Is Your Boiler the Best It Can Be?
Few energy devices are as universally used in industrial plants as are boilers. Because they are seen as a mature technology, boilers often don’t receive the regular attention given other industrial equipment. Yet because it is the largest single energy consuming device in the plant, the industrial boiler should be at the center of interest in most energy efficiency improvement plans. At a minimum, you should bring your existing boiler plant up to date with today’s technology. It may be that a boiler replacement will pay for itself in short order.
A Major Energy User
The U.S. Department of Energy (DOE) suggests that plant owners begin by determining the efficiency of the steam generation system, based on steam energy output to fuel input. DOE says, “Steam generation needs to be measured with accurate, well maintained and calibrated flow measurement devices and reconciled with a rigorous steam balance.”
DOE also recommends that owners optimize excess combustion air to improve steam generation efficiency. An oft-stated rule of thumb is that boiler efficiency can be increased 1% for each 15% reduction in excess air or 40°F reduction in stack gas temperature. To achieve these improvements, better boiler controls are often needed. Older boilers typically have mechanical linkages that easily drift out of calibration, and are, under the best conditions, imprecise. Modern controls use digital sensing and direct-drive mechanisms to allow burners to meet tight performance standards and remain accurate, even with changing fuel, air and furnace conditions.
Keeping Boiler Surfaces Clean
Another critical aspect to boiler efficiency is maintaining clean fire-side and water-side boiler heat transfer surfaces. A good deposit control program is necessary to do this. It is important to perform boiler blowdowns regularly to reduce dissolved solids in the system. Follow boiler manufacturer and feedwater additives provider recommendations to optimize these blowdowns.
The significance of boiler water-side scale was emphasized in a presentation at a recent Energy Solutions Center’s Technology Marketing Assessment Forum by Jason Smith from Miura Boiler, one of the world’s largest boiler manufacturers. Smith noted that just a 1/32" inch deposit of scale on boiler tubes causes a 10% reduction in boiler efficiency. He further pointed out that such an increased level of scale on all U.S. boilers would cause a waste of $81 billion in fuel costs, and an additional 405 million excess tons of carbon dioxide emissions. The case for appropriate feedwater treatment and adequate blowdowns is obvious.
Newer boilers are designed to minimize fire-side deposits. One of the main advantages of natural gas is that the potential for these deposits is minimal to begin with. Optimizing fuel-air ratios further reduces fire-side problems.
Heat Recovery Essential Today
As fuel prices have increased, it has become increasingly important to recover energy that would otherwise be wasted. Economizers can be added in the boiler’s exhaust gas stream to recover heat that would otherwise be lost to the atmosphere. This heat can be used to preheat boiler makeup water or feedwater. Heat recovery from boiler blowdown equipment is also a practical way of recovering heat that would otherwise go down the drain. Downstream of the boiler, more energy can be conserved by adjusting or replacing steam traps, and by improving condensate return systems.
Condition of Existing Boiler Critical
According to Steve Connor, Marketing Services Director at Cleaver-Brooks, the decision whether to retrofit or replace a boiler can be complex. He points out, “The most important consideration is the condition of the existing boiler, especially the pressure vessel %%MDASSML%% shell, furnace and tubes.” He explains, “If during the annual inspection wherein the waterside and fireside surfaces are revealed, it doesn’t show signs of heavy scaling, pitting, cracking or stress, the pressure vessel is most probably in good shape and in a position to deliver many more years of dependable life.”
Summertime Presents a Challenge
Connor points out that often even if a boiler performs well through the efficiency analysis, there still may be losses due to improper sizing. He indicates that this often results from a boiler being oversized in summer months of the year, or at times when some process steam is needed but major industrial processes are shut down. “During these times, the boiler ‘loafs’ along in low-fire most of the time, cycling several times an hour. This is extremely inefficient operation and drives up radiation and convection losses as a percent of input while increasing excess air levels, reducing combustion efficiency.” In cases like this, the best solution may be the addition of a small “summertime” boiler, sized for these reduced load conditions.
Smith from Miura explains that the penalties from part-load operation can be significant, especially with older boilers with inadequate designs for high-turndown operation. Even the most modern boilers decline in efficiency in high-turndown situations. The solution may be multiple boilers. Smith notes that a group of smaller high-efficiency boilers with modern controls can operate as a single boiler with no turndown penalty, with boilers being quickly brought online as needed.
Boiler Cost Dwarfed by Fuel Cost
Where the existing boiler has reached the end of its useful life with poor pressure vessel conditions and overall poor performance, replacement is the logical step. Connor points out that boilers on average consume four times their cost in fuel every year, so selection of the replacement boiler needs to be well thought out. Industrial users must consider operational requirements, physical conditions of the facility, and financial values.
Hospitals, though actually institutional energy users rather than industrial users, face many of the same decisions as industry because their need for steam is continuous but variable throughout the day and throughout the year. Thus, their decision process is instructive. Rockville General Hospital in Connecticut is a 233,000 square foot facility that had recently gone through a consolidation process and was looking to improve its competitive position without adversely affecting patients. The hospital’s engineering director, John Lombardi, and HVAC engineer, Scott Roman, looked at improving the efficiency of the facility’s steam generation.
Changing Facility Requirements
Overall steam requirements had declined due to outsourcing of some of the facility’s laundry and sterilization needs. They chose to retain one of the hospital’s existing firetube boilers as a standby unit, and they supplemented this unit with a new Optimized Cleaver-Brooks firetube boiler with a high-turndown modulating burner with electronic controls and parallel positioning actuators for precise air and fuel metering.
The new unit also included an advanced stack economizer for heating feedwater and a second condensing stage for preheating cold makeup water. Because this second stage captures the latent energy in the flue gas, it helps the boiler offer up to 90% fuel-to-steam efficiency. In addition to boosting boiler efficiency from 75% or less to more than 90%, the new system delivers NOx emissions of less than 9 ppm and an annual reduction in CO2 emissions of approximately 600,000 lbs.
Making the Right Decision
Using this hospital as an example, Cleaver-Brooks’ Connor stresses, “Looking at a boiler’s in-service efficiency potential and coupling it with basic engineering knowledge will lead to the best solution for any process or heating application. It’s a matter of putting the right shoe on the right foot or, in Rockville’s case, putting the right boiler type, properly sized and configured, in the proper place to do the job.”
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