BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to abrasive media blasting and more particularly to shot and sand blasting devices and associated methods.

2. Background Information

Abrasive media blasting have been used extensively for many years and generally may be described as forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. A pressurized fluid, typically compressed air, or a centrifugal wheel is used to propel the blasting material (often called the media).

The first abrasive blasting process was patented by Benjamin Tilghman in 1870. Regarding abrasive media, the most abrasive are typically metal shot (known as shot blasting) and sand (known as sandblasting). Moderately abrasive media variants include glass beads. Plastic pellets, ground-up plastic stock, walnut shells and corncobs. A mild abrasive media include baking soda and some ice and dry ice media.

In the field of abrasive blast cleaning, constant-area or straight-bore nozzles were primarily used up until the 1950s. British patent No. 722,464 disclosed an abrasive blast cleaning nozzle with a converging-diverging configuration, sometimes referred to as the Mead nozzle.

Most current abrasive blast cleaning nozzles utilize what is called the Laval-type configuration, frequently (and some in the industry say mistakenly) known as “Venturi” nozzles (which term is intended to only refer to nozzles associated with low-speed fluids).

There are also what are known as “long-venturi” nozzles as described below which are commonly used in abrasive blast cleaning of all types, especially including steel structures, such as bridges and the like, to remove paint and corrosion.

Typical methods pick up the media by induction of the media into low to moderate velocity air (or other carrier). The air carries the media to the inlet of a nozzle of various styles. The nozzle accelerates the air though a restricted throat plus the subsequent expansion region of the nozzle until the media exits the nozzle. In practice, these tend to be generally referred to long or short nozzles. The longer nozzles tend to be elongated various amounts beyond the throat while the shorter nozzles have much less length beyond the throat. For the same air mass flow rates, the best performing nozzles are usually referred to as long venturi nozzles, which are also called long nozzles. As the media passes through the nozzle throat and then the remainder of the nozzle, the media velocity is increased by the action of the higher velocity air upon the slower media. This is done to increase the effectiveness of the media to perform it's various purposes. It is generally recognized that higher media velocity produces greater blasting effectiveness for those applications that require greater effectiveness and/or reduction in application time.

A significant problem associated with the long-venturi nozzle is that it is remarkably inefficient with regard to the transfer of energy from the compressed airstream flowing through the nozzle to the abrasive particles entrained therein. This energy transfer efficiency is typically only on the order of about 10%. See, for example, “A Scientific View of the Productivity of the Abrasive Blasting Nozzles,” Journal of Protective coatings and Linings, April 1995, pages 28-41 and 101-102. In some common commercial nozzle designs, the exit velocity of the air is more than 4.5 times that of the media and as kinetic energy is proportional to the square of velocity, the kinetic energy of the media particles exiting the nozzle is about 5% that of the air stream at equal air and media mass flow rates. Thus, 95% of the kinetic energy of the air stream is being lost in such nozzle designs.

For further background on blast nozzle configurations see U.S. Pat. Nos. 6,293,857; 6,390,899; and 9,518,358 which are incorporated herein by reference.

An abrasive blast cleaning nozzle design is identified in Penn State University U.S. Pat. No. 5,975,996, which is incorporated herein by reference, and which attempts to address some of these concerns. This discloses a nozzle, also herein called a PS Nozzle or Penn State Nozzle, for an abrasive blast cleaning apparatus consisting of a short, relatively rapidly converging inlet section, a constant-area throat, a rapidly diverging first diverging section, and a long second diverging section that diverges less rapidly than the first diverging section. The inlet section quickly accelerates the abrasive particles after entering the PS nozzle, while the first diverging section rapidly brings the relative velocity of the air stream and the abrasive particles to about Mach 1.4. The second diverging section helps to maintain the relative Mach number while the abrasive particles continue to accelerate. This makes abrasive blast cleaning more efficient, particularly because the kinetic energy of the abrasive particles emerging from the nozzle is significantly increased. The PS nozzle is similar to a similar long venturi nozzle geometry and have the same entrance diameter and the same throat diameter, but the PS nozzle has a different exit area, and substantial additional length after the throat. The increase in length was determined by use of Newton's equation F=(m) (A), converted to F=(Cd)(p) (Vrelt2 /2), wherein the developers of the PS nozzle surmised that the prior venturi nozzle in this field was of the general form of a DeLaval (supersonic) nozzle, and, that this relationship could be applied to determine a longer length for the expanding PS nozzle portion that would meet specific (if somewhat arbitrary) nozzle design requirements. This lengthening would increase the velocity of the media exiting the PS nozzle by a calculated 35% over the media exit velocity of the comparable long venturi nozzle. The '966 patent also describes other small adjustments to the PS nozzle throat geometry, but not the throat area.

The media blast PS nozzle design of the '966 patent is a step forward in the design of such nozzles, but, it failed to solve at least three of the historic difficulties of these and other shot, sand or hard media blasting nozzles. These are: (1) nozzle wear at the throat, (2) continuously wear through the nozzle expansion region and (3) wear at the exit diameter, all caused by the media passing through the nozzle. Since the internal air velocity within the nozzle expansion region is what increases the speed of the media particles, and this improves application effectiveness, this degradation of the nozzle function begins immediately with the first nozzle use and continues with each subsequent use.

Further as each internal diameter between the throat and the nozzle exit is initially different from every other internal diameter for appropriate velocity development, even uniform internal nozzle wear, which will not occur, results in disproportional wear from a design and performance perspective. Nozzle wear will not be uniform since it is a function of initial and subsequent velocity plus initial and subsequent diameter at every location along the nozzle axis between the throat and the nozzle exit.

These issues are well known in the industry, and lead to material composition, hardness and cost trade-offs in nozzle design and frequency of nozzle replacement. No material lasts forever in this type of application especially when media materials such as steel shot and sand are employed.

Further, blasting nozzles are generally rated by their throat diameter: (#5, 5/16 Inch diameter), (#7, 7/16 Inch diameter), etc. which is arbitrary and not with any appropriate or apparent connection to any specific air compressor rating. This produces serious mismatches between available compressor power and nozzle configuration resulting in continuous wasted power during almost every blasting nozzle use, regardless of type.

The remains a need for an effective abrasive media blasting nozzle that minimizes wear in an effective and cost efficient manner.

SUMMARY OF THE INVENTION

The advantages of the present invention are achieved with an abrasive media blasting apparatus including a supersonic nozzle having a nozzle entrance configured to be coupled to a pressurized fluid source, a supersonic nozzle throat, and a nozzle exit; and an acceleration chamber coupled to the nozzle exit and having a uniform internal cross sectional area along an axial length of the acceleration chamber and the acceleration chamber having an abrasive media inlet in the acceleration chamber adjacent the nozzle exit and configured to be coupled to a source of abrasive media

One embodiment of the present invention provides a shot media blasting apparatus including a supersonic nozzle having a nozzle entrance configured to be coupled to a pressurized air source, a supersonic nozzle throat, and a nozzle exit; and an acceleration chamber coupled to the nozzle exit and having a uniform internal cross sectional area along an axial length of the acceleration chamber, wherein the uniform internal cross sectional area of the acceleration chamber matches a cross sectional area of the nozzle exit, and wherein the acceleration chamber having a shot inlet in the acceleration chamber adjacent the nozzle exit and configured to be coupled to a source of shot.

One aspect of the present invention provides a method of abrasive media blasting comprising the steps of: Introducing pressurized fluid into the nozzle entrance of a supersonic nozzle; Accelerating the fluid through the supersonic nozzle to a supersonic nozzle throat; Passing the fluid from the supersonic throat to an exit of the supersonic nozzle; Introducing abrasive media into the flow of fluid downstream of the nozzle exit in an acceleration chamber coupled to the nozzle exit via an abrasive media inlet; Accelerating the media within the acceleration chamber along a uniform internal cross sectional area along an axial length of the acceleration chamber.

This new invention utilizes a supersonic nozzle to produce the high velocity that is beneficial to the blasting application but avoids the complication and disadvantages of media wear on the expensive nozzle material which is at the heart of the present technology. This is because, the shot or other media does not travel though the supersonic nozzle, but only through a uniform area/diameter acceleration chamber, into which the supersonic nozzle discharges it's high velocity air. Since there is no media, only air, traveling through this nozzle, and it does not wear and can be much shorter and manufactured from less expensive material. Thus the only function of the supersonic nozzle is to convert compressed air to supersonic air, as efficiently as possible. This nozzle is much shorter than current abrasive media blasting nozzles, because the media acceleration function is decoupled from the internal structure of the nozzle. Further, the acceleration chamber is substantially more effective in increasing media acceleration since only the highest velocity available air from the nozzle is applied to media acceleration in the acceleration chamber. Further because the nozzle is decoupled from internal media acceleration, both the nozzle and the acceleration chamber can be configured independently to maximize the specific function of each, and which reduces the size of each to an extent that the total length of the two functions combined is substantially smaller than the traditional long venturi nozzle and the much longer nozzle designed in accordance with the teachings of the '966 patent.

Only the maximum velocity output of the supersonic nozzle is used to accelerate media, therefore the performance of this configuration is also substantially improved over conventional prior art nozzle designs. Thus the industry will get more performance for less investment. This design also frees the nozzle and thus the complete blasting system to be configured to correspond to the peak power output of the specific fluid source that supplies the nozzle.

The high velocity output of the nozzle is closely coupled to an acceleration chamber into which the media is independently inserted. This acceleration chamber is of the same or similar diameter and or shape as the outlet diameter of the nozzle so that the high nozzle exit velocity is not diminished by entry into this acceleration chamber. It is only diminished to the extent the high velocity air transfers energy to the media, which increases media velocity. There is a small loss to air friction or drag in the chamber, but because the chamber is much shorter than current and earlier technologies, and does not have any restrictions, it is a minor loss.

The length of the acceleration chamber can be selected to optimize (maximize) the energy transfer from the much higher air velocity to the media, or to reduce the exit velocity of the media by using a shorter chamber for more sensitive applications. This is determined by either calculation or test, and this simple chamber will wear but is less expensive than current abrasive media blasting nozzles, and is far less expensive than the more expensive long nozzles presently used. Since the maximum exit velocity of the supersonic nozzle is used to accelerate the media, the length of this energy transfer element is minimized, with obvious advantages in cost and wear.

Furthermore, based upon the nature of air conveyance in uniform area conduits, the wear is going to be very small, and restricted to the region of the chamber in the vicinity of the media introduction.

In this system there are alternative mechanisms of introducing abrasive media such as shot. The simplest is gravity insertion from a few feet above the nozzle through a small diameter hose. The media hose diameter is much smaller than the usual combination abrasive media/air supply current or older nozzles require, because air propulsion is not required either to or within the supersonic nozzle. Simple introduction of media, such as namely metal shot, under the combination of entry at the highest velocity region of the primary air flow (Bernoulli affect) plus gravity is sufficient for many hand held applications. Introducing abrasive media in this way through a shot tube open to the atmosphere also introduces a small quantity of atmospheric air. If this is to be avoided for larger applications, a closed container can be used or the cross section of the acceleration chamber can be adjusted along it's axis to suit. A valve in the shot tube can be either manual or remotely operated and the valve is provided to avoid media accumulation in the acceleration chamber when no air is flowing.

This system includes a replaceable wear tip at the exit of the acceleration chamber. This will be the location of greatest wear as the result of the proximity of the system's higher velocity abrasive media exit to the surface being blasted. This abrasive media, particularly with metal shot, rebound will be very energetic. Further an adjustable and flexible shield protects the operator.

