CNKS - P/S/G Feed Tip Size

I happen to run across a chart today that gives the recommended flow capacities of different fluid tips. The info is from DeVilbiss but should be representative. A common flow capacity for typical guns using automotive paints is up to 12 oz/min. Go to the bottom of this page and you will see a comparison of pressure, suction and gravity feed applications for different sized tips. For the common flow capacity, the following are indicated as equivalent:

Pressure feed - 0.7mm
Gravity feed - 1.4mm
Suction feed - 1.8mm

Rod

I also noticed the same DeVilbiss GTI HVLP gun in gravity feed comes standard with 1.3 and 1.4mm tips. It has eight different fluid tips available (1.2 - 2.2mm) but there is only one air cap that serves them all. No optional air caps are available for the gun. For HVLP the cap pressure is fixed at a legal maximum of 10 psig. That tends to make me think that the atomization of the paint is much more a function of the cfm airflow than of the air pressure available at the cap. I have always wondered where the heck the HV is in HVLP, at least as it pertains to the so-called conversion guns that are supplied by compressors and not turbines. The airflow requirements in cfm of modern, professional, full size HVLP guns are really not significantly different than similar conventional guns of 30 years ago (10-15 cfm). A higher air pressure at the cap would increase the air velocity and cfm through a constant orifice size. If, however, the pressure is decreased and the orifice size is increased, the same cfm can be obtained with a lower cap pressure. I don't know if the orifices are larger in an HVLP air cap than in a similar non-HVLP air cap. I don't have any from similar guns to compare.

I think the turbine guns were the first to use the term HVLP. I can understand that. From what little I have read about them the airflow is up in the 100 cfm area. I am beginning to think that HVLP as a term applied to the conversion guns is nothing more than a catchy phrase. If the average transfer efficiency of 65% (HVLP requirement, I believe) can be obtained by lowering the cap pressure and increasing the orifice size for a conventional gun while still getting satisfactory atomization, I guess one can call that HVLP but I am at a loss to explain the HV in that term.

My guess is that the lower cap pressure reduces the air velocity thereby also reducing the "throw" and the "bounceback" from the surface being painted. This would tend to increase the transfer efficiency and would explain why one has to get closer with HVLP than conventional (6-8 inches rather than 8-10 inches). Not sure of this but it seems to make sense at the moment :o).

I agree that transfer efficiency may be overstated. I suppose a lot depends on how the actual work compares with whatever standardized test arrangement is used by the regulatory compliance crowd. When I started to use my HVLP spot gun it was my first use of any gun marketed as "HVLP compliant". I really didn't notice any obvious difference in amount of overspray from what I was accustomed to with a non-HVLP spot gun. My biggest surprise was that I had to get closer to the surface and travel slower than I was used to. That was immediately obvious. Perhaps I was too unimpressed with the gun speed to notice. I just never got the impression that "wow, this gun puts a lot more paint on the surface than the air". To me transfer efficiency is not an important consideration anyway. I don't do enough work with the more expensive paints to make it matter much. Most of my waste in painting materials are in quantities left over, either from initial purchase amounts or individual mixes caused by my intermittent work schedule. If I were "in the business" or did this sort of thing regularly, I might feel differently.