Air conditioners (ACs) and HVAC systems are essential in many facilities and homes today. These systems provide cold air inside an enclosed space by removing heat and humidity from the indoor air and keeping the space from feeling stuffy.

However, while ACs come in various shapes and sizes, they all have the same basic components and operate on the same premise. In addition, there are several factors an engineer must consider before choosing an air conditioning unit for a particular application.

Figure 1: Air conditioners come in various shapes and sizes, but they all have the same basic components and operate on the same premise. Source: zetha_work/Adobe StockFigure 1: Air conditioners come in various shapes and sizes, but they all have the same basic components and operate on the same premise. Source: zetha_work/Adobe Stock

Basic construction and working principle of an AC

An AC is made up of four essential parts:

  • Evaporator
  • Compressor
  • Condenser
  • Expansion valve

The operation of the AC relies on the ability of a refrigerant to absorb heat once converted from a gaseous state to a liquid state. And all of these components work together to manage the refrigerant and provide cool air inside an enclosed space.

#1 The evaporator

The evaporator is the part of the system typically found in the indoor unit in a mini-split AC. It is made up of coils through which low-temperature refrigerant flows and a fan that draws the warm air from the indoor space to the coil. As a result, the low-temperature refrigerant absorbs heat from the indoor space, cooling the air. The fan then blows this cool air back into the indoor space.

Refrigerant usually enters the evaporator coil as a low-temperature liquid but leaves as a vapor due to the heat absorbed from the surrounding warm air. Therefore, there needs to be a way to change the refrigerant to its original state (low-temperature liquid) before it returns to the evaporator coils. This original refrigerant state can be achieved through a series of additional processes performed by the compressor, condenser and expansion valve.

#2 The compressor

The compressor, which is usually located in the outdoor unit of central and split AC units, is the heart of an AC unit. And as its name implies, a compressor compresses the gaseous refrigerant coming from the evaporator coils. This operation raises the pressure of the refrigerant, which changes the refrigerant to a superheated vapor before the refrigerant enters the condenser.

[Learn more about air compressors on]

#3 The condenser

The condenser of a typical AC unit is quite similar to the evaporator; they feature hollow coils through which the refrigerant travels. However, unlike the evaporator coils that absorbs heat from the surrounding, the condenser coils absorb heat from the refrigerant and eject it to the outside environment. Condensers are typically located in the outdoor unit of ACs.

The operation of the condenser on the refrigerant causes it to change its state from superheated vapor into hot liquid. This hot liquid refrigerant is then made to pass through the expansion valve.

#4 The expansion valve

The expansion valve regulates the amount of refrigerant going inside the evaporator. However, remember that the refrigerant coming from the condenser is still at a high temperature. The expansion valve reduces the pressure and temperature of the refrigerant, changing its state to gas. The gaseous, cooled refrigerant goes to the evaporator to repeat the air conditioning process.

Basics of sizing air conditioners

The cooling capacity is an important parameter used in choosing an AC for an application. Cooling capacity measures the air conditioner’s ability to remove heat from an enclosed space, measured in Tonnage (ton) or BTU (British Thermal Unit).

ACs used for cooling small buildings usually have a tonnage rating between 1.5 tons and 5 tons, while commercial units can have a rating of up to 30 tons. To make the best choice, engineers can take these three basic steps.

#1 Calculate the room size

The first step to take is to calculate the area of space the AC is required to cool. For rectangular or square-shaped rooms, this can be done by simply multiplying the length and width of the room, as shown below.

However, engineers might want to divide the room into small sections and sum up all the areas for complex shapes.

#2 Estimate the tonnage required

The U.S. Department of Energy recommends that an AC needs 20 BTU (approximately 0.0016 tons) for each square foot of living space. Therefore, an ideal AC size for a 900 sq ft room would be approximately 1.5 tons, as shown below:

#3 Consider the number of people in the space

The human body generates heat, which tends to add unwanted heat gain to the cooling space. Therefore, it is recommended that engineers increase the cooling capacity of an AC depending on the number of people that will be in the space. For instance, engineers can add 0.0317 tons (or 380 BTU) per person that will be in the cooling space.

Other factors that engineers must consider when choosing a system for an application include the number of windows in the space, the height of the room, and the region’s general climate and humidity levels. Therefore, engineers are advised to reach out to AC suppliers to discuss their application needs.

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