A brief overview of the refrigeration cycle

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Fri, 06/24/2022 - 4:45pm
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In my last article, I offered an introduction to the operating principles of refrigeration. Let’s continue where we left off. I believe a closer examination of the thermodynamic cycle and the construction of the applicable components will offer greater utility for consumers who want a deeper understanding of the operating principles of their appliances.
First, let’s clarify what a thermodynamic cycle is: a series of thermodynamic processes that are linked to the previous and subsequent process. Heat energy, or work, might be transferred to or from the system during these steps. The sequence of operations repeats in a closed loop.
The refrigeration cycle is one example commonly found in use today. Its purpose is to facilitate the transfer of heat from an area of low temperature to one at a higher temperature. Intuitively, we know that heat will not tend to flow spontaneously in this direction, a fact that is formalized by the second law of thermodynamics. Consider a mug of piping hot coffee brought into a cold
room. It would seem exceptionally odd to claim that the coffee could act to decrease the temperature of the room, or that its being in the cold room might somehow heat the coffee. Instead, the temperature of the coffee will tend to diminish over time, while the room’s temperature will rise infinitesimally, though the latter may be difficult to measure. This transfer of heat will continue
until thermal equilibrium is achieved: when the coffee reaches a temperature equal to the air in the room.
While there are several different refrigeration cycles in use today, the most common is the vapor-compression cycle. There are four main components to such a cycle: a compressor, a condenser, an evaporator, and an expansion valve. The condenser and evaporator are both heat exchangers, often similar in design. They transfer heat energy from and to the working fluid of the devices. The condenser and evaporator are arranged such that they are thermally isolated, since, if they were to coexist in proximity while sharing a thermal reservoir, they would fail in transferring heat from an area of low temperature to one of high temperature. A compressor is a mechanical device that increases the pressure and therefore temperature of a gaseous fluid by decreasing the volume it occupies. An air compressor, used for inflating tires, is a common example. Finally, there is the expansion valve. While far more complex and efficient designs exist, a simplistic expansion valve is basically a small orifice or restriction that the working fluid is forced through. To illustrate its operating principle, consider holding one’s finger over the end of a garden hose; significant pressure builds behind the finger, a feeling that is detectable by said finger. Downstream, as the water flows forth, the pressure is far lower and, in fact, equal to that of the atmosphere.
Now we can revisit the vapor-compression refrigeration cycle. Since the process is cyclical, we can begin at any point, but let’s start at the inlet of the compressor. Refrigerant enters the compressor as a low- pressure, low-temperature gas; the compressor does work on the fluid, significantly increasing both its temperature and pressure before it enters the condenser. In the condenser, the working fluid is cooled and condenses into a high-pressure liquid. In this step, heat is rejected from the system into the high temperature thermal reservoir. The now-liquid refrigerant passes through the expansion valve, where its pressure is drastically reduced. This abrupt drop in pressure causes much of the fluid to vaporize and the temperature to drop precipitously. As the mixture of frigid liquid and gaseous refrigerant moves through the evaporator, heat energy is removed from the low temperature thermal reservoir in which that component is installed. During this step, any remaining liquid refrigerant evaporates into a gas. Leaving the evaporator, the working fluid, now a low pressure, low temperature gas makes its way back into the compressor to repeat its intricate thermal dance once again.
Many appliances make use of the vapor-compression cycle; refrigerators, air conditioners and heat pumps are examples. A heat pump differs only in its mission and the orientation of its heat exchangers. A refrigerator places the evaporator inside a thermally isolated chamber whose temperature we wish to decrease, while the condenser is placed so waste heat can be rejected to the environment, typically to the room where the device is placed. In contrast, a heat pump, operating to warm a living space during cold weather, places the condenser inside one’s home, while the evaporator lives outside where heat is absorbed from the environs. Modern heat pumps include a reversing valve that allows them to operate for both heating and cooling. This mechanism effectively reverses the position of the two heat exchangers, placing them in a fashion that mimics the arrangement found in a refrigerator or air conditioner.
The Solar Initiative offers subsidies to support the adoption of heat pumps for residences on Block Island. To learn more about our programs, please visit our website www.thesolarinitiativebi.com or email Wade Ortel at wade@thesolarinitiativebi.com