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Theoretical Spray Angles

Theoretical Spread and Spray Angle Coverage

Although nozzle spray patterns are expressed in degrees of spread, these figures are not precisely accurate as the distance between the cleaning surface and the nozzle increases, which is why the terms theoretical spread or theoretical coverage are used. Theoretical spray angles indicate approximate spray coverage’s based on water velocity. In actual use the spray angle varies with distance.

Formulas for Common Nozzles

The theoretical coverage (fan width) for distances other than those given in the table above can be calculated from these formulas:

There are two types of fan or flat spray nozzles, elliptical orifice or a round orifice set at an angle to a deflecting surface. The elliptical design is by far the most common in the pressure cleaning industry. It produces a spray pattern with tapering edges. Wider droplet dispersion over the surface area is found at the edges of the spray pattern. This works best when the operator sprays in overlapping patterns, making the overall spray more uniform. The deflector type, on the other hand, produces a spray with even edges and may be used in cleaning applications, which require uniform impact over the pattern width without overlapping.

Using the right nozzle – the one that suits both your operator and the application, is most important for achieving good results from a pressure cleaning operation.

Material and Wear

Pressure washer nozzles are generally made of steel, often hardened stainless steel, although some new, high-pressure nozzles are made with tungsten carbide inserts to lengthen nozzle life. Some nozzles used in the industry are made of ceramic, which is very resistant to wear but is easily shattered.

Wear on the nozzle will eventually result in pressure loss as the flow of water slowly enlarges the orifice. In a fan spray type pattern, the most common in the industry, a narrowing of the spray pattern will also be noticed. Because of the variety of variables such as type of chemicals used, hardness of the water and the velocity of the spray, to name three, engineers are reluctant to give estimates on nozzle life.

Additionally, the amount of wear permissible varies according to application and user preferences. Manufacturer’s tolerances vary as well. Two factors, the orifice or opening diameter and the spray angle, are important in differentiating between nozzles and making a decision on what kind of nozzle you need.

Orifice Diameter

Orifice diameter determines the pressure produced at a particular flow. A smaller orifice will produce a high pressure or psi figure at a specific flow or gpm than a larger orifice. Fan spray nozzle orifices are generally elliptical rather than round.

This is the result of the method of manufacture. Fan spray nozzles are made from blanks that are pretty much already machined to their final shape. A round hole is then drilled through the blank from the back and a cut is made across the front. This is called drilling and milling and the result is a three dimensional elliptical opening. The milling cut determines the spray angle.

Nozzle Sizes

Nozzle orifice dimensions are given for a round hole of equivalent area to the actual elliptical opening and are measured in hundredths and thousandths of an inch. Nozzle sizes are generally expressed as a whole number rather than a fraction. The whole number is not necessarily an expression of the actual equivalent diameter. In other words a 65 or 065 nozzle under the most accepted system has an equivalent diameter of .060 inch. A 4 or 040 nozzle has an equivalent diameter of .047 inch.

The nomenclature is somewhat confusing. Historically, the basis of nozzle sizing is the "four, four, four rule." Simply, the rule states that a 40 nozzle at 4 gpm will produce 4000 psi. "As nozzle orifice apertures increase in size the system comes back around and a 10 nozzle has a .075 equivalent aperture diameter. A 20 nozzle could have an aperture of either .106 (large) or .36 (small)and a 40 could be 0.150 or .047."

In technical literature the smaller orifices are designated with a preceding zero but when nozzle sizes are discussed it is generally assumed that a reference to a "40 nozzle" is a reference to a smaller equivalent diameter.

To make it even more befuddling, different manufacturers may use slightly different sizing systems. One manufacturer rounds off equivalent diameters to the nearest hundredth of an inch, and, consequently lists three different sets of flow and pressure characteristics for nozzles of the same apparent equivalent diameter.

Thus, nozzle sizes based on specifications alone will show discrepancies. One of the most popular pressure washers today is a 22 gpm, 1000 psi machine. However, you will discover, when checking standard nozzle sizes, using nozzle charts prepared by the leading United States nozzle manufacturer, that one size produces 1000 psi at 2 gpm and the next larger size produces 1000 psi at 2.3 gpm. The same is true of many other standard sizes. (A comparison of nozzle charts prepared by different nozzle manufacturers shows a number of differences in rated flow and pressure for nozzles of the same apparent size. We have relied on the Spraying Systems specifications for their popular Washjet nozzle.)

Flow and Pressure Differences

To illustrate the difference that the orifice diameter makes in flow and pressure, let’s compare a 65 and a 40 nozzle. At a flow in the vicinity of 2 gpm (really 2.1) the 65 nozzle produces a pressure of 400 psi. The 40 nozzle, however, produces 1000 psi at a 2 gpm flow.

The higher the flow and pressure the more impact and cleaning power-with certain qualifications. For example: hot water at high pressures-3000 psi and above-may lose cleaning power through vaporization and dissipation.

Arguments continue in the industry over whether flow or pressure is more important. One side argues that a higher pressure will produce better cleaning results while the other maintains that after a certain pressure is reached; improved results can best be obtained by increasing flow.

Popular Nozzle Angles

The most popular nozzle is the 25 degree nozzle, which is considered to represent the best compromise between spray pattern coverage and impact.

Some experienced operators prefer a 15 degree nozzle, trading off greater impact for less coverage and making up for reduced coverage with skill in using the equipment. When a machine comes with more than one nozzle the patterns are generally zero degree, 25 degree and 40 degree. The wider-angle nozzles are generally used for detergent application or rinses.

Other Nozzle Types

Nozzles that allow variable patterns, often with three or four different nozzle tips set to rotate into position, are available for lower pressure-generally no more than 1000 psi. In order to increase impact, some nozzles are made to create oscillating or intermittent flows with a turbine or cam arrangement.

However, most nozzles can be used at a higher pressure than they are rated for, although such use is not recommended by manufacturers and may cause problems.