Red Seal Refrigeration & AC Mechanic key terms

The quantity of heat a substance absorbs or releases when it changes between liquid and vapour at constant temperature and pressure. In refrigeration it is the dominant cooling effect produced in the evaporator.

Heat that changes the temperature of a substance without changing its state. It is measured with thermometers and accounts for the temperature rise or fall of air, water, or refrigerant liquid and vapour.

The number of degrees a vapour is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding enough refrigerant to protect the compressor.

The number of degrees a liquid is below its saturation temperature at a given pressure. Measured at the condenser outlet, it confirms the liquid line is solid and the metering device will receive bubble-free refrigerant.

The temperature at which a refrigerant boils or condenses at a stated pressure. On a pressure-enthalpy diagram it is the point where the refrigerant is a mixture of liquid and vapour.

A chart plotting refrigerant pressure against enthalpy, used to trace the four basic cycle processes and calculate capacity, compressor work, and coefficient of performance from measured pressures and temperatures.

The total heat content of a refrigerant per unit mass, combining internal energy and flow work. Differences in enthalpy between two points on a chart give the refrigerating effect or the work of compression.

The ratio of refrigerating effect produced to the energy input required by the compressor. A higher value means a more efficient cycle; it falls as condensing temperature rises or evaporating temperature drops.

The temperature at which moist air becomes saturated and water vapour begins to condense. In cooling coils it marks where condensate forms, and in buildings it indicates risk of surface condensation.

The lowest temperature air can reach by evaporative cooling alone, read from a thermometer with a wetted sock in moving air. It indicates the total heat content of moist air and limits evaporative condenser performance.

The ordinary air temperature measured by a dry thermometer, unaffected by moisture content or air motion. It is the value used for comfort setpoints and equipment entering-air ratings.

The amount of water vapour in air expressed as a percentage of the maximum the air could hold at that temperature. It changes with temperature even when the actual moisture content stays constant.

A graph relating dry-bulb temperature, wet-bulb temperature, humidity ratio, relative humidity, and enthalpy of moist air. Technicians use it to plot air-conditioning processes and size coils and airflow.

A metering device that regulates refrigerant flow into the evaporator in response to bulb-measured superheat. It maintains a nearly constant evaporator outlet superheat across changing loads and pressures.

A stepper-motor-driven metering device controlled by a circuit board that responds to temperature and pressure sensors. It holds very tight superheat and is common on inverter and variable-capacity systems.

The heat exchanger in which low-pressure liquid refrigerant boils and absorbs heat from the space or product being cooled. Its performance depends on airflow, coil cleanliness, and correct refrigerant feed.

The heat exchanger that rejects heat from hot high-pressure refrigerant vapour to the surrounding air or water, causing it to condense to liquid. Types include air-cooled, water-cooled, and evaporative.

The machine that raises refrigerant pressure and temperature by drawing in low-pressure vapour and discharging it at condenser pressure. Common types are reciprocating, scroll, screw, and centrifugal.

The absolute discharge pressure divided by the absolute suction pressure. High ratios raise discharge temperature and reduce capacity, and they often signal the need for two-stage or compound compression.

The low-side pressure measured at the compressor inlet, which sets the evaporating temperature and therefore the coil temperature. It is the primary reading used to check system charge and load.

The high-side pressure measured at the compressor outlet, which sets the condensing temperature. Elevated readings point to dirty condensers, non-condensable gases, or overcharge.

Air or another gas that will not liquefy at normal condensing conditions and accumulates in the high side. It raises discharge pressure and power draw, and shows as a large condensing approach temperature.

An electric resistance heater wrapped around or inserted in the compressor sump that keeps oil warm when the compressor is off. It prevents refrigerant from migrating and diluting the oil, avoiding lubrication failure on start-up.

The movement of liquid refrigerant into the compressor crankcase or other cold parts during an off cycle. It dilutes oil, causes foaming and slugging, and is prevented by crankcase heaters, pump-down, and check valves.

A control sequence that closes the liquid line solenoid and lets the compressor run until suction pressure drops to a setpoint, storing refrigerant in the receiver. It protects against liquid slugging and migration.

A vessel on the high side that stores liquid refrigerant and provides a reserve for varying load. It allows the condenser to drain freely and holds the charge during pump-down.

A vessel installed in the suction line that traps liquid refrigerant and oil and meters them back to the compressor as vapour. It protects the compressor from slugging on flooded or heat-pump systems.

The pipe carrying high-pressure liquid refrigerant from the condenser or receiver to the metering device. It must stay free of flashing and is fitted with a filter-drier and sight glass.

A device in the liquid line containing a desiccant and filter that removes moisture, acid, and particulate matter from the refrigerant. It protects the metering device and compressor from contamination and ice blockage.

A small window in the liquid line that shows whether the refrigerant is solid or flashing. Many include a colour-changing moisture indicator that signals unsafe water content.

A colour-change element in a sight glass or filter-drier that reacts to dissolved water in the refrigerant. Its reading warns when moisture has exceeded the desiccant's capacity and acid formation is likely.

The process of removing air and moisture from a system with a vacuum pump to a deep vacuum, usually verified with a micron gauge. Proper evacuation prevents non-condensables, ice, and acid from forming.

An electronic instrument that measures absolute pressure in microns to confirm the depth of vacuum reached during evacuation. It detects moisture and leaks that a manifold gauge cannot show.

A two-valve service tool with hoses that connects to suction, discharge, and centre ports to read pressures and direct refrigerant flow. It is used for charging, recovery, and evacuation setup.

A compressor-driven unit that removes refrigerant from a system into a recovery cylinder, often with an oil separator and filter. It must be used before opening a system for service.

A refillable pressure vessel, usually grey with a yellow top, that is DOT-approved for transporting recovered refrigerant. It must never be filled above 80 percent of its capacity by liquid volume.

Reprocessing recovered refrigerant to new-product purity by chemical analysis and distillation, verified against AHRI Standard 700. Reclaimed refrigerant can be resold and used in any system.

A formal procedure that isolates energy sources with a padlock and tag before service work begins. It prevents accidental start-up or release of stored energy, and only the person who applied the lock removes it.

Wearable safety gear such as safety glasses, face shield, insulated gloves, and respirator selected for the task. For refrigerant work it protects against frostbite, liquid splash, and oxygen displacement.

An ASHRAE classification combining toxicity (A or B) and flammability (1, 2L, 2, or 3). It tells a technician what ventilation, ignition controls, and leak detection a refrigerant requires.

A refrigerant rated A2L, A2, or A3, such as R-32 or R-290, that can ignite under the right concentration. Handling requires no ignition sources, proper ventilation, and leak detection.

A charging technique for fixed-orifice and piston systems in which refrigerant is added until evaporator outlet superheat matches the manufacturer's target at the measured indoor and outdoor conditions.

A charging technique for systems with a thermostatic expansion valve in which refrigerant is adjusted until liquid-line subcooling matches the manufacturer's target. It is the preferred method for TXV systems.

The documented process of verifying that a newly installed system meets design airflow, charge, controls, and safety requirements before handover. It includes start-up checks, readings, and client training.

The volume of air moving through a system per unit time, usually stated in litres per second or cubic feet per minute. Correct airflow is essential for coil performance, charge verification, and comfort.

The outward force per unit area exerted by air on duct walls, measured in pascals or inches of water. Total external static pressure indicates duct restriction and blower loading.

An instrument that measures small pressure differences, often as a digital or inclined water column. Technicians use it for duct static pressure, filter pressure drop, and gas train readings.

A timed or demand-initiated sequence that melts frost from an evaporator coil using heaters, hot gas, or off-cycle air. It restores heat transfer and airflow in low-temperature refrigeration.

Oil formulated to lubricate the compressor and remain miscible with the refrigerant. Mineral oil suits CFC and HCFC systems, while POE and PVE oils are required for HFC and HFO systems.

A property of a substance that readily absorbs moisture from the air. Polyol ester lubricant is strongly hygroscopic, so it must be kept sealed and system exposure times kept short.

A joining process in which filler metal melts above 450 degrees Celsius and flows into the joint by capillary action without melting the base metal. Most refrigerant piping joints are brazed with silver alloy.

The practice of flowing dry nitrogen through piping while brazing to exclude oxygen. It prevents internal oxidation and scale, which would otherwise contaminate the refrigerant circuit.

A mechanical joint in which a cone-shaped nut compresses a flared tube end against a matching seat. Common on small split systems, it must be torqued correctly to avoid leaks.

The systematic search for refrigerant leaks using electronic detectors, pressure tests, dyes, or soap bubbles. Early detection protects the environment, reduces cost, and maintains system capacity.