Thermodynamic Diagrams

In a thermodynamic diagram, the temperature, pressure, volume, enthalpy, and entropy relationships of substances are represented. The most frequently used diagrams are temperature-entropy and pressure-enthalpy diagrams. In these diagrams, it is important to know whether the system being described is one phase or multiphase and which phases are present. This is most readily achieved by plotting the saturation lines, which represent, in a system capable of existing as liquid or as vapor (ie, a fluid), the point at which the first bubble of vapor appears in the liquid and the point at which the last drop of liquid evaporates to vapor. Thus, the saturation lines delineate the boundary between the one-phase and the two-phase regions. The one phase may be liquid or vapor, or the two phases may be liquid and vapor in coexistence. At some temperature and pressure, known as the critical point, liquid and vapor can no longer be distinguished.

To calculate heat and work in a mechanical refrigeration system, it is necessary to have knowledge of the thermodynamic properties of the working fluid, or refrigerant. This can readily be represented by means of a pressure-enthalpy diagram. A schematic pressure-enthalpy diagram is shown in Figure 1. The line that describes the property of saturated liquid (g), the line that describes the property of saturated vapor (h), and the critical point (*) are marked. To the left oig is a single-phase region of liquid. The g describes the liquid at the point when vapor just begins to boil off. The h describes the vapor as the last drop of liquid vaporizes. Between g and his & two-phase region of mixed liquid and vapor. Beyond h, the single phase is vapor. Vertical lines describe conditions of constant enthalpy, where the heat content of the refrigerant is unchanging. Horizontal lines describe conditions of constant pressure. The line c describes a constant temperature path. Note that in the two-phase region, this line is horizontal, that is, at constant pressure. At constant pressure, a pure liquid boils at constant temperature. As pressure increases, the boiling point increases. The line e describes a path of constant entropy. Additional lines, such as constant quality (ie, constant liquid/vapor ratio) and constant density lines can also be plotted. Diagrams such as these are readily available for all common refrigerants. Similar

Figure 1. A schematic pressure-enthalpy diagram.

Enthalpy

Figure 1. A schematic pressure-enthalpy diagram.

Figure 3. Pressure-enthalpy diagram for the mechanical refrigeration system shown in Figure 2.

Hi H2 H3

Enthalpy

Figure 3. Pressure-enthalpy diagram for the mechanical refrigeration system shown in Figure 2.

diagrams for temperature-entropy relations can be drawn. The key lines for understanding refrigeration systems are those that describe the saturated liquid and the saturated vapor.

Compression Refrigeration Cycles

Figure 2 schematically shows a mechanical refrigeration system. In the evaporator, a liquid under low pressure boils as it absorbs heat entering from the surroundings. The vapor travels to a mechanical compressor, which raises its pressure. The temperature increases. The high-pressure vapor passes through a condenser, where it rejects the heat to the surroundings. As heat is lost, the vapor condenses. The resulting liquid is held in a reservoir. As required, liquid from the reservoir passes through an expansion valve into the low-pressure side and enters the evaporator, where the cycle begins again. An operating principle of such a device, which uses work to remove heat in a cyclical process, may be described by a reversed Carnot cycle. Figures 3 and 4 are the basic thermodynamic diagrams that describe the process. In Figure 3, the changes in unit mass of refrigerant fluid are as follows. Point e represents the saturated liquid in the reservoir. It has a heat content of Hi, pressure of P2, and temperature of T2. As it passes through the expansion valve, the pressure drops, and some vaporization occurs, lowering the temperature and bringing the fluid to a, at temperature Tlt pressure P1 and heat content still Hj. As the refrigerant takes in heat from the

Figure 2. A mechanical refrigeration system, schematic.

Figure 4. Temperature-entropy diagram for the mechanical refrigeration system shown in Figure 2.

Figure 2. A mechanical refrigeration system, schematic.

Entropy

Figure 4. Temperature-entropy diagram for the mechanical refrigeration system shown in Figure 2.

surroundings (at a temperature greater than 71,), the heat content increases. At b, the heat content is now H2, with the pressure still and the temperature 7\. This point is on the saturation line and describes the boiling of the last drop of the liquid. From b to c describes the compression of the initially saturated vapor, an adiabatic (or isentropic) step. The increase in heat content is the work supplied by the compressor to increase the pressure. At c, the temperature is T3, the heat content is Hz, and the pressure is P2. The vapor enters the condenser and loses heat to the surroundings. At d, on the saturation line, condensation just starts to occur. The pressure is still P2, the temperature has dropped to T2 (still above the surroundings), and the heat content is now H2. As heat continues to leave the refrigerant, it enters the two-phase region, until the saturated liquid is reached at e. At e, the condition of the liquid reservoir, the heat content is Hlt the pressure is P2, and the temperature is T2. Thus, in one pass through the cycle, the unit mass of refrigerant has picked up an amount of heat, H2 — Hlt in the evaporator at temperature T1 and has rejected this heat in addition to the heat representing the work of compression, H3 — H2 in the condenser. Work is being used to enable heat to be transferred from a lower temperature to a higher temperature. A refrigerant is chosen so that the pressure required for a boiling point at or below the refrigeration temperature is not too low and so that the pressure required to have a boiling point high enough to reject heat into the surroundings of the condenser is not too high. Table 2 lists appropriate temperatures and pressures for some common refrigerants. The major components of a vapor compression system are the compressor, the condenser, the evaporator, and the controls. Any standard type of compressor is satisfactory; reciprocating compressors are commonly used, but rotary and centrifugal compressors can also be used. Condensers and evaporators are simply heat exchangers. The condenser, which serves to reject heat from the system, may use air cooling, water cooling, or evaporative cooling. Water-cooled condensers flow the water through one of three configurations: shell and tube, shell and coil, and tube in tube. An air-cooled condenser circulates the high-temperature, high-pressure refrigerant through the condenser coil and uses air moved by free convection, wind, or fans to remove the heat. An evaporative condenser transfers heat from the coil to a film of water that evaporates into an air stream passing over it. In the evaporator, the liquid refrigerant boils at low temperature and pressure within the coil, and the medium to be cooled is in contact with the exterior of the coil.

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