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# How To Use a P-T Chart

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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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• This presentation is intended to educate on how to calculate superheat and subcooling. The main points to be covered are: How refrigerant exists in a system, Basic principles of P-T charts, Saturated refrigerant, calculating Superheat and calculating Subcooling. To understand how to calculate Superheat or Subcooling, an understanding of refrigerant phase change in the system and the pressure-temperature relationship is necessary.
• Refrigerant exists in the system in three forms or “phases”. That is all vapor, all liquid or a mixture of both vapor and liquid. The diagram illustrates the form in which refrigerant is found at various points in the system. From the outlet of the evaporator to the compressor and from the compressor to the inlet of the condenser there will normally be only vapor. From the outlet of the condenser to the inlet of the receiver and from the receiver to the metering device there will normally be only liquid. In the evaporator and condenser there is refrigerant in transition from a liquid to a vapor (in the evaporator) and from a vapor to a liquid (in the condenser). In other words, mixed vapor and liquid. Regardless of the percentage of vapor to liquid or the pressure at which it exists, where refrigerant vapor and liquid is mixed we say it is “saturated”. The pressure exerted on this saturated refrigerant determines the actual temperature of this mixture. Refrigerant in the receiver is also comprised of both liquid, on the bottom, and vapor, at the top. However during normal operation, there is a liquid to vapor interface at saturation temperature only at the surface of the liquid. Since refrigerant is drawn out of the receiver from below the surface interface, only liquid is provided to the metering device.
• Mixed vapor and liquid refrigerant is referred to as Saturated. Pure vapor at temperatures above saturation is designated as Superheated Vapor. Pure liquid at temperatures below saturation is designated as Subcooled Liquid. Some refrigerants are blends consisting of two or more components. They exhibit temperature glide. When a refrigerant blend is designated on a P-T card, its Bubble Point from the P-T card should be used to determine Subcooling and its Dew Point should be used to determine Superheat.
• The pressure-temperature card simply relates the temperatures at which a saturated refrigerant changes form, each to a given pressure. This relationship is expressed in table form on a P-T card or wall chart. In the example shown, pressures are listed in a column at the left, along with corresponding boiling temperatures for several refrigerants in columns on the right. The refrigerant pressure-temperature relationship is listed on some P-T cards with temperatures in a column at the left and corresponding pressures in columns under their respective refrigerants on the right. Regardless of how the information is presented, the corresponding related pressure-temperature provides an accurate relationship for a given refrigerant wherever liquid and vapor are both present together, defined as saturation.
• At any point in refrigerant system where only vapor is present, the actual temperature will be at or above the saturation temperature corresponding to the pressure at or near that point as read from a P-T card. Vapor, at temperatures above saturation or boiling point at a given pressure, is known as superheated vapor.
• To accurately calculate superheat, an electronic thermometer should be used to measure the actual temperature at the site and a pressure gauge must be used to measure the refrigerant pressure as near to the site as practical. This pressure reading will be converted to a saturated or boiling temperature using a P-T card to allow a temperature difference to be determined.
• Superheated vapor is indicated when the measured temperature is greater than the saturation temperature corresponding to the system pressure reading as designated on the P/T chart. The amount of superheat will be the difference between these two temperatures. Superheated vapor is present in the suction and discharge lines of a normally operating system.
• At any point in refrigeration system where only liquid is present, the actual temperature will be at or below the saturation temperature corresponding to the pressure at or near that point as read from a P-T card. Liquid at temperatures below saturation or boiling point at a given pressure is known as subcooled liquid.
• Subcooled liquid is indicated when the measured temperature is lower than the saturation temperature corresponding to the system pressure reading as designated on the P/T chart. The amount of subcooling will be the difference between these two temperatures. Subcooled liquid is present in the liquid line from the condenser to the receiver and in the liquid supply line from the receiver to the metering device of a normally operating system.
• If a liquid and vapor are both present, the refrigerant mixture is said to be 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.
• ### How To Use a P-T Chart

1. 1. How to Use a P-T Chart EdUTech
2. 2. P-T Charts - Superheat and Subcooling1. How refrigerant exists in a system2. Basic principles for P-T charts3. Saturated refrigerant4. Measuring Superheat5. Measuring Subcooling
3. 3. Refrigerant Phase Change in a Basic System In this Diagram, Three Colors Evaporator Represent the Forms or Phases of the Refrigerant Condenser All Vapor Compressor All Liquid Receiver Mixture of Liquid & Vapor
4. 4. Refrigerant Phase Change for Subcooling Superheated Vapor use Bubble Point Saturated Mixture for Superheat Subcooled Liquiduse Dew Point
5. 5. Basic principles for P-T Charts Pressur ratur e Tempe Tempe e ratur e rPressue
6. 6. Superheated Vapor Sensible Temperature Above 45° F• Suction line from 130 psig 130 evaporator to compressor Sensible Temperature Above 111° F 372 psig 392• Discharge line from 128 psig 130 R-410A compressor to Sensible condenser Temperature Above 44° F
7. 7. Electronic Thermometer & PressureGauge Measuring TEV Superheat
8. 8. 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
9. 9. Subcooled Liquid Sensible Temperature Below 109.7° F• Liquid line from the 365 psig condenser to the receiver R-410A• Liquid supply line from the receiver to the metering device 366 psig 366 psig Sensible Temperature Below 109.9° F
10. 10. Subcooled Liquid P-T Equivalent to 365 psig 110° F Measured Temperature 106° F Subcooling 4° F• S 365 psig u 106° F b t r a c t R-410A• The difference is Subcooling t h 105° F e 366 psig a c P-T Equivalent to 366psig 110° F t Measured Temperature 105° F Subcooling 5° F u a
11. 11. 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
12. 12. EdUTech