The Thermodynamics of Refrigeration
The Thermodynamics of Refrigeration
Abstract
Thermodynamics as a scientific concept is best explained using mathematical equations. It involves processes of transferring heat given varying pressures and temperatures among other variables.
Introduction and Background Information
The main purpose of a refrigeration plant is to cool down articles or a substance down to, and/or maintain these substances at a temperature which is lower than the ambient.
Among the oldest and most popular refrigerants known to man include water, air and ice. in the beginning, the main purpose of cooling down substances was for food preservation. The Chinese take the credit as the first people to discover that ice increased the life of food and additionally, it improved the tastes of different drinks. The Eskimos also have also preserved their food for centuries by freezing it in ice.
In the beginning of the last century, science has made a tremendous step and terms such as enzymes, bacteria or yeast came to light for man, it was established that micro-organisms depended on the temperatures of their surrounding for their colonies to grow, and the growth level becomes really low at temperatures below 10 degrees Celsius. As a result of this knowledge, it is now common practice to use refrigeration as a convenient method for food preservation.
“Thermodynamics simply describes the movement of heat. Thermodynamics is derived from thermo, meaning heat, and dynamics, (literally means “power”), and is used to describe the movement or change of a process due to heat flow. Heat is the flow of energy from one object or system, to another object or system”, (Dolin 1).
According to Dolin, Cold is “made” by removing heat from an object or system and cold is not a substance just as heat is not a substance (1). Heat is one of the principles one has to know in order to understand thermodynamics. Other principles include:-
Temperature – this is the measure of kinetic energy of an object kinetic energy being the “warmness” or “coldness” of the object
Closed system – in this kind of a system, no mass crosses the boundaries of the system but energy is transferable between the system and its surrounding.
Isolated system – in the system both mass and energy cannot cross the boundaries of the system.
Open system – in this system mass and energy can freely move beyond the boundaries of the system and the surrounding.
Laws of thermodynamics.
1. The Zeroth Law
The states that if body A and body B both have temperatures equal to that of body C, then the temperature of body A equals the temperature of body B. Note that this law seems extremely obvious, but still remains the basis for temperature measurement.
2. First law
The law states that energy can neither be created nor destroyed but instead can change form and its location (Dolin 3). It formula is:-
dU = dQ + dW (1)
Where dU is the infinitesimal change of the internal energy. dQ is a small amount of heat added to or removed from the considered system (e.g. Fig. 1), or created by a magnetic internal source (magnetocaloric effect). dW denotes the differential of the work performed on the system, or extracted from it (Kitanovski & Elgolf 3).
3. Second law
The law states that energy must flow from a higher state to a lower state. That is, heat must always flow from the warmer object to a cooler object and not from the cooler object to the warmer object (Dolin 3).
Some heat is wasted when converting heat into mechanical energy. The entropy of the universe is increasing. The mathematical expression is:-
QH/TH = QL/ TL (2)
4. Third law
The third law states that the entropy of a perfect crystal approaches zero as the absolute temperature approaches zero thus providing an absolute reference point for the determination of entropy (Dolin 3).
Energy conversion
In the refrigeration cycle electrical energy is converted into mechanical energy through an electric motor. The refrigerant is then converted from gas to liquid and back to gas again by compression. After that the endothermic process of liquid to gas is balanced by an exothermic process. The lower temperature heat source is pumped into a higher temperature heat sink. Insulation reduces the work and energy required to achieve the lowered temperature. Internal energy (U) will not spontaneously flow from a cold to hot region. Thus, it must be forced by doing work on the system. This takes place via a phase change in the refrigerant (Osterberg & Ulness 1).
(3)
Vapor compression refrigeration
The refrigerant gas is compressed by use of a compressor that is usually powered by an external energy source for example electricity. The gas enters from the evaporator to a temperature that exceeds the surroundings of the condensing element. The refrigerant then enters the condenser in the form of vapor. The heat exchanging coils and fins of the condenser dissipate the heat caused by pressurization to the surroundings.
The refrigerant then slowly cools as it passes through the condenser coils, and condenses into liquid form. The pressure inside the condenser remains significant as the liquid refrigerant nears the throttling expansion valve.
The throttling expansion valve controls the rate at which the liquid refrigerant flows from the high pressure condenser element into the low pressure evaporator coil that resides with the subject of the refrigeration process. An expansion valve may be as simple as a capillary tube that simply restricts the flow of liquid into the evaporator. The pressure drop between the condenser and the evaporator causes the liquid refrigerant to immediately boil and evaporate, thus drawing heat from itself, resulting in a reduction of temperature of the evaporator coil.
The coil-fin arrangement of the evaporator is distinctly similar to the condenser, except that the evaporating gas inside the coil absorbs heat from the surroundings of the evaporator. The refrigerant then returns to the condenser, where the compression cycle begins once more. Note that the pressure difference required for cooling is maintained since the compressor continually sucks gas from the “out” end of the evaporator.
Conclusion
While the phase changes for water work will in a direct heat engine application like a power plant, the circulating heat transfer medium in a refrigerator must possess specific characteristics. The refrigerant must be recyclable: that is, it can be condensed and evaporated repeatedly. Furthermore, it must be able to easily absorb and expend heat at normal operating temperatures for the device. Water fails as an efficient refrigerant: its boiling point greatly exceeds normal operating conditions, and its freezing point also is too high for good cooling.
References
(1) Andrej Kitanovski, Peter W. Egolf, Thermodynamics of magnetic refrigeration, (2005) pp 1-25.
Karin Osterberg Dr. Darin Ulness, The Thermodynamics of a Refrigerator Concordia College (2005): pp. 1-19.
Brian Dolin, Basic thermodynamics for refrigeration and air conditioning – part 1, Washington, (2010): pp. 1-3
Black, William Z. & James G. Hartley. Thermodynamics. New York: Harper & Row , (1985): pp. 1-13.
