I botched the title. There's the full one.

Did you look at my suggestion to use tomatos grown over the air intake?

Misting systems can be efficent as well, I am not sure if they are as efficient as a swap cooler, but do work on the same principle.

I was wondering if anyone has tried using a fountain (Yeah I know, where am I gonna fit a water fountain in my greenhouse. But it might be a Nice decorative way to help cool a greenhouse. Perhaps have a Waterfall that is covered with Swamp cooler hay (Yeah i know, they do not use hay, but you know what I mean) They circulation fans might provide enough enough air movement to increase the evaporative cooling

jimmy: yep, any evaporation of water will resulting in cooling. What you really want is a fan connected to a humidistat to draw out the saturated air. But people should have this anyway to prevent mould.

Misting works fine, but it uses a lot of water. If you could recycle the water with say a pond pump it would be quite practical. I suggested to someone a while back using an old shower rose with a pond pump and a fan. These are called 'bong coolers' in the overclocking universe.

Here is an evaporative cooler good for up to 400sq. ft. Note the 5% discount offer.

Here is a link that might be useful: Convair Artic Breeze 1500

Yes, I saw the post about the tomatos, something to think about but I'm not sure it's the way I want to go. I mean, if something happened to those tomatoes, the greenhouse would quickly become uninhabitable for any plants.

quote:
yep, any evaporation of water will resulting in cooling. What you really want is a fan connected to a humidistat to draw out the saturated air. But people should have this anyway to prevent mould.

Could you please elaborate on this? How did you mean "draw out"? Do you mean a vent fan to the outside?

Do you mean a vent fan to the outside?

yes.

Thanks for the link cottagefarmer! That's a hundred bucks less than even some of the pre-made misting fans. If I can't find a better deal, I'll probably go with that.

A CFM calculator I found suggested around a 1200 CFM air exchange, but I've read here that more is better.
1) Just how much fan power (in one or two fans) should I get for my 10x12 in the summer heat?
2) Is there any good way to utilize the ridge vents with fan-based ventilation?'

and one about shade cloth:
3) Just how much shade cloth do I need for the killer Texas sun? Winds get fairly strong around here, so I'm going to want to attach it directly to the roof of the greenhouse, but should I also place it on the walls that get direct sun?

Here's something I found on a link I got here, groworganic.com, when I was searching for shade cloth info:

General field guidelines: for vegetables, such as tomatoes and peppers, use 30% shadecloth in areas with very hot summers; for lettuce, spinach and cole crops, use 47% in hot areas, 30% in northern or coastal climates. Use 63% for shade-loving plants. Shadecloth is also used over greenhouses in summer to reduce the need for expensive ventilation. Maximum shade density (80%) is often used over patios and skylights to cool people as well as plants.

Would yall say this is accurate?

It sounds similar to what I've heard here. I usually put 50% over the greenhouse in last spring.

Fan power is misleading, you need to get a certain number of CFM. A 1foot diameter fan might need to be 300W to move the same number of CFM as a 30W 3foot diameter fan.

A large hole in the side of your greenhouse can pretty much do away with the need for a fan. If you can add a chimney to the top, even more so.

I think people equate a breeze on the face as equivalent to cfm, and so tend to buy small, noisy inefficient fans instead of large, quiet efficient fans.

I use a 12" 1650 cfm fan mounted through the wall of a 10x12 GH. In the summer, mid-July, the fan could not keep up with the heat, even with a 60% shade cloth (passes 40%). Ended up leaving the four roof vents open all the time too. Here is a picture of the fan:

{{gwi:294482}}

I have better recirc fans this year and am planning on putting in a misting system, but still need to run a 200' trench from the nearest water.

Farmtek sells a 12" fan that does 1050 cfm for about $165. The link is below. You need a motorized vent also to allow thermostat control.

Here is a link that might be useful: ValuTek 12

That fan looks too small. Get a fan that fills the whole square and it'll do a lot better. The efficiency of fans is basically the radius in inches, according to a friend. So that fan would be about 6% efficient... Spring loaded (passive) louvres also dramatically reduce the performance (because the fan needs to work against the spring)

I would look for a 2' 30W fan for the 10x12 HFGH.

Cottage, thanks again but if 1650 CFM isn't doing the job in your zone 4 10x12, then 1050 cfm just won't cut the mustard here in Texas.

Nathan, do you grow terrestrial orchids in your greenhouse with 50% shade cloth? Would my paphs and tomatos alike enjoy that amount of light?

After looking around a bit, perhaps a 16" or larger fan might be the way to go. You say that larger for the same CFM is quieter, and that makes good sense to me. Still, I'm not sure how many CFM's I should shoot for. Something over 2000 certainly, but how many would I be using before it overpowers (for example) that 400 square foot evaporative cooler, if I placed the cooler near the intakes?

yeah, I grow terrestrials (thelymitra, spiranthes, pterostylis) and epiphytes(mostly dendros, sarcs and weird australian only genuses). Paphs like it warmer I think. Tomatos seem to deal with as much heat as you can provide, as long as they get enough water. I've had tomatos up to about 50C with a suitable VPD without deaths. You need lots of CO2 in that case though. We get about 1100W/m^2 in the middle of summer, and about 400W/m^2 in the middle of winter. So 50% shade in spring and summer is still brighter than winter sun.

Humid air rises, so put the evap cooler on the ground, and vent at the ceiling. for CFM perhaps you could work out the CFage of your greenhouse (if it is a box, just multiply wxhxd. The volume of a tent shape (triangular prism) is lxwxh/2. Then you can decide how many air changes per minute you want and thus how many cfm.

For example, a 10x12x8 greenhouse with a 3' triangle on the top would have a volume of 960 + 3x12x10/2 = 1140cf so a 1050cfm fan would have 1 air change per minute. I'd be guessing RH100% at 40C is about 4kPa or 30g/m^3 of water = 0.85g/cf. Assuming a starting humidity of 50% gives a cooling effect of about 0.9btu/cf moved.

Thus, if you could evaporate enough water, the 1050cfm fan would be equivalent to a 4 ton chiller (using units(1)):

You have: (15g/m^3) * (2.2MJ/kg) * (1050 foot^3/minute)
You want: tonne * 335kJ/kg day
* 4.21
However, that would require your evap cooler to evaporate 27kg (6.5gallons) of water per hour which is probably beyond a little commercial unit. You probably don't need 4.2 tons of cooling though, so just run the fan when the temperature gets too high (thermostat), and the cooler whenever the humidity drops below 90% (humidistat).

Apparently I didn't make a clear point. The fan is sized for the cubic volume of your greenhouse per minute. If you have a 10x12x10.5 then you need a fan of at least 1050 cfm. Voila! The smaller fan is "big enough". The point I was trying to make is that no fan is good enough by itself as a cooling solution under these conditions. The problem lies in the cooling medium which is hot air. We will let Nathan work out the math using the Second Law of Thermodynamics. You need multiple methods of cooling including reducing the energy intake, active cooling through misting or a swamp cooler, and air exchange. Also keep in mind the plants have an extremely wide "dynamic range" regarding temperature as long as they have enough humidity. Any form of ventilation reduces humidity so this requires a more intelligent control mechanism so that you don't vent while misting, as an example.

Hrm, intelligent control mechanism. Can you give an example? I was just going to wire it open up and turn on when it gets too hot with a standard 2-stage controller box (I think I used the right nomenclature there)

So maybe, with the evaporative cooler, it might be enough to have 1050 CFM?

Minimally, if the humidistat is calling for humidity, it has to disconnect power to the vent fan control so the vent fan won't run. This would require one relay, or if your humidistat has an SPDT contact set, you could wire the vent fan power to the Normally closed side of the humidistat. If it doesn't have this capacity or uses 24VAC for control or has any other complexity then the relay will be needed.

I don't see what that is necessary. Why not humidify when too dry, and vent when too hot? What does an exclusion achieve? Commercial house evap coolers vent and humidify at the same time.

(And I don't think we need to involve the second law of thermodynamics here, just basic arithmetic and the laws about specific and latent heat)

So, I need a thermostat for the fan(s) and a humidistat for the humidifier?
I must say that the 4.21 tonne of cooling calculation only helped in the sense that you calculated the rough volume of the GH, which I'd calc'd previously at 1100, glad for something to compare my numbers against. I guess I'll go for a 2000 cfm kinda fan and hope that'll do the job well. Maybe a 2-speed.

I was suggesting the other way around: humidistat for fan and thermostat for humidifier, but I can't convince myself that there's an advantage either way. It makes sense when you are cooling people, because people are sensitive to higher humidities, but plants don't really care (up to the VPD limit anyway, which you probably won't achieve).

I can't exactly work out what you want, because you haven't said exactly what you want. Do you have good climate data for your area?

Why is 1 air change per minute good? My old greenhouse was more like 0.1 air change per minute without any problems. Can someone work out the CO2 needs and thus compute the minimum air change rate?

We seem to be wandering aimlessly around in the dark, not knowing what we want, or how to achieve whatever that might be...

Why is 1 air change per minute good?

Because when Mr Sunshine is shining as hard as he can then 1 air change per minute gives you a greenhouse temp of just about what it is outdoors.

If you are activly COOLING the incomong air and you are blowing that out at one greenhouse volume per minute then make sure you are not venting colder air than the outside air is!

Perhaps you're right Nathan, I'm not sure how this works or exactly what I'm looking for. That was my sole purpose for joining here. I'm bright enough, if not very informed about actually setting up my greenhouse. I've gotten some real help and thanks again to the fellow who sent me the PDF on the 10x12 HFGH. It's been nice to get an overview before I rip open that steel cage, plan how I want to make the foundation.

What I do know is that Dallas, TX hovers around 40-50% rh and gets hellishly hot in the bright Texas sun. I know that I don't want my greenhouse to kill anything I put in it. I know that I want a higher rh environment to start seeds and plants, and a protected environment for veggies, and a rain-free environment for my cacti in the cooler months. And finally I know that I'd like something I can stand to walk around in during the summer.

chris: I just sketched the problem out, sunlight is W/m^2, so assuming a 2.4m high greenhouse, 1acm in full sun would stabilise at 15C warmer than outside, which seems rather high (I assumed 1kW/m^2 with 80% absorption of light, R1 glazing). This matches cottage's experience with not enough fan - 15C (30F) warmer than 40C (100F) is way too hot. Rather than acm we really want to measure area equivalent air changes per minute.

But I think you're right, it should be: vent with a thermostat and humidify with humidistat so as not to waste power and water.

gardenertx, I'm not picking, I'm just trying to work out how we can model the problem - we can either try to model the problem and work out what we need, or just buy things and hope. the advantage of modeling is that we might be able to save considerable money at the cost of a bit of thought. Multiplied by all the greenhouse owners this might have a significant impact.

Down under, most commercial greenhouse owners just leave one or both doors open during summer to keep temperatures down - a door open at each end will vent far more hot than the biggest fan you could afford/find. If you can make your greenhouse act like a chimney (with an electric actuated vent at the bottom, electric vent at the top) you might be able to avoid using a fan altogether.

It sounds like heating is not going to be a problem for you, thankfully!