US2659987A

US2659987A - Sheet moisture drying machine control mechanism

Info

Publication number: US2659987A
Application number: US325874A
Authority: US
Inventors: Andrew E Bennett
Original assignee: MASON NEILAN REGULATOR Co
Current assignee: MASON NEILAN REGULATOR Co; MASON-NEILAN REGULATOR Co
Priority date: 1952-12-13
Filing date: 1952-12-13
Publication date: 1953-11-24
Anticipated expiration: 1970-11-24

Description

NOV. 24, 1953 BENNETT 2,659,987

SHEET MOISTURE DRYING MACHINE CONTROL MECHANISM Filed Dec. 1:5, 1952 2 Sheets-Sheet 1 Nov. 24, 19:53 A. E. BENNETT SHEET MOISTURE DRYING MACHINE CONTROL MECHANISM 2 Sheets-Sheet 2 Filed Dec. 115, 1952 :WENTOR: H/VDREW Cl BEA/N577 BY CWM wu n L Tatented Nov. 24, 1953 SHEET MOISTURE DRYING MACHINE CONTROL MECHANISM Andrew E. Bennett, Weymouth, Mass., assignor to Mason-Neilan Regulator Company,

Boston,

Mass., a voluntary association of Massachusetts Application December 13, 1952, Serial No. 325,874

17 Claims. (01. 34-48) This invention relates to control mechanism for use in connection with drying machines commonly employed in industries concerned with the manufacture of paper, textiles, synthetic material and the like for reducing the moisture content of such material to a predetermined value. Drying machines of the type referred to comprise a plurality of heating elements, usually in the form of rotating drums or rolls, commonly arranged in one or more main drying sections, over which the material passes from the so-called wet end of the dry end of the machine, it being understood that the material being dried by machines of this type may be in the form of solid sheets such as paper, felt, cloth or the like, or may consist of sheets in the form of strands as, for example, are commonly termed the warp in themanufacture of cloth. And the control mechanism to which this invention relates is adapted to govern the heat input to a drying roll or rolls in one or more main drying sections to reduce the moisture content of the sheet material passing through the machine to a predetermined value. More particularly this invention relates to that type of sheet drying control mechanism wherein a pilot dryer section in the form of one or more drying rolls, associated with a main drying roll section, is used to reflect or indicate changes in sheet moisture, to which changes the mechanism embodying this invention responds and varies the heat input, as for example steam, to a main section of drying rolls as required to maintain the desired moisture content in the sheet as it leaves the section. In controls of this type it is customary to supply steam to the indicating roll or what may be termed the pilot dryer section, independently of the steam supplied to the main drying roll section, and by responding to changes in the rate of condensation of steam in an indicating roll, occasioned by changes in the moisture content of the sheet passing over the roll, the flow of steam to the main drying roll section is varied eating section and changes in pressure in said section are utilized to vary the heat input to the main drying section. Inasmuch as the range of a fixed orifice is limited, each orifice must be sized accurately to the immediate working conditions, and it therefore becomes necessary to provide a number of orifices to meet the variety of working conditions commonly encountered in normal mill operation. Furthermore, as is more fully pointed out herein, a definite time lag is introduced into measurement of a change in sheet moisture, which time lag together with the lag inherent in the control loop of a sheet material drying machine is a major factor in creating instability of control. Therefore sensitive responses to changes in sheet moisture cannot be obtained without a definite tendency to throw the system into a cycle under normal operating conditions.

I have discovered that by providing a variable orifice in the conduit to the indicating section, and maintaining a constant pressure-upstream of the orifice, if the orifice size is automatically varied as required to maintain a substantially constant differential pressure across the orifice, I am able to utilize the orifice size or opening as a measure of sheet moisture load, thereby providing means for measuring sheet moisture changes over a wide range of load change, such as may be encountered in the usual mill operation, and at the same time I am also able to obtain an immediate measurement of sheet moisture change. By this means the difficulties encountered with the fixed orifice type of control, namely, limited rangeability, and the time lag inherent in that form of control, are avoided and a stable and extremely; sensitive control may be provided which is suited to a wide rang of operating conditions.

The mechanism embodying this invention also functions without the use of additional equipment to reduce the drying machine temperature during the interval after a sheet break occurs and sheet material is not passing through the machine. The additional equipment referred to is the subject of my copending application for United States Letters Patent filed January 28, 1952 under Serial No. 268,618, now Patent No. 2,629,939 dated March 3, 1953, and is disclosed in connection with flow meter types of sheet moisture drying machine controls, this application being a, continuation-in-part, subtractive, of the aforesaid copending application. And in this connection I have also found that since the measurement lag in substantially eliminated by the means embodying this invention, after a break in sheet material has occurred and the machine temperature has been automatically reduced to a selected value, the time required to increase the heat input to the main drying section after the sheet has again been introduced to the machine is less than one-fifth the time interval required with the flow meter and fixed orifice form of control. And since modern sheet material dry-machines may have a roll surface speed of 2,000 feet per minute or more, I am able to effect a very considerable saving in time and in the quantity of sheet material which has passed through the machine without the desired moisture content.

It is an object of this invention to provide a control mechaism which may be used to govern the moisture content of sheet material within extremely close limits as the sheet leaves the dry end of a drying machine or the dry end of one or more drying sections thereof.

It is an object of this invention to provide an improved mechanism which responds to minute changes in moisture in a sheet of material passing over one or more drying machine rolls having a separate steam supply from that which supplies the rolls of a main drying section, for the purpose of varying the heat input to the said main drying section and thereby maintaining a predetermined sheet moisture content.

It is an object of this invention to provide a sheet moisture drying machine control mechanism which is suitable for application to multiplesection machines and which may operate from a moisture content of 60 or '70 percent at the wet end section and the moisture may be thereafter gradually reduced through the intermediate sections to a moisture content of 2 or 3 percent at the dry end of the machine.

It is an object of this invention to provide a means of measuring moisture changes in sheet material passing through a drying machine without any substantial measurement time lag which, when present and combined with the time lag inherent in the control of a sheet material drying machine, materially increases the instability of control. 7

It is an object of this invention to provide an improved control mechanism which functions both to maintain a selected moisture content in sheet material passing through a drying machine and also to reduce automatically the temperature of the machine when a break in sheet material occurs, thereby preventing overdrying of the sheet and consequent damage thereto when the sheet is again introduced to the machine.

While there is herein shown and described a preferred form of control mechanism wherein a controller which governs the stem input to a main drying roll section of a sheet material drying machine is responsive to roll pressure, it will be understood by those skilled in the art that the said heat input may also be governed by a controller responsive to roll temperature in said section or to the rate of flow of steam to the rolls of said section; and it will be further understood that while there is herein shown and described a preferred form of control mechanism, including means responsive to changes in steam pressure in a sample roll, for the purpose of indicating changes in moisture in sheet material passing over the roll, as indicated in modified views such means may be responsive to roll temperature if desired, since changes in sheet moisture are reflected by both pressure and temperature.

Further objects and advantages of this invention will be apparent from the following description when taken in connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view, partly in perspective, of a control mechanism embodying this invention as applied to a paper drying machine.

Fig. 2 shows, in perspective, a modified construction of part of the mechanism shown in Fig. 1; and

Fig. 3 shows, in end elevation and enlarged, a modified construction of the mechanism shown in Fig; 2. Having reference to the drawings and particularly to Fig. 1, there is illustrated a portion of a drying roll section, generally indicated at 9, of a paper drying machine. It will be understood that the drying machine may comprise a plurality of drying sections similar to that shown in Fig. 1, and that a multi-section machine may be provided at selected points between its wet and dry end with suitable breaker rolls and coating or sizing rolls, not shown, which may be required for certain grades and types of paper as is well known to those skilled in the art.

The section 9 includes a plurality of rolls [0 which are connected in the usual manner by pipes l I with a main steam header l2, supplied from a source, not shown, by a conduit 13, and are also connected by means of condensate pickup pipes, not illustrated, and pipes 14 with a condensate conduit l5 leading to a separator, also not shown. The drying rolls i 8 are rotated in unison by means not illustrated, and are engaged by a paper sheet l6 which passes over and under the rolls in the direction indicated by the arrow from the wet end to the dry end of the machine all as shown, the usual felt being omitted to simplify this description.

While there is herein illustrated a plurality of drying roll l0 forming part of a drying roll section, it will be understood by those skilled in the art that the drying roll section may consist of one large roll, such as that employed in a Yankee Dryer commonly used for drying light tissue, or ma comprise any other form of drying means suitable to the particular type of sheet material being processed.

Means for governing the flow of steam to the rolls H) to provide the temperature required to reduce the sheet moisture content to a predetermined value as the sheet leaves the drying machine section is herein illustrated as a pressure controller, generally indicated at 20, which functions in connection with a variable orifice in the form of a motor valve 2| to vary the flow of steam through the supply conduit [3 as required to provide a selected sheet moisture content. The controller 29 may be of any Well-known construction and for the purpose of this description, is shown diagrammatically in simplified form. It includes a spiral Bourdon coil 22, a control couple consisting of a nozzle 23 and flapper 24, a pneumatic proportioning device 25 and a pneumatic control point reset device 25. While the controller is shown without a booster pilot for speeding up the motor valve action and without a reset bellows or its equivalent for eliminating so-called drift, it will be understood that either or both of these devices may be employed if desired.

The Bourdon coil 22 is mounted with one end secured to a circular back plate 21 which in turn is rotatably mounted on the controller back plate 21 indicated in broken lines. The spiral coil is provided with a central shaft 28 journalled in the plate 21 at its inner end and at its outer end in an arm 29 which extends radially over the coil and is rigidly attached to the said plate 21. A motion transmitting arm 30 is secured to the shaft 28 which in turn is in operative connection with the free end of the coil. The fixed end 3| of the coil communicates with a flexible tube 32 which, as herein shown, connects with the header l2, the parts being arranged so that on an increase of pressure in the header, thecoil unwinds turning the arm 30 clockwise, and on a decrease source not shown, through pipes 33 and 34, the

former having a restriction 35 of less capacity than that of the nozzle orifice so that the nozzle pressure is governed by the relative position of the flapper in respect to the nozzle. The proportioning device includes a bellows 36 having a fixed mounting 31 to which one end of the bellows is connected, the interior of the bellows being in communication with pipes 33 and 34 by means of a pipe 38. Secured to the free end 39 of the bellows is a post 46 which is also attached to a disc 4|, an expansion spring 42 being confined between the disc and a fixed member 43 so that the end of the flapper 24, V which is pivotally connected to the post at 44, assumes a position which is proportional to the fluid pressure in the bellows 36, the overall bellows movement being determined by the rate selected for the spring 42. The other'end of the flapper 24 is engaged at 45 by one end of a lever 46 pivotally mounted at 46 on a post 41 forming part of the control point reset device 26 to be described. The other end of the; lever 46 is engaged by the free end of the coil arm' and the parts are So arranged that an increase in fluid pressure in the steam header l2 causes the coil to unwind and turn the arm 30 clockwise, thereby moving the flapper 24 away from the nozzle 23. This movement results in a decrease in pressure in the proportioning bellows 36 causing the bellows to contract and return the flapper to a throttling relation with the nozzle at a lower pressure, which pressure is proporactuated by a diaphragm 5i defining one wall of a, fluid pressure operating chamber 52, the pressure in which is opposed by a spring 53. The diaphragm chamber 52 is connected with the controller piping 33 and 34 by means of a pipe 34 the valve parts being arranged so that the valve member 49 moves toward closed position on a decrease in operating pressure. Thus when the pressure in the steam header I2 increases or decreases, the pressure in the nozzle 23 decreases or increases, respectively, in proportion thereto, thereby providing a proportional throttling action to the valve, and varying the flow of steam to the header l2 in a direction to return the pressure in the drying rolls 10 to a set value.

It will be understood by those skilled in the art that the pressure at which the controller is set depends on the initial position assumed by the flapper in relation to the nozzle and that this in turn depends both on the radial position assumed by the coil arm 30 at a selected pressure in the header I 2 and on the vertical position of the pivotal connection 46 between the lever 46 and post 41 of the pneumatic reset device 26.

Suitable means for manually adjusting the radial position of the coil arm 30 is in the form of a hand operated gear 54 which meshes with a toothed section 55 on the plate 21, the pipe 32 being provided with a flexible portion 56 to permit the coil to be rotated by the gear to any selected position within the range of control setting adjustment. When the coil is rotated counterclockwise the pressure setting is raised and vice versa. The manual control point settling device just described functions in connection with the pneumatic control point resetting device 26 in a manner to be hereinafter set forth.

Means for adjusting the proportioning device 25 to vary the width of the proportioning band of the controller may be in the form of a rack 51 on which the nozzle 23 is mounted, movement toward the connection 45 between the flapper 24 and the lever 46 being effective to narrow the proportional band and vice versa. To permit of this adjustment, the pipe 34 is provided with a flexible portion as indicated at 58.

The apparatus just described illustrates a simple form of control mechanism for governing the valve 2| to maintain a set pressure in the header I2 and functions with apparatus to be described to vary the pressure setting and thereby the roll temperature as required to maintain the moisture content of the sheet at a selected value as it leaves a main drying section. And it will be understood that while I have shown a pressure controller by Way of illustration, a temperature or rate of flow controller can be utilized, the object being to provide a roll temperature such as may be required for the purpose.

Referring again to Fig. 1, the drying section 9 includes an indicating roll 60 which is preferably at or near the dry end of the section and which may consist of one or more rolls if desired. The roll 60 is segregated from the main drying section rolls Ill in that it is supplied with steam under pressure through a separate conduit 6| which connects with a suitable source of supply, not shown. The roll 60 is provided with means for removing the condensate as by an internally mounted pickup pipe, not illustrated, which connects with a condensate discharge pipe 62 having a steam trap 63 which in turn communicates with a discharge conduit 64. In order that the steam pressure in conduit 6| may be held reasonably constant, I provide a reducing regulator which may be of any usual construction. The regulator shown is of the so-called direct acting type and is generally similar in construction to the valve 2|. It has a diaphragm chamber 9| which is connected with the conduit 6| on the downstream side of the valve by means of a pipe 92. The pressure in the diaphragm chamber 9! is opposed by a spring 93 which may be tensioned to provide a selected reduced pressure on the downstream side of the valve, and since an increase of pressure tends to close the valve and a decrease in pressure tends to open it, a reasonably constant pressure is thereby maintained.

The equipment insofar as hereinabove described is similar to that shown and described in my copending application referred to above.

pressure in the pilot dryer 60 within selected limits. However-instead of providing a fixed orifice in the conduit 6I as shown and described in the said application, the fixed orifice is eliminated and the valve 66 functions as a variable orifice. And since the regulator 90 maintains the pressure upstream of the variable orifice substantially constant,the controller-transmitter 95 functions with the regulator 90 to maintain the pressure drop across the variable orifice constant within said selected limits. Therefore by maintaining a constant pressure upstream of the variable orifice, the orifice opening may be utilized as a measure of the rate of heat transfer through the wall of the dryer 60 both when the sheet is ofi the machine, as may be occasioned by a break in sheet material, and when the sheet is passing through the machine, as in normal operation.

The valve 56 may be similar in construction to the valve 2I in the conduit I3 and comprises a reverse acting valve member 61 positioned by a stem 68 connected with a diaphragm 69 which is subjected to fluid operating pressure in a chamber I9 on one side and to the force of a compression spring II on the other side. A valve positioner, generally indicated at I2, is preferably employed to position the valve member 61 in respect to the valve seats on a linear basis with changes in a controlling pressure. The valve positioner may be similar to that shown in the United States patent to John Robins, No. 2,588,988, dated March 11, 1952. It receives a supply of fluid pressure from a source not shown through a pipe I3, and is actuated by the valve stem 68 to vary fluid pressure in the diaphragm chamber I0, to which it is connected by a pipe It, as required to position the valve member 6? in linear relation to changes in controlling pressure received through a pipe I5 connected with the controller-transmitter 95, now to be described.

Referring again to Fig. 1, the controller-transmitter 95, which also may be of any well known construction, is employed to respond to roll pressure changes and to provide an output pressure proportional thereto. The controller-transmitter includes a spiral coil 96, having a flapper actuating arm 9?, and a manual control reset mechanism generally indicated at 68 both of which may be similar to the coil 22 and manual reset 54-2I described in connection with the pressure controller 20. The controller-transmitter also has a proportioning device I of similar construction to the proportionihg device 25, shown in connection with the controller 20, together with a flapper IDI which is positioned by the coil arm 97 and proportioning device I00 in respect to a nozzle I02. Since the nozzle I02 is herein shown as being mounted above the flapper IOI, the bellows I03 and spring I04 are in the reverse position from that shown in the controller proportioning device 25. A sensitive or proportion ing band adjustment I05 likewise similar to that in the controller 20, is also provided. The fixed end of the coil 96 communicates with the steam conduit 6I by means of a pipe 99 at a point between the valve 66 and the roll 60. The controller nozzle I62 is connected with a regulated pressure supply of fluid from a source not shown through a pipe I06 having a restriction I01 of less capacity than that of the nozzle, pipe I06 being connected with a pipe I06 which communicates with the nozzle. A connection I08 between the pipe I06 and proportioning bellows I03 serves to communicate the nozzle pressure with the interior of the bellows I03 in the usual manner. The 'nozzle pressure is also connected with the valve positioner I2 by means of the pipe I5 referred to above. It will be understood that a decrease of pressure in the roll 60 and therefore in the coil 96, occasioned by an increase in moisture load, causes the coil to wind up, thereby turning the arm 91 counterclockwise and moving the flapper IIlI towards the nozzle I02 to increase the nozzle pressure. The resulting expansion of the bellows I03 moves the flapper away from the nozzle and returns the parts to a throttling relation to provide an increase in the nozzle pressure in the pipe I06 or in other words an increase in the controller-transmitter output pressure, on a basis which is proportional to the decrease in roll pressure. And it will also be understood that on a decrease in roll pressure, aproportional increase in controller-transmitter output ressure occurs. These changes in output pressure may be utilized both to vary steam input to the dryer 60 as required to maintain the dryer pressure within selected limits as deter-- mined by the setting of the proportioning band, and also as a measure of the valve opening which varies in direct proportional relation to changes in sheet moisture load. And by means of mechanism hereinafter to be described which functions with the controller-transmitter 95 and with the controller 20, the proportioning adjustment of the controller-transmitter 95 may be set so that there is no substantial tendency of the pressure in the dryer 60 to cycle and at the same time the controller 20 may be effective to vary the heat input to the main drying section in proportional relation with minute changes in sheet moisture load.

The mechanism just referred to is in the form of a receiver-controller indicated at 8, which is herein shown as generally similar in construction to the controller-transmitter 95. It includes a coil I6 which is responsive to changes in the output pressure of the controller-transmitter 95 by means of a pipe 11 which connects with the output pressure line I5 of the controller. It also includes a coil actuated arm I8, a flapper I9, a cooperating nozzle 80, and a proportioning device 6i which comprises a bellows 82 and an opposing spring 03 by the combined action of which and of the arm I8 the flapper is positioned in respect to the nozzle. Th nozzle is supplied from a source of operating fluid, not shown and preferably under regulated pressure, through a pipe 0 3 having the usual restriction 85, a pipe 86 serving to connect the nozzle with the pipe 84 to which the bellows is also connected by means of a pipe 87. The nozzle 80 is movably mounted on a rack 38 to permit the proportiohing band of the receiver to be adjusted, a flexible portion 89 being provided in the pipe 86 for the purpose. It will be noted that the parts just described are in the reverse position to like parts in the controllertransmitter 95. Thus an increase in pressure in the coil I6 rotates the arm I8 clockwise to provide a proportional increase in nozzle pressure in the pipe 86. And it will be understood that a decrease in pressure in the coil I6 rotates the arm 18 counterclockwise to provide a proportional decrease in nozzle pressure in the pipe 86. The receiver-controller 8 functions together with the controller-transmitter 95 and with the control point setting device 26 of the controller 29 to increase the sensitivity of the system in a manner to be hereinafter set forth.

The control point resetting device 26 has a bellows H having a fixed mount HI through which the interior of the bellows is connected with the receiver-controller output pressure pipe 86 by means of a pipe 90. The post 41 on which the lever 46 is pivotally mounted as hereinabove described, is secured to the free end H3 of the bellows and to a disc H4 between which and the fixed mount H5 an expansion spring 146 is confined. Since the vertical position of the pivot 46 determines the control setting value of the controller 20 in respect to the setting established by the rotary position of the coil arm 30 as provided by the manuallyoperated gear 54, the bellows H0 functions to vary the control setting above and below the value established by the manually set mechanism on a basis which is inversely proportional to changes in roll pressure. Thus when the sheet moisture increases and the pressure in the roll 60 starts to drop, the proportional increase in output pressure in the pipe of the controller-transmitter 95 causes the coil 16 of the receiver-controller 9 to unwind, thereby rotating the arm 18 clockwise and causing the flapper 19 to approach the nozzle 89 to increase the nozzle pressure. The increase in nozzle pressure expands the proportioning bellows 82 and returns the flapper 19 to a. throttling relation with the nozzle at an increase in pressure, which increase is proportional to the increase in controller-transmitter output pressure in the pipe 15, it being understood that the extent to which the pressure increases. is dependent upon the position assumed'by the nozzle 80 lengthwiseof the rack 88. This increase in pressure, which is communicated to the bellows H0 of the pneumatic set device 26 by the pipe 90, expands the bellows, thereby moving the flapper 24 of the controller 20 towards the nozzle 23 and raising the pressure setting of the controller by an amount which is proportional to the increase in receiver output pressure in the pipe 90.

In the operation of the control system, let it be assumed that the paper sheet I6 is passing through the drying machine, that the controller 29 is maintaining a pressure in the main drying section rolls E0 of p. s. i., and that the controller-transmitter 95 has a pressure setting for the pilot dryer of 25 p. s. i. Let it be assumed further that the coil 96 of the controller-transmitter 95 has a range of 060 p. s. i., that the coil 16 of receiver-controller 8 has an overall operating range of 3-15 p. s. i., that the coil 22 of the controller 20 has a range of 0-100 p. s. i., and that the output pressure of the controllertransmitter 95, the receiver-controller 8 and the controller 20 operates over a range of 3-15 p. s. i. Upon an increase in moisture in the sheet IS, the pressure in the roll 60 starts to drop and the controller-transmitter 95 increases its output pressure to the valve 65, thereby opening the valve to increase the steam input to the roll 69 by an amount which tends to return the roll pressure to the setting of 25 p. s. i. and therefore the pressure drop across the orifice to a constant value. Thus the increase in valve opening is a measure of the increase of steam flow required and the increase in output pressure in the pipe I06 of the controller-transmitter 95 is communicated to the coil 16 of the receiver-controller 8. Inasmuch as the coil 96 of the controllertransmitter functions over a 60# range and the coil 16 of the receiver-controller 8 functions over a 12# range, with the proportioning band adjustment of the controller-transmitter at 100 percent the response of the coil 16 is basically five times as sensitive as the response of the coil 96. On this basis of sensitivity the increase in pressure in the receiver-controller coil 15. provides a proportional increase in output pressure in the pipe 86 of the receiver-controllr 8 and this increase in pressure is communicated to the control point setting bellows H0 in the controller 29 by means of the pipe 90, thereby raising the pressure setting of the controller 20 to reduce the moisture content of the sheet 16. It will be understood that on a decrease in sheet moisture, the opposite occurs.

I have found from actual tests on a commercial paper drying machine that sheet moisture may be maintained within minute limits by employing a 5 percent proportioning band for the controller-transmitter 95, a 10 percent proportioning band for the receiver-controller 8, a 20 percent proportioning band for the controller 20, and a 100 percent operating range for the control point setting device 26, without any tendency of the control to become unstable under drastic changes in moisture load. I have also found from actual tests on the same paper drying machine that with the flow meter type of moisture control, wherein a constant steam input to the pilot dryer is maintained across a fixed orifice, an instrument responsive to changes in pilot dryer pressure, and utilized to reset the pressure setting of a controller governing the steam pressure in a main drying section, requires a proportioning band setting of 30 percent in order to avoid cycling under normal operating conditions. Thus in the control system embodying this invention, since the proportioning band is 5 percent as compared with 30 percent in the control system just referred to, the initial response to sheet moisture change is not only six times as sensitive as the corresponding response in the flow meter type of control, but the receiver-controller 6 may be also utilized to increase the sensitivity still more without creating any tendency to instability under drastic changes in sheet moisture.

It will be understood by those skilled in the art that the control loop in a sheet moisture drying machine control system is subject to an inherent lag since a substantial period of time is required between the initial response to a moisture change and the return of sheet moisture towards a selected value by means of a change in heat input to the main drying section. This time lag tends to cause instability and is characteristic of all sheet material drying machines so far as known to me. It therefore becomes necessary to provide a sensitive response to sheet moisture change in order to avoid introducing additional time lag in the measurement per se, which time lag added to the time lag inherent in the control loop is extremely unfavorable to stability.

In carrying out various tests on a commercial drying machine, I find that there is a definite measurement lag with the flow meter form of control, whereas in the control embodying this invention this lag is substantially eliminated. For example, in the flow meter system wherein a constant steam input is maintained to the pilot dryer and a moisture change is measured by a change in pilot dryer pressure, the pressure in the dryer can only be changed by changing the B. t. u. heat content of steam in the dryer. Since this change must be accomplished by the paper, a definite quantity of paper of a given moisture content and temperature must pass over the dryer to effect a given change in dryer pressure. This quantity or weight of paper requires a definite time to pass over the dryer. As an example, disregarding the specific heat of the dryer shell, assume that a sheet having a percent moisture content is passing over the dryer, and that the dryer is subjected to 25 p. s. i. steam pressure. Assume further that each 1 p. s. i. change in pressure requires a 1,000 B. t. u. change. If the moisture content of the sheet is suddenly changed to percent, the pressure in the dryer will start to decrease and the decrease will continue until a new equilibrium pressure is finally obtained at say 24 p. s. i. During this time interval a 1,000 B. t. u. heat transfer from the dryer to the sheet material has occured. The new value of dryer pressure would be reached as an exponential function of time. Thus the time lag involved in the response to sheet moisture change is combined with the time lag inherent in the control loop, and the tendency to instability is materially increased.

Referring to the flow meter control system of the type wherein a set pressure is maintained upstream of a fixed orifice and the change in pressure drop across the orifice is used as a measure of an increase in sheet moisture, while the flow through the fixed orifice increases somewhat as the pressure drops in the dryer and therefore the final pressure change is not as great as that required in the flow meter type of control wherein a constant steam input is maintained, nevertheless a definite pressure drop is required which takes place over the period of time determined bp the cooling effect accorded by whatever quantity of paper has to pass over the pilot dryer until the temperature equilibrium is reached. Thus in either the constant steam input system, or in the system which responds to changes in rate of flow across a fixed orifice,

there can be no immediate measurement of the extent of the moisture change, and therefore the time lag involved is combined with the time lag inherent in the control loop of the machine to definitely increase the tendency to instability.

In the control system embodying this invention, since a slight drop in dryer pressure is immediately responded to by increasing the valve opening to an amount required to maintain the pressure Within reasonably close selected limits, the valve opening obtaining for this purpose is an immediate measure of the extent of the increase in sheet moisture, and by means of this system of control, the valve governing the heat input to the main drying roll section is at once positioned at an opening required to offset the change in sheet moisture and maintain the moisture content of the sheet as it leaves the drying section at a selected value. And I have found that by means of the receiver-controller 8 which functions in connection with the controllertransmitter 95 and the set point device 26 in the controller 20 to vary the heat input to the main drying section on an extremely sensitive basis, I am able to maintain the sheet moisture at the desired value within minute limits.

When a break in sheet material occurs, the

operation of the control system embodying this invention is similar to that described above. As

soon as the sheet passes off of the machine, the sheet moisture load is eliminated, the pilot dryer pressure starts to rise, and the opening of the valve 6% is immediately reduced until the transfer of heat through the dryer shell, caused by the atmosphere and by the felt when employed, is equalled by the steam input required to maintain a substantially constant set pressure in the dryer. Thus the valve opening becomes a measure of the heat transfer and the main valve 2I is immediately positioned at such opening as may be needed to maintain the main drying section pressure at a value that is proportional to the rate of heat transfer across the drying rolls.

It will be understood that when the sheet is again introduced to the machine, the pressure of the pilot dryer starts to drop and the opening assumed by the valve 66 is again the measure of the change in moisture load. Thus the valve 2| is again positioned at such opening as may be required to maintain the selected sheet moisture content. By this means the sheet when again introduced to the machine is not damaged by overheated rolls and a considerable waste of material is avoided. Thus the same apparatus functions both as a means of obtaining a desired sheet moisture content and as a means for automatically maintaining the drying section at a suitable temperature during the period when there is no material passing through the machine.

Fig. 2 illustrates an alternate construction wherein the controller-transmitter coil 96 responds to the internal temperature of the roll 6!! instead of to the roll pressure as shown in Fig. 1. The roll 60 to which the steam supply pipe BI and condensate discharge pipe 62 are connected as shown in Fig. 1, is provided with a temperature bulb I25 having the usual capillary tubing [26 which connects with the fixed end of the coil 9%, thereby providing a sealed thermal system which may be filled with a suitable expansible fluid. The bulb I25 extends into the interior of the roll 60 and is sensitive to changes of temperature therein.

The operation of the controller-transmitter is substantially identical to that described in Fig. 1, wherein the controller-receiver responds to changes in the roll pressure. Thus the device functions in the same Way as the pressure response shown in Fig. 1 since the coil 96 responds to changes in thermal system pressure which is substantially the equivalent to changes ,in roll pressure,

In Fig. 3 there is shown a means for responding to changes in the surface temperature of that portion of the cylindrical surface of the roll 60 which is engaged by the sheet I6, the roll 60 being connected with the steam inlet conduit BI and the discharge pipe 62 as hereinabove described. For this purpose a surface temperature measuring device I30 of any well-known suitable construction is employed, preferably a measuring device of the type shown and described in the application for United States Letters Patent of Andrew E. Bennett and Daniel S. Brown, filed February 16, 1951, under Ser. No. 211,336. The measuring device I30 may be mounted on a stand I3I at the cylindrical surface of the roll 60 and at a selected position on that portion of the surface which is engaged by the paper sheet. A capillary tube I26 which corresponds to the tube I26 in Fig. 2 is connected to a bulb I121 in the measuring device I30, a suitable expansible fluid being employed to vary the fluid pressure in the coil 96 in response to changes in the roll surface 13 temperature. It will be noted that this means of response to changes in roll temperature functions generally in a manner described in connection with Fig. 2.

In reference to the description of Figs. 2 and 3, it will be understood that an equivalent temperature measuring means may be utilized for the purpose as, for example, a thermocouple or other electrically operated means.

Having thus described my invention, what I claim as new therein and desire to secure by Letters Patent of the United States is:

1. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section and a sheet moisture indicating section, and

each section has a conduit connecting with a source of heating fluid under pressure, in combination, a first variable orifice in the main section conduit, a second variable orifice in the indicating section conduit, means maintaining a constant pressure in the indicating section conduit upstream of said second orifice, mechanism responsive to changes in the rate of heat transfer in the indicating section for varying the opening of said second orifice to maintain a constant pressure differential across said orifice within bination, a first variable orifice in the main sec-.

tion conduit, a second variable orifice in the indicating section conduit, means maintaining a constant pressure in the indicating section conduit upstream of said second orifice, first mechanism responsive to changes in the heat value of the main section for varying the opening of said first orifice to maintain a selected heat value in said main section, second mechanism responsive to changes in the rate of heat transfer in said indicating section for varying the opening of said second orifice to maintain a constant pressure differential across said orifice within selected limits, and means associated witheach mechanism for measuring changes in the opening of said second orifice and varying theheat value selected for said main section in proportional relation with changes in the opening of said second orifice.

3. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section and a sheet moisture indicating section, and each section has a conduit connecting with a source of heating fluid under pressure, in combination, a first variable orifice in the main section conduit, a second variable orifice in the indicating section conduit, means maintaining a constant pressure in the indicating section conduit upstream of said second orifice, first mechanism responsive to changes in the heat value of the main section for varying the opening of said first orifice to maintain a selected heat value in said main section, second mechanism responsive to changes in the rate of heat transfer in the indicating section for varying the opening of said second orifice to maintain a constant pressure difierential across said orifice within selected limits, means associated with said second mechanism for measuring changes in the opening of said second orifice, third mechanism responsive to measurements provided by said second mechanism and proportionally increasing the effect of said measurements, and means associated, with said third mechanism and with said first mechanism for varying the heat value selected for the main section in proportional relation to the measurements efiected by said third mechanism.

4. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section having a source of heat supply and a sheet moisture indicating section having a conduit connecting with a source of heating fluid under pressure, in combination, first means governing the said heat supply and automatically maintaining a predetermined heat content in the main drying section, a variable orifice in the said conduit, second means for maintaining a constant fluid pressure in the said conduit upstream of said orifice, third means responsive to changes in the rate of heat transfer in the sheet moisture indicating section for varying the orifice opening to maintain a constant pressure differential across the orifice within selected limits, fourth means associated with said first means for maintaining a predetermined heat value in the main drying section, and fifth means associated with said third means and said fourth means for measuring changes in the orifice opening and varying the predetermined heat value in the main drying section in proportional relation thereto.

5. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section and a sheet moisture indicating section, and each section has a conduit connecting with a source of heating fiuid under pressure, in combination, a first power operated variable orifice in the main section conduit, 5], second power operated variable orifice in the indicating section conduit, means maintaining a constant pressure in the indicating section conduit upstream of said second orifice, a controller having a source of power and varying power from said source to said second power operated orifice in response to changes in the rate of heat transfer in the indicating section to maintain a constant pressure differential across said orifice within selected limits, the power governed by said controller being a, measure of the opening of said second orifice, and means responsive to changes in said power for operating said first power operated orifice to vary the orifice opening in proportional relation to changes in the opening of said second orifice.

6. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section and a sheet moisture indicating section, and each section has a conduit connecting with a source of heating fluid under pressure, in combination, a first power operated valve in the main section conduit, a second power operated valve in the indicating section conduit, means maintaining a constant pressure in the indicating section conduit upstream of said second valve, a controller-transmitter having a source of power and varying power from said source to said second valve in response to changes in the rate of heat transfer in the indicating section to maintain a constant pressure difierential across said orifice within selected limits, the power governed by said controller-transmitter being a measure of the opening of said second valve, a receivercontroller having a source of power and responsive to changes in the power governed by said controller-transmitter, said receiver-controller varying power from said source in multip1e relation to changes in power governed by said controller-transmitter, and means responsive to changes in the power from said receiver-controller for operating said first valve to vary the flow of fluid to said main section in multiple relation to changes in the opening of said second valve.

'7. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section and a sheet moisture indicating section, and each section has a conduit connecting with a source of heating fluid under pressure, in combination, a first power operated valve in said main section conduit, a second power operated valve in said indicating section conduit, means maintaining a constant pressure in said indicating section conduit upstream of said second valve, a controller-transmitter having a source of power and varying power from said source to said second valve in response to changes in the rate of heat transfer in said indicating section to maintain a constant pressure difierential across said second valve within selected limits. the power governed by said controller-transmitter being a measure of the opening of said second valve, a. receiver-controller having a source of power and responsive to changes in the power governed by said controller-transmitter, said receiver-controller varying power from said source in multiple relation to changes in power governed by said controller-transmitter, and a controller having a source of power and responsive to changes in heat value in the main section and varying power from said source to said first power operated valve to maintain a predetermined heat value setting in the said main section, said controller being responsive to changes in the power governed by said receiver-controller and varying said predetermined heat value setting in multiple proportional relation to changes in the opening of said second valve.

8. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section and a sheet moisture indicating section, and each section has a conduit connecting with a source of heating fluid under pressure, in combination, a first valve in the main section conduit, a second valve in the indicating section conduit, a fluid pressure operated motor for varying the opening of said second valve, means maintaining a constant pressure in the indicating section conduit upstream of said second valve, a pneumatic controller-transmitter having a source of operating fluid under pressure and including an element responsive to changes in pressure over a selected range of pressure change, said changes in pressure being governed by changes in the rate of heat transfer in the indicating section, said element of said contro1ler-' transmitter varying the output pressure to the motor of said second valve in proportional relation to said pressure changes and over a lesser range of pressure change than that selected for said element to vary the opening of said valve as measured by said output pressure changes and maintain a constant pressure difierential across said valve within selected limits, and means for operating said first valve and responding to changes in said output pressure to vary the opening of said first valve in proportional relation to changes in the opening of said second valve.

9. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section and a sheet moisture indicating section, and each section has a conduit connecting with a source of heating fluid under pressure, in combination, a first valve in the main section conduit, a second valve in the indicating section conduit, a fluid pressure operated motor for varying the opening of said second valve, means maintaining a constant pressure in the indicating section conduit upstream of said second valve, a pneumatic controller-transmitter including a first element responsive over a selected range of pressure change to variations in pressure governed by changes in the rate of heat transfer in the indicating section, a first proportioning device and adjustment means therefor, and a first operating pressure control valve having a source of fluid pressure jointly operated by said first element and by said first proport'ioning device to vary an output pressure to the motor of said second valve over a lesser range of pressure change than the range of pressure change to which said first element is responsive for maintaining a constant pressure differential across said second valve within limits determined by the said adjustment means, a receiver-controller element including a second element responsive to changes in the output pressure of said controller-transmitter, a second proportioning device and adjustment means therefor, and a second operating pressure valve having a source of fluid pressure and jointly operated by said second element and by said second proportioning device to vary the output pressure from said receiver-controller inproportionailrelationto changes in the output pressure from said controller-transmitter and within limits determined by said adjustment means, and means for operating said first valve and responding to changes in the output pressure from said receiver-controller to vary the opening of said first valve in proportional relation to said changes in output pressure.

10. In control mechanism for use in connection with a machine for drying sheet material, wherein the machine includes a main drying section having a source of heat supply and a sheet moisture indicating section, means varying the supply of heat from said source to the main section, and a source of heating fluid and a conduit connecting said source with the indicating section, in combination, a valve in said conduit, a constant pressure device upstream of said valve, a fluid pressure operated motor for said valve, a source of fluid operating pressure, a controller responsive to changes in the rate of transfer of heat in the indicating section, means associated with said controller for varying operating pressure from said source to said motor to vary the opening of said valve and maintain a constant pressure differential across the said valve within selected limits, and means associated with said controller and with the means for varying the supply of heat to the main section to vary the said supply of heat in proportional relation to changes in the opening of said valve.

11. A control mechanism according to claim 1, wherein the mechanism responsive to changes v in the rate of heat transfer in the indicating section comprises first control mechanism responsive to said changes and in operative connection with the second orifice, and the means associated with the mechanism for measuring changes in the opening of the second orifice include second control mechanism which is in operative connection with the first orifice and is governed by the first control mechanism.

12. A control mechanism according to claim 11, wherein the first control mechanism is responsive to changes in the rate of heat transfer in the indicating section by responding to changes in indicating section pressure,

13. A control mechanism according to claim 11, wherein the first control mechanism is responsive to changes in the rate of heat transfer in the indicating section by responding to changes in indicating section internal temperature.

14. A control mechanism according to claim 11, wherein the first control mechanism is responsive to changes in the rate of heat transfer in the indicating section by responding to changes in indicating section surface temperature.

15. A control mechanism according to claim 10, wherein the controller includes an element responsive to changes in the rate of heat transfer in the indicating section, the means associated 'with the controller for varying operating pressure to the motor being a second valve connected with the source of fluid operating pressure and governed by the said element to vary operating pressure to the motor in proportional relation to changes in the rate of heat transfer, and the means associated with the controller and with the means for varying the supply of heat to the main section being responsive to the said variations in operating pressure.

16. A control mechanism according to claim 15, wherein the element is responsive to changes in the rate of heat transfer in the indicating section by responding to changes in indicating section pressure.

1'7. A control mechanism according to claim 15, wherein the element is responsive to changesin the rate of heat transfer in the indicating section by responding to changes in indicating section temperature.

ANDREW E. BENNETT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,142,037 Stamm Dec. 27, 1938 2,148,720 Armstrong Feb. 20, 1939 2,208,784 Armstrong July 23, 1940 2,629,939 Bennett Mar. 3, 1953