Air Conditioning and Physics 101

Jan. 1, 2020
Simply put, Bernoulli's Equation states that when the velocity of a fluid increases, its pressure decreases. This explains what is known as the venturi effect. For the mathematicians among us the equation is:

What does a fellow named Daniel Bernouilli who developed an equation back in 1738 have to do with air conditioning?

Simply put, Bernoulli's Equation states that when the velocity of a fluid increases, its pressure decreases. This explains what is known as the venturi effect. For the mathematicians among us the equation is:

Where P is pressure; p is density; v is velocity; g is acceleration of gravity (32 ft./sec./sec.); and h is height.

If you want to work the math, be my guest. I'd rather let the engineers do that, because all I need to know is the principle behind the venturi effect. That's what makes an old carburetor work and is what provides the lift to an airplane wing.

But what does all that have to do with air conditioning? To understand the answer to that question let's delve a little bit into some definitions and principles of the science of p:

1. A fluid is defined as a substance that conforms to the shape of its container. Gases and liquids are fluids.

2. Gases can be compressed while liquids are virtually incompressible.

3. When a gas is compressed, its temperature increases.

4. There are three ways heat can be transferred:

A. Radiation – indirect transfer, as when something close to, but not touching, a hot exhaust manifold gets hot without air movement.

B. Conduction – direct transfer, as when you burn your arm on a hot exhaust manifold.

C. Convection – by air movement, as when you blow on your arm after burning it on a hot exhaust manifold.

4. Heat always flows to less heat.

5. As the pressure on a liquid increases, its boiling point also increases.

6. As the pressure on a liquid decreases, its boiling point also decreases.

7. When a gas changes states to a liquid (condenses), it releases heat.

8. When a liquid changes states to a gas (boils), it absorbs heat.

Now, let's use these definitions and principles along with Bernoulli's Equation to see how an air conditioning system works.


An air conditioning system starts with a compressor where the refrigerant, which is in a gaseous state, is compressed to a high-pressure gas. From the compressor, the high-pressure gas is routed to the condenser, where the heat that it absorbed when it was compressed is transferred to the atmosphere – first by conduction and then by convection.

When the heat transfer takes place, the high-pressure gas changes states (condenses) into a liquid under high pressure. As you will recall, increasing the pressure on a liquid increases its boiling point, so the liquid will not boil until the pressure/temperature combination drops to the proper point.

This is where Bernoulli's Equation comes into effect. The volume of fluid flowing through the system in a given amount of time remains constant. Therefore, if a restriction is placed somewhere in the liquid section of the system, the velocity of the liquid must increase in order for the volume flowing to remain constant. When the velocity increases, the pressure decreases, which also decreases the boiling point.

Refrigerants have a low boiling point, hence the need for high pressures. The restriction in the line is normally either a fixed orifice of small diameter or a variable orifice whose diameter can be controlled either thermostatically or electrically.


As the velocity of the refrigerant increases and its pressure decreases, it reaches its boiling point. As it boils and changes back to a gas, it absorbs heat – a lot of heat. Most of this change occurs in the evaporator core where air is directed across the fins of the core and into the passenger compartment. Because heat always flows to less heat, the heat in the air that is directed across the evaporator core fins is transferred to the refrigerant by convection and conduction. This cooler air then absorbs the heat that is in the passenger compartment by convection. The refrigerant, now in a gaseous state, is routed back to the compressor where the whole process is repeated.

There are several other components used that enhance the various systems, such as mufflers that lower the noise level, accumulators that store any refrigerant not needed at the time and serve to dampen pulsations from the compressor, and driers that remove any moisture that may find its way into the system, eliminating orifice freeze-up and acid formation. But, right now we're talking about physics, not enhancements.


The volume of boiling refrigerant flowing through the evaporator core must be controlled to prevent ice buildup on the fins of the core. Remember how the hot gaseous refrigerant enters the condenser, is cooled, condenses and exits the condenser as a liquid? As the air passes through the cooling fins on the evaporator core, the lower temperature of the core causes the moisture in the air to condense, thereby drying the air. That drier air can absorb more moisture from our skin and thus feels cooler to us. That's why it's called an air conditioner not an air cooler.

However, if the temperature in the evaporator core drops below the freezing point of the moisture in the air, which will be somewhere around 32°F (0°C) depending on how many contaminants are in the moisture, the resulting formation of ice can inhibit or even totally block the flow of air through the fins on the core. The resulting loss of efficiency can be dramatic, especially if the vehicle is a dark color and the temperature and humidity are both approaching 100.

Today's automotive air conditioning systems use several different methods to control the flow of refrigerant through the system, and they each have their own particular strengths and weaknesses. In some systems the compressor runs constantly at a constant volume. The volume of boiling refrigerant in the evaporator is controlled by varying the size of the orifice in an expansion valve.

Most of these expansion valves are controlled thermostatically: A tube containing a substance with a relatively high expansion/contraction rate is connected to a diaphragm in the expansion valve and acts on a pintle valve. As the temperature in the evaporator core increases, the substance in the tube expands and opens the pintle valve, allowing a larger volume of boiling refrigerant into the evaporator core.

Conversely, as the temperature in the evaporator core decreases, the pintle valve closes down, allowing less refrigerant into the evaporator core. This type of system provides steady cooling and prevents shock loading of the compressor, belt and other accessories. But it adds a constant load to the engine and, therefore, lowers the fuel mileage.

A cycling compressor system uses a fixed orifice and a constant volume compressor that is turned on and off (cycled) to control the volume of boiling refrigerant that enters the evaporator core. The compressor is normally cycled by use of a pressure switch somewhere on the low-pressure or cold side of the system.

When the pressure in the low-pressure side of the sys-tem drops to a predetermined level, the compressor is shut off, totally stopping refrigerant flow. This pressure drop can be caused by low temperature or a low refrigerant level. If the low pressure is caused by a low refrigerant level, shutting off the compressor will save it from damage due to oil starvation.

This type of system doesn't add a constant load to the engine, so the fuel mileage is not as adversely affected as it is with a constant run compressor. However, the cooling is not very steady. The temperature at the passenger compartment air outlets varies with compressor operation. There is also shock loading of the compressor, belt and other accessories every time the compressor engages.

In a variable displacement compressor system, the displacement of the compressor is varied to increase or decrease the volume of refrigerant flowing through the system. This type of system also uses a fixed orifice. When air conditioning is requested, the compressor runs constantly. However, the volume of boiling refrigerant required to maintain a steady passenger compartment temperature is controlled by varying the displacement of the compressor.

This system provides steady cooling and doesn't add the shock load factor, plus the load on the engine is varied so the effect on fuel mileage is as minimal as possible. However, the additional components required for the compressor and the compressor controls add significant cost to the system.


Any substance that is in its gaseous state under normal conditions can be used as a refrigerant, provided you can keep it under enough pressure to prevent it from boiling. During the great fluorocarbon debate, many substances were rumored to be under consideration as a replacement for CFC12, more commonly known as R-12: carbon dioxide (CO2), oxygen (O2), and nitrogen (N).

More than one can of beer has been chilled very quickly using Bernoulli's Equation and a CO2 fire extinguisher, but because the boiling point of CO2 at normal pressure is extremely low at -109.3°F (-78.5°C) it would take extremely high pressure to raise the boiling point enough to be used as a refrigerant. Imagine high-pressure refrigerant lines and a condenser under the hood of a vehicle that are as thick as and as heavy as the walls of a CO2 fire extinguisher. A compressor capable of developing that much pressure would rob quite a bit of power from the engine as well.

The boiling points of oxygen and nitrogen at normal pressures are even lower. O2 boils at -297.4°F (-183°C) and N at -320.4 °F (-195.8°C) While these gases may work as refrigerants, very few people would want to drive a Ford Focus that weighs as much as an Abrams Tank with a fuel economy to match.

As a result of the fluorocarbon debate, the use of R-12, as well as all other chlorofluorocarbons (CFCs) in new products, was banned. While the relatively high boiling point of R-12, -21.6 °F (-29.8 °C), means that the pressure required to keep it from boiling is low enough that its use as a refrigerant is feasible, when it is set loose in the lower atmosphere, it rises and depletes the protective ozone layer in the upper atmosphere.

Its use was banned in order to protect the lives of our children, grandchildren and all the generations to come. Whether you agree or disagree with the ban, it doesn't change the fact that R-12 can no longer be used except to fill original R-12 systems.

Luckily for the motoring public, there are other substances whose boiling points are in the correct temperature range and which are usable in motor vehicles. HFC134a, more commonly known as R-134a, was the compound chosen to replace R-12 in post-ban vehicles. It does not have near the ozone depleting characteristics of R-12, and with a boiling point of -15.7°F (-26.5°C), the pressures required are comparable.

While R-134a is more environmentally friendly than R-12, it will eventually be replaced by another substance. But, no matter what substance is chosen as a replacement, it will most likely rely on Bernoulli's Equation to provide conditioned air.

As you can see, physics plays a major role in air conditioning. But then again, physics plays a major in almost everything automotive. I have always found that understanding the basic laws and principles of physics involved in a particular vehicle system helps immensely when it comes to diagnosing that system.

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