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ption for plants that need a source of steam or hot water. The waste heat boiler is similaronventional boilers with one exception: it is heated by the exhaust gas stream from arocess oven or furnace instead of its own burner. Waste heat boilers may be the answer flants seeking added steam capacity. Users should remember, however, that the boilerenerates steam only when the process is running.

Not all processes are candidates for waste heat recovery. Exhaust volumes and temperaturmay be too low to provide financial justification. A comprehensive program for reducing ov

r furnace energy consumption involves achieving the best performance from the existingquipment and adding modifications and upgrades that can make substantial reductions innergy consumption.

Department of Energy, Industrial Technologies Program (ITP)his article by the Department of Energy's Industrial Technologies Program (ITP), was reprinted withermission. For more ways to save, visit the Department of Energy's Industrial Technologies Program's wte at http://www.eere.energy.gov/industry/.

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heat storage, wa

conveyor or 

radiation losses

will be multiplied

by the available

heat factor 

 A recuperator is

gas-to-gas heat 

exchanger place

on the stack of the oven or 

furnace. It 

transfers heat 

from the outgoin

exhaust gas to

the incoming

combustion air 

while keeping th

two streams from

mixing.

where W is the mass of the exhaust gases; CP is the specific heat of the

xhaust gases; TEXHAUST is the flue gas temperature entering the furnace

xhaust system (stack); and TAMBIENT is the ambient temperature.

The highest priority is to minimize exhaust gas temperature and mass orolume of exhaust gases. The heat processing equipment exhaust gas temperature dependn many factors associated with the equipment operation and heat losses. It can be

measured directly, or it can be assumed to be 100 to 200°F (55 to 111°C) above the contemperature for the heated zone where the flue gases are exhausted.

The exhaust mass flow depends on the combustion airflow, fuel flow and the air leakage inhe heat processing equipment. Measurement of fuel flow, together with the percentage of xygen in the flue gases, can be used to estimate mass or volume of exhaust gases.

The flue gas specific heat (CP) for most gaseous fuel-fired furnaces can be assumed to be.25 BTU/lb/°F or 0.02 BTU/scf/°F for a reasonably accurate estimate of flue gas heat losse

Minimize Exhaust Gas Temperatures. Excessive gas temperatures can be the result of 

oor heat transfer in the heat processing equipment. If the combustion gases are unable toransfer the maximum possible heat to the oven, furnace or other heat processing equipme- and its contents -- they will leave the equipment at higher temperatures than necessary

Overloading heat processing equipment also can lead to excessive stack temperatures. To he proper rate of heat transfer, combustion gases must be held in the heating chamber fohe right amount of time. The natural tendency of an overloaded oven or furnace is to runolder than optimal unless the temperature is set artificially high. This causes the burners tperate at higher than normal firing rates, which increases combustion gas volumes. Theigher the gas flow rates and shorter time in the heat processing equipment cause poor heransfer, resulting in higher temperature for the flue gases. Increased volumes of higher

emperature flue gases lead to sharply increased heat losses.

Minimizing Exhaust Gas Volumes. Avoiding overloading and optimizingeat transfer are two ways to lower waste gas flows, but there are others.

The most potent way is to closely control fuel-to-air ratios. Operating theven or furnace near the optimum fuel-to-air ratio for the process alsoontrols fuel consumption. The best part is that it usually can be done withxisting control equipment and a little maintenance attention.

Some reduction in exhaust volumes will be the indirect result of efficienciespplied elsewhere. For instance, flue gas losses are a fixed percentage of he total heat input to the oven or furnace. Any reduction in heat storage,

wall, conveyor or radiation losses will be multiplied by the available heatactor.

Use of Oxygen Enriched Combustion Air. Ambient air containspproximately 21 percent oxygen, with nitrogen and other inert gases ashe balance. The total volume of exhaust gases could be reduced byncreasing the oxygen content of the combustion air, either by mixing extraxygen into the ambient air or by using 100 percent oxygen. The reduced