Case Study

Driven to efficiency

variable frequency drives
,
Managing Editor
Variable frequency drives help Sundial Orchards improve the operations and energy costs of its walnut dryers.

When fall rolls around, Sundial Orchards doesn’t have any time to waste. Because the harvest season for walnuts is a mere six weeks, the Gridley, California-based company has to work quickly to process the 18 million to 19 million pounds of walnuts it receives from local growers every year. To meet the demands of a rigorous processing schedule, Sundial installed variable frequency drives to optimize operations and cut energy costs. 

Sundial retrofitted the fan motors of its 14 dryers with variable frequency drives (VFD) from Danfoss, the Nordborg, Denmark-based manufacturer of controls, compressors and drives, to improve the speed and energy efficiency of its walnut drying equipment. The VFDs allow Sundial to regulate the air flow, temperature and energy of the dryers, which play a crucial role in processing the walnuts. 

From mid-September through October, Sundial processes up to 400 tons of walnuts a day. When the walnuts are delivered to the company’s 20-acre facility, workers must start the hulling, cleaning and drying processes within 24 hours of arrival. Otherwise, the high-moisture content in the nut will degrade and mold the walnuts. 

At Sundial, the walnuts are dropped into a hopper and conveyed to a huller, which removes the hulls of the walnuts as well any other foreign debris, such as rocks, leaves and sticks. Then the walnuts are submerged in a water bath and sprayed with a sanitizing solution. After passing through an optical inspection system, the walnuts are conveyed to metal drying bins, which are heated to 105°F and each hold about 6,600 lb of walnuts. The walnuts remain in the bins for about 12 to 24 hours until they reach the required moisture level. Finally, they are shipped to food processors for shelling, packaging and retail distribution.

Drying principles

During the drying process, Sundial carefully monitors the temperature and air pressure of the bins because moisture content is critical to walnut quality. Natural gas burners produce heat. Centrifugal fans then circulate the hot air through a tunnel and into the drying bin. Sundial uses sensors to control the temperature of each tunnel and the burner’s firing rate. 

“Drying time for walnuts varies depending on the climate, temperature, humidity and other factors,” says Justin Leishman, operations manager for Sundial. “If walnuts get too dry, the shell gets brittle and can break more easily, causing lower grades for the grower. Ideal walnut moisture level is about 8 percent. Drying them down any lower reduces weight, which cuts into profitability to both the grower and Sundial Orchards — and adds hours in the bins, which also reduces profit by wasting time, gas and electricity.”

To create optimal drying conditions for the walnuts, the dryers must maintain a static pressure of 1.5 in. of water column. Less pressure will increase the drying time, while more pressure will increase energy costs without gaining any drying benefits. 

Before the fan motors were retrofitted with the VFDs, the drying bins often encountered fluctuations in the static pressure. Although workers used sensors to regulate the temperature in the tunnels and firing rate of the burners, they couldn’t adjust the speed of the fan motors, which control the dryers. As a result, Sundial had to run the dryers at full speed even when the bins weren’t filled to capacity with product. It increased static pressure and wasted energy. In addition, because the fans were running constantly at full speed, they created higher air pressure that made it difficult to close some of the dryer doors. The doors of the bins must be closed when the walnuts reach the right moisture level, so they don’t get exposed to any more heat and can be unloaded to a conveyor. If the doors are not closed when the walnuts achieve the desired moisture content, it risks drying out the meat of the walnuts and reduces the efficiency of the drying process. 

To maintain a consistent static pressure, Jose Rodriguez, a field electrical specialist from California Industrial Rubber, a vendor for Sundial and a Danfoss distributor, suggested using a Danfoss VLT variable frequency drive.

Rodriguez retrofitted two stadium-style dryers inside the Sundial facility and 12 dryers located outside with Danfoss VFDs. He plugged each motor’s pressure transducer into a VFD. Then he programmed the VFDs with parameters for energy-efficient operations, including maintaining a static pressure of 1.5 in. of water column no matter how much product is in the bins and whether the bin doors are opened or closed. 

“When the pressure changes, the VFDs back down the fans when the bin doors close,” Leishman says. “And when the fan rpms go down, the temperature can be lowered so the burners can turn down, saving electricity and gas.”

Total control

The VFDs are also helping Sundial reduce electricity costs by controlling how much energy is used to start up the motors. Previously Sundial used about 300 horsepower to ramp up each fan’s 75 horsepower motor, according to Leishman. With the VFDs now retrofitted on the fan motors, Sundial can ramp down the frequency and voltage needed in order to slowly start the motors.  

“The VFD allows me to choose how long I want for my motor to ramp up to speed. By doing that, I don’t have a huge surge of power. And so the motor never pulls more than the 75 horsepower that it requires to run,” Leishman says. “It’ll start out with just a few amps to get it moving, and then it’ll increase the amperage draw for the calculations in the VFD until it reaches capacity. And then it just continues there, unless I make adjustments. So then my demand charges will not be reflected as big spikes.”

And because the VFDs are helping the fan motors operate efficiently, Leishman expects increased longevity of the dryers and its parts. “Through using the VFDs, it’s minimizing the strain that’s put on them,” he says.

Leishman says he’s pleased with how fast he has seen results from the VFDs. “With the Danfoss VFDs, I’m getting the intelligence to optimize pressure and fan rpms to cut electricity and gas costs,” he says. “Retrofitting the motors in our walnut dryers with VFDs was a complete no-brainer.”

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