Using Waste Heat for External Processes Page: 2 of 2
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" Matching the timing of the heat supply from the primary process and the heat
demand in the secondary process.
" Placement ofprimary and secondary heating equipment. The closer the primary
and secondary process can be situated, the better.
Figure 2 shows some
heating processes that
commonly use waste
heat from a higher
and the approximate
range of waste gas
gases can be used if
the heat recovery
device is deliberately
Space Heating (Air)
0 500 1,000 1,500 2,000 2,500
Waste Gas Temperature (F)
A plant uses a furnace with a firing rate of 10 MMBtu/hr, which discharges flue
gases at 1,400 F (primary process). The plant also has a drying oven that operates at
400 F and requires 2.5 MMBtu/hr of heat (secondary process). The recoverable heat
can be estimated using Figure 1. At 1400 F, the heat content of the exhaust gases (at
10% excess air) is about 42% of the heat furnace input. Again using Figure 1, the
heat content of exhaust gases at 400 F is approximately 20% (at 10% excess air).
The approximate amount of heat that can be saved is 42% - 20% = 22% of the heat
input to the primary process. The net heat available for the secondary process is
approximately 0.22 x 10 MMBtu/hr = 2.2 MMBtu/hr. Actual savings would be
greater than this because the available heat at the 400 F exhaust gas temperature is
approximately 80% (see Figure 1 in Process Heating Tip Sheet #9, Load Preheating
Using Flue Gases from a Fuel-Fired Heating System). The actual savings for the
oven are thus 2.2/0.8 = 2.75 MMBtu/hr.
In this case, there is more than enough heat to meet the heat demand for the drying
oven. It would be necessary to use additional heat in the oven if the exhaust gas heat
from the furnace were not sufficient to meet the oven heat demand. At a fuel cost of
$8.00 per MMBtu, the company can save $22.00 in fuel costs per hour. Assuming
8,000 hours of operation per year, annual savings are $175,000.
BestPractices is part of the Industrial
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BestPractices emphasizes plant systems,
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easy access to near-term and long-term
solutions for improving the performance of
motor, steam, compressed air, and process
heating systems. In addition, the Industrial
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FOR ADDITIONAL INFORMATION,
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and Renewable Energy
U.S. Department of Energy
Washington, DC 20585-0121
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1 Calculations by Richard Bennett, Janus Technology Group.
2 Figure by Richard Bennett, Janus Technology Group.
Process Heating Tip Sheet #10
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Using Waste Heat for External Processes, report, January 1, 2006; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc878974/m1/2/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.