If you have a lubricated screw compressor running in load/unload mode, you should be aware that the compressor cycle times greatly affect the efficiency of your compressed air system.
The cycle time is the amount of time it takes for the compressor to load and unload while supplying your compressed air needs. The faster the compressor cycles, the more energy it consumes for a given compressed air output.
Do a check during your average loading period. When the compressor loads (causing the pressure to rise), start timing the cycle with a stopwatch (most smartphones have this feature). When the compressor unloads, the compressor will sound different and the pressure will fall. Time the unloaded time, too. Best bet is to do this test for about 5 or so consecutive cycles.
Calculate the average cycle time (loaded plus unloaded). Is it less than a minute? If it is, your compressor will be running inefficiently compared to one that is running with a cycle time of two minutes or longer. Depending on how heavily loaded the compressor was during the test, the comparison could show a significant difference in average power, as shown by the chart above. The chart is shown in gallons per cfm because the size of the storage receiver connected to the compressor affects cycle time.
Some tips about cycle time:
- Always try to have effective storage receiver capacity of 5 gal per cfm or higher. This storage is based on the largest compressor in the system not the total compressor capacity.
- The width of the compressor pressure band also affects cycle time; widening the pressure band from 10 psi to 20 will reduce cycle time by half.
- Sometimes pressure differentials such as excessive dryer and filter loss, or undersized piping will cause the compressors to cycle excessively. Addressing this problem can increase cycle time.
- Long cycle times of 10 minutes or more may allow the compressors to turn off between load cycles, saving more power.