Advances in dryer design
Industrial dryers are overcoming unique challenges and making improvements in energy efficiency, process control, and drying uniformity.
Industrial dryers are among the most versatile pieces of equipment within the industries they serve. There are many configurations for many applications. 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.
Overcoming unique drying challenges
Food and feed dryers can be complex because of the need for energy efficiency, process control, drying uniformity, and sanitary designs. Food dryer technology changes constantly. The most recent improvements are seen in energy efficiency, process control, and drying uniformity. Developments in controls that allow more precise control of temperatures throughout the process account for energy efficiency advances.
"We have unique applications, such as gelatin, where we must have intimate process knowledge to control the rate of drying without degrading or destroying the material," said Frank Poandl, associate chief engineer at Buhler Aeroglide, a major manufacturer of drying equipment. "Drying gelatin noodles is counterintuitive to most drying applications. For most wet products, we gradually use a high-to-low air temperature profile because the high inlet moisture evaporates and keeps the product temperature cool. However, gelatin is extremely sensitive to the air temperature and we must slowly increase the product temperature because the gel has a low melting point. If the gel begins to melt, then the air gaps between the noodles seal and mass transfer of water from the product to the air is inhibited."
Food and livestock feed aren’t the only applications that present unique drying challenges. Kiln-drying lumber is another demanding drying application. Drying lumber requires many days, and aggressive attempts to reduce the drying time can result in the wood either splitting or cracking.
Overcoming drying challenges, such as uniformly drying nonuniform products and reducing the drying time of heat-sensitive products, also can be difficult. For example, molded-fiber packaging products, such as egg cartons and protective packaging used during transit, need ample drying time. Thin-wall products can dry in less than 10 minutes, whereas engineered thick-wall protective packaging could take up to an hour to dry. For every pound of dry fiber that enters a dryer, there typically is up to four times as much water by weight, which requires a lot of energy to adequately dry. Advanced controls, such as proportional-integral-derivative (PID) temperature controllers that can run sophisticated ramp-and-soak profiles typically overcome these types of challenges.
Matching the dryer with the application
"Conveyor dryers are best suited for steady state continuous processes, where the product has a uniform shape and size distribution," Poandl said. "Our conveyor dryers gently handle products and expose them to conditions that are carefully monitored and controlled for individual zone control. In these zones, we are able to adjust process parameters such as temperature, humidity, and airflow to control the rate of drying."
"Rotary dryers are best suited for random-sized bulk materials where size degradation during drying is not a concern," Michael Whaley, principle process engineer at Buhler Aeroglide, which offers both continuous conveyor and rotary dryers. "The combination of air movement and mechanical forces create a differential in retention time in rotary dryers that allows the different sized particles to achieve uniform moisture content at the discharge."
Keeping an eye on efficiency
Most dryer users in North America are interested in reducing energy costs and exhaust volumes, according to Whaley. Many of the large multinational companies have directives to reduce the greenhouse gas emissions, he said.
Poandl says that Buhler Aeroglide is addressing energy efficiency by (depending on the process) exploring options for energy conservation through reusing exhaust air from cooler, cascading airflow inside the dryer, as well as heat recovery through external features, such as flash steam systems, air-to-air heat exchangers, air-to-water/glycol coils, heat pumps, and hot waste water/fluid from other processes. Some of the applications with relatively high evaporation rates financially justify the additional equipment cost with an excellent return on investment and payback periods.
Poandl explains that the principle design feature of the Buhler Aeroglide dryers is that a large portion of the air used for drying is recirculated. "We introduce dry makeup air and expel wet exhaust air as necessary to promote energy efficient drying. Energy consumption depends highly on the evaporation rate from the product so it is difficult to generalize across markets and show a fixed quantity or percentage. We balance exhaust and make-up air volumes to maintain absolute humidity levels that are optimal for drying and energy efficiency," he said.
Because the evaporation of water is an energy intensive physical phenomenon, Whaley suggests investigating methods to reduce the moisture content of the product prior to drying.
Keep maintenance a priority
As with any industrial equipment, 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 often is 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. After 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.
This article originally appeared in the Gas Technology Spring 2018 issue.