Walker Filtration WFH75PE 75 SCFM Particulate/Coalescer Filter Element Often called the pre-filter since it is placed on the air line just prior to the air dryer, a coalescing filter assembly removes oil, particles and moisture before it can come in contact with the desiccant in the dryer and ruin it. Coalescing filters capture oil, moisture and particles, binding them together until their mass is heavy enough for gravity to send this waste to a drain valve. A coalescer filter is often called a pre-filter because it is usually located prior to an air dryer to prevent oil and particulate from soiling the desiccant.