Refrigeration Cycle Fundamentals: Refrigerant States and Saturation Concepts

Key Terms You Will Use Constantly

Refrigerant is the working fluid that changes pressure, temperature, and phase as it moves through the system.

Phase describes whether the refrigerant is liquid, vapor, or a combination of both.

• Subcooled liquid: liquid refrigerant at a temperature below its saturation temperature for the same pressure.
• Saturated mixture (two-phase): liquid and vapor exist together at the saturation condition; boiling or condensing can occur without changing temperature (at constant pressure).
• Superheated vapor: vapor refrigerant at a temperature above its saturation temperature for the same pressure.

Saturation temperature (T_sat) is the temperature at which a refrigerant boils (evaporates) or condenses at a given pressure.

Saturation pressure (P_sat) is the pressure at which a refrigerant boils or condenses at a given temperature.

Boiling vs. condensing: At the saturation condition, adding heat tends to boil liquid into vapor; removing heat tends to condense vapor into liquid. The key is that both processes happen at the same T_sat for that pressure (assuming steady pressure).

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The One Rule That Unlocks Most State Questions

For a given refrigerant, pressure and saturation temperature are linked. If you know the refrigerant and either pressure or temperature, you can find the corresponding saturation partner from a P–T relationship (often a P–T chart or table).

• If T < T_sat at that pressure → subcooled liquid (not boiling).
• If T = T_sat at that pressure → saturated mixture (boiling or condensing is possible).
• If T > T_sat at that pressure → superheated vapor (not condensing).

Understanding Saturation Without Memorizing Numbers

Think of saturation as the “borderline” between liquid and vapor. At that borderline, the refrigerant can exist as both liquid and vapor at the same time. The exact borderline temperature depends on the pressure.

Higher pressure → higher saturation temperature. This is why raising pressure in a condenser allows the refrigerant to condense at a higher temperature, and lowering pressure in an evaporator allows it to boil at a lower temperature.

What a P–T Table Is Telling You

A P–T table lists saturation temperature for various pressures (or saturation pressure for various temperatures) for a specific refrigerant. You do not need to memorize the table; you need to interpret it.

Example of a generic saturation P–T relationship (illustrative only):

Notice the pattern: as pressure increases, the saturation temperature increases. That pattern is what you rely on when diagnosing states.

Step-by-Step: How to Decide the Refrigerant State from P and T

When you have a pressure and a temperature at a point in the system, use this procedure.

1. Identify the refrigerant (because each refrigerant has its own P–T curve).
2. Use the measured pressure to find T_sat from a P–T table (or use measured temperature to find P_sat).
3. Compare actual temperature to saturation temperature at that pressure.
4. Classify the state: subcooled liquid, saturated mixture, or superheated vapor.
5. Infer what is likely happening: if it’s saturated mixture, it may be boiling (evaporator) or condensing (condenser) depending on where you are and whether heat is being absorbed or rejected.

Worked Example 1 (Interpretation Focus)

You measure at a location and find:

• Pressure: 800 kPa
• Temperature: 30°C

From the table above, at 800 kPa, T_sat ≈ 25°C. Since 30°C > 25°C, the refrigerant is superheated vapor. That means it is not in the middle of condensing; it is fully vapor and hotter than its condensing/boiling temperature at that pressure.

Worked Example 2 (Saturated Mixture)

You measure:

• Pressure: 400 kPa
• Temperature: 5°C

From the table, at 400 kPa, T_sat ≈ 5°C. Since T = T_sat, the refrigerant is at the saturation condition, meaning it could be a saturated mixture. Whether it is boiling or condensing depends on the direction of heat transfer at that location.

Connecting the Three Common Refrigerant “States” to Real Measurements

1) Subcooled Liquid (Below Saturation)

If a liquid is subcooled, it is “safely liquid.” It will not flash into vapor unless pressure drops or heat is added enough to reach saturation.

Measurement clue: at a given pressure, the measured temperature is lower than T_sat.

At pressure P:  T_measured < T_sat(P)  → subcooled liquid

2) Saturated Mixture (At Saturation)

At saturation, temperature is pinned to T_sat for that pressure while phase change occurs. In this region, you cannot determine “how much is liquid vs vapor” from P and T alone; you would need quality (mass fraction vapor) or enthalpy information. For this chapter, focus on recognizing that it is in the phase-change zone.

At pressure P:  T_measured = T_sat(P)  → saturated (often two-phase)

3) Superheated Vapor (Above Saturation)

Superheated vapor is “safely vapor.” It will not condense unless temperature drops to T_sat (at that pressure) or pressure rises enough that T_sat exceeds the vapor temperature.

At pressure P:  T_measured > T_sat(P)  → superheated vapor

Mini-Exercises: Boiling, Condensing, or Neither?

Use the generic P–T table shown earlier (illustrative). For each case: (1) find T_sat at the given pressure, (2) compare, (3) decide the likely state. Then answer whether it is likely boiling, condensing, or neither based on the state alone (hint: boiling/condensing requires saturation).

Exercise Set A (State Identification)

• A1: P = 200 kPa, T = -15°C
• A2: P = 200 kPa, T = -10°C
• A3: P = 200 kPa, T = -5°C

Check yourself (interpretation):

• A1: At 200 kPa, T_sat ≈ -10°C. Since -15 < -10 → subcooled liquid → neither boiling nor condensing.
• A2: T = T_sat → saturated → boiling or condensing is possible (depends on heat flow/location).
• A3: -5 > -10 → superheated vapor → neither boiling nor condensing.

Exercise Set B (Spot the Saturation Condition)

• B1: P = 1200 kPa, T = 40°C
• B2: P = 1200 kPa, T = 35°C
• B3: P = 800 kPa, T = 25°C

Check yourself:

• B1: At 1200 kPa, T_sat ≈ 40°C → saturated → boiling/condensing possible.
• B2: At 1200 kPa, T_sat ≈ 40°C. Since 35 < 40 → subcooled liquid → neither.
• B3: At 800 kPa, T_sat ≈ 25°C → saturated → boiling/condensing possible.

Exercise Set C (Reasoning About Pressure Changes)

Answer without doing detailed math—use the rule “higher pressure → higher saturation temperature.”

• C1: If pressure increases while temperature stays the same, does the refrigerant move toward saturation, away from saturation into subcooled liquid, or away from saturation into superheated vapor?
• C2: If pressure decreases while temperature stays the same, does the refrigerant move toward subcooled liquid or toward superheated vapor?

Check yourself:

• C1: Increasing pressure raises T_sat. With the same actual T, it becomes more likely that T < T_sat → moves toward subcooled liquid (or toward condensing if it reaches saturation from the vapor side).
• C2: Decreasing pressure lowers T_sat. With the same actual T, it becomes more likely that T > T_sat → moves toward superheated vapor (or toward boiling if it reaches saturation from the liquid side).

Common Interpretation Mistakes to Avoid

• Mistake: “If it’s cold, it must be liquid.” Reality: at low pressure, refrigerant can boil at very low temperatures, so it may be vapor or a mixture even when cold.
• Mistake: “If pressure is high, it must be liquid.” Reality: high pressure can still be vapor if the temperature is above T_sat (superheated vapor).
• Mistake: “At saturation, it’s either all liquid or all vapor.” Reality: saturation is the phase-change boundary; it is often a mixture unless you know it is exactly saturated liquid or saturated vapor (requires more information than P and T alone).