Improving steam system efficiency
Steam system efficiency sometimes doesn’t get the attention it deserves. Perhaps it’s because the boiler plant is hidden in a lower level mechanical room. Perhaps it’s because today’s gas-fired steam systems run unobtrusively, without the need for constant attendance. But these systems are often the largest energy user in industrial facilities and deserve at least as much attention as lighting or window leaks.
Natural gas-fired boilers are clean and have high inherent efficiency because of the ideal nature of the fuel. Yet a systematic approach to saving energy at the boiler plant and through the system can yield significant additional energy savings, not to mention improved reliability and lowered greenhouse gas emissions. A major improvement can often be made by addition of a boiler economizer. Starting with the most basic, what steps should be taken to make your steam plant more efficient?
Find the Leaks
A first step is to find steam and condensate leaks. Puddles on the mechanical room floor or elsewhere in the plant are often signs of leaky steam traps or valve packing, or problems in the condensate return system. You don’t need special equipment to find these problem areas. Trace them to the source and fix them. Hot spots along steam and condensate lines are also obvious indications of either steam leaks or inadequate insulation.
If you have a trained infrared sensor team and the right equipment, they can find these hot spots. Alternatively, help is available from infrared specialists. A single large uninsulated valve could be costing your company hundreds or even thousands of energy budget dollars each year. Custom insulation companies make removable
covers for pipes and fittings that are efficient and easy to replace after service work.
Condensate Return Systems
In older plants, condensate return lines were often incomplete and these are seldom adequately inspected or kept in repair. With today’s tight energy budgets, this is unacceptable. You need to study the entire condensate system and extend return lines throughout the plant. Just as importantly, plan to inspect the entire system with regularity. Failure rates of steam traps and valve packings make such systematic inspection essential. By taking advantage of improved condensate return you not only salvage the heat from the condensate, but also reduce your costs for makeup water treatment and feedwater pre-heating.
Boiler Blowdown Heat Recovery
If you are not currently salvaging waste heat from your boiler blowdown system, this should be considered. Blowdown heat recovery is particularly effective for higher-pressure industrial boilers. Recovered heat is often used for preheating makeup water, feedwater, or for building potable water systems. Package heat recovery systems are available for a wide range of blowdown systems sizes. This chart illustrates potential heat recovery values.
Boiler Blowdown Heat Recovery Potential
Boiler Burner Review
Even if your boiler is in excellent condition, it can’t perform efficiently without a high-efficiency burner in peak operating trim. Some owners are improving the efficiency of their boiler by replacing older burners that have slack mechanical controls with newer models that feature precise digital controls and fuel-air mixture feedback devices. Not only does this reduce the need for constant combustion recalibration, but it also permits the boiler to operate with an optimum mixture at all firing levels.
You’ll probably want an industrial boiler specialist to evaluate the burner-boiler operation. Provide this consultant with a complete profile of your varying steam and hot water requirements throughout the week and throughout the year. The payback on a burner replacement can be remarkably short – sometimes just a few months. Don’t postpone this decision.
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.