The document describes an innovative on-line cleaning method for air coolers called the D.D.T. process. It is an efficient, safe, and environmentally friendly dry-chemical cleaning method that does not require dismantling or damaging the fins. The process sprays dry chemicals from bottom to top and then rinses with water from top to bottom, penetrating through the bundle without damage. It provides visible cleaning results and increased efficiency compared to other methods that can damage fins or have limited penetration.