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The Pressure Enthalpy Diagram

( Key to the diagram features)

The pressure scale on the diagram is in "absolute" pressure. Some systems run in a vacuum which would be at an absolute pressure less than 15 PSIA and all compression ratios are calculated using absolute pressures. To convert from Absolute (PSIA ) to a gauge pressure (PSIG ) subtract 15 from the Absolute pressure to find Gauge pressure. To convert from Gauge to Absolute add 15 to the Gauge pressure. PSIA is the "Pressure" reference of the "Pressure Enthalpy" diagram

The enthalpy scale is shown at the bottom of the diagram. Any movement to the right indicates an increase in enthalpy. Enthalpy is defined as: Total heat in a substance above an accepted base temperature. In the case of refrigerants the accepted base is -40°F. If you use the R-22 diagram provided you will see plotting a line from -40 on the "saturated liquid line down to the enthalpy scale will give you a "0" enthalpy. This is the "Enthalpy" reference of the "Pressure Enthalpy" diagram.

The saturated condition "envelope" is outlined here. Anything inside this crooked "bell" shape is in a saturated condition. A saturated condition is defined as: Liquid and vapor in the same place under the same conditions of pressure and temperature. As you will see, the two pressures and temperatures in a refrigeration cycle are saturated conditions. Everything to the left of the bell is in a liquid state, everything to the right of the bell is in a vapor state. Inside the bell is where the cycle moves the majority of the heat energy. This is where the cycle does the work. The refrigeration cycle is shown imposed upon the refrigerant pressure enthalpy diagram. The arrows indicate the movement of the heat energy (enthalpy) and pressure while the refrigeration cycle is operating. Any movement to the right is increasing in enthalpy, any movement to the left is decreasing in enthalpy. Any movement upward is an increase in pressure, any movement downwards is a decrease in pressure. If you really wish to understand the "mechanical compression refrigeration cycle" spend some time with the pressure enthalpy chart. As you move through the modules of this course you will find each part of the cycle plotted on the pressure enthalpy diagram. This is an introduction to that diagram for your reference.

Pressure in PSIA


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