Venting tents and small greenhouses

GreatLife4All;1822998 said:

Since you are, in essence, trying to vent a greenhouse, let me give you some data from one of my books.

These numbers are based on emperical research in outdoor greenhouses in full sun at the hottest point of the day. Therefore, these will be sufficient for all but the most intensive lighting systems.

The first column is the amount of air that must be moved from outside to inside to cause a temperature drop whose range is given in the second column. There are issues with this table, but I have found it very helpful in setting up my tents.

[table="width: 500, class: outer_border, align: center"]
[tr]
[td]Air Change Rate,[/td]
[td]Inside Temperature,[/td]
[/tr]
[tr]
[td]Volumes/Minute[/td]
[td]Degrees F Above Outside[/td]
[/tr]
[tr]
[td]1/2[/td]
[td]16 to 18[/td]
[/tr]
[tr]
[td]3/4[/td]
[td]11 to 13[/td]
[/tr]
[tr]
[td]1[/td]
[td]8 to 10[/td]
[/tr]
[tr]
[td]2[/td]
[td]4 to 6[/td]
[/tr]
[/table]

So lets make some assumptions and see where this table leads us.

A1 - Indoor Temp Goal 75F

A2 - Outdoor Temp Range 68-72

A3 - Tent Size 4x4x6'
​

Based on our assumptions, we know that we are trying to ventilate a space of 96 cubic feet (4' x 4' x 6').

We also know that we want to maintain a 3 to 7 degree temperature differential between inside the tent and outside the tent. So lets look that up in the table above... and we see that it is close to the same range for the last row in the table. So we need to change the air somewhere around 2 x each minute in order to effect the desired temperature drop.

So lets round a little and say that we need a fan that runs at 200 CFM (100 CF x 2 cycles/min).

but you will find that this is not sufficient. Why? Because you will NEVER get the rated capacity out of a fan once you attach it into your system. The numbers that I used above are taken from measuring the exhaust coming out of the greenhouse - not the rated capabilities of the motors. And this makes a difference.

In theory, you can use the type of duct work, the length of duct, the number of corners, to calculate the loss of air flow. In general, more flexible duct has greater loss than rigid... longer runs have greater loss than shorter... and the more corners you put in the more flow you lose (and this is a BIG loss). So you want to design your ventilation to run as short and straight as possible.

So... to make things simple. I just double what I need and keep all the duct work as short and straight as possible. Seems to be a pretty good metric that works in practice.

So... for a 4 x 4 x 6' tent, running a 1,000W light, you will need a 400CFM fan. Add 10% for errors... and both of my tents actually have 440CFM fans. I have had no issues with mold or humidity.

You can run this same math for your tent size and get a very good estimate for what you will need. A 600W light will not heat the tent as much as a 1,000... but more than a 400. So you will need to adjust slightly if you are running less than 1,000W. But if you just run the numbers like I did and stay below 1,000W - you will have more than enough.

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