Quiz Corner: Energy savings of operating pump at reduced speed

Approximately how much energy is saved by operating a pump motor at 85% speed as compared to operating the pump motor at full speed to provide the same liquid flow rate?    

A.  15%

B.  30%

C.  45%

D.  60%

Operating the pump motor at 85% speed will reduce its discharge pressure and energy consumption by approximately the square and cube of the speed respectively. Therefore, the discharge pressure will be approximately 72% (0.85 x 0.85) of the full speed discharge pressure. The energy consumption will be approximately 61% (0.85 x 0.85 x 0.85) of the full speed energy consumption, saving almost 40% of the full speed energy consumption. Therefore, Answer C is reasonable.

Additional complicating factors

There are few complications in the problem as stated. However, innovative process changes that are customized to the process can sometimes provide significant additional savings.

David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.