Load Preheating Using Flue Gases from a Fuel-Fired Heating System Page: 1 of 2
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Questions to ask if your furnace
can be adapted to load preheating
(not all can be):
1. Would combustion air preheating
or some other savings measure be
cost-effective?
2. How large a preheating chamber
is needed?
3. Do you have enough space for a
preheater that size?
4. You might have to restrict exhaust
gas paths so they will come in
contact with the load. Will this
interfere with exhaust gas flow and
cause too much backpressure in the
furnace chamber?
5. How will incoming parts move
through the preheating chamber?
If conveying equipment is needed,
can it withstand exhaust gas
temperatures?
Questions to ask before adding a
separate load preheat section or
chamber:
1. How would flue gases move to the
heating chamber? Will a fan or
blower be needed to overcome
pressure drops in ducts?
2. Does heat demand equal heat
supply during most of the heating
cycle time?
3. How would the hot load be
transferred to the main furnace?
Would the heat loss be considerable?
4. What type of controls are required
to maintain the desired temperature
in the preheat chamber? Will an
auxiliary heating system be needed?
Resources
See also the ASM Handbook,
Volumes 1 (1990) and 2 (1991);
Materials Park, OH: ASM
International.
U.S. Department of Energy-
For additional information on
process heating system efficiency,
to obtain DOE's publications and
Process Heating Assessment and
Survey Tool (PHAST) software,
or to learn more about training,
visit the BestPractices Web site
at www.eere.energy.gov/industry/
bestpractices.Load Preheating Using Flue Gases from a
Fuel-Fired Heating System
The thermal efficiency of a heating system can be improved significantly by using
heat contained in furnace flue gases to preheat the furnace load (material coming
into the furnace). If exhaust gases leaving a fuel-fired furnace can be brought into
contact with a relatively cool incoming load, heat will be transferred directly to the
load. Since there is no intermediate step, like air or gas preheating, in the heat
recovery process, this can be the best approach to capturing waste heat. Load
preheating is best suited for continuous processes, but it can sometimes also be used
successfully with intermittently operated or batch furnaces. Load preheating can be
achieved in a variety of ways, including these:
" Use of an unfired load preheat section, in which furnace flue gases are brought in
contact with the incoming load in an extended part of the furnace.
" Use of an external box, in which high-temperature furnace flue gases are used to
dry and/or preheat the charge before loading in a furnace.
" Use of a counter-current flow design in a furnace or a kiln, in which the burner
gases flow in the opposite direction of the load being heated.m
x
m
R
a'
C
m
0
m
a100
90
80
70
60
50
40
30
20
10
0Fuel: Birmingham Natural Gas (1,002 Btu/cu ft, 0.6 sp gr)
% Excess Air (%-2)
800 (18s.8 50 (7.4) 10 (2.1)
150 (13.1)
1 25 (4.5)
NN00 (17.1)
1200(19.4) 300 (161) 100 (11)
600 (18.2) 1
\ 200 (14.4)1000
Exhaust Gas Temperature ( F)0
2000
3000
The amount of energy savings obtained by using load preheating is higher than the
amount of actual heat transferred to the load. The "net" heat delivered to the load
has to account for the efficiency of the furnace. Since the furnace efficiency is
always less than 100%, the resulting energy savings exceed the energy picked up by
the load. Load preheating can result in higher production from the same furnace.
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Load Preheating Using Flue Gases from a Fuel-Fired Heating System, report, January 1, 2006; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc877647/m1/1/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.