• SkillCat Team

Refrigeration Equipments: Compressor Properties & Types

Updated: Jul 1

Refrigeration Equipment : Chapter 2


Compressor


In this module, we will go over the compressor component in more detail. Skip to quiz!


Review: Compressor Function


In the refrigeration cycle, the compressor comes after the evaporator. Remember that in the evaporator, refrigerant absorbs heat and changes phase from a liquid to a vapor. This means that the refrigerant entering the compressor is completely vapor.


The compressor’s job is to increase the refrigerant’s pressure. By increasing the refrigerant’s pressure, we also increase its temperature. So in the compressor, the refrigerant turns into a high pressure, high temperature vapor.


In the compressor, we need materials that can handle the high pressures and high temperatures of refrigerants. This is why we need components made of strong materials like hardened steel or stainless steel in the compressor.


Compressors have different types of physical enclosures. Physical enclosures are the outer housing that contain the compressor. This comes in a few different forms.


Depending on the type of physical enclosure the compressor has, it can be split into one of three categories:

  1. Hermetic Compressors,

  2. Semi-Hermetic Compressors, or

  3. Open Compressors.



Hermetic Compressors


Hermetic compressors are completely sealed and welded together. This means that the motor and the compressor are contained and sealed inside a single shell.

Because hermetic compressors are sealed from the outside, they have several advantages, including:

  • Less leaks,

  • Less contamination,

  • Less maintenance,

  • Compact size, and

  • Lower noise.

Because their components are welded together, hermetic compressors are compact in size. They are also relatively quiet, which makes them ideal for household use. We commonly see hermetic compressors in household refrigerators.


A disadvantage is that if there is any problem with the compressor, the whole component has to be replaced.


This is because it would need to be opened to detect any internal problems within the compressor. There are no screws or bolts that can be quickly unscrewed and then screwed back so we have to replace the component.



Semi-Hermetic Compressors


In semi-hermetic compressors, the motor and compressor are not housed in a single shell. The motor and compressor are contained in different shells.

Compared with the hermetic compressor, the main difference is that the semi-hermetic compressor enclosure is bolted rather than welded.


This makes opening the compressor easier, which allows us to make repairs instead of having to replace the entire component.


The drawback with the layout in semi-hermetic compressors is that there is a larger chance of leaking. Since the compressor and motor are joined by bolts and rubber seals, the system is not as sealed off as in hermetic systems. This can cause more leaks.


Open Compressors


In open compressors, the motor is housed completely separately from the compressor. The motor drives a different wheel which is connected to the compressor by a series of gears or belts as shown here.

The main advantage of an open drive compressor is that it does not necessarily need an electric motor to function.It can be driven by non-electric sources such as a diesel engine or a steam turbine.


A disadvantage of open compressors is that there are high chances of gas leaks from the compressor. As the compressor motor is open to outside air and dust, regular maintenance is necessary.


Open compressors tend to be seen in

  • Older appliances, and

  • Industrial settings— like in chemical plants or large factory settings.


Since the motor is housed separately in an open compressor, a single motor can be used to operate multiple systems. This works for large industrial or factory settings, since there would be multiple systems to operate.


In this module, we discussed the different types of housings evaporators have and what they are used for. We now know what hermetic, semi hermetic, and open compressors are.




Compressor Parts


In this module, we will go over the compressor component in more detail. Skip to quiz!


Overview

The compressor’s main job is to increase the pressure of the refrigerant. It transfers refrigerant from the evaporator, which is low pressure, to the condenser, which is high pressure.


The compressor reduces the volume of the refrigerant, which increases its pressure and temperature. Let’s now take a look at what is actually inside a compressor.


In order to do its job, the compressor has several key components:

  • Inlet and Outlet Pipes

  • Motor

  • Crankcase

  • Housing, and

  • Compressor.

Inlet and Outlet Pipes

The inlet and outlet pipes bring refrigerant into and out of the compressor. In the refrigeration cycle, remember that refrigerant flow from the evaporator to the compressor through the suction line. The refrigerant then enters the compressor through its inlet pipe.


When refrigerant enters the compressor, it is a low pressure vapor. After the compressor increases its pressure, it then becomes a high pressure vapor.


The outlet pipe brings high pressure vapor refrigerant out of the compressor to go to the condenser. Coming out of the outlet pipe, the refrigerant then flows through the discharge line.


Motor


The purpose of the motor is to power the compressor. The motor converts electricity into mechanical rotation in the compressor.


Depending on the type of compressor, the mechanism used to increase pressure will be different. We will talk about these different mechanisms as we look at specific types of compressors in later modules.


This mechanical compressor is what causes the volume to decrease, which increases the pressure and temperature of the refrigerant.



Crankcase

The compressor’s crankcase acts like a container that collects refrigerant oil in the refrigeration system.


When the system is not running, the refrigerant and refrigerant oil does not have any force or pressure acting on it. Because of this, it will naturally flow to where there is least pressure.


Imagine juggling some oranges. As you are juggling, the oranges are moving in a circle in the air because your hands are moving them in a circular motion. If your hands stop moving, the oranges will fall to the floor. That is the natural force of gravity acting on them.


In the same way, if the refrigeration system is not running, refrigerant oil will flow to the place of least pressure. The place of lowest pressure is designed to be at the crankcase.


The oil pressure in the crankcase is lower than the refrigerant pressure. This is why refrigerant will migrate to the crankcase. This is also similar to how heat will travel from high to low, as we discussed in the Refrigeration Cycle.


To reiterate, the refrigerant oil will migrate to the crankcase in the compressor because that is the place with the lowest pressure in the system. The crankcase collects this oil so it does not leak out.


Housing

We talked about the types of housing in the previous module. We can classify compressors by how they are packaged together.


The housing is the part that contains all these components. It protects the components from outside elements and prevents against leaks.


Hermetic compressors are where the compressors and motor are completely welded together. This makes the component more compact and less prone to leaks. But it also makes them hard to repair, so most of the time they are replaced.


Semi-hermetic compressors are bolted together rather than welded together. This is the main difference between hermetic and semi-hermetic compressors.


And in open compressors, the compressor and motor are completely separate.


Compressor


There are different mechanisms that we can use in the compressor to decrease volume. The different mechanisms is what defines the different types of compressors that there are.


In compressors, we can decrease volume by:

  • Using pistons to push down on a space, or

  • Rolling a piston.

We will talk more about each of these types of compressors in a following module.


In this module, we discussed the different parts of compressors and what they are used for.





Compressor Properties


In this module, we will go over the compressor component in more detail. We will talk about

- Principles behind how compressors decrease volume,

- Compressor oil, and

- Compressor safety Skip to quiz!


Review: Compressor Function


The compressor’s job is to decrease volume so we can increase pressure of the refrigerant.

There are two ways compressors do this. There is:

  • Positive displacement, and

  • Non-positive displacement.



Positive Displacement


In positive displacement compressors, gas is sucked into a chamber. Then, a part such as a piston presses down on the gas. In this way, the piston part is “displacing” the vapor, which is why we call it positive displacement.


As the piston pushes down on the gas, the space inside the container gets smaller. This decreases the volume inside. And remember, decreasing volume increases the pressure of the refrigerant gas!


In positive displacement, low pressure refrigerant enters the compressor through the suction valve. This is because the line that connects the evaporator to the compressor is called the suction line.


Refrigerant is sucked through the suction valve because the pressure inside the compressor chamber is lower. Substances will always travel from high pressure to low pressure regions.


In the compressor, the refrigerant’s pressure is increased. Then, this high pressure refrigerant leaves the compressor through the discharge valve. This is because the tube leading refrigerant out from the compressor to the condenser is called the discharge line.


The discharge valve opens when the pressure acting on the valve is high enough. We need to compress refrigerant to this pressure in order to open the discharge valve.



Non-Positive Displacement


The other way a compressor works is by non-positive displacement. In non-positive displacement, we are not displacing air. Instead, they work by churning the refrigerant at very high speeds.


Non-positive displacement works because of centrifugal force. Centrifugal force is the force that comes from circular motion. In non-positive displacement, this centrifugal force acts on the refrigerant to increase its pressure.


To visualize how non-positive displacement works, let’s imagine we are rotating a glass of liquid. As we see in this video, an empty space forms in the middle of the glass. This is because the circular motion is pushing against the liquid, forcing it to move to the sides of the glass.


This is what is happening inside a compressor that works using non-positive displacement. Refrigerant is compressed using this centrifugal force instead of a piston-like part.


Non positive displacement is seen in a centrifugal compressor, which is also called a turbo compressor. We will talk about more about how centrifugal compressors work in the next module.


Both positive and non-positive displacement work by decreasing the volume of the refrigerant. They just decrease the volume in different ways. The reduction in volume increases the pressure of the gas, which is the compressor’s job.



Lubrication

Compressors generally need lubrication or oil to work properly. In this way, compressors are kind of like cars.


Recall that compressors are complex and have many moving parts. When these parts move or slide over each other, there is a lot of friction between them. The more friction there is, the more likely the compressor’s components will fail or break.


Lubrication oil helps reduce the friction between the moving or sliding surfaces in the compressor. This oil also prevents the compressor from overheating because it makes it easier for the compressor to move. We also call this compressor oil.


It is important to follow manufacturer instructions when it comes to compressor oils. We should only use the compressor oil recommended by the manufacturer. This is because different types of oils provide different types of lubrication.


Using the wrong compressor oil can damage components and cause failure. They can also interact with refrigerant and cause harmful combinations. Always use the recommended oil.


Compressor Safety

In an HVAC(R) appliance, a compressor is one of the hottest parts. This is because we are increasing the pressure of the refrigerant, which increases its temperature as well.


The most common type of accident working with compressors is skin burns from touching a hot compressor. In order to avoid this, we want to wait for the compressor to cool down before servicing. We also need to wear gloves.


The compressor also uses electricity to operate its motor. There is risk of electric shock or burn if operated improperly.


In this module, we discussed the different types of evaporators and compared what they are used for.




Compressor Types - Part 1


In this module, we will go over the different types of compressors and how they work. Skip to quiz!


Review: Compressor Function


The compressor’s main job is to increase the pressure of the refrigerant. There are different ways we can increase the pressure of the refrigerant.


There are different types of compressors depending on what mechanism is used to increase pressure of the refrigerant.


The different types of compressors are:

  • Reciprocating Compressors,

  • Rotary Compressors,

  • Scroll Compressors,

  • Screw Compressors, and

  • Centrifugal Compressors

We’ll talk about each of these in more detail.



Reciprocating Compressors


Reciprocating compressors have been used in refrigeration systems for a long time. The outer casing of reciprocating compressors can be hermetic, semi-hermetic, or open.


Reciprocating compressors are generally used in smaller systems. This includes:

  • Domestic refrigeration

  • Small commercial units, and

  • Small industrial units.

These compressors work by using positive displacement. Recall that positive displacement is when we use piston part to push on refrigerant to decrease its volume. Let’s take a look at a video to see how reciprocating compressors work.



Rotary Compressors


Rotary compressors are one of the most widely used types of compressors used in small HVAC appliances. Let’s take a look at a video to get familiar with how rotary compressors work.


In a rotary compressor, the rolling motion of the inner shaft is what compresses the refrigerant. The outer shell is stationary while the inner shaft rotates. As the inner shaft rotates, it pushes on the refrigerant inside the compressor chamber.


The inner shaft pushes on the refrigerant, which

  • Reduces volume, and

  • Increases pressure.

Reducing the refrigerant’s volume causes its pressure to increase. Since rotary compressors reduce refrigerant volume, they work on positive displacement.


Rotary compressors can be seen in refrigeration appliances, window ACs, and ductless split AC systems.


In general we find rotary compressors in appliances that are:

  • Small, and

  • Hermetically sealed



Scroll Compressor


Like rotary compressors, scroll compressors also use a circular sort of motion to compress the refrigerant. But its components look a little different. Let’s take a look at a video to get familiar with how scroll compressors work.


In a scroll compressor, there are two matching scrolls as shown in the videos. The scrolls like spirals that are placed together.


One scroll is stationary and does not move. The second scroll moves and is rotated by a motor.


Refrigerant travels from the outside in.It starts from the outside of the scrolls and works its way to the inside of the scrolls.


When the refrigerant reaches the center of the two scrolls, it gets pushed up to leave the compressor. The discharge outlet is on top of the compressor.


The second scroll moves around the first scroll. As the second scroll moves, the refrigerant gas is squeezed between the two scrolls and is compressed.


The refrigerant gas is trapped in the pockets between the two scrolls and is compressed between their circular motion. This means the volume of the refrigerant decreases and the pressure increases.


There are new and improved versions of the scroll compressor. These are the:

  • Two Stage Scroll Compressor, and the

  • Digital Scroll Compressor.

These newer scroll compressors provide more accurate compression and precise control of the refrigerant flow. They also control room temperature and are more efficient than a regular scroll compressor.


The two-stage scroll is also called a step scroll. A two-stage scroll compressor also has two scrolls. However, the moving scroll can return halfway during the rotation or complete the full rotation as required.


The smaller rotation is for normal use, and a larger one for hot days when the compressor has to work harder to cool. A normal scroll compressor will pretty much work at a constant capacity at which it is designed by the manufacturer.


A digital scroll can either operate at

  • 100 percent, or

  • 0 percent.

A digital scroll compressor can quickly switch between fully on to fully off.


Digital scroll compressors are very efficient. They provide the same cooling as a normal scroll compressor but only turn on 50% of the time. The rest of the time when they are not needed, they are off, which conserves energy.


This means that if we need more cooling, the digital scroll compressor will be on and working.

But if it’s too cool, we can just turn the digital scroll compressor off. This saves on your electricity bill!


In this module, we discussed about a few different types of compressors and what they are used for. These were:

  • The Reciprocating Compressor

  • The Rotary Compressor

  • The Scroll Compressor

We also learned how a reciprocating compressor uses a piston to compress gases.


We learned how a rotary compressor uses a rolling shaft to compress gases. We also discussed about the two scroll in a scroll compressor. We will learn about the remaining two types of compressors in the next topic.





Compressor Types - Part 2


In this module, we will go over the remaining two types of compressors

- The Screw Compressor

- The Centrifugal Compressor

We will also learn how they work. Skip to quiz!


Review: Compressor Function


The compressor’s main job is to increase the pressure of the refrigerant. There are different ways we can increase the pressure of the refrigerant.


There are different types of compressors depending on what mechanism is used to increase pressure of the refrigerant.


The different types of compressors are:

  • Reciprocating Compressors,

  • Rotary Compressors,

  • Scroll Compressors,

  • Screw Compressors, and

  • Centrifugal Compressors

We’ll talk about the remaining two of these in more detail.



Screw Compressor


Screw compressors also use a circular motion to compress refrigerant. This is why they are sometimes called rotary screw compressors. Let’s take a look at a video to get familiar with how scroll compressors work.


Rotary screw compressors are generally used for systems that provide large cooling capacities. Screw compressors can run well continuously and can cool large amounts of refrigerant in industrial settings.


Screws are the two shafts that are shaped like wood screws. In screw compressors, we can have one or two screws that rotate to compress refrigerant. In this video, we see that there are two screws that rotate.


Motors power the screws’ rotating motion. The outer housing of the screw compressor is tapered at one end. As the screws are rotated, the gas is sucked inside through the inlet port.


The screws rotate and the refrigerant flows in the space between the threading. As the screws rotate, this pushes the gas forward towards the discharge.


And as the refrigerant flows toward the discharge outlet, the volume reduces. This increases the pressure of the refrigerant. This is how compression happens in the screw compressor.



Centrifugal Compressor


A centrifugal compressor compresses the gas using centrifugal force. In the previous module, we talked about how centrifugal force compresses refrigerant. This is non-positive displacement, since we are not changing the volume.


To visualize how non-positive displacement works, let’s imagine we are rotating a glass of liquid. As we see in this video, an empty space forms in the middle of the glass. This is because the circular motion is pushing against the liquid, forcing it to move to the sides of the glass.


A centrifugal compressor uses this outward acting force to compress the gas in it. Let’s take a closer look at how centrifugal compressors work.


Centrifugal compressors are used for larger systems. Most centrifugal systems are used in chillers in large, commercial buildings.


A centrifugal compressor has three parts:

  1. Impeller

  2. Diffuser

  3. Volute Casing

The impeller is a circular disc that has curved blades. These blades turn at high speeds to. As the impeller turns, refrigerant is sucked into the center and then is pushed away from the center by centrifugal force.


We can see that the impeller is the component that increases the pressure of the refrigerant. It is here in the impeller that the refrigerant is compressed.


After the impeller moves the refrigerant outwards, the diffuser comes in to change the direction the refrigerant is moving in. The diffuser is a circular container around the impeller.


The diffuser changes the direction of the refrigerant to a circular motion. It also slows down the refrigerant.


The volute casing is a container around the diffuser. It is connected to the discharge outlet.

We can think of the volute casing as a funnel that directs the refrigerant out of the compressor.


In this module, we discussed about two types of compressors and what they are used for.

These were:

  • The Screw Compressor

  • The Centrifugal Compressor


We learned how two screws rotate within each other to compress gases. We also discussed about the centrifugal force used in centrifugal compressors. We learned how different components of a centrifugal compressor assist each other to compress the gas.






Question #1: The compressor’s job is to increase the pressure of the refrigerant.

  1. True

  2. False

Scroll down for the answer...













Answer: True

This is true. The compressor’s job is to increase the refrigerant’s pressure.


Question #2: The advantages of a hermetic compressor include all of the following except:

  1. Less leaks and contamination

  2. Compact size

  3. Less refrigerant

  4. Less noise

Scroll down for the answer...









Answer: Less refrigerant

Because hermetic compressors are sealed from the outside, they have several advantages, including:

  • Less leaks,

  • Less contamination,

  • Less maintenance,

  • Compact size, and

  • Lower noise.

Less refrigerant is incorrect because the type of compressor does not determine the amount of refrigerant in the system.


Question #3: The disadvantages of a hermetic compressor include:

  1. Can’t repair easily

  2. Have to replace if broken

  3. Both of these.

Scroll down for the answer...









Answer: Both of these.

Both of these are disadvantages of hermetic compressors.


Question #4: Which of the following is an advantage of a semi-hermetic compressor?

  1. Less refrigerant

  2. More refrigerant

  3. We can replace the nuts and bolts that hold it together

  4. We can repair it because it can be taken apart

Scroll down for the answer...












Answer: We can repair it because it can be taken apart

The major advantage of a semi-hermetic compressor is that we can repair if since it can be taken apart.


Question #5: Because semi-hermetic compressors are less sealed off than hermetic compressors, they are more likely to develop leaks.

  1. True

  2. False

Scroll down for the answer...












Answer: True

True. Leaks can happen through the nuts and bolts that hold semi-hermetic compressors together.


Question #6: In open compressors, the motor and compressor are

  1. Not both needed

  2. Housed together

  3. House separately

  4. All of these

Scroll down for the answer...












Answer: House separately

In open compressors, the motor and compressor are housed separately.


Question #7: The motor in the compressor provides electrical power so that the compressor can

  1. Decrease volume

  2. Increase pressure of the refrigerant

  3. Increase temperature of the refrigerant

  4. All of these

Scroll down for the answer...














Answer: All of these

All of these are correct.


Question #8: The purpose of the crankcase is to collect refrigerant oil.

  1. True

  2. False

Scroll down for the answer...













Answer: True

This is true. The compressor’s crankcase acts like a container that collects refrigerant oil in the refrigeration system.


Question #9: Positive displacement is one way compressors can increase the pressure of the refrigerant.

  1. True

  2. False

Scroll down for the answer...












Answer: True

This is true. In positive displacement compressors, gas is sucked into a chamber and a piston presses down on the gas. The piston pushes down on the gas, and decreases the volume inside. Decreasing volume increases the pressure of the refrigerant gas!


Question #10: Positive displacement

  1. Decreases volume of refrigerant

  2. Increases pressure of refrigerant

  3. Uses a piston to push on the refrigerant

  4. All of these

Scroll down for the answer...












Answer: All of these

All of these are correct. Both methods of displacement will decrease volume and increase pressure of the refrigerant. Positive displacement uses a piston-like device to push on the refrigerant, which is why it is called positive displacement.


Question #11: Non-positive displacement works by using

  1. Negative force

  2. Negative and centrifugal force

  3. Centrifugal force only

  4. All of these

Scroll down for the answer...












Answer: Centrifugal force only

Non-positive displacement works by using centrifugal force.


Question #12: Compressor oil acts to

  1. Help decrease volume of the refrigerant

  2. Help increase volume of the refrigerant

  3. Reduce friction between the moving parts of the compressor

  4. All of these

Scroll down for the answer...








Answer: Reduce friction between the moving parts of the compressor

The purpose of compressor oil is to reduce friction between the moving parts of the compressor.


Question #13: Compressor oil makes it hard to decrease volume of the refrigerant because the parts can get too slippery. This is why we rarely use compressor oil.

  1. True

  2. False

Scroll down for the answer...












Answer: False

This is false. Compressor oil is needed to make sure all parts of the compressor work well together. When the compressor works properly, it will compressor the refrigerant better.


Question #14: We can use olive oil in our compressor if we are out of options. Any oil is better than no oil.

  1. False

  2. True

Scroll down for the answer...












Answer: False

This is false. When selecting the type of oil to use, we must follow manufacturer instructions. Using any other type of oil than the oil that is recommended can result in equipment damage or failure.


Question #15: Which of the following are safety risks when working with compressors?

  1. Electric shock

  2. Skin burns

  3. Both of these

  4. None of these

Scroll down for the answer...












Answer: Both of these

Both electric shock and skin burns are safety risks associated with compressors.