SIMPLE
SETUP
Things
to remember:
1)
Let your air cool off the compressor. Big shops have refrigerated air coolers.
A minimum of 25' to your air control units.
2)
If using Pipes slope your pipe toward your air drains so everything flows downward
toward your drains.
3)
Use an air Drying, Filtering System.
4)
If you have very little air, use a high
flow air hose or couplers. - Call for Couplers.
5)
Use a finishing Filter at the Gun to remove any last contaminant.
6)
Filter your paint as you pour it into the Gun or pot.
CLICK
HERE
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.
