Extract Water from Air: DIY Methods & Solutions

[russ_watters]

This is a lot of what I do for a living (HVAC engineering), so I have some tools available at work I can use to help.

The thermodynamics of air is a science called psychrometrics: http://en.wikipedia.org/wiki/Psychrometrics
The wiki has a sample chart and there are apps you can download or us online to plug in some conditions and get the rest. I use this one: http://www.tranecds.com//COMMERCIAL/DNA/View.aspx?i=1250

So the humidifier generates 20.5 oz per day (0.053 lb/hr) of moisture from 86F air at 80% RH. Plugging into a psychrometric chart gives a dew point of 79F and an absolute humidity of 152 grains (I'll explain that in a min...).

According to published climate data I have, the most humid it ever gets in absolute terms in Irvine (.4% of the time) is a dew point of 70F at 79 degrees (111 grains per pound of air). So we'll try to calculate the peak real output of this dehumidifier that you'll see for those 35 hours a year. By comparison, the most humid areas in the US are in the rainforests of southern Florida, where they get a dew point of up to 81F (matching the desing conditions of your dehumidifier).

A dehumidifier works by cooling air down below its dew point, squeezing moisture out as it does. The surface temperature of the peltier device in the dehumidifier is probably about 40F to avoid freezing. This will vary though; a little lower when it is cooler and drier, a little higher when it is warmer and wetter. That temperature will also vary with airflow, so if the dehumidifier gets clogged with dust, that temperature will also drop. For our calcs, we'll assume it is a constant 40F (37 grains).

The amount of moisture in the air is absolute humidity. For laymen, it is the dew point; your dehumidifier lowers the temperature and therefore dew point of the air flowing through it from our starting point of 70F to 40F. In psychrometrics, absolute humidity is measured in grains, which is the actual mass of water in air: 1 grain is 1/7000th of a pound. Relative humidity is the ratio between the moisture carrying capacity of the air (which varies with temperature) and the actual amount of moisture.

So your dehumidifier will drop the moisture in the air from 111 grains per pound to 37 grains per pound. Or, for every pound of air that moves through it, (152-37)/7000 = 0.016 lb of moisture will be squezed out. So that requires an airflow of 0.053 / 0.016 = 3.3 lb per hour (about 0.7 cubic feet per minute).

Reversing the calcultion using your acutal humidity yields (111-37)/7000 * 3.3 = 0.035 lb / hr or 13 ounces per day.

 

[Travis_King]

You might find this interesting: http://bigthink.com/design-for-good...e-world-to-make-drinking-water-out-of-thin-ai

 

[perplexabot]

First of all thanks to everyone for helping out and for the many good ideas and suggestions. Physics forums is where all the cool people are at

Check https://blumatsystems.com/category/3/Tropf-Blumat-Drip-Blumat

We used them here. They worked nicely in the winter, but were unable to keep up in the summer. However, perhaps using two or a larger model will do the trick, we never checked.

Interesting component. Not exactly what I need though, but may come in handy at some point. Thanks.

This is a lot of what I do for a living (HVAC engineering), so I have some tools available at work I can use to help.

The thermodynamics of air is a science called psychrometrics: http://en.wikipedia.org/wiki/Psychrometrics
The wiki has a sample chart and there are apps you can download or us online to plug in some conditions and get the rest. I use this one: http://www.tranecds.com//COMMERCIAL/DNA/View.aspx?i=1250

So the humidifier generates 20.5 oz per day (0.053 lb/hr) of moisture from 86F air at 80% RH. Plugging into a psychrometric chart gives a dew point of 79F and an absolute humidity of 152 grains (I'll explain that in a min...).

According to published climate data I have, the most humid it ever gets in absolute terms in Irvine (.4% of the time) is a dew point of 70F at 79 degrees (111 grains per pound of air). So we'll try to calculate the peak real output of this dehumidifier that you'll see for those 35 hours a year. By comparison, the most humid areas in the US are in the rainforests of southern Florida, where they get a dew point of up to 81F (matching the desing conditions of your dehumidifier).

A dehumidifier works by cooling air down below its dew point, squeezing moisture out as it does. The surface temperature of the peltier device in the dehumidifier is probably about 40F to avoid freezing. This will vary though; a little lower when it is cooler and drier, a little higher when it is warmer and wetter. That temperature will also vary with airflow, so if the dehumidifier gets clogged with dust, that temperature will also drop. For our calcs, we'll assume it is a constant 40F (37 grains).

The amount of moisture in the air is absolute humidity. For laymen, it is the dew point; your dehumidifier lowers the temperature and therefore dew point of the air flowing through it from our starting point of 70F to 40F. In psychrometrics, absolute humidity is measured in grains, which is the actual mass of water in air: 1 grain is 1/7000th of a pound. Relative humidity is the ratio between the moisture carrying capacity of the air (which varies with temperature) and the actual amount of moisture.

So your dehumidifier will drop the moisture in the air from 111 grains per pound to 37 grains per pound. Or, for every pound of air that moves through it, (152-37)/7000 = 0.016 lb of moisture will be squezed out. So that requires an airflow of 0.053 / 0.016 = 3.3 lb per hour (about 0.7 cubic feet per minute).

Reversing the calcultion using your acutal humidity yields (111-37)/7000 * 3.3 = 0.035 lb / hr or 13 ounces per day.

Wow. First of all, I definitely learned a lot from that post so I want to thank you for that. I was able to follow the math up until you calculating air flow : ( Will get back to it again, for sure. So according to your calculations the max yield (which is only 35hrs a year!) is around 13oz/day.
So I am assuming, I should be able to follow your calculations (and use the table) to see how much water this dehumidifier will obtain for my current weather?

I will give it a go.
PS: That is one crazy looking chart

 

[perplexabot]

Ok, so here it goes:
Given: average temperature = 70F and average humidity = 13%
Plugging DB=70F and RH=13%, I get W = 21.4 (grain)
W < 37 => On average I will get nothing out of the dehumidifier, under current weather conditions.
Is that true?