By Ron Marshall
A compressed air system used for engine start duty needed a new air compressor. The old unit had been troublesome and it was difficult to get parts. The purchasing department was asked for quotes for a new unit and obtained three bids from various suppliers.
In comparing the prices for the replacement, the purchasing department representative saw that two of the bids were similar in price, one of which was the same lubricated screw compressor they had installed in another system that had exactly the same application. But the third unit had 20% lower cost, and the supplier had glowing references about the quality and long life their units provided. Purchasing felt it was a no-brainer, they went with the lowest bid, saving $7,000 on the sale price.
About a year after installation, a compressed air auditor did some routine measurements to see how the systems in the facility were operating. Measurements revealed that the two engine start systems, one of which used the new compressor, had quite different operating costs. While one cost only about $2,000 per year in electrical consumption, the other, using the new compressor, consumed $25,000 per year in electricity.
Unknown to the purchasing department, the new compressor had been purchased stripped bare of any energy saving features. This compressor, producing very little average air flow, ran constantly in a control mode called modulation and had no other operating regime than “constant run.” In modulation, this compressor runs constantly, and while doing so consumes no less than 70% of its full load power.
The other system used a brand of compressor that runs in the more efficient load/unload mode, and these compressors were equipped with auto shutdown, which allows the compressor to completely shut down, using minimal power, between load cycles. As a result, the compressor spends most of its time turned off.
The purchasing agent had saved the company $7,000 in purchase costs, but his decision to buy based only on the initial price cost the company $22,000 in extra electricity costs for every year of compressor life. This is false economy at its extreme. When purchasing air compressors, especially the lubricated screw variant, total life cycle costs need to be taken into account!