Abrasive blasting has been around for as long as man could throw a mineral abrasive, such as silica sand onto an object. The reasons for the surface preparation vary from removing an existing coating to preparing the surface to accept a new coating. The idea is simple, and the industry was born with the advent of the air compressor.
An abrasive blast room is the core to any modern abrasive blast system. Confining the blasting operation to a controlled clean environment enables efficient abrasive recycling.
The design criteria required for a properly sized blast room system includes the size of the largest workpiece, the weight of the largest workpiece, the material handling method, the number of hours of blasting per day, and the base material of the workpiece. Each of these items needs to be addressed in order to finalize the configuration of the blast room.
The size of the largest piece will determine the dimensions of the blast room enclosure. The width of the room is determined by adding four to five feet on each side of the workpiece. This space is required for the blast operator to maneuver around the part and blast the part from various angles.
The height of the blast room is also determined by the workpiece height, but the material handling of the part must also be considered. For example, if a work car on a track is the material handling method, then the height of the work car must be taken into consideration to determine proper clearance of the blast room roof panels. Again, a four- to five-foot clear area will be required for the blast operator or up to seven feet clear if the operator will walk on top of the part while blasting (e.g. a tank in the railcar industry).
The length of the blast room is determined by adding four to five feet on each end of the workpiece to allow for operator clearance.
Ventilating a blast room can be done by three different methods of air-flow design. The three air-flow designs are “down-draft,” “end-to-center,” and “cross-draft” ventilation.
The various room air speeds are determined by the abrasive that will be used in the blast room and the method the room is to be ventilated. Table I is provided by ANSI and outlined in ANSI Z9.4-1985, “Abrasive Blasting Operations–Ventilation and Safe Practices.”
The most common and economical method of ventilation is “cross-draft.” Basically, the calculation for the dust collector size is determined by the following formula: width of room x height of room x crosssectional air speed (fpm) = cfm. Note: The cross sectional air speed is typically 50 fpm for steel grit abrasive and 60 fpm for nonferrous mineral abrasives.
For example, the dust collector sizing for a room 16 x 16 x 60 ft. that is using steel grit abrasive is calculated as follows: 16 x 16 ft. x 50 fpm = 12,800 cfm required for a room air-flow rate of 50 fpm.
The reclaim system adds an additional air volume to the dust collector that will range between 500 to 1,200 cfm; therefore, the resultant dust collector will be sized for (12,800 cfm + 500 cfm.)—a total of 13,300 cfm.
When selecting the proper abrasive for the blast room, it is important to look at the entire spectrum of parts that will be blasted in the facility.
For example, a job shop blasting operation may see one type of work for a period of time and another type of fabricated aluminum parts for another period of time. In this type of application, you would want to select a type of abrasive that is applicable to both types of base material, i.e. steel (ferrous) and aluminum (nonferrous), such as garnet, star blast, aluminum oxide, etc.
While a mineral abrasive gives you the flexibility to accept a variety of work into your shop, it does break down at a much faster rate than steel grit abrasive. Typically, you can recycle mineral abrasives about three to six times, based on operating pressures at the nozzle and the initial size and hardness of the abrasive.
Steel grit abrasive also comes in a variety of sizes and hardnesses. The typical recycle rate for steel grit (G-40) is 150 to 200 times. Again, this can vary based on operating pressure at the nozzle.
Disposal costs of spent abrasives are a major factor in abrasive selection. The amount of waste that must by removed is directly related to the recyclability of the specific abrasive and the volume of abrasive that is used. This can result in a large quantity of waste material that must be dealt with in regards to “cost of disposal.” This cost will vary based on the blasting operation and coatings being removed.
If lead paint or zinc primer is being removed, the disposal costs can be up to $500 per 55-gallon drum of waste. The return on investment (ROI) for a blast room facility is the key element when making the investment in a blast room facility.
Recycling abrasives and minimizing waste disposal costs is the single most important element in achieving a good ROI on the capital purchase.
The reclaim system is comprised of a floor reclaim and an abrasive separator. The floor reclaim design can vary from simple “sweep-in” designs to “full” floor reclaims, which recover all the abrasive through a grated floor.
Reclaim designs vary based on the manufacturer and type of abrasive that is being reclaimed.
When a very light abrasive, such as small glass beads or a fine aluminum oxide (i.e. 120 mesh or smaller) is being used, the most common method of reclaim is a vacuum floor and cyclone separator. This method is also used in most hand cabinets that use suction or pressure blasting.
Single-screw partial reclaim system.
Single-screw partial reclaim system.
The most reliable and cost-effective method of reclaim floor is the mechanical screw floor with a belt and bucket elevator, rotary scalping drum, and air-wash separator. There are four different designs for a reclaim floor. They are single screw, “H”-shaped, “U”-shaped, and full reclaim. The selection of the proper design is based on production needs as well as economic concerns. The following is a description and plan view drawing of each reclaim floor design.
A single-screw partial reclaim system is the most economical floor design available. The system contains the major components found in reclaim systems including metering shed plates; heavy-duty screw, belt, and bucket elevator; airwash separator; perforated plate rotary scalping drum separator; and abrasive storage hopper with a caged man ladder and handrail. This reclaim package can be expanded to an”H,” “U,” or full-floor reclaim design. This floor design is for low- to medium-production levels.
H-Shaped partial reclaim system
The “H”-shaped partial reclaim system adds two longitudinal metered screw assemblies along each side wall of the blast room. The position of the screw assemblies allows the abrasive delivered from the blasting nozzle, which is either blown or rebounded off the work piece, to strike the side walls and fall into the screws, reclaiming approximately 60 to 90% of the blast media. The remaining abrasive on the floor is pushed into the screw assemblies at the end of the work shift. The screws are protected from an overload by metering shed plates. The “H”-shaped floor design is typically utilized in a “flow thru” room configuration where heavy workpieces or material handling devices can drive into the room and position the workpiece on the steel-covered concrete floor located between the screws. This floor design is best suited for medium to high production.
The “U”-shaped partial reclaim system adds two longitudinal metered screw assemblies along each side wall of the blast room and positions the cross screw along the back wall of the blast room. The position of the screw assemblies allows the abrasive delivered from the blasting nozzle, which is either blown or rebounded off the work piece, to strike the side walls and back wall of the blast room and fall into the reclaim system. A “U”-shaped floor design will reclaim 60 to 90% of the blast media. The remaining abrasive on the floor is pushed into the screw assemblies.
U-Shaped partial reclaim system
The screws are protected from an overload by metering shed plates. The “U”-shaped floor design is typically utilized in an “in–out” room configuration where heavy workpieces or material handling devices can drive into the room and position the workpiece on the steel- covered concrete floor. This floor design is best suited for medium to high production.
The full-floor reclaim system utilizes multiple screw assemblies to create a fully automatic abrasive reclaim floor system. All the abrasive that is blasted is returned to the separator system. The full-floor reclaim design requires that the material handling of the workpiece be intricately designed into the configuration of the room.
Material handling of the workpiece includes a work car/track system, an overhead monorail crane, an overhead bridge crane, or heavy-duty floor grating and support steel sized to allow a forklift to drive onto the reclaim floor. The full-floor reclaim design can be used with any room configuration. This system is best suited for high-production requirements.
The partial floor reclaim systems will have an added “clean-up”cost associated with the ROI that must be calculated into the justification for capital expenditure. Choosing the right floor configuration to best meet production and cost requirements will result in the quickest return on your investment.
The material handling method of moving the workpiece through your facility or just through the blast room must be considered so the room is designed for that specific handling device.
As mentioned earlier, the partial reclaim floor designs lend themselves to forklift trucks or driving the workpiece directly into the blast room, such as construction equipment or trailers.
Full-floor reclaim system
The full-reclaim floor design can be configured for a work car track system, overhead monorail or bridge crane system, or a combination of both. The blast room configuration will vary based on your plant layout and the material handling method for the workpiece.
A “flow-thru” room configuration is designed for work to enter one end of the room and exit the room on the opposite end. This configuration is typically used for an “in-line” production flow of the workpieces.
A “flow-thru” design requires more floor space in your facility, in that you typically allow for an in-bound and out-bound staging area prior to the next production phase, such as paint.
An “in–out” room configuration is designed for work to both enter and exit the same end of the blast room. This configuration is typically used because of space or production flow considerations. The blast room and paint booth are usually side-by-side. This configuration allows for both rooms to be presented the workpiece from a variety of sources and directions, while minimizing factory floor space requirements.
The amount of abrasive blasting that is done in an eight-hour period and the number of blasters required for that production level is another factor that dictates a blast room design.
For example, if you are currently operating with four blasters in an outdoor sandblasting operation, you will require a blast room with enough room, abrasive capacity, and reclaim equipment to accommodate this production level.
The reclaim systems and room designs will vary in size and capacity based on production levels.
Conclusion
A properly designed blast room facility will provide your company the tools it needs to meet the demands for a clean environment as well as providing a good ROI. Recycling blast abrasives saves money by reducing waste. The enclosed blast room saves money by allowing production to continue regardless of outdoor weather conditions.
By Eric G. Thomas
Eric. G. Thomas has been involved in the abrasive air blast industry since 1987 and oversaw the installation of over 1,500 air blast facilities.
