By Ron Marshall
A building products manufacturer was running out of air pressure during their production shift and were wanting to upgrade their compressor capacity. Before spending any money on equipment, they called in a compressed air auditor to give an opinion on the size they need. As part of the study the auditor placed instruments on the system to measure pressure, power and flow. These are basic measurements done by all compressed air system assessment professionals and can clearly show system characteristics.
The original system was designed to be extremely efficient, using VSD technology, a central compressor controller, large storage, efficient cycling drying and low loss filters — to ensure the cost of production of the compressed air is as low as possible, as directed by the previous Facility Manager.
The data captured by the logging instruments showed that high efficiency was no longer being achieved, however, and that there was an excessive pressure loss of about 20 psi across the air dryers and filters. Compressor selection was also a problem, causing more energy to be expended than desired. Some changes had taken place in the recent past that cut into the original premium design.
The original air dryer, well sized with double redundancy at 2,400 cfm to cover the total capacity of all of the systems four compressors had been replaced with a single unit about one third the capacity. The system low loss mist eliminator filter had been left unmaintained for years, and had developed a high pressure loss. And the compressor controller that orchestrated the operation of the four compressors had been bypassed, allowing the VSD compressor to run near minimum speed most of the time, greatly reducing system efficiency, and likely contributing to maintenance issues.
Further to this, the average production flow of the plant was measured at an average of 480 cfm. Night and weekend load, however, appeared to be 400 cfm. Doing some quick calculations, these measurements suggest a non-productive leakage load that could be approaching 80% of the system average flow.
It would appear the plant does not likely need more compressor capacity, but should focus on the demand side, pressure loss reduction, and renewing compressor control.
By measuring their system first, before acting, the plant management will be able to defer the cost of a new compressor, and save energy by attacking previously unknown system inefficiencies.