Arkansas HVACR NewsMagazine November 2024

HVACR NewsMagazine November 2024

Tech News

I'll also bring out the old coffee analogy: a sugar packet will mix into a cup of hot coffee or tea quite easily, but a lot of that sugar will sink to the bottom and not dissolve in a cup of iced coffee or tea. "Wringing Out" the Air When air moves over the evaporator coil, heat moves out of it, which brings down the temperature. As the temperature goes down, the air can't hold as much water, so water vapor condenses onto the coil and drains out.

like letting a sponge expand back to its original size after squeezing it; it'll have a lot less moisture. The air is at 100% RH (or close to it) when it goes into the supply plenum, but it will absorb heat and be able to hold more moisture by the time it mixes with the air in the space. As a result, the RH will drop significantly, all without taking any more humidity out of the air. Dehumidifiers apply the same principle; they consist of an evaporator and condensing coil

all in one box. The evaporator takes the moisture out of the air while cooling it, and the condensing coil provides that reheat effect. That's how dehumidifiers can help us control latent BTUs but at the cost of some sensible BTU gains.

That "sponge" gets smaller, and it stays small as it moves into the supply plenum; it may be carrying a lot less total moisture than it was before, but it's probably holding as much water vapor as it can possibly hold. That's why the RH is usually close to 100%. (It won't be exactly 100% due to bypass air that doesn't make good contact with the coil.)

Magical Reheat

So, how do we go from 100% RH to <60% without a dehumidifier? The air reheats when it moves through the ductwork and mixes with the air in the rest of the space. Reheating is