Evaporative Cooling

[johnhilgeman]

Hi all, I am actually an architect testing for my licensure. I am using a Kaplan study guide and I am really stuck on thier explaination of how evaporative cooling works and I am feeling that it is oversimplified.

My understanding of a swamp cooler is that a fan passes air over moistened strips of cloth - the moisture absorbs heat and evaporates, dropping the temperature of the cloth strips and forcing moist air out into the space. My question is how exactly, on an atomic level, does the air itself feel cooler? It would seem that the new air has a lot of the heat trapped as latent heat, and the rest of the air is cooler (which we feel).

Kaplan explains that simply raising the relative humidity makes a person feel cooler. They also say that there is no change in the total heat of the air (which makes sense as some of it is trapped as latent heat). However, it seems that it is not the humidity itself that makes a person feel cooler, but the fact that it has been specifically used to hold heat in a form that is not sensory.

Is this analysis close to correct? I would love any explanation or input.

oh, here is another statement from Kaplan that confuses me: "The moist air is then delivered to the indoor space, where it cools the body and helps evaporation of body moisture." It would be great if anyone could explain how that works!

 

[rmw]

You need to google " Psychrometric Chart " and find a link that explains how to use it. That will help you understand what is going on in the swamp cooler. I don't understand Kaplan's statement because I live in the deep south of the USA where it is humid (and swamp coolers don't work) and the way we make air feel cooler, or more comfortable, is to dry it so it will evaporate more off of the skin. The last thing we would want to do is moisten it. Maybe people who live in very dry climates will have an answer for what Kaplan has stated.

rmw

 

[zdas04]

I live in northwest New Mexico (elevation 5,300 ft) and the relative humidity is generally between 5% and 20% (right now at 11:00 pm, it is 73F, 6% RH, and a 1F dewpoint according to Yahoo), swamp coolers work wonderfully and amazingly cheaply (normal cooling bill on the hottest month of the year is $50/month).

The evaporative cooler can only get about a 15-20F depression in temperature (i.e., on a 100F day the air out of a properly functioning cooler will be 80-85F) which can sometimes be less than enough. Most days it is downright cold.

The warm dry air is pulled through saturated pads (almost never "cloth", mostly people use aspen bark or a blue synthetic material with a huge surface area for water to contact the air). Some amount of the water vaporizes (look up the enthalpy of water at 70F and 13 psia, then vapor at 80F and 13 psia, the difference is the latent heat of vaporization that left the air stream, lowering its temperature, you may need a psychometric chart to get the values). The cooled, moister air is blown into the house through (in my case) central HVAC ducting and you get a wind chill of higher humidity air that is quite cooling. The extra humidity increases the specific heat of the air and it is able to transfer more heat away from a body or other heat source (than the drier air would).

They work especially well at elevation because the low-pressure low-humidity air up here has very poor convective heat transfer properties--raising the pressure a few mBar in the fan and adding 20-30% humidity improves those properties considerably.

I haven't calculated it, but I would guess that the maximum RH where evaporate coolers are effective would be around 50% RH.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[Compositepro]

Your thinking is "fuzzy" because you don't clearly understand the meaning of the words you are using. It seems that Kaplan, too is fuzzy. You "feel" is temperature, which is a measure of the kinetic energy in molecules. Latent heat is not heat, but the energy required to change a liquid to its gas phase with no change in temperature.

The important principle in evaporation is that at any given temperature liquid water has a certain vapor pressure. If you put water in a chamber with no air, then the pressure in that chamber will be equal to the vapor pressure of water. now if you let air into the chamber it does not change the water vapor pressure it just mixes with it. The relative humidity of that air will be 100%. If you blow air through the chamber, water will evaporate to try to re-establish the equilibrium vapor concentration. Due to the "heat of vaporization" (latent heat), the temperature of the water will fall. This cools the air in contact with the water.

Humid air will not feel cool because it is humid. Air feels cool when it is removing heat from your skin. It will remove heat by being lower in temperature and flowing across the skin (convection or "wind-chill"). Evaporation of sweat will significantly enhance cooling. With higher humidity, of course, evaporation is less effective at cooling. Many have a mistaken belief the fans always cool. If the air they are moving is hotter than an object the fan will heat that object, so fans are used in ovens, too. Sweating does allow the body to cool below ambient air temperature and is simply nature's "swamp cooler".

When air passes through a swamp cooler there is essentially no change in the total energy of the air and water involved, but the temperature drops. You are being confused in this case by calling energy heat.