Essential question of humidity vs temperature

[osolemio]

Regarding the plants ... I am thinking of certain types of plants where part of the plan absorbs moisture, while other parts expels it. This could be like a tree taking water using the roots, which in turn evaporates from the leaves (and other parts).

If you could have either part of the plant outside, and part inside, or have a kind of small green house around the part of the plant which expels moisture (and vent that outside), you could have the plant work for you - entirely for free!

Besides, many plants clean the air of certain pollutants, as well as increase oxygen leves. There must be a huge potential here ...

[ThomSjay]

My plants shrivel up if I don't water them. And then, as you mentioned, the leaves expirate the moisture.

A former co-worker used to put tall cans spaced around in his shop. On the top was, I believe a funnel-shaped lid that had, if I'm right, calcium that would absorb the moisture. The moisture would collect and funnel down into the can, which could, in turn, be dumped later.

[mackerel]

Before worrying about taking moisture out of the air, have folks given a thought to where it's coming from? As an energy auditor, I see a lot of houses here in Maine that have damp basements. Many run dehumidifiers. Many just put up with a musty smell that makes everyone sick. I tell people over and over: Use a fan over the kitchen stove, and in the bathroom. Get an energy star fan with a humidistat switch. Paint the basement concrete so it can't be a source for ground moisture evaporation. If you have a dirt floor crawlspace or basement (all too common here), put a membrane (usually builders plastic, but occasionally something better like EDPM) over it. Seal the moisture down.

I deal with a heating climate. Few of my clients use air conditioners before I get there, and I hope none do afterwards. If they do, I look for problems with ceiling/attic insulation. Fix that and AC isn't needed here.

If cool dry air comes into a house and warms up, the relative humidity drops. That makes the air suck moisture out of wood, skin, plants, whatever. When it escapes, at some point it reaches the dew point and water condenses out, regardless of venting.

In this maritime climate in the summer, if people have windows open, especially basement windows, the moist outside air comes in and condenses in the crawl space or basement.

So really the best way to dry air would be a heat exchanger system that first cools it, removing water, and then warms it again by running it past incoming air that's being cooled, or taking waste heat off the cooling machinery. Ideally, this would have very little energy consumption in the process. I'm thinking of the water purification distillation units in Gaviotas that left the clean water only 3°C warmer than the incoming water, despite having just been through a solar evaporator.

[S-F]

Originally Posted by mackerel

Before worrying about taking moisture out of the air, have folks given a thought to where it's coming from? As an energy auditor, I see a lot of houses here in Maine that have damp basements. Many run dehumidifiers. Many just put up with a musty smell that makes everyone sick. I tell people over and over: Use a fan over the kitchen stove, and in the bathroom. Get an energy star fan with a humidistat switch. Paint the basement concrete so it can't be a source for ground moisture evaporation. If you have a dirt floor crawlspace or basement (all too common here), put a membrane (usually builders plastic, but occasionally something better like EDPM) over it. Seal the moisture down.

I deal with a heating climate. Few of my clients use air conditioners before I get there, and I hope none do afterwards. If they do, I look for problems with ceiling/attic insulation. Fix that and AC isn't needed here.

If cool dry air comes into a house and warms up, the relative humidity drops. That makes the air suck moisture out of wood, skin, plants, whatever. When it escapes, at some point it reaches the dew point and water condenses out, regardless of venting.

In this maritime climate in the summer, if people have windows open, especially basement windows, the moist outside air comes in and condenses in the crawl space or basement.

So really the best way to dry air would be a heat exchanger system that first cools it, removing water, and then warms it again by running it past incoming air that's being cooled, or taking waste heat off the cooling machinery. Ideally, this would have very little energy consumption in the process. I'm thinking of the water purification distillation units in Gaviotas that left the clean water only 3°C warmer than the incoming water, despite having just been through a solar evaporator.

Bang!









Apparently "Bang!' is too short a message to post so I have to add this filler to get my "Bang!" out there.

[mackerel]

I had a couple more thoughts.

If the machinery was outside the conditioned space, unlike a standard dehumidifier, you won't have the waste heat inside.

Also, from a simple thermodynamics point of view, you should be able to calculate how much water you're condensing out of the air.

Say you have 85° air with 100% humidity, and you hope to have 85° air with 50% humidity. You're removing 90 grains of water per pound of dry air. You're going from 180 grains total to 90 grains total. You're bringing the air down to 65°, where 100%RH means 90 grains of moisture as vapor, and the rest condneses, and then you're warming this air back to room temperature by cooling the air coming into the dehumidifier system.

A grain is .000143 pounds. A pound of air is about 13 cubic feet. Say your house is 2000 square feet, with 8' ceilings, and gets the recommended .35 air changes per hour natural ventilation. That's 5600 cubic feet. That's 430 pounds of air. That's .062 pounds of water to condense. The latent heat of evaporation is 970 BTU/lb for water, so we're talking about ~60 BTU per hour, which is virtually nothing.

EDIT: I found my math error: 5600 ft3 -> 430 #air -> 38800 grains h2o -> 5.54 #h2o -> 5400 BTU -> 1.6 kWh. Something like a 20 amp air conditioner, including equipment inefficiencies.

There will also be efficiency losses, and the air will be cooled in the process. If we can keep the cooling to 5°F, at .0182 BTU/(ft3*°F), that's another 500 BTU, and the room is down to 80°F, but up to about 57% relative humidity.

Somebody (STILL) check my math. If I want to be certain, I use a spreadsheet and a second person to check things. What I calculated is all air. If there are solid surface (through the insulation) heat gains, those will have to be dealt with separately. I also didn't account for the moisture you've added to the air from normal human activity, like breathing and doing dishes.