Tag: adiabatic

Air conditioning was about humidity control from the very start. Willis Carrier’s very first air conditioning system was all about controlling humidity with the side effect that it also could reduce the sensible temperature. Theaters caught on that this new fangled 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 (humidify) and remove water from the air (dehumidify) as part of our work but let’s take a different look at what happens when we do that, but 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 are measuring dry bulb temperature. It is a measurement of the average movement of the molecules in the air you are measuring with no consideration for the amount of water in it. We also call dry bulb temperature sensible temperature and changes in dry bulb, 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. This heat the moves during a change of state is called 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 by itself it doesn’t tell you how much energy or water is in the glass.

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 was 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 is can be used 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, in the USA that is usually BTUs per LB. 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 often we 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 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.

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°F.  Let’s say it’s 100°F in Phoenix and 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 – 98.7°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 but the temperature of the leaving air is still lower temperature.

Take a look at the two sets of air conditions shown above. If you were to use a dessicant 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 dessicant dehumidifiers every day.

Now…

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 than there will be an enthalpy change, if the dessicant wheel is a different temperature than the air passing over it then 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

 

 

Just so you don’t get bored and quit reading let’s go straight to the point.

When the blower runs for more than a few minutes after the system has cycled off in cool mode the air may continue to be “cooler” (lower sensible temperature) coming out of the supply but the heat content of the air will remain unchanged. 

The only reason I say “may” be cooler instead of “will” be cooler is that we are assuming there is moisture on the coil and/or in the pan and the indoor RH is less than 100%.

Translation: When you run the blower once the system has gone off in cool mode you will continue to cool for a while, but that extended cooling comes from the evaporation of water out off of the coil and out of the pan. This results in sensible cooling and greater sensible efficiency but also increased indoor humidity.

Translation of the translation: It may feel cooler but there ain’t any less heat in the air by the time you figure for humidity.

Translation of the translation translation: If you live in a humid place run shorter off-cycle run times and think twice before running the fan in the “on” position. If you are in a dry place then let it blow until your heart is content.

Whenever cooling occurs by direct evaporation of a substance into an airstream (think a swamp cooler) it occurs at no net decrease to the heat content in the air. The heat is just going from sensible (what you can measure with a thermometer) to latent resulting in higher relative humidity air.

If you go below this line it is going to get nerdy… BEWARE


Now let’s talk about why, but first some terms.

Heat = Molecular energy or total molecular movement within a substance
Temperature = Molecular velocity, the speed that the molecules are moving
Adiabatic Process = A change in temperature without a change in heat content

Think of adiabatic process like this – You have a whole room full of ping pong balls bouncing around in a zero-gravity room. The balls are molecules, their total motion is the amount of heat and the speed they move is temperature. If you were to change the size of the room by bringing in one of the walls the balls the balls would bounce faster because the available space was decreased so the “temperature” would increase but the number of balls and the total motion would remain constant (this is what happens to refrigerant in a compressor by the way). If you were to move a wall outward and increase the size of the room the speed of the of the molecules would decrease, resulting in less speed and lower “temperature”. All the while the number of balls and the total motion remained constant (which is what occurs at the outlet of the metering device). In both of these examples temperature (Sensible heat) changes but the total heat content does not change, these are both examples of an adiabatic process due to compression and expansion of contained molecules.

An adiabatic process can also occur in uncontained systems like open airstreams, and evaporation of water is one such example.

Evaporation of water is a process where heat is absorbed into water molecules as they evaporate from liquid water and become entrained in the air as a vapor displacing some of the nitrogen and oxygen in the air. When that heat is absorbed from the air into the water it results in lower sensible temperature, but the water is still CONTAINED IN THE AIR. This means that while the air may be cooler it still has all the heat contained in it in the form of water vapor.

Now for the real shock..

Water vapor is NOT more dense than dry air at the same temperature it is actually less dense / lighter than dry air, however, is does contain more heat (enthalpy for you nerds like me). This means that when you run the blower after a cooling cycle the moisture on the coil and in the pan are evaporated back into the space and depending on the RH of the air it will lead to sensible cooling but latent gains. This means cooler but higher RH and this is due to the higher heat content of higher RH air at the same temperature.

Once again, depending on where you live this may be positive or negative.

In Arizona or Colorado? Run that blower after the cooling cycle.

Florida? May wanna shut it off right after the cycle or maybe 90 seconds at most and leaving the fan in the “on” position will likely result in a small increase of indoor RH.

— Bryan

 

 

P.S. – I also did a Facebook Live Video about it today

also… Here are some great videos on the subject by Jim Bergmann

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