by Ron Joseph
Paint Efficiencies for HVLP Sprayers
Q1: I am researching the transfer efficiencies for HVLP sprayers. The specific models I am reviewing tout a minimum transfer efficiency of 65%. Can this efficiency be decreased by inexperience on the part of the user, or is the HVLP design largely unaffected by the experience level. With other sprayers the two influences on efficiency are more characterized; however with HVLP, the answer is simply 65%.
A1: There is no such thing as a minimum transfer efficiency and any company that makes such a claim is untruthful. Transfer efficiency depends on many factors, such as type of spray gun, atomizing air pressure, fluid pressure, fluid flow rate, gun-target distance, fan width, coating viscosity, coating rheology, size and shape of the part to be painted, and more. The painter's technique probably plays more of a role in determining TE than any of the other factors combined. Using the same paint and spray gun setup, one painter can get a low TE while another painter can achieve a noticeably higher TE. For some parts, such as wire products, small parts, tubular steel, etc., it might not be possible to achieve a TE of 65% under any reasonable circumstances. By "reasonable" I mean that the application is consistent with normal production speeds and practices. Clearly, if the painter narrows his fan to no more than 1 inch, stands within 2-3 inches of the parts and moves at a snail's pace, he might achieve a high TE, but that is not consistent with normal painting practices.
Finally, don't be fooled by the 65% value. This is not a scientifically derived number. It was literally "pulled out of the air" by an engineer at the EPA in the late 1970's. A friend of mine at the EPA who participated in writing the first guideline documents that discussed transfer efficiency has tried to locate the source of information that lead his former boss to use that number, but to no avail. In all likelihood his former boss spoke to one of the technical folks working for a spray gun manufacturer and asked what the typical TE would be for an electrostatic spray gun (HVLP did not exist in the late 1970's). The guesstimate was 65% and that number stuck. When HVLP was introduced in the mid-1980's the South Coast Air Quality Management District (SCAQMD), the first to regulate HVLP spray guns, intended to require HVLP to be at least as efficient as electrostatic. Hence the 65% number was carried over to include electrostatic and HVLP spray guns. In fact, the regulations do not require HVLP spray guns to be 65% efficient. Rather, if an end user wishes to use another type of spray gun, such as airless, air-assisted airless, etc., other than electrostatic or HVLP, he is required to demonstrate that the alternative gun will achieve >65% efficiency.By definition, an HVLP spray gun MUST be set up so that the atomizing air pressure at the cap of the gun is equal to or less than 10 psig. If a painter exceeds this value, (and there is nothing on the design of the gun body to prevent this) the spray gun no longer meets the definition of HVLP.
Q2: Your explanation helps greatly. SC DHEC-BAQ (the Air Quality Bureau for SC) handles a lot of air permits for body shops and this can be a grey area. Your explanation puts the industry's processes in context for us trying to figure out to write the permits.
Am I to understand that efficiency could be more influenced (potentially) by experience and technique versus unit design? What would you assume to be a fair transfer efficiency, for both experienced and inexperienced painters? Do you know of any research that has tested the validity of published efficiencies? (Even something as simple as figuring out mass concentration captured on air filters from fugitive PM versus total paint sprayed could be a decent first guess.)
Thank you so much for your help!
A2: There is no such thing as an average TE. If you intend to permit users of HVLP spray guns please do NOT require them to meet a value, such as 65%, since this unenforceable. Rather, require that they comply with the definition of HVLP; namely that the atomizing pressure at the cap of the HVLP spray gun shall not exceed 10 psig. This is both measurable and enforceable. In any case, there is no mechanism by which inspectors can verify the transfer efficiency of a painting operation, since the TE varies from moment to moment.
Every paint operator can be expected to purchase a kit that allows him to measure the pressure at the cap of the spray gun. In fact, you can make that a permit condition. In other words, you can state that the permit holder must be able to demonstrate compliance to the inspector, and this is accomplished by using a pressure gauge kit that must match the operating air cap of the HVLP spray gun. The kit is only used to demonstrate compliance. It is not used during the painting operation.
Please visit www.exponent.com/paint where you will see that I run a class that will be most useful to you and your colleagues. Please also review the course agenda. The class, which was presented a few weeks ago, will again be run at the end of September 2008 in the same location.