One of the first decisions frequently made when developing a new air blasting process is determining whether the equipment should be suction blast equipment or pressure blast equipment. Suction blast and pressure abrasive blast machines can both be economical and productive solutions to a surface preparation application – selection of one or the other is dependent on a number of factors.

First some quick definitions:

• Suction Abrasive Blast Equipment: Uses the venturi principle to draw abrasive from a non-pressurized hopper to the blast gun where it is combined with the compressed air stream and propelled against the work surface. Suction blast machines are most easily distinguished by having two hoses (one for air and one for abrasive) running to the blast gun.
• Pressure Abrasive Blast Equipment: Uses a pressurized vessel or “blast pot” that holds the abrasive media. In a pressure blast machine the air pressure in the vessel and in the “pusher” line is the same. This means that there is no pressure differential between the blast pot and the blast line – this allows the abrasive media to meter into the air stream at the blast pot and then run the length of the blast hose out to the nozzle. Pressure abrasive blast equipment has a single hose running to the blast nozzle.

Note that it is a common misnomer that in a pressure blast system the abrasive is pushed out of the blast pot and into the blast hose. That really isn’t what’s happening, at least not in a well functioning pressure blast machine. Instead, the abrasive is metering through an abrasive “grit” valve into a compressed air stream that is running underneath the grit valve. The blast pot needs to be under pressure so that there is not a difference in pressure between this compressed air stream and the vessel holding the abrasive. This allows the abrasive to meter into the compressed air stream primarily via gravity.

Automated abrasive blast equipment and abrasive blast cabinets can generally be suction or pressure. Note that portable abrasive blast machines are almost always pressure.

So how do we choose one over the other? Well, as you’ll see below, there are advantages and drawbacks to both types of systems:

Pressure Abrasive Blast Equipment

Advantages

• Greater media velocity. A pressure blast machine produces greater abrasive particle velocity than a suction blast machine which means faster cleaning of tough contaminants.
• Moves more media. More abrasive particles are propelled against the work surface per minute, for a given nozzle size. Again, this translates into greater productivity, particularly on difficult applications
• Greater stand-off distance. Because the abrasive particles are moving faster to begin with, you can increase the stand-off distance of the blast nozzle from the work surface, producing a larger blast pattern while still achieving velocities great enough to clean the work. Again, this translates into greater overall productivity than a suction blast machine.
• More productive. As a general rule of thumb, pressure abrasive blast machines are four times as productive as suction abrasive blast machines.

Sounds like a no-brainer for pressure abrasive blast machines, right? Well, not so fast...

Suction Abrasive Blast Equipment

Advantages

• Lower capital equipment cost. Suction abrasive blast cabinets are lower cost than the same sized pressure blast cabinet.
• Easier maintenance. There are far fewer moving parts and valves in a suction abrasive blast machine than a pressure abrasive blast unit. This translates into much easier maintenance and trouble shooting.
• Less air and abrasive demand. A suction abrasive blast gun has its air demand governed by the abrasive air jet, which is typically half the size of the blast nozzle diameter. This means that for a given nozzle size, a suction abrasive blast machine uses a fraction of the amount of air volume (CFM) than is required for a pressure abrasive blast machine, when both units are blasting at the same pressure (PSI).
• Continuous blasting. Because there is no pressure blast pot in a suction abrasive blast system, there is no reason to stop, depressurize the vessel and allow abrasive to refill the pot (there is no pressure vessel, remember?). In a pressure blast system, the length of continuous blasting time is dictated by the amount of abrasive media the pot will hold at one time.

So, there are clearly advantages and drawbacks to both a suction and pressure abrasive blaster. Confused? How can you decide which one is right for your job?

• Evaluate the application. If a suction media blast system is capable of producing the level of cleanliness, surface finish, and productivity that your application requires, it will almost always be the better choice than a pressure media blast system because of the lower capital equipment investment, air demand and ease of maintenance. Ask to have your work-piece test blasted to determine whether suction can do the job or if pressure blasting is required. If your supplier can’t provide sample processing for this type of analysis, find one that will.
• Does the job require automation? Because of lower air consumption and the ability to continuously blast, suction abrasive blasting is almost always the preferred method over pressure blasting in a multiple nozzle, automated system. Multiple nozzle pressure abrasive blast machines with continuously operating “dual chamber” blast pots are available, and for some jobs they are by far the best solution. These systems can tend to be somewhat complex, more maintenance intensive, require a larger air compressor, and tend to have a higher capital investment than a comparable suction blast system, however.
• What blasting abrasive are you going to use? Some abrasives require the high velocity associated with a pressure abrasive blast machine in order to work effectively. This is particularly true of lightweight abrasives like sodium bicarbonate (soda), corn cob, walnut shells, and plastic blast media. If you’re using one of these abrasives, you will probably require a pressure blast machine to get acceptable results.
• Does the job demand pressure? For some types of contaminants, very hard work surfaces, or specific applications like most shot peening, pressure abrasive blasting is the only technology that will do the job.
• Do you need a high degree of flexibility? For the highest volume output, (remember the rule of thumb that pressure is 4 times more productive than suction) pressure abrasive blasting is the way to go. Are you only going to own one abrasive blasting cabinet and want it to be capable of everything that you ever throw at it? A pressure abrasive blaster will offer the ultimate flexibility for the widest range of applications.

Without considering the specific needs of the application, neither technology – suction abrasive blasting or pressure abrasive blasting is “better” than the other. While this is a hotly debated subject among many in the surface preparation industry, and often a source of confusion among buyers, the key is to always match the application and production needs with the right tool to do the job. This approach will produce the shortest return on investment and best fit for your business needs.

Bob Kraber is the Vice President and General Manager of International Surface Preparation, a full line distributor of abrasives, replacement parts, and equipment for abrasive blasting, wheel blast cleaning, and vibratory finishing applications. ISP has nine sales and service locations throughout the Southern United States and Canada that stock the full line of ISP products. International Surface Preparation features abrasive blasting equipment by Empire, Zero Products, Clemco Industries, and Schmidt Manufacturing. ISP can be reached toll free at 1-800-374-4043 or on the web at www.surfacepreparation.com.