NATE CR Service key terms

The number of degrees a refrigerant vapor is heated above its saturation temperature at a given pressure. Measured at the evaporator outlet to confirm the expansion valve is feeding the coil correctly.

The number of degrees a liquid refrigerant is cooled below its saturation temperature at a given pressure. Measured at the condenser outlet to verify the condenser is fully condensing and the liquid line is solid.

The temperature at which a refrigerant changes state between liquid and vapor at a specific pressure. Used as the reference point when calculating superheat and subcooling from gauge readings.

A reference table listing the saturation temperature of a refrigerant at each pressure. Technicians use it to convert gauge readings into temperatures for superheat and subcooling calculations.

A modulating metering device that regulates refrigerant flow into the evaporator in response to superheat at the coil outlet. It uses a sensing bulb, diaphragm, and spring to balance opening forces.

The charged element clamped to the evaporator suction line that senses outlet temperature and transmits pressure to the expansion valve diaphragm. Poor contact with the line causes inaccurate superheat control.

A line connecting the expansion valve diaphragm to the evaporator outlet that compensates for pressure drop through the coil. Required on evaporators with distributors or large pressure drops so superheat stays accurate.

The heat exchanger where low-pressure liquid refrigerant absorbs heat from the refrigerated space and boils into vapor. Its capacity depends on airflow, coil surface, and refrigerant feed rate.

The heat exchanger that rejects heat from hot high-pressure refrigerant vapor to the surroundings, condensing it to liquid. Air-cooled and evaporative types are common in commercial refrigeration.

The pump that raises refrigerant vapor pressure and temperature, moving it from the evaporator to the condenser. Common commercial types include reciprocating, scroll, semi-hermetic, and screw compressors.

The discharge pressure at the compressor outlet, corresponding to condensing temperature. High head pressure reduces capacity and efficiency and may trip high-pressure safety controls.

The low-side pressure entering the compressor, corresponding to evaporating temperature. It indicates evaporator load and is the basis for low-pressure controls and superheat calculations.

The pipe carrying condensed liquid refrigerant from the receiver or condenser to the metering device. It should be solid liquid; flash gas here indicates restrictions or insufficient subcooling.

The pipe carrying low-pressure vapor from the evaporator back to the compressor. Its insulation and sizing affect superheat, pressure drop, and compressor reliability.

The pipe carrying hot high-pressure vapor from the compressor to the condenser. It is used to route heat reclaim, oil separators, and discharge-line accessories.

A device in the liquid line containing a desiccant and filter that removes moisture and particulate contamination from refrigerant. A pressure drop across it signals restriction or saturation.

A small window in the liquid line used to observe refrigerant flow. Bubbles may indicate insufficient subcooling or a restriction, though they are not a reliable charge indicator alone.

A color-changing element in a sight glass that reacts to moisture in the refrigerant. It helps detect contamination that can form acids and damage compressors.

A manifold at the evaporator inlet that divides liquid refrigerant evenly among coil circuits. Incorrect nozzle sizing or installation causes uneven frosting and poor coil performance.

A timed or demand-initiated sequence that melts frost from the evaporator using electric heaters, hot gas, or off-cycle air. Proper termination prevents coil icing and product temperature swings.

The control that ends a defrost cycle when the evaporator reaches a set temperature or time limit. Failure can leave ice or overheat the coil and raise case temperatures.

A device that schedules defrost cycles at set intervals and durations. Modern controls use electronic timers that may also initiate demand defrost based on coil conditions.

An electrical heater on the compressor crankcase that keeps oil warm during the off cycle. It prevents refrigerant migration and oil dilution that can cause compressor damage.

A vessel in the suction line that traps liquid refrigerant and oil and releases them gradually to the compressor. It protects the compressor from liquid slugging.

The condition where liquid refrigerant enters the compressor, causing mechanical damage, oil dilution, and high discharge pressures. Often caused by overcharge, low superheat, or migration.

A control sequence that pumps refrigerant from the evaporator into the receiver before shutdown. It prevents liquid migration and reduces the risk of slugging on restart.

A tank in the high side that stores liquid refrigerant and accommodates changes in system charge. It supplies the liquid line and is used in pump-down arrangements.

A switch that opens or closes the compressor circuit based on suction pressure. It provides pump-down control and low-charge protection.

A safety control that opens the compressor circuit when discharge pressure exceeds a set limit. It protects against condenser fan failure, blocked airflow, or overcharge.

A device on the discharge line that removes oil from refrigerant vapor and returns it to the compressor crankcase. It helps maintain lubrication and reduces oil logging in the evaporator.

A control that monitors net oil pressure and shuts down the compressor if lubrication is inadequate. It uses a time delay to avoid nuisance trips during startup.

A test of compressor oil or refrigerant that detects acid formation from moisture, heat, or motor burnout. It guides decisions on filter-drier replacement and system cleanup.

Ice crystals that form on evaporator surfaces when coil temperature is below freezing and moisture condenses. Excessive frost reduces airflow and requires defrosting.

The amount of refrigerant contained in a system. Correct charging is verified by superheat, subcooling, and sight glass observations rather than pressure alone.

The movement of refrigerant vapor or liquid to the compressor crankcase during the off cycle. It dilutes oil and can cause slugging on startup; crankcase heaters and pump-down help prevent it.

The difference in pressure between two points in a refrigerant circuit. Excessive drop across filters, valves, or coils reduces capacity and can cause flash gas.

Vapor that forms when liquid refrigerant pressure drops below its saturation point before reaching the metering device. It reduces valve capacity and cooling performance.

A heat rejection device that combines air movement and water evaporation over a coil to condense refrigerant. It requires water treatment and maintenance to control scale and biological growth.