Dryers: Controlling Moisture
November 01, 2009
by Shane Whitaker
Snack manufacturers and bakers continue to develop a wide variety of new and unique products, and drying is critical to ensure proper texture, moisture levels and consistency of the product. Cookies, crackers, cereals, extruded snacks, breadsticks, pretzels, pita and snack chips and croutons rely on dryers to help control moisture in the final product.
Processors require new dryers be flexible and easier to sanitize and operate with respect to changing process variables, according to Terry Midden, industry manager, CPM Wolverine Proctor LLC, Horsham, PA. “Customers today want the ability to easily change all process parameters, not just temperature,” he said. “We routinely supply units with variable-speed drives on all fans and conveyors.”
David Kuipers, vice-president, sales and marketing, Reading Bakery Systems (RBS), Robesonia, PA, noted that, “Snack manufacturers are looking for low-cost, flexible and efficient systems that can be easily cleaned. For dryers, efficiency means the ability to reduce the product moisture evenly and quickly. Flexibility can be related to cleaning — the drying equipment must be able to be cleaned quickly without risking product cross-contamination.”
Dryers must feature proper air handling and distribution to ensure consistent and uniform product drying, according to Dennis Hill, president, The Lanly Co., Cleveland, OH. “Many processes require air velocities and exhaust rates to be controlled through a PLC to accommodate differences in products or product recipes,” he said. “Proper zoning of the dryer is critical for temperature and moisture control. Zoning allows for an optimum thermal and moisture profile through the length of the drying process.”
Applying a standard dryer is not enough to ensure the product develops the proper texture, moisture and consistency, according to Mr. Hill. The equipment manufacturer works with its customers to test their products in the Lanly Development Lab to determine the most critical dryer parameters to produce the desired products consistently and at high production rates.
Dryers and ovens are essentially the same in many respects, but they perform different functions. Post-baking dryers reduce residual moisture in products. Whereas ovens allow chemical reactions to occur in products that impart color, shape and flavor, dryers use higher airflows and lower temperatures to evaporate water from snack and baked foods.
A variety of convection dryers that use air to transfer heat are available. Aeroglide Corp., Raleigh, NC, manufactures four different types including conveyor, impingement, fluid-bed and rotary dryers, according to Paul McKeithan, the company’s market manager. “They all use air as the medium for mass and heat transfer,” he said. “Other than that they are quite different in their function.”
Wolverine Proctor also manufactures a wide range of conveyor, fluidized-bed and impingement systems, according to Mr. Midden. “Each is custom designed and manufactured to meet a customer’s specific requirements,” he added.
A conveyor dryer can take on many different configurations and typically has a relatively low airflow velocity through the product bed (100 to 200 ft per minute) and relative low temperatures (150-350°F), according to Mr. McKeithan. “The primary purpose of a conveyor drier is to gently convey the product while evenly removing moisture,” he explained. “It is commonly used as a finish drier in most snack applications. It is more economical to remove the final moisture points of a snack using a finish drier than to accomplish this with an oven.”
Impingement dryers can either use a conveyor bed or a vibratory pan to advance the product, and the airflow impingement velocity is much greater than a typical convection conveyor drier, generally somewhere between 800 and 5,000 ft per minute. “It also typically operates at a much higher temperature,” Mr. McKeithan said. “The impingement technology rapidly increases the heat transfer to the product. It is most commonly used with a mono layer of product to set the structure or to develop certain product characteristics.”
Wolverine Proctor’s impingement dryers and ovens use delivery devices such as tubes, slots or nozzles to increase air velocity, which helps shorten baking or drying times. Mr. Midden said in some applications, impingement dryers’ temperatures may exceed 600°F. “Because of cost, impingement technology lends itself to high-temperature, s h or t - t i m e processing,” he added.
A fluidbed dryer achieves an increased heat transfer to the product by passing enough airflow through the product to gently lift it. “The product literally looks and behaves like a fluid,” Mr. McKeithan said. “It has the potential of decreasing the drying time for a particular product by exposing the entire surface to the heated air.”
Rotary dryers are like clothes dryers at home but product moves through the dryer. “The rotary dryer is very efficient at drying products that are rugged or the shape doesn’t matter and is often used to dry products like rice, extruder scrap, powders, fruit pumice and other ingredients.”
A new offering from Aeroglide is the AeroExpander, a continuous hot air expander. The AeroExpander uses a modified version of the company’s rapid heat transfer technology to continuously achieve unprecedented uniform expansion of pellets. “This allows manufactures to produce a healthier snack with less oil content,” Mr. McKeithan noted.
SINGLE AND MULTIPLE PASS.
RBS uses two primary drying systems on its snack lines. A single-pass drying kiln is positioned beneath the oven, or for applications where more time is required or the product drying benefits from having the bed turned, it offers a multi-pass product dryer, according to Mr. Kuipers. “In all cases the product dryer is the same width as the oven system,” he said.
The dryers are normally operated with temperatures of 200°F (95°C), which is less than typical baking temperatures. “The processes of baking are accomplished in the oven; the function of the dryer is to gently remove moisture so the product can be packaged,” Mr. Kuipers explained. “Outputs of dryers are matched to the forming and baking system, but typically range from 200 to 900 kg per hour for baked snack products.”
One of its latest introductions is the RBS SPECTRUM Multi-Pass Dryer, which uses balanced air plenums to deliver uniform airflow on each pass of the product through the drying system. A burner fires continuously into each section of the penthouse that is mounted on top of the dryer heating the circulated air, and a formula programmed into the control system regulates the rate of the fire. “The operator has control over temperature, air velocity and time,” he said.
In addition, the penthouse is split into two chambers so that two zones can be supplied with air circulated at different speeds and temperatures. The SPECTRUM multi-pass dryer can accommodate three to five passes, requiring a minimal footprint in plants.
Multi-pass configurations reduce floor space and potentially reduces energy consumption per pound of water removed, according to Mr. McKeithan. “Aeroglide provides multiple pass systems ranging from two to five passes, although two and three pass configurations are most common,” he said.
Because multi-pass dryers are used quite often in plants, a common misconception is that turning the product over is the key to uniform moisture removal, according to Mr. McKeithan. “A single-pass drier can also achieve this, if it is designed correctly,” he said. “A singlepass drier can be more sanitary simply because of better access and fewer parts. Of course a multi-pass dryer can make achieving uniformity easier.”
Multi-pass dryers are also constructed with sanitation in mind. Mr. Kuipers pointed out that large access doors on both sides of the SPECTRUM multi-pass dryer permit complete access to the carrying mesh and the plenums themselves, so sanitation can be done quickly.
Wolverine Proctor’s most recent introduction was a line of ovens and dryers equipped with complete clean-in-place (CIP) systems.
Accessibility for cleaning is always important, according to Mr. Hill. “Lanly uses double-gasketed, insulated fullheight doors to allow the cleaning crew walk-in access,” he said. “A flush-threshold design allows debris to simply be swept out without obstructions.”
One of the fastest-changing areas in dryer technology, according to Mr. Hill, is improvements to energy efficiency. Temperature, moisture levels, air circulation and exhaust rates are all controlled with PLCs, and gas and electrical consumption are monitored and data logged simultaneously with the dryer process parameters, he said. “By monitoring and controlling temperature and moisture levels, product quality and uniformity is improved and the process can be optimized for minimum energy consumption,” he said. “Also, improved insulation and construction techniques minimize heat loss into the bakery. Various types of heat recovery systems can be employed to make use of heat lost at the exhaust stacks and end openings.”
Lanly performs energy audits on existing dryers and ovens to determine their energy balance, and once the audit is complete, the company recommends changes and improvements to reduce overall energy consumption. Mr. Hill said the company developed a networked gas metering system for a major snack manufacturer allowing it to simultaneously monitor and log real-time data from more than 22 pieces of equipment. “The monitoring is done through a desktop computer located in the production office,” he said. “Trends in excessive energy consumption versus production rates can be readily identified, thus allowing for faster corrective action.”
While the cookie and cracker market is not seeing much growth, baked snacks segment is a growing, and dryers play an important function in removing moisture and developing flavor and texture in these products. Equipment manufacturers are focused on building dryers that meet the needs of these customers, and frequently, dryers are built on a custom basis to meet eachcustomer’s exact parameters.