Basic Principles of Air Conditioning

You don’t have to be an air-conditioning expert to use Spot Coolers, but understanding the basics of air conditioning will help you understand how Spot Coolers work.

This section highlights some of the basic principles of air conditioning for those who would benefit from additional knowledge in this area.


Heat Transfer

Heat is a form of energy. Every object on earth has some heat energy. The less heat an object has, the colder we say it is. Cooling is the process of transferring heat from one object to another. When an air-conditioning system cools, it is actually removing heat and transferring it somewhere else. This can be demonstrated by turning on a Spot Cooler and placing one hand in front of the cold air nozzle and the other over the warm air exhaust. You will feel the action of the transfer of heat.


There are two forms of heat energy: sensible heat and latent heat.

Sensible heat is the form of heat energy which is most commonly understood because it is sensed by touch or measured directly with a thermometer. When weather reporters say it will be 90 degrees, they are referring to sensible heat.
Latent heat cannot be sensed by touch or measured with a thermometer. Latent heat causes an object to change its properties. For example, when enough latent heat is removed from water vapor (steam or humidity), it condenses into water (liquid).

If enough latent heat is removed from water (liquid), it will eventually freeze. This process is reversed when latent heat is added.


An object that changes from a solid to a liquid or liquid to vapor is referred to as a change of state. When an object changes state, it transfers heat rapidly.


Moisture in the air is called humidity. The ability of air to hold moisture directly relates to its temperature.

The warmer air is, the more moisture it is capable of holding. Relative humidity is the percentage of moisture in the air compared to the amount of moisture it can hold. A moisture content of 70°F air with 50% relative humidity is lower than 80°F air with 50% relative humidity.

When the humidity is low, sweat evaporates from your body more quickly. This allows you to cool off faster. High humidity conditions do not allow sweat to evaporate as well because the air is at its maximum capacity.

Humidity is also a form of latent heat. When air contains more humidity, it has more latent heat.


Refrigerants are substances used by air conditioners to transfer heat and create a cooling effect. Air-conditioning systems use specially formulated refrigerants designed to change state at specific temperatures providing optimum cooling.

Portables use a refrigerant called R-22 or HCFC-22. HCFC stands for hydrochlorofluorocarbon.This is currently the most common refrigerant used by air-conditioning systems.


Many of the current forms of refrigerants used today are being phased out based on concern for depletion of the ozone layer. Portables use R-22, which has been deemed acceptable for use by the EPA until the year 2010. By that time, an ozone-friendly refrigerant that can be easily substituted for R-22 will be readily available.


The system by which air conditioners provide cooling is called the Refrigerant Cycle. This system has four major components common to all air-conditioning systems (see figure below). These components and their basic functions are listed below.

1. Compressor

Refrigerant is drawn from the evaporator and pumped to the condenser by the compressor. The compressor also pressurizes the refrigerant vapor so that it will change state (condense) readily.

2. Condenser

The high-pressure refrigerant vapor releases heat through the condenser coils as it condenses into liquid refrigerant. making it easier to vaporize.

3. Metering Device

(capillary tube, txv valve) The metering device restricts the flow of liquid refrigerant from the condenser to the evaporator. As refrigerant passes through the metering device, its pressure decreases.

4. Evaporator

The low-pressure liquid refrigerant absorbs heat as it vaporizes in the evaporator coils.

The process described above is the Refrigerant System or Refrigerant Cycle. It is the system on which virtually all modern Air-Conditioning and refrigeration is based.


Airflow Diagram

Room air or ambient air is drawn in through the evaporator intake in front. The air is cooled as it passes over the evaporator coils and leaves through the cool air discharge. The condenser intake on the side or the back of the unit draws in air to cool the condenser coils. This warm air is then discharged out of the condenser exhaust on top.

Common Types of Air Conditioning Systems

There are many types of air conditioners specifically designed for different types of applications. Listed on the right are some of the more common systems and their applications.


The difference between a water-cooled air conditioner and an air-cooled system is in how the heat is removed from the condenser. Water-cooled systems flow water over the condenser coils to remove heat from the refrigerant inside, and air-cooled systems blow air across the condenser to remove heat.


These systems cool by pumping a liquid over evaporator coils to provide cooling. These units are used for large buildings and to provide direct internal cooling to special types of equipment.


The components of this system are separated. The condenser is located outside and is connected to single or multiple evaporator units inside. These evaporator units are connected to the condenser unit by small flexible hoses and do not use a central air duct. This system is used for residential and light commercial buildings.


All components are packaged together in one enclosure (e.g., window unit, rooftop unit, spot cooler).


This is a packaged system that mounts on the roof and connects directly to central ducting. It is typically used for medium to small office buildings and commercial buildings.


This is a small packaged unit designed to fit in a window, primarily for residential use.

(Central Air Conditioning)

The components of this system are sparated in two different locations.
The evaporator is located inside and provides cooling through a central air duct. The compressor and condenser are located outside. This system is most commonly used for residential applications.


These coolers do not use a refrigerant system but cool by evaporating water into the air. The units vary greatly in size and are used for anything from residential use to large industrial buildings. They work best in dry climates.