The answer to the question of which spray is right for you is heavily dependent on your situation. Armed with the right information, approach equipment suppliers and together, you can come to a decision on which spray technology would work for you.
By Ed Bals
Odor and dust control are becoming increasingly important within the waste industry. The growth of waste management sites and ever-expanding urban areas has resulted in increased scrutiny on both worker exposure and offsite emissions. Odor and dust mitigation strategies and equipment are a key part of new facility design and existing site upgrades.
There are a variety of different strategies and technologies for odor and dust control, with the core requirement of any method being the need to capture/neutralize odor and dust before they escape the confines of the facility.
For strategies incorporating spray atomization, control can take two forms: airborne suppression, where spray is released in order to combat emissions present in the air, and topical applications, in which spray is applied to a surface in order to prevent airborne movement from occurring at all. This article will mainly focus on sprays used for airborne suppression, as this application allows for an in-depth discussion of spray characteristics.
What is a Spray?
According to the dictionary definition, a spray is: “A mass of very small drops of liquid carried in the air.” However, there is a lack of clarity on what is meant by ‘very small’. So, for now this can be simplified to: “A mass of liquid droplets carried in the air.” This is sufficiently broad to cover all bases, but there are additional methods of characterizing sprays, which are required in order to distinguish between one spray technology and another. Following are a few important differentiating factors.
This is the physical volume of liquid dispersed by the equipment over a unit of time. There is obviously a theoretical minimum amount of liquid required for control, set by the actual volumes of odor or dust produced. However, inefficiencies in application and environmental conditions mean that more than this theoretical minimum will need to be applied in order to achieve effective control.
Volume applied is also an important factor in the operation of standalone units. The size of their bowsers will place a limit on their operational period before requiring refilling. Filling up (usually not easily accessible) bowsers often adds appreciable workload onto site operations, especially when high volumes are used. Efficient spray technology for standalone units is absolutely a key factor.
While the volume of applied liquid can be a useful indicator, it must be used in conjunction with the other characteristics of the spray technology used. If just volume applied was used to assess application, then a 1,000-gallon tank with the tap open above a drain and an atomized spray of that same volume would be equivalent control strategies of airborne odor and dust. This is clearly not the case.
There are many different formulations for odor and dust control on the market. Normal use of these products requires dilution with water in a tank mix or metering with a dosing pump prior to spray application. For airborne dust control, plain water is often used (though there are additives available) and for airborne odor control, the addition of an odor neutralizing agent is normal practice, although some equipment now applies undiluted product requiring no water.
These formulations, and the water used to apply them, often present a large ongoing cost to waste management operations. It is, therefore, vital that they are applied in an efficient and effective way. While the choice of formulation is an important one, the equipment used to apply it is equally, if not more, important as this choice can make the difference between effective and ineffective control.
Droplet size is one of the most important factors and will greatly affect efficacy of a spray technology. It is generally understood that smaller droplet sizes are preferable for airborne odor and dust control as they have a greater likelihood of interacting with airborne odor and dust particles. However, there is a balancing act between droplet size and other factors, such as environmental conditions and the technology used for spray atomization.
Very often, droplet size is quoted for spray equipment, but it is not always clear what it is referring to as all sprays are made up of a spread/range of droplet sizes. Sometimes the phrase ‘Droplets as small as…’ is used, and, as we all know from sales with ‘Prices as low as…’, this is not necessarily representative of all the droplets/prices. Other equipment states that they produce a single droplet size, which will certainly not be the case. Equipment that states a range of droplet sizes or an average droplet size are closer to reality, but some additional information is still required in order to compare different spray systems.
The best way to look at the droplet sizes present within a spray is by testing spray technologies using droplet sizing lab equipment. The results from these measurements can then be used to determine the span/spread of droplet sizes within a given spray as well as for calculation of an average droplet size. However, there are a variety of different averages which can be used for characterization.
Two common averages used are the Volume Mean Diameter (VMD/ DV0,5) and the Sauter Mean Diameter (SMD/ D32). SMD is the more relevant of these two measures for airborne odor and dust control, as it takes into account the surface area presented by the spray. Surface area is a significant factor in determining the level of control achievable as this plays a part in determining the likelihood of spray interaction with odor or dust particles. This article will stick with this brief explanation of spray averages, as a full discussion of this topic would need another full article.
A Spray Example
Let’s look at an example of a spray. We will use sports balls instead of micron sized droplets for simplicity, with 10 balls representing a ‘spray’ with each ball a separate spray droplet. As a simple example, we will say nine balls are tennis balls and one is a medicine ball. The medicine ball has a diameter about five times larger than each tennis ball, and a volume 125 times larger than each tennis ball. Therefore, even though there are nine times the number of smaller tennis balls (droplets) in the spray, there is 14 times more volume in the single large medicine ball (droplet) than for the nine smaller ones combined.
For our ‘spray’ then, 7 percent of the total volume is made up of tennis balls and 93 percent is the volume of the single medicine ball. This means that, looking at volumes, saying that droplets as small as tennis balls are produced would be a slightly misleading way of describing this spray, as the majority of the volume is actually contained in that single medicine ball. Even quoting a range of spray droplet sizes (from tennis balls to medicine balls for this example) may give the wrong impression, since it says nothing about the distribution of droplet sizes.
In the context of airborne odor and dust suppression, this could mean that a spray with a very small minimum droplet size quoted could be wasting the majority of the spray as large droplets, which rapidly fall to the floor.
To better evaluate the distribution of droplet sizes within a spray, its droplet spectrum is usually used. This gives either the number or, more usually, the volume of the total spray contained in certain drop size classes. The data that makes up these graphs can then be used to derive average droplet sizes as well as the span/spread of droplet sizes produced. With these two pieces of information, some meaningful evaluation of a spray and comparison between sprays is achievable.
Having now examined what a spray physically is, let’s look at how they are produced. Physics dictates that energy is required in order to break up bulk liquid into spray droplets. There are a few different ways of supplying this energy, with different spray technologies capable of producing different droplet size distributions. Let’s look at how some of the available spray technology solutions work.
Hydraulic nozzles use pressure to force liquid through a small orifice, with pressure energy converted to the kinetic energy of the spray. At the nozzle tip a sheet of liquid is produced, which then breaks up into droplets. As the breakup of this sheet is random, this leads to a wide spread of droplet sizes produced. Higher pressures and smaller orifices lead to smaller average droplet sizes, but there remains a relatively wide spread around this average. Hydraulic nozzles are widely available and only need a high-pressure pump (with appropriate filtration) to feed liquid to them.
In these types of spraying systems, liquid and air are mixed for atomization. There are various methods used, but, in general, these produce smaller droplet sizes than hydraulic nozzles. However, they are generally quite inefficient, requiring large amounts of energy for atomization usually supplied from air compressors or blowers. There is a degree of this atomization present in spray cannon units due to the air shear from the fan airflow passing over the nozzle, reducing droplet size compared to a situation with no air stream. Vapor systems are often variations of these systems, mixing air with undiluted odor control formulations. These produce droplets, which are then carried in an airstream along distribution pipes with small outlets for dispersal of the spray.
Ultrasonic atomizers produce very small droplets by vibrating an immersed piezoelectric transducer at high frequencies, using the same technology as found in household tabletop humidifiers. However, these systems are limited to atomizing very small volumes per transducer. They do not scale up as easily as other technologies and so they are rarely used for large scale odor and dust control.
Rotary atomizers use centrifugal energy to atomize liquids. Liquid is fed to a rotating disc or gauze and spray is produced from the periphery. Due to the nature of atomization employed, this produces sprays with a much tighter control of droplet size and is very energy efficient compared to other methods. Droplet size can be adjusted by changing the peripheral speed of the disc or gauze, either by varying RPM or by using atomizers with different diameters. Higher peripheral speeds lead to smaller produced droplet sizes. When mounted in cannon units these systems also benefit from the air shear of the air stream, tending to produce smaller droplets than if operated in still air.
A variation of these systems exists where liquid is fed to a fan assembly, which accelerates the liquid, releasing it near its tip. These designs then shatter the liquid on a series of pins, producing both large and small droplets. Any smaller droplets are carried off in the airstream produced by the fan, with larger droplets caught by a recirculation system for re-use.
What Kind of Spray Do We Need Anyway?
There is not a clear consensus. The generally accepted wisdom: the smaller the droplet size, the better. However, in the context of trying to apply a spray directed in a particular location, smaller droplets (with less mass) will be harder to apply as they will rapidly lose any momentum supplied to them (for a real-world example, think about throwing a ping pong ball versus throwing a tennis ball).
Smaller droplets will follow the direction of any air movement present and can be used for dispersal of a spray. However, at least for outside application, to use this effectively requires careful placement of permanent spray equipment with wind-activated sensors or otherwise requires mobile platforms that can be placed upwind as and when they are needed. In this case, large cannon units are simply giving the spray a higher effective release height to allow it to be dispersed by the wind over a greater distance.
Conversely, larger droplets have a greater mass and so, do not lose imparted momentum as quickly, but they do fall faster to the ground and may be too large to effectively capture/neutralize airborne odor or dust particles.
The answer to the question of which spray is right for you is heavily dependent on your situation. Armed with the information in this article, approach equipment suppliers and together you can come to a decision on which spray technology would work for you. | WA
Ed Bals is the Industrial Rotary Atomizer Lead at Micron Sprayers Ltd. (Bromyard, UK). Ed joined Micron in 2015, after gaining a Physics degree at Imperial College London and is responsible for testing and development of the industrial range of rotary atomizers, which complement Micron’s leading ranges of rotary atomizers for use in agriculture, public health and pest control. For more information, e-mail firstname.lastname@example.org or visit www.microngroup.com.