As a dedicated unit or added to a truck or trailer, pull-behind sprayers come in many designs, configurations and sizes. Sometimes called "tow-sprayers," most units are skid-mounted. The skid, which also holds the pump and engine, includes two runners that cradle the tank in the bed of a truck or the back of a utility vehicle. In addition, you can convert most skids into trailers by adding an axle, tire-and-wheel assembly and a tow hitch. When considering the purchase of a sprayer, you must specify which options you need for your applications. The sprayer itself consists of a tank and pumping and delivery systems. The weight that these tanks must support is extraordinary, and they require support units such as cradles or saddles.
Tank material Tanks are primarily made of fiberglass and polyethlene. Steel or stainless-steel tanks are still used, but they are not as common as they once were. Fiberglass tanks are made from custom molds using chopped fiberglass and hand-rolled for maximum density and strength. They are more durable than plastic tanks. Versatile manufacturing methods allow many fiberglass-tank shapes. The life expectancy of fiberglass tanks (barring any unusual damage) is 30 years.
Plastic tanks are made from castings or molds. They have more porosity and a shorter life expectancy (approximately 15 years) but also are less expensive than fiberglass units.
Both types have good chemical resistance. However, when damage occurs, plastic tanks are difficult to repair, and, if punctured, a new tank may be necessary because glues and sealants do not adhere to their surfaces. Fiberglass tanks, on the other hand, require only simple and effective patching for punctures.
Jet agitation You need to keep your tank mix agitated to avoid settling. Even when properly mixed, chemicals tend to settle in the corners and at the bottom of your tank.
Jet agitation relies on pressurized water to stir the tank mix. A pump takes a percentage of the liquid-chemical mix and recycles it back into the tank through a series of strategically placed jets-similar to those in a whirlpool. This common, non-mechanical agitation process is used primarily on plastic tanks that cannot support the weight of mechanical-agitation devices.
Mechanical agitation Mechanical agitation (considered the most effective mixing method) requires a tank that can support bearings and the force created by the mixing unit. Most fiberglass tanks support mechanical agitators. The typical mechanical system is belt driven and spins at relatively slow speeds. Paddles or propellers on the shaft mix the chemical and carrier. The size of your tank and the types of chemicals you use dictate the number of paddles per unit.
Most mechanical-agitation units have stainless-steel or aluminum paddles because they are strong and not easily effected by chemicals.
Tank support You must be sure that your tank has proper support for its intended use. As I mentioned, most tanks (including stainless steel, fiberglass and plastic) mount on a skid that rests on two runners-usually on the bed of a truck or trailer.
You also can mount tanks in a cradle or other supporting structure on your vehicle or towing unit upon which its heavy weight will rest. A 300-gallon tank, filled with water, weighs (at 8 pounds per gallon) more than 1 ton. The design, moldings and fittings of the tank-not to mention your vehicle or towing unit-must be able to accommodate this weight.
If you are considering the installation of a 300-gallon tank in the back of your half-ton pickup, reconsider. A tank of this size is best carried by a 1-ton truck or larger. It is not just a matter of your truck's springs or bed being strong enough to hold a tank of this size. Proper brake sizing is required to enable your truck to stop when carrying such extreme weight. If you include the weight of the rest of the sprayer, this unit will weigh approximately 3,000 pounds. You simply cannot tow (or stop) that kind of weight with a standard truck or tractor.
On-road units Spray units that operate on city roads and streets require additional equipment to accommodate extra weight and stopping ability. Towing any tank requires proper axles, shocks, wheels, tires, skids, lights, fenders and brakes. City and state departmentsof transportation (DOT), parks-and-recreation crews and landscaping contractors commonly use these types of units. Most manufacturers offer on-road-towing packages, but you must know whether your truck will require additional gear for your desired unit. Generally, tanks under 200 gallons fit in the back of a standard half-ton truck and do not require additional equipment.
Off-road units Off-road systems are much simpler and less expensive, typically consisting of an axle (sized for the load) bolted directly to the cradle, with frame, wheels, tires and rims designed for its potential weight. Most off-road trailers use a simple drop pin that hooks to the draw bar of a tractor. You must properly size and equip these towing units as well.
Pumping systems A pump is the heart of your spraying system and comes in various types. Pumping systems include various components: the pump, regulator, plumbing, strainer and drive. Custom-made spraying units fit almost any special requirements you have. The engine must be sized to pump and pressure as needed. You can choose from gasoline and diesel models. Pump output is measured in terms of gallons per minute (gpm) and pounds per square inch (psi).
Variable-output pumps A mainstay in our industry, variable-output or centrifugal pumps allow you to pump more material as the speed of pump rotation increases-within the design limits of the pump. Although considered low-pressure units (under 150 psi), they pump large volumes of material with low horsepower requirements. They are favored when a low-pressure boom or gun is the delivery-system choice. In addition, they serve as transfer pumps for transferring liquid to or from storage tanks.
Units with a plumbing system should have a bypass or a properlysized regulator designed to control and handle the pressure created when you seal off their output for short amounts of time. Some manufacturers claim that you must not leave the output valve shut off for more than 10 minutes because overheating can result, causing internal damage. The mechanical seal has a ceramic seat that gets hot if fresh water does not cool it. When activated, the delivery system cools this ceramic seat with a constant flow of water. However, if you turn the system back on after it has become hot, the water might crack the seat.
Fixed-output pumps Remember, if you buy a fixed-output pump thinking it will suit your needs-but discover later that it doesn't-there is no way to increase its output by increasing rpm. You must buy a larger pump if you need greater output. Three common types of fixed-output pumps are available: * Diaphragm pumps produce fixed-volume outputs based on their chamber size. Depending on their design, they produce pressures from 250 to 750 psi and volumes from 2 to 50 gpm or more. * Roller pumps (primarily used in agriculture) produce medium pressures (300 psi), are easy to maintain and relatively inexpensive. However, powdered and granular chemicals tend to damage them. * Piston pumps, commonly used for tree applications, create high pressure (over 800 psi) but are considered high-maintenance and can be expensive to repair.
Fixed-output pumps, also called positive-displacement pumps, pump material as long as there is material to pump. They require properly sized relief valves because if you seal off all lines without turning the pump off, it will continue to produce pressure until something gives-either the drive system or a plumbing component.
Diaphragm pumps are good replacements for piston pumps and perform similar applications. They are both capable of pumping abrasive materials consistently and repair costs are reasonable. The secret to reducing damage to sprayers is proper use, cleaning, maintenance and ensuring chemicals never contact the pump mechanism.
Both diaphragm and piston pumps have a crankshaft, connecting rods and pistons that operate in an oil-filled cavity. The piston or diaphragm pulls liquid into a chamber and then pushes it out, using valves to keep it flowing in one direction-much like the pistons and chambers in an engine. When these units require repair, it is usually only a matter of replacing a valve, diaphragm or piston.
Delivery systems The entire delivery system consists of the tank, mixing system, pump and spray gun or boom. Delivery systems must suit the type of application you want to perform. The chemicals you plan to deliver play an integral part of this decision. Low-pressure units with a 20- to 40-psi boom (with properly spaced nozzles) spray large turf areas. Walking booms enable you to park your utility vehicle away from weight-sensitive turf areas and deliver applications on foot. In newer units, a computer controls the amount of material you apply and monitors proper mixing in the tank via sensors. Electronic sprayer controls specify rates of chemical applications based on your calculations and input. These controls are extremely accurate and reliable, and calculate variables such as ground speed, nozzle size, chemical-to-carrier ratios and application rate.
Modern sprayers are high-tech pieces of equipment and have long life expectancies. Do not rush into purchase decisions. You may know what you need from a sprayer, but you should contact a variety of manufacturers for specific information.
Safety Sprayers produce high pressures and a broken line can be dangerous, even deadly. If your unit needs repair, redesigning or rebuilding, have a certified mechanic or machinist perform that work. Any plumbing components must be of sufficient strength for the type of pump the sprayer uses and sized to provide enough flow without undue restriction. Of course, always wear proper safety equipment when working with chemicals and machinery. Be sure to follow all instructions on the label of the chemical you are applying, including directions about appropriate personal protective equipment. If you properly use and maintain it, a pull-behind sprayer is a safe, valuable and long-lasting piece of equipment.
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