Dehumidified storage

jonr · August 2016

I need to dehumidify a shipping container based storage/tool shed where there is no utility power. Primarily to prevent rust, but without any dehumidification, mold would be an issue too.

Some theories:

a) paint the place black and have lots of interior thermal mass and the %RH will stay low due to solar heating
b) needs to use large batteries to power a dehumidifier 24x7 (although it will cycle on/off per the humidistat).
c) could use a system with little or no batteries that runs the dehumidifier only while PV solar power is available with dehumidification being stored in the form of bentonite clay granules.

I don''t know what to believe, although I'd like to avoid b) for cost reasons. I can envision c) using a DC-DC converter, supercapacitors (for the motor start surge) and a DC->120VAC inverter.

BB. · August 2016

Here is a very nice write-up on storing stuff in a cargo container:

http://www.dampstick.com/for-shipping-containers/

The above company sells an ANHYDROUS CALCIUM CHLORIDE water vapor collection system. This chemical system should be available from many sources.

You do not want temperature changes--An insulated container is going to be much less of a humidity problem.

When the interior is hot, and the sides/top are cold--You will have "container rain"--Condensation on the roof dripping onto your goods.

Also, many containers (virtually all?) have wooden floors (not really moisture proof)... Raising the container off of the ground (good air circulation under the container) is going to help too.

A super cap is not going to help anything.

-Bill

Vic · August 2016

Cargo containers are reasonably well sealed. Customarily, there are shrouded vents at the top of containers, one on each corner of 20-footers. These vents are three or four small holes through the steel side walls, and probably really just venting to atmospheric pressure.

EDIT: Should add, upon further inspection, today, there is more variability in the number of vents (2 - 4 ea), the number of holes in the walls under these vent shrouds (3 - 9), and there is some variation in the hole sizes on the 20-foot containers in use, here.<

Almost all modern containers that I've seen have Luan plywood floors, about 30 MM thick. But, there is an Undercoating spayed onto the bottom, on the outside. This makes the bottoms essentially waterproof, for many years, especially, if, as BB Bill mentioned the containers are set up, off of the ground, and the containers are not dragged around with the bottom scraping along on the ground.

We do have one container with a plastic floor -- plastic (was said to be PVC) flooring screwed onto longitudinal stringers, and then sealed with caulk on the inside joints between the plastic and the stringers.

In the fairly arid climate, here, have two louvered vent panels low on the side walls at each end of the container, and one roof vent on one end (yes, in the center would probably be better, ... but).

FWIW, Vic

Marc Kurth · August 2016

@jonr - don't forget that % RH is not the same thing as actual moisture content of the air.

In simplified terms:
Dehumidifiers (chemical or refrigeration) actually remove water from the air.
Good insulation and maintaining temperature control can be effective in maintaining a lower %RH and therefore reducing condensation - but if you are storing something that readily absorbs water from the air, it may not be effective.

Many substances will absorb moisture when the surrounding air is measured at seemingly low "RH" numbers. Putting it another way, many things absorb moisture without ever having condensation on them. Dried food products and gun powder are examples of this.

Dehumidified storage

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