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Ventilation 101



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.







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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. 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 provideventilation 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.****************************** ****************************** ****************************** *****CFM = 3.16*Watts/deltaTwhere deltaT = 10 *FIntake Area = 2 x Exhaust AreaFans - Axial/Computer Fans due to minimal pressure lossExample: 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.Easy to buildFans - axialplants stink up the placeplants 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 keepup with the dehydration caused by living in a wind tunnel.CFM = 3.16*(Total Watts)/deltaTwhere deltaT = 10 *FIntake Area = 2 x Exhaust AreaFans - Blower or Inline for high pressure drop across scrubberExample: 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.single fan design less equipment to purchaseFan operates at 50% or less airflow due to large pressure drop across scrubberHigh SP requires a Centrifugal Blower, won't work with Axial fansFan/Filter Curves are needed to determine working airflowScrubber is many times larger than Ventilation specs to get light cooling airflowmany restrictions in multi chamber designs require special attention to intakes and intrachamber airflowhigh airflow for to cool lights creates stress on plantshigh airflow through scrubber reduces effectivenessCentrifugal Blower and Inline Fans are very noisy 60-70 dbCFM = 3.16*(Total Watts)/deltaTwhere deltaT = 20 to 30 for cooltubeIntake Area = 2 x Exhaust AreaFans - Blower or Inline for high pressure drop across scrubberExample: 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.single fan design means less equipment to purchaseCooltube/Ventilated Hood improves cooling significantly by trapping most of the heat in the hoodLess airflow through the grow chamber compared to the above designs but still high.Fan operates at 50% or less airflow due to large pressure drop across scrubberHigh SP requires a Centrifugal Blower, won't work with Axial fansFan/Filter Curves are needed to determine working airflowScrubber is many times larger than Ventilation specs to get light cooling airflowmany restrictions in multi chamber designs require special attention to intakes and intrachamber airflowhigh airflow for to cool lights creates stress on plants where only 0.2 ACpM is neededhigh airflow through scrubber reduces effectivenessCentrifugal Blower and Inline Fans are very noisy 60-70 dbLIGHTING CFM = 3.16*(Total Watts)/deltaTwhere deltaT = 20 to 30 *FVENTILATION = 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 sectionFans - Axial for Lighting maximum airflow, minimal pressure lossSmall blower, inline or axial for scrubberExample: 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.unrestricted airflow through lighting allows fan to operate at maximum rated flowlow air flow rates allow for smallest scrubberuse of less expensive fans as design is more efficientLighting exhaust deltaT can be 20-30*F while maintaining low grow chamber tempsMuch quieter as axial fans operate at 20-30 db compared to 60-70 for centrifugal fansmore equipmentmore intake and exhausts require more light proofingventilation design is more complexLIGHTING CFM = 3.16*(Total Watts)/deltaTwhere deltaT = 20 to 30 *F per ventilated hoodRule of thumb: 20*F temp rise for no more that 4 hoods chained together for a combined rise of 80*FVENTILATION = 1 - 5 Min/Ch (Minutes per Air Changes)Intake Area = 2 x Exhaust Area for EACH sectionFans - Axial for Lighting maximum airflow, minimal pressure lossSmall blower, inline or axial for scrubberExample: 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.unrestricted airflow through lighting allows fan to operate at maximum rated flowlow air flow rates allow for smallest scrubberuse of less expensive fans as design is more efficientLighting exhaust deltaT can be 20-30*F while maintaining low grow chamber tempsMuch quieter as axial fans operate at 20-30 db compared to 60-70 for centrifugal fansmore equipmentmore intake and exhausts require more light proofingventilation design is more complex- airflow to maintain temperature within 10*F of ambient- 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.- 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.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)^2/4Thanks 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.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 04:00 AM ..