HVAC Excellence Refrigeration key terms

Heat that changes the temperature of a substance without changing its state. In refrigeration work it is measured as the difference between entering and leaving air temperatures across a coil.

Heat absorbed or released when a substance changes state at constant temperature, such as refrigerant boiling in an evaporator or condensing in a condenser. It does not register on a thermometer.

The sum of sensible and latent heat content of a substance, also called enthalpy. Psychrometric charts and refrigerant pressure-enthalpy diagrams use it to calculate coil capacity.

The temperature at which a refrigerant boils or condenses at a given pressure. On a pressure-temperature chart it pairs one pressure with one temperature while both liquid and vapor are present.

The number of degrees a vapor is above its saturation temperature at the measured pressure. Suction superheat confirms the evaporator is fully fed and protects the compressor from liquid slugging.

The number of degrees a liquid is below its saturation temperature at the measured pressure. Measured at the condenser outlet, it verifies the metering device is receiving solid liquid refrigerant.

Pressure measured from a perfect vacuum, equal to gauge pressure plus atmospheric pressure. Refrigerant pressure-temperature charts require absolute pressure, so gauge readings must be converted before use.

Pressure read on a manifold or gauge that ignores atmospheric pressure, reading zero at sea-level ambient. Most service instruments display gauge pressure, which must be converted for saturation calculations.

A chart plotting refrigerant pressure against heat content that shows the evaporating, condensing, and compression processes. Technicians use it to calculate capacity, mass flow, and compressor work.

The repeating sequence in which refrigerant evaporates at low pressure, is compressed, condenses at high pressure, and is metered back down. Each step moves heat from a cooler space to a warmer one.

The component that raises refrigerant vapor from evaporator pressure to condensing pressure, adding energy and creating the pressure difference that drives the cycle. Common types include reciprocating, scroll, and rotary.

A restriction that drops refrigerant from condensing pressure to evaporating pressure while regulating flow into the evaporator. Common designs are thermostatic expansion valves, electronic expansion valves, and fixed orifices.

The heat-exchange coil where low-pressure refrigerant boils and absorbs heat from the conditioned space or product. Its capacity depends on airflow, refrigerant feed, and the temperature difference across the coil.

The heat-exchange coil where high-pressure refrigerant vapor rejects heat and condenses to liquid. It may be air-cooled, water-cooled, or evaporative, and its cleanliness directly affects head pressure.

A modulating metering device that controls refrigerant flow to hold a set superheat at the evaporator outlet, using a sensing bulb and diaphragm. It responds to load changes automatically.

The pipe carrying low-pressure vapor from the evaporator back to the compressor. Suction line pressure and temperature readings are the primary indicators of evaporator performance and charge.

The pipe carrying condensed liquid refrigerant from the receiver or condenser to the metering device. Sight glasses, filter-driers, and subcooling checks are typically located on this line.

The pipe carrying hot high-pressure vapor from the compressor to the condenser. Abnormally high discharge temperatures often signal overheating, low charge, or a condenser problem.

A device in the liquid line containing a desiccant and screen that traps moisture, acid, and particulate contamination before it reaches the metering device. It protects the system from ice and sludge.

A sight glass with a color-changing element that reveals moisture content and shows whether liquid refrigerant is present in the liquid line. It is a quick diagnostic for charge and contamination issues.

The high-side pressure at the compressor discharge, set largely by condensing temperature and ambient conditions. High head pressure reduces capacity and raises compressor current draw.

A mixture of two or more refrigerants formulated to achieve desired properties, available as azeotropic or zeotropic. Zeotropic blends fractionate, so they must be charged as liquid and leak-tested carefully.

The temperature change that occurs while a zeotropic blend evaporates or condenses at constant pressure. Technicians must average coil temperatures and account for glide when setting superheat.

Removing refrigerant from a system into a recovery cylinder or machine rather than venting it to the atmosphere, as required by environmental regulations. Recovery is the first step of most repairs.

Pulling a deep vacuum with a pump to remove air, moisture, and non-condensables from a system before charging. A standing vacuum test confirms the system holds and is dry.

An electronic instrument that reads very low absolute pressures in microns during evacuation. It is far more accurate than a manifold gauge for confirming dehydration.

Air or other gases that do not condense at normal system temperatures, raising head pressure and reducing efficiency. They are removed by proper evacuation and purging of hoses and gauges.

Lubricant circulated with refrigerant to protect compressor bearings and seals. Its viscosity, miscibility, and moisture tolerance must match the refrigerant, especially when retrofitting.

A synthetic lubricant used with HFC refrigerants that is highly hygroscopic, readily absorbing moisture from air. Systems using it require tight evacuation and prompt sealing of open lines.

A field test of compressor oil that detects acidic byproducts formed when refrigerant breaks down under high temperature or moisture. Positive results indicate contamination requiring oil change or cleanup.

A charging procedure that sets refrigerant charge by measuring evaporator outlet superheat against the manufacturer's target at known indoor and outdoor conditions. It is preferred on fixed-orifice systems.

A charging procedure that sets charge by measuring liquid-line subcooling against the manufacturer's target for the current conditions. It is the standard approach for thermostatic expansion valve systems.

A service tool with two gauges, hoses, and valves that reads high-side and low-side pressures and allows evacuation, charging, and recovery. Hoses must be purged to avoid introducing air.

Liquid refrigerant returning to the compressor, which can wash out oil and damage valves or bearings. It is caused by overcharge, low superheat, or excessive evaporator load.

Locating escaping refrigerant using electronic detectors, ultraviolet dye, pressure tests, or bubble solution. Early detection preserves charge, protects the environment, and prevents compressor damage.

A condition where the expansion valve alternately overfeeds and underfeeds the evaporator, causing suction pressure and superheat to swing. Oversized valves, poor bulb contact, or low load usually cause it.

A safety device, often a bimetal or internal thermostat, that opens the compressor circuit when current or temperature exceeds limits. Tripping indicates high head pressure, low voltage, or a failing winding.