By Pier-Luc Vinet-Thibault Eng.,
The notion of "cooling" items or spaces has been developed and tested as early as the 18th century. During this era, people generally gathered ice blocks from various bodies of water to preserve their food for longer periods. A number of companies were known for their thermal isolation products, as the first concern of consumers was conserving the ice, not creating it.
In the first half of the 19th century, an economically viable solution to produce artificial refrigeration transformed the prospects of numerous domains where ice and cooling were imperative. Nowadays, houses and buildings commonly include cooling systems meant to provide for the occupants' needs. Those systems are beneficial for food and organic matter preservation, machinery cooling, air conditioning and even the practice of sports.
Most of the modern cooling systems use the principle of the refrigeration cycle to extract heat from a room and expel it elsewhere. This cycle is based on the Second law of thermodynamics: heat always flows spontaneously from regions of higher temperature to regions of lower temperature in order to attain a thermal balance, hence both regions reaching a similar temperature. During this process, there is an energy exchange from the warmer region to the colder region.
Temperature and energy
Before analyzing the process of the refrigeration cycle, one must understand the relationship between energy and temperature. In fact, energy and heat are ever-present in objects, even in extremely cold temperatures. From this perspective, "cold" does not mean "without heat" but simply "of a low heat", hence "with little energy". To be precise, only when the temperature reaches 0 kelvin (K) (-273 Celsius degrees (ºC)) is the energy almost null. By applying the Second law of thermodynamics, if we wish to extract the energy of an object whose temperature is 40 ºC, we must simply use a colder object in order to absorb the energy by temperature difference.
The exchange of energy in a phase change also represents a major phenomenon in refrigeration. An easy way to illustrate the energy absorption phenomenon in a phase change, like in evaporation, is to place a bit of water on a table. After a while, the water will evaporate completely and thus have reached a state of equilibrium with the ambient air. During the evaporation process, the water will have absorbed energy (heat) from the table and will leave a cold region on its surface. In fact, a considerable quantity of energy is necessary in a phase change and that specific aspect is useful in the refrigeration cycle. For example, it takes seven times more energy to bring water from a liquid state to evaporation at 100 ºC, than it takes to increase the same liquid's temperature from 20 ºC (ambient temperature) to 100 ºC. A similar analogy can be made regarding solidification or freezing water. Water will release four times more energy in order to solidify (freeze) at 0 ºC than it would for its temperature to drop from 20 ºC to 0 ºC.
The refrigeration cycle
The refrigeration cycle exploits both previously described phenomena, in order to absorb energy (temperature) from one place and expel it elsewhere. A cold fluid is used to absorb heat through temperature difference and the absorption of energy is maximised as the fluid changes phase to gas in the process.
The usage of a specific fluid that would be subject to a phase change during the heat transfer is crucial; hence numerous substances designated as "refrigerants" were conceived. The most popular refrigerants are the R-12, R-22, R-134a and R-410a, where R-12 and R-22 are progressively replaced as they present a danger for the environment. Ammonia (R717) and Carbon Dioxide (R744) are also used as refrigerants since they are less harmful for the environment, but they are toxic to humans.
In a refrigeration cycle, there are four principal elements corresponding to four physical states of the refrigerant. Figure 1 illustrates each step of the cycle.
State 1: The refrigerant is cold and in a vapour state (low pressure). A compressor is used to compress the refrigerant. The compressor is controlled by pressure sensors and maintains a differential pressure in the refrigeration system.
State 2: The refrigerant is hot and in vapour form (high pressure). It contains a maximum amount of energy. The refrigerant goes through a condenser acting as a heat exchanger and releases its energy. A fluid (water, air or other), which is at a lower temperature than the refrigerant, absorbs the energy (heat) of the refrigerant by temperature difference. As the refrigerant expels its energy, its temperature decreases and eventually reaches its condensation temperature. When the refrigerant condenses, even more energy is released by phase change.
State 3: The refrigerant is a liquid at high temperature. An expansion valve is used to control the flow of refrigerant through the evaporator. The thermostatic valve opens or closes automatically depending on the refrigerant's temperature at the outlet of the evaporator. The valve reduces the refrigerant pressure and allows it to cool by expansion before entering the evaporator. It also ensures that the refrigerant exiting the evaporator is completely gaseous and that no liquid (incompressible) damages the compressor.
State 4: The refrigerant is in the form of a liquid/gas mixture, but it has now cooled. It contains a minimum amount of energy. The cold refrigerant flows through a second heat exchanger called evaporator and absorbs the energy (heat) from the element to be cooled (room, object or fluid). Since the refrigerant is colder than the element, it absorbs the energy (heat) by temperature difference. As its temperature decreases, the refrigerant reaches its evaporation temperature and absorbs even more energy during the phase change.
Knowledge of the refrigeration cycle and its applications have enabled us to greatly improve our life quality by moving energy where we wanted, which permit us to control the ambient temperature for our greatest comfort.
Moreover, the application of the principle of heat absorption and rejection of the refrigeration cycle is extremely interesting in a context where there is concern about energy saving and efficiency. For instance, systems have been developed that expel the accumulated heat of a house into an outdoor swimming pool in summer, or absorb heat from the ground to heat a building in winter (geothermal).
Despite the many advantages brought by refrigeration systems, there are also disadvantages resulting from mechanical failures due to operating conditions, maintenance, design or manufacturing of these complex systems. These failures can cause significant damage to the items preserved, or to the buildings in which the refrigeration systems are installed.
Consequently, these failures can lead to disputes and litigation requiring the analysis of the technical aspects involved. In this regard, the experts of CEP Forensic Engineering inc. are able to conduct various technical analyses in order to establish the root causes of these failures.