GAS TECHNOLOGY: Dryer technology on the move

heat exchangers are installed on dryer exhaust air. The heat recovered is used to pre-heat dryer makeup air. This type of heat recovery typically reduces energy usage by 10-15%, but savings can be much higher in certain applications.

July 20, 2010

Few pieces of industrial equipment serve as many purposes and are available in as many configurations as dryers. They are widely used for drying wood, paper, painted surfaces, plastics, livestock feed, pharmaceuticals, food products and many more materials. They range from large conveyor dryers to small batch units and are most commonly fueled by natural gas.

According to Shelly Ryder of Aeroglide Corporation, a major manufacturer of food drying equipment, “The majority of our dryers are gas-fired, and have been for many years. We consider the gas-fired dryer to be the norm rather than the exception.”

Food Drying a Special Challenge

Food and feed dryers are often the most complex in the dryer family, and today these are the beneficiaries of major innovations. Ryder explains that food dryer technology is continuously changing. She notes, “Although most dryers have not fundamentally changed over the past 10 years, today’s food dryers are significantly improved in terms of energy efficiency, process control and drying uniformity.”  Many of the advances in energy efficiency are a result of the advancements in controls, allowing more precise control of temperatures through the process. 

Ryder points out, “Ten years ago, many dryers were operated improperly, resulting in high energy consumption. Today’s advanced digital controls allow us to ensure that the dryer is always operating at optimum efficiency.”  Food dryers have not only progressed in efficiency, but also in terms of sanitary design. Ryder emphasizes, “This, for the food producer, is number one on the list of priorities.”

Cleanability a Key Feature

Sanitary design involves minimizing ledges, ensuring good access, and using materials that can be cleaned, but Ryder stresses, “Many food producers are looking for dryer manufacturers to offer innovative solutions that take sanitation to the next level.”  Some newer designs offer improved access for both video and physical inspection, and to simplify cleaning. Materials widely used in construction include new specialty plastics and stainless steel for better cleanability and longer life.

Another aspect of food dryers that is receiving increasing attention is improving operating efficiency through heat recovery. Most frequently, heat exchangers are installed on dryer exhaust air. The heat recovered is used to pre-heat dryer makeup air. This type of heat recovery typically reduces energy usage by 10-15%, but savings can be much higher in certain applications. Newer dryers are also designed to stage heat at different levels through the machine, eliminating waste heat and over-drying.

Assure Dryer Can Handle Product

When purchasing a dryer for any application, industrial operators will want to assure that the machine can handle its specific product. Most dryer manufacturers encourage owners to submit samples of the product, which can then be tested in the specific dryer that the owner is considering.

Aeroglide, for example, operates a Technical Center that houses a wide range of dryers.  The facility is used both for product development and for customer test drying. Ryder adds, “All of our lab equipment is also transportable, so when product or process dictates, test units can be shipped to the customer site. In addition, we have portable lab units permanently located in both the European and Asia Pacific regions.”

Importance of Handling Product Gently

Food dryers can take many different forms, including batch cabinets, tray and belt conveyors, single and multiple drum or barrel dryers, and very specialized equipment such as spray dryers for milk, whey and other food products. In general, the goal is to produce food product with as little effect on color, aroma and chemical makeup as possible.

An interesting innovation in food drying is the Refractance Window® concept introduced by MCD Technologies, a dryer manufacturer headquartered in Tacoma, Washington. This product was featured at a recent Technology and Market Assessment Forum sponsored by the Energy Solutions Center. In this dryer design, a slurry of liquid product is evenly applied to the top surface of a continuous sheet of transparent plastic. This impervious conveyor belt floats on a surface of hot water at temperatures of 210°F or less.

 “Window” Solution Uses Infrared Characteristic

The “window” aspect of the transparent sheet allows infrared energy from the hot water to pass rapidly and directly into the slurry. Heat is also conducted through the belt, which helps to evaporate moisture in the product. Evaporate is carried away by mechanically boosted airflow. The transfer of infrared energy through the transparent sheet slows as the product loses moisture, creating a controlled drying process that retains nutrition, flavor, color and aroma.

The Refractance Window dryer is also suitable for pharmaceuticals, nutraceuticals, cosmetics, pigments and byproducts such as whey and fish oil. According to MCD, besides its gentle drying characteristic, Washington State University research has calculated that the Refractance Window dryer is 99% efficient in its heat transfer. Its automated operation reduces labor costs and the transparent belt design allows quick cleanup and minimizes maintenance.

 Airless Drying with Steam

Another innovation in food and feed drying technology is an “airless” dryer that uses superheated steam at atmospheric pressure rather than heated air as the drying medium. This product was developed by the Dupps Company, another major dryer manufacturer. According to Robert Horton, process engineer from Dupps, this dryer has about the same energy consumption as conventional direct-fired dryers. “However,” adds Horton, “The exhaust contains latent heat, which can be recovered.” He adds that the exhaust can be condensed and the water potentially reused in the process.

The airless dryer minimizes risks of fires and dust explosions because it is an oxygen-starved drying environment. Horton points out that the effluent gas emission rate is significantly lower than that of conventional dryers. “This yields better odor control and reduces the need for scrubbing.” No plume of condensed vapor is visible with this design, and because it operates at atmospheric pressure, a pressure vessel is not required.

This is a large-volume drum type dryer, and is suitable for drying a wide range of feed products and animal protein materials. These products are less prone to bacteriological contamination because they are dried with superheated steam instead of air. Dupps offers trial drying on customer materials.

 Priority on Maintenance

Preventive maintenance is an important aspect of dryer operation. Most of today’s dryers are built to a high reliability standard, but this can breed complacency in performing recommended inspections or activities such as lubrication and belt replacement. Because drying is often a key step in a longer process, a dryer outage can stop an entire line, cause loss of product and require considerable labor to get the process moving again.

After initial dryer startup, a regular PM program should be established to eliminate unscheduled downtime. Regular inspections should be scheduled by a qualified engineering expert with a goal of anticipating mechanical or structural failure.

 A Long-Term Relationship

Because dryers have proprietary design and operation features, the process of selecting a dryer manufacturer should be carefully done, with a look toward the long term. Once the dryer is selected, owners will likely maintain a relationship with its manufacturer for years. For that reason, industrial buyers should look not only at the equipment, but at the manufacturer’s ability to support the product, promptly provide parts and service, and train plant operators and maintenance staff.

Look for a provider within the industry that has a history of customer support. Take advantage of the energy-conserving features of the newest dryer designs, as these will become increasingly important. Because you will be living with it for a long time, make sure your new dryer is a good match for your product.