In the economizer system that incorporates the TMC technology, sensible heat, latent heat and water are all recovered. The efficiency gain can be up to 15 percent with as little as 25 percent cold makeup flow.
A study conducted in 2005 showed that at that time, more than 162,000 industrial and commercial boilers were in operation throughout the United States. Of those boilers, roughly 15 percent were generating steam in the 10 million BTU to 100 million BTU range - many in process applications.
Fast forward to 2011. Much of the installed boiler inventory in this particular segment is more than 25 years old. This includes natural circulation firetube boilers - the most prevalent in this category - watertube boilers and forced circulation steam generators. These systems primarily are fired by natural gas although light oil often is used as an alternate or backup fuel.
Many boiler designs exist to meet a customer’s needs - insofar as quantity of steam output and pressure, desired fuel and operating characteristics. However, the basic design of these boilers is standardized from the standpoint of:
- The actual combustion process.
- Heat transfer surface per horsepower.
- General shell-and-tube configurations.
- Basic controls and safety features.
These are mostly complete packaged systems, factory assembled and tested, having fuel-to-steam efficiencies in the low 80 percent range.
While the basic boiler itself has not changed much, there have been a number of technological advances in the overall system over the last 20 years. End-users have asked for - and received:
- Higher efficiencies.
- Lower emissions.
- Ease of maintenance.
- Greater safety.
- Less operator intervention.
- Longer useful life.
- Best use of available space.
In response to this wish list, boiler and auxiliary equipment manufacturers improved existing technologies and developed new ones. Many of these improvements can be applied to a large number of the existing installed boiler base. To take full advantage of these improvements though, in some cases, a complete boiler replacement may make more sense and provide a respectable return on the investment.
What can you expect to find in a modern boiler setup? Consider these common boiler design improvements.
Current firetube boiler designs are optimized for peak efficiency vs. cost to manufacture. One improvement has been the use of extended-surface boiler tubes, which are usually internally ribbed. This internal extended surface produces a tube with a heat transfer rate significantly greater than that of a plain tube. The use of this type of tubing reduces the overall size requirement for a boiler and can result in reduced boiler cost. Replacing existing boiler tubes with ribbed tubes can increase the output of existing equipment - without a corresponding increase in fuel cost - or reduce fuel requirements at the current load.
Excess air is required for the safe, complete combustion of fuels. Too much excess air results in a loss of combustion efficiency while not enough leads to incomplete combustion and safety issues. Proper control of the fuel-to-air ratio is the key to optimizing combustion efficiency.
The most common system of control has been the use of a single rotating shaft and mechanical linkages operating the fuel valve and combustion air damper in tandem (single-point positioning). This system is set initially at startup and usually is adjusted once or twice a year thereafter. Normally, the setup is performed to reach a 5 percent to 7 percent O2 reading at the stack to compensate for atmospheric changes and mechanical variances in the linkage system that normally occur between tuneups. Although the amount of excess air is higher than what is required for the most efficient combustion, in order to remain at safe levels without constant monitoring and adjustment, there is a built-in efficiency loss.
The use of microprocessor-based controls combined with an O2 analyzer and transmitter in the flue gas, individual servo or stepper motors directly controlling the gas valve and air damper (parallel positioning), or a variable-speed drive controlling fan speed, have allowed owners to comfortably run their boilers with as low as a 2 percent O2 reading at the stack. The parallel-positioning setup gives owners confidence that the air-to-fuel ratio is constantly monitored and adjusted for optimum efficiency and safety. By using this method of control to minimize excess air, most users can expect a 2 percent to 3 percent increase in efficiency.
Low NOX Burners
To meet the ever-tightening clean air regulations, boiler and burner manufacturers have been required to develop technology that will allow them to provide a package with the lowest NOX emissions possible - without post-combustion treatment. Several generations of low NOX designs - including steam injection, flue gas recirculation, premixing, two-stage combustion and fiber mat combustion head - have allowed manufacturers to now supply equipment with single digit NOX emissions (on natural gas), with some claiming as low as 5 ppm.
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Most new industrial boilers are said to be somewhere in the low 80 percent range in fuel-to steam-efficiency, meaning anywhere from 16 percent to 20 percent of the heat input is lost up the stack. Traditional stack economizers, in various forms, have been installed in boiler stacks almost as long as there have been boilers. Most modern economizers consist of a series of extended surface tubes installed in the boiler flue gas stream. Warm boiler feedwater is circulated through the tubes, where wasted heat is exchanged from the flue gas to the feedwater, which is then fed directly to the boiler. The conventional stack economizer will provide about a 2 percent to 4 percent gain in efficiency.
More recently, with improvements in design and materials selection, condensing economizers are more in demand. These units are similar to a conventional economizer; however, by circulating cold water through the tubes, using boiler makeup water, the stack temperature can be lowered enough to condense water vapor in the flue gas. This allows the unit to recover both sensible heat and latent heat.
These units are usually only found on natural gas-fired boilers, many times in combination with a conventional economizer in the same housing. They usually require all stainless steel construction to prevent corrosion from the condensate. While the initial investment can be substantially higher than a conventional unit, it is possible to gain up to an additional 10 percent in efficiency.
There are many variations of the condensing economizer, particularly in off-boiler applications where cold process water, in much greater volume than boiler makeup, is the heat sink. Typically, these are floor- or skid-mounted units consisting of a single- or multiple-stage economizer and include an induced-draft fan with controls to draw hot flue gas as needed from the boiler stack. These systems are invisible to the boiler and burner operation and can be turned on and off to meet demand. Generally, with large volumes of very cold water, the recovery rate can be calculated to an equivalent of greater than a 10 percent increase in efficiency.
Another development in boiler heat recovery incorporates both conventional and condensing economizers as well as a patented heat exchanger called a Transport Membrane Condenser (TMC). The TMC was invented by Gas Technology Institute, Des Plaines, Ill., as a part of the U.S. Department of Energy’s (DOE) Super Boiler program.
In the economizer system that incorporates the TMC technology, the TMC section utilizes porous ceramic tubes coated with selective membranes. When cold makeup water is passed through the tubes, a capillary condensation occurs within the tube wall. The makeup water is kept under negative pressure, allowing this warm condensate to be added to the boiler water stream.
With this economizer system, sensible heat, latent heat and water are all recovered. The efficiency gain can be up to 15 percent with as little as 25 percent cold makeup flow. The design allows nearly any boiler system to take the greatest advantage of condensing without the need for a large cold water process flow as a heat sink. TMC technology can be retrofitted on any clean fuel boiler to raise the efficiency to as much as 95 percent fuel-to-steam efficiency while allowing the return clean water to the system.
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Currently, there are many oversized boiler systems operating in the United States. Oversized boilers can lead to great operational inefficiencies. Generally, boilers become less efficient when operated at less than their maximum rating. Of particular concern is when a larger boiler that is not designed to cycle is required to turn on and off in response to steam load. The loss of efficiency due to oversizing can make a system using boilers rated at greater than 80 percent efficiency actually produce steam with a system operational fuel-to-steam efficiency as low as 50 percent. Even a “right sized” boiler system will operate at an efficiency less than its rating unless operated at or near its capacity most of the time.
Systems utilizing multiples of small footprint, low water content, forced circulation steam generators are gaining in popularity. In this type of system, the boilers are designed to turn on and off, via microprocessor-based control, in response to steam load demand. These boilers are designed to be unaffected by the thermal shock associated with boiler cycling, and because of the small physical size and low water content, normal cycling losses are minimized. When set up properly, a modular system will operate at or near its reported efficiency throughout its operating range.
Reliable performance also is a factor in overall efficiency. Poorly performing equipment not only wastes fuel but impacts maintenance and service budgets as well. Some boiler manufacturer offer remote monitoring services, where each installed boiler is connected directly to a computer monitoring system at the manufacturer’s offices. The system sends continuous data on burner history, fault history, fuel consumption, scale buildup, and a number of other items. The manufacturer monitors each boiler and reports to the owner immediately when a caution occurs. This allows the owner to respond to issues before they become major. Also, a monthly report may be submitted, summarizing key boiler operating parameters and offering recommendations to assist the owner in keeping boilers at peak operating condition.