• SkillCat Team

Refrigeration Cycle

Updated: Apr 30

EPA 608 Core Chapter 4

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Essential Chemistry

In this module, we will go over concepts in chemistry that will help you better understand how HVAC systems work. Skip to quiz!


Atoms and Molecules


Recall that an atom is the basic building block of all things. Everything around us can be broken down into their smallest unit of atoms. You can think of atoms as lego blocks that can be arranged to form everything we see around us.


Clusters of atoms are known as molecules. If atoms are lego blocks, molecules would be a stack of lego blocks in a particular quantity and order.


For example, a water molecule, H2O, is made of

- Two hydrogen atoms, and

- One oxygen atom.


This particular arrangement of two hydrogen atoms and one oxygen atom is what makes the molecule water. If we had two hydrogen atoms and two oxygen atoms, this molecule would no longer be water. So it is the unique combination of atoms that makes the molecule what it is.


Physical States of Matter


Everything in the world is made of matter, from your TV, to the oceans, to the air you breathe. Matter is anything that has a volume and a mass. Matter is made up of atoms and molecules.


Matter exists primarily in three states:

- Solid

- Liquid

- Vapor (Gas)

Note that gas and vapor refer to the same state of matter. For purposes of the EPA exam, we will refer to this state as just vapor.


The melting point is the temperature at which a solid changes to liquid. For example, the melting point of ice is normally 32°F. This means that at any temperature below 32°F, ice will stay solid. But if the surrounding temperature is 32°F or higher, the ice will start to melt, changing state from solid to liquid.


The boiling point is the temperature at which a liquid changes to gas. The boiling point of ice is 212°F or 100°C. This means that at any temperature below 212°F, water remains liquid. For example, at a room temperature of 72°F, a glass of water is just liquid.


When we boil water, we notice that it stays liquid until it reaches 212°F, which is when it starts bubbling. The bubbling is a sign that water is changing phase to gas.


Note that 212°F or 100°C is the temperature at which pure water starts to boil. If you measure the temperature at which water boils in your kitchen, you may see that the actual temperature is a little off. This is due to the minerals like calcium that are naturally dissolved in your water.


Effects of Temperature


We commonly think of temperature in terms of units like Fahrenheit or Celsius.


Temperature is a measure of how fast the molecules in a substance is moving, on average.



Effects of Pressure


Pressure is defined as the force exerted over a certain area. Applying pressure on matter can change their properties, including their temperature.


From the previous videos, we see that as pressure increases, a substance’s boiling point increases. Conversely, we see that as pressure decreases, a substance’s boiling point decreases.


Matter exists primarily in three states: solid, liquid, and gas. When temperature is increased or decreased, a substance changes state. This occurs at the melting or boiling point. When pressure is increased, a substance’s boiling point increases.


Understanding these essential concepts will allow us to better understand the Refrigeration Cycle.



Physical States of Matter


In this module, we will go over the physical concepts that are the foundation of how refrigeration equipment works. Skip to quiz!


Review: Physical States


From our Essential Chemistry module, we saw that there are three primary states of matter.


- Solid

- Liquid

- Vapor (Gas)


The state that a substance is in depends on both its temperature and pressure. If we change the pressure, temperature, or both, we can change the state of the substance.


For example, an ice cube is in solid form. If you were to put an ice cube in a pan and heat it up, its temperature will increase until it becomes a puddle of water. It is then in liquid form.


As you continue to heat up the water, it will increase in temperature until it bubbles up and becomes steam. This is water vapor.


We can also change the state of a substance by applying or removing pressure.


As mentioned in the video, increasing the pressure acting on a bubble changes the vapor to liquid. Decreasing the pressure acting on the bubble changes liquid to vapor.


Boiling


Recall that temperature is a measure of how fast the molecules are moving in a substance, on average. The higher the temperature, the faster the molecules are moving inside the substance.


In the refrigeration cycle, the process of changing from a liquid to a vapor is called boiling or vaporization. Note that you need to add heat to a substance for it to change state from liquid to vapor.


Recall from Essential Chemistry that there are bonds between water molecules that keeps them together and forms a mass of water. This is part of what makes a tangible mass that we recognize as water.


When we add heat, we are increasing the temperature and making the molecules move faster until they overcome those bonds between them. When the molecules overcome their bonds, they turn to water vapor. This is what is happening when we see steam coming up from a pot.


Boiling Point


Boiling or vaporization occurs at a temperature called the boiling point. Each substance has its unique boiling point depending on its physical properties.

The boiling point of pure water, for example, is 212 °F or 100 °C. That’s the temperature where water becomes vapor at a pressure of 1 atm, which is the standard pressure of the air around us at sea level.


Recall from Essential Chemistry that the boiling point depends on pressure. If we want to boil water at a higher pressure, the boiling point will be a higher temperature. This means it will take more heat to make the water change phase at a higher pressure.


Note that the boiling point will also be different if your substance is not pure water. For example, if there is salt or other minerals dissolved in the water, the boiling point will change.


If we are heating up pure water and it doesn’t reach 212 °F, the water will not change to vapor. For example, if we heat up the water to 180 °F, the water becomes hot but it does not bubble. Bubbling is an indication of changing from liquid to vapor.


If the temperature of the water is close to 212 °F, you will see a few bubbles forming. This is because the bottom of the pan is in direct contact with the heat source, so it gets hot faster than the top of the pot. So the bubbles indicate that some molecules on the bottom have reached 212 °F and are changing phase.


The condensation point is the temperature at which vapor turns to liquid. Note that the condensation point is the same as the boiling point. That means if you have pure water vapor at 250 °F and you let it cool, the water vapor will condense into liquid at 212 °F.


Both the boiling point and condensation point are properties of pure substances only. We will discuss the boiling temperature for mixtures when we discuss refrigerant blends.

Condensation


As temperature of a substance is decreased, the substance also changes state. For our purposes, the state change we are most concerned with is condensation.


In the video, ice cubes are placed on top of the cup to remove heat from air under the cup. Condensation occurs when temperature is decreased, changing vapor to liquid. To reiterate, you need to remove heat from a substance to change state from vapor to liquid.


A decreasing temperature is the opposite of boiling. A decreasing temperature means a slower average speed of the substance. As molecules slow down, they start to form bonds with other molecules around them, forming a liquid.


We see this commonly on windows on a cold day. Outside air is much colder than inside air, making the windows also cold. Since the cold window is in contact with warm indoor air, heat transfers from water vapor in the air to the cold windows, condensing the water vapor to liquid droplets.


In this chapter, we reviewed the three main states of matter. We also discussed boiling and condensation. Understanding how increasing and decreasing temperature is related to the different states of matter will help us understand how HVAC systems can provide cooling.



Refrigeration Cycle Overview


In this module, we will go over the basic concepts behind the refrigeration cycle. We will discuss how the refrigeration cycle powers your air conditioning and refrigerator. Skip to quiz!


How Does the Refrigeration Cycle Work?


In your daily life, you are likely surrounded by comforts made possible by the refrigeration cycle — especially during the hot summer months. The refrigeration cycle is what powers your air conditioning unit and, on a smaller scale, your refrigerator.


As mentioned in the video, air conditioning works by absorbing heat from your indoor air and ejecting it outdoors.


It does this by passing a refrigerant with a low boiling point through a series of coils that come in contact with warm indoor air. Recall that a boiling point is the temperature at which liquid turns to vapor.


In the refrigeration cycle, we use refrigerants with low boiling points. This means that the refrigerant will boil at a lower temperature than your indoor temperature on a hot day. This is so that your indoor air is hot enough to make the refrigerant boil.


The most commonly used refrigeration system is called the direct expansion vapor compression system. For example, this type of system is found in both your air conditioning as well as your refrigerator.


Direct expansion systems are systems where the cooling effect comes from the expansion of a liquid to a vapor. If we think about water vaporizing to steam, the steam rises up and fills the air. This is the “expansion” behavior of vapors.


Vaporization


As the refrigerant boils, it changes state from liquid to vapor. As mentioned in the review, this process is also called vaporization, as the refrigerant is changing from liquid to vapor.


Recall that during vaporization, or boiling, you need to add continuous heat until all liquid has changed state to vapor.


Think about boiling a pot of water. You apply heat to it by turning on the stove, which increases the temperature of the water. Once the water reaches its boiling point, it starts to bubble and turn into steam, which leaves the pot and dissipates into the air.

If you keep the stove on, you are continuously applying heat to water that is already at its boiling temperature. More and more water molecules will become steam, which is also known as water vapor.


If the stove is left on, the water in the pot will decrease until all the water eventually turns to vapor. You’ve probably seen this if you’ve accidentally left the stove on too long. Either the pot has completely dried up or there’s much less water than before. This is because water has changed state into vapor.


If you turn off the stove after the water starts to boil, the water will stop boiling and remain in the pot. This demonstrates that you need to continuously apply heat in order for all of the water to become vapor.


In the refrigeration cycle, instead of water, you have refrigerant boiling. And instead of a stove flame, it is the hot indoor air that is providing heat for refrigerant to completely boil. This process is what draws heat away from your indoor air and provides cooling.


To reiterate, it is the boiling or vaporization of refrigerant that provides cooling in a refrigeration system.


As the refrigerant moves through the refrigeration cycle, it effectively transfers heat from inside the house to outside the house.


The refrigeration cycle is what powers cooling in your air conditioning system or refrigerator.


Next, we’ll go into more detail about the components of the refrigeration cycle and what happens in each component.


In the refrigeration cycle, refrigerant absorbs heat from your indoor air by using that heat to boil, changing state from liquid to vapor. We choose refrigerants with a low boiling point in order for this to happen.



Refrigeration Cycle Components


In this module, we will go over the individual components of the refrigeration cycle. We will also discuss what happens to the refrigerant in each component and the function of each component. Skip to quiz!


Overview of Components


Aside from the refrigerant flowing through the system, the four key components of a refrigeration cycle are:

1. Evaporator

2. Compressor

3. Condenser

4. Metering Device — also known as a Thermal Expansion Valve (TXV)


Let’s now look at each of the components on their own, starting with the evaporator.


Evaporator


The evaporator is located inside the home. This is where refrigerant absorbs heat from indoor air, changing from liquid to vapor.


As seen in this video, the evaporator coils provide more surface area for hot indoor air to pass over. This increased surface area allows more heat to be transferred to refrigerant inside the coils.


We also see in this schematic that as refrigerant in the evaporator coils absorb heat, it changes state from liquid to vapor. Refrigerant gets heated up past its boiling point, which makes it super-heated vapor by the time it leaves the evaporator.


This change into a superheated vapor state is indicated in the color change of the coils from blue (liquid) to red and yellow (vapor). Vapor refrigerant is let out from the evaporator to the next component, the compressor.



Compressor


After the refrigerant is boiled to a vapor in the evaporator, it is sent through the suction line to the compressor.


The compressor’s job is to increase the pressure of the refrigerant. Recall from our Essential Chemistry module that as pressure increases, temperature increases. This is exactly what the compressor does. We saw a visual representation of that in the previous video.


Refrigerant is then sent from the compressor to the next component, the condenser.


Condenser


Refrigerant is sent from the compressor to the condenser via the hot gas line, since refrigerant exits the compressor as a hot, high pressure gas. As seen in the video, the condenser is housed in the same unit as the compressor.

The condenser is located outside the home so that coils containing the refrigerant can come into contact with outdoor air.


Since the compressor increased the temperature of the refrigerant, the refrigerant is now hotter than the outside air. This causes heat to transfer from the refrigerant in the condenser coil to the outdoor air, cooling the refrigerant.


In this video, we see blue arrows going into the unit. This represents cooler outdoor air flowing over the condenser coils. Recall that this does not mean outdoor air is “cool”. It just means that it is at a lower temperature than the compressed refrigerant inside the coils.


As outdoor air passes over the condenser coils, it absorbs heat. Air that is exiting the unit is hotter because it has absorbed heat from refrigerant in the coils. This is why the arrows leaving the unit are red.


Think of a steaming mug of coffee. Because it is hotter than the surrounding air, heat will transfer from the coffee to its surroundings. You can feel the heat transfer out when you wrap your hands around the mug. Recall that heat transfers from high heat to low heat. This is basically what happens in condenser coils.


Recall that as temperature decreases in a substance, it will condense, which is the process of changing from vapor to liquid. As discussed earlier, we see this commonly on windows on a cold day. Heat transfers from water vapor in the air to the cold windows, condensing the water vapor to liquid droplets.


In the condenser, heat transfers from the refrigerant to outside air. Refrigerant inside the condenser cools until it changes state from vapor to liquid.


Refrigerant is then sent from the condenser to the next component, the metering device/ thermal expansion valve (TXV).


Metering Device


Refrigerant enters the metering device from the condenser via the liquid line, since refrigerant has cooled enough to completely change state to a liquid.


The metering device decreases the pressure of the refrigerant so that it can be sent back to the evaporator. There, it can repeat the cycle and continue absorbing heat from inside.


There are four main components to the refrigeration cycle: evaporator, compressor, condenser, and metering device/ thermal expansion valve (TXV). The main function of the refrigeration cycle is to absorb heat from indoor air and dispel it outdoors.


Different refrigerants can be used in the refrigeration cycle. In the next module, we will discuss the different types of refrigerants that can be used.



State and Pressure Changes


In this module, we show how refrigerant state and pressure changes through each piece of refrigeration equipment. Skip to quiz!


State Changes


Let’s look at a video to get a brief summary of the state changes within the different components of the refrigeration cycle.


In the evaporator, refrigerant absorbs heat from the indoor air. This heat increases the temperature of the refrigerant until it boils. Heat from the indoor air raises the refrigerant temperature past its boiling point, making it a superheated vapor.


Refrigerant changes from liquid to vapor here, so the evaporator contains both liquid and vapor.


In the compressor, refrigerant is fully vapor. Because the compressor increases the pressure of the refrigerant vapor, it increases in temperature but stays as a vapor.


Refrigerant enters the condenser as a high pressure, high temperature vapor. It cools as it the coils come in contact with outside air, cooling into a liquid. So in the condenser, there is both vapor and liquid.


In the metering device, refrigerant is fully liquid as it changes from high to low pressure liquid. It passes back to the evaporator and repeats the process.


This chart summarizes the states of refrigerant in each component of the refrigeration cycle.

Component State of Refrigerant

Evaporator Vapor + Liquid

Compressor Vapor

Condenser Vapor + Liquid

Metering Device / TXV Liquid


Pressure Changes


Changing pressure plays an important part in the refrigeration cycle, as was discussed in the videos. Changing pressure allows us to move heat from inside the house to outside.


We can separate the components of the refrigeration cycle into a high pressure side and a low pressure side. On the high pressure side, we have the compressor and the condenser. The high pressure allows for heat to be dispelled from the system to the outside air.

Recall that increased pressure increases temperature. The higher temperature in the refrigerant allows for heat to be released from the system to the outside air.


On the low pressure side, we have the metering device and the evaporator. This is useful for controlling conditions within the evaporator so that it’s ideal to absorb heat from your indoor air.


As mentioned in previous videos, the follow components change the pressure of the refrigerant:

- Compressor

- Increases pressure

- Metering Device

- Decreases pressure


Here, we have added the pressure of the refrigerant in each component to the chart we created before.

Component State of Refrigerant Pressure

Evaporator Vapor + Liquid Low

Compressor Vapor High

Condenser Vapor + Liquid High

Metering Device / TXV Liquid Low


In this module, we discussed how the state and pressure change happens in different components of the refrigeration cycle and how it contributes to cooling.



Question #1: As the temperature of a liquid increases, what phase does it turn into?

(Assume constant pressure)

  1. Solid

  2. Liquid

  3. Vapor

  4. Water

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Answer: Vapor

As the temperature of a liquid increases, it turn into Vapor. Think about water. As water boils, it becomes steam, which is water vapor.


Question #2: Which of the following states has the highest temperature?

(Assume constant pressure)

  1. Solid

  2. Liquid

  3. Vapor

  4. Water

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Answer: Vapor

Vapor has the highest temperature of the three states.


Question #3: If we increase the pressure acting on a liquid, what state can the substance change to? (Assume constant temperature)

  1. Solid

  2. Liquid

  3. Vapor

  4. Water

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Answer: Solid

If we increase the pressure acting on a liquid, the molecules will move closer and closer together. Eventually, if the pressure keeps increasing, the liquid can turn to a solid.


Question #4: Boiling requires the addition of heat.

  1. True

  2. False

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Answer: True

Adding heat will increase the temperature of the substance until it reaches it boiling point, which changes its state from liquid to vapor.


Question #5: For the same substance, the average speed of the molecules in a vapor is faster than the average speed of molecules in a liquid.

  1. True

  2. False

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Answer: True

This is true. Recall that temperature is a measure of the average speed at which molecules are moving. The higher the temperature, the higher the speed of the molecules. The temperature of a vapor will be greater than the temperature of a liquid of the same substance. Since the temperature is higher, so is the average speed of the molecules.


Question #6: The boiling point is the temperature of a substance where it turns from liquid to vapor.

  1. True

  2. False

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Answer: True

This is the definition of the boiling point.


Question #7: The boiling point of most refrigerants is 212 °F.

  1. True

  2. False

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Answer: False

The boiling point of water is 212 °F. Different substances generally have different boiling points. This means they will change from liquid to vapor at different temperatures. We choose substances with much lower boiling points to use as refrigerants. We’ll see why in the next module.


Question #8: Condensation happens when a gas or vapor cools down and becomes liquid.

  1. True

  2. False

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Answer: True

This is true. Condensation is the state change from vapor to liquid. This is because the temperature of the vapor decreases.


Question #9: If we decrease the pressure of a substance, its boiling point decreases.

  1. True

  2. False

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Answer: True

This is true. Decreasing pressure decreases the boiling point of a substance.


Question #10: You are boiling a pot of water with the lid on. As the water heats up and starts to boil, you notice that drops of water form on the bottom of the pot lid. This is an example of condensation.

  1. True

  2. False

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Answer: True

In this example, as the water starts to boil, some molecules have become gas. These vapor molecules rise up from the pot. But they come into contact with the lid. The lid is at a lower temperature because it is in contact with the air outside the pot. Because of this, heat transfers from the vapor molecules to the lid. As the vapor molecules lose heat, they turn back to liquid form. This is why water drops form on the bottom of a pot lid.


Question #11: Your air conditioning works by adding cold air into your house.

  1. True

  2. False

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Answer: False

This is false. Your air conditioning works by absorbing heat from your indoor air, not by adding cold air.


Question #12: Refrigerant absorbs heat from your indoor air in order to (Select all that apply)

  1. Change state from vapor to liquid

  2. Change state from liquid to vapor

  3. Make your house cooler

  4. Evaporate into the air

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Answer: Change state from liquid to vapor.

Make your house cooler.

Refrigerant absorbs heat from your hot indoor air in. This is what makes your house cooler. As temperature of the refrigerant increases, it changes state from vapor to liquid.


Question #13: When the refrigerant enters the evaporator, what state is it in?

  1. Solid

  2. Liquid

  3. Vapor

  4. Water

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Answer: Liquid

Refrigerant starts off as a liquid in the evaporator. Here, the refrigerant absorbs heat from the air to change state from liquid to vapor. This means that it starts off as a liquid.


Question #14: When the refrigerant exits the evaporator, what state is it in?

  1. Solid

  2. Liquid

  3. Vapor

  4. Water

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Answer: Vapor

In the evaporator, refrigerant absorbs heat from the indoor air to change state from liquid to vapor. This means that when the refrigerant leaves the evaporator, it is fully in vapor state.


Question #15: After the evaporator, which component does the refrigerant go to next?

  1. Condenser

  2. Compressor

  3. Metering Device

  4. Your AC Unit

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Answer: Compressor

After the evaporator, refrigerant goes to the compressor.


Question #16: When the refrigerant enters the compressor, what state is it in?

  1. Supercooled vapor

  2. Supercooled liquid

  3. Superheated Vapor

  4. Superheated liquid

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Answer: Superheated Vapor

After the evaporator, all of the refrigerant has heated beyond its boiling point, making it a superheated vapor. This means that from the evaporator, entering into the compressor, the refrigerant is in superheated vapor state.


Question #17: What is the compressor’s job?

  1. To cool the superheated vapor

  2. To increase the temperature

  3. To increase the pressure

  4. To decrease the temperature

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Answer: To increase the pressure

The job of the compressor is to increase the pressure of the refrigerant.


Question #18: The vapor leaving the compressor is lower in pressure than vapor entering the compressor.

  1. True

  2. False

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Answer: False

The job of the compressor is to increase the pressure of the refrigerant. So the vapor refrigerant leaving the compressor would be higher pressure than the vapor refrigerant entering it.


Question #19: Does the refrigerant change state in the compressor?

  1. Yes, it changes from a vapor to a liquid

  2. Yes it changes from a liquid to a vapor

  3. Yes, it changes from a liquid to a solid

  4. No, it does not change state

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Answer: No, it does not change state

The refrigerant does not change state in the compressor. It enters the compressor as a low pressure vapor, and exits as a high pressure vapor. So, even though the pressure has increased, the state has not changed.


Question #20: What is the condenser’s job?

  1. To cool the high pressure vapor

  2. To heat the high pressure vapor

  3. To increase the pressure

  4. To decrease the pressure