In order to understand how hvacr systems work, it is important to familiarize yourself with the fundamentals of heat. For example, air conditioning systems work by taking heat from one area and placing it in another. To be more specific, your air conditioner is taking indoor heat and placing it outdoors. Many people believe that their a/c units are pumping cool air into their homes, when really they are only displacing the heat.

Laws of Thermodynamics

First, you should know the two laws regarding heat. These laws, known as the laws of thermodynamics, are the frameworks for heat. When you know these laws, you will understand how to control and move heat.

The First Law of Thermodynamics: Energy cannot be created or destroyed, it can only be converted from one form to another. This means that to generate heat, it must be taken from another form of energy. An example would be electrical energy converted into heat energy for an oven or a hot water tank.

The Second Law of Thermodynamics tells us that heat always travels from a hot object to a cooler object. One way to illustrate this would be putting a spoon into water that has been boiled. The spoon will begin to get warmer because it absorbs the heat in the water.



Moving Heat Along

Heat can travel in three different ways.

Convection: This is when heat moves due to a carrier, such as air or water. The principle of convection is used in forced-air heating systems, where the furnace heats the air and moves it through the ducts, and in hydronic systems, in which water conveys the heat.

Conduction: This happens when heat moves through a substance. The heat starts at one end of an object and travels to the other end. Types of metal are good at conducting heat.

Radiation: This is the travel of heat through waves. A good example would be rays from the sun heating an object on earth.



Physical States

Although heat travels in different ways, it is all done through molecules. Molecules are the smallest particles of a substance. The state of an object, whether it is a solid, liquid, or gas, depends on the movement and speed of the molecules.

When water, for instance, is heated or frozen, the molecules are forced to rearrange themselves into another pattern. This new pattern creates the new physical state.

  • When we freeze water, the molecules slow down to the point that the water hardens into ice.
  • If we want to turn water into a gas or steam, we must heat the water to the point that the molecules are moving so rapidly that the change occurs.

    • These changes in physical state have more technical terms:

    Fusion: Water to ice or ice to water.

    Vaporization: Water to vapor or vapor to water.

    Sublimation: Vapor to ice or ice to vapor.

    There are also two different kinds of heat that change the properties of an object:

    Sensible heat — This occurs when the temperature of an object changes, but not its physical state. For example, if you cool water to the point that its temperature drops but it does not change to ice, the change is in its sensible heat.

    Latent heat — This occurs when the physical state of an object changes but its temperature does not. This may sound confusing, but here is one way to look at it. If you fill a glass with ice and let it melt, the temperature will not change until all of the ice has melted. The combination of ice and water as the melting process continues will stay at the same temperature.

    There are four types of latent heat:

    Latent heat of fusion — Process of changing a liquid to a solid.

    Latent heat of melting — Process of changing a solid to a liquid.

    Latent heat of vaporization — Process of changing a liquid to a gas.

    Latent heat of condensation — Process of changing a gas to a liquid.



    Other Terms to Know

    Saturation point:The point when an object has enough pressure and temperature to change its state.

    Superheat: A temperature above the saturation point of an object.

    Subcooling: A temperature below the saturation point of an object.

    Specific heat: The amount of heat needed to raise the temperature of an object one degree Fahrenheit (1 degrees F).

    Absolute zero: The point at which all of the heat has been removed from a substance. This happens at -460 degrees Farenheit or -273 degrees Celsius.

    Publication date: 08/20/2001