Humidifying and Dehumifying

When we remove water from the air with an air conditioner or a typical dehumidifier, we are cooling the air sensibly until it hits dewpoint. The evaporated water in the air will then begin to condense on the surface of the evaporator coil and will 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.

Changing Air Humidity, but Not Temperature

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 dessicant 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 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 98.7°F but the outside of your skin is cooler than that, often more like 93°. Let’s say it’s 100° in Phoenix and 40 percent 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? It would need to be somewhere between 93° – 98.7F right? So how can 93° 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 but the temperature of the leaving air is still a lower temperature.

If you were to use a dessicant dehumidifier that could decrease the total moisture content of 75° 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° to 95°. This can and does occur in dessicant dehumidifiers every day.

To Sum Up

Rarely do processes in real life abide by idealistic conditions plotted on a psychrometric chart. If the water is a different temperature than the surrounding air in a swamp cooler, then there will be an enthalpy change. If the dessicant wheel is a different temperature than the air passing over it, then there will also 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.