Agitation in the Spray Tank – A Basic Necessity for Uniform Spray Application

Most agricultural spray materials do not mix well in water – their carrier.

Most pesticides and other materials sprayed on crops are formulated to be as emulsifiable as possible, but many contain elements that either do not dissolve (wettable powders-WP), petroleum distillates (Emulsifiable Concentrates- EC) or just precipitate as they are heavier than the water (fertilizers, powdered metals, etc).

spray tank design hurts the ability for good agitation in the spray tank

We will not get into the chemical compositions of the materials, but how to maintain them in suspension in the spray tank during the entire spray operation. Adjuvants play a major role in bonding the otherwise un-mixable spray materials with water, but there are some cases in which even the best adjuvant cannot keep these materials afloat.

We are growing evermore conscious of the need for effective tank agitation and most of the better sprayer manufacturers are constantly improving their designs to optimize the efficiency of their machines in that area. But the finest and most advanced agitation system in a sprayer will not assure a uniform application unless the operator does his job properly.

One of the most common causes of uneven application due to poor agitation lies with the spay applicator…

The habit of:

  • Not agitating during Mixing/Loading
  • Turning off the PTO drive that operates the sprayer pump and agitator during the drive to the field
  • Turning off the PTO when reaching the end of a row or field
  • Shutting down a sprayer with material in the tank when going on break or driving from field to field
  • Restricting return agitation lines when the pump is wom, so as to be able to maintain pressure
  • Not periodically inspecting the agitation systems (mechanical or hydraulic) to prevent failure, i.e. propeller wear, seal wear, nozzle wear, corrosion, etc
  • All or any of the above by believing the adjuvants are enough to keep everything mixed.

Another of the most common causes of poor agitation is the actual design of the sprayer. In my years as a sprayer specialist I have come to appraise that probably 70% of the sprayers manufactured in the US have poor agitation systems, basically because of the lack of hands-on experience of the people that design and build them.

Most manufacturers pay attention to tanks, chassis/frames, pumps and delivery systems and put what they think is an agitator in the tank. Be it mechanical or hydraulic (return agitation) the fact that they are able to create turbulence in the water covers that subject for them…….. WRONG!!!

The foremost and basic requirement for effective tank agitation is for the movement of water (turbulence) to actually “sweep” the bottom of the tank so that any precipitated material is picked up and re-mixed with the rest of the solution. This is also important because most of the tanks discharge to the pump from the very bottom.

Some even have sumps to catch up to the last drops of material. So, if we have precipitation of material in the bottom of the tank, the pump is going to suck up a concentrate that could be many times what the label recommends or allows, with the subsequent problems:

  • A – too much material in one area and not enough on the rest of the crop.
  • B – concentrated chemical can cause permanent damage to crops.
  • C – lack of control due to material being too diluted. A wasted effort.
  • D – violation of Federal and State Law.

The key to effective agitation is to move the contents of the tank in one direction, such as a swirl. Many sprayer manufacturers make the mistake of orienting their hydraulic return nozzles in two or more directions in the tank.

This creates a total confusion of movement in the water and, although it looks effective, does not properly cover the critical area, which is the bottom of the tank. I know, because when I was designing and building sprayers, I was an advocate of this sort of agitation until, as a grower, I realized it did not do the job.

Mechanical agitation systems also have a drawback, especially in cylindrical tanks. Because of location of shafts and blades, the agitators cannot sweep the bottom of the tank and, although they do produce the “swirl” effect, there is always some material that precipitates and stays out of their reach.

The fact that they are considerably ineffective when the tank is in its last quarter is redeemed by the fact that any precipitation at that stage would not be as concentrated as when there is a full load of chemicals in the tank.

Mechanical agitators require attention. Replacement of seals (they are always leaking through the tank bulkhead fittings: a waste, also a no-no from a safety and environmental standpoint), blade breakage, belts, etc. Be sure you inspect them regularly.

Cylindrical and Oval tanks are the ideal configuration for the “sparger” type hydraulic return agitation system. This system consists of a tube located longitudinally along the wall of the tank, some 6 to 10 inches above the lower centerline, with volume booster nozzles aimed at that centerline so that they sweep across the bottom and produce a swirl with the axis of the tank as the center, this assures full agitation at all times (as long as it is operating!)

Irregularly shaped tanks (which tends to show us that the designers are thinking first of appearance and size, rather than functional operation) are the most difficult to assure effective agitation. There again, the bottom of the tank must be swept to keep concentrations of precipitants from getting to the pump and the crops.

The most effective agitators for these tanks are specifically placed and oriented volume boosters mainly aimed across the bottom, but away from the pump line inlet, so as not to create cavitation (injection of air) that could cause inefficiency and even damage to the pump.

Vertical cylindrical tanks are also difficult to properly agitate. Most manufacturers of sprayers with these tanks tend to install vertical nozzle agitators that create a “fountain” effect that does show up nicely on the surface when one looks down to see if the agitator is working, but fails miserably when it comes to mixing near the bottom.

Other sprayer designers have put horizontal agitators up to 1 foot above the bottom: this does not sweep. Others have them pointing in three directions, which causes a lot of turbulence but also fails to assure that the bottom will be clean.

Consequently, the key to good agitation is always the bottom of the tank. Look at it. If you have to modify it, you may want to do it yourself: extending pipes, changing orientation of nozzles, putting in spargers or volume boosters. All this can be done relatively simply with pvc pipe and fittings from your local plumbing supply.

If your tank has hydraulic return agitation, but does not have a volume booster nozzle, order one and install it. Volume booster nozzles take a small amount of water pumped into their venturi chamber and create a vacuum that draws 3 to 4 times the volume from the surrounding water and expels it out the end.

Consequently, with a return line generating 2 gpm you have a total output of 8gpm, and so forth. This means that you can have a healthy agitation volume without taking too much away from your pump system.

Look at agitation as stirring the sugar into your coffee. It is the same principle, except that, unlike sugar, many of the materials do not dissolve in the water and you have to keep stirring the spoon the entire time you have them in the tank.


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