Multipass corrosion proof air heater

A multipass, corrosionproof air heater comprising:

• a group of tier passes forming a package with each group of tier passes having banks of tubes forming heating surfaces, and said tubes being arranged in a plurality of tier passes successively connected in series by way of connecting air ducts to form the airside arrangement of said air heater;
• a plurality of further air ducts for feeding cold air to the first bank of tubes;
• said first bank of tubes having a plurality of passes;
• one of said further air ducts including means for preheating an initial comparatively small fraction of the cold air which is to be fed into the initial pass of said plurality of passes in said first bank of tubes while the remaining air ducts of said plurality of further air ducts are connected to respective connecting air ducts for forming with uptake gases a parallel connection of said plurality of passes within said first bank of tubes;
• said first bank of tubes being located in a zone of the same gas temperatures and being exposed to the same flow of gases wherein the relationship between the temperature of air and that of gases is apt to provoke corrosion;
• and one of said further air ducts feeding the balance of the cold air connected downstream of the last pass of said plurality of passes in said first bank of tubes to the connecting air duct communicating with the next tier pass of said air heater.