The Agitation over Agitation

Sprayers101 recently received a couple of seemingly unrelated questions about airblast sprayers.

What are the advantages and disadvantages of mechanical versus hydraulic agitation? Why would someone want a stainless tank versus the cheaper poly or fiberglass options?

Recognizing that each manufacturer has their own reasons for the features and materials used in their sprayers, we posed these questions to Mr. Kim Blagborne of Slimline Manufacturing. The following article was written from Kim’s response, and it turns out these two questions are very much related. Kim writes:

This is a great debate among customers and manufacturers, and it’s difficult to stay neutral. Let’s consider the following:

Hydraulic Agitation

The flow required for hydraulic agitation requires about 30% of the pumps total capacity. This is very important because many sprayers cannot achieve, or maintain, this minimum requirement whilst spraying. This may be why it’s rare for a sales person to demonstrate agitation while the sprayer is spraying; quite often, the agitation slows or even stops. And, of course, because everyone gets wet.

Let’s say an airblast sprayer has a pump with a manufacturer-listed capacity of 26 gallons per minute (gpm) (Click to download the spec sheet for the pump). The figure in that output chart is determined on a bench at 540 rpm and at 50 psi. However, when an operator uses that pump in the field, they run it at ~150 psi, and that brings the pump capacity down a bit to 25.5 gpm.

Now we build in the line pressure drop associated with the sprayer’s plumbing. Effectively, another 8-10% of the pump’s output is lost to plumbing (a figure easily measured by collecting the total output capacity of the pump). Let’s say we are now down to a practical capacity of 23 gpm.

If the operator’s crops are on 14 foot rows, it would be reasonable to spray 200 gpa at a travel speed of 3 mph at 150 psi. With both booms spraying that’s a required flow of 16.8 gpm.

Remember, our hypothetical 26 gpm pump can only provide 23 gpm in the field. When we subtract the 16.8 gpm required for spraying, we’re left with 6.3 gpm excess capacity for agitation. But, we said we needed 30% of the pump’s 26 gpm capacity, and that comes out to 7.8 gpm. We’re short by 1.5 gpm, or stated differently, we’re about 20% short of what we need.

Why don’t we see that deficit? Because the flow to the booms is prioritized, and therefore the sprayer output matches the calibration, so everything seems OK. But no one sees the reduced return flow through the regulator, and certainly no one peeks into the tank while spraying to see that the hydraulic agitation is greatly reduced.

And so, while everything looked great during loading, the spray mix (especially SC and WDG formulations) may not stay suspended correctly during spraying. In extreme cases, that could lead to burning a crop (high concentration) at the start of a spray job, and reduced efficacy (low concentration) at the end. We’re quick to blame the chemical, but no one ever thinks to question hydraulic agitation.

Let’s consider it from another angle: TeeJet suggests a model number 62905c-5 jet agitator for a sprayer with a 250 US gallon tank. To correctly agitate the contents of this tank, we will need 30 psi and 7.6 gpm (see the chart below).

Unfortunately, there is no simple way for an operator to measure the agitation pressure or the flow, so it goes unchecked. The only way to determine if the flow demand is satisfied is to apply the generic rule of 30% of pump capacity and make an estimate. That’s pretty loose math since we’ve already established that the listed capacity may not reflect reality.

Still another angle: Many operators now employ the Gear Up, Throttle Down (GUTD) approach to match their sprayer air settings to the crop canopy. However, when we reduce PTO input speed we also reduce pump capacity. Remember our piston diaphragm pump with the 26 gpm capacity at 540 rpm? We still need 16.8 gpm to spray, but reducing the rpm’s by 100, per GUTD, drops our pump output to only 23.16 gpm.

23.16 minus 16.8 equals 6.36, and we needed 7.8 gpm to maintain sufficient hydraulic agitation. Oops.

Mechanical Agitation and Tank Material

There are definite advantages to mechanical agitation. It is not affected by the PTO speed because it is already excessive at 540 rpm. This means there is no pump capacity issue and it allows the operator to take advantage of GUTD.

There are also a few disadvantages. Unlike a hydraulic system, mechanical agitation requires maintenance, such as regular (daily?) greasing. The packing where the the system inserts into the spray tank also requires occasional inspection and adjustment to prevent leaks.

And of course there’s sticker shock. Many European manufacturers offer hydraulic agitation because it is ~$500.00 CAD less expensive. Further, mechanical agitation creates vibrational stress on tanks walls, which fiberglass or plastic tanks can’t handle for long. The solution is stainless tanks, which is a more expensive material. Further, stainless cannot be moulded around pumps and rotating parts, so more steel is required, adding to expense and weight.

In my opinion, there is sufficient benefit to stainless to easily recover the investment. Beyond permitting mechanical agitation, there’s durability. We have stainless tanks built in 1948 that are still operating today, and we’ve never found a plastic or fiberglass tank that can claim that. There’s also sprayer sanitation. It has long been know that stainless cleans more easily and more reliably that plastic or fiberglass, especially as the tanks begin to age.

Closing

The decision to buy a sprayer with hydraulic agitation or mechanical agitation lies, ultimately, with the consumer. But be sure to look past the price tag, and under the hood. Ensure that you have sufficient agitation to properly suspend your tank mix, and give you the flexibility to Gear Up and Throttle Down to improve your spray coverage and efficacy.