Steam Systems
Quick Tip: Maintaining proper combustion efficiency (i.e. fuel-to-air ratio) is the most common boiler energy savings recommendation. When was your boiler last tuned?
General Description: [1]
Steam is generated in a boiler or a heat recovery steam generator by transferring the heat of combustion gases to water. When water absorbs enough heat, it changes phase from liquid to steam. In some boilers, a superheater further increases the energy content of the steam. Under pressure, the steam then flows from the boiler or steam generator and into the distribution system.
The distribution system carries steam from the boiler or generator to the points of end use. Many distribution systems have several take-off lines that operate at different pressures. These distribution lines are separated by various types of isolation valves, pressure-regulating valves, and, sometimes, backpressure turbines. A properly performing distribution system delivers sufficient quantities of high quality steam at the right pressures and temperatures to the end uses. Effective distribution system performance requires proper steam pressure balance, good condensate drainage, adequate insulation, and effective pressure regulation.
In a heat exchanger, the steam transfers its latent heat to a process fluid. The steam is held in the heat exchanger by a steam trap until it condenses, at which point the trap passes the condensate into the condensate return system. In a turbine, the steam transforms its energy to mechanical work to drive rotating machinery such as pumps, compressors, or electric generators. In fractionating towers, steam facilitates the separation of various components of a process fluid. In stripping applications, the steam pulls contaminants out of a process fluid. Steam is also used as a source of water for certain chemical reactions.
The condensate return system sends the condensate back to the boiler. The condensate is returned to a collection tank. Sometimes the makeup water and chemicals are added here while other times this is done in the deaerator. From the collection tank the condensate is pumped to the deaerator, which strips oxygen and non-condensable gases. The boiler feed pumps increase the feedwater pressure to above boiler pressure and inject it into the boiler to complete the cycle.
Potential Energy and Cost Savings Opportunities:
The below energy conservation opportunities or energy efficiency actions (EE ACTIONS) are provided as a partial list of potential savings opportunities in your plant. They are grouped as no-to-low cost, moderate cost, and long-term cost investments. Consider each for your plant and feel free to contact us for clarification or any assistance you may need in assessing specific projects.
No-to-low cost investment
EE ACTION: Minimize Excess Air [2]
Operating your boiler with an optimum amount of excess air will minimize heat loss up the stack and improve combustion efficiency. Combustion efficiency is a measure of how effectively the heat content of a fuel is transferred into usable heat. Given complete mixing, a precise amount of air is required to completely react with a given quantity of fuel. In practice, combustion conditions are never ideal, and small amounts of additional or “excess” air must be supplied to completely burn the fuel.
Inadequate excess air results in unburned combustibles (fuel, soot, smoke, and carbon monoxide) while too much results in heat lost due to the increased flue gas flow—thus lowering the overall boiler fuel-to-steam efficiency. The correct amount of excess air is determined from analyzing flue gas temperature and oxygen concentrations. The clearinghouse can help you better understand this process and the instrumentation needed. Please contact us for more information.
EE ACTION: Clean Boiler Waterside Heat Transfer Surfaces [2]
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% for water-tube boilers and up to 5% in fire-tube boilers.
EE ACTION: Minimize Boiler Blowdown [2]
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.
Moderate investment
EE ACTION: Inspect and Repair Steam Traps [2]
In steam systems that have not been maintained for 3 to 5 years, 15% to 30% of the installed steam traps may have failed—thus allowing live steam to escape into the condensate return system and significantly increasing the steam load. In systems with a regularly scheduled maintenance program, leaking traps should account for less than 5% of the trap population.
EE ACTION: Insulate Steam Distribution and Condensate Return Lines [2]
Uninsulated steam distribution and condensate return lines are a constant source of wasted energy. Insulation can typically reduce energy losses by 90% and help ensure proper steam pressure at plant equipment. Where possible, any surface over 120°F should be insulated, including boiler surfaces, steam and condensate return piping, and fittings.
Insulation frequently becomes damaged or is removed and never replaced during steam system repair. Damaged or wet insulation should be repaired or immediately replaced to avoid compromising the insulating value. Eliminate sources of moisture prior to insulation replacement. Causes of wet insulation include leaking valves, external pipe leaks, tube leaks, or leaks from adjacent equipment. After steam lines are insulated, changes in heat flows can influence other parts of the steam system.
EE ACTION: Return Condensate to the Boiler [2]
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, makeup 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 makeup water (50°F to 60°F) that must be heated.
EE ACTION: Recover Heat from Boiler Blowdown [2]
Heat can be recovered from boiler blowdown by using a heat exchanger to preheat boiler makeup water. Any boiler with continuous blowdown exceeding 5% of the steam rate is a good candidate for the introduction of blowdown waste heat recovery. Larger energy savings occur with high-pressure boilers. The following table shows the potential for heat recovery from boiler blowdown.
References
[1] Exerpts from Improving Steam System Performance: A Sourcebook for Industry, Industrial Technologies Program (U.S.), 2004, United States Department of Energy, Washington, D. C., pp. 108 (cited 9/13/10)
[2] Energy Tips – Steam Systems, Industrial Technologies Program (U.S.), United States Department of Energy, Washington, D. C., (cited 9/13/10). To find more tip sheets go to EERE Publication and Product Library.
