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I agree with your recommended compressor capacity for "serious" use. You can make do with less but will be limited. I did not see any compressor pressure rating that the 6.6 cfm is referred to. I presume that the 15.2 cfm gun requirement is with the recommended pressure at the gun, which is probably the same as your SATA 2000 (about 30 psig). As long as the compressor pressure at 6.6 cfm is high enough over the 30 to compensate for all the losses between tank and gun at a flow of 15.2 cfm, I suppose it doesn't matter. Standard compressor cfm ratings are at 90, 100, 125 and 175 psig. Some of the smaller home-shop models also have a rating at 40 psig in addition to that at 90. I discard any cfm rating at such a low pressure . I consider any rating at less than 90 to be pretty much useless for most work, since most tools are performance rated at 90 at the throttle. For that to happen, it's got to be more than that back at the storage tank - sometimes a lot more. You are correct that the cfm delivered changes slightly with discharge pressure for any reciprocating compressor, increasing with decreasing pressure. That's due to the changing volumetric efficiency characteristic. But it's not that great an amount. If the compressor was 6.6 cfm at 90 psig, I would expect something like 8 cfm at 40. Certainly no more than that. That's not enough to support a 15.2 cfm gun for long in any practical installation. Their calculation seems to be set up to accomodate multiple guns in a relatively large painting facility, although you can run the thing for a single gun. Whenever multiple users share a common utility, there is a diversity factor involved to account for the reality that not all users are going to be using the utility at the same time. That could possibly be what the 1/3 "time factor" is about. However, for a single user, there is no diversity factor involved at all and the utility must meet the needs of at least that single user. The calculation doesn't seem to address this fact. It could also be that the 1/3 time factor stated may be the result of having the gun trigger pulled only 1/3 of the elapsed time and the cfm used over the total time is averaged. I don't think that's very practical unless you consider time extending beyond the actual painting process. And that doesn't make any sense at all. For me, the "average" time the trigger is pulled during any single-coat period is much higher than 33%. It's probably closer to 85%. Here's a hypothetical: 60 gal tank, 6.6 cfm @ 90 psig compressor, 8.0 cfm @ 40 psig, 125 psig cut-out pressure, 95 psig cut-in pressure, avg tank temp of 100 degF, 15.2 cfm consumption with 30 psig at the gun and 20 psig drop between tank and gun. Starting with a full tank at 125 psig, that will give you just one minute of usage before the compressor starts. From there on, since the compressor delivery is less than the consumption, there will be a continuing drop in tank pressure, even with the compressor running continuously. After about another three minutes, the tank will drop below 50 psig and the gun pressure of 30 psig will not be maintained. Counting a trigger pull time percentage of 85% during a single-coat painting process, that would yield about 4.5 minutes total usage time before the gun pressure could no longer be maintained. It's been a long time since I did an overall paint job on a full sized car but I think it took longer than 4.5 minutes to do a single coat. Perhaps one of the pros here can give a time estimate for applying a single coat to a full sized car, assuming no mid-coat stops. If you're doing only panel painting, there sould be no problem with the hypothetical for a single painter. Overalls need a longer time for continued air capacity in my way of thinking.
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Rod