How to Read P-H Diagram Notes

Introduction to P-H Diagram

P-H Diagram Definition:
- A graphical representation of the thermodynamic properties of a refrigerant.
- Illustrates the relationship between pressure and enthalpy.
- Enthalpy is measured in BTU/lb (British Thermal Units per pound).
- Pressure is measured in psi (pounds per square inch).

Importance of Using P-H Diagram

Clear Understanding:
- Provides a clear understanding of how pressure and enthalpy change as the refrigerant moves through the refrigeration system, from compression to expansion.
Troubleshooting:
- Aids in troubleshooting mechanical refrigeration systems.
Function of Components:
- Helps visualize the function of each component and how they work together to change pressures and move heat.
Predicting Pressures and Temperatures:
- Enables the prediction of pressures and temperatures expected at various points in the system.
System Design:
- Assists in designing various and energy-efficient combinations of system design.

Comparison Between P-H Diagram and T-H Diagram

P-H Diagram:
- Tool in refrigeration and air conditioning.
- Designed to depict the relationship between pressure and enthalpy.
- Visualizes changes in a refrigerant's state during the refrigeration cycle.
T-H Diagram:
- Focuses on illustrating the relationship between temperature and enthalpy of a substance.
- Allows for the analysis of heat transfer processes.
- Shows how temperature changes with enthalpy variations.

Components of a P-H Diagram

Axes of the P-H Diagram

X-Axis (Horizontal Axis) – Enthalpy (H):
- Measured in BTU/lb (British Thermal Units per pound) or kJ/kg (Kilojoules per kilogram).
- Represents the heat energy content of the refrigerant.
- Increases from left (low enthalpy) to right (high enthalpy).
Y-Axis (Vertical Axis) – Pressure (P):
- Measured in PSIA (Pounds per Square Inch Absolute) or bar/kPa.
- Represents the pressure of the refrigerant in the system.
- Higher pressure is at the top, lower pressure at the bottom.

Key Regions of the P-H Diagram

Saturated Liquid Line (Left Curve):
- Represents refrigerant in a pure liquid state.
- Before this line, the refrigerant is subcooled (cooler than boiling temperature).
Saturated Vapor Line (Right Curve):
- Represents refrigerant in a pure vapor state.
- After this line, the refrigerant is superheated (hotter than boiling temperature).
Dome-Shaped Area (Wet Region / Two-Phase Region):
- Area between the Saturated Liquid Line and Saturated Vapor Line.
- Refrigerant exists as a mixture of liquid and vapor.
- The closer to the left, the more liquid present; the closer to the right, the more vapor present.
Superheated Region (Right of the Saturated Vapor Line):
- Refrigerant is in 100% vapor state.
- Occurs after the evaporator and before compression.
Subcooled Region (Left of the Saturated Liquid Line):
- Refrigerant is in 100% liquid state.
- Occurs after the condenser and before expansion.

Critical Points and Key Terms

Critical Point:
- The top peak of the dome where liquid and vapor become indistinguishable.
- Beyond this point, refrigerant acts as a supercritical fluid.
Superheat:
- The extra heat added to the refrigerant after it has turned into vapor.
- Happens in the evaporator and compressor discharge.
Subcooling:
- The extra cooling applied to the refrigerant after it has turned into liquid.
- Happens in the condenser before the expansion valve.
Isenthalpic Process:
- A process where enthalpy remains constant (vertical movement on the diagram).
- Example: Expansion valve operation.
Isentropic Process:
- A process where entropy remains constant (compression process).

Importance of P-H Diagrams in Refrigeration

Essential Tool:
- The Pressure-Enthalpy (P-h) Diagram is essential in refrigeration because it visually represents the thermodynamic processes of the refrigeration cycle.
- Learning the P-H diagram is a valuable skill because it helps in troubleshooting, system analysis, predicting performance, and understanding advanced refrigeration concepts.
Troubleshooting Refrigeration Systems Efficiently:
- The P-h diagram helps diagnose system issues such as incorrect refrigerant charge, inefficiencies in compression, or improper heat transfer.
- By plotting real system pressures and enthalpy values, technicians can pinpoint operational problems and make precise adjustments.
Understanding the Functions of Refrigeration Components:
- Each component of a refrigeration system-compressor, condenser, expansion valve, and evaporator- affects pressure and enthalpy in a specific way.
- The P-H diagram visually shows how these parts work together to move heat and regulate pressure, making it easier to understand how each component contributes to the overall process.
Predicting Expected Pressures and Temperatures at Key Points:
- With experience, the P-h diagram allows you to estimate expected values for pressure and temperature at different locations in the cycle.
- This skill helps in:
  - System diagnostics.
  - Ensuring system components are operating within safe and efficient limits.
  - Detecting abnormal pressure drops or increases.

Interpreting Pressure-Temperature Relationship

Direct Relationship:
- The pressure and temperature of a refrigerant are directly related, as stated by the gas law: when volume is constant, an increase in pressure results in an increase in temperature.
- This principle is essential in refrigeration, as it explains how refrigerants behave during compression, expansion, and heat exchange.
Effect of Pressure Changes on Boiling Point:
- Boiling point is directly affected by pressure changes.
- Higher pressure increases the boiling point.
- Lower pressure decreases the boiling point.
- The condenser and evaporator regulate temperature.
- Refrigerant changes state and temperature as it moves through the system.
- System efficiency depends on the refrigerant’s boiling point.
- Lower pressure and temperature lead to a lower boiling point.
- Low boiling point refrigerants require less energy for phase change, improving efficiency.
Lower Boiling Point Efficiency:
- Phase Change Efficiency: Refrigerants with low boiling points evaporate at lower temperatures, enhancing heat absorption and cooling efficiency.
- Heat Absorption: Evaporation at low temperatures allows refrigerants to absorb significant heat, maintaining effective cooling in air conditioners and refrigerators.
- Environmental Impact: Modern low-boiling-point refrigerants are designed to reduce ozone depletion and greenhouse gas emissions.
Relationship Between Temperature and Refrigerant State:
- The relationship between pressure and the state of the refrigerant is fundamental in refrigeration and air conditioning systems.
- The Pressure- Enthalpy (P-H) diagram helps visualize how refrigerants behave under varying pressure conditions.
- Refrigerant states depend on both pressure and temperature.
- The refrigerant state changes based on pressure levels:
  - Higher pressure increases the boiling (saturation) temperature.
  - Lower pressure decreases the boiling (saturation) temperature.
- What is the temperature which a refrigerant changes it’s state? Refrigerant changes its states depending on the type of refrigerant, but typically change between $4$ ° to $10$ °C upon entering the evaporator and around $47$ °C in the condenser.
Using PSIA Instead of PSIG in P-H Diagram:
- PSIA (Pounds per Square Inch Absolute) and PSIG (Pounds per Square Inch Gauge) are the two different ways to measure pressure.
- Understanding PSIA vs. PSIG is crucial for accurate pressure measurement in refrigeration systems.
- What’s the difference between PSIA and PSIG?
  - PSIG measures pressure relative to atmospheric pressure (does not include atmospheric pressure).
  - Used in practical use or in field measurements, such as pressure gauges on refrigeration systems.
  - PSIA includes atmospheric pressure ( $14.7$ PSI at sea level).
  - It’s the literal amount of weight the fluid is placing on the measuring apparatus.
  - Always used in P-H diagrams to provide absolute values.
Conversion formula for PSIA: $PSIA = PSIG + 14.7$ .
- Example: A refrigeration system operating at $55.3$ PSIG has an absolute pressure of: $55.3$ PSIG + $14.7$ PSI = $70$ PSIA.
- A vacuum of $-10$ PSIG means the absolute pressure is: $-10 + 14.7 = 4.7$ PSIA.
Why is it important to measure pressure?
- Pressure directly affects the refrigerant’s state which is involve in the cooling or absorption of heat in a refrigeration cycle

Identifying Refrigerant States on a P-H Diagram

Subcooled liquid
- Subcooled liquid state is found on the left side of the dome on the p-h diagram which is in the Liquid region.
- A subcooled liquid is a liquid that is cooled below its saturation temperature at a given pressure and is not yet ready to vaporize.
Saturated liquid-vapor mixture
- At the left edge of the saturation dome, the substance is in a saturated liquid state is a liquid that is about to vaporize.
- On the other side, the right edge of the saturation dome, the substance is in a saturated vapor state, meaning that it’s a vapor that is ready to condense at the given temperature and pressure.
- Saturated liquid-vapor mixture is said to coexist in equilibrium, meaning that the amount of refrigerant in evaporation process is equal to the amount of the refrigerant in condensation process.
Superheated Gas
- In contrast, superheated gas is a gas heated above its saturation temperature, or the boiling point to make sure that only pure gas or steam will enter in the compressor.
- However, there is a certain degree ( $5-10$ degrees Celsius) that is needed in this process to avoid low-superheat which can cause the liquid refrigerant to enter the compressor and cause damage, and the high-superheat which could result to be difficult for compressor to compress the refrigerant as it needs more energy and it could also result in overheating and could damage the other components.
How to determine refrigerant condition?
- To summarize it, when refrigerant is in subcooled liquid state, it is in a fully liquid condition and not yet ready to vaporize.
- Then, in saturated liquid-vapor mixture state wherein the rate of refrigerant in vaporization process and condensation process is balanced and does not change through transitioning process.
- Lastly, the superheated gas, which is the opposite of subcooled liquid, this needs a certain degree of heat above the boiling point to completely turn the refrigerant into steam or vapor.

Using P-H Diagrams for System Analysis

Visualizing Refrigerant Flow Using P-H Diagram
- The Pressure-Enthalpy (P-H) Diagram is an essential tool for understanding how refrigerant flows through a refrigeration cycle.
- It plots pressure on the vertical axis and enthalpy on the horizontal axis, making it easier to visualize changes in pressure and heat energy.
- The diagram outlines the four main stages of the refrigeration cycle—evaporation, compression, condensation, and expansion—using a specific refrigerant, such as R-134a.
STEP 1: Evaporator
- The refrigerant entering the evaporator is a cold, partial liquid-vapor mixture.
- The operating pressure and temperature of the evaporator is called the suction pressure and suction temperature.
- The suction line is the piping that routes refrigerant gas from the evaporator to the compressor.
- As the evaporator transfers heat to the refrigerant, there is no gain in temperature, since all the heat is used to convert the remaining liquid to a gas.
- In an ideal evaporator, there is just enough heat transfer to convert all the liquid to gas and nothing more.
- Thus, the output of an ideal evaporator is 100% vapor at the same entering temperature, Low.
STEP 2: Compressor
- The compressor draws in low-pressure, low-temperature vapor and compresses it isentropically, raising both pressure and temperature.
- It discharges the high-pressure vapor into the condenser.
- The enthalpy difference represents the work done by the compressor.
STEP 3: Condenser
- The refrigerant entering the condenser is now a hot, high pressure refrigerant gas.
- The condenser is shown on the pressure-enthalpy diagram as a horizontal line.
- This horizontal line is a line of constant pressure, corresponding to the discharge pressure of the compressor.
STEP 3: Expansion Device
- The expansion device receives high- pressure liquid refrigerant from the condenser, then expands it adiabatically (constant enthalpy), shown as a vertical line on the P-H diagram.
- This process drops the refrigerant’s pressure and temperature, creating a liquid-vapor mixture, ready for the evaporator.
How Each Component Affects Pressure and Heat Content?
- Each component of the refrigeration cycle affects the pressure and heat content of the refrigerant in specific ways:
  - EVAPORATOR
    - Pressure: Remains constant (suction pressure).
    - Heat Content: Increases as the refrigerant absorbs heat from the surroundings, causing it to evaporate.
  - COMPRESSOR
    - Pressure: Increases significantly as the refrigerant is compressed.
    - Heat Content: Increases due to the work done on the refrigerant during compression.
  - CONDENSER
    - Pressure: Remains constant (discharge pressure).
    - Heat Content: Decreases as the refrigerant releases heat to surroundings, causing it to condense.
  - EXPANSION DEVICE
    - Pressure: Decreases significantly as the refrigerant expands.
    - Heat Content: Remains constant (adiabatic expansion), but temperature drops, and some of the liquid refrigerant vaporizes.

Plotting the Refrigeration Cycle on a P-H Diagram

What is P-H Diagram Refrigerant Cycle?
- Pressure (P): The vertical axis represents the absolute pressure of the refrigerant. It is typically measured in psia (pounds per square inch absolute) *imperial units or kPa (kilopascals) *metric units.
- Enthalpy (H): The horizontal axis represents the enthalpy of the refrigerant. It is typically measured in Btu/lb (British thermal units per pound) *imperial units or kJ/kg (kilojoules per kilogram) *metric units.
How to Plot?
- Identify refrigerant properties from a thermodynamic table or software.
- Locate the four main states (1, 2, 3, 4) on the P-h diagram.
- Connect them with appropriate process lines (vertical for isenthalpic expansion, steep upward for compression, and horizontal for heat exchange).
- Add the saturation dome (liquid-vapor region) for reference.
Evaporator $1-2$ (Heat Absorption & Enthalpy Change)
- Refrigerant enters the Evaporator low pressure low temperature liquid
- Heat Absorption causing the refrigerant to Vaporize Enthalpy and Temperature increases
- Plotting: (from point 1 to point 2) lower left side of the graph to right.
- $1-2$ Evaporator (Constant Pressure)
  - Pressure (psia) : $72.52 -> 72.52$
  - Enthalpy (BTU/lb): $42.99 -> 98.88$
Compressor (Pressure & Enthalpy increase)
- Compression occur, pressure and enthalpy increase
- ISENTROPIC ( no heat loss) entropy remains constant
  - PRESSURE (psia) : $72.52 -> 261.07$
  - ENTHALPY (BTU/lb): $98.88 -> 116.07$
- Plotting: diagonal line from point 2 to point 3.
CONDENSER
- Heat is release to the surroundings Enthalpy decreases
- Superheated vapor -> saturated liquid
- PRESSURE (psia) : $261.07$ (CONSTANT PRESSURE)
  - ENTHALPY (BTU/lb): $116.07 -> 42.99$
- Plotting: horizontal line to the left (heat rejection) from point 3 to point 4
METERING DEVICE (Expansion Valve/ Capillary Tube)
- ISENTHALPIC ( constant enthalpy) Pressure drops
  - PRESSURE (psia) : $261.07 -> 72.52$
  - ENTHALPY (BTU/lb): $42.99 -> 42.99$
- Plotting: vertical line downwards from point 4 to point 1

Practical Applications of P-H Diagrams

How technician use PH Diagrams in practical applications?
- Technicians use pressure-enthalpy (ph) diagrams to visually analyze refrigerant systems.
- These diagrams help them determine the pressure and temperature of the refrigerant at various locations within the system.
- They can demonstrate the performance of the system and how each component works and changes.
- By comparing system performance against ideal cycles, technicians can quickly identify areas where the system is not operating as efficiently as it could be.
Identifying inefficiencies and potential failures
- Malfunctioning expansion valves: P-H diagrams assist technicians in identifying inefficiencies, such as superheating, subcooling, or incorrect settings of the expansion valve.
- Refrigerant leaks: P-H diagrams can help technicians determine if there have been refrigerant leaks by analyzing the system's performance compared to the ideal cycle.
Improving refrigeration system performance
- Pressure-enthalpy (P-H) diagrams are valuable tools for troubleshooting and maintaining refrigeration systems.
- Technicians can identify and address potential issues before they lead to more bigger problems
- Optimizing key components such as expansion valves, compressors, and evaporators, technicians can improve the performance of refrigerants.

Common Troubleshooting Using P-H Diagrams

Identifying Superheat and Subcooling Points on the Diagram
- The P-H Diagram provides a virtual method for understanding the concepts of superheat and subcooling within a refrigeration system.
- Superheat - refers to the condition where the refrigerant vapor at the outlet of the evaporator ( and typically the inlet of the compressor) has been heated above its saturation temperature at the measured pressure.
  - Incorrect Superheat: A superheat point too far right on the P-H diagram indicates high superheat, while one near or within the saturated vapor line suggests low superheat.
  - Severe low superheat can lead to liquid refrigerant entering the compressor, risking damage.
- Subcooling – refers to the condition where the liquid refrigerant at the outlet of the condenser (and typically the inlet of the expansion valve) has been below its saturation temperature at the measured pressure.
  - Incorrect Sub-cooling: if the Subcooling point on the P-H diagram is significantly further to the left in the subcooled region than expected, it indicates high subcooling.
  - On the other hand, if the Sub-cooling point is close to or even within the saturated liquid line, i n d i c a t i n g l o w S u b - c o o l i n g , i t s u g g e s t s t h a t t h e r e f r i g e r a n t i s n o t b e i n g a d e q u a t e l y c o o l e d i n t h e c o n d e n s e r.
Detecting Compressor Inefficiencies through P-H Diagram Analysis
- Identifying Indicators of Internal Leaks and Valve Issues
  - Compressor inefficiencies can result from internal leaks or faulty valves.
  - Leaks between high- and low-pressure sides reduce pressure difference and may increase discharge temperature due to gas recompression.
- Analyzing the Impact on the Refrigeration Cycle
  - Inefficiencies, whether from increased entropy during compression or mechanical issues, add heat to the refrigerant, increasing condenser load.
  - This raises condensing pressure and temperature, ultimately lowering the system's coefficient of performance (COP).
Evaluating Refrigerant Charge Levels by Interpreting P-H Diagrams.
- The Relationship Between Refrigerant Charge and the P-H Diagram
  - The amount of refrigerant in an HVAC system is critical for proper operation.
  - An incorrect charge, whether overcharged or undercharged, can cause performance issues or system damage.
  - The P-H diagram helps evaluate refrigerant charge levels by analyzing the position and characteristics of the plotted refrigeration cycle.
- Identifying Symptoms Of Overcharge And Undercharge
  - Overcharge: In overcharge system, there is an excessive amount of refrigerant.
    - This often leads to a higher-than-normal condensing pressure and temperature.
    - An overcharge can also lead to a flooded evaporator, where liquid refrigerant does not fully vaporize, resulting in lower-than- normal superheat at the evaporator outlet.
  - Undercharge: An undercharged system has an insufficient amount of refrigerant.
    - This typically results in lower-than-normal evaporating and condensing pressures and temperatures.
    - An undercharge often leads to a starved evaporator, where the refrigerant boils off quickly, resulting in higher-than- normal superheat at the evaporator outlet as the vapor absorbs additional heat.

Summary

They visualize the refrigeration cycle, simplifying the tracking of refrigerant behavior.
It helps determine whether the refrigerant is in a subcooled liquid, saturated mixture, or which is essential for accurate system control and diagnostics.