Factors to consider when choosing your blast nozzle.
Proper blast nozzle selection for each application is simply a matter of understanding the variables driving cleaning efficacy, efficiency and ultimately job profitability.
Ensuring the optimization of these variables is as simple as providing answers to the following four basic questions:
1. What blast pattern is desired?
2. What (nozzle) can your compressed air source support?
Blast nozzle bore shape refers to the path inside of the nozzle – it determines the blast pattern, hot spot size, and velocity of the nozzle. Selecting the proper bore shape can greatly improve blasting efficiency.
Generally, blast nozzle bore shapes can be classified as either: angled/curved, straight, or Venturi.
Several variations of the Venturi-type available. Venturi bore nozzles create wide blast patterns and increase abrasive velocity as much as 100%, making them the best choice for achieving greatest productivity when blasting large surface areas. Long-Venturi bore nozzles yield a ~40% increase in blasting productivity & a ~40% decrease in abrasive consumption vs. straight bore nozzles.
The various nozzle bore types are summarized below:
Production Rate
Blast Pattern 3'' @ 18''
Straight bore nozzles are the most basic type of nozzle shape consisting of a tapered converging entry, a parallel throat section, and a full-length straight bore and straight exit.
As compressed air passes through the converging entry end of a straight bore nozzle it accelerates, in turn accelerating the abrasive particles suspended in the flow. The particles exit the nozzle in a tight stream and produce a tight, concentrated blast pattern with little over-blast upon impact great for spot blasting or blast cabinet work.
Straight bore nozzles are best for smaller jobs such as parts cleaning, weld seam shaping, cleaning handrails, steps, grillwork, or carving stone/other materials, and are poorly suited for blasting large surfaces.
Production Rate
Blast Pattern 3'' @ 18''
Venturi bore nozzles feature a converging entry, a short, straight section, and a long diverging end that widens as you reach the exit of the nozzle. Venturi nozzles produce a large blast pattern and a more uniform particle distribution than a standard bore nozzle.
This design is intended to produce an effect which greatly accelerates the air flow and particles, creating a wide blast pattern and increasing abrasive velocity by as much as 100% for a given pressure.
Venturi nozzles are the best choice for ensuring high productivity when blasting larger surfaces. Compared to straight bore nozzles, Venturi style nozzles yield a ~40% increase in productivity while reducing abrasive consumption by ~40%.
Production Rate
Blast Pattern 3'' @ 18''
A double Venturi nozzle is a de Laval nozzle with an extra wide exit opening (wider than a standard Venturi nozzle) and holes at the diverging end. It can be thought of as two nozzles in series with a gap and holes in between to allow the insertion of atmospheric air into the downstream segment of the nozzle.
According to the Venturi effect, as the velocity of a flow increases, the pressure drops, creating a vacuum between the shockwave and the throat, reducing abrasive velocity. With a double Venturi nozzle, atmospheric air is drawn through the holes into low pressure area, expanding the air flow to produce a wider blast pattern and minimizing the loss of abrasive velocity.
Double Venturi nozzles are designed to be used on jobs where medium cutting action is required with a more even dispersion of abrasive throughout the larger blast pattern. These nozzles are excellent choices for plastic or agricultural abrasives.
Production Rate
Blast Pattern 3'' @ 18''
Wide throat nozzles are de Laval nozzles featuring a large entry throat, an extra-1/4'' wide converging section, and a large diverging exit bore.
When matched with a hose with the same size ID (and with a corresponding air volume increase), they can provide a 15% increase in productivity over nozzles with a smaller throat (e.g. long venturi types).
Wide throat nozzles featuring a larger diverging exit bore can be used at higher pressures to yield up to a 60% larger pattern with lower abrasive use.
Production Rate
Blast Pattern varies
It's a good idea to have angled/curved nozzles available for blasting tight areas (e.g. bridge lattice, inside pipes, behind ledges, the flanges of beams, inside cavities, etc.)
Many operators frequently wait for abrasive to ricochet, wasting both abrasive and time – the minimal amount of time required to switch nozzles is quickly recovered and total job time reduced.
To achieve the desired surface finish, a balance must be struck between compressor, hose and nozzle sizes – air supply pressure and volume must be adequate for ensuring abrasive velocity is maximized.
As a general rule, the air supply system should be able to provide at least 50% more air volume (CFM) than a new nozzle in order to develop the required working blast pressure. This ensures nozzles continue to provide good performance even after being slightly worn. However, excessive nozzle wear should be avoided in all circumstances to prevent a dramatic decrease in productivity.
In addition, nozzle throat entry diameter should match the inside diameter of your air supply hose. The wrong size combination can lead to wear points, pressure drop, and excessive internal turbulence.
Compressor requirements for various blast pressures by nozzle size can be found in Table 1.
To find the optimal nozzle size your system can support, determine what nozzle pressure (PSI) you need to maintain for productive blasting and what compressed air volume (CFM) you have available, then consult the Table 1 to find the nozzle size that meets those parameters.
| NOZZLE SIZE | NOZZLE PRESSURE (psi) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | 140 | 150 | ||
| #2 | 1/8″ | 14 | 17 | 19 | 21 | 24 | 26 | 28 | 30 | 32 | 34 | 37 |
| #3 | 3/16″ | 32 | 37 | 42 | 47 | 52 | 57 | 62 | 67 | 72 | 77 | 83 |
| #4 | 1/4″ | 57 | 66 | 75 | 84 | 93 | 103 | 111 | 119 | 127 | 136 | 185 |
| #5 | 5/16″ | 89 | 103 | 117 | 131 | 145 | 158 | 172 | 186 | 200 | 214 | 229 |
| #6 | 3/8″ | 129 | 149 | 169 | 189 | 209 | 229 | 249 | 269 | 289 | 309 | 330 |
| #7 | 7/16″ | 176 | 203 | 230 | 258 | 285 | 312 | 339 | 367 | 394 | 422 | 451 |
| #8 | 1/2″ | 229 | 265 | 300 | 336 | 371 | 407 | 442 | 478 | 513 | 549 | 586 |
| #10 | 5/8″ | 356 | 412 | 468 | 524 | 580 | 632 | 688 | 744 | 800 | 856 | 914 |
| #12 | 3/4″ | 516 | 596 | 676 | 756 | 836 | 916 | 996 | 1,076 | 1,156 | 1,236 | 1,318 |
| EFFICIENCY | 47% | 55% | 64% | 74% | 86% | 100% | 115% | 130% | 145% | 165% | 175% | |
Blast nozzle orifice size affects blasting productivity – the larger the bore, the bigger the area blasted, facilitating higher production rates. For each increase in nozzle size, an increase of up to 10% in your blast pattern size will occur.
However, nozzle shape, rather than orifice size, has the greatest impact on blast pattern size.
For maximum productivity, select the largest nozzle bore size that the available compressed air (pressure and flow) will support for the intended blast pressure, given the surface characteristics and specifications of the task, while taking into account that there will be a significant reduction in pressure as your nozzle wears to a larger diameter. Finding the sweet spot where your nozzle can be productive over its useful life span is key to getting the most value out of your nozzle.
Choosing a nozzle with a bore smaller than your system will support will sacrifice blasting capacity and may impact media flow. Conversely, choosing a bore size beyond what your system can support will result in your inability to blast effectively and may result in the rapid wear on the blast hose.
Example:
You are running a 375 CFM compressor at 80% capacity. In addition to the blast nozzle, the compressor is supplying air to various components (air helmet, air motors, pneumatic controls, etc.) leaving 250 CFM available for the nozzle. Referring to Table 1, you can see that 250 CFM is sufficient for a #6 (3/8'') nozzle operating at 100 PSI. A larger nozzle, or a worn #6 (3/8'') nozzle, will require more air flow to maintain a blast pressure of 100 PSI. This extra flow requirement will either overwork your compressor or decrease productivity. On the other hand, choosing a nozzle with a bore smaller than your compressor can supply will result in less than maximum productivity from the system.
Additionally, it is important that your blast nozzle be sized to the blast hose it is to be used with – blast hose ID should be at least 3-4 times larger than the bore size of your nozzle (e.g. a 3/8'' nozzle requires a minimum 1-1/4'' blast hose, a 1/2'' nozzle requires a minimum 1-1/2'' blast hose, etc.). Wrong size combinations may lead to excessive internal turbulence, pressure drop & wear points.
Typically, a 3/8'' nozzle is sufficiently constricted to produce an effective blast pressure with a 185 CFM compressor, and a 1/2'' nozzle is sufficient with a 375 CFM compressor.
The fourth consideration of nozzle selection is the composition of the material lining inside the bore. Durability, impact resistance, and cost are the three most important elements to consider when choosing the right nozzle bore material.
Nozzle bore material selection is a function of the type of abrasive used, blasting frequency, the size of the job, and the rigors of the job site.
While harder materials are more resistant to wear, they are more brittle and prone to cracking under rough handling and more expensive to replace.
Below is a summary of the various nozzle materials available and general application guidelines for each.
Durability
Impact Resistance
Price $
Tungsten carbide nozzles are the least durable of the carbide nozzles, but relatively cheap and resistant to impact. Suitable for glass, mineral and slag abrasives.
Note: boride tungsten carbide nozzles feature top wear grade material and thick-wall construction.
Durability
Impact Resistance
Price $
Silicon carbide nozzles offer service life and durability similar to tungsten carbide nozzles but at one-third of the weight, reducing operator fatigue.
Silicon carbide composite nozzles are an excellent choice when operators are on the job for long periods and prefer a lightweight nozzle.
Durability
Impact Resistance
Price $$$
Boron carbide nozzles are extremely hard, offer the longest wear life of any standard nozzle material. Peak nozzle production performance is maintained for longer periods with optimum consumption of air and abrasive.
Boron carbide is ideal for aggressive abrasives such as aluminum oxide and selected mineral aggregates when rough handling can be avoided as boron carbide nozzles are brittle.
Boron carbide will typically outlast tungsten carbide nozzles by 5-10 times and silicon carbide nozzles by 2-3 times when aggressive abrasives are used.
Durability
Impact Resistance
Price $$$
Some manufacturers produce their own composite carbide nozzles, which are even harder than boron carbide so keep that in mind. Also, research has shown that blast productivity gradually increases with abrasive feeding rate until a critical value is reached, after which productivity maintains constant. So after that value has been surpassed, consuming more media actually reduces particle velocity, wastes abrasive and lowers efficiency.
Nozzles wear out – abrasive passing through the nozzle will eat the lining away, enlarging the orifice.
More air volume per minute is required to maintain your target pressure with a worn nozzle because the volume of air passing through the enlarged orifice becomes inadequate to maintain the pressure.
Nozzles should be replaced once the orifice has worn 1/16'' beyond its original size.
Production losses of 1-1.5% are incurred with every PSI of pressure loss.
Pressure drop between the compressor and the nozzle can be significant – up to one PSI per 50' section of 1'' ID hose – due to friction created within the hose.
In addition, pressure drop increases with each bend or change of direction in the hose: the shortest, straightest hose configuration is best to minimize pressure drop.
Lastly, ill-fitting couplings or leaks in the hose could further contribute to additional, and potentially total, pressure losses.
Smaller ID air hoses are more restrictive and generate more friction, resulting in greater pressure drop as illustrated in Table 2, below. Therefore, larger air hoses are recommended to minimize pressure losses.
| Hose ID (in) | Pressure Loss (psi) | Production Loss (%) |
|---|---|---|
| 3/4 | 11.1 | 16.60 |
| 1 | 2.4 | 3.60 |
| 1-1/4 | 0.7 | 1.00 |
| 1-1/2 | 0.2 | 0.30 |
Air flows best through straight, hard lines – any directional changes and/or protrusions will impact air flow/pressure drop. Properly sized metal piping or plastic tubing can convey air with minimal friction loss, which is common to rubber airlines.
Keep hoses and piping as short as possible, use only the length of air line required, and avoid erratic bends. When bends are unavoidable, make them as gradual as possible.
Using the correctly sized air line is critical to get the most from your compressor and blast pot. The ID (inner diameter) of the air line should be consistent with the ID of all fittings to allow for smooth air flow. Air line ID should be at least four times the ID of the nozzle orifice.
If choosing between long air hoses and long blast hoses – keep the blast hoses as short as possible.
A summary of the minimum recommended air hose size for each nozzle can be found in Table 3, below:
| Nozzle Size | Nozzle ID | Min. Air Hose ID |
|---|---|---|
| #3 | 3/16' | 1.00' |
| #4 | 1/4' | 1.00' |
| #5 | 5/16' | 1.25' |
| #6 | 3/8' | 1.50' |
| #7 | 7/16' | 2.00' |
| #8 | 1/2' | 2.00' |
| #10 | 5/8' | 2.50' |
| #12 | 3/4' | 3.00' |
