Refrigeration Cycle Fundamentals for Beginners

New course

10 pages

All courses > Professional courses > Refrigeration and air conditioning ::

Refrigeration Cycle Fundamentals: Evaporator Heat Absorption and Superheat

Capítulo 7

Estimated reading time: 7 minutes

+ Exercise

The Evaporator’s Job: Absorb Heat by Boiling Refrigerant

The evaporator is the heat-absorption component of the system. Inside it, refrigerant enters at low pressure and low temperature and absorbs heat from the air, water, or product being cooled. That absorbed heat is used primarily to boil the refrigerant (change it from liquid to vapor) at roughly constant temperature while it is in the boiling region.

Two ideas matter most for understanding evaporator behavior:

Boiling (evaporation) happens at the saturation temperature that corresponds to the evaporator pressure.
Superheat is the “extra” temperature above saturation after all liquid has boiled off. It is a safety margin that helps ensure only vapor returns to the compressor.

Evaporator Zones: Boiling Region and Superheat Region

1) Boiling (Two-Phase) Region

In the first part of the evaporator, refrigerant is a mixture of liquid and vapor. As it absorbs heat, more liquid turns into vapor. In this zone:

Pressure is low and relatively steady along the coil (with some pressure drop).
Temperature is close to the saturation temperature for that pressure.
Heat absorbed mostly goes into changing state (latent heat), not raising temperature.

Because boiling is very effective at absorbing heat, this region typically provides most of the evaporator’s cooling capacity.

2) Superheat (Vapor-Only) Region

Once the last droplet of liquid has boiled off, any additional heat absorbed raises the temperature of the vapor. This portion of the evaporator (and sometimes part of the suction line) is the superheat region. In this zone:

Continue in our app.

You can listen to the audiobook with the screen off, receive a free certificate for this course, and also have access to 5,000 other free online courses.

Or continue reading below...

Download the app

Refrigerant is 100% vapor.
Temperature rises above saturation at the same pressure.
The main purpose is protection: keeping liquid out of the compressor.

Think of superheat as a buffer: enough to ensure “dry vapor” reaches the compressor even when load changes, airflow changes, or refrigerant distribution is imperfect.

What “Superheat” Means (Concept and Why It Matters)

Superheat is defined as:

Superheat (°) = Actual suction temperature − Saturation temperature at suction pressure

Interpreting it physically:

If superheat is 0°, the refrigerant is right at saturation: it may be vapor, but it is on the edge of condensing/boiling. Any small disturbance can allow liquid to remain.
If superheat is greater than 0°, the refrigerant is definitely vapor (and warmer than its boiling point at that pressure).

Superheat is not “extra cooling.” It is primarily a reliability and system-behavior indicator that tells you where the boiling process finishes and how safely vapor is returning.

Step-by-Step: Calculating Superheat from Suction Pressure and Suction Line Temperature

To calculate superheat, you need two measurements taken as close as practical to the evaporator outlet (or at the suction line location you are evaluating):

Suction pressure (to determine saturation temperature).
Suction line temperature (actual refrigerant line temperature).

Step 1: Measure suction pressure

Read suction pressure at the evaporator outlet/suction service port. Record it in the units your P–T reference uses (psig, bar, etc.).

Step 2: Convert suction pressure to saturation temperature

Using the correct refrigerant P–T chart (or a digital gauge set to the correct refrigerant), find the saturation temperature corresponding to that suction pressure. This is the boiling temperature of the refrigerant in the evaporator at that pressure.

Step 3: Measure suction line temperature

Clamp a temperature probe firmly to the suction line (clean metal contact, insulated from ambient air if possible). Record the actual line temperature.

Step 4: Subtract to get superheat

Superheat = Suction line temperature − Saturation temperature

If the result is near zero or negative, treat it as a warning sign that liquid may be present or measurement technique/location may be misleading.

Worked Examples (Understanding System Behavior)

Example A: Normal-looking superheat

Given:

Refrigerant: (use the system’s refrigerant in your P–T reference)
Suction pressure: 58 psig
P–T chart says saturation temperature at 58 psig: 32°F
Suction line temperature measured: 42°F

Calculation:

Superheat = 42°F − 32°F = 10°F

Interpretation: The refrigerant has finished boiling before it reaches the measurement point, and the vapor has warmed above saturation. This suggests a reasonable vapor-only return condition. The evaporator likely contains both a boiling region and a superheat region.

Example B: Low or near-zero superheat (risk of liquid carryover)

Given:

Suction pressure: 40 psig
Saturation temperature at 40 psig: 22°F
Suction line temperature: 23°F

Calculation:

Superheat = 23°F − 22°F = 1°F

Interpretation: The vapor is only barely above saturation. This can mean the boiling process is finishing very late in the evaporator (or even in the suction line). System behavior you might observe in such a condition includes:

Higher chance of unstable readings as load changes (superheat can swing quickly).
Potential for liquid droplets to leave the evaporator under certain conditions (especially during transients), which is undesirable for compressor reliability.
Very strong evaporator utilization (large boiling region), but with reduced safety margin.

Important note: A reading near zero can also come from measurement issues (probe not insulated, pressure drop between coil outlet and gauge location, or reading pressure at a different point than temperature).

Example C: Negative “superheat” (what it usually means)

Given:

Suction pressure: 50 psig
Saturation temperature at 50 psig: 28°F
Suction line temperature: 26°F

Calculation:

Superheat = 26°F − 28°F = −2°F

Interpretation: In a properly interpreted measurement at the same location, true negative superheat would imply the refrigerant is below saturation at that pressure, which would indicate some liquid presence or that the pressure/temperature are not being referenced to the same point. In practice, negative values most often point to:

Pressure measured at a different location than temperature (pressure drop effects).
Temperature probe influenced by cold airflow or poor contact.
System conditions where liquid is indeed returning (a serious reliability concern).

Example D: High superheat (vapor is “too warm” leaving the evaporator)

Given:

Suction pressure: 55 psig
Saturation temperature at 55 psig: 30°F
Suction line temperature: 60°F

Calculation:

Superheat = 60°F − 30°F = 30°F

Interpretation: The refrigerant finished boiling earlier in the evaporator, leaving a larger portion of the coil (and/or suction line) devoted to heating vapor rather than boiling liquid. System behavior commonly associated with high superheat includes:

Reduced effective use of the evaporator’s boiling potential (less two-phase area).
Warmer suction vapor returning to the compressor, which can contribute to higher discharge temperatures.
Often a sign that the evaporator is being fed less liquid than it could boil under the current load (or that heat load/airflow conditions are unusual).

Connecting Superheat to What the Evaporator Is “Doing”

What you observe	What it suggests about evaporator zones	System behavior focus
Low/near-zero superheat	Boiling region extends very close to outlet; superheat region is tiny	Strong boiling utilization but small safety margin; higher risk of liquid leaving during changes
Moderate superheat	Clear boiling region plus a controlled superheat region	Good balance between capacity use and compressor protection
High superheat	Boiling ends early; large superheat region	Less boiling area; suction vapor warmer; may indicate underfeeding or high load effects

Practical Measurement Notes (So the Number Means What You Think It Means)

Match pressure and temperature locations as closely as possible. Pressure drop between evaporator outlet and compressor can make saturation temperature from the gauge differ from the saturation temperature at the temperature clamp point.
Insulate the temperature probe. Suction lines are often colder than ambient; without insulation, the probe can read closer to air temperature than pipe temperature.
Allow the system to stabilize before interpreting superheat as “system behavior,” especially after door openings, defrost termination, fan cycling, or load swings.
Remember what superheat is telling you: where boiling finished and how confidently vapor (not liquid) is returning.

Now answer the exercise about the content:

A technician measures suction pressure and converts it to a saturation temperature of 32°F. The suction line temperature at the same location is 42°F. What does this result indicate about evaporator operation?

You are right! Congratulations, now go to the next page

You missed! Try again.

Superheat equals actual suction temperature minus saturation temperature: 42°F − 32°F = 10°F. A positive value indicates the refrigerant is vapor and has warmed above saturation after all liquid boiled off, helping prevent liquid from reaching the compressor.