Most people charged with the responsibility of designing the compressed air system or installation of a correctly sized air dryer, typically match the inlet capacity with the compressor maximum output. This is especially incorrect when sizing for an air-cooled reciprocating air compressor. Reciprocating air compressors are typically air cooled, and use pitched vanes on the pump flywheel which drive cooling air across finned cylinders to allow/assist in the air cooling and heat dissipation. These pumps are typically designed for a maximum duty cycle of between 50 to 75%.
If the duty cycle of a compressor that produces 20 CFM is 65%, then the maximum output or constant should be based upon no greater than 13 CFM, therefore the matching dryer size would only require 13 CFM (plus dryer purge air consumption) if the total dry air requirement had to match the requirement of the air compressor. The reality is that the compressor should be adequately sized for maximum flow based upon the duty cycle, and this often exceeds the actual compressed air requirement of the plant or application.
For example if the total air demand for the plant application/process has been calculated to be only 5 CFM in the above example, the dryer need only be large enough to provide a NET output (after purge loss) of 5 CFM to the plant or application. If the dryer is being used at 100 PSIG, the dryer inlet would only see 6 CFM and this 13 CFM compressor would have no trouble keeping up with the requirement and the purchase would require a 6 CFM dryer only. Considering that most twintower desiccant regenerative air dryers require 15 to 20% purge air consumption, excessive purging and increased compressor wear and tear will result if the dryer is over-sized. Not matching the dryer size to the application could cost considerably more if the incorrect sizing method is used.