3.2 System Components in Detail
Key Takeaways
- Filter-driers remove moisture and contaminants — should be replaced whenever the system is opened
- Sight glass moisture indicator: Green = dry (good), Yellow = wet (problem)
- Accumulators protect the compressor from liquid slugging; located in suction line
- PRVs reseat automatically; rupture discs and fusible plugs are one-time use devices
- Centrifugal compressors are used in large Type III low-pressure chiller systems
Last updated: March 2026
3.2 System Components in Detail
Beyond the four primary cycle components, refrigeration systems include many secondary components that are critical for proper operation, safety, and EPA 608 exam questions.
Compressor Types
| Type | How It Works | Common Applications | Key Feature |
|---|---|---|---|
| Reciprocating | Pistons compress vapor in cylinders | Commercial refrigeration, residential AC | Most common in small-medium systems |
| Scroll | Two spiral-shaped scrolls trap and compress vapor | Residential/light commercial AC, heat pumps | Quiet, efficient, fewer moving parts |
| Screw (Helical Rotary) | Two meshing helical rotors compress vapor | Large commercial, industrial | High capacity, continuous compression |
| Centrifugal | Impeller spins at high speed, using centrifugal force | Large chillers (100+ tons) | Highest capacity; used in Type III systems |
| Rotary (Vane) | Vanes in a rotating drum compress vapor | Window AC, small refrigerators | Compact, lightweight |
Filter-Drier
- Function: Removes moisture and particulates from the refrigerant
- Location: In the liquid line between the condenser and metering device
- Why it matters: Moisture in a system causes acid formation, which corrodes internal components. Moisture also can freeze at the metering device, blocking refrigerant flow (a condition called "icing at the metering device")
- Replacement: Should be replaced whenever a system is opened for service
Sight Glass/Moisture Indicator
- Function: Allows visual inspection of refrigerant flow and moisture content
- Location: In the liquid line, typically after the filter-drier
- Reading the indicator:
- Clear glass: System properly charged (full liquid flow)
- Bubbles in glass: Possible undercharge, restriction, or low subcooling
- Green dot: Dry system (acceptable moisture level)
- Yellow dot: Wet system (excessive moisture — system needs attention)
Accumulator
- Function: Prevents liquid refrigerant from reaching the compressor (prevents liquid slugging)
- Location: In the suction line between the evaporator and compressor
- How it works: Catches any liquid refrigerant that leaves the evaporator, allowing it to boil off before reaching the compressor
- Common in: Heat pump systems (which reverse flow direction)
Receiver
- Function: Stores liquid refrigerant after it leaves the condenser
- Location: In the liquid line between the condenser and metering device
- Purpose: Provides storage for excess refrigerant and ensures a solid column of liquid reaches the metering device
- Key fact: Pressure relief devices are required on receivers
Service Valves and Ports
- Suction service valve: Located on the compressor suction (low-side) connection
- Discharge service valve: Located on the compressor discharge (high-side) connection
- Schrader valve: Spring-loaded valve (like a tire valve) used as a service port; the core depressor in the service hose opens it
- King valve: A valve at the outlet of the receiver that allows isolation of the charge in the condenser and receiver
Pressure Safety Devices
All refrigeration systems require safety devices to prevent dangerous over-pressure conditions:
| Device | Function | Reset |
|---|---|---|
| Pressure relief valve (PRV) | Opens at set pressure to relieve excess pressure | Reseats automatically (can be used multiple times) |
| Rupture disc (burst disc) | Bursts at set pressure to relieve excess pressure | One-time use — must be replaced after activation |
| Fusible plug | Melts at a specific temperature to relieve pressure | One-time use — must be replaced after activation |
For the Exam: Know the difference between PRVs (resetable), rupture discs (one-time), and fusible plugs (temperature-activated, one-time). Also know that pressure relief devices must NEVER be valved off or bypassed during system operation.
Test Your Knowledge
What type of compressor is most commonly used in large centrifugal chillers (Type III systems)?
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Test Your Knowledge
What color does the moisture indicator in a sight glass turn when the system contains excessive moisture?
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Test Your Knowledge
What is the function of an accumulator in a refrigeration system?
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