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Knapsack Mistblower Sprayers
The powered knapsack sprayer, also called a mistblower, has a small engine and fan. It is actually a small, back-carried air-blast sprayer.
Powered knapsack sprayers are equipped with two-stroke cycle, three- to five-horsepower engines. These relatively lightweight engines require that oil be mixed with the fuel. They operate at 5,800 to 8,000 revolutions per minute and are rather noisy; operators should wear ear protection. The sprayers weigh between 17 and 25 pounds when empty and are much heavier than the manual models (Figure 2.3).
The engine propels a centrifugal fan that delivers 200 to 450 cubic feet of air per minute (cfm) at a discharge velocity higher than 200 mph. With this high velocity, air shear nozzles are sometimes used. Hydraulic orifice and rotary nozzles also are popular because they can easily form and inject droplets into the airstream. The air from the fan is fed through a flexible tube with an air nozzle on the end that the operator directs to deliver the spray. Because of the high discharge velocity, the air nozzle should be kept at least 6 feet from the trees. The airstream should be aimed downwind so that air currents can assist in carrying the droplets away from the operator. Mistblowers should not be used to apply herbicides.
Powered knapsack sprayers can spray trees much faster than the manual sprayers. The airstream will assist in delivery and coverage even at lower application rates. The number of trees that can be sprayed, however, is still limited because the sprayer tanks are small. Because a tankful will cover only a relatively small area or a few trees, refilling and measuring the chemicals is time consuming.
