The AA Superheater Co. has invented, tested, and patented an annular superheater designed specifically for package firetube boilers. The superheater is designed to increase the steam temperature without increasing the steam pressure. Superheat temperatures up to 650°F (343°C) are possible, enabling continued use of the existing steam piping system.

Superheated steam contains greater energy than saturated steam of the same mass. This greater energy provides users with increased production and the potential for reduced fuel use and greenhouse gas emissions. These improvements also aid the user company’s environmental, social, and governance (ESG) rating. 

The AA Superheater uses modular, annular (a tube within a tube) superheater elements designed and arranged to fit the existing tube pattern used in the firetube boiler. This feature enables existing firetube package boilers and newly manufactured boilers to be converted to produce superheated steam using a simple conversion without any significant alteration to the boiler. The superheater elements fit directly into the boiler tubes selected for them.

The modular design makes it a drop-in installation that can be performed in a few days. It also simplifies future repairs, element replacement, and boiler tube maintenance because the superheater elements are accessible and easily removed or replaced. 

Since about 1900, the advantages of superheated steam have been well-known for increasing steam use efficiencies. Patent records are flush with ideas to provide superheat. However, most were miserable failures. In 1916, a process was invented to forge superheater return bends from two singular tubes that proved reliable for steam locomotive use. Within three years, most locomotives were leaving the factory with superheaters installed, and large locomotives built in the previous 20 years were being retrofitted. The superheating increased locomotive thermal efficiency by about 30%.

After World War II, the utilitarian concept of a “packaged” boiler became common. Uniformity and simplicity became the fundamental design forces. The locomotive-style superheaters were too complex for integration in common package firetube boilers. Attempts to install integral superheaters in package boilers failed. These failures reinforced the boiler industry’s opinion that integral superheaters are not practical in firetube package boilers.

The AA Superheater concept overcomes these design difficulties. Its annular configuration allows it to be accommodated in tube patterns of existing boilers. In addition, the design takes advantage of advancements in materials, combined with computer modeling, to overcome the problems that past superheaters experienced when installed into package boilers. 

FIGURE 2. The multiple flow paths of furnace gases and steam in the operating superheater.

Three of the AA Superheater design features are:

  1. The superheater’s elements are designed using annular steam flow. The heat flux is always from higher temperatures to lower temperatures in the direction of lower-temperature steam.
  2. The steam velocity flow rates are among the highest practical available to take advantage of high heat transfer rates. 
  3. The formed return end cap is designed using materials selected to handle high-velocity pressure and flow rates while providing high wear resistance for long service life at operating temperatures.

Description of the AA Superheater

The AA Superheater elements use an annular design in which an outer tube fits over an inner tube, forming an annular passage through which the steam passes. 

The superheater assembly that is installed in the boiler consists of several individual superheater elements. The superheater elements are sized to allow the high-temperature furnace combustion gas to flow over them as the gas passes through the boiler tube. 

Each inner and outer tube set is connected at its end by a formed return that transfers the steam from the outer tube to the inner tube. The formed return end is located opposite the boiler from both steam headers and close to the tube sheet. 

Not every boiler tube needs to be populated with a superheater element. Each 1 ½-inch outer diameter (OD) element is sized to superheat 10 boiler horsepower (BHP). A choke thimble is an optional item that would aid in balancing gas flow through the boiler tubes, should this be necessary.

At the superheater’s front end, located at the boiler’s burner end, the outer tube connects to the saturated steam header, and the inner tube connects to the superheated steam header. Both headers fit in the interior cavity formed by the removable boiler casing head at the burner end of the boiler and the boiler tube sheet. Both are shaped to connect to all superheater elements.

The steam flow path consists of the saturated steam being discharged from the boiler before entering into the saturated steam header and then the outer tube. The saturated steam flows through the annular space formed by the outer tube’s interior wall and the inner tube’s exterior wall. The saturated steam is heated by the high-temperature furnace combustion gas during this passage and becomes superheated. The superheated steam is then directed by the formed return end into the interior of the inner tube, where it flows back to the superheated steam header and is then discharged from the boiler through the superheated steam discharge pipe.

The temperature of the superheated steam depends on the geometry of the superheater installation and the boiler’s firing rate. If necessary, a water spray-type de-superheater is located on the steam supply pipe at the boiler exterior to control the superheated steam and prevent the superheated steam temperature from exceeding the maximum needed for the plant’s process equipment. 

Use and Benefits of Superheated Steam

The general purpose of steam in the industry is to convert the fuel energy to heat energy and then transport that heat energy safely and efficiently from the boiler to a distant point where the heat is used for a specific purpose. 

The purpose of a boiler superheater is to increase the heat energy of the steam by raising the temperature of the saturated steam that exits the boiler to a higher temperature while maintaining the same boiler pressure. This is accomplished using the additional heat energy present in the furnace combustion gases to heat and be absorbed by the steam. The superheating process converts the saturated steam into superheated steam, which can then be used by the plant’s process equipment. 

The advantage of using superheated steam in industrial processes is the superheated steam contains and transfers more heat energy in the same mass. This means that less water must be boiled to transfer the same heat from the boiler to the plant’s end-use equipment. For example, when 150-psi (1.034 MPa) steam is superheated to 650°F (343°C), 12.9% less water must be boiled to transmit the same heat energy.

FIGURE 3. Thermocouple leads used in instrumenting prototype superheater in testing.

In addition, the use of superheated steam provides faster heating in many industrial processes and consequently reduces the heating time required. This occurs because the heat flow across a boundary, such as an industrial machine’s heating surface, is not just faster with a higher heat differential but exponentially faster. Superheated steam heat can provide measurably shorter process times than saturated steam in industrial processes, such as cooking, laundry drying, chemical processing, etc.. It still takes the same number of Btu to complete the heating process, but the time period can be reduced by 10%-30%, producing cost savings and/or an increase in production. 

The main benefit of using superheated steam in the plant process equipment is that it aids plant production by reducing the heating time needed for many processes while increasing the heat flow rate across heating surface boundaries. In some cases, the time reduction can potentially reduce fuel use, thereby reducing greenhouse gas emissions.

Use of Superheated Steam to Generate In-House Plant Electricity

Perhaps the most effective and profitable application of equipping a firetube package boiler to produce superheated steam is the ability to generate and sell significant electricity with a fast return on investment. 

This is accomplished by the plant installing a steam turbine-electricity generator set. The superheated steam would first be supplied to the plant’s steam turbine-electricity generator, where it would generate electricity that would be supplied to the plant’s electric supply lines for its use. The turbine exhaust steam would then be discharged to the plant’s processing equipment. 

A 1,500 BHP (20,006 MHP) firetube package boiler designed to operate at 350 psi (2.41 MPa) can be equipped with a superheater making 750°F (399°C) steam. Steam at this pressure and temperature can then be sent through a one-megawatt steam turbine-generator set and will exhaust steam at 100 psi (0.69 MPa) and 570°F (299°C). 

FIGURE 4. Installation of a prototype superheater in a typical package boiler.

With no other alterations to the plant, except installing the high-pressure superheater equipped package boiler and the steam turbine-generator set, 1 megawatt (1 MW) of electricity can be generated in addition to the moderate superheated steam supplied to the plant as process steam. 

This installation provides the plant a rapid payback and a reliable source of electricity that could also be sold to a utility for profit during periods of low plant electricity demand.    

 Matt Austin is president of AA Superheater Co. Inc.