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This Sporlan training module will demonstrate the proper use of the pressure-temperature pocket card or wall chart. It will illustrate how pressure and temperature measurements from a refrigeration unit may be useful for system analysis. Included will be a discussion about superheated vapor, subcooled liquid and saturated vapor/liquid mixtures in an operating refrigeration system.
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How to Use a P-T Chart
EdUTech
P-T Charts - Superheat and Subcooling
1. How refrigerant exists in a system
2. Basic principles for P-T charts
3. Saturated refrigerant
4. Measuring Superheat
5. Measuring Subcooling
Evaporator
Condenser
Receiver
All Vapor
All Liquid
Mixture of Liquid & Vapor
Compressor
In this Diagram, Three ColorsRepresent the Forms or Phases of the Refrigerant
Refrigerant Phase Change in a Basic System
Refrigerant Phase Change
Superheated Vapor
Saturated Mixture
for Subcooling use Bubble Point
for Superheatuse Dew Point
Subcooled Liquid
Basic principles for P-T Charts
Pressur
e
Temperature
Temperatur
e
Pressure
Superheated Vapor
• Suction line from evaporator to compressor
R-410A
130
130
392
R-410A
130 psig
128 psig
372 psig
Sensible Temperature Above 45° F
Sensible Temperature Above 111° F
Sensible Temperature Above 44° F
• Discharge line from compressor to condenser
Electronic Thermometer & Pressure Gauge
MeasuringTEV Superheat
Measuring Superheat• Subtract the saturated
temperature which corresponds to the pressure, as read from the P-T card, from the actual measured temperature
• The difference is Superheat
Subcooled Liquid
• Liquid line from the condenser to the receiver
• Liquid supply line from the receiver to the metering device
366 psig
366 psig
365 psig
R-410A
Sensible Temperature Below 109.9° F
Sensible Temperature Below 109.7° F
Subcooled Liquid
• The difference is Subcooling
105° F
106° F
365 psig
R-410A
366 psig
P-T Equivalent to 366psig 110° F Measured Temperature 105° F Subcooling 5° F
P-T Equivalent to 365 psig 110° F Measured Temperature 106° F Subcooling 4° F
• Subtract the actual measured temperature from the saturated temperature which corresponds to the pressure, as read from the P-T card
3 Rules to Determine Refrigerant Condition
If a liquid and vapor are both present, the refrigerant is saturated
If the measured temperature is greater than the saturation temperature corresponding to the pressure, the refrigerant vapor is superheated
If the measured temperature is less than the saturation temperature corresponding to the pressure, the refrigerant liquid is subcooled
EdUTech
