By Ron Marshall
A leakage assessment was done at a small industrial shop and a constant flow was found in a CNC machine. This machine was very rarely used, perhaps running less than 1% of the time — however, it was consuming a significant flow for internal purging.
While testing this flow, a duty cycle test was done using the compressor. During this test, the loaded versus running time was measured with a stopwatch to estimate flow. (The test is done both with and without the potential inappropriate use of compressed air to estimate the flow.) This helps estimate the flow of the usage — which might be called an “abandoned use,” because air flow is only necessary when the machine is actually operating.
When this test was done, the CNC flow was determined to be only 3 cfm, enough to cost about $650 in energy per year. However, a strange thing was noticed. When this flow was “on,” the 10-hp compressor ran constantly loaded at about 10%, but running unloaded the remaining 90% of the time. When the flow was turned “off,” the compressor load reduced by about 8%, but the compressor turned fully off between load cycles. This compressor consumes about 3.5 kW while running unloaded — but produces no air and the estimated annual hours in this condition is about 7,845 in wasted operation.
The loaded power consumption of this compressor cost about $745 per year, but the unloaded power consumption was costing $2,760 per year. Turning off the CNC flow saves significantly more than expected, due to the reduction in compressor run time.
An automatic valve was installed on the machine to turn off the purge flow when there is no production. This is expected to save much more annually than the $300 cost of the shutoff valve. Even more savings would be gained if system storage was increased to slow down the compressor cycles.
This is another example of what can be gained by simply turning off the flow of compressed air when not required!