NATE LC Refrigeration flashcards
NATE LC Refrigeration · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Sensible heat
- Heat that changes the temperature of a substance without changing its state; measured with a thermometer and expressed in Btu or joules.
- Latent heat
- Heat absorbed or released during a phase change at constant temperature, such as boiling or condensing refrigerant in the evaporator and condenser.
- Superheat
- The difference between the actual suction line temperature and the evaporating saturation temperature; it confirms that only vapor reaches the compressor.
- Subcooling
- The difference between the condensing saturation temperature and the liquid line temperature; it confirms that liquid, not flash gas, leaves the condenser.
- Saturation temperature
- The temperature at which a refrigerant boils or condenses at a given pressure; on a saturated system, pressure and temperature move together.
- Metering device function
- It reduces refrigerant pressure from condensing to evaporating pressure and regulates the flow of liquid refrigerant into the evaporator.
- Positive displacement compressor
- A compressor that traps and squeezes a fixed volume of vapor per revolution, such as reciprocating, scroll, and rotary designs.
- Zeotropic blend charging
- A refrigerant blend whose components boil at different temperatures; it must be charged as liquid to maintain the designed composition.
- Evacuation target
- A tight system is evacuated to 500 microns or lower and held with the pump isolated; a rising vacuum indicates moisture or a leak.
- Flowing nitrogen while brazing
- Dry nitrogen displaces air inside the tubing during brazing, preventing internal oxidation scale that can clog the metering device.
- Liquid line sizing
- Liquid lines are sized for low pressure drop so liquid reaches the metering device without flashing, which would reduce capacity.
- Suction line sizing
- Suction lines are sized to keep refrigerant velocity high enough to return oil while limiting excessive pressure drop.
- Filter-drier location
- The liquid line filter-drier is installed with the flow arrow toward the metering device; a suction drier is temporary after a burnout.
- Commissioning amp check
- Record compressor and fan amp draw and compare to nameplate values after startup to confirm the system is operating within design limits.
- Suction riser oil trap
- An inverted P-trap on a tall suction riser collects oil and uses refrigerant velocity to lift it back to the compressor.
- High superheat diagnosis
- High superheat with low suction pressure indicates a starved evaporator from undercharge, restriction, or a faulty metering device.
- Low superheat diagnosis
- Low superheat or frost at the compressor indicates liquid floodback from overcharge, an oversized valve, low load, or a stuck valve.
- High discharge pressure
- High discharge pressure with normal suction usually means condenser problems such as a dirty coil, fan failure, or non-condensables.
- Filter-drier pressure drop
- A pressure drop across the filter-drier greater than its rated value, often about 2 to 3 psi, signals a restriction that requires replacement.
- Non-condensable gases
- Air or nitrogen trapped in the system raises discharge pressure and subcooling without another cause; recover, evacuate, and recharge.
- Hunting expansion valve
- A valve that repeatedly overfeeds and underfeeds the evaporator, often from a lost bulb charge or poor bulb location.
- Refrigerant recovery
- Refrigerant must be recovered before opening a system; recovery cylinders are filled by weight, typically to no more than 80 percent.
- Burnout cleanup
- After a compressor burnout, install a suction filter-drier, flush contaminated components, and verify acid levels before removing the temporary drier.
- POE oil handling
- Polyol ester oil is highly hygroscopic and must be kept sealed; it is not interchangeable with mineral oil in a system.
- Leak detection sequence
- Inspect visually, use an electronic detector at joints, then confirm with pressure or vacuum decay testing and soap bubbles.
- Brazing heat protection
- Wet rags or heat-sink paste protect valves and controls from overheating while brazing nearby joints in the refrigerant circuit.
- Ohm's law
- Voltage equals current times resistance (E = I x R); power in a resistive circuit equals current times voltage (P = I x E).
- Capacitor discharge
- A capacitor can retain voltage after power is removed, so it must be discharged through a resistor before handling.
- Locked rotor amps
- The very high current drawn when a compressor rotor cannot turn; it is far above running load amps and trips the overload.
- Start components
- A start capacitor and potential relay provide extra starting torque; failure makes the compressor hum and trip its overload.
- Lockout/tagout
- Service requires isolating the disconnect, locking and tagging it, and verifying zero energy before touching electrical components.
- Refrigerant PPE
- Gloves and eye protection are required because liquid refrigerant causes freeze burns, and vapor can displace oxygen in low areas.