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The Pressure Washer
Probably the easiest way to understand how a pressure washer functions is to diagnose a unit and see how it is constructed. We have selected a cold water portable unit for this purpose.
1.First you have your power source (engine or motor) this is what created the energy to drive the pump. It and the nozzle are what determines the cleaning effect.
2.The heart of any pressure washer system is the high pressure pump. They are a positive displacement reciprocating type pump - either plunger or piston version. Plunger is the most common as they are more efficient and have longer life. Plungers are usually made of ceramic which is very hard and less prone to wear. Plunger pump can be described as a plunger that moves back and forth against a stationary seal or packing. Whereas a piston pump is where the packing is attached to the piston rod and moves against a smooth cylinder wall. Piston pumps generally have better priming characteristics but cannot operate at as high a pressure and have shorter life.
An important feature of the piston pump is that the volume capacity of pump depends on pump speed. So long as pump is rotating the same volume of liquid that goes in must come out. Unlike a centrifugal pump where restriction or increased head creates reduced flow a piston pump has a very flat flow performance curve no matter how high the pressure goes. This is why blockage can be dangerous and why safety devices such as unloader valves must be incorporated into system.
As the plunger moves back, water is drawn into pump cylinder cavity through inlet check valves. When plunger moves forward, the inlet valves close, forcing water to travel through a series of outlet valves and to the discharge port of pump.
Most plunger pumps are either duplex or triplex. Two-piston pumps have higher pulsations and are harder on system components and the operator. Triplex are the most common used for pressure washers as they result in smoother flow.
Newer higher speed pumps also result in less pressure fluctuations and require less starting torque; however, they result in faster wear, have less suction lift capability and sometime noise is high pitched and can be irritating.
Unfortunately since pump is the core of the pressure washer it is generally blamed for any system failing. However, generally 90% of pressure washer problems are not the cause of the pump. Most can be attributed to system restrictions or component wear such as nozzle, etc. Pump starvation of liquid is probably the leading cause of premature pump failure. Inlet line restrictions does not allow adequate water to enter pump resulting in cavitation. Cavitation is when air mixes with water causing small explosions to occur when pressurized which will tear away at packing or piston surface.
Briefly let's talk about proper selection of a pressure washer for your application and types that are available to meet your needs. To better understand this we'll compare high pressure cleaning to washing your hands. Depending upon the degree of dirt and sanitation required you need: water, rubbing action, soap and heat. Relating this to a pressure washer and in tented application you should consider:
1.Flow - measured in gallons per minute (GPM)
2.Pressure - measured in Pounds per square inch (PSI)
3.Chemical - amount and type needed to break down dirt
4.Heat - speeds up cleaning action, particularly for oil and grease applications.
And I'm adding three more considerations:
1.Time - how much time do you have or can afford to pay to get a proper cleaning job done.
2.Cost - What is budgeted for equipment to accomplish above factors.
3.Power - Available AMPS - H.P.
Pressure Washers are generally broken into two (2) Classes: Hot or Cold Water
Cold Water are the most popular, probably primarily due to cost and ease in manufacturing and use. They are generally limited to 140?F - temperature of your hot water tap. They are broken into two types: electric or gas engine.
1.Electric are more compact and lower performance when electric current is limited to 115V. They are more economical and sold to Accessories market or for semi-professional use.
2.Gas engine units are more portable and used primarily outside when noise and gas fumes do not present a problem.
Performance is limited by price you are prepared to pay to get the job done.
Hot Water units are primarily used for commercial or Pumps applications where high temperature are required to break down dirt more effectively. Because they consist of an oil burner and heating coil they tend to be more expensive and complicated to operate. They have however enjoyed probably the largest percent sales growth in the industry due to more efficient manufacturing and promotion.
Most pumps have a dual packing design with a water recycling system which allows water to lubricate packings and bleed back minor leakage from slightly worn packings. When excessive wear occurs and pressure drops significantly packings should be changed. Remember a new pump will sometimes weep slightly until packing take a good set against plungers.
Chemical Injectors - Detergents or additives can be easily injected to the flow of water on a pressure washer almost automatically with an injector. This can be done either upstream (apply chemical at full pressure) or downstream (apply chemical at lower pressure) of the pump. Downstream is the most preferred as chemical does not enter pump and do any damage to it. An injectors works like a jet pump. Water enters the injector and accelerates into a jet through the nozzle. The high velocity jet creates a vacuum which causes fluid to be drawn through the suction tube and into the injector. The mixture then flows into a diverging (venturi) passage where suction tube prevents back flow into chemical container when rinsing or at high pressure. Adequate pressure drop based on flow operating pressure must be made to activate venturi principle. This is done by changing to a larger nozzle or diverting flow into other openings which reduces pressure. Injectors are sized by flow and 80% pressure drop must occur to function. Chemical viscosity must also be considered. Too long or too small a diameter of hose or fitting restrictions may cause excessive pressure loss which will not allow venturi to operate.
Trigger Gun - Guns are fairly simple mechanisms designed around a trigger-operated ball valve. The valve ball is held in a closed or forward position by flow of water and blocks flow of water through gun to the nozzle. When trigger is depressed it pushes a pin against ball, forcing the ball off its seat and opening a path for water to flow to the nozzle. When trigger is released gain a spring returns the ball to its seating and flow is again blocked. Gun should be comfortable for operator to use based on the performance
Generally front entry type guns are used on lower pressure units and are less expensive.
Rear entry guns are more comfortable as they have less kick-back and the hose does not get in the operators way.
The gun is probably the most abused component of the pressure washer and thereby the most frequently replaced. It pays to offer a quality one thereby avoiding expensive warranty cost.
It is useful to have various lengths of lances available to match the particular job at hand. The most common lengths are the 36” and 48”. To determine which lance is best for each application you need to look at the job. There are obvious reasons for choosing a lance, for example, when you are washing a two story house you would want to use a 60”, 72” or telescoping lance to avoid working from a ladder. Or perhaps you are cleaning an engine or confined space and a 12” or flexible lance is a more practical choice. Be sure to take into account other conditions such as potential splash back and space considerations.
For washing fleets a 72” lance is preferred because it allows you to stay back from the water and still reach the top of the fairings quite easily. However, the problem becomes space, if the trucks are not moved or staggered the longer wand becomes too clumsy to work with between the trucks and washing becomes more difficult and time consuming. When working with a row of parked vehicles the 36” or 48” lances are a much better choice allowing for maneuverability between the vehicles and still providing adequate reach.
When working with flat work or decks the most common length is the 48” lance. This particular length lance allows you to stand in a comfortable upright position while allowing proper distance from the surface of the nozzle, helping to prevent splash back and providing adequate reach.
Lances may be equipped with a small bend in the end of the lance. This bend serves two purposes for the operator. First when under pressure the water flow and pressure will lift the end of the lance. This causes the weight of the lance to be transferred back to the gun and hose providing greater balance and less operator fatigue. Second, the bend allows the spray from the nozzle to be applied to the surface evenly. When the operator is reaching up to spray the bend helps to compensate for the angle and applies the fan pattern at a flatter angle. This helps to equalize the surface impact of the fan and prevent what is known as cut marks or lap marks on the surface.
Lances are made with many types of metals and other materials. The most common materials used in construction are stainless steel, zinc plated steel and aluminum. Some of the lances are sold in different schedules (thickness), such as schedule 40 and schedule 80. Typically, the steel and stainless steel lances are sold in a schedule 40 as the material is thick enough and strong enough to support the flows and pressures at that rating. The thinner material also allows for the lance to have a reduced weight. Aluminum lances on the other hand will typically be a schedule 80 as the additional thickness is required to withstand the higher pressures and add strength to the softer metal.
These lances may be equipped with a molded or pressed on grip, a removable grip or no grip at all. The grip will usually have ribs or vents in the construction. The purpose is to allow for airflow around the hand to help keep the hands dry allowing for a better grip on the lance, as well as, providing some structural strength to the lance. In addition, the grip provides protection from heat buildup due to hot water.
Lances/Dual (Variable Pressure Wand)
Dual lances are attached to the trigger gun with a short section of lance that is approximately 12”-14” in length. This section of lance connects to a valve body that has two outlets in which two lances of equal length are attached. The valve body will have a side handle that operates a needle valve and when opened allows the water to be diverted down the secondary tube. This needle valve allows the operator to open the valve a little or a lot. The operator has the ability to vary the pressure, which is where the term “variable pressure wand” comes from. Some dual lances have a trigger assembly instead of the needle valve (side handle) which allows the operator to achieve the same purpose of diverting the water down the secondary tube, however, they do not offer the fine adjustments in pressure that the side handle versions do.
Dual lances are typically used in applications where downstream soap injectors are used or when the operator needs varying pressures on the same job. Dual lances are available in lengths form 12” to 72” and may be equipped with 1/8” or 1/4” outlet pipes. While 1/8” pipes will allow for less weight, the 1/4” pipes allow for greater chemical draw and strength of material.
Flexible lances allow the operator to bend the lance in multiple directions and steep angles while keeping the gun in a single position. This lance is a good choice for those in car wash operations or for those who degrease engines and other equipment. One style of flex lance is equipped with two solid pipes on each end with a flexible rubber grip in the center. Under this grip is a short section of flexible hose that is connected to each side of the pipe. The end of the lance can be bent in any direction and moved easily while the water is spraying. A second style is the McCauley spray lance. It uses a section of hose encased in a flexible metal housing. This unique housing allows the operator to bend the lance to a particular angle and have it maintain its shape under pressure. This feature enables the operator to attach the lance to the end of a pipe for spraying at strange angles. This is especially useful for hood cleaners who often have to spray ducts that turn in many directions. It can also be used for cleaning gutters.
The Nozzle - Next to the pump this is the most important component that determines pressure and cleaning efficiency. The smaller the spray pattern the higher the impact. For example a #5 nozzle changes from an impact of 5.6 lbs. At 15? to 2.6 lbs. With a 40? spray angle. This is why turbo nozzles have become so popular recently. Actually they don't increase pressure but merely use a Zero degree pattern with an oscillating action that covers a larger area faster than you could move a Zero nozzle.
Hose - Hose is generally rated by hose manufacturer for operating pressure with a 4 times burst factor. They are braided and have crimped ends and hose guards at ends to prevent kinking at connections. Diameter and length of hose should be sized to flow and pressure operating at. Pressure or friction loss can increase significantly as flow increases. For example at 3 GPM on a 50 foot length of 1/4" hose the pressure loss is 200 PSI, whereas a 3/8" hose has only a 25 PSI pressure loss.
Pump crankcase- can be trouble free like an automobile if properly maintained. Oil should be changed after initial 50 hour break in period and every 500 hours or annually thereafter or when oil gets milky. This could be caused by condensation from temperature changes or oil seal leakage which may occur if vented crankcase plug is not installed and seals are sucked in from vacuum created when oil heats up.
Filters - a good inlet filter is a cheap price to pay to protect your pump from impurities that are in water. Rust, scale and sand can easily clog valves or scratch plungers. Use a 60-100 mesh screen of adequate size for flow and be sure it is checked and cleaned periodically.
Pressure Reducing Valves - are only required if pump is sensitive to high inlet water pressure when city water pressure is in excess of 60 psi. Sometimes chemical injector will not function properly if inlet pressure is too high.
Pressure Gauges - are generally not used by most manufacturers as they tend to malfunction easily unless more expensive glycerin filled ones are used and they add to machine cost. They do however, serve an important role as a ready warning to nozzle wear or pump seal damage which reducing cleaning effect and potentially more expensive repair later.
Unloader Valves - this is the main safety component of a pressure washer. It along with the trigger gun literally controls the traffic flow of water in the system. Without an unloader valve when gun is shut off the pressure will continue to rise until either the motor - engine stalls or pump, hose or gun ruptures. Unloaders divert all or part of the flow to a supply tank or the inlet of pump to prevent pressure from building up. When gun is opened water moves from outlet of pump through hose to gun and nozzle. When trigger is released the valve closes and the unloader is activated either by an increase in pressure (pressure actuated type) or a reduction in flow (flow actuated type). Water flows into channel and pushes piston down which opens bypass valve. The unloader diverts flow of water from outlet side of pump back to inlet side causing water to flow back to pump virtually under no pressure.
Advantages of flow actuated unloaders is that hose and pump pressure is reduced in unload mode. This is safer for hot water coil and when operator does not want fast kick back when gun trigger is depressed. It however, cannot be used on multi gun systems and is generally more expensive and sensitive to adjust.
Pressure actuated type is the common. It traps pressure in hose during unload mode so immediate pressure is available when gun is opened.
Pressure of system can be controlled to some extend by adjusting the tension on the spring holding the piston in the valve in place. Changing nozzle is preferred method of controlling pressure and unloader should only be adjusted to fine tune the system.
If unloader valve goes on/off when gun is shut off either the spring is not properly adjusted or there is a leak in hose, gun or connections. Too tight a spring tension can create a safety problem due to high pressure spikes before it unloads. Sleight bypass of liquid prevents valve erosion.
Rotary Brush - Mounted on end of lance and spins with water pressure providing brushing action required for certain application such as removing road haze from cars where excessive pressure or chemicals could damage paint surface or pin stripping.
Foamers - A foaming nozzle permits the flow of liquid to suck in atmospheric or compressed air into the flow causing it to form tiny air bubbles. This enables the detergent to cling to the surface longer and penetrate the dirt.
Sand Blast Heads - Small amount of sand in high pressure water stream can significantly increase its ability to strip paint. Must have adequate pressure to work effectively, i.e. 2 GPM at 1000 PSI or 3 at 3000 PSI.
Be certain that sand inlet tube is always at top to prevent water from running back into the sand when gun is shut off which will cause it to clog.
Drain Cleaning Nozzle - Excellent for cleaning clogged drains or pipes. Nozzles have a forward jet and several jets at a backward angle which causes nozzle and hose to propel forward knocking off dirt as it travels through pipe.