If the compressor operates at a suction pressure of 49.7 psia, then the corresponding evaporator pressure is also 49.7 psia and the evaporator temperature is 40 ☏. It is important to note that in the mix region, the pressure and temperature are dependent variables.įor example, if a compressor operates at a suction pressure of 36.8 psia, then the corresponding evaporator pressure is 36.8 psia and the corresponding evaporator temperature is 25 ☏, see below figure for points A and B (Values are for Refrigerant R-134a). The suction line is the piping that routes refrigerant gas from the evaporator to the compressor. The operating pressure and temperature of the evaporator is called the suction pressure and suction temperature. The refrigerant entering the evaporator is a cold, partial liquid-vapor mixture. A sample R-134a diagram is shown below, with a sample refrigeration cycle, identifying (Step 1) Evaporator, (Step 2) Compressor, (Step 3) Condenser and (Step 4) Expansion Device. These diagrams can be found in the ASHRAE Fundamentals book. It is recommended that the engineer get a copy of the P-H diagram for R-134a and the other common refrigerants. Throughout this explanation the refrigerant R-134a is used as an example. The following sections will show each specific part of the refrigeration cycle on the pressure-enthalpy diagram and it will also highlight the important points and calculations needed. One of the most important skills needed for the professional engineer in the HVAC and Refrigeration field is navigating the refrigeration cycle on a pressure-enthalpy diagram. The skeletal graph shown below shows the pressure-enthalpy lines. The enthalpy lines are the vertical lines. The x-y axes of the P-H diagram are the pressure lines running from left to right. The previous figure, Figure 7, indicates the quality lines. The 0.9 or 90% line, which is located near the saturated vapor line, describes points that have 90% vapor by mass. For example quality of 0.1 or 10%, which is located near the saturated liquid line, describes points that have 10% vapor by mass. Quality is a measure of the ratio of vapor mass to total mass. There are also upward sloping curves which indicate quality. Figure 8 illustrates the horizontal temperature lines in the mix region. The mix region is the phase change region, where any addition of enthalpy will cause additional liquid to vaporize instead of raising the temperature. (3) Liquid-Vapor Mix Region: In this region, the P-H diagram shows horizontal temperature lines, which indicate constant temperature. Entropy is the measure of the amount of disorder in the system. There are also lines of constant entropy, which are also important. In this region there are vertical temperature lines, which increase as enthalpy is increased. (2) Vapor Region: The vapor region is also known as the super heated region. Figure 8 is a simplified P-H diagram illustrating the constant temperature lines. (1) Liquid Region: The liquid region is also known as the sub-cooled region. The curves break up the diagram into three regions (1) Liquid, (2) Vapor and (3) Mix. A simplified pressure-enthalpy diagram is shown below, describing this information. The importance of this point is that at any point above, no additional pressure will change the vapor into a liquid. The point at which the two curves meet is called the critical point. The locations to the left of the saturated liquid curve indicate that the refrigerant is in liquid form and locations to the right of the saturated vapor curve indicate that the refrigerant is in vapor form. The region in between the two curves describe refrigerant states that contain a mixture of both liquid and vapor. The left vertical curve indicates the saturated liquid curve and the right vertical curve indicates the saturated vapor curve. The upside down U figure shown on the diagram designates the points at which the refrigerant changes phase. Typically enthalpy is in units of Btu/lb and pressure is in units of pounds per square inch (psi). On the P-H diagram, pressure is indicated on the y-axis and enthalpy is indicated on the x-axis.
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