Environmental Engineering Reference
In-Depth Information
Process heat and steam systems
This section is based on the recommendations to improve efficiency of heating and steam
systems made by the US Department of Energy (“ 'Top 10' ways to improve process heating
and steam systems,” 2007a). For steam systems, improvements are summarized as follows:
1. Improve boiler efficiency.
2. Minimize heat loss in blowdown water.
3. Improve insulation.
4.
Return condensate to boilers.
5.
Fix steam leaks.
For direct-fired heating systems, efficiency improvements are classified in the following:
1.
Recover heat from processes.
2.
Use proper heating methods.
3.
Maintain heat transfer surfaces.
4.
Insulate and maintain equipment.
Improve boiler efficiency
Heat transfer efficiency is an intrinsic property of a boiler design that indicates the amount
of heat that is transferred from the fuel to the water/steam. It is expressed as a ratio of
energy output divided by energy input. Boiler efficiencies are usually in the 75- to 85-percent
range and efficiencies above 90 percent can be found in newer high-efficiency boilers
(Sperber, 2008).
Boiler efficiencies decrease with use as deposits form on heat transfer surfaces. On the
fireside, soot is the main deposit, and on the waterside, they are calcium, magnesium, and
silica. Deposits from these three minerals produce scale formation, which is a continuous
layer of material with heat transfer coefficients at least 10 times less than steel. This layer
retards heat transfer and results not only in a loss of heat through the flue but also in overheat-
ing of boiler tubes, which can produce premature failure. Detection of a scale deposit can be
performed by visual inspection during maintenance or by continuous monitoring of flue gas
temperatures. Rises in flue gas temperatures, when all the other variables are constant, are
indicative of scale formation, which can be avoided by softening water, and once formed can
be removed by acid cleaning or mechanical means (DOE, 2006a).
Combustion efficiency is another parameter indicative of the utilization proficiency of
energy contained in the fuel. Combustion efficiency is related to the amount of air that is made
available to the combustion also known as “air-fuel ratio.” Theoretically, a stoichiometric
amount of oxygen (contained in air) is necessary to burn the fuel completely. However, in
practice, a modest excess is needed to assure a complete combustion. An amount of air less
than required results in unburned fuel and in formation of soot, smoke, and excessive carbon
monoxide, which may produce fouling, increased pollution, loss of efficiency, and potential
for explosions. Conversely, an excess of air more, than the required, leads to heat loss through
flue gases.
Setting the correct air-to-fuel ratio requires monitoring of flue gases either with handheld
systems or with in-line dedicated meters that monitor oxygen and temperature in flue stack
gases. Data from handheld devices are used to adjust the air-to-fuel ratio manually, whereas
data from in-line monitoring systems serve as real time feedback for automatic air-to-fuel
ratio adjustment (DOE, 2006b).
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