Air cooled condensers operate at a high temperature and the air entering them is rarely, if ever, filtered. They are often exposed to extremes of environmental conditions eg. in coastal areas, aluminium finned condensers particularly, are subject to corrosion by the saline atmosphere.
The dirt that accumulates on these coils comes from airborne particles, insects, leaves etc. which in conjunction with the high temperature of the coil can become “baked” on and prove very difficult to remove if not maintained on a regular basis.