One of the occupational hazards of an emergency vehicle technician (EVT) Trainer who emphasizes basic principles is the complaint that the information is not focused primarily on emergency vehicles. This happened to me many years ago when an evaluation sheet turned in by a student complained that my description of the internal workings of an alternator could have been about a Honda alternator and had no place in an EVT training class. No one likes to get a negative review, but it didn’t take me long to see the humor in it. Basic principles really are basic principles and until we get electrons to flow differently, most alternators will operate on similar principles.
The same goes for the HVAC System. The concept is how can we move the heat from inside the box to outside the box? It really doesn’t matter what the box is. It could be a refrigerator, a warehouse, a fire truck, or a classic car. The combination of ambient temperature, sun load, engine heat, and a few other things all add heat to the inside of the box. To understand how to remove it we need to start with this.
WHAT IS HEAT?
Oxford Languages online gives this about heat: “Noun; The quality of being hot, high temperature.” Now, I’m a strong proponent of dictionaries, but this might be the worst definition I have ever read. My old Funk and Wagnalls, which I usually like, is the same. Even worse is the infamous Websters Third: The state of a body or of matter that is perceived as opposed to cold. Yikes. What does perception have to do with a definition of a physical quantity that can be measured?
Then we come to The American Heritage Dictionary of the English Language; “A form of energy associated with the motion of atoms and molecules and capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation.” This is close to perfect. There is no substitute for a good dictionary. Everyone needs more than one.
Heat is molecular motion. What follows needs your undivided attention. You add heat to a balloon, and it expands due to the increased motion of the molecules of air. You add heat to an ice cube and the molecules move faster until they move fast enough for the ice to no longer maintain solid structure. It melts. You add heat to that melted water and the molecules move faster until they move fast enough to break through the surface tension. It boils.
All these examples are accurate because what we are used to is a fixed pressure system. It’s called an atmosphere. An HVAC system is not fixed pressure but fixed volume. More on that in a later article.
Understanding heat to be molecular motion is step one in understanding what is happening inside and HVAC system. Step two is understanding that changing state from solid to liquid and from liquid to gas is the Pac-Man of energy consumption. The energy seems to just disappear. For example, the quantity of heat needed to turn 212ºF water into 212ºF steam is astonishing.
Heat, for our purposes, can be measured in British Thermal Units (Btus). One Btu added to liquid water raises the temperature (a measure of the intensity of heat, not the amount of heat) of one pound of water by 1°F. Since the freezing point is 32ºF, and the boiling point is 212ºF, it takes 180 Btus to raise water from 32°F to 212°F.
However, if we want to boil that water, we don’t have to raise the temperature. Think about that. We could convert 212ºF water into 212ºF steam. No big deal, right? Wrong, this is a very big deal. Since raising the temperature from 211ºF to 212ºF only took one Btu, how much could it take to go from 212ºF water to 212ºF steam? We are not raising the temperature.
Did I say that the energy seems to disappear? It actually does disappear. We call this latent heat. Latent means hidden. This is added heat that cannot be measured with at thermometer. This is why most of the above dictionary definitions of heat were so wrong.
“You can add heat without changing the temperature.” When I teach HVAC, that statement is written on the board before class, and I refer to it for the next eight hours. To understand HVAC, you must get your head wrapped around this principle: that of added heat (latent heat) that does not raise the temperature. The latent heat accomplishes a change of state but is not measurable with a thermometer. Since liquid water and steam can coexist at 212ºF, the added heat serves only to convert more of the water into steam. Forgive the wordiness of this explanation, but understanding this concept is essential to progress down the path of system understanding. At the end of that path is the ability to look at the system, first visually, then with temperature tools, and finally with pressure gauges to discover what is wrong with any improperly functioning HVAC system on your ambulance, fire apparatus, or admin vehicle. They all work pretty much alike.
So, after raising the temperature of the water from 211ºF to 212ºF with one Btu, how much heat does it take to change one pound of water to one pound of steam? 970 Btus. It only took 180 Btus to raise the temperature from 32ºF to 212ºF. It takes more than five times as much to change a pound of water to steam. It is because of this property of refrigerant that we avoid either in an unwieldy large system or a 1,400-psi system. Next time I will cover what the HVAC system needs.
