Cedar Rapids Heat Recovery
To drive additional energy savings at its Cedar Rapids plant and to support the company’s corporate goal of a 20% improvement in energy intensity over 10 years, General Mills is conducting a comprehensive heat recovery project that is expected to reduce energy usage at the plant by about 5%. In combination with other recent efforts, this will lead to a total energy intensity improvement of about 23% over the last four years at the plant.More
General Mills' Cedar Rapids, Iowa, plant is its largest production facility, producing over 70 million cases of ready-to-eat food annually, including cereals, fruit snacks, and frosting. Built in 1968, the facility has a successful history of energy reduction, the product of General Mills’ five-step process that benchmarks, identifies, develops, executes, and then validates energy reduction projects.
Energy reduction efforts implemented within the last four years at the Cedar Rapids plant have already reduced the facility’s energy intensity by 18%. These efforts include:
- Heating, ventilation, and cooling (HVAC) controls and optimization
- Cereal dryer optimization, including installing: controls to minimize hot exhaust air by recirculating this air back into the process; automatic controls of product spread to ensure maximum heat transfer surface area; and new product temperature sensors to eliminate energy overuse
- Boiler efficiency improvements through combustion controls and flash steam recovery
- Lighting retrofits
- Air compressor automation
- HVAC ducting optimization
- Combustion efficiency improvements through burner re-design, heat recovery, and optimizing steam superheaters
- Mechanical insulation upgrades
- Wireless steam trap monitoring
- Fan speed reduction via variable frequency drives (VFDs)
General Mills’ heat recovery project revolves around three improvements: heating ingredient water using “free” waste heat in multiple stages; using flash steam to regenerate a desiccant wheel for packaging dehumidification; and installing coil run-around loops to reduce dehumidification costs. All together, the heat recovery projects are expected to save General Mills about half a million dollars a year in energy costs, resulting in a payback period of a little over three years.More
The three heat recovery improvements work as follows:
- The product’s ingredient water is heated to the required temperature using “free” waste heat in multiple stages. First, a desuperheater, which lowers gas (in this case the ammonia refrigerant) temperature, heats the water 30 degrees Fahrenheit (F). The desuperheater was installed on the refrigeration cycle after the compressor generates hot gas and before the evaporative condensers remove that heat. The desuperheater has the additional benefit of reducing the load and energy required from the evaporative condensers. Since air compressors are inherently inefficient and therefore produce significant heat, the plant is capturing that waste heat as the second stage of heat recovery, adding another 25 degrees F to the water. This step also reduces the load to cool the compressors, leading to additional energy savings. Finally, a condensing economizer heats the water to the final temperature needed for the product. A condensing economizer is stainless steel and designed to handle the harsh conditions that exist when lowering exhaust combustion gas below its dew point, allowing for additional heat recovery compared to traditional economizers.
- Flash steam is used to regenerate the desiccant wheel for packaging dehumidification. Flash steam is created when high pressure condensate from a process is collected in an atmospheric tank and pumped back to the boiler. As the condensate pressure is reduced, the liquid condensate “flashes” back into steam. It is commonly seen venting from roofs in manufacturing plants. Instead of venting this flash steam, the Cedar Rapids plant uses it to heat the air that regenerates a desiccant wheel, which is required to meet the humidity requirements of the production process.Schematic of Waste Heat Recovery Using a Desiccant Wheel
- Coil run-around loops are being installed at the plant to reduce dehumidification costs for product HVAC. In traditional HVAC dehumidification systems, after the air is cooled to remove moisture, it is heated to lower the relative humidity of the supply air. The coil run-around loops provide free heating for this purpose as well as free process cooling. The system works by first pumping glycol through the first (pre-cooling) coil of the HVAC unit. This cools the hot outside air while warming the cold glycol. Next, the warm glycol is circulated through a pre-heating coil after the conditioned air exits the primary cooling coil, thus cooling the glycol back down and heating the air to lower its relative humidity. Run-around loops reduce the HVAC tonnage required, reducing capital when installed on new units.
Schematic of a Coil Run-around LoopLess
General Mills is evaluating similar heat recovery projects at 14 other sites, which could lead to annual savings of about $2 million. The heat recovery project is also reducing water consumption by 2.2 million gallons per year and preventing the release of approximately 5,500 tons of CO2 emissions from the Cedar Rapids site annually.More
*Baseline energy costs represent expected costs in the absence of any energy efficiency measures.