Page 373 - Mechanical Engineers' Handbook (Volume 4)

Mechanical Engineers’ Handbook: Energy and Power, Volume 4, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.
CHAPTER 10
AIR HEATING
Richard J. Reed
North American Manufacturing Company
Cleveland, Ohio
1 AIR-HEATING PROCESSES 362 4 BENEFITS 365
2 COSTS 364 REFERENCES 370
3 WARNINGS 365

1 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 1 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/ft , 910 net
3
Btu/ft , and stoichiometric air/gas ratio of 9.4 1. The oxygen for combustion is supplied
3
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 heats are about 0.24, a simplified heat balance can be used to evaluate the mixing
conditions:
Heat content of the waste stream Heat content of the fresh air Heat content of the
mixture or
WT WT WT (W W ) T m
ww
w
mm
ƒƒ
ƒ
where W weight and T temperature of waste gas, fresh air, and mixture (subscripts w,
ƒ, and m).
Example 1
If a 600 F waste gas stream flowing at 100 lb/hr is available to mix with 10 F fresh air and
fuel, how many pounds per hour of 110 F makeup air can be produced?
362