6.4.1 Construction
Mistblowers typically consist of a 35 - 70 cc two-stroke engine, which drives a centrifugal fan. The larger size engine is required to drive a fan with a greater output of air volume.
These heavier sprayers are needed to spray taller trees, as the greater volume of air emitted can project droplets higher than the small mistblowers. It is rarely possible to project droplets higher than 10m vertically, even with the larger motorised knapsacks.
The engine and fan unit are attached by anti-vibration mountings to a knapsack frame, designed to allow the sprayer to stand upright on the ground.
The frame, with straps, also carries a pesticide tank, spray delivery tube, fuel tank and an air delivery hose. A nozzle is mounted at the end of the air delivery tube.
The volume of spray liquid emitted is controlled by a variable or fixed restrictor, and there is an on/off tap also attached to the air delivery tube. The tank is usually of 10-12 litre capacity. Some with larger tanks are made, but the extra weight in addition to the fan and engine, is considered unacceptable.
The tank has a wide opening to facilitate pouring liquid into it. The floor of the tank should also slope to a low outlet point. Some air is fed from the fan into the spray tank and usually ducted to the base of the filter at the filler opening to provide low pressure (25 kPa) for delivering the spray liquid to the nozzle.
This air pressure is most important if the standard air delivery tube is pointed upwards, when the nozzle may be above the level of liquid in the tank. The large lid on the tank must therefore have an air-tight fit.
On some machines, instead of relying on this air pressure, there is a separate pump, which is usually mounted directly on the fan drive shaft.
Mistblowers fitted with formulation pumps produce a considerably more reliable flow (especially at low rates) and are easier to calibrate, but retrofitting pumps is expensive (usually >€/$100).
The simplest of mistblowers have a single tube to direct the spray liquid into the high velocity air stream.
However, several manufacturers have developed alternative ways of spreading the liquid thinly into the air stream.
On some machines, rotary nozzles are fitted; these can provide a more uniform spray droplet size distribution, but the quality and price of these devices vary considerably.
Flow rate and air velocity have a major effect on droplet size and mistblowers must always be operated at full throttle.
The flow rate is not determined by the nozzle, but by a restrictor mounted in line with the nozzle. On many sprayers, there is a variable restrictor, often with a number of settings.
Users will frequently set this restrictor to the maximum open setting to empty the tank as quickly as possible. This may lead to poor atomisation, so the recommendation is to use a sprayer with separate fixed restrictors.
When the appropriate restrictor is in place, it cannot (and should not) be changed by operators in the field.
