Improving steam system efficiency
The Heart of the Boiler
If you aren’t doing a complete and systematic boiler internal inspection at least once a year, you are missing another opportunity to keep the boiler in trim. Inspection by qualified personnel will reveal cracked or broken tubes, broken refractory material or corrosion on boiler tubes. If you are partially or completely firing with oil, wood waste or other fuels, then regular sootblowing and periodic combustion-side cleaning will be necessary.
Most gas-fired boilers need little periodic cleaning on the combustion side, but the water-steam side needs close watching. Tube inspection will note scale buildup from feedwater. Just an eighth-inch of scale can drop heat exchange efficiency by several percent. Inspection may reveal the need for changes in feedwater treatment.
Economizers, the Grand Opportunity
Possibly the greatest opportunity for significant plant energy saving is in boiler exhaust heat recovery with the right economizer system. An economizer is an exhaust gas-to-liquid heat exchanger downstream from the boiler that takes the last shot at capturing heat that would otherwise be discharged to the atmosphere. The liquid collecting the waste heat – usually water – can be boiler feedwater or makeup water, building potable hot water, process hot water, space heating water, or any process fluid that requires heating.
Combustion & Energy Systems, Ltd. Of Markham, Ontario is a major provider of combustion and heat recovery systems for industrial plants. Cameron Veitch from this firm was a recent presenter at a Technology & Market Assessment Forum sponsored by the Energy Solutions Center. He noted that economizers are a major opportunity for many industrial and institutional steam plants. His company designs and manages the installation of ConDex condensing economizers. These capture both the sensible and latent heat from a boiler exhaust and use it to heat water or other process fluids.
By passing the exhaust gas over water-cooled tubing, sensible heat as well as significant latent energy from combustion is captured. According to Veitch, the most common uses for the recovered energy is to heat boiler makeup water, district heating water or plant process water for various uses. He notes, “By using recovered energy to heat the water instead of using ‘live’ fuel, savings are found.”
Cool Water Needed
To capture energy from exhaust condensation, water temperatures of approximately 130° F or less are required. At higher temperatures, much of the sensible heat is captured but not the latent heat. Veitch emphasizes “The colder the water the better! This will maximize the fuel utilization efficiency of the heat input to the boiler.”
Where the operating efficiency of a modern non-condensing boiler without an economizer may be 80%, additional savings of more than 10% can be achieved with a condensing economizer. Veitch indicates, “Typical ConDex units result in a boiler efficiency of 90-97%. We seldom see anything below a 10% efficiency improvement in a condensing economizer application.” He adds that the most common payback for the systems is 1.5 years, sometimes less. He points out that most plant operators have a pretty good idea of where they are currently using the most live energy to heat water, and can target those uses for the recovered heat.
A Modular Approach
Another interesting approach to the economizer solution is the development of modular economizing units, such as the HeatSponge™ family of units, manufactured by Boilerroom Equipment, Inc. of Export, Pennsylvania. According to Vince Sands from that company, the modular approach to economizer units allows standardization of design and manufacturing efficiencies, along with the quality control that comes with shipping complete units rather than requiring site assembly. Multiple units can be installed to match the exhaust and cold water flow of the project. “The modular approach offers flexibility in system design, and quality in the manufacturing cycle because of repetition and our quality assurance program.”
Sands points out that there are places for both condensing and non-condensing economizers. “If you don’t have the cold water source, there’s no advantage to going with the condensing design.” He explains that because of the acidic character of the condensate, stainless steel is the best heat-exchange medium for condensing units. “But carbon steel is a better and more economical choice for non-condensing. We stress the importance of accurate information on the exhaust and cold water conditions so we can optimize the units for the site.”
Alternative to a Condensing Boiler
He notes that the payback with HeatSponge units can range from a few months to three years, rarely more. He also points out that the added efficiency from a condensing economizer can make conversion to a condensing boiler unnecessary. “We are accomplishing the same thing at a lower cost.
Boilerroom Equipment offers assistance with “Bruce”, a robotic on-line sales engineer, to help owners select a HeatSponge economizer and generate predicted performance, pricing, and even a proposal on-line within a matter of minutes. According to Sands, this feature provides access to many years of experience designing and optimizing heat recovery systems suitable for most industrial applications.
Time to Get Started
The key to boiler efficiency is optimizing combustion and letting the least possible amount of heat go out the exhaust stack. Beyond the boiler, paybacks on leaks, faulty insulation and condensate return improvements are remarkably short. Today’s natural gas being used as an industrial fuel is attractive economically, but not so inexpensive that you can afford to waste the energy you are buying. Beyond this, improvements in steam system and boiler efficiency will certainly reduce your carbon emissions. Don’t delay in putting your system at peak efficiency.
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In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.
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