Fluid Dynamics.

[DrPhoton]

Need someone who knows about fluid dynamics. I have a greenhouse which i have installed a airflow system. I originally was going to apply a negative pressure system but i was unable to pressurize the room because of too much leakage. So i decided to move air with haf fans in series. This is the best solution i could find. Running them in series throughout the room relys on the velocity and throw distance to move and cycle the air. Now im pretty close to my target, another 5 degrees and i will be happy. Because of the throw dispersion, having a completely open exhaust was best as theres no pressure to direct airflow. Same with the intake.

Now theres only a couple things i can think of that could help improve airflow. Because theres no static pressure difference, not all the air gets picked up except for the air closest to the HAF fans. Also fresh air is not pulled in as well due to very little pressure difference. If i somehow increase static pressure by closing up air leaks, i could create more airflow.

Or, i add more fans in parallel to increase max air flow. Adding in parallel only increases pressure which doesnt help me. But id be using more power than id like.

Anyone got any ideas how i could optimize what i have now ? Maybe by fan placement, distance, inlet/exhaust size. Etc. I feel i have almost reached the optimal effeciency but perhaps theres something iv missed.

Details on airflow system.
29x7.5x7.5 greenhouse (1630 cubic feet volume)
3x 2000cfm HAF fans (5 feet apart)
Thermostat, humidistat controlled.
50°F/10°C temperature difference to outside. (Without airflow system temperature difference is 86°F/30°C)

Im so close....

[DrPhoton]

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[DW2]

Danofdanger, I'm not sure that I understand what it is that you want to do with the air in your greenhouse. Do you wish to exchange the air inside with the air outside? I worked in the cotton ginning and industrial air filtration for about 14 years. If want to remove the air inside and replace it with outside air then either set your fans to exhaust the air out and provide an air intake opposite of the exhaust fans, or it can be reversed where the fans pull the outside air in and you provide air exhaust on the opposite side.
If you wish to circulate the air only, then mount your fans on opposite sides facing opposite each other to get a circular rotation.
Of course, the direction of rotation, is dependent on whether you are in the northern or southern hemisphere! LOL!!! Just a funnin' ya!
I would like to have an area that spacious to grow in,
Good luck

[South Sierra Dude]

Impressive. I don't see many set up like that around here. Seems advanced.

[DrPhoton]

Sparky mate, never can be too clever
I live in a colder part of south australia so temps get a little hot still. But still gets cold as well, so greenhouse it is.

[OzBud]

That's a pretty sweet greenhouse.

[DrPhoton]

Danofdanger, I'm not sure that I understand what it is that you want to do with the air in your greenhouse. Do you wish to exchange the air inside with the air outside? I worked in the cotton ginning and industrial air filtration for about 14 years. If want to remove the air inside and replace it with outside air then either set your fans to exhaust the air out and provide an air intake opposite of the exhaust fans, or it can be reversed where the fans pull the outside air in and you provide air exhaust on the opposite side.
If you wish to circulate the air only, then mount your fans on opposite sides facing opposite each other to get a circular rotation.
Of course, the direction of rotation, is dependent on whether you are in the northern or southern hemisphere! LOL!!! Just a funnin' ya!
I would like to have an area that spacious to grow in,
Good luck

I am wanting to exchange inside air with outside air. Like i said i have reduced the temperature difference by quite a margin from 86°F to 50°F. I just need another 10°F. I cannot pressurize the greenhouse so negative or positive pressure has very little effect as the air will not flow and be limited by the velocity and throw distance. This is why i have opted for a wind tunnel design. By having 3 fans a certain distance apart, air can be kicked along.
What i am trying to figure out is how to improve this type of airflow system further to gain that special extra 10°F i need. Wether it comes from a added fan someplace, vent sizing, fan placement etc. This is where understanding fluid dynamics probably helps.

[DW2]

Danofdanger,
The only way to get to that extra difference would be to increase the CFM capacity of the fans, the more CFMs moving thru the green house, the greater the heat exchange. You have enough CFMs with the three fans (6000 CFM) to completely change out the air every 16.3 minuets (6000 CFM / 1630 CF = 3.680 times per hour ). The only way to get the air in the green house to exchange with the outside air, is to create a pressure differential between the green house and the atmospheric pressure outside. Mount your fans in the wall on one end blowing into the green house. Make the exhaust vents (preferably screened) on the far end wall, to get the air flow moving thru and around your plants. The fans set (as in the photo) can only circulate the air within the green house, not exchange it.

[DrPhoton]

Danofdanger,
The only way to get to that extra difference would be to increase the CFM capacity of the fans, the more CFMs moving thru the green house, the greater the heat exchange.

Yea i thought that.

You have enough CFMs with the three fans (6000 CFM) to completely change out the air every 16.3 minuets (6000 CFM / 1630 CF = 3.680 times per hour ).

I thought being cubic feet per minute that 6000cfm would exchange air 3.68 times every minute ?.

The only way to get the air in the green house to exchange with the outside air, is to create a pressure differential between the green house and the atmospheric pressure outside.

Yes this is the problem i have, due to air leakages i am having to fight against. I cannot pressurize the greenhouse as air escapes or enters, there is a gap running along the top corner where air is pulled in preventing negative pressure. Without sufficient static pressure its like trying to pressurize open air outside.

Mount your fans in the wall on one end blowing into the green house. Make the exhaust vents (preferably screened) on the far end wall, to get the air flow moving thru and around your plants.

Im curious why you suggest positive pressure. With my basic understanding of fluid dynamics, negative pressure creates a more even internal flow as its negative pressure is pulling air through at a more even rate. Positive pressure has more turbulent flow due to its velocity and dispersion characteristics inside.
Link
This creates hot spots which is not ideal for grow rooms.

The fans set (as in the photo) can only circulate the air within the green house, not exchange it.

Im not sure what you exactly mean by this but fans inline inside a enclosure can move and exchange air, this works just like a inline fan for ducting. Just because its inside the enclosure does not mean it doesnt. It just wouldnt create a pressurized room as air is being pulled in and out evenly. This is why they are in series as to keep air moving, another fan needs to help kick along the previous fans flow as the dispersion and throw cannot complete the entire length. Creating a sort of wind tunnel effect.
Also because it doesnt cover the entire diameter of the flow direction, it doesnt move all the air the same. There is slower air movement towards the outer walls. Then having them in series means they cannot move more than 2000cfm, they do not add together. Only pressure difference is increased. Only in parallel does airflow add together.

Without the current setup on, temperature differences is around 90°F with internal temperatures reaching over 125°F on sunny days. With the airflow system on, temperature difference is down to about 50°F with internal temperatures reaching as high as 98°F. Big difference!.
I just need another 10°F roughly for a peak temperature of 89°F.

You have made some good points, i may try putting all fans in parallel on the exhaust or intake but i fear the low static pressure difference will prevent a proper airflow.
Appreciate your input, if you have any more to add please do. This all helps.

[DW2]

Danofdanger,
The shroud, like duct work, can prevent slippage at the tips of the fan blades. Much like the turbulence on the end of an airplane wing, the higher pressure air at the front of the blade tries to 'slip' around the tip of the blade to the lower pressure back side of the blade creating turbulence, friction, and reducing the efficiency of the fan. The closer the fit (smaller the gap) between the blade and shroud or duct, the better the flow. The shroud will tend to keep the stream of air going in the same direction (like a nozzle on a hose).
A higher rated 'static' pressure fan creates the same pressure on the intake (negative pressure) as it creates on the discharge side (positive pressure). Air flow alone does not necessarily create pressure, although, flow does create velocity pressure. A tire has pressure, but does not have any flow.
Keep on a truckin'

[DrPhoton]

Yep i'm with ya.
Well i was able to build negative pressure today after applying a cover around the exhausts so theres no leakage. Soon as i turn the fans on i seen the black polyethelene sheets suck in and the toy windmill start to spin. But i can also see air bleeding through the long gap accross the top. Being 6000cfm it may be enough to bring negative pressure from the leaks and from my passive intake. It really all depends how well it mixes and drags. It might end up being i cover up my passive intake and let the top running gap as the intake. Its all trial from here. Il let you know after today how the temps are.
Peace out.

[DW2]

It sounds like You are getting a handle on it. Just don't change too many things at once and, I have found that keeping good records of changes and results, help me out with deciding what to do to obtain the results that I wish.
I envy You, having all of that space, I'm growing in a space 71 inches wide by 22 inches deep, and 84 inches tall (a closet!!), but every situation has it's own problems. Like Roseanne Rosanna Danna used to say on Saturday Night Live (in the 70's) "It' all ways something".

[StickyFingers]

Danofdanger...Swamp cooler on one end for incoming air, big fan on other end to exhaust. Guaranteed to get you your extra 10 degrees plus. I managed a retail nursery once upon a time. We had a greenhouse that was probably 20' x 60'. It was cooled by one big swamp cooler, nothing else needed. It routinely gets into the low 100s here in the summer and the greenhouse probably stayed between 70-75 F all day. We'd go in there to cool off. You'd have to watch the humidity. If it gets too humid, shut off the water to the swamp cooler and just run it as a fan. A window air conditioner would also do the trick and solve the humidity problem but might cost more to run than you want to spend. Shade cloth comes in varying densities. If you were to put some on your roof it would also cut the heat way down. You would lose more or less light depending on your shade cloth. Nice greenhouse, everything in there looks very healthy. Good job.

[DW2]

This is a very good idea StickyFingers. the cooler can lower the ambient temperature by up to 40 deg. F (depending on the relative humidity). The cooler would be a good choice.

[DrPhoton]

Yes this is a great idea, il do some research and get back to you..

[DrPhoton]

What device is used to measure static pressure ?

[DW2]

Dwyer Instruments make gauges that measure both 'static' and velocity pressure. They are called 'magnahelic' gauges (may be miss spelled!). They come in ranges of 0.0 - 1.0 inches of water to 30 plus inches and are pricey.

[DrPhoton]

Yea they are expensive. I think id rather buy a barometer and just manually calculate the difference.

[9fingerleafs]

What about misting the fans?

[9fingerleafs]

no worries man, jorge is also the authority for me on this subject, i bought the new book, canabis encyclopedia and it blew my mind you gona have a great time watching him explain everything, also check my grandaddy threat i just updated the second harvest, it was all CFL and a lot of training

[StickyFingers]

9fingerleafs...I went to an outdoor summertime concert once that was put on by a guy whose father was a rich walnut grower. He talked his dad into fronting him the money for the concert. He rented the Madera, CA 1/4 mile dirt track Speedway, hired Maria Muldaur, New Riders, Dan Hicks and several other bands to play on what turned out to be a very hot day. His dad had 2 giant 8 or 10 feet diameter fans mounted on 30 feet tall hydraulic rams, mounted on trailers. Guess he used them in his orchard if frost threatened. Anyway, they made up a pipe that bolted onto the top of the fans, capped one end, drilled holes, and attached the other end to a fire hose which was then attached to a nearby fire hydrant. Those things squirted water at least a couple of hundred feet and saved the day. Without them several hundred hippies would have died from heatstroke that day. One of the most fun concerts I ever attended.

[9fingerleafs]

Haha I'm sure it was a big help. In hot weather I've seen handheld fans with a little water mister right in the center so when you press the button it squirts. On a hot day under the sun the water is evaporated very quickly by the wind. Taking with it the heat from whatever surface it landed. After long periods of time using the fan your face is freezing cold no matter how hot it is

[DrPhoton]

Well i placed the fans at the back venting out, i still have to cover the surrounding space so air isnt pulled from around the fans from outside. But with it like this, there doesnt appear to be any negative pressure at all. I bought a windmill toy and placed it at the intake. Im using this as a rough indication of intake airflow. Obviously its only useful with no wind factor but i have a treeline which windbreaks. With the wind tunnel design the windmill spins 80x a minute, with the 3 fans as a negative pressure theres no spin. I think this is because of what i have been complaining about with air leakages. The running gap across the top measures 0.32x29 feet which is 9 squared feet opening. All rough recommendations for the cfm i use suggest no bigger than 2.5 to 3 squared feet opening. It is highly likely this leakage is going to prevent pressurization unless i seal it. What makes it worse is the prevailing wind runs across the roof and runs down this gap. This makes negative pressure harder to aquire. I may try positive pressure if it fails tomorrow.

[DrPhoton]

Well it was a long one. Its safe to say i was in or at the greenhouse for most of the sun out day. Will be expecting sun burn.

Anyways, i preceded to cover up the exhaust which ended up pretty good (picture supplied). I was still noticing drafts coming from the gap accross the top so i decided to block it up with some insulating batts. This done a good job and i could tell after this that i had a proper negative pressure.
Heres a video of me testing the negative pressure effect.


I went about testing passive intake sizes. I started really big, more than the recommended and calculated. It wasnt until i narrowed it down to about 10 squared feet that i noticed airflow and the windmill spin. This is about the calculated amount when you devide the cfm by 600. I settled on 5 sq feet as this had a more desired air velocity. This video demonstrates how important vent sizing is for pressurization.


The result however was less pleasing. After retrieving the numbers and calculating the difference, i ended up no better if not worse than 50°F. It just means i was getting good airflow with the tunnel design and i think its a tad better. It also has good circulation and mixing which is a added bonus.
I clearly just need more airflow. Even with the combined 6000cfm it wasnt enough. However these haf fans are not built for pressurized enviroments so that probably doesnt help. I even tried adding a 500cfm mixed flow fan to see if it would help with the stress. But it was negligable.

What i think i will do is add another fan in parallel with the last fan near the exhaust. This will increase negative pressure as it throws more air out. I think if i put it at the front by the intake this would barely do anything as the air cannot travel that far. I would need two more to carry the energy all the way through. At least this way i can keep energy costs down and also have a higher negative pressure. This combination may be the ticket.

[DW2]

Danofdanger, It looks like your going in the right direction. The addition of a evaporative cooler might be a way to get the results you wish. First, the type of fan that they use (squirrel cage, or, technically, a centrifugal blower) is capable of producing higher pressures than a vane axel flow (fan type) blower, and, with the addition of the evaporative cooling, could lower your temperatures it better levels. I do not know what the weather is like in your location, but you may need to not use the evaporative cooling during the bloom to keep from having mold problems.

[DrPhoton]

Yea i wouldnt have to use it during flowering. I am just trying to figure out how much power they use, in my location i only have available to me a portable 10L evap cooler. Its relatively cheap but it all depends how much power it uses. I just discovered too that lean to greenhouses are particularly difficult to cool as they generate more heat because of the extra surface area. You can pretty much consider them to be twice the size of a regular free standing greenhouse. So its no wonder this is requiring so much effort. It just tests my skills is all .
So my options so far are evap cooler or two new fans. Or perhaps both.
Thanks for your help so far, iv learnt a lot.

[DW2]

The wattage of the motor on the cooler can tell you how much power it will consume. Multiply the wattage of the motor times the estimated running time, divide this by 1000 to get the Kw (Kilo watts), then multiply by your Kw/hr. cost. I the motor only lists the amperage and voltage you can get the wattage by multiplying the volts times the amperes.
May be You could tack up some plastic sheeting up on the 'solid' wall to reflect the sun light back towards the plants and to also provide an air gap thermal barrier to help cut down on the radiant heat from the wall? It might help and it does not get much cheaper than plastic sheeting, and NO batteries required!!! LOL !

[DrPhoton]

Me a sparky but na theres no power specifications on the website or any information online. I dont think a reflector would work unless it was angled out so the enegy is not placed inside. I dont have room for anything else either unfortunately. All good ideas though.

[StickyFingers]

Hi Danofdanger and DW2...swamp coolers are very inexpensive to run, Only a little pump and the fan. The fan can be run without the pump being on. Don't think it would cost any more to run than one of your big fans, maybe less. Much less than an air conditioner. I don't think a little portable cooler would get the job done Danofdanger I know you're not here in the US, but you need something like this even if you have to go to a larger town to find one:

[DrPhoton]

Thats really good to know, thanks sticky fingers. I will be looking into that if my last attempt doesnt work as i do not think two extra fans will cut it still. You really have to double your airflow to reduce temperatures by half.
However i have come up with using my 500cfm mixed flow fan as a intake seen as it does nothing on as exhaust. I figure because i run a wind tunnel which has very little to no negative pressure that using the fan (which shouldnt be done if your trying to pressurize as it destabalizes it) as a intake might do something. It has excellent throw as well.
Well see.

[DrPhoton]

Made some good progress today, the mixed flow intake worked well. Before, the best temperature difference compared to my outdoor thermometer was 48°F. Now im getting 37°F. Not bad. Its a pretty good summers day out and it only gets a little hotter throughout the rest of the summer but the max temp iv been getting is around 95°F. This is acceptable for me, 90 max would be ideal but i feel comfortable enough with this. Maybe with some tweaking i could make a little headroom who knows.

To conclude i feel the intake has helped because theres not enough pressure to bring in new air, so bringing it in does its job for it. I also think the HAF fans must have been bogging down under the static pressure when used as negative pressure. Although i had pressurized the room, id say the airflow rate had decreased so much that it was struggling to cycle fast enough. If i had a 2000cfm high torque exhaust fan it would probably do well and perform close to what i jave now.
I prefer my setup however because i am using less power (250 watts). It doubles as circulation fans and mixes the air well.

The lesson of the day is, if you cant pressurize a room, use a balanced pressure system. Plain and simple.

Thanks dw2, sticky fingers, fingerleaf. Been a great trip. Learnt a lot.