Project OverviewTo optimize performance of the plant's glycol cooling system, facility management at the La Crosse brewery hired a consulting engineering firm to perform a feasibility analysis and make performance optimization recommendations. This decision followed a preliminary screening by the Energy Center of Wisconsin, Madison. WI. The project called for the consultant to document the systems, analyze and evaluate the energy-saving opportunities, implement cost-effective projects, and conduct followup measurements to confirm savings.
Brewing at the La Crosse facility is conducted on a weekly cycle beginning Sunday night, though not all tanks are cycled each week. After brewing is completed, the beer is cooled in a heat exchanger to 54°F (12°C) and moved to storage tanks. The beer is further cooled in the storage tanks by a glycol cooling system. A solution of water and 36% propylene glycol is pumped through a 400-ton chiller that uses ammonia from the central refrigeration plant to cool the solution to 22 to 24°F (-4 to -5°C). The glycol solution then is channeled through an intricate piping and pumping system to cool the beer storage tanks. This pumping system was the focus of the energy-saving efforts.
The original glycol pumping system consisted of three parallel pumps directly coupled to three 150 hp electric motors. Originally, each pump had a 17" impeller dia. Prior to this project, one impeller was trimmed to 14.75" dia. and its motor was replaced with a 75 hp unit. The 75 hp motor, however, could not handle the pumping load under all conditions; therefore, it was no longer in use. To compensate, one of the 150 hp pumps operated continuously. To prevent the motor from exceeding its rated amps, the gate valve on the discharge was closed substantially.
The Systems ApproachTo improve the energy efficiency and performance of its beer cooling system in an environmentally conscious manner, the Stroh Brewery project team developed a feasibility study based on the systems approach. The project consisted of the following eight steps:
- Review system documentation, perform field verification and discuss systems operation with brewery staff.
- Prepare a detailed system description.
- Prepare a measurement plan, identifying measurements (flow, temperature, pressure, kW, etc.) to be taken and under what conditions.
- Measure system operation according to the measurement plan.
- Evaluate current system performance by comparing measured data to design information (e.g., pump curves).
- Identify technical options to increase system efficiency and, if possible, ability to meet production needs.
- Analyze each technical option to establish feasibility, estimate cost and energy savings, and determine cost effectiveness.
- Prepare a report summarizing analysis results.
In addition, brewery staff conducted follow-up metering after implementation to verify projected savings and ensure that system operation was satisfactory.
Project ImplementationFollowing a thorough examination of the facility's glycol cooling system, three optimization measures were evaluated and considered for implementation:
- Trimming the pump impeller.
- Installing a new pump that matched the existing system.
- Installing a new pump with a variable speed drive.
Based on measured data and calculations, the team concluded that the first measure presented the greatest return for the least cost. This decision was based on the team's finding that approximately 70% of the 300 ft of head produced by the pump was being consumed by the substantially closed gate valve on the pump discharge. Coupled with the finding that the system only required 90 ft of head and 1,200 gal/min to operate at peak system pressure drop (maximum required flow), the team concluded it could significantly improve the system's performance and move it closer to its best efficiency point by simply reducing the impeller's size.
The project team used a seven-step procedure to trim the impeller. The steps, which took an estimated 15 hr to accomplish, were:
- Isolate pump and drain down casing.
- Remove top half casing.
- Remove rotor.
- Set up and trim impeller on shaft.
- Deburr and grind trailing edge of vanes.
- Balance rotor.
- Reassemble pump.
This adjustment had been previously attempted unsuccessfully, and the team found that the earlier attempt did not trim enough from the diameter of the impeller. Trimming the impeller to 14.75" dia. had failed to provide the necessary conditions for the gate valve to be fully opened. Furthermore, with the smaller motor - 75 hp instead of 150 hp - the flow rate decreased to 1,000 gal/min. By trimming the impeller to 11.75", however, the discharge valve could be completely opened, and a normal constant flow rate, either equal to or greater than the flow rate achieved with a 150 hp motor and 17" dia. impeller, could be maintained.
ResultsField measurements conducted by brewery personnel after the impeller was trimmed to 11.75" dia. confirmed the success of the adjustment. Not only did the new system's flow rate increase by 15%, from 1,200 gal/min to 1,380 gal/min, but the smaller motor reduced electricity demand by more than 50% (from 112 kW to 54 kW). Extrapolating these results shows that the system's annual electricity consumption will fall from 981,000 kW-hr to an estimated 473,000 kW-hr.
This relatively minor adjustment resulted in substantial cost savings for the facility. With demand charges for electricity of $6.26 per kW/month and energy charges of $0.0288 per kWh (the rates are based on a blend of on- and off-peak hours), the adjustment cut $19,000 from the original annual energy cost of $36,700, a savings of more than 50%. Project costs, consisting primarily of labor to trim the impeller, were estimated at $1,500. Based on the annual savings of $19,000, the simple payback for the adjustment is about one month.
Additional benefits realized by Stroh Brewery as a result of this minor adjustment included: greater available cooling for peak load periods, extended equipment life and decreased maintenance for pumps and valves as a result of the reduced power input, and increased efficiency of the operating system, and reduced nuisance overheats.