Apparatus and Method for Preventing Ink Resoftening on a Printed Web as the Web Travels Over a Chill Roll

1. Field of the Invention

The invention relates to an apparatus and method for cooling an endless paper web after it exits a dryer of an offset printing system and, more particularly, relates to an apparatus and method for preventing the ink on the web from resoftening as the web traverses the first downstream chill roll and for preventing dew or frost formation on chilled components of the system when the system is taken off-line.

2. Discussion of the Related Art

In high speed offset printing processes, an endless paper web up to 72" wide is fed through an offset press at speeds up to 3000 feet per minute (34 mph), where it is printed on at least one and typically both sides with a thermoplastic ink. The printed web is then drawn through a dryer which dries the web by evaporating most of the solvents from the ink. It is important to note, however, that dryers are not intended to and do not evaporate all solvents from ink. Were this the case, the ink would become brittle and crack and fall off from the web, thus forming a nonusable product. Industry standard therefore is to evaporate only 75-95% (typically 80-90%) of the solvents from the ink, thereby improving the finished product.

In the industry standard process, the printed and dried web exits the dryer at a temperature of about 280.degree.-325.degree. F. and enters an insulated sheet metal housing or "smokehood" which traps solvent vapors which are emitted by the still-hot web. The hot web then travels alternately over and under a series of cooled chill rolls which cool the web to or near room temperature.

Referring now to FIG. 1, when a rapidly moving web W emerges from the dryer and smokehood and makes apparent contact with a first downstream chill roll C, a layer of air A is formed between the web W and the surface of the chill roll C, causing a web-to-chill roll surface clearance Ho on the order of 0.001"-0.002". It is with this air layer A and associated clearance that one aspect of the present invention is concerned, and the manner in which they are formed and the problems produced thereby will now be described.

A widely held misconception is that the air layer A is formed by a boundary layer of air following the moving web and/or rotating chill roll. While such boundary layers do exist, they form little or no part in the formation of the air layer A because the average speed of the air following the web or roll decreases rapidly with distance from the web and hence exhibits a sharply decreasing low pressure flow profile. The resulting boundary layer is easily eliminated. Indeed, it has been proven by calculations that, at web speeds of 2000 feet per minute, the boundary layer produced by the moving web W can be eliminated by increasing the tension T on the web by less than 3%. The boundary layer following the chill roll surface can be eliminated even more easily because it is much smaller than the boundary layer following the web surface due to the fact that it has a very short distance in which to form, i.e., only that portion of the chill roll which is not contacted by the moving web--typically less than 180.degree..

Others have theorized that the air layer A between the web W and the first downstream chill roll C is the result of centrifugal forces produced by the web W as it bends around the chill roll C. These forces were theorized to throw the web outwardly away from the chill roll surface. However, it has been mathematically proven that the centrifugal forces actually present in the typical chill roll stand are of the same magnitude of the boundary layer effect and can be accommodated just as easily as the boundary layer.

It has been discovered that the air layer A is actually formed by a hydrodynamic pumping action occurring as the web W approaches the chill roll C. Specifically, air following the converging surfaces of the web W and chill roll C is drawn into a wedge which rapidly decreases in thickness as the web W approaches the chill roll C. Drawing air into this area of rapidly decreasing cross section acts as a pump which compresses the air to form the very thin but relatively high pressure air layer A between the web W and the chill roll C. Unlike boundary layers which are at extremely low pressure and can be eliminated quite easily, this relatively high pressure air layer cannot be removed simply by increasing web tension a few percent. Indeed, web tension T could be increased to the web breaking point without sufficiently reducing the thickness of the air layer A. This problem is exacerbated by the fact that the pumping action produced by the converging web and chill roll surfaces increases with increased speeds, resulting in higher-pressure and thicker air layers at higher press speeds.

The presence of the air layer A between the web W and the chill roll C produces at least two problems. First, and probably most obvious, chill roll performance is degraded because the cool surface of the chill roll is not in intimate contact with the web, thus decreasing heat transfer efficiency. This decrease is rather dramatic because air is a relatively poor heat conductor. Accordingly, more and/or larger chill rolls are required for complete web cooling than would be required if the web W were always in intimate contact with the first downstream chill roll C.

A second and more insidious problem arising from the formation of an air layer A between the web W and the first downstream chill roll C is solvent condensation and resulting ink resoftening and "picking." As discussed above, the web W is still very hot as it approaches the chill roll C, and residual solvents continue to evaporate from the hot web surfaces as the rapidly moving web W makes apparent contact with the first chill roll C. The solvent vapors in the air layer A quickly condense and accumulate on the relatively cold outer peripheral surface of the chill roll C. The accumulated solvents are then reabsorbed by the surface S of the previously-dried web W, thus resoftening the ink. The resoftened ink is then offset or "picked" on the next downstream surface to be contacted by the surface S of the web, typically the third chill roll on the chill roll stand. The defects caused by this picking or offsetting are referred to as "condensate streaks."

Condensate streaking is exacerbated by the fact that it does not necessarily take place only on the first chill roll. As discussed above, the cooling efficiency of the first downstream chill roll C is decreased due to the insulating effect of the air layer A. This decreased efficiency may prevent the web from being cooled sufficiently on the first chill roll C to prevent further solvent condensation and the resulting condensate streaking on subsequent chill rolls.

The need thus has been established to eliminate the air layer formed between a web exiting a dryer and the first downstream chill roll over which the web travels, or to at least eliminate the condensate streaking resulting from this air layer. Many have recognized that the air layer could be eliminated by pressing the web into intimate contact with the chill roll. However, all previous efforts to this effect have proven unsuccessful.

For instance, U.S. Pat. No. 4,369,584 to Daane attempted to eliminate the problem of condensate accumulation by preventing the air layer between the web and the first downstream chill roll from ever forming by blowing high pressure air on the moving web from a nozzle or orifice. Intimate contact between the web and chill roll is never achieved with this device; the air gap is only reduced in thickness. The Daane '584 patent teaches that the nozzle outlet should be located within 0.5" of the line of tangency between the web and the chill roll to optimize jet utility. In actual practice, the orifice has to be installed several inches downstream of the tangent line so as to prevent the air from the orifice from causing the web to move or flutter as the web exits the smokehood. Except for relatively low press speeds (below 1500-1700 fpm), the Daane device did not achieve its goal. At press speeds of 1800 fpm and above (in common use today), excessive power is required to minimize the thickness of the air layer.

The Daane '584 patent also discusses the use of a mechanical nip roll to eliminate the air layer, but only as it applies to films or other webs that can be contacted without damage. There is no discussion of contacting a hot moving printed web without damaging the web or overheating the nip roll.

Others have recognized that the only practical way to achieve true intimate contact between the web and first downstream chill roll is to mechanically press the web directly onto the chill roll. One such device, disclosed in U.S. Pat. No. 3,442,211 to Beacham, pressed the web into intimate contact with the chill roll using a "squeegee-roll . . . coated or covered . . . with a layer of ink-resistant material such as a silicone compound or a synthetic plastic such as polytetrafluorethylene." The device failed to perform as predicted because the "squeegee-roll" surface absorbed heat from the endless web and quickly overheated. The overheated surface remelted the ink on the web, causing the ink to adhere to the hot surface and damage the printed product. Beacham attempted to overcome this deficiency by locating his nip roll at the point where the web was partially cooled and was leaving the chill roll rather than at the point of first web contact. Thus, nip rolls such as those proposed by Beacham damaged the printed web even worse than condensate streaking with no nip roll.

U.S. Pat. No. 4,476,636 to Gross describes a device which is designed to eliminate as much air as possible between the web and the chill roll surface. Gross states that the purpose of his invention is to achieve an air layer reduction rather than elimination of the air layer. Gross's use of a rubber covered "squeeze" roller applied directly to the surface of the chill roll has little or no effect on the cause of web flotation over the chill roll surface.

U.S. Pat. No. 5,111,595 to Bessinger is yet another attempt to overcome the problem of condensate formation on the first downstream chill roll causing ink resoftening. Bessinger speaks of a pressure roller to squeeze the web against the roller (chill roll). He admits to the impossibility of its use in practice because ". . . the web surface that faces away from the roller (chill roll) to be contacted cannot tolerate engagement by a solid object." Bessinger specifically wants to avoid contact with the outward web surface of a printed web traveling over a chill roll.

U.S. Pat. No. 5,184,555 to Quadracci is another attempt to solve the problem of condensate streaking at the chill roll. The Quadracci device attempts to cure the problem without direct contact to the chill roll or web. This device does not work in actual practice because it does nothing to eliminate the formation of condensate in the annular air space between the web and chill roll surface. So long as the printed web remains above a temperature of about 200.degree. F., solvent will continue to evaporate from the web and condense on the chill roll surface.

The Quadracci patent also describes the Baldwin chill roll wiper device in use today on many web offset press systems. This device helps alleviate the condensate problem, but does not eliminate it. The Baldwin device uses a porous, absorbent cloth material that makes contact with the first downstream chill roll surface in the area left between where the web leaves the chill roll and the point of first web contact with the chill roll. Examination has shown that this device removes a portion of the condensate, but not all of it.

Still another solution to the air layer problem was proposed in U.S. Pat. No. 5,121,560 to Daane (the Daane '560 patent), which sought acceptable ways to cool an elastomer-coated nip roll in order to overcome the problems produced by the Beacham process. The Daane '560 patent is assigned to the assignee of thee present application, and the inventors of the present invention were familiar with Daane's efforts. Daane points out the problems encountered when attempting to use an all-metal pressure roll or nip roll to press a web into intimate contact with a chill roll. Daane also discusses the problems involved when attempting to use an elastomeric pressure roll and states "the index of contact temperature preservation for an elastomeric pressure roll is very low and not effective for cooling the opposite side of the web which it contacts. In operation the elastomeric surface immediately becomes hot and does not cool and set the ink. Instead the pressure roll will pick and smear the ink to destroy the readability of the print . . . . Thus efficient cooling of the unset ink, especially on both sides of the web in high-speed printing, remains a significant unsolved problem."

Daane attempted to solve this problem by cooling the nip roll peripheral surface from the exterior. Accordingly, the Daane '560 patent proposed a technique of positioning a doctor roll adjacent the nip roll to provide a metered amount of coolant on the outer peripheral surface of the nip roll upstream from the web. A later commercial embodiment achieved the same effect using a doctor spray bar.

Two problems were associated with the externally cooled nip roll proposed by the Daane '560 patent. First, the coolant had to be applied to the undersurface of the web to prevent coolant from dripping onto the web and ruining the product. This technique required that the web contact the first downstream chill roll from below. Unfortunately, as many as 99% of existing chill roll stands contact the first downstream chill roll from above and thus are incompatible with Daane's technique. Second, it has been discovered that moisture is inevitably transferred to the web by the damp nip roll and that this moisture quickly accumulates on downstream chill roll surfaces. The web absorbs the accumulated moisture to the point that it becomes saturated and unusable. The process proposed by the Daane '560 patent thus solved the second problem produced by the Beacham technique only to produce a third problem which can be solved only at such great expense as to make the device unmarketable.

Another problem associated with many fluid-cooled rolls is that frost tends to build up on the outer surface of the nip roll if the system is temporarily taken off-line due to the condensation of moisture on the cold surface of the roll. Upon subsequent restart, this frost melts as the hot web traverses the roll, thereby wetting the web and causing ink streaking or even web breakage.

Many of the problems discussed above are addressed in parent application Ser. No. 08/406,572, and the objects of the invention aimed at addressing those problems will not be repeated herein. Additional objects of the invention, will, however be detailed. Specifically:

It is an object of the invention to provide an offset printing system which reliably reduces moisture condensation and resultant frost build-up on the chilled nip rolls and possibly other chilled rolls of the system which could otherwise occur when the system is taken off-line.

Another object of the invention is to provide an offset printing system which has the characteristics discussed above and which can be brought back on line relatively quickly.

In accordance with a first aspect of the invention, these objects are achieved by providing 1) a system comprising a coolant-chilled roll having an outer peripheral surface, and 2) a coolant circuit. The coolant circuit is dimensioned and configured to selectively a) continuously circulate a chilled coolant through the roll when the system is on-line, thereby cooling the roll and b) circulate a heated fluid through the roll for at least a limited period of time when the system is taken off-line, thereby warming the roll and preventing frost accumulation or moisture condensation on the outer peripheral surface thereof.

Preferably, the coolant circuit comprises a chilled coolant source, a heat source, and first and second valve assemblies. The first valve assembly is switchable from a first state permitting the flow of chilled coolant from the chilled coolant source through the roll to a second state prohibiting the flow of chilled coolant from the chilled coolant source through the roll. The second valve assembly is switchable from a first state permitting the flow of heated fluid from the heat source through the roll to a second state prohibiting the flow of heated fluid from the heat source through the roll.

Once the roll is heated above the frost and dew points of the surrounding air, further heat transfer to the roll is neither required nor desired. Measures are accordingly taken to discontinue heated fluid circulation after nip roll heating. This object can be accomplished using a temperature controlled circuit or, in accordance with a preferred form of the invention, by providing a timer which causes the second valve assembly to switch from the first state to the second state after a designated period of time and by providing a third valve assembly which is responsive to the timer and which is switchable, at the end of the designated period of time, from a first state prohibiting chilled coolant bypass flow around the roll to a second state permitting chilled coolant bypass flow around the roll.

An actuator is preferably provided which selectively moves the nip roll from the operative position to an inoperative position in which the nip roll is located remote from the chill roll. In order to accommodate web splices and other so-called "tension upset" conditions within the system, a mechanism is preferably incorporated for alternatively and selectively 1) retracting the actuator to place the nip roll in the inoperative position, 2) extending the actuator sufficiently to place the nip roll into contact with the web but not sufficiently to apply sufficient pressure on the web to place the nip roll in the operative position, and 3) extending the actuator sufficiently to place the nip roll in the operative position.

The coolant system can be used with more than one nip roll. In this case, the system may further comprise a second coolant-chilled roll having an outer peripheral surface, and a second coolant circuit which is dimensioned and configured to selectively a) continuously circulate a chilled coolant through the second roll when the system is on-line, thereby cooling the second roll and b) circulate a heated fluid through the second roll for at least a limited period of time when the system is taken off-line, thereby warming the second roll and preventing frost accumulation or moisture condensation on the outer peripheral surface thereof.

Still another object of the invention is to provide an offset printing system which has one or more of the characteristics discussed above and which utilizes waste-heat from the system to prevent moisture condensation and frost formation on the nip roll.

In accordance with still another aspect of the invention, this object is achieved by using a chiller evaporator as the chilled coolant source and by using waste heat from the chiller evaporator to heat coolant flowing through the warmup loop. Preferably, a flow switch is included which prevents the second valve assembly from switching from the first state to the second state when the chiller evaporator is not operating.

Yet another object of the invention is to provide a process in which moisture condensation and resultant frost build-up on the cooled nip rolls of an offset printing system, which could otherwise occur when the system is taken off-line, are prevented.

In accordance with yet another aspect of the invention, a method is provided which includes applying ink to at least one side of a moving endless paper web to form a printed web, then conveying the printed web through a dryer to form a dried web, then conveying the dried web over a cooled chill roll, thereby cooling the dried web, and pressing the dried web into intimate contact with the chill roll, via operation of a nip roll, so as to at least inhibit solvent condensate formation on the chill roll. A further step includes selectively (1) cooling an outer peripheral surface of the nip roll by circulating a chilled coolant therethrough, and (2) heating the outer peripheral surface of the nip roll by circulating a heated fluid therethrough.

Preferably, the heating step comprises conveying an ethylene glycol solution through the nip roll at a temperature of between 50.degree. F. and 100.degree. F. and at a flow rate of approximately two gallons per minute. The heating step may further comprise, prior to the step of conveying the solution through the nip roll, conveying the solution through a primary flow path of a liquid-to-liquid heat exchanger to heat the solution from a temperature of between 15.degree. F. and 20.degree. F. to the temperature of between 50.degree. F. and 100.degree. F. The heating step may further comprise conveying water through a secondary flow path of the heat exchanger at a temperature of between 60.degree. F. and 140.degree. F. and at a flow rate of approximately 3 gallons per minute, thereby heating the solution in the primary flow path.

In order to avoid heating the nip roll more than is necessary, the heating step preferably comprises maintaining heated fluid flow through the nip roll for approximately 3 to 8 minutes and thereafter preventing any fluid flow through the nip roll.

Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:

FIG. 1 schematically represents the operation of a prior art chill roll, appropriately labeled "Prior Art";

FIG. 2 schematically represents an offset printing system including a chill roll stand and nip roll constructed in accordance with a preferred embodiment of the present invention;

FIG. 3 is a sectional end view of the chill roll stand of FIG. 2;

FIG. 4 is an enlarged fragmentary side elevation view of the nip roll of FIGS. 1 and 2 and of the cooperating portions of the chill roll stand;

FIG. 5 is an enlarged fragmentary side sectional elevation view of the confronting portions of the chill roll and nip roll of FIGS. 2-4;

FIG. 6 is a partially cut-away perspective view of the nip roll of FIGS. 2-5;

FIG. 7 schematically represents a first preferred closed loop coolant circuit for cooling the nip roll of FIGS. 2-6;

FIGS. 8 and 9 are schematic hardwired and PLC ladder diagrams, respectively, for controlling the operation of the control circuit of FIG. 7;

FIG. 10 schematically represents a second preferred closed loop coolant circuit for cooling the nip roll of FIGS. 2-6;

FIGS. 11 and 12 are schematic hardwired and PLC ladder diagrams, respectively, for controlling the operation of the control circuit of FIG. 10; and

FIG. 13 schematically represents a preferred closed loop coolant circuit for simultaneously cooling two nip rolls of FIGS. 2-6;

1. Resume

Pursuant to the invention, an offset printing apparatus is provided having a fluid-cooled nip roll which, in use, is placed in intimate contact with a hot, rapidly moving, endless printed and dried paper web so as to improve the cooling efficiency of the first downstream chill roll, permit the use of fewer chill rolls, and avoid condensate streaking. The apparatus takes the form of a nip roll specially designed to press the web into intimate contact with the first downstream chill roll while avoiding damage to the web, nip roll, or chill roll, and while avoiding ink picking by the web surface contacted by the nip roll. Employing the nip roll in the system also permits the use of a substantially shorter dryer and fewer chill roll is as compared to a conventional system operating at the same web speed. The coolant circuit for the nip roll includes a warmup loop which temporarily circulates warm fluid through the nip roll when the system is first taken off-line, thereby preventing condensate formation or frost formation on the nip roll. The warmup loop preferably includes a heat exchanger using waste heat from the coolant circuit to heat the warmup fluid. Two or more nip rolls can be cooled and heated by a single coolant circuit.

2. System Construction

Referring to FIGS. 2-6, an offset printing system 10 is provided which prints and dries an endless paper web 14 as it is conveyed through the system at speeds up to 2500 to 3000 feet per minute. The system 10 includes a plurality of ink units 12 each of which applies a basic color of ink to at least one and usually both sides of the web 14 as it is drawn through the system 10 by drive rolls (not shown). A flotation dryer 16 is located downstream from the ink units 12 for drying the printed web 14, and a chill roll stand 18 is located downstream from the dryer 16 for cooling the dried web. A nip roll assembly 50 is mounted on the chill roll stand 18 for pressing the web 14 into intimate contact with the first chill roll 20 contacted by the web 14 after it leaves the dryer 16 (called the "first downstream chill roll"). The ink units 12, flotation dryer 16, and associated but not illustrated devices, such as a smokehood, are conventional and will not be described in further detail.

The chill roll stand 18 typically comprises a plurality--four in the illustrated embodiment--of internally cooled metal chill rolls 20, 22, 24, 26 over and under which the web 14 passes after it leaves the dryer 16. The chill rolls 20, 22, 24, and 26 are coaxially and rotatably mounted on a support assembly formed from a pair of opposed vertical side plates 28, 30 connected to one another by suitable cross braces 32, 34. In the illustrated embodiment, the bottom surface of the web 14 is cooled by chill rolls 20 and 24 and the top surface is cooled by chill rolls 22 and 26. An idler roll 38 is also mounted on the chill roll stand 18 for guiding the dried and cooled web 14 through the chill roll stand 18 and out of the offset printing system 10.

The chill rolls 20, 22, 24, and 26 are, per se, well known. Each preferably takes the form of a hollow steel drum having internal channels 40 (FIG. 5) through which a coolant--typically water chilled to between 50.degree. and 70.degree. F.--flows for cooling the web 14 as it travels over the outer peripheral surface of the roll.

The nip roll assembly 50 is designed to apply sufficient pressure (typically about 20-35 pounds per linear inch or pli) to the web 14 to press the web into intimate contact with the first downstream chill roll 20. The nip roll assembly 50 includes a nip roll 52, a support assembly, and an actuator assembly.

The support assembly may comprise any structure capable of supporting the nip roll 52 and actuator assembly on the chill roll stand 18. In the illustrated embodiment, the support assembly comprises a pair of opposed side plates 54, 56 connected to one another by an upper cross brace 58. The side plates 54, 56 are welded or otherwise fixed to the upper portions of the chill roll stand side plates 28, 30 at a location adjacent the first downstream chill roll 20. Stand outs 57 and 59 protrude inwardly from plates 54, 56 for supporting mounts 68 and 70.

The actuator assembly may comprise any structure capable of selectively moving the nip roll from its operative position illustrated in solid lines in FIG. 4 to its inoperative position illustrated in phantom lines in FIG. 4. In the illustrated embodiment, the actuator assembly comprises (1) a pair of actuators 60, 62 disposed proximate the opposed ends of the nip roll 52, and (2) a mating pair of pivot arms 64, 66. Each of the pivot arms 64, 66 rotatably receives a respective end of the nip roll 52 at a first end and is pivotally mounted at a second end to a pivot shaft 69 journaled on the mounts 68, 70. Each actuator 60, 62 preferably comprises a double-acting pneumatic cylinder pressurization of which is controlled by a pair of electronically controlled solenoids (seen only schematically in FIG. 8). Each of the cylinders 60, 62 has a cylinder portion pivotally connected to a bracket 72, 74 suspended from the upper cross brace 58 and a rod portion pivotally connected to a respective pivot arm 64, 66 at a location between the nip roll 52 and the bracket 68, 70.

In a particularly sophisticated embodiment, the solenoid valves associated with each cylinder could be "feathered" or proportionally controlled to permit the pressure exerted by the nip roller to be varied on the fly and thus reduced as necessary to permit passage of a web splice or any other so-called "tension upset" condition in which signature upset occurs. Specifically, the pressure would be released without retracting the cylinders during passage of a web splice or the like. Pressure release reduces signature upset, while retaining the nip roll near its operative position negates the need to re-engage the web with the nip roll on the fly--an operation which could break the web at high web speeds.

The nip roll 52 is rotatably mounted on the side plates 54, 56 via suitable bearing arrangements 76, 78 and is designed to press the web 14 into intimate contact with the chill roll 20 without damaging the web 14, nip roll 52, or chill roll 20 and without causing ink picking on the surfaces of downstream equipment. To this end, the nip roll 52 takes the form of a hollow metal (steel in the preferred embodiment) shell 80 covered with a layer 82 of elastomeric material bonded to the metal shell. Except for receiving the outer elastomer layer 82, the nip roll 52 is of standard construction and is commercially available, e.g., from F. R. Gross Company, Inc. of Stow, Ohio, and Webex, Inc. of Neenah, Wis. As is standard with rolls of this type, the shell 80 has an axial coolant inlet 84 and an opposed axial coolant outlet 86. A rod or barrel-like member 88 (FIG. 6) is disposed in the shell 80 and has a peripheral spiral ring 90 provided thereon which promotes spiral turbulent flow of coolant through a channel 92 in contact with the inner peripheral surface of the shell 80, thereby enhancing heat transfer through the shell 80 and elastomer layer 82.

The elastomer layer 82 must be employed to prevent damage to the web 14 and to the rolls 20 and 52 and to promote the application of uniform pressures along the width of the web 14. However, all known elastomers are extremely poor heat conductors (or good insulators). It was heretofore thought that the insulating properties of such elastomer layers would prevent the use of an internally cooled nip roll. The inventors have found, however, that if the properties and dimensions of the elastomer layer 82 are carefully selected, and if the coolant properties and flow rates are carefully controlled, the elastomer layer 82 can be adequately cooled to reduce the steady state operating temperature of its outer peripheral surface to an acceptably low level. Considerations which must be addressed and solutions obtained include:

(1) Elastomer Property. The elastomeric material should exhibit relatively high thermal conductive properties, should be capable of being ground into a very thin layer while still being capable of being bonded to the steel shell, and must withstand temperatures in excess of 130.degree. F. (the maximum desired temperature of the outer peripheral surface of the nip roll). The elastomer must also be sufficiently hard to assure intimate contact between the web and the chill roll along the entire length of the chill roll 20. EPDM (a terpolymer made from ethylene-propylene diene monomer) has been found to be the most suitable for these purposes and is the preferred material. However, other elastomeric materials exhibit satisfactory combinations of these characteristics and could also be used. Metallic powders or other heat conductive materials could also be mixed into the elastomer layer to increase heat transfer with the coolant in the nip roll.

(2) Elastomer Layer Thickness. The elastomer layer should be as thin as possible so as to minimize the insulation effect of the layer and promote heat transfer therethrough. However, the layer must be thick enough so as not to peel off from the shell 80 in use. Layers of 0.10-0.15", and preferably 0.10", are considered suitable for these purposes. It is contemplated that these thicknesses will decrease with advances in elastomer technology.

(3) Nip Roll Diameter. The nip roll should be as large as practical to minimize the surface area percentage contacting and receiving heat from the web at any particular time. The nip roll must also be large enough to permit coolant to remain in the nip roll long enough to provide adequate heat transfer with the elastomer layer. The inventors have found that nip rolls of 8"-12" diameters meet these criteria, and a 10" diameter nip roll is preferred at this time. Less than 1% of the surface area of such a nip roll will contact a web at any given time. The remaining 99% accordingly is available for heat transfer from the elastomer cover to the coolant.

(4) Coolant Properties. Standard chill rolls employ water as their coolant or employ ethylene glycol solutions operating at temperatures typically in excess of 45.degree. F. It has been discovered that such coolant temperatures do not provide adequate cooling of the nip roll. Ethylene glycol solutions can be cooled below the freezing point of water but exhibit lower heat transfer efficiency than water, and thus must be cooled still further and/or pumped through the nip roll at higher rates to provide adequate heat transfer. The flow rate of coolant through the nip roll also must be controlled so as to maintain the temperature change (.DELTA. T) within acceptably low limits.

It has been discovered that, assuming an elastomer having the properties discussed above is employed, sufficient heat transfer is achieved if a 30% ethylene glycol solution is used as the coolant, if the coolant enters the nip roll at a temperature of between 15.degree.-20.degree. F., and if coolant flows through the nip roll at a rate of 10-12 gallons per minute. These parameters assume a press speed of 2500 feet per minute. If the press speed were to be increased to 3000 feet per minute, thus requiring more nip roll cooling, the fluid flow rate should increase to 14-15 gallons per minute.

(5) Dryer Temperature. Although not a component of nip roll design per se, dryer temperature design is also an important consideration when designing the overall system. As discussed above, standard industry practice is to overheat the web to a temperature up to 320.degree.-350.degree. F. in an attempt to avoid condensate streaking. Such overheating not only overdries the ink and wastes energy, but also increases heat transfer to the nip roll from the web and exacerbates the nip cooling problem. Indeed, the temperature of the web 14 leaving the dryer 16 has more effect on the temperature of the nip roll 52 than any other external parameter because, other conditions being equal, the temperature on the outer peripheral surface of the nip roll would increase proportionally with the web temperature. Since the nip roll 52 completely eliminates condensate streaking, there is no need to overheat the web. It has been discovered that controlling the dryer 16 to heat web 14 to a temperature of 240.degree.-260.degree. F. adequately dries the web 14 and yet reduces heat transfer to the nip roll and reduces the nip roll cooling load.

In summary, assuming a web speed of 2500 feet per minute, the current preferred embodiment is to convey the web 14 out of the dryer 16 and to the first downstream chill roll 20 at a temperature of 240.degree.-260.degree. F. A 10" diameter nip roll 52 is pressed into contact with the web 14 under about 35 pli, thereby forcing the web 14 into intimate contact with the chill roll 20. The outer periphery of the nip roll 52 is formed from a 0.10" thick layer of EPDM. A coolant consisting of a 30% ethylene glycol solution is circulated through the nip roll at a temperature of 15.degree.-20.degree. F. and at a flow rate of 10-12 gallons per minute, thereby maintaining the temperature of the outer peripheral surface of the nip roll 52 below 130.degree. F.

The coolant circuit for controlling the flow of coolant through nip roll 52 may comprise any system for forcing coolant through the nip roll 52 as described in Section 2 above. Two preferred coolant circuits suitable for this purpose will now be described.

3. First Coolant Circuit

The first preferred coolant circuit, illustrated in FIG. 7, is closed loop and includes as its major components a conventional chiller evaporator 102, a coolant reservoir 104, and a pump 106 arranged in series. An inlet conduit 108 leads from the pump 106 to the coolant inlet 84 of the nip roll 52, and an outlet conduit 110 leads from the coolant outlet 86 of the nip roll 52 to the chiller evaporator 102.

A flow valve 112 is disposed in the outlet conduit 110, and a bypass valve 114 is disposed in a branch line 116 connecting the inlet conduit 108 to the outlet conduit 110 at a location downstream from the flow valve 112. The flow and bypass valves 112 and 114 are solenoid operated and controlled by the circuit 150 and logic 200 detailed in Section 4. The flow valve 112 is preferably normally open and the bypass valve 114 normally closed.

Many changes could be made to valves 112 and 114 without affecting the operation of the system 10. For instance, the flow valve 112 could be located in the inlet conduit 108 between the branch conduit 116 and the coolant inlet 84 of nip roll 52. The valves 112 and 114 could also be combined as a single three-way valve selectively causing coolant to flow through and bypass the nip roll 52.

Several safety and monitoring devices are also provided in coolant circuit 100. For instance, a safety bypass restrictor 118 and bypass line 120 are provided in parallel with branch conduit 116 to permit limited coolant circulation through the chiller evaporator 102 and thereby to prevent damage to the chiller evaporator 102 if both valves 112 and 114 are closed or if flow adjustment valve 130 is closed. A standard low level switch 122 and flow switch 124 are also provided to shut down the system 100 and prevent damage to the chiller evaporator 102 in the event of a coolant leak. A temperature probe 126 is provided downstream of the chiller evaporator 102 for monitoring operation of the chiller evaporator. In addition, in order to maintain coolant flow rates in the preferred range of 10-12 gallons per minute, a coolant flow meter 128 is located in the inlet conduit 108 downstream from the conduits 116 and 120 to permit an operator to manually set a coolant flow adjustment valve 130 to maintain flow rates in the desired range.

4. First Control Circuit

Referring now to FIGS. 8 and 9, a preferred hardwired system schematic 150 and PLC ladder diagram 200 are illustrated for controlling operation of the coolant circuit 100 and the nip actuators 60, 62. The construction and operation of devices constructed from these diagrams are believed to be self-explanatory from the drawings and will be discussed only briefly.

The hardwired circuit 150 of FIG. 8 is preferably powered by the same source supplying power to the chiller evaporator 102. Inputs for the circuit 150 include a manually operated on/off switch 152, a first safety switch 154 located at the main console of the offset printing system 10, a second safety switch 156 located at the chill roll stand 18, and a high temperature limit switch 158. The safety switches 154, 156 are designed to prevent any operation of the nip roll 52 when personnel are in the vicinity of the chill roll stand 18. The high temperature limit switch 158 is responsive to an IR sensor 160 or the like (FIG. 7) monitoring the temperature of the outer peripheral surface of the nip roll 52. PLC input switches 204, 206 and 208 are closed upon closure of switches 152, 154, and 156. Similarly, a PLC switch 210 is activated if an alarm contact 188 is closed in response to the detection of an unacceptably high coolant temperature by probe 126, and a PLC switch 212 is activated by a conventional press run contact 190 monitoring press speed. PLC switches 220, 222 and 228 serve as outputs for the circuit 150 and actuate a first solenoid 180 controlling extension of the pneumatic cylinders 60 and 62, a second solenoid 182 controlling retraction of the pneumatic cylinders 60, 62, and solenoids 184, 186 for the bypass and flow valves 112 and 114, respectively.

Referring to FIG. 9, the PLC program usable in conjunction with the hardwired circuit of FIG. 8 is represented by a ladder diagram 200 the first rung of which includes a CHILLER START output 204 signal generated whenever the evaporator chiller 102 is active as detected by a logic switch 202. A NIP ON switch 218 is closed to generate a signal 220 energizing solenoid 180 in FIG. 8 and triggering extension of the actuating cylinders 60, 62 only if PLC switches 204, 206, 208, 210, 212, and 214 indicate that certain conditions are met. These conditions are: (1) the chiller evaporator 102 must be operational, (2) the safety switches 154 and 156 must not depressed, (3) coolant temperature as monitored by sensor 126 must be below an acceptable temperature of, for example, 20.degree. F., (4) the press must be running at an acceptably high speed (as monitored by a press run contact 190 in FIG. 8), and (5) the temperature of the outer peripheral surface of the nip roll 52 as detected by the IR sensor 160 must be below a designated temperature of, for example, 150.degree. F. If any one of these conditions are not met, the NIP OFF PLC signal 222 is generated at the next rung to cause the circuit 150 to energize solenoid 182 and retract the actuators 60, 62, thus placing the nip roll 52 in its inoperative condition.

The PLC ladder diagram 200 also illustrates the generation of a flow timer signal 228 in response to operation of switches 204, 218, 224, and 226. Generation of the flow timer signal 228 closes a corresponding switch which, for reasons detailed below, energizes the solenoids 184 and 186 in FIG. 8 to alternately and intermittently cycle the nip coolant circuit 100 between flow and bypass conditions (under the control of switches 224 and 228) when the nip roll 52 is in its inoperative position. Finally, a system status signal 232 may be generated in response to the operation of logic switches 206, 208, 226, and 214 to provide a visual and/or audible indication of the operational state of the system 100.

5. Operation of System with First Coolant Circuit and First Control Circuit

In operation, an endless paper web 14 is coated by printing units 12, dried by dryer 16, and conveyed out of the dryer 16 and through the chill stand 18, as illustrated in FIG. 1, at speeds up to 2000-3000 feet per minute. As the web 14 travels over the first downstream chill roll 20, it is pressed into intimate contact with the chill roll 20 by the nip roll 52, which preferably engages the web 14 near the point at which it first contacts the chill roll 20 as illustrated in FIG. 5. However, because the air layer between the web 14 and chill roll 20 is completely eliminated and intimate contact achieved, nip roll contact at this location is not essential, and the nip roll 52 could be located considerably downstream from this point if necessitated by system parameters. The intimate contact prevents condensate formation and the resulting condensate streaking, thus dramatically improving the product. Moreover, because an insulating air layer between the chill roll 20 and web 14 is absent, web cooling at the first chill roll 20 is dramatically enhanced, even if coolant is circulated through the chill roll 20 at a relatively high temperature of about 70.degree. F. Indeed, it has been discovered that the temperature of the web 14 is reduced by up to 150.degree. as it traverses chill roll 20, an additional temperature drop of of 55.degree.-60.degree. F. as compared to conventional processes in which intimate contact is not achieved.

When the nip roll 52 is in its operative position, the web 14 exiting the first downstream chill roll 20 is typically at a temperature of 100.degree.-150.degree. F. (assuming a dryer web exit temperature of 240.degree.-280.degree. F.) and exhibits no danger of ink softening or condensate streaking on subsequent chill rolls. Even if dryer 16 is operating at a higher web exit temperature range of 280.degree. to 320.degree. F., the web 14 will have a temperature of less than 200.degree. F. as it leaves the chill roll 20, but solvent evaporation is reduced sufficiently to eliminate any condensate buildup on downstream chill rolls 22, 24, 26. The paper wetting problems associated with externally cooled nip rolls of the type disclosed in Daane '560 patent are also eliminated because the nip roll 52 is cooled internally rather than externally.

The web continues to pass over and under successive chill rolls until it leaves the last roll at or near room temperature, i.e., 70.degree.-90.degree. F. Because of increased cooling at the first downstream chill roll 20, fewer chill rolls are required than are normally employed in the industry. As few as two or even one chill roll would be required in many instances.

The beneficial effects of the inventive nip roll can be appreciated by comparing the inventive system having at least one nip roll to a conventional system lacking any nip rolls. A conventional system operating at 2,500 fpm requires a 38 to 48 foot dryer followed by 8 chill rolls, and a conventional system operating at 3,000 fpm requires a 48 to 58 foot dryer followed by 9 chill rolls. In contrast, a system employing a nip roll constructed in accordance with the present invention and operating at 2,500 fpm requires only a 23 to 24 foot dryer followed by 2 chill rolls. These benefits are even more pronounced at 3,000 fpm, in which the iventive system requires only a 28 to 30 foot dryer followed by 4 chill rolls.

So long as the actuators 60, 62 are extended to press the web 14 into intimate contact with the first downstream chill roll 20, the bypass and flow valves 114 and 112 will be closed and opened, respectively, to assure coolant flow through the nip roll 52 at a temperature of 15.degree.-20.degree. F. and at a flow rate of 10-12 gallons per minute (assuming a 30% ethylene glycol coolant solution and a press speed of 2500 feet per minute). Sufficient heat is transferred through the elastomer layer 82 to maintain the steady state operating temperature of the outer peripheral surface of the nip roll 52 below 130.degree. F., and for the most part below 80.degree.-120.degree. F., thereby avoiding any ink softening by the nip roll 52 and ink picking on the downstream equipment.

If for any reason the circuit 150, 200 retracts the actuators 60, 62 to take the nip roll 52 off line, e.g., because a safety switch 154 or 156 is activated or because the coolant temperature as monitored by probe 126 is unacceptably high or the temperature of the outer peripheral surface of the nip roll 52 as monitored by sensor 160 is unacceptably high or the press run contact 190 is opened when the system 10 stops, the timing logic of the PLC 200 will control the valves 112, 114 to intermittently cause coolant to bypass and then flow through the nip roll 52. The purpose for this intermittent flow is two-fold. First, cooling of the outer peripheral surface of the nip roll 52 should be retained to permit restart upon short notice if the nip roll 52 is temporarily taken off line. Second, steady state coolant flow through the nip roll 52 must be terminated to prevent excess cooling and frost buildup on the outer surfaces of the nip roll 52. Intermittently circulating coolant through and bypassing the nip roll 52 for about 30 seconds to one minute intervals has been found to achieve these goals.

The coolant circuit 100 and accompanying control circuit 200 detailed above operate satisfactorily under many operating conditions. It has been discovered, however, that merely intermittently cutting off the flow of coolant through the nip roll 52 when it is taken off-line may not prevent moisture condensation and frost build-up on the nip roll 52 because coolant at temperatures as low as 15.degree. F. are present in the nip roll 52. A coolant circuit and accompanying control circuit which address this potential problem will now be disclosed.

6. Second Coolant Circuit

Referring to FIG. 10, a second coolant circuit 300 is illustrated which differs from the first coolant circuit 100 primarily in that a different mechanism is provided to prevent condensation and frost accumulation on the nip roll 52 when it is taken off-line. Elements in FIG. 10 corresponding to those in FIG. 7 are accordingly designated by the same reference numerals, incremented by 200.

The coolant circuit 300 is closed loop and includes as its major components a conventional chiller evaporator 302, a coolant reservoir 304, and a pump 306 arranged in series. An inlet conduit 308 leads from the pump 306 to the coolant inlet 84 of the nip roll 52, and an outlet conduit 310 leads from the coolant outlet 86 of the nip roll 52 to the chiller evaporator 302.

A flow valve 312 is disposed in the inlet conduit 308, and a bypass valve 314 is disposed in a branch line 316 connecting the outlet conduit 310 to the inlet conduit 308 at a location upstream of the flow valve 312. A warmup loop 331 extends from the upstream side of bypass valve 314 to the inlet conduit 308 downstream of the flow valve 312. Disposed in the warmup loop are a flow restrictor valve or flow restrictor 333, a heat exchanger 335, and a warmup valve 337.

The flow restrictor 333 may be any device which restricts the flow of coolant sufficiently for adequate operation of the heat exchanger 335. In the illustrated embodiment, the flow restrictor restricts the coolant flow rate to about two gallons per minute.

The heat exchanger 335 preferably comprises a tinned liquid-to-liquid heat exchanger having 1) a primary flow path through which the coolant in the warmup loop 331 passes and 2) a secondary flow path through which a heated fluid enters via a conduit 341 and exits via a conduit 343. The heated fluid flowing through the secondary flow path preferably, but not necessarily, comprises water and, in the illustrated embodiment, is heated in a closed loop by receiving waste heat from the chiller evaporator 302 as detailed below. A flow switch 345 is located in the conduit 343. As will be detailed below, flow switch 345 disables operation of the warmup loop 331 when the chiller evaporator 302 is not operating.

The flow valve 312, bypass vane 314, and warmup vane 337 are solenoid operated two-way/two-position vanes controlled by the circuit 350 and logic 400 detailed in Section 7. The flow vane 312 is preferably normally open and the bypass and warmup vanes vane 314 and 337 normally closed.

Many changes could be made to valves 312, 314 and 331 without affecting the operation of the system 300. For instance, all valves could be normally closed, normally open, or any combination thereof. Moreover, the flow valve 312 could be located in the outlet conduit 310 between the branch conduit 316 and the coolant outlet 86 of nip roll 52. The valves 312 and 314 could also be combined as a single three-way valve selectively causing coolant to flow through and bypass the nip roll 52.

Several safety and monitoring devices are also provided in coolant circuit 300. For instance, a safety bypass restrictor 318 and bypass line 320 are provided in parallel with branch conduit 316 to permit limited coolant circulation through the chiller evaporator 302 and thereby to prevent damage to the chiller evaporator 302 if all of valves 312, 314, and 337 are closed or if flow adjustment valve 330 is closed. A standard low level switch 322 and a standard flow switch 324 are also provided to shut down the system 300 and prevent damage to the chiller evaporator 302 in the event of a coolant leak. A temperature probe 326 is provided downstream of the chiller evaporator 302 for monitoring the chiller evaporator 302, and a temperature probe 327 is provided at the outlet of the coolant reservoir 304 and is used as an input for the down-to-temperature switch 410 and the alarm generator 388 discussed below. In addition, in order to maintain coolant flow rates in the preferred range of 10-12 gallons per minute, a coolant flow meter 328 is located in the outlet conduit 310 prior to the conduits 316 and 320 to permit an operator to manually set a coolant flow adjustment valve 330 to maintain flow rates in the desired range.

7. Second Control Circuit

Referring now to FIGS. 11 and 12, a preferred hardwired system schematic 350 and PLC ladder diagram 400 are illustrated for controlling operation of the coolant circuit 300 and the nip actuators 60, 62. The construction and operation of devices constructed from these diagrams are believed to be self-explanatory from the drawings and will be discussed only briefly.

The hardwired circuit 350 of FIG. 11 is preferably powered by the same source supplying power to the chiller evaporator 302 of FIG. 10. Inputs for the circuit 350 include a manually operated on/off switch 352, a first safety switch 354 located at the main console of the offset printing system 10, a second safety switch 356 located at the chill roll stand 18, the warm water flow switch 345, and a high temperature limit switch 358. The safety switches 354, 356 are designed to prevent any operation of the nip roll 52 when personnel are in the vicinity of the chill roll stand 18. The flow switch 345 prevents fluid flow through the warmup loop 331 when warm water is not flowing through the heat exchanger 335 from the secondary flow path inlet conduit 341 to the secondary flow path outlet conduit 343. The high temperature limit switch 358 is responsive to an IR sensor 360 or the like (FIG. 10) monitoring the temperature of the outer peripheral surface of the nip roll 52.

PLC input switches 401, 404, 406 and 408 are closed upon closure of switches 345, 352, 354, and 356, respectively. Similarly, a PLC switch 410 is activated if an alarm contact 388 is closed in response to the detection of an unacceptably high coolant temperature by probe 327, and a PLC switch 412 is activated by a conventional press run contact 390 monitoring press speed. PLC switches 420, 423, 425, and 427 serve as outputs for the circuit 350 and actuate a first solenoid 380 controlling extension of the pneumatic cylinders 60 and 62, and solenoids 386, 384, and 387 for the flow, bypass, and warmup valves 312, 314, and 337, respectively.

Referring to FIG. 12, the PLC program usable in conjunction with the hardwired circuit of FIG. 11 is represented by a ladder diagram 400 the first rung of which includes a CHILLER START output signal 404 generated whenever the chiller evaporator 302 is active as detected by a logic switch 402. A NIP ON switch 418 is closed to generate a signal 420 energizing solenoid 380 in FIG. 11 and triggers extension of the actuating cylinders 60, 62 only if PLC switches 404, 406, 408, 410, 412, and 414 indicate that certain conditions are met. These conditions are: (1) the chiller evaporator 302 must be operational, (2) the safety switches 354 and 356 must not depressed, (3) coolant temperature as monitored by probe 327 must be below an acceptable temperature of, for example, 20.degree. F., (4) the press 10 must be running at an acceptably high speed (as monitored by a press run contact 390 in FIG. 11), and (5) the temperature of the outer peripheral surface of the nip roll 52 as detected by the IR sensor 360 must be below a designated temperature of, for example, 150.degree. F. If any one of these conditions are not met, the NIP ON PLC signal 420 is not generated, causing the circuit 350 to de-energize solenoid 380 and retract the actuators 60, 62, thus placing the nip roll 52 in its inoperative condition.

The PLC ladder diagram 400 also illustrates the coolant flow control logic for the nip roll 52. When all of the conditions specified in the preceding paragraph are met, the NIP ON switch 418 is closed, causing the generation of a signal 423 de-energizing the flow solenoid 386 and allowing unrestricted coolant flow through the nip roll 52.

When the NIP ON switch 418 is open, indicating that the system 10 is off-line and causing energization of flow solenoid 386 and the retraction of cylinders 60 and 62, a warmup timer signal 429 is generated. Generation of the warmup timer signal 429 results in energization of warmup solenoid 387 and opens warmup valve 337 for a designated period of time during which coolant flows from the chiller evaporator 302, through the restrictor 333, through the primary flow path of heat exchanger 335, through the nip roll 52, and back to the chiller evaporator 302. When the warmup timer 429 time sequence is complete, a bypass signal 425 is generated to energize the bypass solenoid 384, thereby opening valve 314 and allowing unrestricted coolant flow from the pump 306 to bypass the nip roll 52 and return directly to the chiller evaporator 302.

The warmup timer 429 and associated circuitry could be replaced by any suitable system which cuts off operation of the warmup loop 331 and triggers operation of the coolant bypass circuit when the temperature of the nip roll 52 exceeds the frost and dew point temperatures of the surrounding air. For instance, signals generated by the temperature sensor 360 could control switching from nip heating to nip bypass at the appropriate time.

Finally, a system status signal 432 may be generated in response to the operation of logic switches 406, 408, 410, and 414 to provide a visual and/or audible indication of the operational state of the system 300.

8. Operation of System with Second Coolant Circuit and Second Control Circuit

In operation, an endless paper web 14 is coated by printing units 12, dried by dryer 16, and conveyed out of the dryer 16 and through the chill stand 18, as illustrated in FIG. 1, at speeds up to 2000-3000 feet per minute. As the web 14 travels over the first downstream chill roll 20, it is pressed into intimate contact with the chill roll 20 by the nip roll 52, which preferably engages the web 14 near the point at which it first contacts the chill roll 20 as illustrated in FIG. 5. However, because the air layer between the web 14 and chill roll 20 is completely eliminated and intimate contact achieved, nip roll contact at this location is not essential, and the nip roll 52 could be located considerably downstream from this point if necessitated by system parameters. The intimate contact prevents condensate formation and the resulting condensate streaking, thus dramatically improving the product. Moreover, because an insulating air layer between the chill roll 20 and web 14 is absent, web cooling at the first chill roll 20 is dramatically enhanced, even if coolant is circulated through the chill roll 20 at a relatively high temperature of about 70.degree. F. Indeed, it has been discovered that the web 14 is cooled up to 150.degree. as it traverses chill roll 20, an increase of 55.degree.-60.degree. F. or more as compared to conventional processes in which intimate contact is not achieved.

The web 14 exiting the first downstream chill roll 20 is typically at a temperature of 100.degree.-150.degree. F. (assuming a dryer web exit temperature of 240.degree.-280.degree. F.) and exhibits no danger of ink softening or condensate streaking on subsequent chill rolls. Even if dryer 16 is operating at a higher web exit temperature range of 280.degree. to 320.degree. F., the web 14 will have a temperature of less than 200.degree. F. as it leaves the chill roll 20, but solvent evaporation is reduced sufficiently to eliminate any condensate buildup on downstream chill rolls 22, 24, 26. The paper wetting problems associated with externally cooled nip rolls of the type disclosed in Daane '560 patent are also eliminated because the nip roll 52 is cooled internally rather than externally.

The web continues to pass over and under successive chill rolls until it leaves the last roll at or near room temperature, i.e., 70.degree.-90.degree. F. Because of increased cooling at the first downstream chill roll 20, fewer chill rolls are required than are normally employed in the industry. As few as two or even one chill roll would be required in many instances.

So long as the actuators 60, 62 are extended to press the web 14 into intimate contact with the first downstream chill roll 20, the bypass and flow valves 314 and 312 will be closed and opened, respectively, to assure coolant flow through the nip roll 52 at a temperature of 15.degree.-20.degree. F. and at a flow rate of 10-12 gallons per minute (assuming a 30% ethylene glycol coolant solution and a press speed of 2500 feet per minute). Sufficient heat is transferred through the elastomer layer 82 to maintain the steady state operating temperature of the outer peripheral surface of the nip roll 52 below 130.degree. F., and for the most part below 80.degree.-120.degree. F., thereby avoiding any ink softening by the nip roll 52 and ink picking on the downstream equipment.

If for any reason the circuit 350, 400 retracts the actuators 60, 62 to take the nip roll 52 off-line, e.g., because a safety switch 354 or 356 is activated or because the coolant temperature as monitored by probe 327 is unacceptably high or the temperature of the outer peripheral surface of the nip roll 52 as monitored by sensor 360 is unacceptably high or the press run contact 390 is opened when the system 10 stops, the timing logic of the PLC 400 will control the valves 312, 314, and 337 to cause nip warming. Specifically, coolant in the warmup loop 331 flows through the primary flow path of the heat exchanger 335 at a rate of about two gallons per minute, where it is warmed from its normal temperature of 15.degree. F. to 20.degree. F. to a temperature between 50.degree. F. and 100.degree. F. via heat transfer with water flowing through the secondary flow path at a rate of about three gallons per minute and at an initial temperature of 60.degree. F. to 140.degree. F. The warmed coolant flowing out of the primary flow path of the heat exchanger 335 flows through the nip roll 52 and then back the chiller evaporator 302, thereby to warm the surface of the roll 52. At the end of the period set by timing circuit 429 (typically about three to eight minutes), the warmup loop 331 will have heated the outer peripheral surface of the nip roll 52 to about 65.degree. F. to 80.degree. F. (above the frost point and dew point of the surrounding air). The timing logic of the PLC 400 then will automatically 1) close warmup valve 337 to prevent further fluid flow through the warmup loop 331 and 2) open the bypass valve 314 to permit continuous, unrestricted recirculation of coolant through the chiller evaporator 302 and maintain the chiller coolant at a sufficiently low temperature (below 20.degree. F.) for rapid restart. The surface of the nip roll 52, being at a temperature above the frost and dew points of the air, and being exposed to room temperature, will remain free of condensation and frost for the remainder of the period that the system 300 is off-line.

When the five conditions specified above are again met and the system is restarted by generation of the NIP ON signal 418, coolant again flows through the nip roll 52 at a temperature of 15.degree. F. to 20.degree. F. and at a flow rate of 10 to 12 gallons minute. This flow is more than adequate 1) to assure that the outer surface of the nip roll 52 is cooled to its operating temperature by the time that the heated printed portion of the web contacts the nip roll 52 several seconds after system restart and 2) to maintain the steady state operating temperature of the outer surface of the nip roll 52 within an acceptable range.

9. Two-Nip Coolant Circuit

Both of the coolant circuits described above are described in conjunction with a single nip roll 52. However, a single coolant circuit could be used for the independent control of two or more nip rolls. Referring to FIG. 13, a coolant circuit 500 is illustrated which differs from the second coolant circuit 300 only in that it controls simultaneously and independently the flow of coolant through two separate nip rolls 52 and 52'. Elements in FIG. 13 corresponding to those in FIG. 10 are accordingly designated by the same reference numerals, incremented by 200.

The coolant circuit 500 is closed loop and includes as its major components a conventional chiller evaporator 502, a coolant reservoir 504, and a pump 506 arranged in series. A first inlet conduit 508 leads from the pump 506 to the coolant inlet 84 of the first nip roll 52, and a second inlet conduit 508' leads from the pump 506 to the coolant inlet 84' of the second nip roll 52'. Similarly, a first outlet conduit 510 leads from the coolant outlet 86 of the nip roll 52 to the chiller evaporator 502, and a second outlet conduit 510' leads from the coolant outlet 86' of the nip roll 52' to the chiller evaporator 502.

A flow valve 512, 512' is disposed in each of the inlet conduits 508, 508', and a bypass valve 514, 514' is disposed in a branch line 516, 516' connecting the outlet conduit 510, 510' associated with each nip roll 52, 52' to the corresponding inlet conduit 508, 508' at a location upstream of the flow valve 512, 512'. A warmup loop 531, 531' is provided for each nip roll 52, 52' and extends from the upstream side of the respective bypass valve 514, 514' to the respective inlet conduit 508, 508' downstream from the flow valve 512, 512'. The warmup loops 531, 531' each include a flow restrictor valve 533, 533' and a warmup valve 537, 537'. Both warmup loops 531, 531' cooperate with a common heat exchanger 535 which additionally includes a heated fluid inlet 541 and a warm fluid outlet 543. As in the embodiment of FIG. 10, the heated fluid flowing through the secondary flow path of the heat exchanger 535 preferably, but not necessarily, comprises water which is heated by the chiller evaporator 502 in a closed loop. Also as in the embodiment of FIG. 10, a flow switch 545 is located in the conduit 543 and disables operation of the warmup loops 531 and 531' when the chiller evaporator 502 is not operating.

The flow valve 512, 512', bypass valve 514, 514', and warmup valve 537, 537' associated with each nip roll 52, 52' are controlled by a separate logic circuit of the type illustrated in FIGS. 11 and 12 and described in Section 7 above.

As in the embodiment of FIG. 10, several safety and monitoring devices are also provided in coolant circuit 500. For instance, a safety bypass restrictor 518 and bypass line 520 are provided in parallel with branch conduits 516 and 516' to permit limited coolant circulation through the chiller evaporator 502 and thereby to prevent damage to the chiller evaporator 502 if all of valves 512, 514, and 537 or 512', 514', and 537' associated with a particular nip roll 52 or 52' are closed or if the flow adjustment valve 530 or 530' is closed. A standard low level switch 522 and flow switch 524 are also provided to shut down the system 500 and prevent damage to the chiller evaporator 502 in the event of a coolant leak. A temperature probe 526 is provided downstream of the chiller evaporator 502 for monitoring operation of the chiller evaporator. A second temperature probe 527 is disposed at the outlet of the reservoir 504 and is used as an input for the alarm contact 388 and down-to-temperature switch 410 discussed in Section 7 above. In addition, in order to maintain coolant flow rates in the preferred range of 10-12 gallons per minute, a coolant flow meter 528, 528' is located in each outlet conduit 510, 510' prior to the conduits 516, 516' and 520 to permit an operatour to manually set a coolant flow adjustment valve 530, 530' to maintain the flow rate through each nip roll 52, 52' in the desired range.

Many changes and modifications could be made to the present invention without departing from the spirit thereof. The scope of such changes will become apparent from the appended claims.