Package Boilers: Save Energy with Brick, Refractory, Insulation and Lagging
Many medical, industrial, college, and government facilities are energy sufficient and generate all or a very large portion of their own electricity or steam for heating. Most of these facilities use a small steam-generating boiler called a "package boiler." A package boiler is a pre-engineered steam-generating boiler with different ranges in size and steam capacity (typically from 10,000 to 200,000 pounds/hour). For these facilities to save energy, they must understand their package boiler design and the important role that brick, refractory, insulation and lagging have in that design.
Though brick, refractory, insulation, and lagging (bril) may be one of the smallest of the components found at a medical, industrial, college, or government facility, it pays to pay attention to your bril. The bril found on these package boilers are key components of the boiler for energy savings and are necessary for personnel protection, required for heat conservation, and vital for efficient boiler operation. A package boiler is operating efficiently only when it’s using the least amount of fuel required to meet its operating conditions.
The bril materials are key components of these package boilers. The bril, just like the tubes that carry the water and/or steam, the soot blowers that keep the unit free of fly ash or dust, and the burners that burn fuel, help keep the package boiler operating in a thermally and energy-efficient manner. The bril used on these package boilers aren’t only necessary for safety and heat conservation, but are also considered a key component of the boiler design for optimizing energy savings. When a package boiler is operating efficiently it will be using the least amount of fuel (energy savings) required to meet its operating conditions.
It’s helpful to understand a little about the package boiler history and design. There are many types of package boilers designed over the past 50 years with many different names. Some of the more common still found in operation are the FM-D series by Babcock & Wilcox, the "A" series by Combustion Engineering (now called ABB), the AG series by Foster Wheeler, the MH series by Riley Stoker (now called Babcock Borsig), the Keystone M series by Zurn/Erie City, the Nebraska package boiler and the Keeler Company DS package boilers.
The package boiler design has been evolving since the late 1940s. The industrial market and the demand for package boilers haven’t stopped growing. This is mainly because they come completely shop-assembled and can be moved from one location to another. It’s difficult to move them but it can be done. They can fit in very small areas of a facility and they can generate a lot of heat and power when maintained and operated properly.
The D-type Package Boiler
The Babcock & Wilcox Company developed the first package boiler in the late 1940s. This package boiler design was developed from their field assembled "F" series boilers (F, FF, FH, FL, FP, and FO). The FO is the actual forerunner to the FM design.
The D-type shop-assemble boiler (i.e. Foster Wheeler AG and the Babcock & Wilcox FM) has a D-shaped furnace area on one side, with the boiler bank on the other, and it utilizes two drums. A super heater bank might be added for higher capacity units. The boiler is fired parallel to the drums toward the rear wall where the gas turns 180 degrees and goes toward the gas outlet.
The front walls of the D-shaped furnace cavity are usually either all brick or all refractory. The front wall is also called the burner wall because the burners are located there. For example, the Babcock & Wilcox FM boilers used 9-inch thick brick walls backed with at least 1-inch thick insulation. Originally this material contained asbestos, but now it would be a mineral wool board class 5 meeting American Society for Testing and Materials (ASTM) C-612. The Foster Wheeler AG series burner wall has a 12-inch thick refractory design and uses a combination of insulating castable and medium weight type castable. When these burner walls were installed in the shop they were usually laid flat on the floor. The whole wall, including the refractory cast burner throat area or tile lined throats, were raised into position by the use of cranes and pulleys. To replace a cast throat design for a burner you will have to change the material from a castable type to a plastic type because it would be impossible to cast a burner throat in a vertically positioned wall.
The rear wall of the furnace area of these D-type boilers started out as either all brick or refractory. These walls ranged from 9 inches thick to 12 inches thick and used a combination of materials (brick, tile, insulation, insulating castable, medium weight castable) to achieve its design thickness and temperature requirements. Later these walls became flat studded or membrane tube walls and required refractory only to seal gaps and openings.
All outer exposed boiler and furnace walls for the D-type boilers were insulated with a fiberglass blanket type material with an outer casing design. Lagging partially replaced the outer casing on many of the larger size units by 1967 and was used around the middle band section of the boilers. The insulation thickness ranged from 2-1/2 inches to 4 inches thick depending on the pressure of the boilers. Boilers up to 700 psi usually had 2-1/2 inch or 3 inch thick insulation and boilers with 701 psi and up had 4-inch thick insulation.
A high duty type brick or tile with a Pyrometric Cone of Equivalent (PCE) of 31-32 was used on the floor inside the furnace area to protect the tubes. Refractory was used to seal the boiler and furnace areas to prevent gas leakage and to protect the super heater headers.
The A-type Package Boiler
The A-type package boiler [i.e. Combustion Engineering (ABB) Company "A- series" and Riley Stoker (Babcock Borsig) MH series] has the shape like a capital "A." Tubes forming the "A" design surround the furnace cavity. The boiler is fired parallel to the tube walls toward the rear wall where the gas turns 180 degrees on both sides and exits out at the gas outlets.
Combustion Engineering (now ABB) developed its A-type package boiler in the 1950s. It was designed to improve package boiler reliability and to reduce the tube replacement cost. The FM design by Babcock & Wilcox had a thinner tube wall than the Combustion Engineering "A" type tube walls. This "A" boiler series had welded tube panels and utilized an outer casing design. The end walls, burner wall and rear wall were an all-refractory castable type design. An insulating and medium weight refractory was used in combination to achieve its design thickness and temperature requirements. No tile or brick was required on the floor of this boiler. Refractory was used in all other areas of this boiler design for sealing gaps to provide maximum heat conservation.
The Riley Stoker (Babcock Borsig) MH series was very similar to the Combustion Engineering (ABB) Boiler except that it used a brick and tile construction for the front and rear walls of the furnace area. These walls backed their brick or tile construction with 4-inch thick insulating board on the front wall and 5-inch thick insulating board on the rear wall. A high duty type square edge tile with a PCE of 31-32 was used on the floor inside the furnace area to protect the tubes.
For either the Combustion Engineering (ABB) or Riley Stoker (Babcock Borsig) package boiler, refractory was used to seal the boiler and furnace areas to prevent gas leakage and to protect the super heater headers. All exposed boiler and furnace walls for the A-type boilers were insulated with a mineral wool board or blanket type material 1-1/2 inch to 2-1/2 inch thick and utilized an outer casing design. The Riley Stoker (Babcock Borsig) MH series also used two layers of 1/4 inch thick asbestos millboard to backup their mineral wool board insulation.
The O-type Package Boiler
The O-type boiler was also introduced to the power generating industrial market in the 1950s. This type of boiler had a two-drum design. The tubes form the "O" and surround the furnace cavity. Just like the A-boilers, this type of boiler is fired parallel to the tube walls toward the rear wall where the gas turns 180 degrees on both sides and exits at the gas outlets.
This package boiler design was manufactured by Erie City/Zurn Boiler Company. The O-type boiler, like the "D" and "A," came in a variety of sizes and steam capacities. Brick, refractory, and insulation were necessary in all areas of this uniquely shaped boiler.
The Erie City/Zurn Boiler design was called their M series or Keystone (probably because they were manufactured in the state of Pennsylvania). The M series used a combination of high duty tile with a PCE of 31-32 and insulating castable on the rear wall and castable refractory and insulating firebrick on the burner or front wall area. The floor area inside the furnace was lined with a high duty tile. Refractory was used in all other areas of this boiler design, as with the D and A boilers, for sealing gaps to provide maximum heat conservation. The boiler and furnace walls were either a membrane or seal welded tube construction and utilized a 2-1/2 inch thick mineral wool board and outer casing construction to meet its temperature requirements.
Mobile Package Boilers
Another type of package boiler is the modulatic industrial boiler design. This type of package boiler is the most mobile of all because it comes on a skid. It may appear to be quite different from the "D," "A," and "O" type package boilers. However, the fundamental similarity is refractory and insulation. Refractory and insulation are still very important for this small package boiler to be thermally and energy efficient. An insulating castable or a high temperature insulation is required between the two shell casings to keep this package boiler thermally and energy efficient.
Brick, Refractory, Insulation and Lagging: Saving Energy
The drawings and schematics of all of these types of package boilers show the brick, refractory, insulation, and/or lagging. Lagging is only found on the very largest of package boilers around the middle of the units. Bril is found in all areas of the boiler and furnace. The manufacturing companies knew from the very beginning that the proper design and installation procedures for their package boilers were critical.
Each boiler design has unique characteristics with the proper calculation of the bril materials to keep their installation costs down in order to be competitive on the market. On a long-term investment the package boiler owner knows the bril materials will keep the boiler operating at peak energy efficiency. With maintenance scheduling and using proper material selection and installation procedures, the boiler will be energy efficient. Only proper upkeep will minimize the amount of heat loss that radiates from the outer casings or lagging surfaces.
Since these boilers are shop assembled it’s critical to know how to repair them. Knowledge of the bril designs is necessary if you intend to fix or maintain them.
Brick and Tile
The firebrick or tile used in the package boilers are either a high duty or super duty quality and will be classified or specified by the PCE testing number referred to in ASTM C-64 method C-24. An insulating type firebrick will be classified by its temperature limits. A high duty firebrick will have a PCE of 31-32 and a super duty firebrick will be 33-34. Firebricks (high duty or super duty quality) are used for the brick walls, for baffling of gas flows inside the furnace, or for protecting the floor tubes. Insulating firebrick will come in temperature ranges from 1,800 degrees Fahrenheit (F) to 3,000 degrees F and the choice will depend on the area of usage.
Brick or tile (high duty, super duty or insulating) will always be laid-up with an air setting mortar unless it’s used on a floor in two or more staggered layers. It will then be laid dry. The brick or tile is dipped in mortar slurry that has been thinned with water to a gravy-like consistency, shaken to remove excess mortar, and then tapped into place with a mallet. Such joints are usually between 1/16 inches and 3/32 inches thick. Diamond saws are used to cut brick to fit exactly. Bricks are bedded in mortar for two purposes: to cause the bricks to adhere to each other and to distribute the pressure uniformly over the whole bed where the bricks are irregular. Great care should be taken that both the bed and the cross-joints are thoroughly filled with mortar.
There are three basic types of refractory (dense, medium and lightweight). All three types use the same common chemicals, yet each will vary considerably to meet their specific use requirement. Some of the most common chemicals for refractory materials used in the boiler industry today are alumina, silica, ferric oxide, titanium oxide, calcium oxide, magnesium oxide, and alkalies. Any and all of these basic refractory materials are used to prevent gas and fire from escaping from the boiler-furnace.
Once you have selected a material based on the operating conditions to which the refractory material is to be exposed, you must consider the installation of the material before your final selection is made. Each refractory material has some unique qualities and some refractory material can only be installed in one particular way. Before you finalize your material selection, it’s important to pay attention to the material installation. Avoid trying to use a single castable for all types of service. There’s no universal castable. When replacing old refractory material, it may be a mistake to automatically use the same material as the original. It’s better to examine the reasons for failure and adjust the selection accordingly. Ask yourself: Did the material spall due to thermal shock? Has it shrunk due to temperatures above its use limit? That gouge may indicate mechanical abuse. If the surface appears "glassy," this may be due to operation at temperatures above the use limit. The old lining may offer several good clues.
Most problems with refractory materials can be traced to improper mixing, curing, or drying. Attention to the five following points, however, should produce a serviceable lining.
Amount of Water
The right amount of water is essential. True, a wetter mix handles more easily, but it robs the refractory material of its needed strength. On the other hand, if the mix is too dry, it’s difficult to place, and it may set to a weak, porous, "popcorn" structure. A proper mix will usually seem on the thick side compared with conventional concrete. One good guide to follow when troweling refractory is the "ball-in-hand" test. Make a small ball of castable and toss it 12 inches into the air. If it breaks apart when it lands on your flat palm, it’s too dry. If it flattens out, it’s too wet. The ball should retain its size and approximate shape for the mix to be right.
Type of Water
Many common industrial compounds can easily contaminate a refractory mix, and seriously affect its end properties. Certain salts, for example, react with the binder to make it useless. Be sure to use clean water, clean mixing and handling equipment and clean forms. Also, it’s best to use potable water because it’s free of the minerals normally found in tap water. Those minerals could affect the ability of the castable from reaching its proper strength.
Though mixing can be done by hand or in concrete mixers, mortar mixers usually give the best results. They are geared to handle fairly thick mixtures. Hand mixing and concrete mixers often require excess water. On big jobs, use two or three mixers to provide a continuous supply of fresh castable. Refractory materials all have a "pot life." Pot life refers to the amount of time after the material is mixed that it’s still good to use at a location. This will vary from castable to castable and can be anywhere from 20 minutes to 60 minutes. Three or four minutes mixing time should be plenty with a mechanical mixer to insure a uniform and homogeneous mixture. The high iron aluminous cements and pure calcium aluminate cements generally have considerably faster setting time than those of calcium silicate (Portland) cements. Over mixing tends to speed up the setting rate and weaken the refractory material. Remixing should never be done.
Cold weather will adversely affect the strength of the refractory if the dry material used is in the freezing range of temperatures, and if mixed with cold water. It’s desirable that both the dry material and the water be in a temperature range of 60 degrees F to 70 degrees F if maximum strength is a consideration. Provisions should be made to maintain a minimum ambient air temperature of 50 degrees F when placing refractory materials. At the same time, any steel that will come into contact with the refractory should be maintained at a temperature not lower than 50 degrees F. The freshly installed castable should be protected against freezing for a minimum of 48 hours or until thoroughly dried. When the temperature is below 50 degree F, the maximum strength of the material can be improved by heating the mixing water.
Curing and Drying
It’s only after the refractory has been cured and dried will the refractory be capable of doing what it was designed to do.
Curing is the process of keeping the refractory material wet or the surrounding atmosphere humid for a period of at least 24 hours after installation. The primary purpose is to create the most favorable conditions for the completion of the chemical reactions of the cement. Curing results in improved strength.
Dry out is the process of drying out the cured refractory material by the use of heat. The dry out process is very important in the refractory application to assure that the refractory reaches its full strength. Unlike the curing of refractory, which is done right after the installation (by the installing contractor), the dry out is done later with no set time limit when it has to be done. However, this doesn’t apply to phosphate-bonded refractory materials, since a phosphate-bonded material must be cured and dried at the same time. A phosphate-bonded material must be cured and dried within the first two to three weeks after installation because a phosphate-bonded material will begin to absorb moisture from the surrounding atmosphere. Eventually, over a period of two or three weeks the material will begin to slump and fall off.
Close attention should be made to the boiler design and the type of insulation used. Some package boilers with an outer casing were designed to compress the insulation (i.e. fiberglass 4 inches thick compressed to 3 inches thick). Knowing this before you substituted a different insulation material for the fiberglass (say a mineral wool type), you would have to run a thermal calculation program, check the thickness of the new insulation so that it fits into the space between the tubes and the outer casing, and check the compressibility of the new type of insulation if maintaining the same original thickness.
The outer casing or the lagging is designed for certain temperature limits and was based on a specific design criteria. It will be important to know those original design requirements (i.e. outer surface temperature, saturated water temperature, external wind velocity, emissivity factor, ambient air temperature) before any change of the insulating material is used.
Where outer lagging is required on a package boiler it will be attached to a support or structural system by the use of fasteners. Fasteners for attaching lagging should be at the least a #14 stainless steel screws zinc plated, self tapping and/or self-drilling with a weather seal neoprene washer. Sheet metal screws should be applied on every other rib, regardless of the material involved (steel or aluminum). On flat sheets, sheet metal screws should be applied on no greater than 2-foot vertical by 3-foot horizontal centers. Common sense should prevail in the use of screws. Excessive numbers are costly and detract from appearance. The lagging screws are installed properly when they are pulled down (screwed) tight. A screw is considered "loose" when you can wiggle the washer with your fingertips.
Health and Safety
When removing or installing brick and refractory, be aware of the chemical composition of the existing materials. Some refractory materials used contained chromium compounds as part of the refractory mixture. During operation some of the chromium compounds would be converted into hexavalent chromium (CR+6). What this means is that the refractory material, when initially installed, didn’t represent a health problem. However, during the operation, some of the chromium compounds may be converted to CR+6. Therefore, when the refractory material is removed, the dust created may transport the hexavalent chromium. Inhaling the CR+6 increases the risk of lung cancer and may also cause other health hazards.
Brick or tile often contain crystalline silica. The Environmental Protection Agency found that crystalline silica, when converted to dust, presents a potential health hazard if inhaled over a period of years. During brick installation the dust, called silica dust, is created by the use of power saws when cutting the bricks. Silica dust is a serious and potentially fatal health threat. To prevent this, one should use, wherever possible, wet saws to cut the brick. Also, respirators should be used and ideally, exhaust fans should be installed for proper air ventilation.
Insulation, like refractory and brick, require special handling during removal or installation. Always check the material safety data sheets before installing or handling insulation material. Any insulation material that contains crystalline silica greater than 0.1 percent by weight requires a cancer warning. The fibers that make up any glassy or vitreous filaments are extremely sharp and can cause skin and upper respiratory irritation. The skin irritation can be caused if the broken ends of the ceramic fibers become embedded in the skin. Breathing dust from such products and materials may cause lung damage or an upper respiratory irritation. The upper respiratory irritation is a reaction by your body to the sharp ends of the broken fibers.
In years past, insulation was manufactured using an asbestos base material. Asbestos insulation materials have been classified as a carcinogen. Special attention and careful removal practices must be adhered to for health and safety reasons. Your maintenance, purchasing, and supplier records, along with your original insulation specifications, should be reviewed to determine whether and/or where the asbestos containing products were used.
Brick, refractory, insulation, and lagging, when designed and installed properly on a package boiler used in a medical, industrial, college, or government facility will help to assure the boiler is operating at peak efficiency. If the bril materials are improperly designed, specified, stored, installed, cured, dried, or removed it will have an adverse affect on three important factors:
- energy usage and energy savings.
- efficient boiler operation.
- your financial bottom line.