Rotary Atomizers vs Competitive Pressure Nozzles

   Figure 6. Varimax Hub operating in 90,000 TDS Produced Water

 

 Figure 7. Pressurized nozzles clogging with buildup

 

Nozzles Used in Snow Cannons

The Spraying Systems Co. 8-15W nozzle is used on a snowmaking-type cannon at 40 psi. The 8-15W nozzle has a mean water droplet average of 500 to 600 microns. The cannon uses over 270 8-15W pressure nozzles to obtain a 25 L/S flow rate.

 

These nozzles are spaced very tightly around the end of the cannon creating multiple collisions of 500-micron droplets due to each nozzle having a 100° cone spray angle. See the nozzle cone spray video that shows the dispersion angle. Several studies regarding the collision coalescence of water droplets prove droplets increase in size as they move away from the nozzle.

 

“During this process, smaller droplets collide, form larger droplets, and lose their momentum due to drag. Thus, the droplet diameter starts increasing away from the nozzle. As the droplets become larger and slower, the probability of subsequent droplet-droplet collision increases, resulting in further coalescence.”  See Figure 8 for a closer look at the coalescence location beyond the initial fracturing from a pressure hollow-core nozzle.
                           

Figure 8. Hollow Pressure Nozzle Coalescence Zone

Here are the critical highlights from the Project showing that the best use of the same nozzles is for dust suppression:

• Evaporation, although beneficial in regards to overspray, is not a significant factor for the VMD droplet sizes produced by the proposed nozzles.
• For the prevention of dust generation for stockpiles, standard hydraulic nozzles that produce drops between 200 and 1,200 μm are generally used.
• The size of the droplet affects the vertical velocity at which it falls. A larger droplet will fall at a greater speed than a smaller droplet in the same conditions.
• 500-micron droplets fall at a speed similar to light rain.
• Water drops that are greater than 300 μm in diameter will experience negligible evaporation from a fall over 1000m in an atmosphere with relative humidity as low as 40% (Pruppacher & Klett, 1980; Gunn & Kinzer, 1951).
• Since the fall distance from the Spray Tower will be no more than 40 m and the VMD is 500 μm, we are basically off the chart ie the effects of evaporation for the VMD can be disregarded.

Figure 9 shows the light rainfall effects of shooting 500-micron water over a large open-pit mine wall, and the results in the image speak for themselves.

 

The question to consider in using snow cannon pressurized nozzle technology is do you have over 1000m of drop on your project site for a 500-micron drop to evaporate?

Figure 9. Snow cannon units operating over open pit mine site.

Figure 10 other snowmaking/evaporation cannon-style systems use pressurized spiral nozzles to obtain droplet fracturing.

Figure 10. TF Series Spiral Nozzle by Bete.

 

Other Types of Nozzles
Bete and Spray Systems manufactures a typical spiral nozzle for disposal purposes. Spiral nozzles need pressures between 40 to 400 psi to gain needed flow rates. The snow cannon system places around 30 of these nozzles on a 2” ring and are placed tightly together and angled towards the inner core of the cannon face.

 

Flow rates vary based on the pressure provided by secondary pump systems. The increased pressure from the pump system increases flow rates through the spiral nozzle and increases atomization.

 

Because pressures are higher with spiral nozzles, wear is a considerable challenge to detect. As these nozzles deteriorate and wear, increased flow rates or lower spraying pressures increase droplet size.

Figure 11 shows how difficult it is to monitor wear in a spiral nozzle. In applications with higher TDS, nozzle tip wear would be difficult to detect and, in essence, create challenges in evaporation droplet sizes, not to mention the increased coalescing of drops.

Figure 11. Tip Wear as shown in Spray Systems Technical Appendix

Figure 12  average droplet sizes produced from spiral nozzles are shown in Spray Systems Technical Reference Appendix with a snapshot shown. Based on typical pressures and flow rates, the average micron calculation for a hollow cone spiral nozzle is between 200 microns at a flow rate of 0.16 GPM to 1250 microns at 38 GPM.

 

The approximate flow rate for a 30-nozzle system would be approximately 3.6 GPM, which would likely place the droplet sizing between 300-400 microns, making this nozzle friendlier for dust suppression.

Figure 12. Hollow Cone Spiral Nozzle Micron Sizing

Bete nozzles provided micron data from their most popular used evaporation and snowmaking spiral nozzle tips. Bete’s engineer further stated the 3/8” TF12XPN is the smallest clog-resistant nozzle purchased for snow cannons or fan guns.

Figure 13 for the data provided by Bete. Bete provided additional data for smaller nozzles that are not as clog-resistant and have D32 droplet sizes at 112 and 140 microns. The models offered were the TF6NN and TF8NN.

                                                         Figure 13. Bete Fan Gun Nozzle Data

The snow cannon design used in many evaporation projects uses the concept of saturating the flow of ambient air with lots of water flow. The coalesced water begins its saturation point at the end of the cannon to cool the temperatures below wet bulb when mixing with outside ambient air. Once the wet-bulb temperatures lower, snowmaking potential rises mostly in high altitudinal locations.

Figure 14 shows the high amounts of saturation at the end of the fan-style evaporator. A two-year consumptive water loss from man-made snow study determined a mean loss of only 6% from evaporation and sublimation during initial loss calculations.

 

When you saturate the outside air to 100% humidity, the remaining water falls back to the surface, similar to rain.

 

Figure 14. Closeup snow cannon

 

Fan-Style Cannon Study

The only in-depth evaporation study found using fan-style cannons was performed in the Southeast United States near Savannah, GA. Due to the significant fallback associated with these styles of evaporation systems, much lower volumes of water were used with the spiral nozzles so that data could be collected.

 

The study concluded if significant fallback was tolerable, then the expectation of evaporation efficiency would be 10-12 GPM from the 70 GPM spray rate. This is a 14% modeled evaporation rate during daylight operation only and was never confirmed directly from the study.

One of the other limiting variables for the fan-style cannon evaporators is when they operate from the land. Overspray concerns from differing wind directions throughout the day cause shutdowns that lower production.

Figure 15, shows the 180 degrees of operational capacity.
                                                         

Figure 15. Land based pressure nozzle cannon shut down zone

Figure 16 land-based cannon configurations have difficulty when winds blow back towards the saturated blast zone, which causes a severe overspray condition back onto the land behind the system.

 

Figure 16. Overspray condition using land-based cannon

 

Figures 17-18, these show the typical way cannons combat overspray to operate without shutting off. The cannons often get lowered to a position of shooting the saturated stream directly back into the ponded water.

Figure 17. Cannon style overspray position

 

Figure 18. Cannon overspray setting in high wind region.

 

Cannon Overspray Considerations
Other considerations during overspray operations are the mist digestion of dissolved solids back into the internals of the cannon fan blades and the fouling of the spiral nozzles, as noted in this Salton Sea study with a few notations from the study bulleted below.  Snow cannons offer an optional vibration shutoff to combat digestion concerns. 

Figures 19-20 show a cannon unit with mist digestion.

• The nozzles on the units are plugged up.
• Mist fouling of the evaporators was a major problem.
• The inline filters had to be cleaned numerous times per day to keep the units in operation.
• Left unattended, enough mist could be digested into the units to force the impeller blades
  out of balance.
• The device had to be shut down every couple of days and pressure washed both inside
  and outside of the housing. This process was time-consuming and was an endless task in the course of project operations.
• Mist fouling of the evaporators was a major problem. Any wind from a non-aligned direction resulted in mist surrounding the units. Much of the mist was sucked into the impellers of the turbofans, resulting in deposits like those shown in figure 5.8. Left unattended, enough mist could be ingested into the units to force the impeller blades out of balance. The devices had to be shut down every couple of days and pressure washed, both inside and outside of the housings. This process was repetitive and time-consuming for project operation.
• During the test, the machine could average 54 GPM. throughput at a power consumption of
  93 kwh. Our machine operates at 80 GPM. using less than 75 kwh. The client must understand that these are gallons pumped through the unit, not water evaporated.
• To reduce, but not eliminate, the risk of mist digestion by the EES units, each
  EES unit needs to robotically slave to multiple wind directions, wind speeds, and wind shear detection systems. Any fouling by mist digestion by a significant number of EES units would be very expensive and time-consuming to clean up.
  

Figure 19. Mist digestion side view

Figure 20. Mist digestion inside the cannon

 

Addition Pressurized Nozzle Types

Figure 21 another type of pressurized nozzle system has recently entered the market. The design places a few pressurized atomization nozzles just above the top of the surface water.

 

This helps the system alleviate some overspray concerns. What the system doesn’t provide is enough evaporative spacing above the water level, where studies have shown higher levels of saturated water vapor interfaces.

 

Based on the Craig & Gordon model from 1965, turbulent, dryer wind is needed to create an uplift into the higher layers from their modeling interfaces.

Figure 21. Pressurized nozzle operating just above the water-to-vapor exchange interface

 

Efficiencies in electrical consumption during evaporation bear mentioning when comparing the differences between rotary atomization and pressurized nozzles for enhanced evaporation projects.

 

Table 2 depicts the electrical differences between e3 Solutions' rotary atomizers and a typical cannon setup. The cannon needs a separate 160kW/h water pump in conjunction with a 75kW motor to spin the cannon blades versus e3 Solutions' Varimax 60 unit only using 9.7kW/h.

 

Table 2. Electrical Cost Differences e3 Solutions vs Snow cannon

                                                    

Performance comparisons in Table 3 show typical evaporation rates in highly humid locations such as the Southeast United States operating 14 hours daily.

Table 3. Performance Differences between e3 Solutions & Snow cannon

                                                            
Table 4 depicts the efficiencies of evaporative volume OPEX cost between the two systems and illustrates that higher water volume throughput doesn’t necessarily equal more evaporation.

 

It does mean it will cost you more per 1,000 gallons of the operational expense to use a snow cannon versus an E3 Varimax accelerated evaporation system to achieve a similar amount of evaporated volume.

 Table 4. Actual Evaporation OPEX Cost
                                  

 

 

Rotary Atomizer Advantages
There are many sustainable advantages related to using rotary atomization versus pressurized nozzles.

 

The electrical cost and cost per 1,000 gallons evaporated show two of the main differentiators in the side-by-side comparisons. The cannon-style evaporator aims large water droplets high into the air with most of them raining back.

 

e3 Solutions' patented rotary atomization technology creates smaller droplets that quickly evaporate and mix with the available unsaturated dry air.

 

Contact one of E3’s evaporation experts to discuss how rotary atomization will effectively respond to any of your accelerated evaporation needs.

Editor's Note: This post was originally published in September 2020 and has been completely revamped and updated for accuracy and comprehensiveness.