1. Field of the Invention
This invention relates generally to an improvement in the processing of fiber slurries which are saturated with a fire retardant particulate organic binder by a beater-saturating technique. More particularly, this invention relates to a method of improving the drainage time of beater-saturated asbestos slurries while yielding fire retardant products requiring no post-saturation.
2. Description of the Prior Art
Most processing plants need to remove rather large quantities of low-value heat. Industrial cooling towers, rectangular structures usually made of redwood and measuring roughly 50 feet high, 60 feet wide and several hundred feet long, have been familiar sights around such plants for many years. Modern cooling towers, i.e. the hyperbolic tower, are, however, gradually replacing the older, less efficient rectangular structures. These towers operate by allowing ambient air to pass over the hot process water, thereby carrying away its process heat. The hot process water, from the plant's condenser and other equipment, splashes down through heat transfer plates, such as against an upward flow of air from ducts at the bottom of the tower. The cooled water is collected at the base of the tower and circulated back to the processing plant for reuse. The heat transfer plates, sometimes called fill materials, are usually of specific shape and dimensions so as to maximize the surface area of the downwardly flowing process water thereby, in turn, maximizing the contact with the upwardly flowing cooling air. While a variety of materials have been used as such heat transfer plates, the most economical have been rigid, corrugated sheets of treated paper-like materials. In order to operate satisfactorily in these cooling systems, it is essential to have available a fill material that is not only economical in cost but also fire retardant and dimensionally stable, e.g., it does not lose its effective heat transfer shape under conditions that arise normally from the operation of cooling towers.
One of the most successful of the paper-like materials used as cooling tower fill are asbestos paper. Such cooling tower fill papers are advantageously made by the widely recognized beater saturation process so commonly used in the paper-making industry. In this process, various binders are added to the beater saturation slurry of asbestos in order to achieve interfiber bonding, such binder precipitating and coating the asbestos fibers. These binders are preferably added to the aqueous slurry as resin emulsions or latices. After draining, the asbestos sheets are saturated with a melamine or phenolic resin to impart rigidity. They are then formed into the desired (usually corrugated) shape, dried and cured so as to give the desired fire-retardant board useful as cooling tower fill. The resulting asbestos sheets adhered with the latices and resin emulsions have proven economically disadvantageous for tower fill substances, principally because of the necessity for the additional post-saturating step after paper formation so as to develop sufficient rigidity in the final product.
In an attempt to obviate this problem, e.g., to eliminate the post-saturation step necessary to give a sufficient rigidity and shape retention characteristics to the fill material, asbestos sheets containing fire-retardant, chlorinated polymers such as poly(vinyl chloride) have been utilized. While these vinyl materials enjoy commercial attraction in terms of the elimination of the post-saturation step, the processing conditions for beater-saturated asbestos fibers using PVC binder latices have been shown to provide drain times that are excessive and, therefore, cannot be economically used on conventional paper-making machines.
Accordingly, there is a need for a cooling tower fill material that is fire retardant as well as rigid at cooling tower operating conditions, such formed by a beater saturation process in which the binder-coated asbestos slurry is quick draining.