Early Monday morning I was alerted to the lag compressor alarm on the instrument air compressor. Both compressors were running and the tank pressure was increasing until it reached cut-out pressure. I inspected for any signs of an air leak but didn't find anything obvious. I observed the compressor and found that when the air dryer swapped desiccant towers it began blowing down a lot of air. The poly-tube connections beneath each dryer had a build-up of ice. I cleared the ice out as best as I could and observed the dryer for another several cycles, but the problem did not reoccur. We will continue to monitor the performance of the air dryer.