In my last article, I offered an introduction to the critical concepts requisite to grasping the operating principle of refrigeration devices. In this article, I will continue from that point by presenting a closer examination of the relevant thermodynamic cycle and the construction of the applicable components so consumers can attain a deeper understanding of the design of their appliances.
First, it is necessary to clarify what a thermodynamic cycle is: a series of thermodynamic processes that are linked to the previous and subsequent process. Energy might be transferred to or from the system during these steps, with the system’s defining state variables also potentially being altered. This sequence of operations repeats in a loop. The refrigeration cycle is a thermodynamic cycle whose 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 placed in a cold room. It would seem exceptionally odd to claim that the presence of 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, as we can easily observe, 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. Heat transfer continues until thermal equilibrium is achieved: when the coffee reaches a temperature equal to that of the room.
While there are several distinct refrigeration cycles in use today, by far the most prevalent is the vapor-compression cycle. Four major components are involved in such a cycle: a compressor, a condenser, an evaporator, and an expansion valve. The condenser and evaporator are both heat exchangers; their role is to transfer heat energy from and to the working fluid of the device, respectively. The condenser and evaporator are arranged so that they are thermally isolated from one another, since, if they were placed in thermal proximity, the resulting device would not be efficacious. A compressor is a mechanical device that acts to increase the pressure and therefore temperature of a gas by decreasing its volume. An air compressor, used for inflating tires, is a common example. Finally, there is the expansion valve. While more complex and efficient designs exist, an expansion valve is fundamentally a small orifice or restriction through which working fluid is forced. 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 of the obstruction, the pressure is lower and equal to that of the atmosphere.
Now we can revisit the vapor-compression cycle. Since it is cyclical, an examination could begin at any point, but we’ll start at the inlet of the compressor. Refrigerant enters the compressor as a low pressure, low temperature gas; the compressor significantly increases both its temperature and pressure before entering 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 then 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 where that component is installed. At this point, any remaining liquid refrigerant evaporates into a gas. Leaving the evaporator, the working fluid, now a low pressure, low temperature gas enters the compressor to repeat its intricate thermal dance.
Many common appliances, such as refrigerators, air conditioners and heat pumps, make use of this technology. Fundamentally, a heat pump differs from other refrigeration devices 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 placement of the condenser facilitates the rejection of waste heat, typically into 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, an insulated zone whose temperature we desire to increase, 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 reverses the position of the heat exchangers, mimicking the arrangement found in a refrigerator.
The Solar Initiative offers subsidies to support the adoption of heat pump technologies for residential applications on Block Island. To learn more about our programs, please click here or email Wade Ortel at wade@thesolarinitiativebi.org