Refrigerator & Refrigeration cycle: How does It Work?
Refrigerators are one of the prime appliances that we utterly rely on in our daily activities. It is considered as an integral part of our household as well as industrial and other activities. To understand how it works, you have to get an idea of the refrigeration cycle’s working principle. The basic principle of refrigeration is simple. You have to pass a colder liquid continuously around the object (let’s say an apple) which needs to be cooled. It will help to take heat outside from the object. The whole refrigeration unit is based on this principle.
Let’s start with the throttling device which is commonly know as capillary tube. It is fit between a condenser and evaporator. It is make up of copper with a diminutive diameter and is also consider as an expansion device.
For effective throttling, the inlet liquid of the capillary tube should be a high pressure and high temperature liquid. As the name implies, the expansion device acts as an obstruction to flow this liquid which causes the pressure on the refrigerant’s molecules to decrease considerably.
Due to drop in pressure, the boiling point of the refrigerant comes down and it starts to evaporate. Heat required for evaporation comes within the refrigerant; so its temperature drops. (Let’s say from 45 degree to -20 degree and pressure from 80 bar to 0.6 bar). So Refrigerator Refrigeration cycle a low pressure and low temperature liquid is achieve after this process. In practical cases, we do not get only liquid refrigerant, but actually a mixture of liquid and vapor of low pressure and low temperature is coming out of the expansion device.
The next phase is just to pass this cold liquid over the refrigerator body (evaporator) to absorb heat. During the heat absorption process, refrigerant further evaporates and transforms into pure vapor. Now if we can use this low pressure vapor into a high pressure liquid state which is require for throttling device, the cycle will be repeat again.
So in order to raise the pressure, a compressor is essential for this purpose that raises the pressure back to its initial value (from 0.6 bar to 8 bar again). But since its compressing gas, the temperature is also increase (by ideal gas law, PV=nRT) along with the pressure inevitably.
Compressors as known as the heart of the refrigeration system play two important roles for the refrigeration cycle. Firstly, It serves the compression of gas in order to gain a high pressure state. Secondly, it helps a continuous flow of refrigerant. Generally reciprocating type compressors are use for domestic purposes.
Now the Refrigerator Refrigeration cycle is high pressure vapor by compressing and we have to convert it to liquid phase. Hence, another heat exchanger known as condenser is place outside the refrigerator that liberates latent heat to the surroundings. Vapor is then condense to the liquid phase and temperature will come down to the normal level. So the refrigerant is again sit to its initial level which is high pressure liquid (80 bar). This process is repeat again and again for continuous refrigeration. Thus a closed loop refrigeration cycle is form.
This cycle is conventionally call vapor compression cycle. To enhance the heat transfer, both evaporator and condenser are having fins attach to it. Since the evaporator is cooling the surface around it, it is common that water is condense over it forming frost. Frequent removal of forming frost is prerequisite to enhance heat transfer popularly known as defrost mechanism.
An energy balanced equation can be write for this purpose. A huge amount of input power is require for the compressor (Pin). Heat is absorb (QABSORBE ) in the evaporation process and is reject (QREJECT) in the condensation process. Therefore, it can written as-
PIN + QABSORBED = QREJECTED
COP (Coefficient of Performance), indicates the efficiency of the refrigerator can be write as-
COP = output/input or QABSORBED /PIN
The value of coefficient of performance is always more than 1 (one).