Use a regular box or room fan. You can buy one for about $20 USD. http://www.acehardware.com/family/index.jsp?categoryId=1260957&cid= I've used them for 16 years. They are cheap, do the job and last 3 to 5 years. You can also build a fan from a salvaged motor from a dishwasher or washing machine. Just buy blades to fit the shaft. The advantage to this is the motors are usually made with ball bearings which can be oiled. They lasty forever.

If your evaporative cooler is a passive model, then you'll need a fairly powerful exhaust fan to create enough vacuum inside your gh to operate it satisfactorily. If you put in too many vents, you'll negate the benefit of your cooler. As air velocity increases, so does evaporation...more vents = less velocity.

It's been my experience that fan CFM is a slippery number. Depending upon how the fan is measured, it might not have an output even close to the number claimed. Based upon the measurements of your gh, I would recommend two Lasko 3300 wind machines mounted near the apex of your roof (if you're dealing with summertime temps of 35 Centigrade or more). That should get you a turnover of approximately once per minute, possibly a little less. Here in the States you can purchase them for around $23 apiece at WalMart.

If you design your airflow properly through the gh, you won't need an oscillating fan. You're going to have microclimates as you move further away from the intake side, both laterally and vertically. Monitor those zones, and place your orchids according to their individual needs.

-Bruce

Thanks for your replies and useful information.

The typical box fans available locally are rounded ones, so I would have to had padded/filled in around them too.

And I came across some more information about air stratification which got me to thinking about air flow, humidity and venting- some people have vents opening in the roof to release the hot air.

This is a discussion where some one talks about horizontal air flow removing 'good' air with hot air staying up the top- though if you had the exhaust fan up at the highest point you would get more of the hotter air being removed I think:

http://209.85.173.132/search?q=cache:AsuB1xJiPZYJ:www.orchidsafari.org/ghse/ghse1.html+greenhouse+exhaust+%22high+velocity+fan%22&hl=en&ct=clnk&cd=1&gl=au

This is interesting in that I read about someone who installed a the exhaust fan on the opposite end wall to an evaporative cooler or wet wall which worked well to remove heat but also removed a fair bit of humidity.

In contradiction I've read turbine ventilators do remove humidity and are installed and used to not only remove heat in summer but to remove moisture from under the roof/ceiling or in attics.
I know some one who uses whirly birds/turbine ventilators though they have said height is really important for them to be effective.

So I ended up not going with fans for exhausting air on the opposite end wall to the cooler and instead got a 30cm turbine ventilator whirly gig thing. I also found a few more exhaust fans [for bathrooms, etc] and got one [25cm AUS $49.00. that will fit under the whirly, extracting air keeping the turbine spinning.

The exhaust fan is not on a thermostat but I figure I could just turn it off and maybe cover it in winter to stop heat escaping.

I will be placing it in/on on the roof nearer towards the highest point where hot air should rise to.

I don't know how effective it will be but with misting under the benches, the evaporative cooler and trickling water down the shadecloth/matting against the fence I'd be happy if it only reduced temps just a few degrees.
Where my masies, dracs, pleuros, oncids, miltonias etc are at the moment is a mucher small potting shed, which I kept totally closed up last summer but I did set up a hose in there to release a small spray of mist [slightly dripping]
and have the evaporate cooler in there [+ fan inside circulating air around].

Despite some days with temps going past 35 degrees celcius
I didn't lose or have any plants visibly suffer or have masie leaf drop except for one plant that was a newbie.

I think it was keeping the humidity up around 80-90% [+ shading] that helped.

My main concern really has been about ondontoglossums and hybrids with the cooler/cold ones like crispum, I have been told and read too that they do better if temps can be kept lower in Summer.

I will post back later on how it works out and put some pics of my turbine/exhaust fan up in my Pleurothallidinae and Oncidiinae Grow House photobucket folder.

Damian.

Here is a link that might be useful: My http://s21.photobucket.com/albums/b253/daemondamian/Orchids/Pleurothallidinae and Oncidiinae Grow House/

Photos of your orchids are beautiful. Want to see more.

They both work. Question is which works better.

I don't have the answer.

However, if you look at the blade and fan designs for true exhaust fans and regular fans, they are somewhat different.

I would think there must be reasons.

dcarch