Technical Library: Sprayers and Spraying

Low-volume Spraying Reduces Worker Exposure

by Frederick C. Dramm

“When chosen and used property,
low-volume application methods
will produce the best results in every greenhouse.”

In recent years chemical application in the greenhouse industry has undergone changes due to pressure from government regulations, lobbying efforts of environmentalists, greater awareness of worker safety and the introduction of integrated Pest Management practices. These changes have forced improvements which include the acceptance of a number of new low-volume chemical application methods. This new wave of equipment and technology is applicable in every greenhouse and can improve worker safety, reduce costly chemical runoff and provide consistently effective application of chemicals.

High-volume Spraying

Conventional high-volume (HV) sprayers apply chemicals diluted in large amounts of water. The high volume of water is necessary because conventional hydraulic sprayers produce large spray droplets (100-400 microns in diameter). The large droplet spectrum does not allow the active chemical to be spread across the crop effectively and will often run off, wasting valuable chemical. Worker exposure to chemicals during the long application times causes concern.

Low-volume Spraying

Low-volume (LV) chemical application involves spraying chemicals at the labeled rate (per acre or square foot area) in greatly reduced quantities of water, resulting in a highly concentrated solution. Thus, low-volume equipment applies the same quantity of chemical to a given area as does high-volume hydraulic spraying equipment. The term low-volume refers to the small amount of water or diluent used to apply the chemical. For example, if the labeled rate for a chemical is 16 ounces mixed into 100 gallons of water, under low-volume techniques this same 16 ounces of chemical would be mixed into a quantity of water or diluent as small as 23 gallons. This small quantity of solution allows either a very fast or an automatic application of the chemical – reducing or eliminating the operator’s exposure to the chemical and reducing chemical runoff.

The LV principle works only if micros pray particles are produced. Research and commercial use has proven that droplets which are less than 100 microns in diameter increase the effectiveness of chemical application. Low-volume sprayers that produce a 10 micron average diameter particle will deposit an average of 19,000 particles or droplets in I cm. HV application (100 micron average diameter particles) would deposit only about 19 droplets in I cm².

Types of Applicators

There is a variety of different types of LV chemical applicators available, ranging from very high pressure hydraulic sprayers (3,000 psi) to thermal foggers and automatic aerosol micro-particle generators. There is no one perfect type of sprayer for the greenhouse. Many growers find that integrating two or more types of sprayers is the ideal. A favorite combination is a thermal fogger and a high pressure or conventional hydraulic sprayer. The thermal logger provides a fast, broad coverage and the hydraulic sprayer affords easy spot treatment.

High Pressure Hydraulic Sprayers

Considered a step up from conventional hydraulic sprayers are the high pressure hydraulic sprayers. These machines have a similar design to conventional spray equipment, but they operate at 1,000-3,000 psi and require a much smaller volume of water added to the chemical.

The piston or diaphragm pump forces the chemical solution through a hand-held spray gun’s nozzle tip. The solution exits the nozzle tip at high pressure and produces an extremely fine spray with much smaller droplets than those created by conventional hydraulic sprayers working at 300-600 psi.

For example, the 601S RWR Coldfogger'” produced by the Dramm Corporation is the only 3,000 psi airless chemical sprayer available for greenhouse use. the 30 micron average diameter particles are thrown 20-25 feet from the spray gun. With a 12-gallon tank of concentrated spray solution growers should be able to treat 45,000 sq. ft. in 45-60 minutes.

This type of sprayer is useful for growers who produce violets and orchids or other crops which may be sensitive to carrier solutions used by thermal foggers. The fine spray settles out of the air rapidly, so it is also effective in shade or saran houses.

Recently introduced electrostatic sprayers are similar to high pressure hydraulic sprayers, but do not use a high pressure pump. These machines produce a fine spray quality that is electrically charged and then air-blasted into the crop. The negatively charged particles are attracted to any surface and can provide coverage which is similar to high pressure hydraulic spraying.

Thermal Fogging Machines

The thermal fogging principle has been in existence for many years. Thermal fogging equipment uses a pulsing jet engine to produce a highly visible fog that can stay suspended in the air for up to six hours. These machines are the fastest sprayers and are ideal for quickly treating lots of individual quonsets or large gutter-connected greenhouses.

Inside thermal foggers, a gasoline and air mixture explodes in the enclosed resonator and the explosion rushes out as a jet stream. A chemical solution is injected into the jet stream and is blown apart into billions of micronized particles that are 0.5-30.0 microns in diameter.

Thermal foggers require specialized carrier solutions to produce a visible fog, eliminate the evaporation of chemical droplets and ensure uniform particle sizes.

Aerosol Micro-particle Generators

New to the U.S. greenhouse market are automatic micro-particle generators. These machines employ an oilless compressor to pump air through a special nozzle to produce super-fine fog particles that contact every plant surface. On most large models a built-in convection fan circulates air in the greenhouse to disperse the fog and a timer allows machines to treat large gutter-connected ranges without an operator present. Smaller models are also automatic, but do not have a convection fan.

The specially designed nozzles can produce particles that are as small as 0.5-10 microns in diameter and stay suspended in the greenhouse for up to six hours. No special carrier is required for diluting the chemical solutions and most commonly used greenhouse chemical formulations, including wettable powders, can be fogged on a wide variety of crops. Most aerosol micro-particle generators have a solution tank agitator.

These new machines are able to treat very large areas of greenhouse from one location. With horizontal airflow fan assistance, the Dramm LVH-10 Autofog can treat upwards of 70,000 sq. It. with a full tank of solution.

The Next Generation

The latest development in aerosol micro-particle generators is a fogging system with stationary logger heads which are computer controlled. This set-up will allow growers to keep one or more fogger heads in a permanent position inside the greenhouse and pipe compressed air to the nozzle from a centrally located compressor. A system like this is best controlled by a greenhouse computer, but it can be controlled by a separate control box.

Low-volume chemical application is going to have a greater role to play in the future of the greenhouse industry Reduced worker exposure, increased worker safety, costly chemical runoff and the need [or consistent, effective applications are going to be issues that will dominate the chemical application scene for many years. When chosen and used properly, low-volume application methods will produce the best results in every greenhouse.