Vapour compression cycle - working, diagrams, uses, COP with PDF

 This is The Mechanical post and today you will be reading about What is vapour compression cycle? its construction, working, C.O.P, P-H & T-S diagrams and applications.

In the end, you can print or download the PDF on Vapour compression cycle by clicking the button provided. 

Vapour compression cycle - working, diagrams, uses, COP with PDF

Before starting off, let's get acquainted with what is refrigeration?

What is meant by refrigeration?

Refrigeration can be defined as, "lowering the temperature of an enclosed space by removing heat from it and transferring it elsewhere". This is obtained by passing a cold liquid around the space to be cooled. The cold liquid absorbs heat from the nearby area resulting in cooling effect or refrigeration. This liquid used is known as a refrigerant.

Introduction to Vapour compression cycle

Most of us use a refrigerator or an air conditioner on daily basis. Their function is to cool, or talking more specifically they are used to produce refrigeration effect. 
Have you ever wondered how do they work? Most of the domestic refrigeration systems work on the vapour compression refrigeration cycle.

So now, let's take a look at its construction and working.

Components of Vapour compression cycle

The vapour compression cycle consists of four major components. These components are given below:

Compressor

A reciprocating compressor is used to compress the vapour refrigerant from low pressure to high pressure. The compressor is also responsible for maintaining the flow of refrigerant in the system.

Condenser

The main function of the condenser is to convert the high-temperature vapour refrigerant into a liquid refrigerant by heat rejection to the surroundings.

Throttling device

MasterTriangle12CC BY-SA 4.0, via Wikimedia Commons


A throttling device is a very important component of the vapour compression refrigeration cycle. The expansion valve (figure shown above) and the capillary tube(figure is shown below) are the two commonly used throttling devices. 

Read more in-depth about the Expansion valve. (link would open up in a new tab, so you can continue reading this article.)
The capillary tube is a long tube with a very small diameter of around 0.5 mm to 2.25 mm. The capillary tube is coiled to occupy less space. 

The main function of the throttling device is to intake liquid refrigerant at high pressure and give a cold mixture of vapour and liquid refrigerant as the output.

Evaporator

The cold mixture of vapour and liquid of the refrigerant is passed through another heat exchanger called the evaporator. The evaporator's main function is to absorb the heat from the surroundings.

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Working of Vapour compression refrigeration cycle

The working of the Vapour compression refrigeration cycle is as follows:

Vapour compression refrigeration cycle diagram


The working of vapour compression refrigeration cycle consists of four main stages namely:
  • Compression
  • Condensation
  • Expansion
  • Evaporisation                                                                                                                                                             

Stage 1 - Compression

In process 1, the refrigerant is in the form of cold & dry vapour and enters the compressor at low pressure and low temperature. This condition is described as "dry saturated vapour". Work input is required in the form of electrical energy to run the compressor.                                                     

The low pressure-temperature refrigerant vapour is converted into high pressure by the compressor. However, during the compression process, the temperature of the vapour increases considerably.

Thus, the refrigerant is converted in a high-pressure, high-temperature vapour. This is then passed through a condenser.

Stage 2 - Condensation

In this process, the vapour refrigerant is converted into liquid refrigerant by the condenser by heat rejection to the surrounding.
After conversion, the refrigerant is in the form of high-pressure liquid. This liquid still contains heat energy in it.

Stage  3 - Expansion

After condensation of the refrigerant, it is passed through the throttling device. If the throttling device is a capillary tube, its small internal diameter, opposes the flow of the liquid refrigerant. And when the expansion valve is used, the poppet valve poses as an obstruction to the flow.

Due to this obstruction, the liquid's pressure drops. As a result, some amount of liquid starts to evaporate. The energy required for this phase change is taken from the heat energy present in the liquid itself.

As a result, the temperature of the whole mixture drops. This drop in temperature is quite noticeable. 

Take for eg: if the inlet temperature is around 30 - 35 degree Celcius, then the outlet temperature will be around -20 degree celsius. (* these values are for understanding purpose, actual values might differ)

Stage 4 - Vapourisation

It is to be noted that the refrigerant when exits the throttling device, is in partial vapour and partial liquid state. This low pressure, low-temperature refrigerant mixture is then passed through the evaporator.

The evaporator is a type of heat exchanger. Since the cold mixtures flowing through the evaporator, it absorbs heat from the surrounding, thus, cooling the surrounding.

Now, this absorbed heat is then used to convert the partial liquid refrigerant into vapour. Since there is a change of phase (i.e. liquid to vapour), the overall temperature of the refrigerant remains the same.

This low-pressure vapour is then passed to the compressor, and the cycle begins once again.

This is how the vapour compression cycle works and refrigeration or cooling effect is produced.

Now let's learn a little about refrigerants.

What is refrigerant?

Refrigerant is the working fluid in the vapour compression cycle. It carries heat from a low-temperature region and delivers to a high-temperature region. Commonly used refrigerants are 

Note: The boiling point of any liquid depends upon the pressure around it. Take for eg. Water at normal atmospheric pressure (1.01 bar) boils at 100-degree celsius. However, when the pressure is increased the boiling point of water too increases. At a pressure of 2 bar, water boils at 120.2 degree celsius. Similarly, as the pressure decreases, the boiling point also decreases.

At 0.06 bar water boils at 36.1-degree celsius. So making use of this property, the pressure of the refrigerant is reduced to such an extent that it boils up due to its own heat in the throttling device. Refrigerants like Ammonia have boiling point -33.3 degree celsius at normal pressure. You can check out boiling points of different refrigerants here.

Coefficient of performance (C.O.P) of a refrigerator

The refrigerator's cooling performance can be measured by the term C.O.P (coefficient of performance).

The coefficient of performance of refrigerator = Output / Input
  
 =  Heat absorbed by the system / Power input to the compressor

P-H and T-S Diagrams of vapour compression cycle

P-H and T-S Diagrams of vapour compression cycle

Explanation:

The dry saturated vapour enters the compressor, this is shown as point 1 in the chart above. This compressor increases the pressure of the dry saturated vapour from the evaporator pressure to condenser pressure. 

During the compression, the dry saturated vapour's temperature increases. This increased temperature condition is known as "Superheated vapour". 

At point 2 the superheated vapour enters the condenser where heat rejection takes place at a constant pressure. As a result, the temperature of the superheated vapour drops resulting in condensation and drop in temperature of the refrigerant.

At point 3 expansion takes place, due to which there is a pressure drop, however, the enthalpy (H) remains constant during the drop. 

As the pressure drops a portion of the refrigerant liquid evaporates as shown at point 4. This liquid and vapour mixture absorbs heat in the evaporator resulting in enthalpy rise at constant pressure and temperature as shown in the process 4  -1.

 Applications of the Vapour compression cycle

The following are the applications of the vapour compression cycle
  • It is used in refrigerators and air conditioners for producing a cooling effect.
  • Public water coolers also work on this cycle.
  • Ice manufacturing plants also make used of this system to manufacture ice.
  • Cold storages is an application of the vapour compression cycle.

That's all about Vapour compression cycle. Comment if you like or have any doubts.

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1 Comments

  1. Thanks for the knowledge about vapour pressure and temperature coprempretion

    ReplyDelete

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