In essence, an HVLP spray gun is still an air spray gun as it uses air as its primary atomization force. Both HVLP and conventional air spray guns use the same two components of compressed air, pressure and volume but in different quantities. The pressure, which is normally noted in terms of pounds per square inch or PSI and volume, which is noted in terms of cubic feet per minute, are both necessary for either gun to work.
Originally HVLP guns where designed to use extremely high volumes of CFM that were generated by turbines rather than compressors. These turbines delivered, in some cases, hundreds of CFM but very little pressure. In order to remain competitive with the turbine HVLP guns, the manufacturers of the traditional air spray guns soon figured out how to convert their guns to HVLP using compressed air rather than turbine air. Today HVLP guns are as common as the old conventional guns.
HVLP technology proved that if you used a lot of volume of air instead of a lot of pressure like conventional guns did, you could not only atomize coatings but also do so more efficiently. The efficiency improvement was mostly related to the fact that the atomized paint particles traveled at a slower speed than paint particles that were atomized with high air pressure. We refer to this characteristic as spray particle velocity. The higher the velocity of the paint particle the lower the efficiency would be.
I like to use the analogy of a tennis ball and a cement wall. The paint particle is representative of the atomized paint particle and the wall represents the substrate being coated. The harder you throw the ball against the wall, the further the ball bounces from the wall. During air spray atomization the paint particle can travel at speeds greater than 30 feet per second or FPS. When the same coating is atomized with HVLP technology, the FPS drops to 10 to 23 FPS. The slower the particle velocity, the less bounce back and over spray, the more efficient the technology will be.
In addition to spray particle velocity, another characteristic that affects guns that use a lot of volume of air such as HVLP is air damming. Here I like to use the analogy of a 2′ high by 2′ wide by 2′ deep box and blocks that are 1′ high by 1′ wide by 1′ deep. In this scenario the box is the inside of a cabinet and the blocks represent a cubic foot of air from your gun. Try to imagine these blocks of air as they exit the spray gun and enter the box. When you spray into the box you are filling it with the blocks of air. The inside area of the box can only hold 8 cubic feet or 8 blocks, but the gun you are using is putting out 20 cubic feet per minute or 20t blocks per minute.
The result is that within seconds you are putting too many blocks into the box and those extra blocks are preventing you from putting in more blocks and they are spilling out all over the place. The volume of air has to go somewhere because it usually can’t pass through the item you are trying to spray. The high volume of air from the gun is preventing more air and most importantly, the coating from getting into the box. This is an example of air damming and the higher the volume of air the more damming you will get.
In addition to air damming there is the problem of air turbulence. Turbulence results from sudden changes in wind direction and velocity. This causes the aircraft to bounce and move rapidly in directions that are not intended. Anyone that has ever flown in an airplane or jet has probably experienced it first hand. As the atomizing air exits the spray gun it begins to interact with the coating and the surrounding air in the atmosphere creating turbulence. Additionally, the air and coating interact with the substrates’ geometry creating eddies and currents which add to the turbulence. All guns create their own turbulence during atomization but guns with high atomizing pressures or high volumes of air will create more turbulence than guns that use low pressures and very low volumes of air.
So aside from these characteristics that are shared by conventional air spray, HVLP, LVLP and RP guns, one should also understand the benefits and limitations of HVLP. For the most part the benefit is reduced coating usage as HVLP has been proven to increase ones’ transfer efficiency. In essence transfer efficiency is than percent of paint solids that leave the spray gun during atomization that actually end up on the substrate and not in the spray booth filters, booth walls or the booths floor. Some manufacturers have seen coating savings per square foot of up to 50% while others may only see savings of 15 to 20%. Much depends on your coating, your environment, your finishers and how well your conventional gun was set up in the first place.
As for the limitations, HVLP guns use substantially more CFM than conventional guns. This means that you will most likely be paying more for electricity to run your compressor at best or you might need to upgrade to a larger compressor at worst. Although the overall atomization of HVLP has greatly improved, atomization quality can be negatively affected by high viscosity and or high flow rates. Most states require that HVLP guns be limited to 10 psi at the air cap because pressures higher than that begin to decrease the guns’ efficiency substantially. So if your material is viscous or if you are running high flow rates you may be hampered by this 10-psi limit or by the declining efficiency should you exceed 10 psi.
Low Volume Low Pressure
To help users with limited compressed air availability, some gun manufacturers have introduced Low Volume Low Pressure guns. These guns are designed to reduce spray particle velocity like HVLP guns, but can actually use less air than a conventional gun. This can help reduce the high electricity costs associated with the high cfm demand of HVLP guns. However, it is recommended that you evaluate these guns thoroughly for finish quality and efficiency before automatically switching to this technology. The money you save in electricity may be outweighed by a loss in efficiency or finish quality. Then again, you may find that some of they are actually more efficient or give you a better finish and save you money on your electric bill. The only way to be sure is to try them at your facility, with your coatings and your finishers.
Another advantage of this technology is their ability to deliver pressures substantially higher than HVLP. For companies or industries that are not restricted by state environmental regulations concerning the 10-psi limit, these guns can handle higher viscosities and flow rates than HVLP guns but most likely at lower efficiencies. However, the efficiencies are usually significantly higher than Conventional Air Spray. LVLP guns are considered HVLP compliant but only if operated at 10 psi or less at the air cap.
All of the above low fluid pressure, air spray technologies are available in siphon, gravity and pressure feed formats. However, keep in mind the limited siphoning ability of some of these guns, especially if your material is viscous or if you need to spray at high flow rates. It is also important that you make sure that the gun you select can be operated within the compliant pressure range (10 psi for HVLP) or that it is recognized as a compliant technology, if you are required to do so by any state environmental regulations.