pressure entalpy

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DOC NO: HVAC-CH-02.3

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Refrigeration System

Learning Objectives

1

Pressure Enthalpy chart


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The pressureenthalpy (P-h) chart plots the properties of a

refrigerantrefrigerant pressure on the vertical axis and enthalpy onthe horizontal axis.

Enthalpy is a measure of heat quantity, both sensible and latent, perpound [kg] of refrigerant. It is typically expressed in terms of Btu/lb

[kJ/kg].


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4

To further demonstrate the use of the Ph chart, let us lookat the process of heating and boiling water, at a constantpressure, on a Ph chart for water.


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5

The distance between the edges of the envelope indicates the quantityof heat required to transform saturated liquid into saturated vapor at a

given pressure.This is called the heat of vaporization.


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6

The P-h chart can be used to analyze the vapor-compressionrefrigeration cycle and determine the conditions of the refrigerant atany point in the cycle. The chart in this example is for R-22.


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7

The change in enthalpy from A to C that occurs inside the evaporator iscalled the refrigeration effect. This is the amount of heat that each

pound [kg] of liquid refrigerant will absorb when it evaporates.


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8

Compressors are designed to compress vapor. Liquid refrigerant cancause damage if drawn into the compressor. In some refrigeration

systems additional heat is added to the saturated vapor (B) in theevaporator to ensure that no liquid is present at the compressor inlet.This additional amount of heat, above saturation, is called superheat.


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9

The compressor draws in the superheated refrigerant vapor (C) andcompresses it to a pressure and temperature (D) high enough that it canreject heat to another fluid. As the volume of the refrigerant is reducedby the compressor, its pressure is increased.


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When the refrigerant vapor is discharged from the compressor, itstemperature is substantially higher than its saturation temperature(the temperature at which the refrigerant would condense). Theincrease in enthalpy from C to D is due to heat added by thecompressor, or the heat of compression.


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Inside of the condenser, heat is transferred from the hot, high-pressure refrigerant vapor (D) to relatively cool ambient air. This

reduction in the enthalpy of the refrigerant vapor causes it todesuperheat. It becomes saturated vapor, condenses into saturatedliquid, and further sub cools before leaving the condenser (G) to go tothe expansion device.


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The primary purpose of the expansion device is to drop the pressure ofthe liquid refrigerant to equal the evaporator pressure. At this lower

pressure, the refrigerant is now inside the saturation envelope where itexists as a mixture of liquid and vapor.


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pressure entalpy

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