Change of State
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Table of Contents
The Physics Behind Air Conditioning
Get a large gathering of automotive people together and ask the group, “How does air conditioning work?” and you will hear a lot of chatter about “Freon” and pressures but very few people really understand how to make the cabin of a car cold. Actually you can’t make cold, you have to remove the heat from the cabin to make it cold. More about that later, but first we need to start at Ground Zero, the magic behind latent heat of vaporization.
Change of State (Solid > Liquid > Gas)
| Section | State | BTU | Section Details |
|---|---|---|---|
| A-B | Solid | 32 | In this section, a one pound block of ice (solid) is 0oF. 32 BTUs of heat are added to bring the ice up to 32oF. The heat added in this section is called sensible heat since the added heat caused a rise in temperature. |
| B-C | Solid > Liquid | 144 | In this section , the ice is holding at 32oF as it is changing from a solid state to a liquid state. 144 BTUs of heat are added in this section to change the block of ice at 32oF to all water at 32oF. The is called latent (hidden) heat since the added head is not causing a rise in temperature. |
| C-D | Liquid | 180 | In this section, 180 BTUs of heat are added to bring the water from 32oF up to 212oF. This is called sensible heat since the added heat caused a rise in temperature. |
| D-E | Liquid > Gas | 970 | In this section, the water is holding at 212oF as it is changing state from a liquid to a gas (steam). 970 BTUs of heat are added in this section causing the water at 212oF to change over to all steam at 212oF. This is again called latent (hidden) heat since the added heat is not causing a rise in temperature. |
| E-F | Gas | In this section, heat is added to take the gas (steam) up to a higher temperature. This is called sensible heat since the added heat is causing a rise in temperature. |
When you look at this information one number ought to really jump off the board, the 970 BTUs it takes to change a pound of water to a pound of steam. The latent heat of vaporization is a huge number and this is the secret behind moving heat. We just need to build a system that will change a liquid to a gas inside the car and then pump it outside and change it back to a liquid. That is exactly what air conditioning systems do!
The “Car Cooler”
Between the 1930s and the 1960s, the “Car Cooler” was available as an aftermarket accessory. It hung in a side window, as shown in Image 1 below. While traveling, outside air would flow through the unit which contained water. The water would evaporate and the change of state would pull heat from the air which would then blow into the cabin of the car.
These units were marginally effective and worked better in hot dry climates which made them popular in the southwestern states of California, Arizona, West Texas, New Mexico, and Nevada. They were ultimately obsoleted by the air conditioning systems that are in use today.
Basic Air Conditioning Cycle
This is a basic explanation of an automotive air conditioning. The goal is to show how the change of state is used to cool the cabin of a car. In reality, automotive heating, ventilating, and air conditioning HVAC systems are complex; however, it helps if you understand the core physics behind it all, the change of state.
| # | Part | Section Description |
|---|---|---|
| 1 | Condenser | The condenser (1) is located on the high-pressure side which starts at the compressor (4) and ends at the restrictor (2). The compressor pumps refrigerant, in the form of a gas, to the condenser where ambient ram air pulls heat out of the refrigerant which then condenses back to a liquid. |
| 2 | Restictor | The restrictor (2) splits the AC system into the high-pressure side from the compressor (4), through the condenser (1), to the restrictor, and the low side that goes from the restrictor through the evaporator (3) and back to the compressor. As the refrigerant goes through the restrictor it goes into the low-pressure side where it can start changing its state to a gas. |
| 3 | Evaporator | The evaporator (3) is located in the cabin of the car. The refrigerant changes state from a liquid to a gas inside the evaporator as it takes on the latent heat of vaporization that was covered in detail earlier in this lesson. |
| 4 | Compressor | The compressor, along with the restrictor, splits the air conditioning system into a high and low pressure closed circuit. The refrigerant leaves the evaporator still on the low-pressure side and enters the compressor where it is pumped onward to the condenser to restart the process. |
Engine Cooling System (Keeping it a Liquid)
The liquid-based cooling system for an automotive engine has always operated above the boiling point and in recent history has continued to rise even more. To keep the coolant predominately water-based, the following features are utilized.
Coolant Solution
While straight water boils at 212 degrees (F), a 50/50 mixture of water and coolant boils at approximately 225 degrees (F).
Pressurized Cooling System
While an air conditioning system reduces the pressure of a liquid to lower the boiling point and force a change of state, with the engine’s cooling system the exact opposite is done.
The engine’s cooling system is pressurized to raise the coolant’s boiling point to approximately 250+ degrees (F). Note the radiator cap shown in the image to the left is rated at 22 PSI. Always replace radiator caps with the correct pressure as defined by the manufacturer.