eli5: how do dehumidifiers make water from air?


eli5: how do dehumidifiers make water from air?

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Its condensation. You know when you have a cold glass of liquid on a hot day drops of water form on the outside of the glass? This is water vapor condensing out of warm humid air.

A dehumidifier works this way. It uses mechanical compression and expansion to create cold coils – these condense water out of the ambient air as it cools. But we had to move the heat from the cold coils somewhere, so else where there is a hot coil, part of the same fluid circuit.

In an air conditioner you’d blow this now cool (and dryer) air back into the room and you blow OTHER air over the hot coils to remove that heat and blow that hot air it out the window (or you blow outside air over the hot coils which you have outside). In a dehumidifier you run the now cool dry air back through the hot coils to pick up the heat again so its the same temperature as it was coming in.

All air has water vapour dissolved in it – that’s what humidity is. The warmer air is, the more water vapour it can hold.

A humidifier takes in warm, moist air from the room. It blows it over a cold coil, and that cools the air down. When the air cools down, it can no longer hold as much water vapour, so some of the water vapour condenses out of the air, so the coil ends up coated in droplets of water. Gravity makes those water droplets fall into a collection bowl, and you periodically empty the bowl down your sink.

The air already had the water in it, that’s why it was called “humid”. The machine cools the air to condense the water out and then warms it back up and releases it in to the room.

It doesn’t make the water, it’s already in there. It’s like a sopping wet towel. If the air is dry, the dehumidifier won’t pull any water out of it.

Remember that heat is energy. Air with energy can carry things along with it. Think of a garden hose turned up high and pushing a lot of leaves and dirt. If the air cools down, it loses that energy, and it drops what it is carrying. This is similar to a hose being turned down to a trickle and it can’t push dirt. The dehumidifier cools the air, it drops the water it is carrying into a basin, and blows the dry air out.

A dehumidifier is actually just an air conditioner, but the waste heat doesn’t go outside.

When air gets cold, it can’t hold as much water. This is why air conditioners usually drip water, it’s water from the air on the cold side condensing. A dehumidifier does the same thing, but without changing the overall temperature of the room. It just takes yhe heat of the air in one spot, letting the water condense and puts that water in a tank or drip pan, and heats the air up in another spot (for conservation of energy) and then repeating the process after a fan has exchanged the dry air for new, wet air.