|Minimize Boiler Blowdown|
|Written by USDOE Office of Industrial Technologies|
|Wednesday, 09 June 2010 07:51|
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.
Assume that the installation of an automatic blowdown control system 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.
Initial = 100,000 / (1 - 0.08) = 108,695 lbs/hr
Final = 100,000 / (1 - 0.06) = 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
Annual Fuel Savings = (2312 lbs/hr x 8760 hrs/yr x 310.5 Btu/lb x $3.00/MBtu) / (0.82 x 106) = $23,007
Annual Water and Chemical Savings = (2312 lbs/hr x 8760 hrs/yr x$0.004/gal) / (8.34 lbs/gal) = $9,714
Annual Cost Savings = $23,007 + $9,714 = $32,721
Review your blowdown practices to identify energy saving opportunities. Examine operating practices for boiler feedwater and blowdown rates developed by the American Society of Mechanical Engineers (ASME). Considerations include operating pressure, steam purity, and deposition control. Consider an automatic blowdown control system (see sidebar).
References and Footnotes
1. “Consensus Operating Practices for Control of Feedwater/Boiler Water Chemistry in Modern Industrial Boilers”, published by the ASME, 1994.
2. “Recommended Rules for the Care and Operation of Heating Boilers”, Section VI of the ASME Boiler and Pressure Vessel Code, 1995.
3. “Recommended Guidelines for the Care of Power Boilers”, Section VII of the ASME Boiler and Pressure Vessel Code, 1995.
About DOE’s Office of Industrial Technologies
The Office of Industrial Technologies (OIT), through partnerships with industry, government, and non-governmental organizations, develops and delivers advanced energy efficiency, renewable energy, and pollution prevention technologies for industrial applications. OIT is part of the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy.
OIT encourages industry-wide efforts to boost resource productivity through a
strategy called Industries of the Future (IOF). IOF focuses on the following nine
energy- and resource-intensive industries:
• Agriculture • Forest Products • Mining
• Aluminum • Glass • Petroleum
• Chemicals • Metal Casting • Steel
OIT and its BestPractices program offer a wide variety of resources to industrial
partners that cover motor, steam, compressed air, and process heating systems.
For example, BestPractices software can help you decide whether to replace or
rewind motors (MotorMaster+), assess the efficiency of pumping systems (PSAT),
or determine optimal insulation thickness for pipes and pressure vessels (3E Plus).
Training is available to help you or your staff learn how to use these software
programs and learn more about industrial systems. Workshops are held around the
country on topics such as “Capturing the Value of Steam Efficiency,” “Fundamentals
and Advanced Management of Compressed Air Systems,” and “Motor System
Management.” Available technical publications range from case studies and tip
sheets to sourcebooks and market assessments. The Energy Matters newsletter, for
example, provides timely articles and information on comprehensive energy systems