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Why extract from the top?

extract fans heat airflow with farty bits

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#1 Budgie

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Posted 08 August 2016 - 04:05 PM

bear with me, I do like to question accepted convention sometimes :)

 

but, why do we extract from the top of the groom? Other than to use the thermodynamic assistance of rising heat?

 

Good fans these days will pull air out of any tents/grooms with ease. But, extracting from the top, doesn't that mean that is fighting against CO2 which is a heavy gas that sinks in air and is equally as important half the time as O2, perhaps more so in hot temps.

 

So, would it not be better to push the intake airflow down through the plants (and over the lights) so they get the most o2 and co2 possible, to the topside leaf stomata? 

 

Does it not actually make a blind bit of difference if it's extract from the top or extract from the bottom?

 

Think about winter time, the cold air is getting sucked up and cold air is hitting the plants. If reversed the lights will warm up the airflow slightly giving more stable temps with less need for oil rads/heating?

 

 

 

Go on.....what's your thoughts?


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#2 Anonymiss

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Posted 08 August 2016 - 04:21 PM

Rather than me regurgitate others' work, have a look at page 7 (93), Direction, of chapter six, Air Movement, in the Plant Growth Chamber Handbook :)


ETA: Here it is :)

DIRECTION

Air in controlled-environment rooms can flow from bottom to top, top to bottom (often called downward or reverse air flow), or horizontal. The chief argument for moving the air from bottom to top is that this method provides a laminar flow of air through the plant growing area. Unfortunately, the laminar flow is quickly disrupted once plants are placed in the chamber, and uniform air flow cannot be maintained unless the plant containers are spaced evenly over the growing area. Moreover, the air movement across the leaf will change considerably depending upon the number and size of the plant containers per unit area of the growing space. For example, a plant growth chamber with an area of 2.97 m² might have an air flow of 0.5 m/sec when empty. If the space were filled with 15-cm pots and about 112 could be used, the reduction in free area conceivably could result in a three-fold increase in air velocity as the air passed between the containers.

From the point of view of both engineering and plant scientists, downward air flow is preferable to upward air flow because temperature gradients are smaller (Morse, 1963). Dimock (1963) noted that the results of intensive study at Cornell University showed that the vertical temperature gradient with downward air flow was only half as great as with upward flow. Matsui et al. (1980) also concluded from their studies on humidity distribution that downward air flow gave the most consistent pattern of air movement. In general, downward movement of air will more closely mimic humidity and temperature profiles found under field conditions. Humidity levels should always increase with depth in the canopy (Allen, 1975). Morse and Evans (1962) reported that plant growth of tomato, lucerne, and subterranean clover was slightly greater when the air flow was downward (Fig. 5).

The argument for using horizontal air flow in a growth chamber is that canopy turbulence more closely resembles natural conditions than with an upward or downward air flow (Monteith, 1964; Doorenbos, 1972), although no data have been presented to verify this assumption. The obvious disadvantage of a horizontal air flow is that in the presence of the radiant lamp load, air temperature and humidity will rise as the air crosses the chamber. The greater the lamp load, the greater the horizontal gradient. Forrester (1979) claims that the plants create turbulence that reduces the gradient to a negligible level. Smeets (1978) claims that the temperature rise is not significant when the radiant heat is reduced by cold water running over the glass barrier that separates the lamps from the growing area. If the distance across a growth chamber is very great, however, this gradient in temperature and humidity may be quite large (Allen, L H. Jr., Gainsville, Florida, Personal communication, 1989). On the other hand, even though a horizontal gradient is present, rooms can be used effectively for research studies by assigning blocks (replicates) along the horizontal gradient. Allen indicates that if humidity is kept low and air temperatures are high enough, the temperature will drop because of evaporative cooling by the plants. However, a horizontal humidity gradient is still produced.
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#3 MDS

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Posted 08 August 2016 - 04:22 PM

Following your logic about CO2 trees shouldn't be tall, but should grow along the ground where you suggest most of the CO2 is likely to be.

 

When we go to bed we should be laying in a pool of CO2 & underground carparks would be extremely dangerous places to go :rr:

 

As I'm sure you're aware this patently isn't the case :)

 

Interesting ideas though...


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#4 Budgie

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Posted 08 August 2016 - 04:32 PM

Rather than me regurgitate others' work, have a look at page 7 (93), Direction, of chapter six, Air Movement, in the Plant Growth Chamber Handbook :)

I'll need to read the whole chapter, but, the first thing I spotted.....was stating that downward air flow is better than upward or horizontal, and that downward airflow grown plants showed significantly greater growth....;)


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#5 Dex

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Posted 08 August 2016 - 04:36 PM

bear with me, I do like to question accepted convention sometimes :)

but, why do we extract from the top of the groom? Other than to use the thermodynamic assistance of rising heat?

Good fans these days will pull air out of any tents/grooms with ease. But, extracting from the top, doesn't that mean that is fighting against CO2 which is a heavy gas that sinks in air and is equally as important half the time as O2, perhaps more so in hot temps.

So, would it not be better to push the intake airflow down through the plants (and over the lights) so they get the most o2 and co2 possible, to the topside leaf stomata?

Does it not actually make a blind bit of difference if it's extract from the top or extract from the bottom?

Think about winter time, the cold air is getting sucked up and cold air is hitting the plants. If reversed the lights will warm up the airflow slightly giving more stable temps with less need for oil rads/heating?



Go on.....what's your thoughts?

Really good thinking buddy coz I do that in cold times we north coast get a nice cold ars breeze and I actually reverse the air flow so my cold air comes in from the top of me tent :) and put me extractor at the bottom valve/hole works good..

And while the extractor is at the bottom of the tent at lights out I shut the valve that's pulling cold air and open the second valve at the bottom of me tent and that drafts room temperature air ;)...

It keeps the lots warm reversing and easier to keep the room temp at lights out manually like but that's how we does it lol..

like captain said cold roots small fruits lol
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#6 Budgie

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Posted 08 August 2016 - 04:38 PM

Following your logic about CO2 trees shouldn't be tall, but should grow along the ground where you suggest most of the CO2 is likely to be.

 

When we go to bed we should be laying in a pool of CO2 & underground carparks would be extremely dangerous places to go :rr:

 

As I'm sure you're aware this patently isn't the case :)

 

Interesting ideas though...

but those trees don't grow in contained environments like a tent or groom?

 

And, our houses are leaky, drafty places so it cancels that build up of CO2. If you ever worked in closed environments there is a need to supply fresh air otherwise you get headaches and such like after a few hours......

 

 

Which funnily enough, could be one of the problems relating to bad-grow-room syndrome? Their are cases when no matter what peeps try to do their plants just fuck up....could this be why, or one of the reasons why I wonder? 


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#7 Anonymiss

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Posted 08 August 2016 - 04:39 PM

It's worth reading the whole book, @Budgie :)

The little 'wind speed' graph on that same page is interesting and suggests that any internal circulation (oscillating fans) should ideally produce an air speed of 0.2 m/s to 0.5 m/s for optimum growth.
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#8 weedtroll

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Posted 08 August 2016 - 05:06 PM

Space.

 

I havent room on the deck for filters and fans as they pretty much take up half the tent space along the L and W,

And I like that stuff to be contained within the tent.

 

I would be surprised if there was much difference from drawing air up through or down from above the plants, but you never know.


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#9 Budgie

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Posted 08 August 2016 - 05:13 PM

...

I would be surprised if there was much difference from drawing air up through or down from above the plants, but you never know.

And ^^that^^ is probably the best answer :)


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#10 Mrs Willy

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Posted 08 August 2016 - 05:16 PM

By default, i extract from under a light. Ive been known to hang ducting to a reflector, just near the socket and suck air out .. into a CF outside the tent.

Its my preferred way of doing it. So it is the top really, but not of a growroom. Its above the plants where the heat is most concentrated.

Heat, smells etc.. out almost straight away.

 

E2A - and the CF has been on the floor as well as on top of tent


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