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Intake Louver Sizing

Intake Louver Sizing

(OP)
The company I work for provides containerized water treatment equipment. The containers are standard sea boxes. Typically we install an exhaust fan in one wall of the container and an intake louver in an opposite wall of the container. I'm trying to find some guidelines for the following:

* for a certain sq. footage of container, what size (cfm) exhaust fan should be provided for proper ventilation.
* based on the cfm of the exhaust fan selected what size intake louver should be provided.


Thank you.

RE: Intake Louver Sizing

Somebody needs to define how much ventilation is proper for the desired result.
Each manufacturer of louvers shows how much flow can go to any specific model and size.
Consider that available area will dictate the max dimensions of your louver, or if you will need to install more than one; same applies to the fan.
Fan and louver(s) will need to prevent rain and pests from entering the container.
Both should be rated for hurricane winds if they will be in such areas.

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

RE: Intake Louver Sizing

2
Based on the information your provided 1 cfm will be the correct flow. Or 1,000,000,000,000 cfm. Or something in between.

RE: Intake Louver Sizing

The first thing you need to work out, is how many air changes an hour do you need in the container ?
Then you can back track from there.
B.E.

You are judged not by what you know, but by what you can do.

RE: Intake Louver Sizing

What is the goal of the ventilation? Maintain temperature? Remove moisture?

RE: Intake Louver Sizing

(OP)
Goal is to maintain temperature.

RE: Intake Louver Sizing

Quote (jkate)

* for a certain sq. footage of container, what size (cfm) exhaust fan should be provided for proper ventilation.
* based on the cfm of the exhaust fan selected what size intake louver should be provided.

Goal is to maintain temperature.

To maintain the temperture, you will need
Q = 1.1 * CFM * ΔT
Where
Q = sum of heat sources in the container (use a load calculation program such as Carrier HAP or Trane Trace).
ΔT = temperature difference between desired temperature and design ambient.
-> solve for CFM

As for louver size, the goal is to prevent water infiltration due to carry over. You need to verify with the manufacture the maximum velocity at which rain infiltration will occur for a specific model; available in their literature. As a loose rule-of-thumb, you want to keep velocity through the louver less than 500 fpm; some louvers can handle up to 750 fpm velocities which is why you need to check with the manufacturer. This works for 90% of the louvers on the market with the notable exception of Miami-Dade hurricane rated louvers.

Bugs, birds and other pests can be kept out with screens.

RE: Intake Louver Sizing

Another note about louvers. Most louvers have a free area of 40-60%. As a starting point in louver size selection I use 50%, then tweak actual size based on free area size table provided by manufacturers.

RE: Intake Louver Sizing

(OP)
Dbill74:

Thank you for you reply, it was very helpful. One question, If our goal is to maintain temperature, does the number of air changes needed come into play in determining the louver size?

RE: Intake Louver Sizing

Consider also the best possible flow of air between the louver(s) and exhaust fan, in order to avoid pockets of stagnation where temperature is going to be higher.
Ideally, the location of the louver should be low and the fan high, because the air will go up as it picks up heat within the box.

If the box has no insulation and big areas are exposed to the Sun, your calculations may show a high rate of ventilation and the available wall's area for installation of louver(s) and fan could limit your capability to keep the internal temperature low enough for worst conditions.

Also verify if dust accumulation and energy consumption or noise could be issues, in which case you may need using air filters in the louvers and a thermostat to control the fan.

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

RE: Intake Louver Sizing

Quote (jkate)

does the number of air changes needed come into play in determining the louver size?
Oh.. how to answer this.......

The critical variable you need is CFM.
From CFM you determine the free area required of your louver(s).
Once you have determined the free area needed you find a louver with the required free area. For this step you need to open a catalog.

From CFM you determine the air change rate (which is used as a check more than anything else). For your application I would expect the air change rate to be one air change every 2-4 minutes.

RE: Intake Louver Sizing

I agree with dbill74..
The basic for select louver size is CFM
You can follow dbill74 information, or you can estimate it.
CMH = Area x Floor to Floor Height x Air Change/hour
CFM = 1.69 x CMH

Then,
you can used catalog for louver selection.

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