Why do you feel cold in Arizona

“I am cold!” We have all said this when we weren’t as warm as we wanted to be.  Temperature is an important part of the overall air balance.

Heat is actually a form of energy that is produced by molecular motion or agitation. The more you agitate the molecules the more heat is produced.  Different people feel cold at different temperatures but most people feel cold when the temperature around them gets near 70 degrees Fahrenheit.

Cooling is like heating because heat continues to be transferred from a higher temperature to a cooler one.  In cooling, the heat travels from the body, which is warmer, to the air which is colder.

There are a number of terms used in a discussion of cooling which we use in measuring changes in the state or temperature of air, water or other materials

First British Thermal Units (BTUs) are a way of measuring heat. One BTU may be defined as the amount of heat required to raise the temperature of one pound water one degree F. Most of this is irrelevant to our discussion.

Sensible heat is the heat which actually changes the temperature of a substance without changing the original form.  We add or remove sensible heat when we heat or cool air.  Sensible heat is measure in degrees on a normal thermometer.  This is called “Dry Bulb” Temperature.

Latent heat will change the form of a substance without changing the original temperature.  You can think of the moment heat changes water into steam.  An example is the cooling which causes water vapor in the air at 75F to condense into the form liquid water at 75F. Another example of latent heat is the cooling required changing liquid water at 32F into ice at 32F. Cooling the liquid water from 75F down to 32F requires the removal of sensible heat not latent heat.

It requires more heat to be removed to change water vapor into liquid water or liquid water into ice than to merely change its temperature. For example 144 BTUs must be removed from 1 lb of liquid water at 32F into ice at 32F.

Heating systems require us only to think about sensible heat. Cooling systems however require us to consider both sensible and latent heat in before sizing the appropriate piece of air conditioning equipment to do the job.

A cooling system must cool the air and remove the moisture from it to maintain a constant relative humidity and a comfortable level to the occupant or piece of equipment. To remove this moisture from the air it requires changing the state of water from gas to liquid. This means that more BTUs will be required in cooling air than in heating air.