Powdered milk falls in line

Transylvania Vocational Services (TVS) in Brevard, NC, likes to do things in bulk. The U.S. Department of Agriculture (USDA) sends 55 lb bags of dry milk to TVS to package into smaller bags for food banks and schools. The challenge for TVS is transforming the bulk bags to the small, 2 lb bags and to do it an efficient manner.

The solution consists of a 48-in.-sq bag-dump station through which two workers unload powder that falls into a 48-in.-diameter circular vibratory separator from Kason Corp. The separator scalps paper pieces from cut bags containing the powdered milk product. The powder falls from the screener's discharge outlet into a 25-cu-ft-capacity floor-mounted hopper, from Flexicon. A 15-ft-long Flexicon flexible screw conveyor, at a 45-degree incline, transports the powder through an interior plant wall to a Hayssen vertical form/fill/seal packaging machine in the adjacent room."The flexible screw conveyor is the easiest method for conveying in this confined space," says Hendrik Colijn, consulting engineer on the project.

Becky Alderman, TVS director of operations, in describing the large components needed to meet the 45 bag/min requirement, explains, "We super-sized the line. A 10-hp motor drives the flexible screw conveyor to draw the high flow of powder through its 6-in. diameter." Charles Merrill, food operations manager for TVS, adds, "We need the high throughput to satisfy our contract requirement of packaging sixteen million pounds per year."

Built to 3-A dairy standards, the system operates in a temperature- and humidity-controlled clean room required for USDA dairy certification. Temperature is maintained between 70 to 72 degrees F; humidity between 27 percent to 35 percent for the dry product (2.5 percent to 3 percent moisture). "Moisture over three percent causes trouble for the packaging machinery," Merrill says.

Density variation
"We're packaging one of the worst products for packaging machines," Merrill continues. Bulk densities change without notice, even in one pallet load, as 55-lb bags of dry milk powder arrive from USDA warehouses from a multitude of suppliers. The varying density requires operators to constantly adjust the vf/f/s machine.

The Vibroscreen® FLO-THRU circular vibratory screener and flexible screw conveyor, however, remain largely unaffected by the bulk density variations.

The low-profile FLO-THRU circular vibratory screener comes equipped with two externally mounted vibratory motors. The motors impart multiplane inertia vibration to two spring-mounted screening decks, causing oversized particles to vibrate across the screen surface in controlled pathways to the screen periphery where they are discharged. Screening efficiency improves by forcing the powder to pass over a maximum amount of the 8-mesh-size screen surface. The 0.0027-in. particles pass through the screen and bottom frame discharge.

The flexible screw conveyor moves the powder through its Ultra High Molecular Weight (UHMW) polyethylene outer tube, enclosing a rugged, flexible stainless-steel screw. The flexible screw is the only moving part contacting the material. As it rotates in the tube, the flexible screw self-centers to provide clearance between the screw and tube wall to prevent grinding or crushing of the product.

To promote flow of the powder into the flexible screw conveyor's intake adapter, a vibrator in the floor hopper is adjusted according to the powder's density. Additionally, the hopper is designed with a steep backwall, and sidewalls that are skewed outward at divergent angles, causing it to topple and flow toward and down the backwall.

The operation
Workers on a 10-ft-high mezzanine remove bags from a pallet, vacuum the bags, and slide them onto a conveyor belt, which moves them to the lip of the bag-dump station. Two workers cut open the bags and pour the contents into the bag-dump station.

Ambient air and dust from dumping activities are drawn onto the exterior of two cartridge filters that derive vacuum from a top-mounted exhaust fan. Dust that accumulates on the filters' exterior surfaces is dislodged by overcoming continuous negative pressure within the cartridge filters, with positive pressure introduced in short blasts on a timed cycle by pulse-jet nozzles. Dislodged material falls into a bin for return to TVS's dust collection system.

To avoid overloading the system and discharging milk powder from the screener as waste, workers must avoid dumping bags too rapidly. TVS trains workers to control their dumping rate and to stop or resume dumping when a light is activated by high-level sensors located at the flexible boot connecting the separator to the hopper. Also, a sensor is located atop the flexible screw conveyor, and a low-level sensor in the floor hopper.

TVS operates the packaging line in two shifts and cleans during the third shift. The Vibroscreen FLO-THRU separator is vacuumed according to 3-A standards. "The screen usually stays clean with minimal material remaining," Merrill says. Prior to cleaning the flexible screw conveyor, a worker removes the cleanout cap and reverses screw rotation to evacuate any residual material. The screw and outer tube are removed for cleaning. The conveyor has no cracks or crevices that can trap particles.

Based on the success of this new line, TVS plans to add a sister unloading and packaging line that will blend milk powder and other ingredients. The system will incorporate another circular vibratory separator and two flexible screw conveyors.

More information is available:

Vibratory separator: Kason Corp., 973/467-8140. Circle No. 276.

Hopper, flexible screw conveyor: Flexicon Corp., 888/353-9426. Circle No. 277.

F/f/s: Hayssen Mfg. Co., 864/486-4000. Circle No. 278.