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.
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:
The working of vapour compression refrigeration cycle consists of four main stages namely:
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
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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