wastewater - fixed or variable costs

Rethinking blower options

Air Demand Analysis reveals significant saving opportunities.

Chapmanville Water Department in Logan County, WV provides the water distribution and wastewater treatment services in the town of Chapmanville, which has a population of 1,200. Its wastewater plant processes about 400,000 gallons per day (gpd). The town planned a major upgrade for 2019 and 2020, including the replacement of three 40-hp multi-stage centrifugal blowers commissioned 25 years earlier.

Problem:

At the beginning of 2018, one of these was no longer in service. Chapmanville planners hoped to limp along on the two remaining blowers, but in the spring of 2018, they lost another one. Their upgrade plans included replacing the three 40-hp centrifugal blowers with two 60-hp blowers of the same type, estimating that one blower could supply both basins with the second blower serving as backup. Now, down to only one blower, they could not wait for the planned upgrade. The department got estimates to fix the blowers, but at $25,000 each this did not seem like a wise choice for what would most likely be a short-term solution.

Solution:

Working with C2G Engineering of Huntington, WV, Kaeser performed an air demand analysis (ADA) to baseline current air use and power consumption and provide a better understanding of the situation. After meeting with a representative from Appalachian Power to discuss potential efficiency rebates, it was clear there were financial incentives if the plant could lower their energy consumption.

The ADA measurement plan included data logging the operation of the existing blower and measuring the power consumption. Kaeser's ADA measured actual kilowatts versus tracking amps and estimating power. The measurement plan also included a flow meter to confirm air volume delivered, a pressure sensor after the blower discharge, and temperature sensors to track blower discharge and ambient temperature.

With this performance data, Kaeser estimated that the town could save roughly $14,000 per year in power costs by installing rotary screw blowers controlled by variable speed drives. With the local utility's incentive they could receive an additional $14,000 once the power consumption was verified.

The collected data collected yielded two options. Both involved replacing the 40-hp centrifugal blowers with 30-hp screw blowers, which delivered more flow than the existing units and with better isentropic efficiency. One option was to install two DBS 220L (30 hp) fixed speed screw blowers and the other was two DBS 220L (30 hp) screw blowers with Sigma Frequency Control VFD. The second option offered even greater energy efficiency and better control of dissolved oxygen.

Results

The town chose the second option, with a total project cost of $112,000. This included the blower equipment, sensors and contracted plumbing, welding, and electrical services. 

After assessing the data provided by the integrated controls on the Kaeser blowers, the utility company concluded that Chapmanville qualified for a one-time incentive payment of $18,298. This was 30% more savings than the initial estimate. With annual savings over $18,000 and an equal up-front incentive payment, the payback for the $112,000 project was a little more than five years, well under the seven year requirement from the utility company. Looking longer term, energy savings added up to nearly $366,000 for a conservative equipment life expectancy of 20 years.

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