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5.2 · Identify the four states of a vapour-compression cycle

Learn to identify the four states of a vapour-compression cycle through clear examples and targeted practice.

University of Alberta MEC E 340: Applied Thermodynamics

Refrigeration and Heat Pumps

MEC E 340 Applied Thermodynamics — Study topic 5.2

Consider a closed refrigeration cycle containing a working fluid, such as a refrigerant. The refrigerant repeatedly passes through four main components: compressor, condenser, expansion device, and evaporator. State numbers mark the fluid at the component connections; they do not, by themselves, specify numerical pressure, temperature, or enthalpy. The standard numbering follows the fluid around the loop: state 1 enters the compressor, state 2 leaves it, state 3 leaves the condenser, and state 4 leaves the expansion device. The evaporator then returns the fluid from state 4 to state 1. This lesson focuses on identifying those four states. Any phase labels below are conditional on the stated simple-cycle model, not universal facts about every refrigeration system.

What you will learn

  • Name the four standard state points in a simple vapour-compression cycle.
  • Locate each state relative to the compressor, condenser, expansion device, and evaporator.
  • Use the cycle direction and component connections to avoid confusing state numbers.
  • Recognize that the actual phase at a state depends on the stated cycle assumptions and property data.

1. Review: identify a state by its place in the loop

A thermodynamic state describes the condition of the working fluid at a particular point. In cycle diagrams and property tables, a number such as 1 or 3 is a label, not a property value. The same numbering convention is useful whether the cycle is shown on a pressure–specific-volume diagram, a temperature–entropy diagram, or a component sketch.
For a simple vapour-compression cycle, trace the refrigerant in the direction it flows. Begin at the compressor inlet and call that state 1. Follow the flow through the compressor to state 2, through the condenser to state 3, through the expansion device to state 4, and through the evaporator back to state 1. The numbered points lie at component connections; the processes occur between them.
The usual first-law balance for a steady-flow component relates heat transfer, work transfer, and changes in fluid energy. For identifying the states, the essential use of this background is qualitative: the compressor raises the refrigerant pressure, the condenser rejects heat, the expansion device lowers pressure, and the evaporator absorbs heat. These component roles help check that the state sequence has been read correctly.
1→2→3→4→11\rightarrow2\rightarrow3\rightarrow4\rightarrow1
  • State 1: compressor inlet and evaporator outlet.
  • State 2: compressor outlet and condenser inlet.
  • State 3: condenser outlet and expansion-device inlet.
  • State 4: expansion-device outlet and evaporator inlet.

2. The four states and their usual phase descriptions

State 1 is the low-pressure refrigerant entering the compressor. In the basic idealized cycle, it is saturated vapour. In an actual cycle or a different stated model, it may instead be superheated vapour. The state number is fixed by the component connection; the exact vapour condition must come from the problem statement or supplied property data.
State 2 is the refrigerant leaving the compressor at the higher pressure and entering the condenser. In the basic idealized cycle, it is superheated vapour. Do not label it saturated vapour unless the supplied information supports that description.
State 3 is the refrigerant leaving the condenser and entering the expansion device. It is commonly modelled as saturated liquid in the basic idealized cycle. A stated subcooled-liquid condition is also possible. The condenser removes heat, but its outlet condition cannot be determined from the component name alone.
State 4 is the refrigerant leaving the expansion device and entering the evaporator. In the usual simple-cycle model, it is a low-pressure liquid–vapour mixture. The expansion process is commonly treated as throttling, so specific enthalpy is unchanged across the device in that model. That relation helps determine properties when data are supplied; it does not change where state 4 is located.
During evaporation, the refrigerant absorbs heat and returns from state 4 to state 1. During condensation, it rejects heat and moves from state 2 to state 3. A cycle diagram is schematic unless enough property information is provided to place states at exact coordinates. Do not infer numerical pressures, phase boundaries, or property values from a sketch.
h3=h4h_3=h_4
  • Use component connections first; use phase descriptions only when assumptions or property data support them.
  • The standard component order is compressor, condenser, expansion device, evaporator.
  • The four state labels are shared connection points between neighbouring components.

3. A reliable identification procedure

First, identify the working fluid and confirm that the system is a vapour-compression cycle. Then locate the four components and follow the refrigerant flow. Start at the compressor inlet, assign state 1, and number each outlet in sequence. This prevents the common error of starting at an arbitrary point or numbering in the reverse direction.
Next, check each state by naming both adjacent components. A state should make sense at both connections: state 2, for example, must be the compressor outlet and condenser inlet. If a diagram shows a different connection, either the flow direction or the numbering has been misread.
Finally, separate identification from property evaluation. If a task asks only for the four states, no property-table lookup or performance calculation is required. If phase labels are requested, use the problem's idealizations or the supplied property data. Do not invent refrigerant properties or assume that every real system has exactly saturated vapour at state 1 and saturated liquid at state 3.
  • Trace flow; do not assign numbers from a component's position on the page.
  • Verify each state using its two neighbouring components.
  • Treat phase conditions as assumptions or data-dependent descriptions.

Worked example

Number a basic cycle from the compressor inlet

A schematic shows refrigerant flowing from an evaporator to a compressor, then to a condenser, an expansion device, and back to the evaporator. Identify the four states. Assume the basic idealized cycle: saturated vapour enters the compressor and saturated liquid leaves the condenser.
Four states of a simple vapour-compression cycle
Four states of a simple vapour-compression cycleSpecific volume, vPressure, PCompressionCondensationExpansionEvaporation1Compressor inlet2Compressor outlet3Condenser outlet4Expansion-device outletSchematic · not to scale

Refrigerant cycle; schematic P–v loop, not to scale. State locations do not provide property values or exact phase boundaries.

  1. Choose the starting point
    State 1 is the compressor inlet. The evaporator outlet connects directly to that inlet, so the same point is also the evaporator outlet.
  2. Follow the refrigerant
    The compressor discharge is state 2, the condenser outlet is state 3, and the expansion-device outlet is state 4. The evaporator carries the fluid from state 4 back to state 1.
    1→2→3→4→11\rightarrow2\rightarrow3\rightarrow4\rightarrow1
  3. Apply the given idealization
    The problem explicitly states the phase assumptions, so state 1 is saturated vapour and state 3 is saturated liquid. The usual simple-cycle description also identifies state 2 as superheated vapour and state 4 as a low-pressure liquid–vapour mixture; these are model descriptions, not numerical property results.
Answer: State 1: evaporator outlet/compressor inlet, saturated vapour under the stated assumption. State 2: compressor outlet/condenser inlet, superheated vapour in the basic idealized model. State 3: condenser outlet/expansion-device inlet, saturated liquid under the stated assumption. State 4: expansion-device outlet/evaporator inlet, a low-pressure liquid–vapour mixture in the usual model.
Check: Every state connects two consecutive components, and the sequence returns to state 1 without skipping a component.

Worked example

Identify states when phase details are not given

A component list gives only the flow sequence compressor → condenser → expansion device → evaporator → compressor. Identify states 1–4, but do not assume any saturation condition.
State numbering from component sequence
State numbering from component sequenceSpecific volume, vPressure, PCompressorCondenserExpansion deviceEvaporator1Compressor inlet2Compressor outlet3Condenser outlet4Expansion-device outletSchematic · not to scale

Refrigerant cycle; schematic P–v loop, not to scale. The sketch identifies flow order only and does not assert phase conditions or property values.

  1. Anchor state 1
    The stated sequence returns to the compressor after the evaporator. Therefore, the point entering the compressor is state 1 and is also the evaporator outlet.
  2. Number the remaining connections
    Moving with the flow, the compressor outlet is state 2, the condenser outlet is state 3, and the expansion-device outlet is state 4. This follows from the component sequence alone.
    1→2→3→4→11\rightarrow2\rightarrow3\rightarrow4\rightarrow1
  3. Keep phase claims limited
    No pressure, temperature, property data, or phase assumptions were supplied. It is valid to identify each location, but not to claim that any state is saturated or to calculate its properties.
Answer: State 1 is the compressor inlet/evaporator outlet; state 2 is the compressor outlet/condenser inlet; state 3 is the condenser outlet/expansion-device inlet; and state 4 is the expansion-device outlet/evaporator inlet. No phase condition can be assigned from the information given.
Check: The answer uses only the flow sequence provided and does not add unsupported property information.

Worked example

Correct a reversed state sequence

A learner labels the condenser outlet as state 1, then labels the expansion-device outlet as state 2, evaporator outlet as state 3, and compressor outlet as state 4. Correct the numbering using the standard convention.
Standard numbering compared with flow direction
Standard numbering compared with flow directionSpecific volume, vPressure, PCompressionCondensationExpansionEvaporation1Evaporator outlet2Compressor outlet3Condenser outlet4Expansion-device outletSchematic · not to scale

Refrigerant cycle; schematic P–v loop, not to scale. State numbers mark component connections; coordinates are illustrative only.

  1. Reset at the compressor inlet
    The standard numbering begins at the compressor inlet, not at an arbitrary convenient point. The compressor inlet is the evaporator outlet, so that connection must be state 1.
  2. Trace one complete circuit
    From state 1, the compressor outlet is state 2, the condenser outlet is state 3, and the expansion-device outlet is state 4. Then the evaporator returns the fluid from state 4 to state 1.
    1→2→3→4→11\rightarrow2\rightarrow3\rightarrow4\rightarrow1
  3. Compare with the learner's labels
    The learner's state 1 is actually state 3, and the learner's state 2 is actually state 4. The learner's state 3 is state 1, while the learner's state 4 is state 2. The issue is the starting point, not a change in the physical direction of flow.
Answer: Correct standard labels: evaporator outlet/compressor inlet is state 1; compressor outlet is state 2; condenser outlet is state 3; expansion-device outlet is state 4.
Check: Starting at the compressor inlet restores the required component order and makes the evaporator the final process back to state 1.

Common mistakes and how to avoid them

Starting at the condenser outlet and calling it state 1.
Correction: Use the standard convention: state 1 is the compressor inlet, which is also the evaporator outlet.
Treating the state number as a phase label, such as assuming state 1 always means saturated vapour.
Correction: A state number identifies location in the loop. Assign phase descriptions only when supported by stated assumptions or supplied data.
Calling the compressor inlet state 4 because the evaporator comes last in a list.
Correction: Trace the refrigerant in the forward direction from state 1: compressor, condenser, expansion device, evaporator.

Lesson summary

  • The standard numbering begins at the compressor inlet.
  • State 1 is compressor inlet/evaporator outlet; state 2 is compressor outlet/condenser inlet; state 3 is condenser outlet/expansion-device inlet; state 4 is expansion-device outlet/evaporator inlet.
  • Common ideal-cycle phase descriptions are conditional; state locations alone do not determine exact properties.
  • A schematic cycle helps show order, but it does not supply numerical state properties.

Check your understanding

Question 1

In the standard numbering, what is state 3?
  1. Compressor inlet
  2. Compressor outlet
  3. Condenser outlet
  4. Evaporator outlet
Show answer and explanation
Condenser outlet
State 3 is the condenser outlet and the inlet to the expansion device.

Question 2

Which connection is state 1?
  1. Expansion-device outlet
  2. Compressor inlet
  3. Condenser inlet
  4. Condenser outlet
Show answer and explanation
Compressor inlet
The standard cycle numbering starts at the compressor inlet, which is also the evaporator outlet.

Question 3

A problem gives only the component sequence and no phase assumptions or property data. What can be identified with confidence?
  1. All four locations in the loop, but not exact phase conditions
  2. The exact refrigerant enthalpy at every state
  3. That state 1 must be saturated vapour
  4. That state 3 must be subcooled liquid
Show answer and explanation
All four locations in the loop, but not exact phase conditions
Component connections determine the state numbering. Phase and numerical properties require assumptions or supplied data.

Key terms

State point
A numbered location in the cycle where the working fluid has a particular thermodynamic condition.
Working fluid
The substance that circulates through the cycle; in this topic, the refrigerant.
Throttling
The usual simple-cycle model for expansion through an expansion device, treated as constant specific enthalpy.
Saturated vapour
Vapour at the condition where it is about to begin condensing; use this label only when the model or data supports it.

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