Consolidated Paper’s Biron, Wis., paper mill installs first-of-its-kind hood, making maintenance easier and eliminating production problems related to condensation

All-Aluminum Modular Dryer Hood Allows High Humidity Operation

In addition to providing an energy-efficient drying enclosure and a quiet, comfortable machine room environment, the hood is designed with dryer maintenance in mind. The large but lightweight modules are quickly removed, reinstalled, and resealed to their original vapor tight condition. The modular design also reduces installation time. With superior insulation qualities and air tightness, condensation and related problems on the No. 25 machine have been eliminated.

THE RIGHT TIME. Paul Fredrickson, paper machine No. 25 line superintendent, says that the time was right to replace the existing 1957-vintage hood. “The hood was in bad shape; corroded in many places. We were facing more and more issues relating to the hood condition, such as condensation, dripping, and black scale.”

The hood replacement was also linked to a machine wet end rebuild project, since the wet end portion of the existing hood needed to be changed to accommodate a new press section. The machine rebuild project was initiated after all state and federal environmental permits were obtained.

Valmet approached Consolidated with a new design for a hood that would improve upon the “high humidity” concepts the company first introduced during the 1980s. The benefits of high humidity drying have been documented —lower energy consumption through lower exhaust and supply air volumes and better exhaust heat recovery potential. Also, with lower exhaust volumes, building makeup air requirements are minimized. Better control over air distribution in the hood can also improve sheet stability and avoid uneven drying.

But the air in a high humidity hood is very hot and heavily loaded with moisture, so any air infiltration can cause localized cooling, condensation, and dripping. A high humidity hood must therefore be sealed exceptionally well, and the same seal integrity must be re-established when it is opened up and re-assembled. Often the sealing of conventional tongue and groove panels deteriorated with age and, once they were removed for dryer section service, the proper resealing of the panels was difficult and seldom achieved.

Since the maintenance of dryer panel sealing is usually not a high priority, many existing hoods have become porous over time, and condensation is often a chronic problem. In many cases, exhaust humidity has been reduced from the original design. Then, the full heat recovery potential from the exhaust cannot be realized.

INPUT FROM MAINTENANCE DEPARTMENT. The Biron mill’s maintenance staff was looking for a way to easily disassemble and reassemble hood panels when they were replacing dryer cylinders or working on other dryer maintenance projects. Removing panels from previous generation modular hoods was no easy task as it required unbolting from both the inside and outside. Ronald Steege, maintenance coordinator, says, “We had a hard time getting it apart. It added a day to a dryer change time.”

Valmet consulted Consolidated maintenance personnel and other customers to devise a “wish list” which would make maintenance easier. The result was the first new generation hood (Figure 1). Before shipping, the panel design was thoroughly tested for insulating qualities, vapor barrier performance, resistance to condensation at a high dewpoint, and strength properties. The tests were conducted at Valmet’s Pansio works in Finland and in Thunder Bay, Canada.

Figure 1: A high humidity, high performance hood is assembled with large but lightweight aluminum modules. The number of joints and the joint perimeter is much less than for a conventional hood.

In the factory, the humidity was raised beyond 0.3 lb of water vapor/lb of dry air before condensation occurred. There were no apparent air leaks at an internal hood pressure of 140 pascals (more than 0.5-in. water gauge). These humidity and pressure levels are far above normal operating conditions.

The new hood was installed at Consolidated in February 1999. The rebuilt machine started production in March 1999.

The aluminum modules need no painting and remain free of corrosion.

The new hood module joints are designed so that original vapor tightness can be maintained after modules are removed and replaced for dryer service.

BUILT FOR HIGH HUMIDITY. The all-aluminum hood modules are constructed to have high strength, good insulating characteristics, and a complete resistance to moisture migration. Like the best window frames for a house, heat transfer through metal to metal contact has been minimized, thereby eliminating localized temperature differences that might cause condensation.

The module core is filled with high R-value insulation, and the inside of each panel is lined with a one-piece aluminum vapor barrier. The roof modules, which incorporate an integral exhaust plenum, are strong enough to walk on. No extra walkways are required on the roof, but an optional perimeter safety railing can be provided. The plenum design incorporates adjustable dampers so that moist air is exhausted evenly over the entire dryer section. Fire sprinkler piping and interior lights are located so that the roof modules can be easily removed.

The roof and side modules are sealed using “controlled joint” technology. All joints are bolted and gasketed from the outside. The panels are fitted with lifting lugs so they can be removed and replaced using the machine room overhead crane. After bolting into place, the original seal integrity can easily be restored by hand fitting from outside the hood. No special tools or “fit and finish” skills are required. After fitting insulation into the joint, the final flashing is applied, providing an attractive and effectively sealed joint.

Most of the hood for the No. 25 machine was assembled using standard modules, but the press section rebuild raised a special design issue. The press felt run would overhang the hood entrance, posing a potential dripping problem.

Valmet worked with the press manufacturer to devise a solution. A saveall pan catches overspray from the felt. A plenum under the saveall is supplied with heated air to eliminate the possibility of condensation and dripping.

SURFACES STAY CLEAN, CORROSION FREE. Aluminum surfaces are naturally corrosion resistant and do not require any painting. Rather than the rafter support structure used for conventional hoods, the modular hood requires a minimum of support steel—only columns that are offset from the machine frame. Longitudinal beams attached to the columns serve as mono-rails for felt roll trolleys. With the absence of rafters, a fully integrated exhaust plenum in the roof modules and smooth interior surfaces, dust buildup is kept to a minimum.

The safety of hood door lifting operations is ensured by several means. A mechanical star wheel catch assembly holds the panel in place if a cable fails. Limit switches stop the individual panel gear motor if the catch assembly is activated or if a cable becomes slack. A photocell system will stop the panel descent when a light beam is broken by a person or an obstruction. As an alternative to a cable lift mechanism, a center guide cylinder lift mechanism can be used.

Ideally, dryer hoods are something that papermakers don’t have to think about. They must provide an efficient drying environment and not cause any operational or quality problems. The startup of the new hood on the No. 25 paper machine has been trouble free and has solved sheet quality problems associated with condensation. As Fredrickson says, “The hood is now nothing to worry about. The black scale has gone away.”

MARK WILLIAMSON is a freelance writer based in Thornhill, Ont.

The hood door lifting mechanisms on the high performance hood are designed to be fail safe.