What is the Refrigeration Cycle, and how is it defined?

Evelyn Maitee Marin
Industrial Engineer, MSc in Physics, and EdD

A refrigeration cycle refers to the repetitive thermodynamic process in which a substance used as a refrigerant it changes phases through a circuit, and its function is to absorb heat from a space to be conditioned to release heat in another region. Unlike other thermodynamic cycles such as power, in systems used for refrigeration, the purpose is not obtaining energy for sale or consumption, but rather the thermal conditioning of a space, for example, a room.

With an operating scheme similar to that of refrigeration cycles, there are heat pumps, which, unlike air Conditioned, they are used to heat a space, that is, to extract heat from the external environment to release it inside the desired premises condition.

From a theoretical point of view, refrigeration cycles can be studied considering the components that make up the system as well as the various stages through which the refrigerant goes through in the different states. The following graph shows the classification of refrigeration systems:

Types of refrigeration systems

Basic components of a refrigeration cycle

According to the second law of the Thermodynamics, the heat flow occurs spontaneously always from a region of greater temperature at a lower temperature, and otherwise, some device or mechanism is required to carry out the refrigeration process. To do this, any refrigeration system requires at least four components:

• Compressor: is a device responsible for increasing the pressure of the refrigerant once it has evaporated. Under ideal conditions, the compression process is at constant entropy, although in practice an increase in entropy occurs.

• Evaporator: this system is basically a heat exchanger through which the refrigerant circulates to absorb heat from the space to be conditioned and as a consequence, the mixture changes to a state of steam.

• Expansion valve: also called capillary tube or expander. Under ideal considerations, this device operates isenthalpic (at enthalpy constant), and its function is to decrease the refrigerant pressure in the mixing zone.

• Condenser: it is formed by a coil through which the refrigerant passes in vapor state, at a pressure and temperature higher than the conditions in the evaporator. During the passage through the condenser, the refrigerant releases heat to the external environment and condenses until it becomes a liquid. Ideally, the process of condensation it occurs at constant pressure, although in reality pressure drops are usually manifested.

Basic components of a refrigerator

Coefficient of Performance (COP)

As already mentioned, to extract heat from a space (cool) and make the refrigerant change phases in a cyclical process, devices are required that operate consuming energy (electric or fuel), and this represents a cost, which should be tried to be minimized, maximizing the function that is expected of them within the cycle.

In power cycles, the thermal or electrical performance is usually determined from the efficiency of the cycle or its components, and as such, this value should always range between 0 and 1 (in percentage terms, it would be between 0 and 100 %). However, in refrigeration cycles, the most common is analyze its performance using as a reference parameter the coefficient of performance, abbreviated by its acronym in English as COP (coefficient of performance), whose value is almost always greater than unity, is always positive, and is determined from the following expression:

Ideal vapor compression refrigeration cycle

With the purpose of maximizing the coefficient of performance of the refrigeration cycles, different methods have been designed, the most The simplest of these is vapor compression refrigeration, which is formed by four states that are shown in the next image):

• An isentropic compression process in the compressor (states 1 – 2),

• A constant pressure heat rejection process in the condenser (states 2 – 3),

• An isenthalpic expansion process in the expansion valve (states 3 – 4),

• A heat absorption process in the evaporator at constant pressure (states 4 – 1).

The following schematic shows a diagram of temperature (T) vs entropy (s) where the different states through which the refrigerant passes are indicated during its passage through the system components to complete the refrigeration cycle by compression of steam:

Q_L is the heat absorbed from the conditioned space, Q_h is the heat rejected to the outside and W_comp represents the work of the compressor.

As described, the working fluid in the refrigeration cycles is the refrigerant, and there is a wide variety of them on the market. Refrigerants have evolved, and currently, the trend is to opt for those that are less polluting, non-toxic and that have the characteristics required in the system to be installed.

Some of the most commonly used types of refrigerants

This group of refrigerants make up 90% of the refrigerants used in the United States.