Don't let an Indian summer fool you. Winter and its cold-induced web breaks lie just ahead.
Proper temperature-conditioning of newsprint rolls before use can reduce production-line halts. Temperature-conditioning is particularly critical during winter, when deliveries may be partially or completely frozen.
Because of the large thermal mass of newsprint rolls, small temperature differences, relatively small heat-transfer coefficients, and the phase change needed to thaw frozen newsprint, adequate temperature-conditioning can easily take two or more weeks. The arrangement of rolls during temperature-conditioning is critical. Interior nested rolls within stacks of frozen newsprint are extremely difficult to temperature-condition in a timely way.
Newspapers in colder-climate regions often try to temperature-condition newsprint rolls in the winter by parking the rolls they receive in either a heated warehouse or in a roll laydown area within the reelroom. One Midwest plant we analyzed used a 12,000-roll capacity heated warehouse. To the west, the warehouse is connected directly to the larger printing plant's reelroom. To the east, the warehouse is adjacent to an enclosed, smaller rail-loading dock. Newsprint rolls are delivered to the dock in unconditioned rail cars from two Canadian paper mills. Shipping time is normally either three days or 18 days.
Newsprint-handling methods render temperature-conditioning a very complex problem that does not offer an "exact" solution. Nonetheless, analytical methods were applied to try to estimate an applicable "bounding" case.
The following data describe the assumed newsprint rolls: largest size, 55" x 45" O.D. x 2" I.D. (2" metal core); approximate mass, 2,200 lbs.; moisture content, 8 percent of paper mass.
Two models were created. The first was of a single newsprint roll exposed to a room maintained at a constant 72°F. The second model was of a cluster of nested rolls representing a typical storage configuration used in the warehouse.
Axial heat transfer in rolls was ignored. The initial temperature of the newsprint was assumed to be uniform at -10°F. Because the newsprint roll is transported by rail car for several days, temperatures will certainly vary within the roll. Because of uncertainties in establishing prior conditions, however, this variation is indeterminate.
Other physical parameters needed for the model: paper conductivity (1995), 0.075 Btu/h-ft-F; paper specific heat (1995), 0.32 Btu/lb-F; heat transfer coefficient (1981), 0.76 Btu/h-ft2-F; heat of fusion--Wtr. (1993), 143.5 Btu/lb @ 32°F.
Researchers assumed the newsprint roll wrappers had the same physical properties as the newsprint and effectively blocked measurable mass (i.e., water vapor) transfer between the ambient air and roll. This limited the energy-transfer problem to the flow of heat via conduction, convection and radiation.
A computer program that estimates transient and steady-state temperature distributions in three dimensional objects was used to model the newsprint. The model predicted a complete thaw after approximately 13 days (310 hrs). To fully temperature-condition the roll to a minimum of, say, 60°F takes four days more.
Actual infrared temperature measurements of newsprint rolls in winter suggest a more realistic initial temperature is 20°F. To completely thaw the paper to above 32°F still requires about 11 days. Complete temperature conditioning to 60°F or above takes an additional four-to-five days.
To test the sensitivity of roll temperature-conditioning to surrounding air temperature, the single-roll model was rerun using 20°F initial newsprint temperature assuming: 1) 45°F air, and 2) 60°F air. As would be expected, air at 45°F is not very effective at conditioning frozen newsprint rolls. After 20 days exposure, the inner one-third of the newsprint roll was still frozen. With 60°F surrounding air, complete thawing takes about 14 days. After 20 days, the newsprint roll was above 50°F at all depths.
The nested multiple-newsprint roll model has, because of its larger mass, a much slower overall thermal response. Given an initial roll temperature of 20°F and 72°F constant surrounding air, interior rolls--those with no surfaces directly exposed to the warehouse air--take at least 40 days before they begin to thaw! Exterior rolls--those partially exposed to the surrounding warehouse air--fare better, taking about 15 days to thaw, but another 35 days on average to be fully temperature-conditioned to 60°F or over.
In light of this finding, "full" temperature-conditioning is probably not practical during prolonged cold snaps, though it should be possible to (at a minimum) fully thaw newsprint rolls. To accomplish this during the coldest winter months, newsprint rolls should be stored for about 15 days in a warehouse heated to 72°F or higher, if feasible. Rolls should also be stacked no more than two rows deep, with space between each group so all rolls can have at least some direct exposure to heated warehouse air.
Doyle is senior vice president and Marseille is a project engineer with Grumman/Butkus Associates in Evanston, Ill. Phone, (847) 328-3555; fax, (847) 328-4550.
©1997 Newspaper Association of America. All rights reserved.