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Fluid Dynamics.

DrPhoton

DrPhoton

New member
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.... ;)
 
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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
 
DW2 said:
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
Click to expand...

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.
 
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.
 
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.
 
DW2 said:
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.
Click to expand...

Yea i thought that.

DW2 said:
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 ).
Click to expand...
I thought being cubic feet per minute that 6000cfm would exchange air 3.68 times every minute ?.

DW2 said:
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.
Click to expand...

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.

DW2 said:
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.
Click to expand...

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.

DW2 said:
The fans set (as in the photo) can only circulate the air within the green house, not exchange it.
Click to expand...
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.
 
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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.
 
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.
 
Danofdanger, a wind tunnel is open on both ends, to create air flow. A fan in an enclosed area only creates a minimal amount of pressure at the discharge side of the fan by the compression of the air by the fan blades and a small negative pressure on the intake, or back side of the blade. In an enclosed area, with no exit for the air , the air will only circulate, moving from the higher pressure side to the lower pressure side. If you placed an air pressure gauge on the green house and started the fans, there would be no increase of air pressure.
The "push or pull" setup of the fans would be determined by the construction of your green house, if the plastic sheeting is on the outside of the structure then an exhaust, or pull, set up would be best. With an opening in one wall and an exhaust fan on an other, the pressure would be very small. The fans that you have are not the type to create a great amount of pressure. A large enough opening, or intake vent, would not allow any significant pressure (negative or positive) to build up in the house.
I'll be more than happy to help you out, if I can with ant more questions.
 
The greenhouse is open at each end. ;)
How do you determine the size of a passive vent ? For there to be negative pressure you need to allow enough resistance to airflow so that more air is leaving than is coming in, or more air coming in than is leaving for positive pressure. If the opening is too large then the static pressure collapses and the room cannot be pressurized, too small and the rooms internal resistance can make it too hard for the airflow system to move sufficient air. As far as i can tell theres no clear cut formula for calculating this due to the complexity of fluid dynamics and the enviroments. With hvac systems they appear to just use on site tools and estimate with simple experiments.

Also do you only need enough static pressure in a room to create airflow ? Is there any benefit to having a higher static resistance, would this allow air to shoot in at a faster velocity.?
 
Oh just to add there is a slight negative pressure at the intake as when the fans are turned on, the screen sucks in and the leaves closest to it wave. However when i had just a negative pressure system this did not happen, this could have been because it was too weak still or the leaks along the top created a different path for the airflow.
 
I did not realize that the green house was open on both ends (completely open?)! At 29 Ft. long, you would have to have a relatively high pressure blower to get movement of the air at that distance, but, that would create more problems than solutions.
Either put in more fans, or larger diameter fans, or you might try to increase the efficiency of the fans that you have by adding a short (approximately 2x the diameter of the fan) shroud or duct to the discharge side to allow the 'column' of air to travel a little farther before dispersing.
If I come up with any 'astounding revelations' , I'll let you know.
Best wishes in your grow!
 
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.
 
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.
 
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