Food security
Drying is one of the most important, and energy-intensive, activities in global food production. Optimized setups, improved control, and advanced energy recovery technologies are helping the sector to squeeze every last drop of efficiency from drying equipment.
Jonathan Ward, June 2024
Where do you get your energy from? If you are a human being or an animal the answer should be simple: from the sun, via your food. The meal on your plate is the last stop in a long chain of chemical reactions that transform solar energy into the fuel that enables us to grow, heal, and live our lives.
In the modern world, however, things are a bit more complicated. The production, distribution, and preparation of food relies on all sorts of additional energy inputs used to make fertilizers, run machinery, transport, process, and cook. Together, these inputs make food production a very energy-intensive business, accounting for about 30 percent of global final energy demand, according to the United Nations Food and Agriculture Organization.
Today, that energy demand is under scrutiny. Population growth and rising wealth are expected to increase food industry energy consumption significantly in the coming years, just as the world seeks to reduce the greenhouse gas emissions associated with energy production from fossil fuels.
Reducing food and food production emissions will require changes across the value chain, from the introduction of agricultural techniques that need fewer fossil-fuel inputs to the use of more energy-efficient appliances at home. In between, food and feed companies are taking a hard look at their own energy consumption, seeking ways to improve efficiency, reduce demand, and cut overall emissions.
For many food and feed companies, drying is the most energy-intensive activity. It’s also one of the most important. Drying is a key process in the production of many types of food, from staples such as grains, pulses, fruits, and vegetables to indulgences such as French fries and breakfast cereals, as well as feed products such as pet food and aqua feed.
“Food companies rely on drying for several reasons,” says Andy Britt, CEO and President of Bühler Aeroglide. “It extends the shelf life of products, which reduces packaging requirements, simplifies supply chains, and reduces waste.
It makes products lighter, which saves energy in transportation, and it plays an important role in determining the texture and palatability of the finished product.” Those benefits come at a significant energy cost. It is estimated that drying accounts for about 25 percent of the total energy used in food production and 70 percent of CO2e emissions.
Much of this energy bill is due to basic physics. At 100ºC, it takes 2,260 kilojoules of energy to turn a kilogram of liquid water into vapor. In industrial food processing, this is typically accomplished in a convection dryer. Food or ingredients are transported through the dryer on a conveyor while hot dry air is blown over them. The heat evaporates the water, and the air carries it away, leaving the dried product ready for freezing, packaging, or further processing.
As a major supplier of drying equipment, Bühler has a clear picture of the energy requirements of modern drying operations and the challenges involved in improving efficiency. “The average dryer we supply can consume 2,300 to 2,600 kilowatts per hour,” says Britt. “For our customers, it’s not uncommon for drying to account for 50 percent of a plant’s total energy consumption.”
Industry-wide, this consumption leads to staggering totals. “If you take just the installed base of dryers that Bühler has supplied around the world, you are looking at an annual energy consumption of about 45 terawatt hours,” says Britt. “That is equivalent to the entire electricity consumption of Hungary per year. If we take the entire food drying market into account, total CO2e emissions amount to around 40 million tons a year.
Drying is such a large part of the food industry’s energy footprint that even small percentage reductions in energy use can have a big impact. Britt and his team aim to help customers maximize the energy efficiency of their drying operations. That work starts on the plant floor. Al Worthington, Director of Process Engineering at Bühler Drying Solutions, and his team spend much of their time conducting performance assessment workshops at customer sites as part of this process.
“We audit existing Bühler and non-Bühler equipment, and help customers find ways to improve their product consistency, reliability, and energy efficiency,” says Worthington. Efficiency is a key issue due to energy price increases and customers’ desire to reduce the carbon footprint of their operations.
These audits often reveal simple changes that can make a significant difference in energy consumption. Ensuring that access doors are closed and sealed, for example, prevents hot air from escaping prematurely from the dryer. Control settings are also important. “Where plants process a lot of different products, we sometimes see dryers permanently set for the most demanding conditions,” he says. “Overdrying is a big waste of energy, so the best practice is to optimize the machine for each type of product.”
Better product handling also helps. “If you put a deeper, more uniform layer of product on the belt, you may be able to run the belt slower without sacrificing throughput,” he explains. “With longer dwell times, you can get the same drying performance with less airflow and lower energy consumption.”
Advances in machine control are the second major area of improvement. Modern dryers are equipped with sensors to monitor temperature and humidity, and actuators and variable speed drives to automatically adjust air flow rates. These components can also be retrofitted to older Bühler machines and non-Bühler machines.
“Controlling the exhaust system to keep air in the machine until it has absorbed as much water as possible is a very effective measure that can be achieved with simple hardware,” he says.
Digital technologies are delivering real improvements to dryer efficiency through precise, closed-loop computer control. Bühler’s DryingPro solution, for example, uses Internet of Things (IoT) technology to measure and adjust drying processes in real time, boosting yields, and reducing waste. A sensor in the dryer’s exit chute continually measures moisture levels in the product, then feeds this information back to the Bühler Insights platform, which analyzes it and adjusts dryer settings automatically.
The energy that goes into a dryer must go somewhere, and most of it is vented into the environment with the moisture-laden exhaust air. To get to the next level of efficiency improvement, food companies are looking for smart ways to recover some of that energy before it escapes. If a plant’s ductwork allows, this can be accomplished with a simple air-to-air heat exchanger. These devices bring the hot, humid exhaust air close to the cool, dry air flowing into the dryer heater. By preheating the supply air, less energy is required to bring it up to operating temperature.
“If the ductwork in the plant wasn’t designed with energy recovery in mind, you can accomplish the same thing with a fluid-filled heat exchanger,” says Worthington. In this approach, a coil containing a heat transfer fluid is installed in the exhaust duct, and the hot fluid is then pumped to a second coil where it heats the supply air. In some food processing plants, dryers can use a similar approach to recover waste heat from other operations or equipment, such as chillers or freezers.
With a high-temperature heat pump system tailored to the processes of your dryer you can:
Reusing waste heat is a compelling idea, but heat exchangers have limitations. “At a given temperature, saturated exhaust air contains more energy than dry air,” says Worthington. Since heat transfers from hot to colder fluid, the supply air can only absorb a fraction of the energy from the exhaust. To overcome this, Bühler engineers are developing a better method. By installing a heat pump in the circuit between exhaust and supply air, they can raise the temperature of the heat exchange fluid at the dryer inlet, making more energy available for recovery.
Heat pump technology is well established: it’s used to heat and cool millions of homes around the world. But the dryer application is at the cutting edge of current heat pump design. “We’re looking at systems that operate at 120ºC or higher, which has been the limit of commercially available heat pumps in recent years,” Worthington says. “We are also taking a close look at emerging heat pump technologies that can provide significantly higher temperatures.”
Getting heat pumps to work well in dryer applications is a tricky engineering challenge too. Unlike a simple air-to-air heat exchanger, these units require energy to operate, so users need to be sure that the pump will recover enough heat to cover its operating costs. They need to know that their heat recovery equipment will continue to operate reliably in a busy production environment, where the buildup of dust and food residue can interfere with efficient heat exchange. Because the most effective heat recovery systems extract enough energy to condense the water vapor in the exhaust air, they may need systems to handle a new wastewater stream.
We’re looking at systems that operate at 120ºC or higher, which has been the limit of commercially available heat pumps in recent years.
Al Worthington,
Director of Process Engineering at Bühler Drying Solutions
These technologies are likely to play a significant role in industrial dryer installations as companies move toward net-zero carbon production. “In future energy systems, more heat will come from electricity,” says Britt. “Switching to renewable and zero-carbon energy sources gives companies a route to carbon-neutral drying, but users will still want to use that electricity as efficiently as possible.”
For now, Britt and his team are focused on making drying cleaner, cheaper, and more effective. “Optimizing the control of a dryer that has been running poorly can reduce energy consumption by 10 to 15 percent,” Britt says. ”With heat recovery, you could save the same amount again. Add a heat pump, and we can cut overall consumption by 30 percent or more and reduce CO2e emissions towards net zero.”
