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BUS-APPL73-03 · Lesson 1 · 10 clock hours · theory

The Refrigeration Cycle and System Components

Objective. Students will be able to trace the refrigeration cycle through each component on a diagram and explain the pressure-temperature relationship for refrigerants used in domestic appliances.

⚠ 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.

How a Refrigerator Moves Heat

A refrigerator does not create cold — it moves heat from inside the cabinet to the room outside. This distinction is fundamental. The refrigeration cycle is a continuous loop in which a refrigerant absorbs heat in one place and releases it in another. Understanding this cycle is the foundation for every diagnosis you will make in this trade.

The Four Main Components

Compressor

The compressor is the heart of the system. It is a motor-driven pump, usually located at the bottom rear of a domestic refrigerator, 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 temperature, which allows it to reject heat to the warmer room air when it reaches the condenser. Hermetic compressors used in household appliances have the motor and pump sealed inside a welded steel shell filled with refrigerant and oil.

Condenser

The condenser is a network of tubing and fins (or a flat coil) located outside the refrigerated space — typically under or behind the cabinet. Hot, high-pressure vapor from the compressor flows into the condenser and loses heat to the surrounding room air. As the vapor cools past its saturation point at that pressure, it condenses into a liquid. This is where the system rejects the heat it collected from inside the cabinet, plus the heat added by the compressor's work.

Metering Device

The metering device controls how much liquid refrigerant enters the evaporator and drops the pressure sharply. Domestic refrigerators almost universally use a capillary tube — a long, narrow copper tube (typically 0.026 to 0.085 inches in internal diameter) that creates a pressure drop through friction and restriction. Some newer or high-efficiency units use a small thermostatic or electronic expansion valve, but capillary tubes remain dominant in household refrigerators. The pressure drop across the metering device is what separates the high-pressure side (compressor and condenser) from the low-pressure side (evaporator).

Evaporator

The evaporator is located inside the refrigerated space, usually behind a panel in the freezer section. Low-pressure liquid refrigerant enters the evaporator and absorbs heat from the cabinet air. As it absorbs heat it boils (evaporates) into a vapor. This boiling at low pressure occurs at a low saturation temperature — cold enough to freeze food or chill a refrigerator compartment. The resulting low-pressure vapor returns to the compressor, completing the cycle.

Refrigerant Lines

Two lines connect the major components. The suction line carries low-pressure vapor from the evaporator back to the compressor. The liquid line (or discharge side) carries high-pressure vapor from the compressor to the condenser, and then high-pressure liquid from the condenser to the metering device. In a properly operating system, the suction line feels cool to the touch while the discharge line and condenser feel warm or hot.

Common Domestic Refrigerants

Two refrigerants are now dominant in household refrigerators sold in the United States.

The Pressure-Temperature Relationship

For any pure refrigerant at saturation (the point where liquid and vapor coexist), pressure and temperature are directly linked. If you know one, you can find the other from a pressure-temperature (P-T) chart. This relationship is the diagnostic cornerstone of refrigeration work.

By reading system pressures with a gauge manifold, a technician can calculate what the evaporator or condenser temperature should be and compare it to actual air temperatures. Deviations point directly to system faults.

Safety Points for This Lesson

Electrical hazard: Refrigerators operate on 120 V AC. Before handling internal components or removing panels on a live unit, follow lockout/tagout (LOTO) procedures — unplug the unit, verify power is off with a non-contact voltage tester, and apply a lock or tag before beginning work. Never assume a disconnected cord is sufficient if others are in the workspace.

Refrigerant handling: EPA Section 608 of the Clean Air Act prohibits knowingly venting refrigerants. R-134a and R-600a must be handled with proper equipment. R-600a is flammable — keep ignition sources away, work in ventilated spaces, and never use standard refrigerant recovery equipment on hydrocarbon systems without confirming equipment compatibility.

PPE: Wear safety glasses when working near refrigerant lines. Liquid refrigerant contacting skin or eyes can cause frostbite. Refrigerant-rated gloves are recommended when connecting or disconnecting service manifolds.

Key terms10

Refrigeration cycleThe continuous loop through which a refrigerant absorbs heat in the evaporator and rejects it through the condenser, driven by the compressor.
CompressorA motor-driven pump that raises refrigerant vapor from low pressure to high pressure, increasing its temperature so heat can be rejected to the surroundings.
CondenserThe heat exchanger on the high-pressure side where hot refrigerant vapor releases heat to the room and condenses into a liquid.
EvaporatorThe heat exchanger inside the refrigerated space where low-pressure liquid refrigerant boils and absorbs heat from the cabinet air, producing the cooling effect.
Capillary tubeA fixed-restriction metering device made of narrow copper tubing that creates a pressure drop between the high-pressure and low-pressure sides of the system in most domestic refrigerators.
Saturation temperatureThe temperature at which a refrigerant boils or condenses at a given pressure; for a pure refrigerant, pressure and saturation temperature are uniquely paired.
R-134aA non-flammable HFC refrigerant widely used in domestic refrigerators; has zero ozone depletion potential but moderate global warming potential.
R-600a (isobutane)A flammable hydrocarbon refrigerant used in many newer energy-efficient domestic refrigerators; charged in very small quantities due to its flammability.
Suction lineThe refrigerant line that carries low-pressure vapor from the evaporator outlet back to the compressor inlet.
Pressure-temperature (P-T) chartA reference table listing the saturation temperature of a refrigerant at each corresponding pressure, used to evaluate system operating conditions.

Worked examples4

1. The low-side gauge on an R-134a refrigerator reads 2 psig. Using a P-T chart, the saturation temperature at 2 psig for R-134a is approximately -13 °F. The actual air temperature inside the freezer is -5 °F. Is the evaporator operating at the expected temperature, and what does the reading suggest?
  1. Step 1 — Read the gauge: low-side pressure = 2 psig.
  2. Step 2 — Look up 2 psig on the R-134a P-T chart: saturation temperature ≈ -13 °F.
  3. Step 3 — Compare to freezer air temperature: freezer air is -5 °F, which is warmer than -13 °F. This is normal; the evaporator runs colder than the air it is cooling so heat flows from the air into the refrigerant.
  4. Step 4 — Interpret: The temperature difference (about 8 °F) is reasonable. The system appears to be operating within a normal range at this operating point.
Answer: The evaporator saturation temperature of approximately -13 °F is colder than the -5 °F freezer air, which is expected and indicates the evaporator is absorbing heat normally. No fault is indicated by this reading alone.
2. An R-134a refrigerator has a high-side (discharge) pressure of 125 psig. The P-T chart shows R-134a saturates at approximately 104 °F at 125 psig. The room temperature is 70 °F. Is this condenser pressure reasonable?
  1. Step 1 — Record high-side pressure: 125 psig.
  2. Step 2 — Look up 125 psig on the R-134a P-T chart: saturation temperature ≈ 104 °F.
  3. Step 3 — Compare condenser saturation temperature to room temperature: 104 °F is significantly higher than the 70 °F room air.
  4. Step 4 — Interpret: For the condenser to reject heat into 70 °F room air, the refrigerant must be hotter than the room. A condenser saturation temperature in the range of 90–110 °F above a 70 °F ambient is typical. This reading falls within a normal operating range.
Answer: A condenser saturation temperature of approximately 104 °F at 125 psig in a 70 °F room is within the expected operating range. The refrigerant is hot enough to reject heat to the room air, confirming normal high-side operation.
3. A technician is checking an R-600a refrigerator. The low-side pressure reads approximately 0 psig. The R-600a P-T chart shows a saturation temperature of approximately 14 °F at 0 psig. The freezer is set to 0 °F. Does the low-side pressure seem consistent with the set point?
  1. Step 1 — Note the low-side pressure: 0 psig.
  2. Step 2 — Look up 0 psig on the R-600a P-T chart: saturation temperature ≈ 14 °F.
  3. Step 3 — Compare to the freezer set point of 0 °F: The evaporator saturation temperature of 14 °F is warmer than the 0 °F target freezer air temperature, which would prevent the freezer from reaching set point.
  4. Step 4 — Interpret: If the freezer cannot reach 0 °F, a low-side reading of 0 psig (saturation ≈ 14 °F) may indicate a low refrigerant charge, a restriction, or a component problem that prevents the evaporator from operating cold enough.
Answer: At 0 psig, R-600a saturates at approximately 14 °F. Because the freezer set point is 0 °F, the evaporator must run colder than 0 °F. A saturation temperature of 14 °F at this pressure is too warm to achieve the set point, suggesting a possible low charge or system restriction warranting further diagnosis.
4. A student reads a P-T chart and notes that R-134a at 10 psig has a saturation temperature of approximately 6 °F, while at 20 psig the saturation temperature is approximately 26 °F. By how many degrees does the saturation temperature increase when pressure rises from 10 to 20 psig?
  1. Step 1 — Identify saturation temperature at 10 psig: ≈ 6 °F.
  2. Step 2 — Identify saturation temperature at 20 psig: ≈ 26 °F.
  3. Step 3 — Subtract: 26 °F − 6 °F = 20 °F.
  4. Step 4 — Interpret: A 10 psig increase in pressure raises the saturation temperature by approximately 20 °F, demonstrating that pressure and saturation temperature rise together.
Answer: The saturation temperature increases by approximately 20 °F when pressure rises from 10 psig to 20 psig for R-134a, illustrating the direct pressure-temperature relationship.
In class: Label a cutaway diagram of a household refrigerator identifying the compressor, condenser, metering device (capillary tube or valve), evaporator, and refrigerant lines, then explain each component's role in a written paragraph reviewed by the instructor. · Use a pressure-temperature chart for R-134a and R-600a to calculate expected saturation temperatures at a range of operating pressures, entering answers in a worksheet that is self-checked against an answer key and reviewed by the instructor. · Watch an instructor-led teardown demonstration of a non-functional refrigerator and match physical components to the cycle diagram, completing a component-identification quiz with a minimum score of 75 percent before moving forward.
Materials: Cutaway refrigerator unit or large poster-quality diagram, pressure-temperature reference charts (R-134a, R-600a), Refrigeration & Air Conditioning Technology (Whitman et al.) textbook, component-identification worksheet, whiteboard.
Check: Component-identification quiz: student must correctly label at least 9 of 12 components and answer at least 3 of 4 pressure-temperature calculation questions correctly (75% pass threshold).

Practice check8

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

Refrigeration systems — PSA Certified Appliance Professional
1. Which component in the refrigeration cycle is responsible for increasing refrigerant pressure and temperature?
Refrigeration systems — PSA Certified Appliance Professional
2. In a domestic refrigerator, where does the refrigerant absorb heat from the food compartment?
Refrigeration systems — PSA Certified Appliance Professional
3. What is the primary function of the capillary tube in a household refrigerator?
Refrigeration systems — PSA Certified Appliance Professional
4. A technician notes that the low-side pressure in an R-134a refrigerator is higher than normal. Based on the pressure-temperature relationship, what would you expect the evaporator temperature to be compared to normal?
EPA 608 — refrigerant safety and environmental regulations
5. Why is R-600a charged in very small quantities in domestic refrigerators?
EPA 608 — regulatory requirements
6. Which EPA regulation requires technicians to be certified before purchasing or handling refrigerants used in appliances?
Safety — PSA Certified Appliance Professional
7. Before removing an access panel from inside a plugged-in refrigerator to inspect the evaporator, the FIRST safety step a technician should take is:
Refrigeration principles — PSA Certified Appliance Professional / EPA 608
8. On an R-134a P-T chart, as pressure increases from 0 psig to 30 psig, what happens to the saturation temperature?
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