heat from lights, need assitance

but guys is what I am doing wrong or ok? I don't have a big heat build up in the reducers or duct for exhaust. closet is at a consistant 81-84 and down to 68 at night time (intake fan is off during darkness). If I use the temperd glass on the hood, room temp drops to 75 to 79. Remeber venting directly from the hood. If I do use the temperd glass, the exhaust duct and reducers get pretty hot. Obvously not enough cool air coming from the vents on the hood.

Plants have been reacting very well to the new conditions. My humidity is still in the low 20's, but the leafs are being sprayed with water. Still in veg cycle so no worries about getting buds wet.

Am I off track or is this ok?

FilthyFletch said:

forget the math reducing the induction size does hinder the flow.Its a simple fact thos in heating and air conditioning can verify.its like a drain of sorts one has a 8 inch opening with an 8 inch drain pipe andother an 8 inch opening with a 4 inch drain pipe,the simple fact is flow speed will increase but total volume has no choice but to wait inline to even get a chance there for the small outlet will increase in out go speed but diminish th total amout of outlet per measurment.Basically the small duct can allow the max flow the bigger duct can even at the accelorated speed.no matter what math you try to use it just is a reducer is simply that...a reducer.

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I'm not sure I understand the last sentence. But, increased velocity will allow more volume to pass through a certain point in a given time. That is a fact. Forgetting about the math is like saying you don't need language to write a book.
And your analogy about the drain is completely unrelated since the the water flowing down the drain is not being pushed or pulled by a machine doing a constant rate of work that must be kept constant; similar to the example heady gave with the straws.

Heady said:

If splifman were correct, we'd all be able to vent our closets with 1" (or less) ducting. Now that's stealthy!

No offense splif, but you ignored the pressure/backpressure factor, which myself and others mentioned.

This is why industrial/commercial centrifugal fans are given Static Pressure ratings, instead of airflow (CFM) ratings.

NASA. Pffffft! Since when are they experts?

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Heady, no offense taken.
I did not ignore disruption of laminar flow and back pressure as I mentioned it in my last response to you. I simply said with the current conditions it is negligible.
However, in the situation that you mentioned (1" tube) it would not be negligible. In this case you would need an exterior force to compress the air. This type of scenario might seriousely hinder the fans ability to pull air at the same rate.

Take a garden hose with running water. Why does the water shoot out faster when you cover half of it with your thumb? However, when you cover 99% of the hole you obviousely are not going to have has much water come out of the hose in a given time period. Not all relationships are linear due to other factors that we haven't even mentioned, like the compressability of a fluid.

Here is an experiment for you: take an empty one gallon milk bottle and see how long it takes the garden hose to fill up the bottle. Then see how long it take for the bottle to get filled when covering 50% of the hole.

Ohh yeah, and I agree NASA may have a long list of fuck ups, but they are usually in situations far more complex than this.

splifman said:

Ohh yeah, and I agree NASA may have a long list of fuck ups, but they are usually in situations far more complex than this.

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You seem to know an awful lot about airflow, which is not an area I know much about so wondered if you knew whether there was a calculation for working out the required size of passive air intakes when using a particular size of cfm fan?

Eg, if a 4 inch inline fan rated at 175cfm were exhausting an area of 0.5m3, what size passive air intakes would be required? Or does it correspond to the area of the exhaust fan?

Cheers.

babygro said:

Eg, if a 4 inch inline fan rated at 175cfm were exhausting an area of 0.5m3, what size passive air intakes would be required? Or does it correspond to the area of the exhaust fan?

Cheers.

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Babygro,

Don’t know that much, but my degree was related.
If I fully understand your question, I seem to remember a post by mogie that was related to this. It said that you want to have the passive air intake to be a little bigger than the exhaust hole, to control the speed at which the air is entering the room. The air is being sucked out at a certain velocity but, depending on your situation you might want the air coming in from the intake to be much slower so as not to cause a wind effect.
By placing your hand by the exhaust hole or duct you can feel how fast the air is blowing out. By using this formula (rTVA)1 = (rTVA)2
Let 1 represent air being pulled out of system and 2 represent air being sucked back in to the system.
r = density
T = temperature
V = velocity
A = area

which becomes (VA)1 ~ (VA)2 because r and T or constant (not necessarily entirely true because hot air being pulled will differ slightly in temp of air coming in, but this is negligible). Yes, there are effects of friction that are not being accounted for, but again, for this type of real life application those affects are also negligible.

Anyway, using the formula you know A1 and you know what V1 feels like. So you might decide that you want V2 to be four times as slow as V1:
V1A1 = (1/4)V1A2 because (1/4)V1 = V2

Therefore, A2 = 4A1, in your example you said your exhaust duct was 4in so A1 = 12.56 in^2 so A2 = 50.24 in^2 therefore, diameter of intake is 8in.
The size of the room being cleared does not need to be known. Also, if you wanted a figure for the velocity of the exhaust, V1, you can use the info given about the inline fan, 175 cfm and the area of the exhaust. I’d have to think about it for a second or two though. Maybe pull out an old book or something. I think integration is involved.

The drain and the air flow are indeed similar with respect to each having its own suction.A reducer will increase velocity but will decrease flow capcity. AAnother example is in cars.Things like restricter plates for manifolds they reduce like a reducer to increase the flow speed but actuall decrease the volume taken in.Or like an exhaust system a small exhaust pipe will work fine but a bigger outlet can increase horsepower by allowing better airflow with less back pressure same as going for 8 inch to 4 inch.Velocity does not mean increase mass flow but in most case will lead to decreased mass movement ie a reducer will limit amount of air that is able to flow at 1 time but will indeed speed the smaller air flow .I will try to find the link to a video file made by Advance Nutrients where they talk about how much more air can be removed via 8" ducting verses a 4" duct and how the larger duct makes the volume of the 4 " seem ridiculously small and will post it here.

thank you very much filthy fletch.

a side not on this. can a hood be puchased that has a flange or an 8" opening? I'm still going to have to use a reducer, but it looks like only a 8 to 6". but if i can go from 8" on the hood through an 8" inline reduced to 6" I think i'll be better off the 8 down to 4. either way it's working ok right now, but i want to make it as efficent as possibe.

FilthyFletch said:

The drain and the air flow are indeed similar with respect to each having its own suction.A reducer will increase velocity but will decrease flow capcity. AAnother example is in cars.Things like restricter plates for manifolds they reduce like a reducer to increase the flow speed but actuall decrease the volume taken in.Or like an exhaust system a small exhaust pipe will work fine but a bigger outlet can increase horsepower by allowing better airflow with less back pressure same as going for 8 inch to 4 inch.Velocity does not mean increase mass flow but in most case will lead to decreased mass movement ie a reducer will limit amount of air that is able to flow at 1 time but will indeed speed the smaller air flow .I will try to find the link to a video file made by Advance Nutrients where they talk about how much more air can be removed via 8" ducting verses a 4" duct and how the larger duct makes the volume of the 4 " seem ridiculously small and will post it here.

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FilthyFetch, you raise some good points but...
First off, again, the drain analogy cannot be used in this scenario as I touched on in my last post. The force pushing the water down the drain is a function of the amount of water going down the drain. F=ma, m is mass, a is acceleration (better known as gravity, g). The smaller drain changes the amount of mass flowing through a given length at a given time, therefore affecting the force.
In the case of the inline fan it is the amount of air going through the tube, along a given length in a given time, that is a function (or is dependent on) the force the fan is pushing or pulling with. In this case the fan is pushing at a constant force. I'm sure you can see how these two situations differ. i.e. in the drain the force is not fixed and is dependent, in the fan system the force IS fixed and constant.

If what you are saying is true, when you converted from 8' to 4' the fan would be forced to slow down. because not as much air could be pulled through. Is this what happens to the fan?

This phrase: "Velocity does not mean increase mass flow but in most case will lead to decreased mass movement" - makes absolutely no sense. If a fluid is moving a given speed through a pipe, how can you say that more mass is not being transferred through the pipe, in a given time, than the same fluid moving at a slower speed?!??! That blows my mind.

About the car analogy; I know absolutely nothing about cars and car exhaust. However, I can say due to very high temperature and pressure conditions of an car engine, I can see how exhaust flow from a car might respond differently due to these more extreme conditions. You see, fluids act very different under extreme conditions. Very hot air is much harder to compress which is what makes jet engines so powerful etc...

Listen, I think we might just have to agree to disagree on this issue. I don't know how else to explain this, short of posting links to published journals on related topics. even then it would take a significant amount of time to read through them to find the appropriate sections that relate to what we are discussing. Not to mention the time it would take for me to dig up these journals.

Peace

splifman said:

If what you are saying is true, when you converted from 8' to 4' the fan would be forced to slow down. because not as much air could be pulled through. Is this what happens to the fan?

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i'll shed a little first hand experience on this question. Less air is moved with these recuders 8" to 4". It's happeneing to me, I can feel air coming back out from the 8" piece. Changing over to a better system in a few days.

What I have does the trick, but it's not effecient as it should be

nightfun said:

i'll shed a little first hand experience on this question. Less air is moved with these recuders 8" to 4". It's happeneing to me, I can feel air coming back out from the 8" piece. Changing over to a better system in a few days.

What I have does the trick, but it's not effecient as it should be

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I have a hydro hut with a 4" exhaust hole that has been a new addition to my grow room in the last two weeks. The only fan that my local store had, that was strong enough to clear the volume of the hut three times per minute was an 8" inline fan. i have had to use a reducer as a result. I do not experience any such problem that you have mentioned above.

I luckily I have more than one 4" intake hole so the air coming in is much slower.

Babygro and others, I don't know about all of the formulas, and I guess it does depend on the CFM of the exhaust fan, but, what I have advised people to use as a general rule for passive intake holes:

Have twice the area of opening for intake that you have for exhaust.

IOW if you have a 6 inch exhaust fan, then you need two 6-inch openings for intake.

HTH