Portable spraying equipment
Motorised knapsack sprayerWe welcome your feedback on this site. Please take a few moments to let me (firstname.lastname@example.org) know what you think: https://www.surveymonkey.co.uk/r/GRB538Y
Many features are shared with lever operated knapsack sprayer. Differences include:
- Pump operated by an internal combustion engine or an electric motor
- Greater range of spraying pressures up to 11 bar
- No manual pumping necessary
- Possibility of mounting a spray gun
- More complex,
- dependency on a power source,
- more expensive and
- often noisy
Notes to slide: The lever-operated pump is replaced by a motor driven one. Avoiding manual pumping is less tiresome for the operator despite the extra weight of the unit. Flow can be expected less “pulsed” and fluctuating in pressure.
Areas of use: Small areas of field and bush-type crops
Products applied: Herbicides, Fungicides and Insecticides
Water spray volumes used: 50 to 500 l/ha
+ No pumping needed during application
- Small tank capacity thus many refills
+ Simple to use and maintain
- Pressure drops during spraying unless pressure control valve fitted
Note: A pressure drop from 5 bar to 2 bar will reduce nozzle output by 58 %
Notes to slide: Specific characteristics of a compression sprayer:
In general compression sprayers are best-suited to applying insecticides and fungicides. The operating pressures are often too high to apply herbicides safely without risk of drift. When applying insecticides or fungicides to most field crops, a lever operated knapsack sprayer is often the better choice as in general these tend to be more durable than compression sprayers. Compression sprayers would be preferred in situations where it might be difficult to operate a lever easily, or where high crops are being treated since lever operated knapsacks cannot easily generate high enough pressures to direct spray upwards for any distance without excessive pumping. Compression sprayers are also the sprayers of choice for many operations relating to the spraying of buildings, such as the control of insect disease vectors, as they are easily used in confined areas and the operator can concentrate on obtaining the correct speed of the nozzle moving across the wall without having to think about pumping rates.
Compression sprayers technical features
Main technical features:
- Pressurized tank of 3 to 7 litres in volume
- Hand operated plunger pump or able to be safely pressurised from an outside source
- One third of tank volume functions as the pressure chamber whilst two thirds may contain the prepared spray solution.
- Pressure control flow valves
- Spray lance & nozzle (Flat fan, Hollow cone, Deflector)
- Decompression valve to enable the spray tank lid to be safely removed.
Notes to slide: With compression, pre-pressurized or pneumatic sprayers there is no pressure chamber as the whole tank is pressurised. This means that to function efficiently they are only partially filled with spray liquid, leaving an air space of about 30% of the tank volume above the liquid. Overfilling the tank with liquid and hence leaving an insufficient air space will cause the pressure in the tank to drop very rapidly once spraying commences, resulting in very uneven flow.
A typical compression sprayer‘s features are:
- Tank - this is strong as it is a pressure vessel
- Lance & nozzle - similar to that on a lever operated knapsack sprayer
- Pump - a piston-type pump to pump AIR
- Water fill line on tank - a suitable air space of 1/3 of the volume is essential
- Pressure gauge - to show tank pressure. Should have indication of too high or too low pressure - eg. colour bands
- Decompression valve - to allow tank lid to be opened safely
Note that not all compression sprayers have a pressure gauge, pressure relief valve or decompression valve, but for a safe, effective sprayer these components should be present. On some sprayers one valve acts as both relief valve and decompression valve.
Notes to slide 25:
After filling the tank to 2/3 of it‘s volume, it is usually pressurised from a hand plunger-type pump, which may either form an integral part of the tank lid or is located separately from the lid on top of the sprayer. The tank should always be fitted with a pressure gauge, so that the operator is aware of the internal pressure, since over-pressurisation of the tank can be dangerous. Some sprayers have no pressure gauges and simply recommend in the accompanying instruction manual the number of pump strokes required to achieve working pressure, but since instruction manuals are easily misplaced, it is always preferable to have a pressure gauge fitted. Once spraying commences the tank pressure falls, which will affect the nozzle output rate and the spray quality. It is important therefore to re-pressurise the tank frequently to ensure a relatively uniform output. Some sprayers are fitted with a pressure regulating valve located between the tank outlet and the lance. This valve is designed to maintain a constant output pressure over a range of tank pressures, and will ensure a uniform output at the nozzle. The sprayer should also be fitted with a pressure relief valve, designed to relive tank pressure at a pre-set pressure level and prevent dangerous over-pressurisation of the tank.
- It is very important to de-pressurise the sprayer BEFORE completely opening the tank lid. This is usually achieved in one of two ways:
There may be a decompression valve or button fitted to the top of the sprayer.
- If no decompression valve exists, the lid should be partially unscrewed with care until the hissing sound of decompression is heard. Once the hissing stops it is safe to open the lid completely. A faceshield and gloves should be worn for this operation in case the pressure is released too quickly and hazardous jets of spray are emitted.
Portable Powered Sprayers: Technical Features
Main technical features:
- May be manufactured or self made
- May be carried or mounted on wheels
- Tanks (often a locally sourced barrel) of various sizes from 50 to 200+ litres which are non pressurized
- Motor operated [often high pressure piston] pump
- Note: Pumps often unnecessarily used at too high pressures -Use spray pressures as stated within label advice
- Pressure regulating valve should be used to limit both spray line and nozzle pressure
- Hoses of 10 to 50 m in length
- Spray gun with trigger and nozzle type that may vary widely
Notes to slide 25: The power sprayer consists of an integrated or external spray tank, a high pressure piston pump usually powered by a petrol engine a pressure regulating valve and a hose of up to 50 m of length. Spray tanks are too big to be carried as a knapsack. The power sprayer is produced in a number of version. Most simple and common is a engine driven pump mounted on a frame without wheels, a 200 l drum and hose and lance. Flow regulation is to be done via a pressure regulating valve and/or by restrictors (basic power sprayer) and the size of the nozzle. At the other end are sprayers mounted on wheels, equipped with pressure regulators. Technically, the power sprayer has a lot in common with the motorized knapsack-sprayer.
Areas of use: bush and trees crops, vegetables, flowers, greenhouse applications. Note: High volumes and/or hose spraying are Not recommended for field crops
Products applied: Fungicides and Insecticides
Water spray volumes used: 1000 to 4000 l/ha (up to run-off point)
No pumping needed during application
Precision of application compromised and often poor
Simple to use and maintain, adaptable
Unnecessarily high pressures greatly increase risks
Ability to cover a longer spray-distance
Long hose – often helpers are needed
High water volumes often used; which may increase losses due to run off
Notes to slides 27: Well suited to treat bush and tree crops the power sprayer gets also used in field crops like vegetables (not ideal). The unit is generally set for high volume spraying, transporting the droplets with high pressure. Hollow cone nozzles are the preferred type of nozzles.
Motorised Mist Blowers
Notes to slide: Starting the engine
The correct oil/petrol mixture must always be used. The manufacturer's literature should be checked for the correct ratio of oil/petrol for that engine. Often the ratio is stamped onto the fuel tank or fuel tank lid, e.g. 1:25, 1:30 etc. The most suitable oil to use is 30 SAE oil. Multigrade oil should NEVER be used because the additives will cause engine failure. Normal or low grade petrol should be used.
The manufacturer's instructions should be checked for starting procedure, but in the absence of this, the following procedure should be used. Once the fuel tanks and spray tanks are filled, any ON/OFF switch on the engine should be set to ON, and the fuel tap opened, allowing the carburettor to fill with fuel. The choke lever is moved to the "CLOSED" position, and the throttle closed. The engine is then started by pulling sharply but evenly on the recoil starter, taking care to rewind the starter rope slowly, not releasing it to snap back. When the engine starts, the choke should be moved to the "open" position and the throttle opened to increase engine speed to maximum revs. At the end of the day the engine should be stopped by closing the fuel tap and allowing the carburettor to run dry, rather than by operating the "engine off" switch.
Motorised mist-blowers: technical features
Main technical features:
- Slightly pressurized tank with a capacity of 10 to 15 litres
- Pump and fan driven by an internal combustion engine
- Air assisted spraying that entrains droplets and propels them to target.
- Used to spray tall trees and larger bushier crops such as tea
- Flow control restrictors and cut-off valves
- Spray lance & air-shear nozzle
- Possibility to apply some dry product formulations (dust application) with many models
Notes to slide: The two-stroke engine is usually either 35cc or 60-70cc capacity. The smaller engine is usually preferred because it is lighter and hence more comfortable to use, but the larger engine may be necessary if the mistblower is to be used for applying vertically into relatively tall tree crops. The engine is air-cooled and connected to the fan or blower by direct drive. There is a throttle to control engine speed, and a stop switch. The fan or blower may be either vertically or horizontally mounted on the knapsack frame, although vertical mounting is more common.
Motorised mist-blowers: droplet formation
Notes to slide:
The airstream from the fan is directed into a flexible hose. The pesticide tank is normally of at least 10 litre capacity and connected to the nozzle (located near the mouth of the flexible hose) by a narrow tube. The spray tank is usually slightly pressurised by a tube bleeding air pressure back from the fan casing, and hence there is a seal on the tank lid to maintain pressure (usually at around 0.2 bar or 3 psi). This pressurisation ensures even flow to the nozzle.
The liquid is delivered to the nozzle through some sort of restrictor to control flow. The metering system is usually of two main types: either a rotating disc with a number of different sized orifices, or a set of interchangeable restrictors. Most mistblowers also have a separate ON/OFF valve to control liquid flow separately from the air blast.
The nozzle is of a type known as an air-shear or twin fluid nozzle. In its simplest form, this consists of a tube delivering pesticide inserted into the airstream. The negative pressure at the orifice causes liquid to flow into the airstream and the liquid jet is then sheared into droplets of varying sizes. An even flow of air over the liquid orifice assists droplet formation, and so some mistblowers feed the liquid to either side of an aerofoil-shaped section mounted across the outlet of the discharge hose. Other sprayers have either a fixed or spinning disc to improve droplet formation by spreading the liquid thinly before it reaches the airstream. At least one manufacturer now produces a spinning disc attachment that can be retrofitted on the outlet of a standard mistblower for ULV spraying.
Areas of use: medium areas of field, bush and tree crops
Products applied: Fungicides and Insecticides
Water spray volumes used: 50 to 100+ l/ha
Low volume applications
Risk of contamination during work due to the spray mist
No pumping needed during application
Spraying taller crops possible
Air flow helps penetration into a crop canopy
Notes to slide: Motorised mistblowers are most suitable for low (LV) and ultra-low volume (ULV) application of insecticides and sometimes fungicides. They are best suited to conditions requiring projection of spray droplets either horizontally or vertically such as into tree canopies when spraying insecticides in cocoa. Some mistblowers can also be adapted for applying granules and dusts. This is usually achieved by allowing the granules to pass directly from the tank into the fan casing and along the flexible hose in the air blast or a wide-bore flexible hose is connected between the tank outlet and the airstream hose where the latter leaves the fan casing. In some areas a 30m long plastic tube has been fitted to the discharge outlet. This is supported by a second operator at the end and has a series of holes along its length through which dusts and microgranules are dispersed. Care must be taken to avoid operator contamination when using lower volumes of pesticide, since the concentration is higher.
Questions to consider at point of purchase:
- Will a mist-blower fulfil your intended use?
- Is that intended use within product label recommendations?
- Are FAO minimum recommendations fulfilled ?
- Is the engine of the correct size ?
- Is maintenance guaranteed ?
- Is the unit comfortable enough ?
- Is the required protective clothing provided ?
Notes to slide: "The small print“
When considering selection of a mistblower in preference to a hand pumped sprayer such as a lever-operated knapsack or compression sprayer the following points should be noted:
- High initial cost of mistblower - the extra cost does not mean that this type of sprayer will do a better job in many spray operations.
- Mistblowers require a higher level of regular maintenance and servicing than hand pumped sprayers, and because of their complexity there is more which is liable to go wrong.
- Unless there is good reason for selecting a mistblower, e.g. to obtain coverage in tall crops or in field crops where walking in the crop is difficult and hence the longer horizontal throw of the mistblower is advantageous, a more uniform coverage can usually be obtained from a hand-pumped sprayer.
- Mistblowers produce a relatively high proportion of small droplets and so are not generally suitable for herbicides. With all products care should be taken to avoid the risk of inhalation.
- Mistblowers are much heavier than hand-pumped sprayers and so are much more uncomfortable and tiring to use over long periods. In addition, there is risk of ear damage unless ear protection is used.
Having decided upon a mistblower as the necessary type of sprayer for the job, a number of other considerations and features are worth noting:
- Engine size - the higher capacity (60-70cc) engines should only be selected if a high vertical throw is needed, for example into a tall tree canopy. The smaller engine (35cc) is usually adequate for most field crops and is lighter to carry.
- For low volume application in tall crops a mistblower with a pump to feed the nozzle will give a more even flow rate.
- Ultra-low volume application - not all mist mistblowers are equipped with restrictors allowing ULV flow rates therefore if ULV spraying is intended, be sure that the mistblower selected will accommodate this.
- Flow control - the type of flow rate control having interchangeable restrictors, only one of which is fitted at any given time, means that there is less likelihood of the flow rate being incorrectly set or changed by inexperienced spraymen.
- A mistblower which can easily be adapted to spray powders and granules as well as liquids may prove a more flexible sprayer.
- A mistblower which has the oil/petrol mix ratio stamped onto the fuel tank or lid is less liable to abuse by inexperienced spraymen using the wrong mixture.
- A horizontally mounted fan, if it is located near the knapsack frame base might cause stones etc. to be sucked into the fan when the mistblower is placed on the ground, resulting in fan damage. A vertically mounted fan is therefore usually more desirable.
- The engine exhaust should be protected by a guard to prevent accidental burning on the hot exhaust when taking the sprayer on and off.
- The straps should be designed to allow the mistblower to be rapidly removed in the event of an engine fire.
- The spark plugs should be easily accessible for regular checks and maintenance.
- Spare parts and service requirements should be readily available along with advice if needed.
Controlled Droplet Application sprayer(CDA),
.... also sometimes called rotary atomisers
Notes to slide: Ultra low volume (ULV) spraying is not a new technique for applying pesticides, but it is only in recent years that it has become commercially popular. It has been defined as the application of less than 5 1/ha for field crops (for tree and bush crops ULV is considered to be less than 50 l/ha) and is generally the application of the minimum volume of spray liquid which can achieve economic control.
Typically (but not exclusively) spray equipment used for ULV application has been of a type known as Controlled Droplet Application (CDA) equipment. CDA involves the production of droplets which are much more uniform in size than those produced by high volume hydraulic sprayers and which are of a size which is best suited to the spray target.
ULV/CDA spraying is applicable to all types of spray equipment including hand-held sprayers, motorised sprayers and tractor sprayers. Only hand-held CDA spray equipment will be considered here. The main advantages of this type of spraying are:
- The absence of any need for large quantities of clean water makes this type of spraying attractive in areas where constant water supply is a problem.
- The reduced volumes make the sprayers much lighter and hence easier and less tiring to use.
- Spray is more accurately targeted. Less wasted product is more cost effective and is less likely to harm beneficial insects. They are therefore ideal for use in IPM programmes.
- Mixing of spray with water is not needed, cutting out a potential source of hazard in the spray operation.
- Mixing of spray with water is not needed, cutting out a potential source of hazard in the spray operation.
- Any spray liquid unused at the end of the spray operation can be stored for future use and does not require disposal.
- The most common types of ULV/CDA equipment in current use are the rotary atomisers or spinning disc sprayers and the 'Electrodyn' electrostatic sprayer.
Basically it comprises a disc, an electric motor to drive the disc, a small spray bottle with a restrictor to feed liquid onto the disc, a long handle which usually houses the batteries for the motor (in some models a separate battery pack is used).
Like the nozzle in a hydraulic sprayer, the disc is the important part of the machine as it is doing the job of producing the droplets. Most discs have a toothed edge to assist droplet formation, and some also have fine grooves on the surface to distribute the liquid evenly to the teeth. Some models also have double discs designed to ensure that any liquid collection on the back of the front disc is caught and re-distributed on the rear disc to prevent splashing.
The spray liquid is held in a small bottle inverted above the disc. The liquid feeds onto the disc by gravity through a restrictor. The restrictors are interchangeable and are usually colour coded; a range of sizes is normally required to cope with spray liquids of varying viscosity. It is important that the liquid bottle is held vertically above the disc, and most sprayers have a pivoting spray head to allow the correct alignment of head and bottle when the handle is held at different angles above the crop. To ensure even liquid flow as the bottle empties there is an air bleed for the bottle. This may be in the form of a separate small pipe or hole, but is also found in the screw-threading between the bottle and bottle holder on the spray head.
Disc designs vary from those used at very high rotational speeds [small drops] to those rotating more slowly [larger drops]
Notes to slide: The key differences are:
High disc speed
Low disc speed
(up to 5-15000 rpm)
(usually about 2000 rpm)
Small droplets <100µm
Larger droplets 200-500µm
Uses wind to place spray
Direct placement spraying
Disc end held upwards
Disc end held down
Spray reservoir at disc end
Spray reservoir at opposite end to disc or on operators back
Spray reservoir 1 litre or less
Spray tank >1 litre
But often connected to a back-pack container e.g. „Handy“ = 5 l.
CDA sprayers; main technical features
- Rotary disc atomizer, electric driven (Different models for herbicide and for fungicide / insecticide spraying)
- Narrow droplet spectrum produced (Droplet sizes changed to intended use)
- Non pressurized tank, various sizes available (For example, 1 litre bottle, 5 litre tank or 10 litre back-pack)
- No pump, liquid supplied by gravitational force
- Flow-rate regulated by a restrictor
- Different spray lance and atomiser for herbicides and insecticides/fungicides
- Sprayers usually light & easy to carry
Notes to slide: The majority of sprayer models use 6 or 8 standard 1.5 volt cells or batteries ('D' size or U2) to drive the electric motor to rotate the disc. Occasionally two 6 volt lantern-type batteries are used, Batteries have a finite lifetime considered to be when individual cells fall to 1 volt output, or when disc speed drops to below approximately 6000 rpm. Battery life is typically about 8 hours spraying time, but it may vary from anything between 5 and 15 hours spraying time depending on the model of sprayer, the liquid flow rate used, the liquid viscosity and the operating temperature. Battering life can be extended by alternating two separate sets of batteries every two hours, since batteries will partially recover if rested.
Alternatives to standard zinc-carbon batteries are nickel-cadmium rechargeable cells. These are actually more effective since they do not show the gradual linear decline in power output with use (resulting in a gradual decrease in disc speed) that the zinc-carbon batteries show. They have a more or less constant power output over about six hours followed by a characteristic sudden decline in power. This is considered an advantage to the operator since it is then obvious that the batteries need recharging. However the initial cost of rechargeable nickel-cadmium batteries and the associated charging pack is relatively high and can only be justified if large areas are to be sprayed in a season.
Solar power has also been considered as a power source for these sprayers, but the very high cost of the solar panels has proved inhibiting, although newer technology may eventually bring the price down and make it a viable option.
Areas of use: concentrated (or undiluted) spray solution supplied under gravity at low flow rates
Products applied: Fungicides, Herbicides, Insecticides
Water spray volumes used: volumes applied can be very low; less than 20 litres/hectare
- Disc rotational speed is set or changed to apply larger drops for herbicide applications or smaller sizes for insecticide/fungicide use.
- Small drop use allows a ‘controlled’ drift application.
- Some designs are engine driven and may include fans to propel spray onto target surfaces.
Notes to slide: These sprayers produce droplets by feeding liquid onto a rapidly spinning disc. Ligaments of liquid are flung off the edge of the disc which then break up to form droplets which are relatively similar in size.
Droplet size is affected by two factors:
- the liquid flow rate onto the disc (this is affected by restrictor size used and liquid viscosity) - higher flow rates produce larger droplets.
- the speed of rotation of the disc - faster disc speeds produce smaller droplets.
If flow rate becomes too high for the size and speed of the disc, the normal atomisation process by ligament break up does not happen and very variable droplet sizes are produced.
Information on droplet size is given in connection with the rpm to be reached at a nominal voltage. A lower voltage than the nominal one results in larger droplets. Depending on the way the unit is used, a reduced in swath-width may be an unwanted effect. Voltage or rpm can be measured with simple tools.
Since insecticides and fungicides require relatively small droplets and herbicides require larger droplets less susceptible to drift, two different types of spinning disc sprayer have evolved to suit these different requirements. The herbicide sprayers are designed to operate at much slower disc speeds and produce droplets in the typically 250-500 µm size range. The design of the sprayer allows the head with the disc to be held near to the ground or weed target, and the liquid bottle is frequently positioned on the back end to counterbalance the weight of the sprayer disc head with motor (some newer sprayers have the spray liquid contained in a pack on the operators back).
CDA sprayer are powered by compressed air rather than electricity and used to produce larger drops for herbicide applications. Note the direction of drops; some are being applied directly to the soil surface whilst others are going upwards, at first, before falling by gravity.
Notes to slide: The BIRKY represented an exceptional CDA sprayer. No batteries were needed, the spinning disk was driven via a turbine and a lever-operated air pump. A whistling noise indicated the correct disk-speed. The spray-unit was behind the operator helping to reduce contamination.
Select correct type of CDA sprayer for the intended use:
Is it to apply a herbicide or fungicide or insecticide? Typical spray volumes are very low: ULV 1 – 5 l/ha; LV 5 – 20 l/ha
Check with label that the product may be applied at these volumes and note PPE requirements
No pumping needed during application
Spraying with very concentrated solutions that may limit product choice
Light and simple construction
Smaller droplets may drift
High work rate for a handheld sprayer
Swath widths may be poorly defined
Little use of water to dilute
Droplet spectrum varies with dropping voltage
Notes to slide: The main features of ULV/CDA spraying are that there is usually no dilution of the product as supplied by the retailer (the exception to this is water based ULV spraying (WULV) which is not common and is detailed in section. The carrier liquid used for spraying is usually a solvent or low volatility oil. As there is no dilution involved, the sprayed formulation is more concentrated than a typical high volume hydraulic spray. This has two implications - it is more hazardous to the operator if it is allowed to contaminate, and the consequences of any errors in calibrations are more serious. The hazards posed by concentrations are, however, offset by the fact that, provided they are used properly, CDA types of spray equipment tend to be safer for the operator to use than high volume hydraulic equipment, in that they are less likely to contaminate the operator during use. This is particularly true of electrostatic sprayers. However they do require initial training in their techniques of use, since these tend to be somewhat different to conventional knapsack or compression sprayers.
Insecticides can be sprayed in wide swaths of 2 – 10 m (ULV) using less time and labour but requiring relatively high levels of skill to operate safely and effectively. Less operator contamination only when used correctly e.g. from an upwind position otherwise the concentrated formulations means greater risk of hazard if contamination occurs as well as more serious consequences of under/over dosing if errors occur.
Often more effective than high volume spraying. Battery requirements can be expensive for smallholders.
Name the key specifications described in the FAO guidelines for knapsack sprayers?
What type of sprayers cannot be accurately calibrated?
Identify the key components of your knapsack?
What factors would you take into consideration when selecting application equipment?