Basic Principles of Air Conditioning

PRINCIPLES OF COOLING
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.

SENSIBLE AND LATENT 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.

CHANGE OF STATE
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.

HUMIDITY
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.

REFRIGERANT
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.

REFRIGERANT PHASE-OUT
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.

AIR-CONDITIONING SYSTEMS
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.