|Return Condensate to the Boiler|
|Written by USDOE Office of Industrial Technologies|
|Wednesday, 09 June 2010 08:48|
Return Condensate to the Boiler
When steam transfers its heat in a manufacturing process, heat exchanger, or heating coil, it reverts to a liquid phase called condensate. An attractive method of improving your power plant’s energy efficiency is to increase the condensate return to the boiler.
Returning hot condensate to the boiler makes sense for several reasons. As more condensate is returned, less make-up water is required, saving fuel, make-up water, and chemicals and treatment costs. Less condensate discharged into a sewer system reduces disposal costs. Return of high purity condensate also reduces energy losses due to boiler blowdown. Significant fuel savings occur as most returned condensate is relatively hot (130°F to 225°F), reducing the amount of cold make-up water (50°F to 60°F) that must be heated.
A simple calculation indicates that energy in the condensate can be more than 10% of the total steam energy content of a typical system. The graph shows the heat remaining in the condensate at various condensate temperatures, for a steam system operating at 100 psig, with make-up water at 55°F.
Consider a steam system that returns an additional 10,000 lbs/hr of condensate at 180°F due to distribution modifications. Assume this system operates 8,000 hours annually with an average boiler efficiency of 82%, and make-up water temperature of 55°F. The water and sewage costs for the plant are $0.002/gal, and the water treatment cost is $0.002/gal. The fuel cost is $3.00 per Million Btu (MMBtu). Assuming a 12% flash steam loss*, calculate the overall annual savings.
Annual Water, Sewage, and Chemicals Savings = (1 – Flash Steam Fraction) x (Condensate Load in lbs/hr) x Annual Operating Hours x (Total Water Costs in $/gal) ÷ (Water Density in lbs/gal)
= ((1 - 0.12) x 10,000 x 8,000 x $0.004)/8.34 = $33,760
*When saturated condensate is reduced to some lower pressure, some condensate flashes off to steam again. This amount is the flash steam loss.
Annual Fuel Savings = (1 – Flash Steam Fraction) x (Condensate Load in lbs/hr) x Annual Operating Hours x (Makeup Water Temperature rise in °F) x (Fuel Cost in $/Btu) ) ÷ Boiler Efficiency
= ((1 - 0.12) x 10,000 x 8,000 x (180 – 55) x $3.00)/ (0.82 x 10^6) = $32,195
Total Annual Savings Due to Return of an Additional 10,000 lbs/hr of Condensate = $33,760 + $32,195 = $65,955
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