The Impact of Adding or Removing Water From Air

Air conditioning was about humidity control from the very start. Willis Carrier's very first air conditioning system was all about controlling the humidity with the side effect that it also could reduce the sensible temperature. Theaters caught on that this newfangled contraption could lead to big summer numbers when they installed it to keep patrons cool, and the rest is history.

We often add water to the air (humidifying it) and remove water from the air (dehumidifying) as part of our work, but let's take a different look at what happens when we do that.

First, let's all get on the same page with some terms. (Skip down past the terms if you already know them because that isn't the point of the article.)

Dry-Bulb Air Temperature

When you measure with a typical thermometer, you measure dry-bulb temperature. It measures the average movement of the molecules in the air being measured with no consideration for the amount of water in it. We also call dry-bulb temperature sensible temperature, and we call changes in dry-bulb temperature “sensible heat change.”

Latent Heat 

When we change the state of matter by boiling, evaporating, or condensing, we cannot track the changes or movement of heat strictly with a thermometer. The heat that moves during a state change but doesn't contribute to a temperature change is latent heat.

Relative Humidity 

The relative humidity is just a percentage that tells us how much water is evaporated in the air in relation to how much there could be at the same temperature. It's like describing how full a glass is; it tells you how full or empty the glass is, but it doesn't tell you how much energy or water is in the glass by itself.

Wet-Bulb Air Temperature

If you were to use an old-school sling psychrometer, you would simply wet a little sock around a bulb thermometer and sling it through the air. If the air humidity were less than 100%, then the wet-bulb temperature would be less than the dry-bulb temperature. The difference between the wet-bulb and dry-bulb temperature is called “wet-bulb depression,” and we can use that to calculate relative humidity.

Dewpoint 

The temperature at which air becomes 100% saturated. Wet-bulb, dry-bulb, and 100% relative humidity lines all intersect at the dewpoint. Dewpoint is the glass 100% full.

Air Enthalpy

A psychrometric chart displays the total amount of heat energy in the air between the dry air AND the evaporated water vapor in the air in heat per mass (usually BTUs per lb in the USA). Wet-bulb temperature and enthalpy of air run VERY CLOSE to right along with one another, so 63° wet-bulb air will have an enthalpy of 28.3 BTU/lb at typical conditions, and we often use wet-bulb as a proxy for enthalpy.

Absolute Humidity/Moisture

We can calculate the total moisture in the air in pounds or grains of moisture per pound of dry air. This is the TOTAL quantity of evaporated water in a pound of dry air by weight and shouldn't be confused with relative humidity.


Here is the part I want to get to:

When we remove water from the air with an air conditioner or a typical dehumidifier, we are cooling the air sensibly until it hits the dewpoint. The evaporated water in the air will then begin to condense on the surface of the evaporator coil. It will then give up latent heat to the coil because the coil temperature is below the dewpoint temperature (at least most of the time on most systems).

When we dehumidify by cooling in an air conditioner, we are dropping the enthalpy, temperature, and absolute moisture of the air all at the same time, and all of that combined heat is entering the coil.

In a dedicated dehumidifier, we do the same thing but then run that air back over the condenser to add back enthalpy via sensible heat. So, the end result is less total moisture with higher air enthalpy to prevent overcooling.

What happens if we humidify or dehumidify the air by increasing or decreasing the total moisture WITHOUT adding or removing heat? This does (mostly) happen with evaporative (swamp) coolers and desiccant dehumidifiers.

When the humidity of air changes WITHOUT a change in enthalpy (total heat content, sensible + latent), the temperature of the air also changes. As a result, this is called an adiabatic process. Adiabatic simply means there's a change in temperature without a change in total energy/heat content within a system.

When you simply add or remove grains or pounds of water vapor to the air, you would obviously change the enthalpy of the air UNLESS the temperature changed to compensate. This may sound like crazy science, but we experience it every day.

The inside of your body is about 98.6°F, but the outside of your skin is cooler than that—often more like 93°F. Let's say it's 100°F in Phoenix with 40% relative humidity. We know that hot goes to cold, so the heat from the air is headed into your skin, and your body reacts by beginning to sweat.

What temperature is the sweat as it leaves your body? Well, it would need to be somewhere between 93°F-98.6°F, right?

So how can 93°F sweat cool you?

It cools you because as it evaporates into the air, the water in your sweat takes energy to make that change, maintaining enthalpy (total heat) in the air around your skin but dropping in temperature.

If you were to measure the wet-bulb or enthalpy change entering and leaving a swamp cooler (evaporative cooler), you would notice that it stays (mostly) constant. However, the temperature of the leaving air is still lower. (Learn more about swamp coolers by reading this article about them.)

Take a look at the two sets of air conditions shown above. If you were to use a desiccant dehumidifier that could decrease the total moisture content of 75°F air from 64.66 grains to 33.88 grains (per lb of dry air) without any exchange of energy (constant enthalpy), the temperature of the air leaving that dehumidifier would increase by 20°F to 95°F. This can and does occur in desiccant dehumidifiers every day.

Now…

Real-life processes rarely abide by idealistic conditions plotted on a psychrometric chart. If the water is at a different temperature than the surrounding air in a swamp cooler, there will be an enthalpy change. If the desiccant wheel is at a different temperature than the air passing over it, there will be an enthalpy change.

The cool thing here is gaining a deeper understanding of the relationship between dry bulb temperature, enthalpy, and total moisture content of air by understanding some of the edge cases many of us don't experience as often.

Evaporation by itself leads to lower sensible air temperature.

Condensation by itself leads to higher sensible air temperature.

—Bryan

 

 

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