Full steam ahead: Packaged boilers
Packaged boilers are factory-assembled and skid-mounted in most cases. This modularity enables quick installation and startup, which offers potential flexibility to end users as original equipment and as replacements. “Most of the steam boilers today are packaged boilers. Very few industrial boilers are field erected,” said Aqeel Zaidi, team lead, Industrial/Residential Technology at Enbridge Gas Distribution Inc. “The steam demand for some of industrial customers has been reduced due to changes in their operation. These facilities used to have large steam plants that would require various levels of full-time stationary engineers and attendants. Some of these plants are converting to smaller packaged boilers to reduce operating and maintenance costs.”
Packaged boiler types
While there are various types of small packaged equipment that produce steam, this article focuses on two primary types: watertube and traditional firetube boilers. In watertube boilers, water flows through the tubes, while combustion gases pass over the tubes. Consequently, the water volume is low, which means less startup time and quicker response to load changes compared with traditional firetube boilers.
In firetube boilers, the combustion gases flow inside the boiler tubes, while water is heated outside the tubes within the vessel, or shell side. Firetube boilers typically contain a large volume of water, and consequently take longer to produce steam than watertube boilers. However, the large water volume enables firetube boilers to respond to load changes with relatively little change in pressure or boiler cycling. “Firetube boilers often are characterized by how many times the flue gases turn inside the boiler before exiting,” Zaidi said. “Each pass sends the flue gases through the tubes in the opposite direction. To make another pass, the gases turn 180° and pass back through the shell. The higher the number of passes, the higher the efficiency. Three and four pass boilers are most common.”
“Although there is no official requirement for any steam generating equipment to be classified as a ‘steam generator,’ a general definition is one that states a steam generator has a once-through, forced flow design. This means water is pumped into the boiler as quickly as it is boiled off into steam,” said Doug MacMaster, senior vice president, U.S. Operations, Miura America Co. “In the case of Miura, water is contained in many vertical tubes, which are surrounded by flame and/or hot combustion gases. The heat is so intense, the water in the tubes boils and converts to steam very quickly. As that water leaves the upper header as steam, more water is pumped in to the lower header to replace it. When these low water content designs first appeared, traditional steam boiler manufacturers used the term ‘steam generator’ negatively in an attempt to classify a steam generator as a less robust or lower quality steam boiler. This is no longer the case. Each type of boiler design has a benefit and application that sets itself apart from other designs.”
The vertical design of the pressure vessel allows for a much smaller foot print. The EX boilers from Miura are some of the safest boilers in the world, according to MacMaster. The largest Miura watertube boiler, the EX300, holds a little more than 100 gallons of water in many small tubes, which are then surrounded by a triple-layer outer shell and casing. A pressure vessel failure in a single tube results in only a small steam leak.
Cleaver-Brooks manufactures both watertube (the FLX) and firetube (the ClearFire-H) boilers. Sean Lobdell, director of sales for Packaged Boilers at Cleaver-Brooks explains that firetube boilers typically have a larger footprint with a large effective heating surface and are capable of delivering large amounts of steam. He said that packaged watertube boilers have a smaller footprint than a firetube boiler and effective heat transfer, but lower water content. Watertube boilers have efficiency equal to that of a firetube if using an economizer. These units will provide a fast load response and can generate steam very quickly if needed.
The Modulatic from Vapor Power International is a once-through watertube boiler with a positive displacement pump that provides a constant feedwater supply. The fuel/air ratio curve changes the amount of fuel and combustion air in direct proportion to the flow of water being sent to the coils. According to Glenn A. Kuhlman, Midwest regional sales at Vapor Power International, 90% of the water is converted into steam in the coils. Dry steam is then produced in the steam separator. The water remaining is removed by a steam trap.
The Circulatic, also from Vapor Power International, is a forced recirculation watertube boiler that uses a drum as a steam separator and a reservoir of water to supply the coils. A recirculating pump draws the water from the drum and forces it through a set of parallel connected coils at the rate of three to four times the maximum steaming rate. The water is then pushed through a steam lance and a series of baffles in the drum where some of the water is flashed into steam and separated. The dry steam is released and the water is recirculated through the coils. The fuel and combustion air are controlled by a modulating motor that responds to steam pressure. The feedwater is controlled by a water level control system and modulating feedwater valve.
MacMaster contends that the low water volume of watertube boilers is safer than traditional pressure vessel type boilers. In a firetube (large volume) boiler, a sudden crack in the shell can cause the entire body of water to be subjected to a sudden and substantial drop in pressure. The huge volume of flash steam that results can cause an explosion of tremendous force. “In a single-pass-through boiler, similar to Miura’s technology, there are isolated tubes, which hold a certain volume of water,” said MacMaster. “If one tube fails, you will have the energy release from that single tube. The chance of all the tubes failing is highly unlikely. We contain those isolated tubes within an outer shell—the pressure vessel itself. There are two layers of protection. If a tube fails, the energy would be released within the outer shell. There is much lower flash steam volume in a single-pass watertube design.”
“Generally, four-pass firetube boilers have high efficiency, however watertube boilers can achieve similar efficiencies by installing a feedwater economizer,” said Zaidi. The ClearFire-H from Cleaver-Brooks is said to have efficiencies up to 85%.
MacMaster said that the efficiency of Miura boilers average in the mid-80% range. “When coupled with a condensing economizer, our overall efficiency increases to more than 90%,” he said. “This is because the small heating surface reduces radiant heat losses and rapid response to fluctuating steam demand reduces energy losses.”
“A steam generator [watertube boiler] can be left cold until just needed,” said Kuhlman. “This results in fuel savings, compared to a firetube that typically needs to be left on low fire. The most significant savings with a steam generator may be realized in applications in which all or part of the boiler operation is in a standby mode. Because of its relatively long startup time, firetube boilers generally will be kept in a ‘hot’ standby condition by maintaining low fire. This results in fuel consumption without the effective use of the energy produced.”
Often plant capacity can be increased while reducing the footprint of the system if replacing an older boiler and auxiliaries with package units. Vertical package boilers are especially economical in their use of floor space.
“Due to their compact size, it is easier to build a steam system using the modular approach,” MacMaster said. “This means the use of several smaller boilers, rather than one large boiler. When multiple units are tied into one system and controlled efficiently with a master controller, the N+1 requirement is met. For example, if a hospital requires 600 hp of steam, using a traditional firetube would require a second 600 hp boiler to be in standby mode just in case the main unit goes offline. With a modular approach, the N+1 requirement could be met by installing just 800 hp, or four 200 hp boilers instead of 1,200 hp with two 600 hp boilers.”
DOE tools available
The U.S. Department of Energy office of Energy Efficiency and Renewable Energy features the Steam System and Boiler MACT sites. Many manufacturing facilities can recapture energy by installing more efficient steam equipment and processes and applying energy management practices.
The Steam System site lists software tools, training, case studies, and publications to optimize performance and save energy. Tools to assess your energy system include “Steam System Modeler” and the “MEASUR Tool.”
DOE currently provides technical assistance on combined heat and power (CHP) technologies to commercial and industrial facilities through its seven regional CHP Technical Assistance Partnerships (CHP TAPs).
Starting in January 2013, DOE supplemented this effort by providing site-specific technical and cost assistance to the major source facilities affected by the Boiler Maximum Achievable Control Technology (Boiler MACT) rule. Through the CHP TAPs, DOE contacted more than 600 facilities with more than 1,500 affected boilers to discuss compliance strategies, as well as to provide information on potential funding and financing opportunities.
More than 50 sites are considering CHP after using DOE technical assistance resources. If all of these sites move forward with installing CHP, they would add more than 700 MW of CHP; three facilities alone are moving forward with 71 MW of CHP. More than 290 sites reported they are already in compliance with the rule. More than half of those sites reported they have switched to natural gas, while the remainder reported they have either changed their operating characteristics to avoid being a major source facility or are converting to biomass. Only 11 sites reported that they plan to install emissions controls to come into compliance and 76 sites have been permanently closed.
– This article appeared in the Gas Technology supplement.