NATE LC Refrigeration key terms

The number of degrees a refrigerant vapor is heated above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding enough refrigerant to fully evaporate the liquid before it reaches the compressor.

The number of degrees a liquid refrigerant is cooled below its saturation temperature at a given pressure. Measured at the condenser outlet, it indicates how much liquid is stacked in the condenser and whether the charge and metering device are correct.

The temperature at which a refrigerant changes between liquid and vapor at a specific pressure. On a pressure-temperature chart it is the reference point used to convert a gauge reading into the corresponding boiling or condensing temperature.

A table or graph listing the saturation pressure of a refrigerant at each temperature. Technicians use it to translate gauge readings into saturation temperatures so superheat and subcooling can be calculated for the refrigerant in the system.

A metering device that controls refrigerant flow into the evaporator by balancing bulb pressure, evaporator pressure, and spring force. It maintains a set superheat so the evaporator is fully used without flooding liquid back to the compressor.

The component that drops refrigerant pressure from the high side to the low side and regulates how much liquid enters the evaporator. Common types include thermostatic expansion valves, electronic expansion valves, capillaries, and fixed orifices.

The heat exchanger where low-pressure liquid refrigerant absorbs heat from the refrigerated space or product and boils into vapor. Its coil, airflow, and defrost condition directly determine the system's capacity and the superheat at its outlet.

The heat exchanger that rejects heat from hot high-pressure refrigerant vapor to the outdoor air, water, or another medium, causing the refrigerant to condense into liquid. Its cleanliness and airflow strongly affect head pressure and system efficiency.

The pump that raises refrigerant vapor from evaporator pressure to condenser pressure and moves it through the circuit. Types include reciprocating, scroll, screw, and rotary, each with specific lubrication and operating limits that technicians must respect.

A vessel on the high side of the system that stores liquid refrigerant and supplies the metering device. It accommodates changes in charge and ensures a solid liquid column reaches the expansion valve in systems that require one.

A vessel installed on the low side, usually at the compressor suction, that traps liquid refrigerant and oil and lets only vapor continue to the compressor. It protects against slugging when the evaporator is flooded or load swings occur.

The pressure of refrigerant vapor at the compressor discharge, which corresponds to the condensing temperature. High head pressure reduces capacity and can trip safety controls; low head pressure can starve the metering device and cause low superheat.

The temperature change that occurs as a zeotropic refrigerant blend boils or condenses at a constant pressure. It means the bubble point and dew point differ, so technicians must know which value to use when checking superheat and subcooling.

A procedure in which the system is evacuated to a deep vacuum, isolated, and watched to see if pressure rises. A fast rise indicates a leak or moisture; a slow rise that levels off often indicates moisture boiling off inside the system.

The process of removing air, nitrogen, and moisture from a refrigeration circuit with a vacuum pump before charging. Reaching and holding a deep vacuum verifies the system is dry and leak-free and prevents acid and ice formation later.

An electronic instrument that measures very low absolute pressure in microns, used to confirm evacuation is complete. It must be connected directly to the system, not through long hoses, to read accurately during and after evacuation.

A leak check in which dry nitrogen is used to pressurize the system to a specified level, then the pressure is monitored for a drop. It is safer than using refrigerant or oxygen and can be combined with soap-bubble or electronic leak detection.

Adding refrigerant by weight using a scale and the manufacturer's nameplate charge, rather than by gauges alone. It is the most accurate method, especially for systems with receivers, long line sets, or blends that must be charged as a liquid.

The pair of copper lines, liquid and suction, that connect the condensing unit to the evaporator. Correct sizing, insulation, routing, and brazing of the line set affect pressure drop, oil return, and overall system capacity.

Joining copper refrigerant piping with a filler alloy and heat, usually while flowing dry nitrogen through the line to prevent internal oxidation. Proper brazing produces leak-free joints and avoids carbon scale that can clog metering devices.

A device in the liquid line containing a desiccant and filter that removes moisture, acid, and particulate contamination from the refrigerant. A pressure drop across it or a cold spot indicates it is restricted and should be replaced.

Removing moisture from a refrigeration system through deep evacuation, sometimes aided by dry nitrogen sweeps or a new filter-drier. Moisture left in the system can freeze at the metering device and form acids that damage the compressor.

A handheld instrument that senses refrigerant vapor and alarms when concentration rises near a joint or component. Sensitivity varies by refrigerant, so the unit must be calibrated and used slowly, close to suspected leak points.

A control sequence that pumps refrigerant out of the low side into the receiver or condenser and stops the compressor at a set low pressure. It limits liquid migration to the compressor during off cycles and is common on larger commercial systems.

Liquid refrigerant or oil entering the compressor crankcase or cylinders in quantity, causing loud knocking, valve damage, and bearing wear. It is usually traced to flooding, low superheat, or migration during off cycles.

An electric heater on the compressor crankcase that keeps oil warm during off cycles so refrigerant does not condense into it. It prevents oil dilution and slugging on startup and should be energized before the compressor runs after a long shutdown.

A switch that opens or closes the compressor circuit based on suction pressure. It can act as a safety to stop the compressor on loss of charge or as an operating control for pump-down and temperature regulation.

A safety switch that stops the compressor when discharge pressure exceeds a set limit, protecting against condenser fan failure, dirty coils, or overcharge. Many are manual reset and must be investigated before being reset.

A partial blockage in the liquid line, filter-drier, or metering device that causes a pressure drop and flash gas. Symptoms include low suction pressure, high superheat, low subcooling, and a frosted or cold spot at the restriction.

A defrost method that stops the compressor and runs the evaporator fans so room air melts frost from the coil. It is used on coolers above freezing and requires a termination control so the coil is cleared without adding excessive heat.

A defrost method using electric resistance heaters mounted on or in the evaporator to melt frost during a timed off cycle. It is common on freezer coils and needs termination and fail-safe controls to prevent overheating.

A temperature switch or sensor that ends the defrost cycle once the coil reaches a set temperature, rather than relying only on time. It saves energy and prevents excessive heat from entering the refrigerated space.

A control that schedules defrost cycles and determines their maximum length. It initiates defrost at set intervals and works with termination and fan-delay controls to return the system to normal cooling safely.

A chemical or electronic test of compressor oil or refrigerant that detects acidity from moisture-driven breakdown or overheating. A positive result indicates contamination and usually calls for oil change, filter-drier replacement, and finding the cause.

Failure of a compressor motor winding or mechanical parts that releases acid, sludge, and carbon into the circuit. Repair requires removing contamination with filter-driers, oil changes, and sometimes line flushing before a new compressor is installed.

Removing refrigerant from a system into a recovery cylinder or machine for reuse, recycling, or disposal instead of venting it. Proper recovery protects the environment and is required by regulation before opening a circuit.

A DOT-approved, color-coded cylinder used to store recovered refrigerant. It must not be filled beyond the rated capacity, and refrigerant type and fill level should be documented to avoid mixing and overfilling.

An electrical component that stores energy and provides phase shift or starting torque for motors. A bulged top, leaking fluid, or failed capacitance test indicates replacement is needed, and it must be discharged before handling.

An electrically operated switch that connects line power to the compressor and fan motors on a call for cooling. Pitted contacts, burned coils, or loose lugs cause voltage drop, single-phasing, and intermittent operation.

A thermal or current-sensing device that opens the motor circuit when it overheats or draws excessive current. Repeated trips point to high head pressure, low voltage, bad capacitors, or failing bearings rather than a faulty protector.

The coils of wire inside a compressor or fan motor that create magnetic fields when energized. Resistance and insulation tests on the windings reveal opens, shorts, and grounds that explain a motor that will not start or trips its protector.

A safety procedure that isolates and locks off electrical or refrigerant energy sources before service work begins. It prevents accidental startup or release of stored energy and protects technicians working on or near moving and energized equipment.