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Old 02-23-2009, 11:43 PM #1
guest 77721
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Ventilation 101

Hey guys,

as you all know if the women don't find you handsome, they better find you handy...

I've got some background experience and have wanted to do a good write up for a while.

I have a collection of source material and will be cutting and pasting from it with a more detailed explanation that is more relevent to our hobby, building cabinets.

One of the lads in the Lodge says that growing leads to carpentry. I'm sure he's right.

****************************** ****************************** ****************************** *****
We are trying to do two things at the same time, sorta like chewing gum and walking. The first thing is to provide
ventilation for the plants living in the grow chamber. The second is to remove the heap of heat generated from our lighting.

This chart gives some recommended ventilation rates for common applications. I recommend 5 Minutes per Air Change and no more than 1-2 MpAC to keep stress down on the plants. High airflow rates cause excessive dehydration which is hard on the plants.



****************************** ****************************** ****************************** *****
COMPARISON OF CABINET DESIGNS

1. Simple Cab - one chamber, no scrubbers or restrictions to airflow.



CFM = 3.16*Watts/deltaT
where deltaT = 10 *F

Intake Area = 2 x Exhaust Area
Fans - Axial/Computer Fans due to minimal pressure loss

Example: A 400 W HPS in a 3 x 3 x 4 cabinet needs 126 CFM of cooling for a 10*F temp rise above room temperature.
The 36 cu ft cabinet is ventilated at over 3 Air Changes Per Minute.

Pro:
Easy to build
Fans - axial

Con:
plants stink up the place
plants are stressed by too much airflow at 3 ACpM.
The plants only need 5 Minutes per Air Changes (MpAC). That's 0.2 Air Changes per Minute (ACpM).

For you hydro guys, this means more rez changes and nute swings because the plants will be drinking alot to keep
up with the dehydration caused by living in a wind tunnel.

2. Simple Cab with Scrubber



CFM = 3.16*(Total Watts)/deltaT
where deltaT = 10 *F

Intake Area = 2 x Exhaust Area
Fans - Blower or Inline for high pressure drop across scrubber

Example: A 400 W HPS in a 3 x 3 x 4 cabinet needs 126 CFM of cooling for a 10*F temp rise above room temperature.
The 36 cuft cabinet is ventilated at over 3 ACpM. The hard part is to match an oversized fan/scrubber combination to get the right airflow.

This design and the simple growbox have cooling requirements that exceed the ventilation requirements of the growchamber by at least 15x. Way too much stress on the plants by living in a wind tunnel.

Pros:
single fan design less equipment to purchase

Cons:
Fan operates at 50% or less airflow due to large pressure drop across scrubber
High SP requires a Centrifugal Blower, won't work with Axial fans
Fan/Filter Curves are needed to determine working airflow
Scrubber is many times larger than Ventilation specs to get light cooling airflow
many restrictions in multi chamber designs require special attention to intakes and intrachamber airflow
high airflow for to cool lights creates stress on plants
high airflow through scrubber reduces effectiveness
Centrifugal Blower and Inline Fans are very noisy 60-70 db

3. Simple Cab with CoolTube/Ventilated Hood and Inline Scrubber



CFM = 3.16*(Total Watts)/deltaT
where deltaT = 20 to 30 for cooltube

Intake Area = 2 x Exhaust Area
Fans - Blower or Inline for high pressure drop across scrubber

Example: A 400 W HPS in a 3 x 3 x 4 cabinet needs 63 - 42 CFM of cooling for a 20-30*F temp rise in the exhaust temps with a 1-2 *F rise in the growbox.

The 36 cuft cabinet is ventilated at 2 ACpM. The hard part is to match an oversized fan/scrubber combination to get the right airflow.

The cooltube design is a big improvement over the Simple and Simple cab with Scrubber designs as the overall airflow is considerably less. However 2 ACpM is still 10x over what the plants need and will stress out the plants from excessive airflow.

Pros:
single fan design means less equipment to purchase
Cooltube/Ventilated Hood improves cooling significantly by trapping most of the heat in the hood
Less airflow through the grow chamber compared to the above designs but still high.

Cons:
Fan operates at 50% or less airflow due to large pressure drop across scrubber
High SP requires a Centrifugal Blower, won't work with Axial fans
Fan/Filter Curves are needed to determine working airflow
Scrubber is many times larger than Ventilation specs to get light cooling airflow
many restrictions in multi chamber designs require special attention to intakes and intrachamber airflow
high airflow for to cool lights creates stress on plants where only 0.2 ACpM is needed
high airflow through scrubber reduces effectiveness
Centrifugal Blower and Inline Fans are very noisy 60-70 db

4. Two Stage Cooling/Ventilation with cooltube and scrubber



LIGHTING CFM = 3.16*(Total Watts)/deltaT
where deltaT = 20 to 30 *F

VENTILATION = 1 - 5 MpAC (Minutes per Air Change) or 0.2 to 1 Air Change Per Minute (ACpM)

Intake Area = 2 x Exhaust Area for EACH section

Fans - Axial for Lighting maximum airflow, minimal pressure loss
Small blower, inline or axial for scrubber

Example: A 400 W HPS in a 3 x 3 x 4 cabinet with a ventilated hood needs 63 - 42 CFM of cooling for a 20-30*F temp rise in the exhaust temps with a 1-2 *F rise in the growbox. 5 Minutes between Air Changes requires only 7 CFM through the scrubber.

Pros:
unrestricted airflow through lighting allows fan to operate at maximum rated flow
low air flow rates allow for smallest scrubber
use of less expensive fans as design is more efficient
Lighting exhaust deltaT can be 20-30*F while maintaining low grow chamber temps
Much quieter as axial fans operate at 20-30 db compared to 60-70 for centrifugal fans

Cons:
more equipment
more intake and exhausts require more light proofing
ventilation design is more complex

5. Multi Chamber Designs



LIGHTING CFM = 3.16*(Total Watts)/deltaT
where deltaT = 20 to 30 *F per ventilated hood

Rule of thumb: 20*F temp rise for no more that 4 hoods chained together for a combined rise of 80*F

VENTILATION = 1 - 5 Min/Ch (Minutes per Air Changes)

Intake Area = 2 x Exhaust Area for EACH section

Fans - Axial for Lighting maximum airflow, minimal pressure loss
Small blower, inline or axial for scrubber

Example: Two 3 x 3 x 4 cabinets with a 400 watt HPS with a ventilated hood needs 63 - 42 CFM of cooling for a 20-30*F temp rise in the exhaust temps of each hood for a total of 40* to 60* temp rise with a 1-2 *F rise in the growbox. 5 Minutes between Air Changes requires only 14 CFM through the scrubber.

Pros:
unrestricted airflow through lighting allows fan to operate at maximum rated flow
low air flow rates allow for smallest scrubber
use of less expensive fans as design is more efficient
Lighting exhaust deltaT can be 20-30*F while maintaining low grow chamber temps
Much quieter as axial fans operate at 20-30 db compared to 60-70 for centrifugal fans

Cons:
more equipment
more intake and exhausts require more light proofing
ventilation design is more complex


Ventilation Chart



RECOMMENDED SPECIFICATIONS FOR GROWBOXES:

Simple Cabinet - airflow to maintain temperature within 10*F of ambient

Cool Tube or Ventilated Hood - exhaust air temp 20 - 30 *F. Hoods can be connected together to a common fan at 20 *F rise per hood, no more than 4 hoods for a total rise of 80 *F.

Grow Chamber Ventilation - 1 to 5 Minutes per Air Change. This is different for every type of light and setup. The more heat that is removed using a cool tube or ventilated heat means you can drop the grow chamber ventilation down further. The plants don't need very much air exchange and over 1 ACpM will cause unnecessary stress.

You will see a variety of equipment used to ventilate growboxes, tents and rooms. My recommendations are to put your money into a cool tube or a ventilated hood. Cooling a bare bulb with a centrifigal fan and scrubber combo is a big waste of money.
There's no need to scrub your light cooling air in a well designed system.

I'm going to show this handy chart again that show Air Flow Rates for Common Bulb Sizes. This chart is useful in two ways. For the cabinet builder, find your bulb size and the amibient temp rise you want and get the air flow needed off the chart.

INTAKE SIZING

Getting the intakes right is the biggest challenge in setting up a growbox. The intake area needs to be at least equal to the exhaust area and up to 2x to reduce losses.

A = pi * (radius)^2 = (pi *diameter)^2/4

Areas for common duct and pipe sizes



Thanks to Freezerboy for the nice picture. This is a common mistake when using a number of small holes to equal one big one. The only solution is to calculate the area for the big hole and divide by the AREA of the smaller holes to get the number of small holes needed.



In this case one 9" hole needs 80 x 1" holes to be equivalent.

I'd like to point out that making one big hole is better than a bunch of little holes as each little hole has it's own Vena Contracta losses which do add up.

How to Check Your Cabinet for Proper Ventilation

For they guy wanting to know how well his cabinet is working, you can get the EFFECTIVE AIR FLOW rate from this chart. Here's how to do it. Measure the temperature of your exhaust air and subtract the ambient temperature to get the temperature rise number. Use the bulb size line and temperature rise to get the EFFECTIVE AIR FLOW RATE.





For example, a 250 W HPS lamp in a growbox with 90*F exhaust in a 70*F room is operating with 40 CMF of effective airflow.

Last edited by redgreenry; 05-24-2011 at 05:00 AM..
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Old 02-23-2009, 11:44 PM #2
guest 77721
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Here is an excellent fan sizing guide from Grainger Fans who produce the Dayton line which many cab builder use.

https://www.grainger.com/images/vent_fundamental.pdf

I found an ventilation engineering course on the EPA website. There is a wealth of information here but it is very technical.

https://www.epa.gov/air/oaqps/eog/bces/toc/full_toc.htm

This site has some good info too
https://www.mil-embedded.com/articles/id/?3281

Fans are a pressure producing device which are basically air pump. Air flow is created by the fan's pressure. It doesn't take much pressure to move air which has little mass compared to water or hydraulic fluid. Air pressure is measured in Inches of Water which are 1/28 of a pound per square inch.

Computer Fans (Axial) produce about 0.2 inches of water pressure and are ideal for unrestricted blowing. They can move a lot of air but don't block them up.

Centrifugal Blowers produce more pressure than an axial fan. Typically the ones used by most cab builders produce about 0.5 inches of water pressure and will work with more static pressure such as scrubbers. The shape of the fan blades make a big difference in the pressure produced by the fan.

Industrial blowers will produce much higher pressures but they require multiple horsepower motors to drive them.




Static Air Pressure is measured with a manometer which is just a U shaped tube filled with water. This picture shows the manometer measuring the intake side of the fan which has 1 inch of water pressure. If you stick a ruler next to the tube one side is 1" higher than the other.




Every fan manufacturer can provide you with a fan curve for your fan which is produced by operating the fan with various static pressures and taking flow measurements. All they do is put a variable damper on a duct and move it from fully open to fully closed.



This fan curve is for 1200 CFM fan that will produce 0.6 inches of water pressure.

When the fan is fully blocked, it creates 0.6 inches of STATIC PRESSURE and 0 CFM AIR FLOW.

When the fan is unrestricted, it creates 0 inches of STATIC PRESSURE and 1200 CFM AIR FLOW.



Now if you have a fan speed controller and can change the speed of your fan this is what happens at different speeds. The fan operates with the same shape curve but with with an identical shaped curve.
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Old 02-23-2009, 11:45 PM #3
guest 77721
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Every ventilation system can be described by a SYSTEM RESISTANCE CURVE where the static pressure and airflow are graphed together.

If you took a big fan and connected it to your growbox and put a duct on the exhaust with a damper so the airflow airflow could be adujsted and measured the static pressure as the air flow is increased, this curve would be made.

Every "SYSTEM" has a unique resistance curve.

AIR FLOW = K * SQRT(PRESSURE) where K is a unique system constant




Point A is my cab with an Axial Fan producing 0.2 in of static pressure and 1000 CFM of air flow (not too realistic eh!)

Point B is my cab with an Centrifical Blower producing 1.0 in pressure and 2000 CFM air flow.



Now let's do something to our cabinet to change the system constant K. Let's say our Cab has two intakes and Curve A is normal. If we block up one intake, we have changed our system constant K to a new number and we get Curve B.

My cab with 2 intakes and an axial fan producing 0.2 in static pressure has an air flow of 700 CFM on curve A

My cab with 1 intake and the same axial fan at 0.2 in static pressure has an air flow of 500 CFM on curve B.

Changing the system will produce a new system curve and a new system constant K.
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Old 02-23-2009, 11:46 PM #4
guest 77721
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The Operating point is where the Fan Performance Curve crosses the System Resistance Curve.



In this case a 0.6 in 1200 CFM fan is operating with 0.25 in of Static Pressure and producing 1000 CFM. The resistance of the system causes reduced performance.

STATIC PRESSURE WORKS AGAINST AIR FLOW AND IS CAUSED BY RESISTANCE



A new operating point is created by changing the speed of the fan without making any changes to the system.
This is the same as upgrading from a low performance fan to a heavy duty industrial type.




Changing the System Resistance moves the operating point along the Fan Performance Curve. In this example I cut a new intake and reduced the System Resistance from Curve A to Curve B that increased air flow from 1000 to 1150 CFM.

A while back I threw out a simple formula for calculating air flow. Here's a better calculation that takes Area into consideration.

CFM = K * SQRT (PRESSURE) * AREA

What's interesting is that doubling the intake area will double the air flow.

You have to change the pressure produced by a fan by 4 times to double the air flow.

If you want to improve your cabinet, adding intakes, increasing duct size and adding more surface area to scrubbers is the way to increase air flow.
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Old 02-23-2009, 11:47 PM #5
guest 77721
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So what actually makes the Air move?

All a fan does is makes a difference in pressure between the intake and exhaust. Let's call this the Total Pressure.

Resistance to the air flow is called Static Pressure and what's left over is called Velocity Pressure.

It's the Velocity Pressure that pushes the air. The Static Pressure is best described as pressure losses in the system.

TOTAL PRESSURE = STATIC PRESSURE + VELOCITY PRESSURE

or another way to look at it is

VELOCITY PRESSURE = TOTAL PRESSURE - STATIC PRESSURE

Static pressure is your enemy in a good ventilation system. If you find your cabinet door hard to open, then you have high static pressure. If you open your cab door and the air flow from the exhaust increases, then you need more intake area.


The fan curve is useful for converting static pressure readings into actual airflow.

Let's say I have a nice fan that is rated at 0.5" WC. Measuring the static pressure at the exhaust I find 0.05" WC and the static pressure at the inlet is 0.25" WC which would give me a Total Static Pressure of 0.3 and an air flow of 950 CFM.

The velocity pressure would be 0.5 - 0.05 - 0.25 = 0.2"



Duct Air Flow Calculations

Here's the full formula for calculating the air velocity in a duct knowing the Velocity Pressure

velocity = 4005 * SQRT( velocity pressure) * SQRT (0.075/air density) where 0.075 is the density of air at 68*F

Let's ignore air density for now.

v = 4005 * SQRT (Vp)

To calculate the air flow in a duct, multiply velocity (ft/min) by the area of the duct (ft*ft)

v * A = 4005 * SQRT(Vp) * A

with velocity in ft/min and area in square feet

Flow = 4005 * SQRT(Vp)*A CFM (cubic feet per minute)



****************************** ****************************** ****************************** ***
Static Pressure Losses



Now let's say I have this growbox with two chambers and some inlet and outlet ducts connected to a 0.5" 1200 CFM Fan

The Total Static Pressure at the fan is 0.3" which will give me a flow of 950 CFM using the fan curve.
The Vp is 0.5-0.3=0.2

The Total Static Pressure loss for the system is 0.3" which is 0.1 for the inlet air ducting, 0.1 between chamber B and A and 0.1 between A and the outlet of the fan.



Now let's try to improve the airflow between chamber A and B by adding a second vent.

Wow, the static pressure in chambers A and B are the same so there is no more loss and it also dropped the static pressure at the fan inlet to 0.2". Going off the fan curve the airflow increased from 950 to 1050 CFM with the velocity pressure increasing from 0.2 to 0.3.



Let's add a second intake. This reduces the Sp down to 0.05" in each intake duct and drops the Sp down to 0.1" at the Fan.

The overall airflow is improved to 1125 CFM which is almost the full rating of the fan at 1200 CFM.

In this example by removing an internal restriction and opening up the intakes, the airflow was improved from 950 to 1125 CFM.

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Old 02-23-2009, 11:48 PM #6
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This is the handiest chart that I've ever seen. It's based on a cooling formula that I've come across in a few places during my research.

CFM = 3.16 * Watts Dissipated / delta T in *F





If I had a dollar for everytime someone asked how many CFM's do I need for my Cabinet....

Here's a handy chart to for ventilation for each type of light bulb. For a simple cabinet use the 10*F curve.

For ventilated hoods and cooltubes, you can size your airflow to the exhaust temps. I'd recommend 20 to 30 *F. The hood traps most of the heat inside. The heat inside the hood stabilizes and is the same as the exhaust temp. With a cooled hood only the radiant heat from the light is transferred to the grow box.

Large rooms that connect several hoods in series to a common fan, 20*F per hood, and no more than 80*F total for the exaust temp rise.






>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>

Cooling lights and providing ventilation for your plants are two different things.

I believe more than One air exchange per minute is excessive and causes undue stress on the plants.

The one pass ventilation and scrubber designs that expose the plants to all the light cooling airflow cause unnecessary stress on the plants as well as requiring oversized fans and scrubbers.

It is better to cool the lights separately using ventilated hoods or cooltubes with unrestricted fans. This drops the requirements down for area ventilation significantly. I sure like putting in small fans and scrubbers which saves $$$ in equipment and operating costs.

One other point of interest, the suggested airflows are in the 5-10 Minutes per Airchange for most uses involving people and habited spaces. High air change rates in the 1 to 5 Air Changes per Minute are found in applications where a lot of fumes need to be ventilated like a paintbooth. Many growboxes are constructed this way leaving the plants exposed to unnecessary stress.

****************************** ****************************** ****************************** ***

Air Conditioner Sizing

BTU/hr = (3.413 * Watts) + (1.25 * Room Area * Delta T)

1 TON = 12000 BTU/hr

For example, a 144 sq ft room with two 600 W lights within 20* F of ambient would require
a 7700 Btu/hr or a 0.64 Ton unit.
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Old 02-24-2009, 12:29 AM #7
Max0r
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Nice post RG! Trying to figure out a working exhaust for my rubbermaid cab. +K!
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Old 02-24-2009, 05:27 AM #8
nag-champa
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i understand exhaust, but i dont understand how the intake come in, do you need a fan for the intake also?

passive intake i beleive is when fresh air comes in from another room, when the exhaust is running.
what is stopping the stinky smells from leaving the passive intake?

Would the exhaust have to run 24/7 on a lower setting ( speed control) and to be overwritten by a thermostat?, this way the room would always be at a negative pressure ( i think this means the air wont escape the room)

can someone tell me what the power usage of a centrifugal fan is, something in the range of 500-700 cfm

thanks for the thread
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Old 02-24-2009, 05:37 AM #9
FreezerBoy
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Originally Posted by nag-champa View Post
i understand exhaust, but i dont understand how the intake come in, do you need a fan for the intake also?

passive intake i beleive is when fresh air comes in from another room, when the exhaust is running.
what is stopping the stinky smells from leaving the passive intake?
What stops the smell is Negative Pressure.

You can use intake fans but you'd better know what you're doing. You can't pump out more than you bring in but you CAN pump in more than you pump out. That's called positive pressure. Where pressure inside is greater than outside the cab. Positive pressure forces air out of the cab through any crack it can find thus eliminating the usefullness of filters.

Traditional passive intake uses 3 equal sized holes. One at the top for a powered exhaust and two passive intakes at the bottom. Now air rushes out as fast as it comes in. No buildup of pressure. Air rushing through the cab creates a pressure drop or, negative pressure. Pressure inside is less than outside the cab (it's why wings lift an airplane or a curveball curves) Because of negative pressure, all air in the vicinity will try to enter the cab. The only way out is through the filter.
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Old 02-24-2009, 05:43 AM #10
FreezerBoy
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Oh, and Red, great post.
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