|Minimize Boiler Blowdown|
|Written by USDOE Office of Industrial Technologies|
|Thursday, 01 July 2010 12:16|
Minimize Boiler Blowdown
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.
Initial = 100,000
(1 – 0.08)
= 108,695 lb/hr
Final = 100,000
(1 – 0.06)
= 106,383 lb/hr
Makeup Water Savings = 108,695 – 106,383 = 2,312 lb/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
Annual Water and Chemical Savings = 2,312lb/hr x 8,760hrs/yr x $0.004/gal
Annual Cost Savings = $62,886 + $9,714
Automatic Blowdown Control Systems
Cycles of Concentration
Adapted from an Energy TIPS fact sheet that was originally published by the Industrial Energy Extension Service of Georgia Tech.
FOR ADDITIONAL INFORMATION, PLEASE CONTACT:
EERE Information Center
Industrial Technologies Program
Energy Efficiency and Renewable Energy
U.S. Department of Energy
Washington, DC 20585-0121