I'm not an orchid guy, so I don't need it "cool" like some here do. Most of what I plan to grow are relatively heat tolerant - citrus, bananas, subtropicals, tropicals etc. I'd be happy keeping summer temps within 5F of outside temps...our average July high is about 87, we do hit the 90s quite a bit, and may top 100 about once every other summer.

If I go with a high enough CFM to match double what is recommended on littlegreehouse.com, i.e., to exchange air every 30 seconds at highest setting, maybe even enough to exchange the air every 15 seconds...and shade maybe 20% - 30% max (most of what I grow will require higher light levels) would I be able to accomplish this? I don't think a deep shade would work esp. for my goal of harvesting fruit...but with that much air exchange could it work?

A swamp cooler would be relatively useless here with our humidity...so that might be off the table.

Then the question of how accurate the "CFM" ratings of fans are...who has some testimonials of ventilation fans (like sold by ACF) that actually meet expectations of CFM performance?

air exchange/min.............no shade..........50% shade
1 .......................................+20...................+9
2 ........................................+11...................+5
3 .........................................+7.....................+3
4..........................................+6.....................+3

source: Greenhouse Engineering, NRAES-33

Here was my thought then. 3 variable-speed fans, all rated at 6200 CFM or thereabouts. 6200 is the number for one exchange/min. Seems that going from 3 to 4 has minimal effect so I'll do three fans, two thermostats - one on a lower one and the other two on a higher setting.

Winter, first thermostat, attached to one fan running at 30%, set at 74 degrees. Second thermostat attached to other two fans, also at 30%, set to 83 degrees...most days, my bet is this fan won't ever kick on.

In spring, I kick the power up to 60%, set thermostats at 82 and 90. In summer, go full power, and set thermostats at 86 and 92, or, since they'll almost certainly all kick on anyway, just set them at the same temp.

I don't know the size of your proposed house, but I am a big fans of shade cloths for heat reduction. They make a major difference and also prolong the life of your films and glazing. You can order about any percentage you want. If you do not want to impact light too much, but still want to reduce heat and also take off the extremes of the SLT between ten and two when the sun is zenith, you might want to consider just installing the cloth over the peaks and partway down the sides. You will still get in good light, but less intense.

The smaller the house, the more you'll struggle with heat. I have large houses but it's not unheard of, even with fans running, and cloth on to hit 105 degrees or on rare occasions higher. G'houses are not friendly for summer growing. Period. I usually transfer any stock to a shade house for the duration. These can be cheaply built, btw and if you are serious about your stock, might want to look into it.

Remember when ventilating, that you will never cool off below outside ambient air unless you combine evaporation somehow. That's what you are sucking in. Also remember that the more airflow you have over your plants, the more they transpire and lose moisture. You can put such a demand on them their roots won't handle it.

For example, a 1000 cubic foot greenhouse with a 1400cfm fan would (in theory) give you ~1 complete airchange per minute.
If the fan duct was 12" diameter,the air velocity would be around 30ft per second.. thats a stiff breeze :)
Now consider a greenhouse profile that acts like an 8ft diameter duct, the same 1400cfm results in an air velocity of just 0.5ft per second.
Unfortunately the airflow wont be evenly spread due to vent/fan locations and the various obstructions so you`ll have radically different airspeeds throughout the greenhouse.