AIR-HEATING PROCESSES Air can be heated by burning fuel or by recovering waste heat from another process. In either case, the heat can be transferred to air directly or indirectly. Indirect air heaters are heat exchangers wherein the products of combustion never contact or mix with the air to be heated. In waste heat recovery, the heat exchanger is termed a recuperator. Direct air heaters or direct-fired air heaters heat the air by intentionally mixing the products or combustion of waste gas with the air to be heated. . | CHAPTER 53 AIR HEATING Richard J. Reed North American Manufacturing Company Cleveland Ohio AIR-HEATING PROCESSES 1641 WARNINGS 1643 COSTS 1643 BENEFITS 1644 AIR-HEATING PROCESSES Air can be heated by burning fuel or by recovering waste heat from another process. In either case the heat can be transferred to air directly or indirectly. Indirect air heaters are heat exchangers wherein the products of combustion never contact or mix with the air to be heated. In waste heat recovery the heat exchanger is termed a recuperator. Direct air heaters or direct-fired air heaters heat the air by intentionally mixing the products or combustion of waste gas with the air to be heated. They are most commonly used for ovens and dryers. It may be impractical to use them for space heating or for preheating combustion air because of lack of oxygen in the resulting mixture vitiated air . In some cases direct-fired air heating may be limited by codes and or by presence of harmful matter of undesirable odors from the heating stream. Direct-fired air heaters have lower first cost and lower operating fuel cost than indirect air heaters. Heat requirements for direct-fired air heating. Table lists the gross Btu of fuel input required to heat one standard cubic foot of air from a given inlet temperature to a given outlet temperature. It is based on natural gas at 60 F having 1000 gross Btu ft3 910 net Btu ft3 and stoichiometric air gas ratio of 1. The oxygen for combustion is supplied by the air that is being heated. The hot outlet air includes combustion products obtained from burning sufficient natural gas to raise the air to the indicated outlet temperature. Recovered waste heat from another nearby heating process can be used for process heating space heating or for preheating combustion air Ref. 4 . If the waste stream is largely nitrogen and if the temperatures of both streams are between 0 and 800 F where specific
