Vapour Compression Cycle

The Vapour Compression Refrigeration Cycle is a process that cools an enclosed space to a temperature lower than the surroundings. To accomplish this, heat must be removed from the enclosed space and dissipated into the surroundings. However, heat tends to flow from an area of high temperature to that of a lower temperature. How can we make the reverse happen? This is where the Vapour Compression Refrigeration Cycle steps in. During the cycle, a substance called the refrigerant circulates continuously through four stages. The first stage is called Evaporation and it is here that the refrigerant cools the enclosed space by absorbing heat. Next during the Compression stage, the pressure of the refrigerant is increased, which raises the temperature above that of the surroundings. As this hot refrigerant moves through the next stage, Condensation, the natural direction of heat flow allows the release of energy into the surrounding air. Finally, during the Expansion phase, the refrigerant temperature is lowered by what is called the auto-refrigeration effect. This cold refrigerant then begins the Evaporation stage again, removing more heat from the enclosed space.

Each of the four stages will now be revisited in detail, explaining the physical changes that occur in the refrigerant and the devices used to accomplish these changes. A visual representation of the cycle is displayed below. With the explanation of each stage, a picture of the part responsible is included to aid in identification.

Evaporation 

During this stage, the refrigerant travels through a device called an evaporator that has a large surface area and typically consists of a coiled tube surrounded by aluminium fins. The cold fluid is a mixture of liquid and vapour refrigerant as it begins this stage. While flowing through the evaporator, all the liquid evaporates and absorbs heat from the enclosed space. The energy absorbed is used to change the state of the refrigerant from liquid to vapour. This lowers the temperature of the space, along with whatever food or beverages are stored in it. The refrigerant exits this stage as a saturated vapour.

Compression

The heat that was absorbed in the Evaporation stage must be released into the surroundings, but this will not happen unless the temperature of the refrigerant is higher than the outside air. This is the purpose of the Compression stage. A device, predictably called a compressor, raises the pressure of the refrigerant vapour. Due to basic thermodynamic principles, this causes the temperature of the refrigerant to rise, leaving the stage as a superheated vapour. Energy is needed to power the compressor, which is why electricity is required to operate a refrigerator  

Condensation

Now that we have increased the temperature of the refrigerant above that of the surroundings, we can dissipate the heat necessary to continue the process. This is accomplished with a evince very similar to the evaporator. It also uses a coiled tube with aluminium fins, but may have different dimensions than the evaporator to accommodate the different state of the refrigerant. As the hot vapour flows through the condenser, the outside air removes energy and the refrigerant becomes a saturated liquid. At this point the slightest drop in pressure will initiate evaporation, which is the basis for the final stage of the process

Expansion

To begin a new cycle, all that must happen is a lowering of the refrigeration temperature to below that of the enclosure. This is the key to the entire cycle, because this was the problem that we started with. However, in this situation we can utilise what is called the auto-refrigeration effect. When a saturated liquid experiences a sudden drop in pressure, a small amount of liquid is instantly vaporised and the temperature of the mixture is drastically reduced. This cold liquid-vapour mixture can now begin a new cycle. The pressure drop is accomplished by the simplest, yet most important, part of the system – a simple flow restriction. This part is commonly called a throttle or expansion valve.

Summary

The Vapour Compression Refrigeration cycle is a simple four stage process that cools a small enclosed area to a temperature lower than the surroundings. During the cycle, a refrigerant removes heat from the enclosed space and dissipates it to the surrounding air. The Evaporator draws heat from the space as the cold liquid-vapour refrigerant evaporates. The Compressor increases the pressure of the vapour, which increases the temperature above that of the surrounding air. The condenser then releases heat to the surroundings as the hot vapour refrigerant condenses. This saturated liquid passes through the Expansion Valve, abruptly lowering the pressure and initiating the auto-refrigeration effect. This drops the temperature of the refrigerant and begins a new cycle.