The flawless finish you set out to achieve is now an orange peel disaster. What
went wrong? Pressure (psi) drop and lack of volume (cfm) are two of the many
causes of imperfections in the finishing process. The air pressure that starts
out as a shout at the air source is barely a whisper by the time it enters the
spray gun, usually resulting in a poor finish. Too often, only atomization pressure
requirements are considered when both pressure and volume are critical to the
proper operation of a spray gun.
What Causes Pressure Drop?
The following factors contribute to pressure drop in conventional air spray
and HVLP (High Volume Low Pressure) systems. Just one weak point may cause a
major loss of pressure.
• Air hose that is too small in diameter
• Air hose that is longer than required or necessary
• Too many quick disconnect fittings in the system
•Restrictive quick disconnect designs
•Low performance air
regulators
•Inadequate capacity air compressors
Quick Air Connectors (Quick Disconnects or QD’s)
These devices are convenient, but depending on design, can be a major source
of pressure drop. Ask yourself, does the application require them? If the air
hose is not removed from the gun more than once or twice per day, these devices
should probably be avoided. Consider the following:
Figure 1 shows areas where pressure loss can occur in a system. A beginning
pressure of 75 psi from the air filter and regulator unit is reduced to 67 psi
after going through a typical quick disconnect. Traveling through 25 ft of 1/4
inch hose at 25 cfm, air pressure drops to 36 psi before going through another
quick disconnect. It drops again to 28 psi at the point it enters the spray
gun after going through another quick disconnect.
 |
| Figure 1: Areas where pressure loss can occur.
|
“Quick disconnect” fittings are part of the sample system in Figure
1. It may be convenient to have a number of quick disconnects throughout a system
for quick, easy detachment of hoses. But, even the best quick disconnect on
the market will result in some pressure loss. With three or four quick disconnects,
it’s possible to choke off the air supply almost entirely. Quick disconnects
should only be used where necessary. A quick disconnect at the wall may only
be used once a week, a steep price to pay for the loss of pressure.
In addition, all quick disconnects aren’t the same. Depending on the pressure
and volume requirements, it is possible to have a pressure drop as little as
1 psi or as great as 25 psi for one quick disconnect. Examine the small opening
in the female portion of the QD. The size of this opening is a good indicator
of just how much potential restriction the QD may put on the air being supplied
to the gun. For example, an opening of 0.305 inch would provide an area 2.4
times the area of an opening of opening in the female portion of the QD. The
size of this opening is a good indicator of just how much potential restriction
the QD may put on the air being supplied to the gun. For example, an opening
of 0.305 inch would provide an area 2.4 times the area of an opening of 0.196
inch. Use “high-flow” quick disconnects with the largest diameter
practical.
Using the Right Air Hose
Selecting the proper hose diameter is an important step in maintaining air pressure.
Perhaps the least understood of all spray system components, air hose diameter
plays a significant role in proper spray gun operation.
TABLE
1: Air Hose Pressure Loss Data
|
| Flow
rates at air cap * |
15 cfm
(0.42 cmm) |
18 cfm
(0.51 cmm) |
20 cfm
(0.57 cmm) |
25 cfm
(0.71 cmm) |
Hose Diameter
and
Length |
|
|
|
|
1/4 inch × 20
ft
(6.3 mm × 6 m) |
20 psi
(137 kPa) |
26 psi
(179 kPa) |
28 psi
(193 kPa) |
34 psi
(234 kPa) |
5/16 inch × 20
ft
(7.9 mm × 6 m) |
7 psi
(48 kPa) |
10 psi
(68 kPa) |
12 psi
(82 kPa) |
20 psi
(137 kPa) |
3/8 inch × 20
ft
(9.5 mm × 6 m) |
2.8 psi
(19 kPa) |
4 psi
(27 kPa) |
4.8 psi
(33 kPa) |
7 psi
(48 kPa) |
| * Inlet
air pressures are 85 to 90 psi (586 kPa to 620 kPa). Air
consumption in cubic feet per minute (cubic meters per minute)
is determined by the air cap design. As the table shows,
the greatest amount of pressure loss occurs when the demand
for air is the highest. For example, with a 1/4 inch hose,
34 psi (234 kPa) of pressure is lost at a volume of 25 cfm
(0.71 cmm). |
With a smaller and more restrictive passage, less air gets through the line.
Using a 1/4 inch (6.3 mm) hose means less pressure at the gun than when using
a 5/16 inch (7.9 mm) hose. A 5/16 inch (7.9 mm) hose delivers less pressure
than a 3/8 inch (9.5 mm) hose. The smaller the inside diameter of the air hose,
the less air pressure delivered to the gun. Table 1 illustrates the loss in
pressure with different hose diameters at different flow rates. For instance,
there is a loss of almost 2 psi per ft (45 kPa/m) with a 1/4 inch (6.3 mm) hose
at 25 cfm (0.71 cmm), compared to a loss of less than 0.5 psi per ft (11 kPa/m)
with the larger 3/8 inch (9.5 mm) hose at 25 cfm (0.71 cmm). Air hose with 1/4
inch inside diameter (ID) is not recommended for use with any air spray guns.
The “right diameter” does not mean that you have to use the biggest
hose. Hose length is also a factor. As air is forced through the line, pressure
decreases the farther the air has to travel. Use the best combination hose ID
and length to prevent over-restricting the air supply.
Select a Quality, High Flow Air Regulator
Air regulator design can also restrict air flow and can create excessive pressure
drop. Check the gauge on the regulator with both static (no flow) and dynamic
(spray gun in use) conditions. If there is a pressure drop reading difference
of more than 5 psi, the regulator may be too restrictive. Select an air regulator
capable of flowing enough air for your spray gun. Regulator capacity specifications
should exceed spray gun demand. In addition, using an air cap test kit (HVLP
systems) to verify the air cap pressure setting can help ensure proper atomization
quality and code compliance.
Select Quality, High Performance Air Compressors
Modern spray guns, particularly HVLP, require greater volumes of air to atomize
today’s high solids coatings. Air compressors must be adequately sized
to handle the volume and pressure requirements throughout the system. This is
especially important considering the restrictions of quick disconnects, small
diameter hose, other air tools, and other compressed air demands.
Special Considerations for HVLP Systems
Unlike conventional air spray guns, modern HVLP spray guns atomize coatings
by using a high volume of air at low air cap pressure, generally in the 1 -
10 psi range. In a typical HVLP design, the regulated 50 psi (345 kPa) of air
supplied to an HVLP spray gun will be reduced to a pressure of no more than
10 psi (68 kPa) as measured at the air cap. In a conventional air spray gun,
air cap pressure is restricted only by compressor capacity, regulator pressure
and normal pressure drop conditions. Under these conditions, conventional air
cap pressure can be considerably higher.
HVLP spray depends on sufficient air pressure and volume at the air cap for
optimum performance. All of the above sources of pressure loss can prevent enough
air from getting to the spray gun to generate the necessary air cap pressure.
In general, avoid using 1/4 inch hose for all spray guns. With the volume requirements
of HVLP spray guns, avoid using 5/16 inch hose to ensure maximum air available
to the spray gun. Use quick disconnects only when the air hose must be removed
from a gun several times per day. Choose QD design carefully to insure that
the least restrictive units are used. Use one QD per gun located at the gun
inlet.
Wall-mounted regulators typically have larger air passages than gun-mounted
regulators. The larger air passages minimize pressure drop compared to the smaller
gun-mounted regulators. These regulators provide a constant air pressure delivered
to the gun. Air pressure controlled with an air-adjustment valve-type restrictor
will first exit the air cap with an initial burst of pressure, then level off
to the adjusted pressure and may fluctuate with changes in system pressure.
Air Cap Test Kits Pay Off
A final suggestion is to use an air cap test kit with HVLP spray guns. Using
the test kit takes the guesswork out of analyzing the amount of air available.
Knowing the amount of air pressure at the air cap allows you to determine whether
there is enough air to atomize the material being sprayed and remain compliant
with environmental regulations.
