Case Study: NAPA Sanitation District

Inefficient Operation & Turndown

NAPA Sanitation District’s consultant QuEST and Herwitt Engineering conducted a study on replacing the plant’s Multistage Centrifugal blowers to address their inefficient operation, operational inflexibility and high energy consumption. Following the recommendation of the study, NAPA Sanitation District replaced their existing blowers with the High Efficiency turbo blowers which helped them secure a rebate by CalPOP.

Although the peak efficiency of the multistage centrifugal blowers that the plant was equipped with was rated at 74% at their design point, the actual annual flows of the plant required the plant operators to blow off excess air to meet turndown requirements, significantly lowering the efficiency of the blowers. Furthermore, although the average annual flow was around 8 MGD, the peak wet weather flows reached up to 32 MGD. The existing multistage blowers were rated for peak flow conditions, resulting in wasted air during typical operation. Wasted air = wasted energy.

Herwitt Engineering and the district’s consultant QuEST evaluated nine months of historical plant operation data to determine potential energy saving alternatives for the Soscol Water Recycling Facility, for Napa Sanitation District. In 2008, the final audit report recommended to replace two of the existing aeration 350 HP multistage centrifugal blowers with higher efficiency Turbo Blowers. The report studied various options for the plant’s blower replacement in order to achieve energy savings. QuEST estimated that installing high efficiency turbo blowers in place of the multistage ones would result in annual energy savings of $114,251. The agency also evaluated the option of replacing the existing blowers with a new positive displacement (P.D.) blower equipped with a VFD to allow for a more flexible operation. Finally, the report concluded that the turbo blower would offer almost twice the energy savings than the proposed P.D. blower at $62,318 in annual savings.
Although the P.D. initially offered a lower capital cost alternative, the net capital investment for the Turbo Blower could be recovered in less than two years due to the difference in energy savings. Moreover, the savings on maintenance, system reliability, and quiet operation confirmed the turbo blower would be the best choice for the plant to maximize overall system efficiency. The district decided to move forward with replacing their multistage blowers with a high efficiency turbo blower and were rewarded a rebate by CalPOP of approximately $110,000.

NAPA Sanitation District followed the evaluated bid process to select and procure their new turbo blowers. This innovative bid process allowed the district to receive competitive bids and make their selection based on evaluation factors. The evaluation committee recommended the selection of APG-Neuros’ turbo blowers based on overall value that took into account the higher energy savings, quality of construction and local support.
In September 2008, the existing multistage centrifugal blowers were replaced with the APG-Neuros high efficiency blowers, immediately reducing the input power by 50HP. The NX300-C080 blowers were designed to meet the plant’s annual flow range and to optimize the blower’s performance to maximize energy savings.The turbo blowers have been in operation since, with the only scheduled maintenance of changing the intake filters as needed. The facility is saving well over one hundred thousand dollars a year from the reduction in energy consumption, labor hours and spare parts.
The benefits from retrofits of conventional blowers such as at Napa Sanitation District, not only lower energy bills, reduce noise, and reduce maintenance requirements, but also contribute to a larger environmental cause: reduce greenhouse gas emissions, demand on electric utility systems, and dependence on oil