Minimize Boiler Blowdown: Office of Industrial Technologies (OIT) Steam Energy Tips Fact Sheet Page: 1 of 2
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Minimize Boiler Blowdown
These systems optimize surface
blowdown by regulating water
volume discharged in relation to
amount of dissolved solids present.
Conductivity, TDS, silica or chlorides
concentrations, and/or alkalinity
are reliable indicators of salts and
other contaminants dissolved in
boiler water. A probe provides
feedback to a controller driving a
modulating blowdown valve. An
alternative is proportional control
-with blowdown rate set
proportional to makeup water flow.
Cycles of Concentration
"Cycles of concentration" refers to
the accumulation of impurities
in the boiler water. If the boiler
water contains ten times the level
of impurities in the makeup
water, it is said to have ten cycles
Minimizing your blowdown rate can substantially reduce energy losses, as the temperature
of the blown-down liquid is the same as that of the steam generated in the boiler. Minimizing
blowdown will also reduce makeup water and chemical treatment costs.
As water evaporates in the boiler steam drum, solids present in the feedwater are left behind.
The suspended solids form sludge or sediments in the boiler, which degrades heat transfer.
Dissolved solids promote foaming and carryover of boiler water into the steam. To reduce
the levels of suspended and total dissolved solids (TDS) to acceptable limits, water is
periodically discharged or blown down from the boiler. Mud or bottom blowdown is usually
a manual procedure done for a few seconds on intervals of several hours. It is designed to
remove suspended solids that settle out of the boiler water and form a heavy sludge. Surface
or skimming blowdown is designed to remove the dissolved solids that concentrate near
the liquid surface. Surface blowdown is often a continuous process.
Insufficient blowdown may lead to carryover of boiler water into the steam, or the formation
of deposits. Excessive blowdown will waste energy, water, and chemicals. The optimum
blowdown rate is determined by various factors including the boiler type, operating
pressure, water treatment, and quality of makeup water. Blowdown rates typically range
from 4% to 8% of boiler feedwater flow rate, but can be as high as 10% when makeup
water has a high solids content.
Assume that the installation of an automatic blowdown control system (see sidebar) reduces
your blowdown rate from 8% to 6%. This example assumes a continuously operating
natural-gas-fired, 150-psig, 100,000-pound-per-hour steam boiler. Assume a makeup water
temperature of 60 F, boiler efficiency of 82%, with fuel valued at $3.00 per million Btu
(MBtu), and the total water, sewage and treatment costs at $0.004 per gallon. Calculate the
total annual cost savings.
Boiler Feedwater: Initial = 100,000
(1 - 0.08)
Final = 100,000
(1 - 0.06)
Adapted from an Energy TIPS
fact sheet that was originally
published by the Industrial
Energy Extension Service of
Georgia Tech. For additional
information on steam system
efficiency measures, contact the
OIT Clearinghouse at
108,695 lbs /hr
106,383 lbs /hr
Makeup Water Savings = 108,695 - 106,383 = 2312 lbs/hr
Enthalpy of boiler water = 338.5 Btu/lb; for makeup water at 60 F = 28 Btu/lb
Thermal Energy Savings = 338.5 - 28 = 310.5 Btu/lb
2312 lbs/hr x 8760 hrs/yr x 310.5 Btu/lb x $3.00/MBtu
ual Fuel Savings - = $23,007
Annual Water and Chemical Savings:
0.82 x 106
2312 lbs/hr x 8760 hrs/yr x$0.004/gal = $9714
Annual Cost Savings = $23,007 + $9,714 = $32,721
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Renfrow, S. Minimize Boiler Blowdown: Office of Industrial Technologies (OIT) Steam Energy Tips Fact Sheet, book, June 18, 2001; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc719344/m1/1/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.