What are headers?
Headers are the boxes at the ends of the tubes, which distribute the fluid from the piping to the tubes.

How are headers constructed?

Almost all headers on air-cooled exchangers are welded rectangular boxes. A vast majority of the headers are of the plug type. This means that there is a shoulder plug opposite each tube, which allows access for inspection and cleaning of individual tubes. They can also be used to plug a leaking tube. The plugholes are used in the manufacturing process for accesses to roller expand the tubes into the headers.
The other common type of header is the cover plate or bonnet type. These are usually used in low-pressure applications (say below 150 PSIG) where complete tube access is desired. This usually means applications where fouling is a potential problem and the tube bundle may require occasional internal cleaning. As the name implies, these have a removable plate on the backside of the header opposite the tubes. The cover plate is attached to the header by a set of studs or through-bolts to a flange around the perimeter of the header. A bonnet header is similar, but opposite in construction. The whole header or bonnet bolts to the tubesheet and comes off. Bonnet headers are sometimes used where the corrosion potential of the process fluid is very high and the tubesheet material is some kind of expensive exotic alloy, such as titanium.
Headers are usually constructed of carbon steel or stainless steel, but sometimes more exotic alloys are used for corrosion resistance. The customer usually makes the selection of materials.