Closed system cooled covered roll and methods of forming and operating cooled covered roll

1. Field of the Invention

The present invention is for use in paper machines having rolls covered with roll covers In particular, the roll covers can be made of polymers, e.g., rubbers, polyurethanes, epoxies, thermosets, thermoplastics, etc., and these paper-machine rolls are internally water cooled in the present state-of-the-art.

2. Discussion of Background Information

Roll covers utilized in critical applications of paper machines are generally water-cooled to counter the internal heating of the cover material as it rolls through the nip. In this regard, the thermo-visco-elasticity of typical roll cover materials increase the internal heating of the cover with increasing nip loads, increasing speeds, and increasing cover thickness. Moreover, with everything else being held constant, the internally generated heat increases for materials having lower thermal conductivity, for materials with lower stiffness, i.e., lower storage modulus “E′,” and for materials that have higher “tangent of the phase angle,” i.e., higher “tan δ.” Thus, it is advantageous to constrain, within limits, the internal and interfacial material properties of the roll cover, e.g., fracture toughness, strength, stiffness, wear resistance, etc., and since the properties of the adhesives used to bond the cover to the roll core decrease with increasing temperature, and since the paper machine process needs a roll cover material with stable properties through time and uniform properties in the cross-machine direction in order to process paper with consistent uniformity.

To ensure an even distribution of temperature from the tending (front) end of the roll to the drive (back) end of the roll, different water-cooling systems have been designed and implemented. A goal of these water cooling designs is to achieve uniform cooling of the roll so as to avoid dead spots, i.e., no flow areas. Generally, the various water-cooling systems utilize water flowing through the roll (in the cross-machine direction) during operation. Accordingly, these designs can require, e.g., flow meters to measure and control a prescribed amount of water flow in and out of the roll, and thermometers and temperature recorders to measure the water temperature in and out of the roll. These flow meters, thermometers and temperature recorders need to be maintained properly, and water pressure and air pressure also need to be periodically monitored. Further, the roll must be continually monitored for too much water, not enough water, and plugged exit ports. The water itself needs to be monitored to ensure it is clean, since, if the water supply contains a large amount of minerals, sludge, or other contaminants, periodic opening may be required to check for scale and mud inside the roll. In the known designs, it is also important, and difficult, to maintain a constant inlet temperature all year round, since available stream water used for cooling these paper machine rolls in most mills can vary more than 22° C. depending on the season. While a heat exchanger can ensure constant incoming cooling water temperatures, this requires additional expenses (including increased energy consumption costs) and increases the maintenance issues.

It is further known, the cooling water in the roll should not be too cold because, if the inside temperature of the cover is significantly cooler than its outside temperature, thermal-gradient-driven permeation may result in debonding of the cover from the roll. This debonding can be caused by condensation of water (driven by permeation through the polymer) at the colder interface. Diffusion of water through the polymer is particularly important for polyurethane covers, since polyurethane is more water-permeable than most rubber compounds. Thus, the cooling water temperature at the inlet of the roll is prescribed to be higher than 24° C. (in the present state of the art) to prevent water permeation damage and hence maintain cover bond integrity. In rolls of this type, regardless of the polymer cover, it is generally recommended to stop the water flow during a shutdown of even short duration. In this regard, if water continues to enter the roll during a shutdown, the roll may fill with water. This can be particularly problematic when the roll during shutdown is partly filled with water colder than the cover temperature, and the top of the roll is not able to cool down to the same temperature as the cold water. As a result, the colder water will pool, by gravity, to the bottom of the roll, which can create a thermal gradient in the roll, i.e., hotter at the top and colder at the bottom. This thermal gradient will produce bending of the roll, e.g., like a banana, with the center of the roll protruding up, i.e., the convex side at the top and the concave side at the bottom. Upon start-up of this distorted roll, non-uniform cross-machine direction contact (and nip pressure profile problems) occurs at the nip, and severe vibrations may arise depending on the severity of the (thermally) bent roll shape.

Therefore, as use of water flowing through a roll to control the heat generation of a cover can create a number of problems and additional costs, it is not surprising that paper mills prefer a roll cover that does not require water flow through the roll to operate. Moreover, the elimination of flow through rolls likewise eliminates the above-noted energy costs, maintenance and operational problems associated with cooling by internal water flow forced through the roll.

The motion of a film of liquid on the inside surface of a roll rotating at a steady rotational speed exhibits a variety of surprisingly profuse flow phenomena. The flow of liquid inside the roll may display various free-surface fluid flow instabilities, pattern formation, non-uniqueness and even non-existence of steady-state fluid flow solutions. The external forces on the fluid film during operation of the roll can include inertia (e.g., centrifugal and Coriolis forces), gravity, viscosity, and surface tension.

At zero rotational speed, the fluid within a partially filled roll is stationary and lies in a pool at the bottom of the roll, and, even at a low speed roll rotation, a liquid film generally lies at the bottom of the partially filled roll, except for a very small amount that travels around with, and wets, the inner wall of the roll. This liquid pool on the bottom of the roll can establish a cellular circulation pattern in the cross-machine or axial direction of the roll. The liquid surface can establish a periodic bore between the liquid and the descending surface of the roll.

As the rotation rate is increased, the surface becomes irregular and can mask this initial periodic pattern. As the fluid film that clings to the surface enters the bottom pool on the receding side, i.e., on the downwardly rotating side of the roll, a sharp straight front is created. An accompanying recirculation region is also formed in the pool that grows in the circumferential direction with increasing rotational speed and the front on the receding side is pulled farther towards the rising side.

With increasing angular speed, the film pulled out of the pool also thickens. The fluid film eventually becomes unstable and the fluid motion changes to a sloshing motion on the rising side of the roll. This falling wave is initially straight in the cross-machine direction of the roll, but at higher rotational speed it breaks up into a number of separate (in the cross-machine direction) gravity waves with approximately parabolic shapes. For a limited range of rotational speed, these gravity waves, sometimes referred to as “pendants,” are practically stationary. At still faster rotational speed, this sloshing instability is overcome by viscosity and the cascading flow becomes essentially two-dimensional.

At high rotational speeds, the front is pulled over the top of the roll, and a “rimming mode” develops where centrifugal forces dominate the flow and the fluid coats the inside surface of the roll. At still higher speeds, and for small enough fill rate of the roll, the fluid coats the inside surface of the roll uniformly and rotates rigidly with the roll. The rotational speed at which the fluid just enters the rimming mode (with increasing rotational speed) is higher than the rotational speed at which the fluid leaves the rimming mode (with decreasing rotational speed). This hysteresis is more pronounced with increasing filling fractions (volume occupied by fluid divided by the total volume inside the roll). Other phenomena associated with the transition regions include the popping or fluttering of surface features associated with strong localized vortex flows inside the fluid sheet, air entrainment at the front, which may lead to avalanches, and shedding of hydroplaning drops. For large filling fractions, curtains or so-called “hygrocysts” spanning the entire cross-machine direction of the roll are formed. For a fluid with small values of viscosity, the flow inside the pool and the rising sheet becomes strongly turbulent. Further, large-scale patterns can persist even in the presence of this (small-scale) turbulent flow.

Therefore, it is known the flow of a homogeneous liquid in the interior of a rotating roll can display a number of different characteristic flow states with various degrees of complexity, including flows that are 3-dimensional with secondary flows in the cross-machine direction of the roll. Moreover, flow states inside rotating rolls have been referred as “flat-front state,” “wavy-front state,” “localized u-shaped structures,” “shark-tooth pattern,” “hydrocysts,” and “cascades.”

For constant kinematic viscosity, the filling fraction level and the rotational speed of the roll are the main determinants of the flow state adopted. For each filling level, the transition between flow states occurs at well defined associated critical rotational speeds. However, the sequence of transition scenarios is not unique, i.e., different transition scenarios can be observed at different locations in the phase plane.

The present invention targets a roll in a paper machine provided with a roll cover. The roll operates as a closed system, i.e., without the need for fluid flowing into and out of the roll.

Moreover, the roll according to the invention can include an amount of coolant, e.g., water, to provide a thickness of fluid in the operating roll greater than in a rimming mode. According to the invention, this greater than rimming thickness of coolant provides cooling in the longitudinal direction of the roll not achievable by a roll, in particular, a closed roll, operating in the rimming mode.

Further, the present invention exploits the fluid instabilities occurring when a fluid is inside a rotating roll, and in particular, exploits secondary flows in the cross-machine or axial direction, to promote the heat transfer of internally generated heat from the roll cover by fluid convection in the axial direction of the roll.

The invention targets a roll including a roll body, a deformable cover coupled to the roll body, a pluggable input, and a coolant contained within the roll body at least in an amount to provide cross-machine direction heat transfer during normal operation of the roll.

According to a feature of the invention, the roll can be a cooled roll operable as a closed system.

In accordance with another feature, the at least an amount of coolant to provide cross-machine direction heat transfer can include an amount greater than the amount of coolant sufficient for at least a rimming mode to occur.

According to still another feature of the present invention, the at least an amount of coolant to provide cross-machine direction heat-transfer may be determined by the difference in steady state power between operation of the roll with fluid flowing through the input and output of the roll and operation of the roll without coolant.