Page 379 - Mechanical Engineers' Handbook (Volume 4)

368 Air Heating

Table 3 Effect of Combustion Air Preheat on Flame Temperature
Adiabatic Flame Temperatured ( F)
With 137,010 With 153,120
With 1000 Btu/gal Btu/gal
Excess Preheated Combustion Btu/scf Distillate Residual
Air (%) Air Temperature ( F) Natural Gas Fuel Oil Fuel Oil
0 60 3468 3532 3627
10 60 3314 3374 3475
10 600 3542 3604 3690
10 700 3581 3643 3727
10 800 3619 3681 3763
10 900 3656 3718 3798
10 1000 3692 3754 3831
10 1100 3727 3789 3864
10 1200 3761 3823 3896
10 1300 3794 3855 3927
10 1400 3826 3887 3957
10 1500 3857 3918 3986
10 1600 3887 3948 4014
10 1700 3917 3978 4042
10 1800 3945 4006 4069
10 1900 3973 4034 4095
10 2000 4000 4060 4121
0 2000 4051 4112 4171

excess air.* The percentage of fuel saved by addition of combustion air preheating equipment
can be calculated by the formula
% fuel saved 100 1 % available heat before
% available heat after

Table 2 lists fuel savings calculated by this method. 4
Preheating combustion air raises the flame temperature and thereby enhances radiation
heat transfer in the furnace, which should lower the exit gas temperature and further improve
fuel efficiency. Table 3 and the x intercepts of Fig. 2 show adiabatic flame temperatures
when operating with 10% excess air,† but it is difficult to quantify the resultant saving from
this effect.

*It is advisable to tune a combustion system for closer to stoichiometric air/fuel ratio before attempting
to preheat combustion air. This is not only a quicker and less costly fuel conservation measure, but it
then allows use of smaller heat-exchange equipment.
†Although 0% excess air (stoichiometric air/fuel ratio) is ideal, practical considerations usually dictate
operation with 5–10% excess air. During changes in firing rate, time lag in valve operation may result
in smoke formation if some excess air is not available prior to the change. Heat exchangers made of
300 series stainless steels may be damaged by alternate oxidation and reduction (particularly in the
presence of sulfur). For these reasons, it is wise to have an accurate air/fuel ratio controller with very
limited time-delay deviation from air/fuel ratio setpoint.