thermodynamics

Refrigeration Cycle or Vapor Compression Cycle

Refrigeration is a system used to transfer heat from a closed space to another place to keep the closed space at a lower temperature. Refrigeration cycle is the process in which refrigeration is done. This cycle uses a refrigerant as the medium to transfer heat. A refrigerant is a chemical fluid substance that is capable of phase changes at low temperatures.

Vapor compression cycle is another name of Refrigeration Cycle. Because the refrigerant is compressed in a compressor to increase the pressure when it is in the vapor state.

Both Refrigerator and Air Conditioner work on the basis of vapor compression cycle.

How Refrigerator works…

refrigeration working principle

 

What does refrigerator do? It keeps food items at a low temperature. Bacterias inside the food items can not grow (or grow quite slowly) at low temperatures. So we can store food for a long time.

Now, we will introduce 4 components of this cycle-

1. Expansion Valve/Throttle Valve/Capillary Tube(to increase volume)

2. Compressor(to reduce volume)

3. Evaporator(to take out heat from food)

4. Condenser(to release the heat outside)

  • The change of three thermodynamic properties (volume, pressure, temperature) of the refrigerant will be considered here. From Gas laws (Volume=V, Pressure=P, Temperature=T)

Boyle’s Law, V∝1/P (when T is constant)

Charles’ Law, V∝T (when P is constant)

Gay-Lussac’s Law, P∝T (when V is constant).

From the combination of these laws, we get, V∝T/P (P, V, T changeable)

Now, Starting the Cycle …

Prepare the Refrigerant to take Heat

  • Think about the refrigerant. There is a receiver in the refrigerator. It stores the refrigerant. In this first step, the refrigerant goes to the expansion valve. The reason is, this valve reduces the pressure. So it takes the sufficient heat away (As P∝T) from the refrigerant. The refrigerant is ready to take heat from food items.

Heat Transfer from Food Items to the Refrigerant

  • The refrigerant then goes to the evaporator. The evaporator is always in contact with the food items. The duty of the evaporator is to pass this cold refrigerant over the food items. The refrigerant takes the heat out from foods. Because heat goes from high temperature to low temperature object. Now, why do we call it evaporator? It is because, by taking the heat, the refrigerant evaporates and goes to its gaseous state here.

Preparing The Refrigerant to Release the Heat

  • We have done our job to keep the food items cool. The refrigerant is now in low pressure and gaseous state. Now, to reuse the refrigerant, we must take it into liquid state. Next, the compressor is used for two reasons,

(1) To reduce the volume of refrigerant. So that it can change its state to liquid easily.

(2) To raise the pressure(V∝1/P). So further in the expansion valve, the refrigerant can be sufficiently cold by reducing pressure. This will help to absorb heat later.

Heat Transfer from the Refrigerant to the Surroundings

  • The last step of refrigeration is to release the heat received from the food items to the surroundings. So, the compressed refrigerant goes to the condenser. The refrigerant changes its form here to its condensed state (from gas to liquid). So the name of this device is condenser. It is always in contact with the surroundings. Then the refrigerant releases the heat to the surroundings. Receiver stores the liquid again.

The refrigeration cycle starts again. After finishing several cycles, when the refrigerator is at the correct temperature, the compressor stops pumping. Then the food items become sufficiently cold.

N.B: Refrigerator uses electrical energy for pumping or compressing gas in the compressor.

How Air Conditioner Works …

 

The basic principle of an air conditioner is same as the refrigerator. There are some additional devices only. Here, there is an evaporator fan and a condenser fan with a single motor.

  •  The evaporator fan takes in the air of high temperature from the room. In a refrigerator, the refrigerant takes heat from food. But in an Air Conditioner, the refrigerant takes heat from the air of the room. This air releases heat, becomes cold and suitable for the room. Then the cold air is released to the room again by recirculation.
  • The condenser fan blows outdoor air to the condenser. It changes the state of the refrigerant from gas to liquid. The outdoor air takes the heat here.
  • Air Conditioner uses electricity for pumping gas in the compressor and to run the fan motor.

Keynotes:

  • Thermodynamic Properties is the characteristics of a matter. They are used to describe its physical condition properly.
  • A thermostat is a device used to send alternating current to the compressor and fan motor in a refrigerator and air conditioner.
  • We can have another question in our mind. How does refrigerant change state to gas at a normal or low temperature of the evaporator? The reason is, the refrigerants used in refrigerator boils at a very low temperature. They have boiling temperatures of minus range. As for example, R22 is a refrigerant with a boiling point of -40.8°C. So, it can evaporate at a very low temperature of the evaporator.
  • Freon is the commercial name of refrigerant.
  • Chlorofluorocarbons (CFC), Hydrochlorofluorocarbons (HCFC) are the refrigerants people used before because they are non-flammable and non-toxic. But they have a bad impact on nature because of helping Ozone Layer Depletion. So now Hydrofluorocarbons (HFC) and Perfluorocarbons (PFC) are the refrigerants that are being used most widely.

For any information, please comment. Thank you !!!

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