Have you ever wondered how something gets cool? How the bottle of water in the freezer comes out chilled after a few minutes? How do all these appliances like the fridge, ac, and water cooler works? Does the water cooler add cold water to the normal water? Did you ever think about how it is possible for any appliance to add cold air to the room?
Well, you must have had these puzzling thoughts. It’s the other thing that you may have never found an answer to all this curiosity. But today you have found one. We are going to answer all your curiosity in this regard.
Most of us, when thinking about a cold room, believe that the cooling appliances like Air-conditioner or other add cold air to the room. Guess what? That is not the case. When a machine makes the room colder, it does not add more cool air to the room or cold water to the water cooler.
Instead, it throws out the heat of the room, and the absence of heat makes the room cooler. It is part of a process that is called ‘The Refrigeration Cycle.’ The refrigeration cycle is the process responsible for cooling. It is the same process that works in all appliances that cool anything, be it air, water, or even ice-cream in the refrigerator.
To understand the Refrigeration Cycle in detail, first, we need to understand some basic principles, terms, and their meaning. Understanding these will help us in understanding any refrigeration cycles and refrigeration systems.
What Is Refrigeration?
While we are talking about the refrigeration cycle, first, we must know what does refrigeration actually means? Refrigeration is simply the process of cooling by removing heat. We know that heat can neither be created nor be destroyed, and therefore, we remove heat by transferring it from one place to another place. Though most of us think of refrigeration as the process of making things cold, it is actually a process of transferring heat from one place to another.
What is Heat?
Heat is a form of energy that can be transferred from one place to another. Heat transfer plays a vital role in the refrigeration cycle. Heat transfer means the amount of heat that is transferred from one place to another. Heat flows from a high energy level to a low energy level.
This means that if two heat sources are at the same energy level connected to each other, the flow of heat won’t occur. Therefore, to ensure that heat flow occurs, it is necessary to have some energy difference. Similarly, a Temperature difference is also compulsory and essential for heat flow.
The heat will flow from only from a high energy source(higher temperature) to a common energy source(lower temperature). The more the difference in temperature, the more is the heat transfer.
- Kinds of heat – There are two kinds of heat, which are described following.
Sensible heat – Sensible heat is the energy of molecular motion. It is measured by temperature and always causes a temperature change in the substance being heated.
For example, when water at temperature 35F is heated to the temperature 212F (boiling temperature), water heats, temperature changes, but the state of water does not change. The heat that changes temperature but does not changes the state of a substance is called sensible heat, and the process is called sensible heating.
Latent heat – Latent heat is the energy of separation of molecules and arrangement. It causes a change of state at a constant temperature.
When water at 212F is further heat, water began to boil. As more heat is added, water continues to boil until all the water changes into a vapor. During this whole process, the temperature of the water does not increase above 212F. Its state changes. This kind of heat that changes the state of the substance without raising its temperature is called the latent heat.
A characteristic of refrigerant in the refrigeration cycle is its ability to boil. That is, to change from a liquid to a gas at a low temperature.
Heat of Vaporization
The amount of energy(heat) that must be added to a liquid substance in order to turn a quantity of that substance into a gas is called the Heat of vaporization. Most often, it is the boiling point of the liquid.
The heat of vaporization is a function of pressure at which the needed transformation takes place. At low pressure, a liquid needs less heat(energy) to boil, while at high pressure, it needs more energy to boil. This means that while the boiling point of a liquid remains the same, the heat required to reach that point varies according to pressure.
Conversely, the gas can also be transformed into a liquid, and this process is known as Condensation. Both vaporization and condensation occur in a refrigeration cycle.
Boiling Point Depends on Pressure.
Pressure changes the boiling point of the liquid. Liquid at high pressure has a high boiling point, while liquid at low pressure has a low boiling point. For example, we know that at the top of the mountain, the pressure is low in comparison to plain areas where pressure is high. Therefore, in the mountains, water will boil at low temperatures while in plain areas, it will need a higher temperature to boil.
What is Cold?
Cold is just the absence of heat. A hot room means it has more heat in its environment. A cold room means nothing but a place with no(or less) heat. Similarly, cold air means it has no heat in it, which can be achieved by absorbing heat from it.
Conservation of Energy
Law of conservation of energy (also known as the first law of thermodynamics) – Energy can neither be created nor be destroyed. It can only be transformed from one form to another form. As heat is also a form of energy, therefore, it can also neither be created nor be destroyed. But it can be transferred from one place to another, which happens in the refrigeration cycle.
Now we are ready to understand the refrigeration cycle. The refrigeration cycle mainly consists of four parts:
1. Compressor –
The first step that occurs in the refrigeration cycle is Compression. The refrigerant, in the form of cold vapor, at low temperature and low pressure, heads to the first component, the compressor. It is in the gaseous state. Here, compression takes place to raise the weather and the pressure of the refrigerant.
The compressor has two functions –
- To create the flow of refrigerant
- To create a pressure difference across the refrigeration cycle.
Parts of the compressor are Crankshaft, Connecting Rod, Piston, Cylinder bore, suction wall, discharge wall, wall plate, and cylinder head.
When Crankshaft moves, the piston moves down, which is known as a suction stroke, and this suction stroke creates a vacuum in the suction stroke. Because of this, the low-temperature and low-pressure vapor refrigerant enter the cylinder bore.
When the piston moves up, it is known as compression stroke. This compresses the refrigerant and increases its pressure because of which discharge wall opens and refrigerant discharges from the compressor.
The compressor forces the refrigerant through the system. It consumes the most power out of the HVAC system and, therefore, is also considered the engine of the refrigeration cycle.
In this process, the cool, gaseous refrigerant is transformed into a very hot and high-pressured vapor.
2. Condenser –
After the compressor, the refrigerant moves to the condenser, whose job is to cool the refrigerant and change it from a gas into a liquid.
The condenser has two functions –
- To move out the heat of the refrigerant
- To convert the vapor refrigerant into a liquid refrigerant.
In the condenser, the refrigerant is at high temperature compared to its outer surrounding because of which the condenser throw out its heat as heat always flow from high temperature to low temperature. Consequently, the vapor refrigerant changes into the liquid. Warm air from outside, when blown across the condenser coil filled with hot gaseous refrigerant, allows the heat transfer between the refrigerant and the outdoor air. This heat transfer turns the gaseous refrigerant into liquid
The condenser coil is wind up through the condenser to maximize the surface area of the piping, which subsequently affects the heat transfer to the air. The Condenser is essentially a heat exchanger. The refrigerant turns from a vapor into a hot liquid due to the high pressure and reduction in temperature. While flowing through the condenser, the refrigerant remains under constant pressure.
In most of the air-cooled refrigeration systems, the design is such that the refrigerant condenses at a temperature of about 25 to 30 degrees above the ambient temperature around the condenser. This means that if the outside temperature is 95F, the refrigerant will condense at a temperature of about 120F to 130F.
In most of the Water-cooled systems, the design is such that the refrigerant condenses at 20 to 25 degrees above the water inlet temperature. This means if the entering water temperature is 80°, the temperature of the refrigerant, right at the point where it first condenses, will be about 105°.
But, in a refrigeration cycle, why do we want the refrigerant to condense at such a relatively high temperature?
So that the air(in air-cooled systems) or water(in water-cooled systems) flowing through the condenser will be very cold relative to the temperature of the discharge vapor, which will allow the heat energy in the vapor to move into that relatively cold air or water and cause the refrigerant to condense.
A low value of temperature at which the refrigerant condenses in a refrigeration cycle also results in a high coefficient of performance.
- Now liquid refrigerant goes to drier which have two functions
- To filter the refrigerant
- To absorb the moisture from the refrigerant.
Drier has filters to filter the refrigerant. Filters remove all the trash(waste) from the refrigerant. After that, the refrigerant moves through the silica-gel, which has the property of absorbing the moisture. Therefore, moving of refrigerant through the silica-gel makes it dry.
3. Expansion device –
Now, the refrigerant enters the expansion device whose sole purpose is to reduce the refrigerant pressure.
By quickly driving the pressure of the refrigerant down, the expansion device makes it easier to boil and evaporate. As a result of this rapid drop in the pressure, the refrigerant turns into a combination of a cold liquid and vapor. Consequently, the temperature drops at this stage.
4. Evaporator –
Now cool and vapor form refrigerant enters the last, but not the least, stage of the refrigeration cycle, that is Evaporator. The evaporator is an important part of the refrigeration cycle.
Evaporator has two functions
- To absorb the heat of the outer surrounding
- To convert the liquid refrigerant into the vapor
In the evaporator, the refrigerant has a lower temperature compared to the surrounding temperature because of which the heat of outer surrounding moves to the evaporator. This creates a cooling effect outside.
When warm air is blown across the evaporator as cold refrigerant moves through the evaporator coil, heat transfers from the air to the refrigerant, which cools the air before it is vented to space. As the refrigerant flows through the coil, and more air comes in contact with the coil, more and more of the liquid refrigerant boils off until all that remains is gas.
The latent heat required to boil off refrigerant from a liquid to a gas is taken from the warm air as it moves through the coil, thus cooling the air. This boiling process is called evaporation.
In other words, the evaporator is responsible for cooling the air by evaporating the refrigerant flowing through it. The liquid refrigerant at low pressure is easily boiled by the warm air blown across the evaporator and heads back to the compressor as a cool gas/vapor. The Evaporator is where we collect the heat we want to remove. Similar to the condenser coil, the evaporator coil also winds through the evaporator in order to maximize heat transfer from the refrigerant to the air.
The evaporation process occurs at low temperatures because of the nature and properties of the refrigerant and the low pressure in this part of the refrigeration system.
The refrigeration system is designed so that the refrigerant will evaporate in the evaporator at a temperature that’s about 10° to 15° below the temperature setting if it’s a refrigerator or freezer, and the temperature will drop to around 0° in the evaporator of an ice machine or ice cream machine.
This process will continue to cool the air, water, or ice cream mix that’s flowing over the evaporator until it reaches the design set point or thermostat setting.
And this is how the refrigeration cycle completes. In short, the refrigeration cycle contains four major components: the compressor, condenser, expansion device, and evaporator. Refrigerant remains piped between these four components and is contained in the refrigerant loop.
Applications of refrigeration cycle –
The refrigeration cycle works in all machines that cool anything. For example, In appliances like Single door refrigerator, double door refrigerator, window air conditioner, split air conditioner, water cooler, cold storage plants, etc., the refrigeration cycle works.