4.2 System Components in Detail
Key Takeaways
- Compressor types include reciprocating (pistons), scroll (two spirals), screw (meshing rotors), centrifugal (impeller, used in large Type III chillers), and rotary/vane (small appliances).
- Metering devices: TXV/EEV modulate flow to control superheat; capillary tubes and fixed orifices/pistons deliver a fixed flow regardless of load.
- Filter-driers remove moisture and particulates and should be replaced whenever a system is opened; sight-glass indicator green = dry, yellow = wet.
- The accumulator (suction line) protects the compressor from liquid slugging; the receiver (liquid line) stores excess liquid for the metering device.
- Oil circulates with the refrigerant to lubricate, seal, and cool the compressor; proper oil-refrigerant miscibility is required so oil returns to the compressor.
The four major components define the cycle, but real equipment contains many secondary parts that the EPA 608 exam tests across every Type section. Knowing what each part does — and where it sits in the loop — lets you answer service, recovery, and safety questions with confidence.
Compressor Types
The compressor is the most expensive and most frequently tested component. Different designs suit different capacities, and the exam connects compressor type to appliance category (especially centrifugal for Type III).
| 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 scrolls orbit to trap and squeeze vapor | Residential/light commercial AC, heat pumps | Quiet, efficient, few moving parts |
| Screw (helical rotary) | Two meshing helical rotors compress vapor | Large commercial, industrial | High capacity, continuous flow |
| Centrifugal | Impeller spins at high speed using centrifugal force | Large chillers (100+ tons) | Highest capacity; used in Type III low-pressure systems |
| Rotary (vane) | Vanes in a rotating drum compress vapor | Window ACs, small refrigerators | Compact, lightweight |
A memory hook: small appliances use rotary; homes and light commercial use scroll or reciprocating; giant low-pressure chillers use centrifugal. That last pairing — centrifugal with Type III — appears repeatedly.
Metering (Expansion) Devices
The metering device sets how much liquid feeds the evaporator. There are two families: modulating valves that adjust to the load, and fixed restrictions that do not.
| Device | Type | How It Controls Flow | Notes |
|---|---|---|---|
| Thermostatic expansion valve (TXV/TEV) | Modulating | A sensing bulb on the evaporator outlet senses temperature and adjusts the valve to hold a target superheat | Most efficient; maintains superheat across varying loads |
| Electronic expansion valve (EEV) | Modulating | A controller and stepper motor adjust the orifice electronically | Precise; common on modern variable-capacity systems |
| Capillary tube | Fixed | Long, thin tube provides constant restriction | No moving parts; cheap; critical charge |
| Fixed orifice / piston | Fixed | A set-diameter hole meters a constant flow | Simple, low cost; flow does not adapt to load |
The big exam distinction: a TXV adjusts refrigerant flow to maintain superheat as load changes, while capillary tubes and fixed orifices deliver the same flow regardless of load. That adaptability is the TXV's main advantage and a frequent test point.
Condenser and Evaporator Variants
- Condensers are air-cooled (most residential), water-cooled (cooling tower or once-through), or evaporative. All reject the heat that the evaporator absorbed plus the compressor's work.
- Evaporators are direct-expansion (DX) coils in most AC, or flooded evaporators in large chillers. The evaporator is always the cold, low-pressure heat-absorbing coil.
Accessories That Show Up on the Exam
Filter-Drier
- Function: Removes moisture and particulates from the refrigerant.
- Location: Liquid line, between the condenser and the metering device.
- Why it matters: Moisture forms acids that corrode internal parts, and can freeze at the metering device (“icing”), blocking flow. Replace the filter-drier whenever the system is opened for service.
Sight Glass / Moisture Indicator
- Location: Liquid line, usually just after the filter-drier.
- Clear glass: Properly charged, full liquid flow.
- Bubbles: Possible undercharge, restriction, or low subcooling.
- Green dot = dry (acceptable moisture); Yellow dot = wet (excess moisture, needs attention).
Accumulator
- Function: Catches liquid leaving the evaporator and lets it boil off before it reaches the compressor — prevents liquid slugging.
- Location: Suction line, between the evaporator and compressor.
- Common in: Heat pumps, which reverse flow direction.
Receiver
- Function: Stores liquid refrigerant after the condenser and ensures a solid column of liquid feeds the metering device.
- Location: Liquid line, between the condenser and metering device.
- Key fact: Receivers require pressure-relief protection.
Service Valves and Pressure-Safety Devices
- Suction service valve / discharge service valve: On the compressor low- and high-side connections.
- Schrader valve: Spring-loaded service port (like a tire valve); the hose core depressor opens it.
- King valve: At the receiver outlet; lets you isolate the charge in the condenser and receiver.
| Safety Device | Function | Reset |
|---|---|---|
| Pressure relief valve (PRV) | Opens at a set pressure to relieve over-pressure | Reseats automatically (reusable) |
| Rupture (burst) disc | Bursts at a set pressure | One-time — replace after it activates |
| Fusible plug | Melts at a set temperature | One-time — replace after it activates |
Never valve off or bypass a pressure-relief device while the system is operating.
The Role of Oil
Refrigerant systems carry lubricating oil that circulates with the refrigerant. The oil lubricates compressor bearings and moving parts, seals the compression chamber against leakage, and helps carry heat away from the compressor.
For the oil to do its job, it must have proper miscibility with the refrigerant — it must mix and travel with it. If oil and refrigerant do not mix well, oil pools in the evaporator and suction line and starves the compressor; if they mix too freely, the oil film thins and lubrication fails. This is why a refrigerant change often requires changing the oil type (for example, mineral oil for older CFC/HCFC systems versus POE oil for HFC systems).
Worked Example: A technician opens a residential R-410A system to replace a failed TXV. Because the sealed loop was exposed to atmosphere, moisture and air entered. Best practice: install a new filter-drier in the liquid line, evacuate the system to remove moisture and non-condensables, then recharge. After running, the sight glass shows a green dot and clear liquid — confirming the drier captured moisture, the charge is correct, and the TXV is feeding the evaporator properly. Skipping the new drier would risk acid formation and metering-device icing.
For the Exam: Centrifugal compressors serve large Type III chillers. TXVs modulate flow to hold superheat; fixed orifices and cap tubes do not. Replace the filter-drier whenever the system is opened. Green = dry, yellow = wet. Accumulators (suction line) stop slugging; receivers (liquid line) store liquid. PRVs reseat; rupture discs and fusible plugs are one-time use.
Match each system component to its primary function.
Match each item on the left with the correct item on the right
Which compressor type is used in the large centrifugal chillers covered by Type III certification?
What is the chief advantage of a thermostatic expansion valve (TXV) over a capillary tube or fixed orifice?
A filter-drier should be replaced:
Why does refrigerant oil need proper miscibility with the refrigerant?