You can't get expansive cooling without dropping the pressure. When you have a tank or compressor supplying the air and go from a smaller pipe to a larger pipe the air doesn't expand, the air supply simply pushes in more air to maintain the pressure. The efficiency of cooling the air with this type of setup is in part determined by the ratio of surface area of the pipe to the volume of air in the pipe. The smaller pipe will have a larger surface area per unit of air volume and therefore will cool more efficiently. So you'll cool the air better by not increasing the pipe diameter. Then add one vertical section of a larger diameter at the end to slow the linear velocity and drop out any water droplets.
