Fundementals of Refrigeration - The Pressure Enthalpy (Heat) Diagram

by Oldrich Bocek(1939-2003)
Thermal Management Expert
Berg Chilling Systems Inc.

The pressure-enthalpy also called pressure-heat diagram is used to describe in engineering terms the interaction of heat, pressure, temperature, heat content, and cooling capacity of a vapor-compression system. This diagram charts pressure along the vertical axis and enthalpy (the heat content of refrigerant compared to reference value) along the horizontal axis.

Table B4 is a pressure-enthalpy diagram showing energy flow and changes of state in the vapor-compression cycle.

The curve for the refrigerant crosses several important lines and areas in the pressure enthalpy diagram:

In the all-liquid area, all refrigerant is a sub-cooled liquid.

In the all-vapor area, all refrigerant is super-heated gas.

In the liquid-vapor area (also called saturated area), refrigerant is a saturated mix of liquid and vapor. This condition is found in both the condenser and the evaporator.

The saturated liquid line separates the liquid-vapor area from the all-liquid area. Refrigerant along this line is not sub-cooled, but it becomes sub-cooled as it enters the all-liquid area.

The saturated vapor line separates the liquid-vapor area from the all-vapor area. Super-heating begins as soon as the gas moves past this line into the all-vapor area.

The line of constant quality is a line along which the refrigerant has constant proportion of gas and liquid.

The line of constant enthalpy (heat) is a vertical line along which refrigerant has equal total heat content.

The line of constant temperature marks locations along which the refrigerant has constant temperature. The line is vertical in all-liquid area, horizontal in saturated vapor area, and nearly vertical in the all-vapor area.

The line of constant pressure is a horizontal line describing location with constant pressure.

The pressure-enthalpy (heat) relationships of the refrigerant are mapped along the polygon ADEFG. The action of a vapor-compression cycle can be understood by referring to table B5.

A    The hot vapor has been compressed in the compressor and is at its maximum pressure temperature, and enthalpy. Without loosing pressure, the vapor enters the condenser.

B    and begins to loose the latent heat of condensation, as shown by the leftward movement (loss of enthalpy) across the diagram. The condenser pressure remains constant while the refrigerant loses heat. At

C    the refrigerant is totally condensed and at the saturated liquid line. The condenser continues drawing heat from refrigerant, which finally becomes a sub-cooled liquid and begins to loose sensible heat. Now the liquid refrigerant enters the liquid line. Temperature and pressure remain constant in the liquid line until

D    the metering device. Now the pressure falls suddenly along the line DE as the refrigerant re-crosses the saturated liquid line and enters the evaporator at

E    the pressure is low and the liquid refrigerant begins to vaporize. After the evaporator pressure stabilizes at E, the refrigerant gains heat and boils until

F    when it is entirely vaporized. Now the refrigerant is entirely vapor. This vapor begins to super-heat in the suction line until it reaches the compressor.

G    The compressor squeezes the refrigerant, raising pressure, temperature, and enthalpy, and the cycle repeats as the refrigerant reaches A.

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