king_of_nothing
- 19
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As some of you may know, I am in the middle of engineering my first garden (RDWC style). Still in the design phase. I want/have to start setting it up in December and hopefully have it running in the same month. Yesterday I had dealt with some stressing questions surrounding nutritional needs in RDWC. Now its time for discussing how to get a nice breeze into the system, also known as HVAC to English speakers.
I was reading an Interesting thread (here). It got me reading into fluid mechanics which - even though having a math, physics and engineering background - still makes me feel uncomfortable. Forgive me if this is going to be a bit elaborate and perhaps basic. But writing like this helps me to clear my mind on the matter and it perhaps makes for easy reading and clearer answers for other beginning growers struggling with the same issues.
The previously mentioned thread so far has been an interesting one to get me further on having a grip on all the important ideas that play a role in refreshing the air volume. It brings about a couple of questions. Some of those are about certain standard practices and heuristics. Others are a bit more fundamental in nature. Here they come.
1. So I have read that the rate of a typical Change of Room(Tent) Volume should be every 2 to 5 minutes. However if you want passive cooling you might even go as frequent as 3 times a minute. However my system is LED based. Other appliances (so far) in my RDWC set up are a water chiller, an air pump and a water pump. So I hope that it is safe to assume that passive cooling won't be much of an issue. Would anybody object to my assumption to stick to the 2 to 5 minutes regime?
2. In deciding on the exact number in that every 2-to-5 minute range, do I understand correctly that more frequent is better for my CO2 input and H2O & heat output. But it makes a more noisy and power consuming system. Am I missing any other aspects in this trade off?
3. Still on the topic of the 2-5 minute rate, I am unsure of its effect on the smell which is an issue for me. Is a fast change-of-air-volume rate better or worse for the aggregated working of a carbon filter?
4. I like tangible concepts. The CFM rate is one of those. So dimensioning the proper ventilator (the larger the better for noise and durability) naively is just a matter of getting its max CFM and then dial down the vent rotation speed with a controller to your actual need. So the simplest system is to have ventilated outlet with a passive inlet. However my tent volume might become pretty big. So say I want to have more outlets and/or ventilated inlets. Is the first step in getting a combined CFM just a matter summing up all individual inlet and outlet CFM's? I am guessing the answer is probably yes. So directly continuing to my real fourth question: if you have your platonic combined CFM. How do you factor in loses due to ducts and filters and what not?? I have read that, for instance a carbon filter slows down the CFM of its connected up to 60%. So the platonic model is not enough. Can somebody point me to an overview of all the relevant heuristics to adjust to a realistic CFM requirement??
5. So the answer or thoughts to question 4 would partially help me to decide on how to scale in vent requirements. But I am also a bit confused on how to decide between the ratio in capacity of inlet to outlet ventilators. This is maybe where one of the more uncomfortable concepts comes into play: (static) pressure. Specifically when it comes to negative vs positive pressure. Ok, I think I understand that you can control the pressure in your tent/room by changing the ratio of your inlet CFM vs your outlet CFM. Higher inlet vs lower outlet means a pressure build up, i.e. positive pressure. Or you could go the other way around and get negative pressure. My question is basically what are the pros & cons of having either negative or positive static pressure in your system. Just so you guys know that I did my homework: I think it fundamentally depends on how you model the shell of your system in terms of air movement isolation. If you treat your shell like a hospital has its hospital rooms set up, a room can only suck in air through a pre-filtered canal. In that case negative pressure is a good thing, because it sucks in your air only through your allowed inlets which - if they are filtered - is nice clean air with next to zero pathogens. On the other end if you assume your tent is not really airtight and you have a filter on your inlet's together with negative pressure, the negative pressure is going to suck in air including nasty pathogens through the shell of your room/tent. This would be completely counter productive to the workings of the inlet filter. So when using filters to keep out pathogens, having a positive pressure is a must. On the other hand, if you assume your ambient air supply is clean enough, a negative pressure is good for keeping in the smell in place which might otherwise be pressed out through the cracks in the shell. A very strong negative pressure is probably undesirable in a tent as it can suck in the tent walls leaving less room for plants to grow and the grower to work in comfortably. So to actually turn out with a question: are there other reasons to go or not go for negative or positive pressure??
6. One heuristic that I read somewhere on the previously mentioned thread, is that the inlet 'hole' should be 4 or 5 times the surface of the outlet 'hole'. If this a recognized rule of thumb in the community, can somebody explain the rationale behind it? I might imagine it has something to do with active-outlet/passive inlet systems. E.g. it helps to obtain a negative but almost negligible pressure: you don't want the hole too big because you want your tent to be modeled as a physically separated container from the rest of its environment. On the other end, if the inlet hole becomes to small, at some point your tent behaves like a 'closed' box with the maximum negative pressure your outlet vent can maintain which leads to infinitesimal CFM.
7. This one is more practical: I saw some picture of carbon filter-vent combi's set up inside the growing tent. Why is that? Aren't you guys then unncessarily increasing the volume of your tent with the volume of the filter-vent installation? What am I missing?
8. Any other general thoughts for me to take into consideration on this topic that I have not scratched myself?
Thanks people!
(PS I am going to bed now. So I might not be there to respond to immediate reactions. But they are highly appreciated nonetheless.)
I was reading an Interesting thread (here). It got me reading into fluid mechanics which - even though having a math, physics and engineering background - still makes me feel uncomfortable. Forgive me if this is going to be a bit elaborate and perhaps basic. But writing like this helps me to clear my mind on the matter and it perhaps makes for easy reading and clearer answers for other beginning growers struggling with the same issues.
The previously mentioned thread so far has been an interesting one to get me further on having a grip on all the important ideas that play a role in refreshing the air volume. It brings about a couple of questions. Some of those are about certain standard practices and heuristics. Others are a bit more fundamental in nature. Here they come.
1. So I have read that the rate of a typical Change of Room(Tent) Volume should be every 2 to 5 minutes. However if you want passive cooling you might even go as frequent as 3 times a minute. However my system is LED based. Other appliances (so far) in my RDWC set up are a water chiller, an air pump and a water pump. So I hope that it is safe to assume that passive cooling won't be much of an issue. Would anybody object to my assumption to stick to the 2 to 5 minutes regime?
2. In deciding on the exact number in that every 2-to-5 minute range, do I understand correctly that more frequent is better for my CO2 input and H2O & heat output. But it makes a more noisy and power consuming system. Am I missing any other aspects in this trade off?
3. Still on the topic of the 2-5 minute rate, I am unsure of its effect on the smell which is an issue for me. Is a fast change-of-air-volume rate better or worse for the aggregated working of a carbon filter?
4. I like tangible concepts. The CFM rate is one of those. So dimensioning the proper ventilator (the larger the better for noise and durability) naively is just a matter of getting its max CFM and then dial down the vent rotation speed with a controller to your actual need. So the simplest system is to have ventilated outlet with a passive inlet. However my tent volume might become pretty big. So say I want to have more outlets and/or ventilated inlets. Is the first step in getting a combined CFM just a matter summing up all individual inlet and outlet CFM's? I am guessing the answer is probably yes. So directly continuing to my real fourth question: if you have your platonic combined CFM. How do you factor in loses due to ducts and filters and what not?? I have read that, for instance a carbon filter slows down the CFM of its connected up to 60%. So the platonic model is not enough. Can somebody point me to an overview of all the relevant heuristics to adjust to a realistic CFM requirement??
5. So the answer or thoughts to question 4 would partially help me to decide on how to scale in vent requirements. But I am also a bit confused on how to decide between the ratio in capacity of inlet to outlet ventilators. This is maybe where one of the more uncomfortable concepts comes into play: (static) pressure. Specifically when it comes to negative vs positive pressure. Ok, I think I understand that you can control the pressure in your tent/room by changing the ratio of your inlet CFM vs your outlet CFM. Higher inlet vs lower outlet means a pressure build up, i.e. positive pressure. Or you could go the other way around and get negative pressure. My question is basically what are the pros & cons of having either negative or positive static pressure in your system. Just so you guys know that I did my homework: I think it fundamentally depends on how you model the shell of your system in terms of air movement isolation. If you treat your shell like a hospital has its hospital rooms set up, a room can only suck in air through a pre-filtered canal. In that case negative pressure is a good thing, because it sucks in your air only through your allowed inlets which - if they are filtered - is nice clean air with next to zero pathogens. On the other end if you assume your tent is not really airtight and you have a filter on your inlet's together with negative pressure, the negative pressure is going to suck in air including nasty pathogens through the shell of your room/tent. This would be completely counter productive to the workings of the inlet filter. So when using filters to keep out pathogens, having a positive pressure is a must. On the other hand, if you assume your ambient air supply is clean enough, a negative pressure is good for keeping in the smell in place which might otherwise be pressed out through the cracks in the shell. A very strong negative pressure is probably undesirable in a tent as it can suck in the tent walls leaving less room for plants to grow and the grower to work in comfortably. So to actually turn out with a question: are there other reasons to go or not go for negative or positive pressure??
6. One heuristic that I read somewhere on the previously mentioned thread, is that the inlet 'hole' should be 4 or 5 times the surface of the outlet 'hole'. If this a recognized rule of thumb in the community, can somebody explain the rationale behind it? I might imagine it has something to do with active-outlet/passive inlet systems. E.g. it helps to obtain a negative but almost negligible pressure: you don't want the hole too big because you want your tent to be modeled as a physically separated container from the rest of its environment. On the other end, if the inlet hole becomes to small, at some point your tent behaves like a 'closed' box with the maximum negative pressure your outlet vent can maintain which leads to infinitesimal CFM.
7. This one is more practical: I saw some picture of carbon filter-vent combi's set up inside the growing tent. Why is that? Aren't you guys then unncessarily increasing the volume of your tent with the volume of the filter-vent installation? What am I missing?
8. Any other general thoughts for me to take into consideration on this topic that I have not scratched myself?
Thanks people!
(PS I am going to bed now. So I might not be there to respond to immediate reactions. But they are highly appreciated nonetheless.)
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