A wallboard manufacturer installs air gravity conveyors to minimize consumption and maintenance.
Temple-Inland Forest Products Corp., located in Diboll, Tex., began gypsum wallboard production in 1964. During the past 37 years, the company has expanded to add a total of four plants to the company. In 1999 Temple completed Standard Gypsum, a joint venture in Cumberland City, Tenn., strategically built adjacent to the Tennessee Valley Authority power plant. Standard Gypsum, used to make gypsum wallboard, produced from the power plant's flue gas desulphurization by-product. Temple receives 66 percent of all company gypsum raw material requirements from this environmentally friendly source.
Standard Gypsum hired Rick Daugherty as part of the construction team assigned to manage the design of the new plant. When Daugherty began working on plans for the Cumberland City plant, he knew he wanted to prevent certain problems and inconveniences experienced at other typical wallboard plants. Key candidates for change were the screw conveyors and bucket elevators the plant used to transfer stucco (plaster of Paris), a dried form of synthetic gypsum, from the calcining mills of storage silos. He explains, "The mechanical conveyors consume considerable horsepower and require frequent maintenance and replacement parts. We wanted to avoid as many problems as possible."
The wallboard production process
The synthetic gypsum begins as an effluent at the bottom of the Tennessee Valley Authority power plant's scrubbers. The effluent is dewatered to a granular state on a fiber belt and then conveyed by a belt conveyor about 800 feet to the Standard Gypsum plant. In the plant, drag conveyors move the material directly into feed bins ahead of the plant's calcining mills. The mills calcine and dry the material to an even finer powder, producing stucco. The material is then collected in dust collectors and conveyed to silos.
From the silos the material is conveyed to a pump, which conveys it to a feed bin. The material flows from the feed bin to a pin mixer, where the stucco and water are blended for 2 to 3 seconds. The mixture flows out onto a sheet of heavy paper, another sheet is placed on top, and forming plates exert pressure on the paper and stucco mixture, producing wet wallboard. After about 3 minutes the wallboard sets. The wallboard is then cut into smaller sections, flipped over, and stacked in a dryer to dry. After the dryer, the wallboard goes through a final cutting, trimming, taping, and stacking operation.
Problems with mechanical conveying
In a more conventional plant, screw conveyors and bucket elevators transfer the stucco from the calcining mills to the silos. Generally, this process consumes considerable horsepower. The conveyors and elevators also have moving parts, including bearings, drives, and motors, making frequent maintenance and regular part replacement unavoidable. In addition, the screw conveyors and bucket elevators aren't closed systems so they emit dust, making the machines less friendly to the environment and to the operators. Daugherty decided up front to avoid these problems in the Cumberland City plant. With a background in the mining and minerals processing industries, Daugherty had always worked with air gravity conveyors. He decided to introduce air gravity conveying at the new synthetic gypsum plant to transfer the stucco from the calcining mills to the process.
"You can vent bucket elevators and screw conveyors and keep a negative pressure on those devices to contain dust, but air gravity conveyor has no moving parts and requires less mechanical maintenance than a screw conveyor, avoiding the mechanical inefficiencies of a big screw turning inside a trough."
Air gravity conveying systems
An air gravity conveyor handles high material tonnage without consuming a lot of energy. The conveyor is a large duct set at a slight incline. Inside the duct, upper and lower compartments are separated by a porous membrane fabric, also called a canvas, that aids in fluidizing the material being conveyed. Fluidizations reduces the duct incline angle of repose, it may not flow inside the duct unless the duct is inclined at a 52-degree angle. However, if the material is fluidized, its aerated angle of repose could be as low as 4 or 5 degrees, allowing the material to be conveyed by gravity alone at a 5- or 6-degree incline.
In the air gravity conveyor, the material becomes fluidized when low-pressure air, provided by a 2.5-horsepower blower, flows from the lower compartment through the porous membrane fabric through the upper compartment and contacts the material. The consistent low-pressure airflow keeps the material in the semi liquid, fluidized state necessary for the conveyor to function properly. Gravity pulls the material on an air cushion, preventing abrasion on the porous membrane fabric or the duct's sides.
"The mechanical conveyors consume considerable horsepower and require frequent maintenance and replacement parts."
Teaming with the conveyor manufacturer
From the beginning, Daugherty knew that he wanted to work with FLSmidth-Pneumatic Transport, Bethlehem, Pa., and their air gravity conveyors. He testifies, "I've been around their equipment since day one and have been very familiar with the company for the past 25 years. They met all my specifications and had competitive prices."
Standard Gypsum and FLSmidth-Pneumatic Transport worked with Mid-South Engineering, a consulting firm in Hot Springs, Ark., to customize the air gravity conveyors and silos for aerating the material prior to conveying. The three companies began by testing several drums of Standard Gypsum's stucco. They performed flow tests, aeration tests, and pumping tests to determine the optimal conveying rates and distances and the necessary conveyor elevations and clearances.
Using the air gravity conveyors at the conveyor manufacturer testing facility, the conveyor manufacturer also determined the conveyors' required horsepower, duct sizing, and incline necessary to maintain the stucco's flow. In this case, the required incline was 6 percent.
The installation
The installation began is September 1997. Standard Gypsum installed 16 Airslide air gravity conveyors. While getting the plant underway had its glitches, Daugherty states, "The conveyors themselves worked well."
The conveyors require air at 1psig to convey material at 100 short t/h and have no moving parts that contact he material, minimizing maintenance and eliminating lubrication requirements. Daugherty explains, "They're pretty low maintenance. Every second or third year we probably have to replace the canvas." Because gravity is doing most of the conveyor's work, power consumption is low. Daugherty compares the air gravity conveyor at a similar plant: "The plant had a screw conveyor that was taking 35 horsepower, but the blower for the air gravity conveyor requires only 2.5 horsepower, " he says.
Storage silos, too
The conveyor manufacturer supplied three flat-bottomed silos with aerated bin bottoms to condition the stucco to flow easily through the air gravity conveyors. The bottoms of the 18-foot-diameter silos are covered with the same porous membrane fabric used in the air gravity conveyors. Each silo rests on a sloped base and has a 5-foot-diameter center discharge. Air is introduced at timed intervals through the fabric to aerate the material and promote its flow out of the discharge to one of the air gravity conveyors.
"You can vent bucket elevators and screw conveyors and keep a negative pressure on those devices to contain dust, but air gravity conveyors are vented by design."
From the conveyor, the material discharges to one of two screw pumps. The pump transfers the material to a weighbin, which weighs the material before it's fed to the wallboard production line. Pumps can also move material that isn't required by the wallboard line back to one of the silos. On the pumps, two-way valves direct the material from the three silos or to the weighbin.
The pumps are the only downside to elevation and air gravity conveying, according to Daugherty: "Bucket elevators generally consume less horsepower than pumps. Pumping for elevation requires twice the horsepower of a bucket elevator, but in this application, the combination of horizontal and vertical conveying help offset the losses of a strictly vertical system evaluation."
"In reality, if you're conveying with an air gravity conveyor versus a screw conveyor, the air gravity conveyor is going to be substantially lower in horsepower consumption."
The money saved on the overall conveying system more than compensates for these costs. Daugherty says, " I think you have to take a more holistic approach to air gravity conveying. In reality, if you're conveying with an air gravity conveyor versus a screw conveyor, the air gravity conveyor is going to be substantially lower in horsepower consumption. Then you take a pump system versus an elevator system, and the pumping system is going to be higher in horsepower. When you look at the overall horsepower draw of air gravity conveyors and pumps versus screw conveyors and bucket elevators you're going to find — at least in our application — an approximate 25-percent increase in horsepower because I use pumps. The real saving comes on the maintenance side. I've got a lot fewer moving parts out there — much less than with traditional mechanical conveying systems. And as long as I take care of my pumps and my Airslides conveyors there's a lot less fugitive dust. It's dust-free."
— Reprinted from PBE, March 2001