Even on small boilers,
the prevention of scale formation can produce substantial energy savings. Scale
deposits occur when calcium, magnesium, and silica, commonly found in most
water supplies, react to form a continuous layer of material on the waterside
of the boiler heat exchange tubes.
Scale creates a problem
because it typically possesses a thermal conductivity an order of magnitude
less than the corresponding value for bare steel. Even thin layers of scale
serve as an effective insulator and retard heat transfer. The result is
overheating of boiler tube metal, tube failures, and loss of energy efficiency.
Fuel waste due to boiler
scale may be 2 percent for water-tube boilers and up to 5 percent in fire-tube
boilers. Energy losses as a function of scale thickness and composition are
given in the table below.
A boiler annually uses
450,000 million Btu (MMBtu) of fuel while operating for 8,000 hours at its
rated capacity of 45,000 pounds per hour (lb/hr) of
150-pounds-per-square-inch-gauge (psig) steam. If scale 1/32nd of an inch thick is allowed to form on the
boiler tubes, and the scale is of “normal” composition, the table indicates a
fuel loss of 2 percent. The increase in operating costs, assuming energy is
priced at $8.00 per million Btu ($8.00/MMBtu), is:
= 450,000 MMBtu/yr x $8.00/MMBtu x 0.02 =
MONITOR FLUE GAS TEMPERATURE
An indirect indicator of
scale or deposit formation is flue gas temperature. If the flue gas temperature
rises (with boiler load and excess air held constant), the effect is possibly
due to the presence of scale.
PERFORM VISUAL INSPECTIONS
Visually inspect boiler
tubes when the unit is shut down for maintenance. Scale removal can be achieved
by mechanical means or acid cleaning. If scale is present, consult with your
local water treatment specialist and consider modifying your feedwater
treatment or chemical additives schedule.
Any scale in a boiler is
undesirable. The best way to deal with scale is not to let it form in the first
place. Prevent scale formation by:
• Pretreating of boiler
makeup water (using water softeners, demineralizers, and reverse osmosis to
remove scale-forming minerals).
• Injecting chemicals
into the boiler feedwater.
• Adopting proper boiler
Department of Energy (DOE)
- DOE’s software, the Steam System Assessment Tool and Steam
System Scoping Tool, can help you evaluate and identify steam system
improvements. In addition, refer to Improving Steam System
Performance: A Sourcebook for Industry
for more information on
steam system efficiency opportunities.
Visit the BestPractices
Website at www.eere.energy.gov/industry/bestpractices to access these and other
from Steam Tip Sheet #7, “Clean Boiler Waterside Heat Transfer Surfaces,” from
the U.S. Department of Energy’s Office of Energy Efficiency and Renewable
Energy. For more information, visit www.eere.energy.gov.