STATE PLANT BOARD
July 1948 ET-258
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Qaarantine
FIELD-MODEL AEROSOL MACHINES
By A. H. Teomans
Division of Control Investigations
Field-model aerosol machines were developed during World War II. These
machines release a foglike aerosol containing insecticide, which drifts with
the wind over large areas. Under good conditions one of these machines can
treat 50 acres per hour with one man as operator.
Since aerosols remain airborne longer than sprays, flying insects collect
more of the insecticide on them than do stationary insects. Aerosol particles,
being smaller than spray particles, penetrate foliage and small openings better.
Aerosol machines are best adapted to applying small quantities of insec-
ticide over large areas, and for this reason a concentrated solution of insea-
ticide is most suitable for use in them. Owing to the reduced weight of con-
centrated solutions, aerosol machines can be used over areas where heavier
equipment for applying dilute sprays would have difficulty in moving. The
aerosol method shows promise for applying DDT to control the tarnished plant
bag on peach trees, Lygus bugs and leafhoppers on alfalfa, flea beetles on
beets, whiteflies on eggplant, arqVworms on truck crops, and as a temporary
control of various species of adult flies and mosquitoes.
One disadvantage of the aerosol method of applying insecticides out of
doors is the requirement of suitable weather conditions. A surface inversion
in the air is necessary to keep Phe aerosol close to the ground. The tempera.
ture near the ground should be 1 3. cooler than at 6 feet above to indicate
a suitable surface inversion. This condition usually occurs on clear nights
between 1 hour after sunset and sunrise and sometimes during the day when the
ground is wet or cold. A light wind of from 1/2 to 8 m.p.h. and steady in
direction is'also necessary.
The amount of deposit on foliage outdoors depends on the contour of the
ground; the density, height, and types of foliage; the particle size of the
aerosol; and the wind velocity and surface inversion. Under the best condi-
tions about 50 percent of the insecticide can be accounted for and the deposit
extends for about 2000 feet, but in poor weather the loss becomes greater and
if the aerosol is released in the heat of the day the appreciable deposit em-
tends less than 100 feet. Under all conditions the' deposit tapers off in In.
tensity as the distance from the machine increases.- Adult mosquitoes flying
through the aerosol have been affected as far as a mile from the release point,
even though there Is no appreciable deposit at this distance.
A great deal of research has been done to determine the optimnaim particle
size to use under various conditions and how to dispense the aerosol with a
minimum loss of insecticide. Generally the optimum particle size for outdoor
applications has been found to be between 10 and 50 microns mass median diam-
eter. The optimum particle size for interior application lies between 1 and
20 microns mass median diameter. Many of the field-model machines are used
for treating interiors of warehouses, barns, and greenhouses. Some poor re-
sults that have been reported in greenhouse applications were probably due to
the use of too large a particle size. Various machines have been developed
for producing aerosols of a controllable particle size.
The new aerosol machines utilize old methods of breaking up the liquid
into small particles, but in addition take advantage of other factors. The
physical qualities of the liquid to be broken up can be utilized. If a vol-
atile liquid is used, the particles shrink as soon as released. To prevent
excessive drift and wastage of insecticide, it is necessary to limit the
shrinkage by adding a small amount of liquid of low volatility to the aero-
sol formula. A volatile liquid, such as xylene, in the insecticide solution
evaporates rapidly, leaving the liquid of low volatility, such as a heavy
oil, and the insecticide to maintain the proper particle size.
There are three general types of field-model aerosol machines-nozzle-
type, thermal, and shattering.
The gas atomizer is the most widely used field machine for producing
aerosols. As in all atomizers, the size of the particles dispersed varies
with the velocity of the gas and the surface tension of the liquid. The
particle size increases rapidly with viscosity of the liquid if the amount
to be broken up is large compared with the amount of gas used to break it
The cold-air atoaizer, such as a paint sprayer, has not been used to
produce aerosols on a large scale, because it is difficult to overcome the
viscosity of large quantities of liquid. Small quantities of oil solutions
can be broken up into aerosols in this manner by adding low-viscosity and
volatile solvents. When a DeVilbiss type WV air gun was used with a setting
of 1 gallon per hour, a mixture of equal parts of deodorized kerosene and
acetone produced ouarticles of 10 microns mass median diameter and 30 microns
maximum diameter. An air pressure of 5 pounds per square inch was used, and
the oil-set screw on the nozzle was open six turns and the air screw two
turns. Cold-air atomizers are used for producing small quantities of fine
sprays in insecticide test work, such as in Peet-Grady chambers, turntable
tests, and some wind-tunnel tests. Aerosols produced by this method are
heterogeneous in particle size. The minimum particle size that can be pro-
duced by this method is about 5 microns mass median diameter. This measure-
ment does not allow for shrinkage due to evaporation after the particle is
formed. Come of the new aist blowers use the air velocity to produce very
fine sprays in the aerosol range, but they are not generally classified as
The hot-gas atomLzer is widely used for producing aerosols, being pre-
ferred to the cold-air type because the heat reduces the viscosity of oil
solutions. To some extent the heat partially vaporizes the ril, which con-
denses into fine particles on contact with cold air. The gasoline-engine
exhaust machine is one of this type on which considerable work was done
during the war. (Fig. 1.) In properly designed exhaust aerosol machines
the oil-injection tie is exposed for several inches to the hot exhaust gases
to lower the viscosity of the oil solution. For this reason the aerosol
nozzle must be attached fairly close to the engine. The exhaust gas is in-
creased to the maximum velocity by using a venturi tube and injecting the oil
solution into the gas at the constriction of the venturi. The most satisfac-
tory method of injecting the oil solution into the gas is through a large
opening at low pressure. If the oil solution is injected into the hot gas
through small openings, clogging anid coking will result.
In this type of apparatus the particle size can be reduced by increasing
the speed of the engine, and by decreasing the flow of liquid. The capacity
of this type of generator depends on the size of the gasoline engine. The
capacity, in gallons per hour, is roughly about one-third the brake-horse-
power of the engine. The minimum constriction to use on the exhaust without
building up too much back pressure is, in square inches, about 1/300 of the
This method of producing aerosols is economical, and the cost of the
equipment is negligible, if the motor is already on hand. When oil solutions
are used, particle sizes from about 2 to 3 microns diameter up to coarse
sprays can be produced. Some of the disadvantages are the difficulty in
breaking up water solutions, and the possibility of thermal decomposition of
This type of apparatus was used successfully during the war on jeep and
aircraft engines (Rice et aL. 2). Since the war smaller engines down to
1-1/2 hp. have been equipped with exhaust nozzles for indoor use or under
canopies (Yeomans and Bodenstein ). For larger areas this method is well
adapted for tractor engines.
A method of atomizing a liquid by heating air and pumping it through a
specially designed nozzle was perfected by a commercial firm. A 6-1/2 hp.
gasoline motor is used to drive a rotary-type air pump that delivers about
150 cupic feet per minute of air. (Fig. 2.) The air passes through and is
heated by a gasoline-burning combustion chamber fired by a constant spark and
regulated to maintain a temperature of 900 F. The hot air then lasses through
special nozzles into which the insecticide solution is injected. A metering
valve is used to regulate the flow of insecticide solution through the nozzle,
and in this way the particle size is regulated. The insecticide pump, a cen-
trifugal type, has capacity enough to supply 50 gallons per hour to the nozzles
and to circulate 200 gallons per hour back to the insecticide tank to keep the
solution agitated. In the specially designed nozzles the liquid and a small
quantity of air are released at the base of the nozzle with a rotating motion.
Most of the air comes in contact with and breaks up the liquid at the tip of
This machine can be used without heat and the liquid is then atomized
into a very fine spray by the air blast. W1*n the air is heated to 900 F.,
the viscosity of the oil is reduced and the oil partially vaporized, and in
this way the particle size is greatly reduced.. Vith a solution of 2/5
xylene .and 3/5 SAE 10 W motor oil, the particle size obtained with the heat
on ranged from 5 microns mass median diameter with the control-valve setting
of 10 gallons per hour to about 50 microns with the maximum setting. Without
heat the particle size ranged from about 25 to 65 microns mass median diameter.
Different particle sizes would be obtained by changing the characteristics of
the solution, such as using liquids of different surface tension and viscos-
ity. This machine consumes about 1/2 gallon of gasoline per hour with heat
off and about 3 gallons oer hour with heat on.
Hot gases produced by combustion of chemicals have been used to weak up
insecticide solutions. One such method-is use of the Comings candle,-'a
small container in which highly combustible material is placed in one com-
partment and the insecticide solution in another. The combustion of the
material in the one end causes hot gases to escape through a venturi tube.
The venturi tube passes through the insecticide solution and as the hot
gases escape through the venturi a plug is melted and the insecticide so-
lution is drawn into the hot gases and is thereby broken up into aerosol
particles. The particle size was very small on the test models (about 1
micron diameter), but the size could be increased by increasing the size of
the oil flow hole. This method is not economical, but in some cases might
be suitable in warfare.
Steam has been used for some time to break up large quantities of oil.
Columbia University, working in cooperation with the Bureau of Entomology
and Plant Quarantine under OSRD funds, developed a pressure-shearing method
in utilizing steam to break up oil. This method, called the Hochberg-La Mer
method, consists in pumping a mixture of water and the insecticide solution
through a coil which is heated in a combustion chamber (LaMer and Hoch-
berg 1). The heat converts the water to steam and lowers the viscosity of
oil solutions. The steam pressure causes a shearing action which breaks up
the insecticide solution as it issues from large jet openings. The combus-
tion chamber is heated with either gasoline or fuel oil, and the temperature
is regulated by a thermostat, which controls the amount of fuel pumped to
the burner. A gasoline engine of about 1-1/2 hp. is used to pump the water
and insecticide solution, also to furnish air and fuel to the burner, and to
furnish a constant spark from its magneto for the burner. The Army model
E-12, used by the Bureau of Entomology and Plant Qup.rntine, is equipped
with two plunger pumps, one to pump the water and the other the insecticide
solution. (Fig. 3) Each pump delivers about 20 gallons per hour. The
water and the insecticide solution are mixed before passing through the
heater coils. This machine is equipped with two nozzles, set at a 60-degree
angle, both 1 inch long with a 1/8-inch diameter opening. The particle size
is controlled by setting the thermostat for temperatures between 300 and
600F. The pressure developed by the steam ranges from 60 to 120 pounds per
square inch. A relief valve is installed in the heater coil line to prevent
.J/ Anderson, L. D., Rogers, E. E., and Latta, Randall. Biological field
tests with insecticidal aerosols generated by modified Comings thermal gener-
ators. OSRD Interim Report C-16, April 26, 1945.
This machine is arranged so that water alone can be pumped through
the heater coil until operating temperature is reached, and also to flush
out the heater coils while cooling after use. This arrangement prevents
waste of insecticide and reduces the danger of coking and clogging of the
When a mixture of 1 part of xylene in 4 parts of SAE 10 W motor oil
was used in a DDT solution,, an average particle size of 10 microns was
obtained at 600F., 15 microns at 500, 20 microns at 400, and 30 microns
at 300. Less viscous oil solutions produced smaller particle sizes.
Insecticide solutions used in this machine should have a higher boiling
point than water; otherwise the particle size would be too smell to be
This machine operates on the principle of keeping the insecticide
solution in liquid form in order to produce effective particle sizes. This
machine uses the nearest approach to the principle used in the aerosol bomb
of any of the field machines.
There are several variations of this machine. Some of the machines
vary the ratio of water to oil-insecticide solution and in this way vary
the particle size without changing the temperature. Most of the machines
are modifications of the thermal smoke generators used by the military
forces. The thermal smoke generators operate at about 900F. with a pres-
sure of about 60 pounds per square inch, and completely vaporize the oil,
which condenses into very fine particles, less than 1 micron in diameter.
This size would be ineffective for insect control. In the smoke generators
only about 10 percent of water is added to the oil to help flush out the
coils. When more water is used, as in the aerosol generators, more fuel
is consumed even though the operating temperature is reduced. Because of
the greater power required in the aerosol machine, larger motors and better
pumps are sometimes necessary when the smoke machines are converted into
Steam can also be used to break up insecticide solutions in the same
way that air or other gases are used in atomizers. A machine to accomplish
this is now available, (Fig. 4 It is similar to the thermal smoke gen-
erator or pressure-shearing machine, except that water alone goes through
the heater coil and is converted into steam. The insecticide solution is
by-passed around the heater and injected into the steam through a nozzle.
A two-plunger pump is generally used-one for the water and one for the
spray solution. This machine is equipped with a 1-1/2 hp. gasoline engine,
and has an output of 38 gallons per hour of insecticide solution and an
equal quantity of water. The burner uses pressure-atomized fuel oil, which
is ignited by an automatic spark from the engine magneto. There are two
50-gallon tanks, one with an agitator for the solution, and the other for
water. The average fuel consumption of this machine is 5 gallons of fuel
oil and 1 quart of gasoline per hour.
A thermal smoke generator manufactured by the same company has been
converted by the Bureau of Entomology and Plant Quarantine to a machine sim-
ilar to the one described. When a mixture of 3 parts of SAE 10 W motor oil
and 2 parts of xylene was used in this machine, a particle size of 100 microns
diameter was obtained at 300F., 65 microns at 350 40 microns at 600, and
20 microns at 800. This type of machine usually operates between 400 and
900 at a pressure between 80 and 160 pounds per square inch. It also re-
quires higher temperatures to produce the same particle size than the
Hochberg-LaMer type, but the insecticide solution has less contact with
heat and there is no danger of clogging the heater coils. This method can
be used to break up water suspensions and liquids of low boiling point, as
well as the oil solutions.
Another method of producing aerosols is by exerting pressure on liquid.
Pressure nozzles are practical only for breaking up liquids of low viscosity.
They generally clog more easily than the gas-atomizing nozzles, and under
ordinary conditions do not produce so fine a particle as the atomizers.
Pressure nozzles, however, are generally simple, small, and inexpensive, and
they usually consume less power than the atomizing type. A method of pro-
ducing aerosols in the field by pressure on liquid is used in South America.
The aerosol machine designed in Argentina uses a Ford engine to compress tin
tetrachloride in one cylinder and the insecticide dissolved in acetone or
similar volatile solvent in another cylinder. (Fig. 5) Both the insecti-
cide solution and the tin tetrachloride are released through a bank of noz-
zles close together so that the two mix and produce a heavy fog. Another
aerosol machine subjects a permanent charge of nitrogen, by hand pump, to
pressures between 350 and 1000 pounds per square inch. The nitrogen exerts
pressure through a piston to the insecticide solution. This high pressure
atomizes the insecticide solution into a super-fine mist or fog as it passes
through the turbulence chamber of the fog nozzle.
Rotating disks are generally useful in breaking up very viscous mate-
rials. A commercial firm has designed and built aerosol machines with ro-
tating disks, a small electrically driven one for indoor use, and a larger
gasoline model for field use. These machines have several disks held to-
gether at the edge, and the insecticide solution is forced out from between
them by centrifugal force as they rotate at high speed. This method of
using the disks limits their use to solutions, and viscosity influences
particle size more than with the open type of disks. Constricted disks are
reported to give finer particles than open ones.
The latest field-model machine uses a 6-1/2-hp. engine to rotate by
means of V belts 21 disks 8 inches in diameter at about 6500 r.p.m.
(Fig. 64 A high-velocity blower with a capacity of about 4500 cubic feet
per minute throws the aerosol out horizontally from the disks. The disks
are mounted on a hollow shaft, through which the insecticide solution is
drawn by the suction produced by the centrifugal force. A separate supply
tank is used and must be mounted at least as high as the disks because the
suction is reduced by a strainer located in the supply line. This machine
will deliver up to about 250 gallons per hour, and the output can be con-
trolled by a metering valve.
The size of particles issuing from the machine is a function of the out-
put. The minimum particle size obtained with a solution of 2/5 xylene and
3/5 SAE 10 W motor oil was about 40 microns mass median diameter. For smaller
particles oil solutions of lower viscosity must be used or volatile solvents
added. The base on which the machine is mounted allows a 20'. tipback with a
3600 rotation. The larger particles produced by this machine compared with
other types do not limit its usefulness for field use except in cases where
the maximum amount of penetration is required. The indoor electrically driven
machine, with a rotation of about 17,000 r.p.m., produces smaller particles,
with a mass median diameter of about 10 microns, when using deobase fly sprays.
Burning, or incomplete combustion, has been used to produce aerosols for
insect control, but generally has not been adopted where other methods are
available. The British have used this method in the field in experimental
work vith some success. The insecticide is mixed with some slow-burning ma-
terial and ignited, and the resulting smoke is drifted over the area to be
treated. This method has the following disadvantages: The percentage of
thermal decomposition of the insecticide is high, the particle size is too
small for optimum efficiency, and the smoke is generally irritating. It has
been used more widely for indoor than for outdoor applications.
A great deal of work has been done on vaporizing insecticide solutions
and then allowing them to condense to produce aerosols. This method has been
used successfully in the laboratory to produce particles up to 25 microns
mass median diameter. Generally the particles are too small to be effective
and under ordinary conditions are less than 1 micron diameter. The number of
nuclei ordinarily found in the air must be reduced to increase the particle
size of the aerosol, and the ratio of air volume to rate of vaporization has
to be controlled carefully. The chance of thermal decomposition of the in-
secticide by this method is one of the disadvantages. Moreover, the insecti-
cide must have a boiling point nearly the same as the solution to maintain
the same concentration in the aerosol as in the solution. The thermal smoke
machines used by the military forces are based on this principle, but when
conversion to aerosol machines was first tried little progress was made be-
cause the rate of condensation could not be controlled enough to produce
effective particle sizes. These machines were later readily converted by
using the method of gas atomization.
Breaking up liquids by shattering has been used to only a limited ex-
tent to produce aerosols. Some tests have been made in which an insecticide
was put into grenades and then exploded. The result did not appear practical
because of the high cost of the explosive and the small quantities of insecti-
Breaker bars and. ipinmping methods of breaking up liquids have not beft
used for producing aerosols in the field. Then pressure nozzles are used ia
conjunction with breaker bars, nozzle clogging and high viscosit are
still problems. This method Is used to break up small quantities of rtert
and the less viscous liquids into the aerosol range. It is used on airplaneas
to produce spreys of particles larger than the aerosol range.
In selecting the proper t"pe of field-model aerosol machine to use,
one imst consider the original cost, the simplicity of design, the operating
cost, the possibliUt of breakdown of insecticide, the types of solution
that the machine will handle, and the range and ease of particle-size control.
There is no machine so far that is outstanding in all these points. A selecst-
ion should therefore be made according to its excellence in the most important
features that apply to the job to be done.
(1) LaMer, V. K., and Hochberg, Seymore.
1945. Hochberg-LaMer aerosol generator inventors model. (OSRD
Report 4901.) U. S. Dept.Com. Off. Pub. Bd. Rpt. 15617.
(2) Rice, R. I., Johnstone, H. F., and Kearns, C. W.
1946. An exhaust aerosol or spray generator for dispersing insecti-
cides. Jour. Econ. Ent. 39: 652-658.
(3) Yeomans, A. H., and Bodenstein, W. G.
1947. An exhaust aerosol generator for 1-1/2 horsepower motors.
U. S. Bur. &it. and Plant Quar. ET-238, 6 pp. (Processed.)
Figure 1.-Exhaust aerosol apparatus mounted oa
engine of sprayer-duster machine.
Figure 2.-Pumped hot-air type of aerosol machine.
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Figure 3.-Pressure-shearing type of steam aerosol machine.
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Figure 4.-Steam-atomizing type of aerosol machine: (A) Aerosol generator
only, mounted on base but without hood, water tank, or solution tank; and
(B) trailer-mounted aerosol generator equipped for towing behind tractor.
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Figure 5.-Liquid-pressure type of aerosol machine.
Figure 6.-Rotating-disk type of aerosol machine.
UNIVERSITY OF FLORIDA
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