• Local electrical costs
• Type of compressor
• Pressure used for spraying
• CFM consumed
• Amount of time the trigger is being pulled
• Number of coats being applied
• Efficiency of the gun/cap/tip combination

Before getting into the cost calculation, let's cover some of the not so obvious factors.

The consumer type compressors that are bought from the big box stores typically do not provide enough air volume/pressure for individual applications.  Two stage compressors provide the higher pressures and volumes required.

The air pressure supplied to the spray gun should be set at the lowest pressure that will allow the atomization required and still allow the operator to accomplish the production required.  Higher than required pressures result in higher air consumption, therefore higher electrical costs.

In general, it is more economical to apply the finish coat in one pass than using two thinner coats.  Less paint is used and less electricity is used.  Be sure to check your coatings Technical Data Sheet for proper application and film builds.

To calculate the daily electrical costs for the air in a spray gun use the following formula:

DC X HC X ED X CFM X .228

DC = DUTY CYCLE = PERCENT OF TIME TRIGGER IS BEING PULLED
HD = HOURS IN WORK DAY
EC = ELECTRICAL COST PER KILOWATT HOUR (.0935 NATIONAL COMMERICAL AVERAGE, JULY '07
CFM = CFM RATING OR AIR CONSUMPTION OF THE GUN
.228 - ELECTRICAL CONSUMPTION OF 1 CFM IN KWh

For example, what is my annual cost to supply the air for my 18 CFM air cap.  How much would I sve if I lowered my pressure to a pressure that consumes 12 CFM or use a different air cap that consumes 12 CFM?

My factors are:

• Duty cycles = 50%
• Work day = 15 hours
• Cost per KWh = $.10
• Days per year = 250
• 18 CFM Air Cap

.5 x 15 x .1 x 18 x .228 x 250 = $769.50 annual electrical costs

Compare the above to a gun/pressure combination consuming 12 CFM

.5 x 15 x .1 x 12 x .228 x 250 = $513.60 annual electrical costs

Lowering air consumption by using lower air pressures or using an air cap with less air consumption would result in an annual savings of $256.50 per spray gun.

The above calculation are based on typical efficiencies and are approximate. The change in cost is

1. Trigger safety is in the SAFETY position. Put trigger safety in SAFETY OFF position. Refer to your Instruction Manual for the operation of your particular spray gun's safety.
2. The gun housing may be blocked from pumping a highly viscous material with a lot of filler that has packed behind the gun needle and will not allow the gun needle to move. Disassemble and thoroughly clean the spray gun.

Fluid tip wear on airless/air assist airless guns is caused by several factors.

1. The pressure being used.
2. The size and shape of the tip.
3. The abrasiveness of the material being sprayed
4. The tip material

Fluid pressure
The higher the fluid pressure, the greater the wear.

Tip size and shape
Smaller tip sizes result in shorter tip life with other factors being equal.
Larger tip sizes result in longer tip life with other factors being equal.
Shorter pattern sizes give longer tip life than taller pattern tips.

Material abrasiveness
Highly abrasive materials will result in a shorter tip life when compared to low or non-abrasive materials.

Tip materials
The quality of the materials will help determine tip life. Higher quality materials like carbide, while more expensive to manufacture, will not wear as quickly as carbon based material.

How does tip wear affect my setup?

Flow rate increases
As the tip wears, the physical opening in the tip increases.  An increase from .015” to .017” (two one-thousands of an inch) may result in a 33% increase in flow rates. How quickly this happens depends on the factors listed above.

Pattern size decreases
The tip will wear out in the top and bottom portions of the tip opening. This will result in a smaller pattern size. It will continue to decrease in size as the tip wears.

How does tip wear affect my spraying?

Tip wear is gradual, usually over days or weeks. The operator will attempt to compensate by doing the following:

1. Increase fluid pressure (an attempt to achieve an acceptable pattern). This will increase fluid delivery even more.
2. Back away from the part (an attempt to achieve a larger pattern). This may result in a dryer spray pattern.
3. Increase gun speed (an attempt to prevent runs and sags).

This depends on what material you're spraying and how many gallons sprayed per day. For example, with lacquers, guns don't need rebuilding as often because lacquers don't have solids in them. In contrast, the high solids in block fillers are abrasive and require more frequent gun rebuilding.

One way to increase gun life before repacking is to thoroughly clean your gun at the end of every day. Be sure to trigger the gun before removing the diffuser and when installing the diffuser. If you don't, the diffuser will score around the ball on the new needle which can lead to premature wear. The gun will develop a leak and this will cause spitting.
 

Tip extensions, extension poles and tip filters can also cause spitting or a stream of fluid after the gun is released due to decompression of the paint trapped between the gun and the tip. Fluid pressure dissipating through the tip causes this spitting or paint stream. An automatic shut-off valve will prevent this problem.
 

The size of an airless fluid tip will depend on the following:

1. The fluid delivery requirements.
2. The size of the pattern required.
3. The viscosity of the material being sprayed.
4. The type and amount of solids in the material beings sprayed.
5. The type of filtration in the system.
6. The pump pressure available

Let’s go through basic tip sizes first.

Fluid tips generally have a size listed in thousandths of an inch. The size in normally the diameter of an equivalent circle. Thus we may have 4 or 5 tips that have identical opening sizes (ie .015”). While the sizes may be all the same, the shape of the opening is not. Imagine taking a 15 in flexible circle and changing the shape from a circle to an oval. How many different size ovals could we make?

Tips may also have an indication of the pattern size or angle of spray. That will vary with the manufacturer.

Looking at Figure 1, the areas of all the circles are the same size.

Tip “A” would provide:

The least amount of resistance
The least amount of "tip plugging"
The least amount of tip wear
The shorter / rounder pattern          

Tip “E” would provide:

The highest amount of resistance
The highest chance of "tip plugging"
The highest amount of tip wear
The taller / narrower pattern

                                   Figure 1

Fluid Delivery Requirements

Most manufacturers have charts available that give flow rates for various tip measured using oils of a specific viscosity at specific pressures. A good rule of thumb for fluid delivery is:

Doubling the fluid pressure will result in about a 40 percent increase in delivery.

Pattern Size Requirements

The choice of pattern size typically correlate directly to the job at hand. Most manufacturers have several pattern sizes available for a given tip opening. Match the size to your job requirements.

Material Viscosity

Higher viscosity materials will result in lower flow rates. Another factor to consider is the temperature of the coating material. Higher solids materials will be easier to spray and atomize better if heated. Check your Technical Data Sheet for the coatings maximum recommended temperature

Material Solids

Material solids directly affect the viscosity of the material. Higher solids material typically require larger tip openings. The particle size of the pigment will also determine how frequently the tip ‘plugs’. Use larger tip sizes if tip plugging happens frequently.

Filtration

Due to the narrow openings of most tips, filtration is highly recommended. Passing the material through a coarse strainer may be adequate on the shorter pattern tips, but a fine filter usually is required for the taller pattern tips.

Pump pressure

Use a pump that has enough pressure available to do the job. You should try to size a pump for no more than 70% of its rated pressure range
 

Typically, a stainless steel mesh filter is placed at the end of the paint pickup tube.  For even better filtration (finer mesh), you might consider wrapping some pantyhose around the stainless steel filter, but first ensure that the pantyhose is not sensitive to any of the paint ingredients, especially solvents.  Some airless spray guns provide a filter in the handle of the spray gun.

Ensure that you maintain the filter and keep it clean.  Also look to see there are not breaks in the mesh.  You can also purchase small filter washers that fit behind the fluid tip.  The mesh size on your airless spray gun might be to large, but you can often purchase other sizes, such as 30, 60, and 100 mesh.  If you use a wooden stick or paddle to mix or stir the paint, do not allow any old dry paint to enter the paint reservoir.  Regular and proper maintenance is the best preventative aid for clogging problems.
 

The selection of a spray gun is an important decision to make in the design of a spray system. Matching the gun and fluid source to the application is of utmost importance. Failure to choose the correct equipment generally results in lower efficiencies, increased coating costs, increased emissions, higher booth filter costs, etc.

To match the spray gun to the application, the following criteria must be known:

Fluid Viscosity:
Generally listed as high, medium and low - manufacturers' charts will usually indicate the capabilities of their air caps and fluid nozzles in terms of viscosity. If one knows the viscosity in centipoise or time in a viscosity cup, it is a simple matter of conversion to the criteria listed.

Fluid Flow:
Generally listed in ounces, cc, ml or gallons per minute - spray gun charts will generally indicate a range of flow rates for a given fluid tip / air cap combination.

Production Rate:
Production speed will dictate what the flow rate of the equipment should be. Obviously, a line running a 30 feet per minute with 6 square foot parts will require a higher flow rate than a line moving at 10 feet per minute with 2 square foot parts.

Available Air (psi & cfm):
In most plants, the available air is adequate to supply the necessary air required by a spray gun. Problems usually show up when:

• Restrictive quick disconnects are used
• Small I.D. air hoses are used (i.e.¼”)
• Air hose longer that necessary is being used.
• Compressors too small in size
• Keep in mind, a typical industrial air cap consumes 20+ CFM

Fan Pattern Size:
Pattern size is one parameter listed for an air cap. It should be matched to the part being sprayed. Pattern sizes range from as small as one inch to several feet.

Atomization Required:
A “Class A” finish is not required for all applications. Applying a finish to a garden utensil would not necessarily be the same as the hood of an automobile. A stain applied to wood that is to be wiped off would dictate a very low level of atomization. If a lower level of atomization is required, an air cap with a lower CFM ($$$) would be selected.

Atomization Type:
Air Atomized:
Conventional Air Spray
Conventional air spray is the lowest in efficiency of the different atomization types, but yields good atomization with difficult to atomize coatings. Conventional air spray may not be permitted by Federal, state or local rules for some industries.

HVLP (High Volume Low Pressure)
An air atomized spray gun with a limit of 10 psi at the air cap. Since there is a legal limit (10 psi) in many cases, choosing the proper air cap for the above criteria is critical.

Hydraulically Atomized:
Airless
Uses high fluid pressure (1000-5000 psi) to atomize. Capable of very high flow rates, lower in atomization capabilities than air atomized equipment.

Air Assist Airless
Uses high fluid pressure (300-1500 psi) to atomize. At these lower pressures (compared to airless) the top and bottom of the pattern have a difficult time atomizing. An air cap is added for the sole purpose of eliminating the “tails”. Quality of finish is between air atomized and airless guns.

Fluid delivery system:
Suction Feed
Suction feed guns have been around for a century. While they are simple to operate, they are the most inefficient of the group. The range of motion is somewhat restricted since the pick-up tube must remain in the fluid to function. It is available in conventional air spray and HVLP

Gravity Feed
Gravity feed guns depend on gravity (cup is on top) to supply fluid to the gun. As a result, some control of the gun is gained over suction feed. Maximum use of the material due to gravity is also a plus.

Pressure Feed
Allows maximum control of the required pressures (air and fluid). The source of the coating is dependent on the amount used during a given time period. Available sources include pressurized attached cups, pressure pots (.5 to 60 gallons) and pumps (both low and high pressure)

Given the above criteria, a competent equipment supplier can match the equipment to the application.

Triggering a spray gun refers to momentarily releasing the trigger when the spray gun has passed the end of the part and is moving to the position for the next spray pass. The amount of paint wasted does not appear to be significant for those who do not release the trigger between passes. The following example shows otherwise:

Example:

Operator spraying 50% of 7 hour day
Flow rate = 10 oz/min
Coating Cost = $10.00 / gallon
Trigger time  = 1/5 (.2) second

Using the above figures:

Wasted Coatings

1 cc lost at each spray stroke end
Part size requires 6 gun strokes
1 gun triggers per stroke, 6 Total
1 cc x 6 strokes = 6 cc lost per part
Production rate = 2 parts/minute
420 minutes x 2 parts / min. = 840 parts
840 Parts x 6 cc = 1.33 Gallons/Gun/Day

3,785 cc / gal
1.33 Gal./Day x 240 Days / Yr. = 319 Gal./Yr.
$10 Cost x 319 Gal. = $3,190 Finish Waste Per Year
$3,190 Lost For One Gun

VOC Reduction

30%  Solids, 70% VOC
8 Lbs. / Gal x 319 Gal. = 2,552 Lbs. Liquid Sprayed / Yr.
2552 Lbs. X 70% = .9 Tons
2,000 Lbs ./ Ton
.9 Tons Emissions Generated by One Gun (applies against your VOC permit)

Waste Disposal Considerations

319 Gal. X 30% Solids = 95.7 Gal. Solid Waste

          95.7             = 1.74 Drums of Waste

55 Gal. / Drum
$200 / Drum Disposal Cost = $348
$348 Waste Cost Generated by one Gun

Total Cost of 1 cc Wasted per Gun

Obviously higher flow rates and higher coating costs would increase the figures above. In addition to the above, increased booth filter usage would be an issue.