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newSchoolWorks.com › VoTechWorks › Business & Cross-Cutting › Appliance Repair › Sealed Systems: Refrigerant Handling, Diagnosis, and Repair › Lesson 1
BUS-APPL73-06 · Lesson 1 · 10 clock hours · theory

Refrigeration Fundamentals and Sealed-System Components

Objective. Students will be able to trace refrigerant flow through the vapor-compression cycle, state the pressure-temperature relationship for common refrigerants (R-134a, R-600a, R-410A), and identify every major sealed-system component and its function without reference materials.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a credentialed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

The Vapor-Compression Refrigeration Cycle

All domestic refrigerators and freezers that use a sealed system operate on the vapor-compression cycle. Understanding this cycle is the foundation of every diagnosis and repair you will make as an appliance technician — and it is tested directly on the EPA 608 Type I exam.

The cycle moves heat from inside the cabinet (where you want it cold) to the room outside the cabinet (where heat is rejected). It does this by circulating a refrigerant that repeatedly changes between liquid and vapor states. Four processes drive the cycle: compression, condensation, expansion, and evaporation.

1. Compression

The compressor is the heart of the sealed system. It is a hermetically sealed motor-pump that draws low-pressure refrigerant vapor from the evaporator and compresses it into a high-pressure, high-temperature vapor. Raising the pressure also raises the refrigerant's saturation (boiling) temperature, which is essential for the next step. The overload protector cuts power to the compressor if the motor overheats, and the start relay provides extra torque during startup.

2. Condensation

Hot, high-pressure vapor travels from the compressor to the condenser coil, located at the rear or underside of the cabinet. Here the refrigerant releases its heat to the surrounding air and condenses into a high-pressure liquid. A condenser fan moves air across the coil in most modern refrigerators; older units rely on natural convection. The refrigerant leaves the condenser as a warm, high-pressure liquid.

3. Expansion

The high-pressure liquid enters the filter-drier, which removes moisture and contaminants that could damage the system or clog the metering device. It then passes through the capillary tube (cap tube), a long, narrow copper tube that acts as the metering device. The restriction of the cap tube causes a rapid pressure drop. As pressure falls, so does the refrigerant's boiling point — setting the refrigerant up to evaporate at a very low temperature inside the cabinet. Domestic refrigerators almost universally use a cap tube rather than a thermostatic expansion valve.

Many refrigerators include a heat exchanger where the cap tube is bonded to the suction line. Warm liquid in the cap tube is pre-cooled by cold suction vapor returning to the compressor, improving efficiency.

4. Evaporation

Low-pressure liquid-vapor refrigerant enters the evaporator coil inside the freezer or fresh-food compartment. It absorbs heat from the air and food in the cabinet, boiling off into a low-pressure vapor. An evaporator fan circulates cabinet air across the coil. This is the process that makes the interior cold. The low-pressure vapor then returns to the compressor and the cycle repeats.

An accumulator may be installed in the suction line between the evaporator and compressor to catch any liquid refrigerant that did not fully evaporate, protecting the compressor from liquid slugging.

The thermostat monitors cabinet temperature and cycles the compressor on and off to maintain the set point.

Pressure-Temperature Relationship

The most important principle for refrigerant work is the pressure-temperature (P-T) relationship: at a given pressure, a pure refrigerant boils (or condenses) at one specific temperature. This is called the saturation temperature. If you know the pressure, you can look up the temperature on a P-T chart, and vice versa.

Common refrigerants used in domestic appliances:

Always use the P-T chart specific to the refrigerant in the appliance. Using the wrong chart will lead to incorrect diagnosis.

Safety

Before any sealed-system work: Follow your employer's lockout/tagout (LOTO) procedure — disconnect and lock out electrical power before opening any sealed-system connections.

R-600a is flammable. Isobutane has a flammable range in air. When working on R-600a systems, eliminate ignition sources, ensure adequate ventilation, and never use an open flame leak detector. Use an electronic leak detector rated for flammable refrigerants.

Refrigerant release is regulated. The Clean Air Act Section 608 prohibits the knowing venting of any refrigerant. Proper recovery equipment must be used whenever refrigerant is removed from a system. This is why EPA 608 certification is required.

PPE: Wear safety glasses whenever connecting or disconnecting refrigerant lines. Liquid refrigerant contacting skin or eyes can cause frostbite. Chemical-resistant gloves are recommended.

Key terms8

Vapor-compression cycleThe four-step thermodynamic cycle — compression, condensation, expansion, evaporation — used by sealed-system appliances to transfer heat from inside the cabinet to the surrounding air.
Saturation temperatureThe temperature at which a refrigerant boils or condenses at a given pressure; the basis of the pressure-temperature relationship used in diagnosis.
Capillary tubeA long, fixed-diameter copper tube used as the metering device in domestic refrigerators; the restriction it creates drops refrigerant pressure and lowers its boiling point before the evaporator.
Filter-drierA sealed component containing desiccant material that removes moisture and contaminants from the refrigerant before it reaches the cap tube, preventing clogs and corrosion.
Superheated vaporRefrigerant vapor whose temperature is above its saturation temperature at a given pressure; the condition of the vapor leaving the evaporator and entering the compressor.
Subcooled liquidRefrigerant liquid whose temperature is below its saturation temperature at a given pressure; the condition of the liquid leaving the condenser.
AccumulatorA vessel installed in the suction line to trap any liquid refrigerant that did not evaporate, protecting the compressor from liquid slugging.
R-600a (isobutane)A flammable hydrocarbon refrigerant used in many modern domestic refrigerators; recognized for low environmental impact but requires special handling precautions due to flammability.

Worked examples4

1. A technician connects gauges to an R-134a system and reads a suction pressure of 22 psig. Using a P-T chart, what is the approximate saturation temperature at this pressure?
  1. Step 1: Locate the R-134a P-T chart. Find the pressure column and identify 22 psig.
  2. Step 2: Read across to the temperature column. On a standard R-134a P-T chart, 22 psig corresponds to approximately 20 °F saturation temperature.
  3. Step 3: Confirm the units — the chart gives gauge pressure (psig) and temperature in °F. No conversion is needed if both are already in those units.
Answer: The saturation temperature for R-134a at 22 psig is approximately 20 °F.
2. A technician is checking an R-134a condenser. The high-side gauge reads 124 psig. What is the approximate condensing (saturation) temperature, and is the condenser operating in a reasonable range for a room at 75 °F?
  1. Step 1: Look up 124 psig on the R-134a P-T chart. This corresponds to approximately 100 °F saturation temperature.
  2. Step 2: Compare the condensing temperature to room temperature. A condensing temperature roughly 20–30 °F above room temperature is typical; 100 °F versus a 75 °F room gives a 25 °F difference.
  3. Step 3: Conclude that the condenser is operating within a normal range. Significantly higher high-side pressure would suggest a dirty condenser coil or failed condenser fan.
Answer: Approximately 100 °F condensing temperature; this is within the normal operating range for an ambient of 75 °F.
3. A P-T chart shows that R-600a has a saturation temperature of approximately 14 °F at 0 psig (atmospheric pressure). A technician sees the evaporator frost pattern only on the upper third of the coil and measures suction pressure below 0 psig (vacuum). What does this indicate?
  1. Step 1: Recall that R-600a boils at about 14 °F at atmospheric pressure. A pressure below 0 psig (a vacuum on the gauge) means the refrigerant is boiling at a temperature colder than 14 °F.
  2. Step 2: Compare this to normal: a well-charged evaporator should show frost across most of its surface, not just the inlet end.
  3. Step 3: A lower-than-normal suction pressure combined with partial frost indicates a low refrigerant charge or a restriction upstream of the evaporator — not a normal condition.
  4. Step 4: Note the safety implication: because R-600a is flammable, the technician must confirm the system is de-energized and area is ventilated before investigating a leak.
Answer: Suction pressure in vacuum on an R-600a system indicates a low charge or restriction; the evaporator is not receiving enough refrigerant to frost completely. Further diagnosis is required.
4. Converting saturation pressure to temperature: An R-410A system shows a low-side pressure of 118 psig. Using a P-T chart, what is the approximate saturation temperature?
  1. Step 1: Select the R-410A column on the P-T chart — do not use R-134a or R-600a values; R-410A operates at much higher pressures.
  2. Step 2: Find 118 psig in the pressure column. On a standard R-410A P-T chart, 118 psig corresponds to approximately 40 °F saturation temperature.
  3. Step 3: Note that 40 °F is a typical evaporating temperature for residential air conditioning but would be too warm for freezer duty — context matters when evaluating whether a reading is normal.
Answer: Approximately 40 °F saturation temperature for R-410A at 118 psig.
In class: Instructor-led diagram walk-through of the vapor-compression cycle using a cutaway refrigerator and a whiteboard pressure-enthalpy (P-H) diagram; students label a blank P-H diagram individually and compare answers with a partner. · Small-group card-sort activity: each group matches 12 component cards (compressor, condenser coil, condenser fan, capillary tube, filter-drier, evaporator coil, evaporator fan, accumulator, heat exchanger, overload protector, start relay, thermostat) to function-description cards and posts results for class debrief. · Students use a pressure-temperature (P-T) chart for R-134a and R-600a to answer a 10-question worksheet converting saturation pressures to temperatures and vice versa, practicing the skill needed on the EPA 608 Type I exam.
Materials: Cutaway domestic refrigerator unit, P-T reference charts for R-134a / R-600a / R-410A, component card-sort sets, blank P-H diagram worksheets, HVACR Fundamentals textbook (Refrigeration and Air Conditioning Technology, Whitman et al., current edition), dry-erase markers
Check: 10-question P-T conversion quiz graded immediately; pass mark is 80%. Instructor reviews labeled P-H diagrams and initials each student's sheet; all components must be correctly identified before the student advances to Lesson 2.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Refrigeration fundamentals / PSA Certified Appliance Professional
1. In the vapor-compression cycle, what is the primary function of the evaporator coil?
Sealed-system components / PSA Certified Appliance Professional
2. Which component in a domestic refrigerator sealed system removes moisture and debris from the refrigerant before it enters the capillary tube?
Refrigerant properties / EPA 608 Type I
3. The pressure-temperature relationship states that for a pure refrigerant at saturation, if pressure increases, the saturation temperature:
Refrigerant safety / EPA 608 Type I
4. A technician is working on a refrigerator labeled as containing R-600a. Which special safety precaution is required that would NOT be needed for R-134a?
Sealed-system components / PSA Certified Appliance Professional
5. Which component protects the compressor motor from liquid refrigerant entering the cylinder during operation?
Regulatory compliance / EPA 608 Type I
6. Under EPA Section 608, a technician who knowingly vents refrigerant to the atmosphere when removing it from an appliance:
Refrigeration fundamentals / PSA Certified Appliance Professional
7. On a pressure-enthalpy (P-H) diagram for the vapor-compression cycle, which process is represented by a horizontal line at high pressure moving from right to left (decreasing enthalpy)?
System diagnosis / PSA Certified Appliance Professional
8. A technician uses an R-134a P-T chart and finds that the suction pressure corresponds to a saturation temperature of 10 °F. The evaporator is in a freezer set to 0 °F. Which statement best describes this reading?
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