Just How Does an Air Conditioning System Work?

Air conditioning systems are truly marvels of engineering, and here is how the work.
Christopher McFadden

If you live in a hot climate there is nothing better than keeping cool using an air conditioning system. But how exactly do they work?

Here we attempt to answer this very question and explore what types of AC systems exist. As Heating, Ventilation and Air Conditioning (HVAC) is a very complex engineering field, we should note that this is not intended to be a comprehensive guide and should be considered as a brief overview. 


How does an air conditioner work?

In short, they work just like your average kitchen refrigerator. The technology behind both air conditioning systems and fridges is exactly the same - the refrigeration cycle

Systems that take advantage of this cycle use special chemicals called refrigerants (water in some systems) to absorb and/or release energy to either heat or cool some air. When these chemicals are compressed by the AC unit's compressor, the refrigerant changes state from gas to liquid and releases heat at the condenser.

When cooling a space, this process occurs outside the space in question. This high pressure, now cool air is pumped to the indoor unit and expanded back to gas using the system's expansion valve.

This, as the name suggests, causes the refrigerant liquid to expand back into gas form. As the refrigerant expands it "sucks" in heat and causes the air to cool in the space in question at the AC system's evaporator

This now-expanded and "hot" gas are further transported to the system's compressor and the cycle begins all over again. 

To visualize this, think of a sponge as the refrigerant and water as "heat". When you squeeze a sodden sponge (the compressor and condenser), water is pushed out and heat is released in our analogy. When you let go of the sponge (the expansion valve and evaporator) it expands and can then absorb more water or heat in our analogy. 

The basic underpinning of this cycle is the scientific principles of Thermodynamics, Boyle's Law, Charles' Law, and Guy-Lussac's Laws.

Primarily the fact "a liquid expanding into a gas extracts or pulls heat from its surroundings." - Goodman Air Conditioning, and Heating

In this sense, AC and fridges work by "moving" or "pumping" energy from one location to another. In most cases, AC units will transfer the "heat" from your room, office or home, and dump it into the air outside your house or office.

how air conditioning works 1
Source: Pixabay

This cycle is reversible and can be used to also heat your room, or your entire home, during colder months but this function is usually reserved for systems called heat pumps

The main difference between a refrigerator and an AC unit is that the unit tends to be split into two discrete parts; the outside condenser (or chiller) unit and the indoor unit.

Refrigerators, on the other hand, are one self-contained unit (though some AC units can also be).

Any heat removed from its interior is dumped into the same room at the rear of the unit. This is the main reason you could never really use a fridge as a DIY AC unit; unless you cut a hole in the wall behind it, of course. 

You can test this by touching (be careful, it can get very hot) the back of the fridge when its in operation. It should feel warm, or hot, to the touch. 

What different types of air conditioning systems are there?

AC units come in many shapes and sizes today, ranging from massive ducted systems in offices and industrial buildings to smaller domestic AC systems that you are probably more familiar with. 

Some of the larger installations have very large outdoor chiller units that can be water or air-cooled or, in older systems, cooling towers. These are linked by insulated pipes to pump refrigerant to temper the air within a large, or set of large, packaged units called air handling units (AHU's).

These systems can be very complex with heating elements and humidifiers, and filters to control very precisely the temperature and air quality for the areas in a building they serve. They also tend to come with complex heat recovery systems to decrease the amount of electricity (or gas) needed to heat/cool air within the system.

They come in two main forms; Constant Air Volume  (CAV) and Variable Air Volume (VAV), which defines the extent to which airflow is controlled around the system's ductwork. 

These can also be controlled by very sophisticated systems of software, sensors, and actuators called Building Management Systems (BMS)

These large HVAC systems "suck" in fresh outdoor air and heat/cool it as necessary before transporting it through ductwork to the areas required. These systems can also have terminal reheat units, or fan coils, to further refine the tempering of supplied air to an area.

More modern installations do away with centralized AHU's in favor of fan coil systems, or "indoor units" that are directly linked to one or more "outdoor" AC units. These are termed Variable Refrigeration Flow (VRF) systems which temper the air directly at the point of use.

But most people are used to split- or multi-split air source heat pumps (ASHP) or single room cooling AC units. These are much more akin to refrigerators and are the systems most often installed in domestic premises.

But it should also be noted that various other systems exist, using the same principle, like ground source heat pumps (GSHP). These use the ground as a "dumping ground", or source of heat,  instead of air or a source of heat. Both ASHPs and GSHPs can also connect to regular radiator systems or underfloor heating systems in place of a regular gas boiler with some modification.

How does air conditioning work in cars?

Put simply, AC in cars works exactly the same as any other AC unit. The only difference being they need to be quite compact in order to fit inside the car.

The chiller part of the system (with the expansion valve and evaporator) is usually mounted behind the car's dashboard. The other business end of the system (the compressor and condenser) tends to be sited near the car's radiator grille - - this is where fresh air is blown in as you drive along). 

Both parts are connected by a circuit of pipes that pass the refrigerant between the units when in operation. Unlike larger units used in buildings, the unit itself in cars tend to be powered by the car's crankshaft, in other words, it is powered by the engine. 

These systems usually also come with heat and dehumidifiers to temper the air as needed. Just like building AC systems, a car's AC unit will convert the coolant between gas and liquid, high and low pressure and high and low temperature as required.

Is it cheaper to leave the air conditioner on all day?

Put simply, no. The reason for this is that by leaving an AC system all day will: 

1. Use energy unnecessarily if you are not at home or rooms/zones are not in use.

2. Running the system puts wear and tear on it. This shortens the lifespan of it. 

You should also ensure windows are closed, or draught-proofing is installed when the AC is in operation. You don't want to "air condition" the world after all. 

You should also ensure that you make use of shading devices (like an awning or strategically planted trees) externally to reduce "solar gain" or heating of your home passively from sunlight. 

Other measures include improving the insulation of your home, keeping AC systems well maintained (especially filters), and using ceiling fans to improve air mixing internally (i.e. preventing stratification of hot air near the ceiling or vice versa).

If you are really concerned about your energy bills related to your AC systems, you might want to make your AC system "smarter". Making use of domestic BMS, smart sensors  (thermostats and weather compensation), zone-control and other energy-efficient measures, you can drastically improve the efficiency and reduce the cost of your AC systems.

You should also make use of "free" cooling and heating solutions by considering using nature to help you. Good use of natural ventilation to either cool down or heat your home will dramatically cut the cost of heating/cooling related energy use by turning it off.

But this is only possible if the air quality outside your home allows for this. Living in a big city with "dirty air", for example, might restrict your ability to make use of this free form of heating and cooling.  

How does reverse cycle air conditioning work?

Reverse cycle air conditioning systems, or heat pumps as they are more commonly known, work much the same as any other form of AC unit. The exception being that they are specifically designed to be able to reverse the cycle at will. 

Like other AC systems, they can also filter and dehumidify the air as needed. 

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