 1,740,681 12/29 Burnham"eta1." 3 446 1,905,694 4/33" Foley" -2 34203 [1,968,754 7 7/34 Harris 34-50 2,509,396 5/50 .Mayer. 2,754,104 7/56 Hess 2665 2,815,552 12/57 Turnbull et al. 22-l96 2,920,398 1'/ 60 Liljenstrom 34-46 2,948,935, 8/601 Carter. 2,961,751 11/60 Operhall et al. 2,964,310 12/60 Van Embden et a1 22196 FOREIGN PATENTS 889,948 2/62 Great Britain.

Claims

1. AN APPARATUS FOR RAPIDLY FORMING AND DRYING LAYERS OF REFRACTORY MATERIAL OVER DESTRUCTIBLE PATTERNS TO FORM THIN-WALLED SHELL MOLDS COMPRISING: A SERIES OF NARROW TUBULAR DRYING TUNNELS, EACH HAVING AN INCOMING LEG AND AN OUTGOING LEG ARRANGED SIDE BY SIDE, THE TWO LEGS OF EACH SAID TUNNEL HAVING THE SAME LENGTH AND BEING OPEN AT ONE END OF SAID APPARATUS, ENCLOSURE MEANS AT ONE END OF SAID APPARATUS DEFINING A DIP-DUST WORK ROOM AND RECEIVING THE OPEN ENDS OF ALL OF SAID LEGS, AN ENDLESS OVERHEAD CONVEYOR EXTENDING THROUGH ALL OF SAID TUNNELS TO CONNECT ALL THE LEGS OF THE TUNNELS IN SERIES AND TO CARRY THE COATED PATTERNS THROUGH ALL OF SAID LEGS AT THE SAME SPEED FROM THE FIRST THROUGH THE LAST TUNNEL, SAID CONVEYOR BEING INSTALLED TO MOVE THE PATTERS IN ONE DIRECTION THROUGH THE INCOMING LEGS AND IN THE OPPOSITE DIRECTION THROUGH THE OUTGOING LEGS, BLOWER MEANS FOR FORCING AIR THROUGH BOTH LEGS OF EACH TUNNEL IN THE SAME DIRECTION AT A VELOCITY OF AT LEAST 1000 FEET PER MINUTE, MEANS FOR REMOVABLY SUSPENDING THE COATED PATTERNS FROM SAID CONVEYING MEANS AND FOR CONTINUALLY ROTATING THE SUSPENDED PATTERNS AS THEY MOVE LONGITUDINALLY THROUGH THE LEGS OF THE TUNNELS, A SERIES OF REFRACTORY COATING DIP TANKS DISPOSED IN SAID ENCLOSURE MEANS FOR COATING THE PATTERNS WITH CERAMIC SLURRY, A SERIES OF CERAMIC DUSTING MEANS DISPOSED IN SAID ENCLOSURE MEANS FOR APPLYING DRY CERAMIC PARTICLES TO THE OUTER SURFACES OF THE COATED PATTERNS AFTER THEY HAVE BEEN COATED WITH THE WET SLURRY, EACH DUSTING MEANS HAVING MEANS FOR CONTINUALLY IMPARTING MOTION TO SAID CERAMIC PARTICLES, ONE OF SAID DIP TANKS AND ONE OF SAID DUSTING MEANS BEING POSITIONED NEAR EACH TUNNEL, MEANS ASSOCIATED WITH THE FIRST TUNNEL FOR REGULATING THE RELATIVE HUMIDITY OF THE AIR IN THE FIRST TUNNEL, MEANS ASSOCIATED WITH THE LAST TUNNEL FOR REGULATING THE RELATIVE HUMIDITY OF THE AIR IN THAT TUNNEL AND MEANS ASSOCIATED WITH THE INTERMEDIATE TUNNELS OF THE SERIES BETWEEN THE FIRST AND LAST TUNNELS FOR REGULATING THE RELATIVE HUMIDITY OF THE AIR IN THE INTERMEDIATE TUNNELS, SAID MEANS FOR CONTROLLING THE RELATIVE HUMIDITY IN SAID TUNNELS ALSO SERVING AS MEANS FOR MAINTAINING THE WET-BULB TEMPERATURE IN ALL OF THE TUNNELS SUBSTANTIALLY CONSTANT, WHEREBY CHANGES IN THE APPARATUS OF THE PATTERNS ARE MINIMIZED.