What's an extra minute?

I am going to outfit the greenhouse with ventilation this year. According to the AFC website calculator I need to move 7000 CFM. That will exchange the air in the greenhouse every minute. My studs are 22 1/2" apart. A single fan and 2 vents sized to move that much air are wider than that and would require reconstructing supports. If I go with units that are 20" wide I can get two fans, but that would require 4 vents and extra expense.
I'd rather do one fan and two vents. A 20" fan will move 4300 CFM and would require two 20" vents. This would exchange the air once every two minutes. Is the extra minute really going to have that much effect?

Thanks,
JB

how long/wide is your grhouse? Usually the width of the fan equals twice the width of one shutter. In two of the grhouses I have 2 (20" wide ) shutters and the fan is 40" across. In the larger grhouse I have a 48" fan and 2 shutters that are 24" wide.

You are saying that you are using a 20" fan and 2 shutters that are 20" across?

The greenhouse is 45 feet long. According to the ACF calculator, http://www.littlegreenhouse.com/fan-calc.shtml, I need two 20" vents for their 20" 4300cfm fan.

This message was edited Feb 20, 2007 10:03 AM

Juniorballoon-I don't think a 20" fan is going to cut it with a grhouse that long-I think you need one bigger. When the temps outside are in the 80's and higher, that size fan isn't going to be able to bring the air through enough. I think the 20" shutters are ok-but I would get a larger fan-of which this company doesn't carry. You can use an agricultural fan-it doesn't have to come from a grhouse supply company. I got mine from Graingers http://www.grainger.com/Grainger/wwg/start.shtml . You are going to have to brace the fan at that gable end anyway, so you will have to make adjustments in your frame-assuming that you get a bigger fan- besides, it is better to have more than less than what you think you will need in this case-especially if you plan on having plants in there when the weather gets to the high 80's outside.

Also, I noticed the diagram on the grhouse at your website has the shutters down low and I disagree with that. I have mine up higher-where the hot air is, and where there won't be any obstacles in front of the air flow. Down low is going to be where the benches are, or plants, and you don't want cold air ( in the winter) blowing on the plants or below the benches.

Interesting post, tigerlilly!! I mounted my intake vanes down low (on top of the knee wall, bottom of the glazing, rear of the GH), on the theory that the coldest outside air would be low to the ground---it's also the shady end of the structure. Mounted the exhaust fan at the highest point possible-opposite (sunny)end, assuming the air up high would be the hottest. My assumption was that I'd be removing the hottest air and replacing it with coolest available. Your scheme seems to suggest that I would be better off just keeping the hot air moving out as fast as possible up high, thus letting the lower levels achieve ambient outside temp. Never even thought about that! But if that is to work, why wouldn't my opening all four roof vents plus the exhaust fan going full-bore work to cool the GH? It didn't; in fact it was the least efficient at cooling... I think I'll try disengaging the lower vanes (leaving them shut) this summer, but opening the roof vents all the way, and watch the min/max. Ain't learning fun???

Stono, I have noticed that if the plastic stapled at the base was loose anywhere, that it affected the pull of the fan, and the shutters would not open all the way (same with the door being open)-I suspect that when your roof vents are open-that it is affecting the pull of the air through the grhouse the same negative manner. I could be wrong here-but I think that a grhouse -esp a larger/longer one is designed to have just the fan and the shutters-not the roof vents as well.

If I understood your other post about your grhouse setup, you have 2 roof vents, 4 24" shutters and a fan that is 24'. I think you need less of the shutters and a larger fan ( although I think a 24" fan is ok for 28' long grhouse. In other words, you are too heavy on bringing in the new air and not enough to remove it-usually 2 shutters is plenty. I would try an experiment this spring and close 2 of the shutters and see if the fan can pull the air through faster-

As to the position of the shutters-regardless of the temps outside ( and I think there is very little difference in 3'-which is really only what it would be if I moved the shutters down-in the spring/early summer)-don't you have plants and benches in front of the shutters? Or do you have to lose that space because of the air coming in? It is fun to try to figure this out! And where in the world do you put 4 shutters on a grhouse that is only 28' long? I only have room for one on either side of the door. I also have the heater in one of the corners there also, and it is vented.

JB, that minute makes a difference...7F to be exact. Here are the stats:

Exchange all the air every 240 seconds (1/4 the volume/min) = GH temp 25F above outside
Exchange all the air every 120 seconds (1/2 the volume/min) = GH temp 15F above outside
Exchange all the air every 60 seconds (1 volume/min) = GH temp 8F above outside
Exchange all the air every 30 seconds (2 volume/min) = GH temp 4F above outside

Source: Growers Handbook for Greenhouse Vegetable Growers, 2007 ©

Mine is sized at one volume per minute. I work hard to get a 4F drop in temp in the summer in other ways (misters, shade cloth, fogger, etc) and if I had known that doubling the fan capacity would get another 4F I probably would have done it.

SB

Yeah, tigerlilly, a larger fan would probably help. But I'm limited by the design of the greenhouse, which uses 24" polycarbonate panels as glazing, so that constricts me to to a 22" fan blade to allow for the structural supports(and that's cutting it mighty close!). But you've made me think (a wonder in itself!)...I COULD put a second exhaust fan on the same front sunny wall (opposite side), and double my air flow. Why didn't I think of that??? Thank you, ma'm! Duh!

Stono-I didn't explain myself correctly! What I think is that the size of your fan is fine for the length of your grhouse-I think the problem is that you have too many places where the air is coming into the grhouse. I would close the roof vent and two of the shutters, and then see how it works. I think you will exchange the air faster this way.

Thanks stressbaby. That is exactly the info I needed. I am going with a fan large enough to move 6125 CFM.

Stono and Tiger, Size of the fan in inches is not the key. Fans are rated to move a certain CFM. You want one that will accomplish the goals you need to keep your greenhouse from becoming a killing zone. That will differ depending on what you grow and where you are in the world.

I have decided to go with a fan that will move 6125 CFM. That will exchange the air once a minute.

Thansk all,
JB

JB, as far as I have always known it-as to how fast a fan can move air thru a grhouse -is the size and maybe amt of fanblades and the size of the motor-what HP it is-is there something else that I am missing? A smaller fan with a 1/4 HP motor is not going to be able to move the air as fast as a larger fan with a larger HP- if I am missing something-I really would like to hear it.I am thinking about building a fourth grhouse. and this info would be revelant

I don't think you're missing anything. Your suppositions are correct. Bigger motors and more fan blades will move more air. The calculation you need to make to determine the correct size of fan and intake shutters is based on the volume of air inside the greenhouse and how often you want to exchange it to achieve a certain amount of cooling. Once you know that number you just match that up with what the fan is rated for.

Good luck,
JB