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.
2. 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
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. Refrigerant Phase Change
for Subcooling
Superheated Vapor use Bubble Point
Saturated Mixture
for Superheat Subcooled Liquid
use Dew Point
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
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. 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. 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. 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
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.