• SkillCat Team

State and Pressure Changes

Updated: Apr 7

Refrigeration Theory: Chapter 4


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


1. 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, the refrigerant absorbs heat from the indoor air. This heat increases the temperature of the refrigerant until it boils. The 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 completely vapors. Because the compressor increases the pressure of the refrigerant vapor, it increases in temperature but stays as a vapor. Recall that for a fixed volume, as pressure increases, temperature increases.


Refrigerant enters the condenser as 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 refrigerants in each component of the refrigeration cycle.



2. 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 the 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 following 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.


3. Conclusion


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.


Different refrigerants can be used in the refrigeration cycle. To learn about the different types of refrigerants, consider taking our EPA 608 course! Getting EPA 608 certified helps you get qualified for jobs that require you to handle refrigerants.



Question #1: When the refrigerant exits the evaporator, what state is it in?

  1. Low pressure liquid

  2. High pressure vapor

  3. Low pressure vapor

  4. High pressure liquid


Scroll down for the answer...










Answer: Low pressure vapor

In the evaporator, the refrigerant absorbs heat from the indoor air to change the state from liquid to vapor. So, when the refrigerant leaves the evaporator, it is fully in a vapor state (low pressure)


Question #2: When the refrigerant enters the condenser, what state is the refrigerant in?

  1. High pressure liquid

  2. Low pressure liquid

  3. High pressure vapor

  4. Low pressure vapor


Scroll down for the answer...











Answer: High pressure vapor

Recall that the compressor increased the pressure of the refrigerant before it reaches the condenser. And the refrigerant was is still a vapor from when it boiled in the evaporator. So refrigerant enters the condenser as a high pressure vapor.



Question #3: When the refrigerant enters the metering device, what is the state of the refrigerant?

  1. Low pressure liquid

  2. High pressure liquid

  3. Low pressure vapor

  4. High pressure vapor


Scroll down for the answer...










Answer: High pressure liquid

The refrigerant enters the metering device as a high pressure liquid. The condenser cools the refrigerant until it changes state to a liquid. As the pressure of the refrigerant had been increased by the compressor, it becomes a high pressure liquid going from the condenser to the metering device.



Question #4: When refrigerant exits the metering device, what is the state of the refrigerant?

  1. Low pressure liquid

  2. High pressure liquid

  3. Low pressure vapor

  4. High pressure vapor


Scroll down for the answer...











Answer: High pressure liquid

A refrigerant is a high pressure liquid when it enters the metering device. The metering device decreased the pressure but didn’t do anything to change the state of the refrigerant, so it is still a liquid.



Question #5: Which components change the refrigerant’s pressure? (Select all that apply)

  1. Evaporator

  2. Compressor

  3. Condenser

  4. Metering Device

Scroll down for the answer...











Answer: Compressor and Metering Device

The compressor increases the refrigerant’s pressure and the metering device decreases the refrigerant’s pressure.



Question #6: Which components are on the high pressure side? (Select all that apply)

  1. Evaporator

  2. Compressor

  3. Condenser

  4. Metering Device


Scroll down for the answer...











Answer: Compressor and Condenser

The compressor increases the refrigerant’s pressure and passes the refrigerant to the condenser. These two components form the high pressure side of the refrigeration cycle.

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