Types of Boilers in Industrial Water Treatment
Boilers and steam production are essential to our modern lives. Boilers are used in almost every industry from generating electricity to heating buildings and everything in between. Due to the diverse demands for steam production, boilers come in various types and sizes.
Firetube boilers are the most common type boilers that we see in industry. In a firetube boiler, hot combustion gases pass through the boiler tubes, and water is on the outside of the tubes. The hot combustion gases usually make 3 to 4 passes through the vessel before being discharged through the flue stack. The heat from combustion is transferred through the walls of the tubes by thermal conduction, convection, and radiation heating the water and ultimately creating steam. Firetube boilers typically have a maximum capacity of 2,000 horsepower [for reference 1 boiler horse power (BHP) will generate 34.5 pounds of steam per hour]. Typically firetube boilers do not exceed a pressure of 350 psig.
Watertube boilers are also common in large industries such as power generation, petrochemical, and oil refining. In a watertube boiler, the water circulates inside of the tubes and is heated by the combustion gases on the outside. The fuel is burned in the furnace section of the boiler. Watertube boilers can utilize natural gas, coal, biomass and other solid products as a fuel source. As the water is heated in the tubes, the heated water travels up the riser tubes (also called generating tubes) to the steam drum. In the steam drum, the steam is drawn off the top, separated from the water and sent into the process. Cooler water at the bottom of the steam drum returns to the mud drum via downcomer tubes. Feedwater is injected into the steam drum to be further heated up to steam pressure and temperature.
Watertube boilers are often rated in pounds per hour of steam produced as opposed to horse power. Watertube boilers may be rated up to pressures of 3,000 psig and can have a steaming capacity up to 250,000 pounds per hour. Watertube boilers operating at high pressures may require advanced pretreatment methods such as reverse osmosis and demineralization. Steam that is used to drive turbines may be superheated to ensure no moisture damages the turbine blades.
Most package watertube boilers produce 30,000 – 100,000 pounds per hour of steam and operate at less than 300 psig. Larger units may need to be field erected. Watertube boilers are available in many different configurations such as D-Type, A-Type and O-Type. Due to the design characteristics of watertube boilers, it is critical that the boilers have consistent soft water feed to prevent deposit formation on the heat exchange surfaces. If deposits are found in the boiler tubes, it is typically necessary that the fouled tubes be water blasted or rattled to remove the deposits. Under most circumstances, severely scaled watertube boilers should not be cleaned “on-line”. Deposits rarely dissolve back into the boiler water where they can be safely evacuated from the boiler through bottom blow downs. Deposits usually come off in chunks and pieces and can plug downcomer tubes. Plugged tubes may lose water flow causing them to fail from thermal stress.
Another common type of boiler we see in industry is the Hurst Hybrid boiler. A Hybrid boiler combines portions of a watertube boiler and a firetube boiler providing the best qualities of both. Hybrid boilers are commonly used in saw mills and other plants that have an abundance of biomass or hog fuel for an inexpensive fuel source.
The watertube portion of the hybrid boiler is similar to an A-Type boiler. The tubes in the watertube portion of the boiler are often referred to as “Chapel Tubes”. The water is heated from the biomass fuels being burned in the furnace section of the boiler and travels up the Chapel tubes. The heated water from the watertube portion of the boiler is directed to the body of the firetube section while the steam is drawn off the top drum into the steam space of the firetube portion of the boiler. The exhaust gases from the furnace are then directed into the tubes of the firetube portion of the hybrid boiler for continued heating and steam production for distribution to the process.
Consistent soft water feed is critical to keeping clean heat exchange surfaces in hybrid boilers.
Vertical boilers are gaining popularity due to their small foot print, low initial startup cost, and rapid steaming rates. Vertical boilers are commonly found in small hospitals, surgery centers, and bakeries. Vertical boilers usually run at a maximum pressure of 15 psig. and usually do not exceed a 50 horsepower steaming capacity. Due to the small steam space of vertical boilers, they are prone to carry over. In order to protect the vertical boiler from oxygen attack, elevated oxygen scavenger residuals are required. Due to single tube design of vertical boilers, it is critical to avoid precipitating internal treatments. Polymer treatment programs must be utilized. Consistent soft water feed is critical to keeping clean heat exchange surfaces in vertical boilers.
As a global leader in custom-designed water treatment programs, Chem-Aqua has the experience, knowledge, and technology to effectively treat any type of boiler. Since 1919, our success has been built upon our Total System Approach providing solutions for water treatment problems and improving water system efficiencies. To learn more, contact Chem-Aqua today!