[Technical Report] Making The Right Nozzle Choice

Editor’s note: This is the second in a three-part series of articles provided by B&G Equipment Company, a Jackson, Ga.-based product manufacturer. For more information about B&G visit the company’s website at www.bgequip.com.

HYDRAULIC PRESSURE NOZZLES. When using hydraulic spray nozzles, many factors have to be taken into consideration in order to select the proper nozzle for your application. The following is a list of criteria that must be taken into consideration when selecting the appropriate spray nozzle.

Spray Pattern. This is the actual pattern of the liquid as it is applied to the surface. The most common spray patterns used in professional pest control to treat surfaces are fan, cone, and pin stream. It is critical to use a proper spray pattern to ensure optimum distribution.

Fan Spray. This spray pattern (photo #1) is used most commonly in pest control applications because it provides the best distribution of the liquid on the substrate. The flat fan spray pattern is best suited for treating flat surfaces, both horizontal and vertical; the width (swath) of the spray pattern can be 18-24 inches (photo #2), depending on the spray angle of the orifice. Because the majority of the liquid is delivered in the center of the pattern, overlapping the swath about one third will result in complete coverage of the surface.

Cone Spray (photo #3). There are basically two types of cone spray nozzles in common use. One has a fixed out-put capacity and spray pattern, the other is an adjustable cone nozzle. In the latter the spray pattern can be adjusted from a cone pattern to solid stream. Cone spray nozzles were originally designed for agricultural use, to spray over the top of plants to get complete coverage (photo #4). They are generally not suitable for treating flat surfaces, and their use can result in over-application of insecticide.

• Fixed capacity cone nozzle – Most commonly used when a small droplet size and low application rate is required. The benefit of this type of nozzle is that the spray pattern and delivery rate are fixed and cannot be changed by the operator.

• Adjustable cone nozzle – This is most commonly found on inexpensive hand and backpack sprayers. These nozzles can be adjusted from a cone spray to pin stream; however, the result of small adjustments can be a large change in delivery rate and spray pattern.

Precaution. The adjustable cone nozzle may seem to have benefits because it can be changed to fit different applications. However, when a service technician changes the pattern, the delivery rate will change. It usually increases, and this can increase costs and result in over-application. For example: The label rate for perimeter application of insecticides is usually 1 gallon/ 1,000 linear feet. A cone-spray nozzle on an inexpensive backpack sprayer can be adjusted to deliver 45 ounces per minute. This gives the technician about 3 minutes to treat 1,000 linear feet — moving (running!) at the pace of 6 feet per second. When the technician moves at a normal pace, the amount of liquid applied can be as much as twice what the insecticide label allows. Professional backpack sprayers, such as the B&G Pestpro, have a 4-way nozzle that has flat fan spray options (coarse and fine fan). The flow rate of the coarse fan spray is 17 ounces per minute. That rate gives the technician about 8 minutes to treat 1,000 feet, which is moving at a reasonable pace.

Pin Stream. This type of spray tip (photo #5) is typically used for crack and crevice applications where a concentrated stream of insecticide is directed at a specific area, such as a harborage for cockroaches or ants. The delivery rate for pin-stream orifices can be nearly equal to that of a fine fan or half the rate of a coarse fan orifice. The crack and crevice straw is the modern equivalent of the classic pin stream orifice. These (plastic) straws extend the pin stream opening several inches so that the inside of a crevice can be treated with precision and without the splash and waste of insecticide. These may also be referred to as solid stream nozzles.

FLOW RATE. This is the output of the spray orifice and it is usually stated in gallons or ounces per minute. On some insecticide labels the application rate will be given in ounces per linear feet. All insecticide labels provide the required spray capacity to achieve maximum effectiveness. By using too small of a capacity spray tip, you will greatly diminish the effectiveness of the insecticide. Using a spray capacity or rate that is above the requirements shown on the insecticide label will not only waste chemical and money it will also place you in violation of the law.

Precaution. Applying more insecticide will not improve effectiveness, only waste time and increase costs. The application rate for modern insecticides is determined by careful laboratory and field evaluations, and considering all aspects of the target pest and conditions. Over-application cannot provide more than 100 percent control, and when it exceeds the label directions it violates state and federal regulations on proper use of insecticides.

Spray Pressure. The operating spray pressure is critical to maintain proper spray droplet size, spray coverage, and application rate. The ideal spray pressure is sufficient to maintain a good spray pattern and distribution. Most hand-pressurized sprayers are capable of maintaining pressure up to 40 psi. Most spray tips work effectively in the pressure range of 30-40 psi.

Precaution. As operating pressure changes, so does the spray tip’s flow rate. For example, a spray tip may have a flow rate .10 gpm at 10 psi, but increasing the operating pressure to 40 psi increases the flow rate to .20 gpm. It’s important to keep application equipment operating at the proper pressures.

Spray Coverage. The spray coverage is determined by angle of the spray as it exits the nozzle orifice. It is usually measured in degrees (such as 50° and 80°) and considered the nozzle’s spray angle. The spray angle determines the amount of spray coverage at a specific distance (this is called the swath [photo #6]) from the spray tip. Generally, a wider angle will treat a larger coverage area. In most pest control applications a spray tip with a spray angle of 0° (for crack & crevice sprays), 50°, 65°, and 80° are used.

Precaution. When spray pressures change so does the nozzle spray angle. Allowing your spray pressure to decrease to a low level will adversely affect the spray angle and coverage area. The labels on pesticides give the required spray coverage to maintain the optimum effectiveness of the chemical being applied.

Spray Distribution. How the spray droplets are distributed over a treated surface is important to proper insecticide application. Using a high quality and tested spray tip is necessary to maintain proper droplet distribution (photo #7). If good distribution is not obtained then the chemical won’t be applied uniformly across the substrate, and this will diminish effectiveness. A fan nozzle provides effective and efficient distribution on most surfaces.

Droplet Size. After the shearing of the liquid as it exits the orifice takes place, spray droplets are created. Different nozzle types and patterns create different particle size. In general, the larger the spray tip out-put capacity the larger the droplet size. When using an external atomizing force (such as air), droplet size tends to be smaller than the droplets formed from hydraulic systems. This is due to the much higher shearing effect provided by the external force. Droplet size is critical to some insecticide applications, especially fogging.

Spray Velocity. This is the force or the speed of the droplets as they exit the nozzle orifice. Typically the higher the velocity, the further spray droplets will travel before effects such as gravity and air resistance, along with features of the external environment, will cause them to fall. Other factors that contribute to spray velocity include droplet size and operating pressure. With external atomizing nozzles, spray velocity can usually be increased by increasing air pressure. For all insecticide applications it is necessary that spray velocity is sufficient to deliver the droplets to the substrate. Typically, the wider the spray angle (such as 80°) the less spray velocity there is from the nozzle.

EXTERNAL PRESSURE NOZZLES. Sprays with external atomizing component use either an external air source or propellant to help break up the spray into very small droplets. Unlike traditional sprays that are used to apply a residual type chemical, these are used primarily with a contact type insecticide to achieve quick knockdown of the target pest. There are basically two types of external pressurizing systems:

• Aerosol cans. Small capacity systems that use an internal air source or propellant.

• Portable aerosol systems and cold foggers. Large capacity systems that use an external air source (e.g., a compressor).

Aerosol Cans. An aerosol spray is an efficient way to deliver small amounts of liquid insecticide to a substrate or void space. Inside the typical aerosol can are the main ingredient (insecticide) and a propellant. The combined ingredients, insecticide product and propellant, are pressure-sealed in a metal can for later release by depressing a valve.

Liquid propellant. These propellants are gases that exist as a liquid under pressure. Since the aerosol is under pressure, the propellant exists mainly as a liquid, but it will also be in the ‘head space’ at the top of the can as a gas. As the product is sprayed from the can, some of the liquid propellant vaporizes to fill the head space. This maintains constant pressure in the can and the quality of the spray delivered is maintained until the can is empty. In aerosol products based on liquid propellants, the propellant is an essential part of the formulation. The insecticide is miscible with the propellant or dissolved with it. When the liquid exits the spray can actuator (valve) as small droplets, the propellant immediately vaporizes. The ultimate droplet size of the spray can be controlled by the amount of propellant, its pressure, and the actuator valve. Special actuators can deliver foam by introducing air into the liquid as it exits.

Compressed gas propellant. These propellants only occupy the head space in the aerosol can; they are not mixed with the insecticide product like the liquid propellants. When the actuator valve is depressed the gas pressure acts to push the liquid out of the can. The amount of gas in the can remains the same, but as the product is dispersed, the pressure in the can will drop. Since there is no solvent type propellant that instantly vaporizes when the spray emerges, only the product is sprayed. The only means of creating small droplets with this type of system is to break up the liquid through mechanical action as it passes through the actuator.

Portable Systems & Cold Foggers. These use an external air source either supplied by an air compressor or in the case of cold foggers by using an air pump. The principle behind the operation of this type system is to use the air source in assisting of the atomization or break-up of the spray into small droplets. These systems create very fine or ULV type spray droplets. In addition, flow rates are generally very small. Most of this type of equipment allows the operator to adjust their liquid so that different flow rates and degrees of atomization can be achieved. Some of the applications where this type of spray is beneficial is treating large spaces, such as storage rooms, and treating small voids, such as behind walls or beneath cabinets. In general, the higher the air pressure in the system (with constant liquid pressure) the lower the flow rate and the smaller the droplet size. The reverse is true for liquid pressure: the higher the liquid pressure (with constant air pressure) the higher the flow rate and the larger the droplet size. For most of the commercially available portable aerosol systems, the air pressure is fixed (not adjustable) and the liquid pressure is adjustable. Thus, reducing the liquid pressure reduces the droplet size and creates the desired cold fog or ULV droplets.

Benefits. The major benefit for using systems with an external atomizing force is to create small droplets and allow these droplets to have the maximum hang time in the air or to assist in maximum penetration into a wall void. The long-term benefits of portable aerosol systems and hand-held (cold) foggers include the cost savings and environmental friendly aspects of using a renewable system versus disposable aerosol cans. These systems have a professional appearance and are adaptable to a variety of commercial and residential locations.

Spray Droplet Size. The first step in understanding spray droplet size is to consider how droplets are measured. There are many different ways to express droplet size, and the same spray can be expressed in different droplet size measurements. The most accurate measurement is VMD (Volume Median Diameter). Consider a spray that has a 15 micron (diameter) droplet size. Under a VMD measurement, 50 percent of the volume (spray capacity) is in droplets that have diameters smaller than 15 microns and 50 percent of the volume is in droplets that have diameters larger than 15 microns. The VMD measurement divides total spray volume into two equal parts, and the droplet size in the exact middle is the VMD. Note: An important feature of all liquid sprays is that they contain a range of droplet sizes—from the very small, perhaps only 5 microns, to very large, 200+ microns. The mechanics of liquids and nozzles prevents a spray from having a fixed droplet size.

This article was written by Dr. William Robinson, technical director of Jackson, Ga.-based B&G Equipment Company, and Cecil Patterson, CEO and president of B&G.
 

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B&G Technical resources

This article was excerpted from "Application of Pesticides in Professional Pest Control," by Cecil Patterson and William Robinson of B&G Equipment Company. The 8-page manual is a review of pesticide applications most commonly used in the United States. Among the topics covered are the development of pesticides; choosing the right equipment; and maintenance tips. "Application of Pesticides in Professional Pest Control" is one of several technical articles and support materials available for downloading on B&G Equipment Company’s website, www.bgequip.com. Other available technical reference materials include "Bait Gun Economics: Technical Manual;" "Microfoam EF Technical Information;" "Portable Aerosol Unit: Training Guide;" and "The Lo-Line Trap For Professional Pest Control."