Plants grow best in an environment that is well ventilated to effectively dissipate heat. All artificial light sources generate heat and grow rooms have to be therefore, provided with some means of cooling. Cooling of grow rooms is best done using blowers, with one mounted on the inside to blow hot air out, and the other mounted on the outside to blow cool air in. This arrangement works best with the hot air exhaust blower mounted up high inside and the cool air blower mounted low on the outside, as hot air being lighter than the surrounding air tends to rise while cool air tends to settle to the bottom.
Exhaust blowers are sized on the basis of the amount of air they can move. Normally blower capacities are quoted in CFM i.e. Cubic Feet per Minute (CFM) of air, like say "315 CFM" which means that the blower will move 315 cubic feet of air in one minute, provided there are no restrictions on the intake or exhaust. But it is not possible to completely eliminate intake or exhaust restrictions. Some restriction will always be there, in the form of hose resistance. When air flows through a hose, it has to overcome the resistance of the hose which heats up the air, it is advisable therefore to select the hose that offers the least resistance to air movement. Aluminum tubing with its smooth inside wall surface offers low resistance to air flow and it therefore ideally suited for such applications.
Apart from the losses due to hose resistance, there are other factors that need to be considered while sizing blowers to suit the requirements of your hydrohut grow room. The blower size will basically depend on the size of the grow tent and the number of air changes that need to be effected per hour. A 6x6x8 (6 feet wide x 6 feet long x 8 feet ceiling height) grow tent that requires to have its air changed every minute or so will require a blower of capacity around 265 CFM. But exhausting the hot air inside will not by itself ensure adequate cooling. It is also necessary to ensure that the hot air inside is replaced by cold air from outside to effect good cooling. That is the reason why it is important to draw in cool air from outside using another blower.
The number of air changes required per hour depends on factors like plant density inside the grow tent, heat build up and other growth factors. Higher air change rates are best suited to large, mature crops in warm, humid conditions. In most cases, however, plants in grow rooms require only about 20-30 air changes per hour. One complete air change every 4-5 minutes is generally acceptable, but if lighting is creating a lot of extra heat that needs to be removed air changes will need to be more frequent probably every 2 -3 minutes.
Conclusion
Air movement with a correctly sized fan plays an important role in determining the success of any indoor crop. Though it is often overlooked, air movement is essential to maintaining optimal conditions for plant growth. It serves to ensure the correct levels of temperature, humidity and CO2 at the leaf surface which is the critical interface for vital plant processes like photosynthesis and transpiration.
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