Sunsweet Growers is a cooperative of prune growers, which operates 481 drying tunnels. Sunsweet Dryers is the largest prune drying company in the United States. Prunes are dried at a relatively high temperature with a maximum temperature of 185°F leaving the burner. Each of the 481 tunnels has two chambers. The drying season is 4 to 5 weeks per year, 24 hours a day, usually from mid July to mid August. Each fan handles approximately 50,000 cfm of air with approximately 20% fresh air and 80% re-circulated air. The burners are rated at 3.2 million BTU’s per hour and are modulating. The roofs of the dryers are not insulated.
A 1,200 ft2 (110 m2 ) SolarWall® system was mounted on the roof of three adjacent dryers and connected to one fan intake. The solar system pre-heats the ambient air 20 to 30°F before it enters the drying chamber. The energy produced by the SolarWall® comes from two sources: The first is the solar energy collected, and the second is the heat recovery from the roof. The air flow rate through the SolarWall® solar collector was designed for 10 cfm per square foot of collector. Monitoring of the gas consumption for the 2004 drying season showed a total fuel savings of 29 therms of gas per day from SolarWall when operated 24 hours a day.SunSweetDryers_SolarWallCropDryingCaseStudy_Y05(PDF)