Experts in advanced refrigeration and air-conditioning

HVAC Terms Glossary


Absolute Pressure (Pabs): is the sum of gauge pressure and atmospheric pressure: Pabs = Pg + Patm

AC – Alternating Current: an electric current that its flow direction is constantly being reversed back and forth. In Europe, it is polarity change from positive to negative 50 cycles per second(50 Hz). In the US and some other countries, the rate of alternation is 60 cycles per second(60 Hz).

Ambient Temperature (Tamb): Temperature of a fluid (usually air), which surrounds an object.

Ampere (A): The unit of current. One ampere is the current flowing through one ohm(Ω) of resistance at one volt(V) potential.


Brazed Plate Heat Exchangers (BPHE): A hermetically sealed heat exchanger in which the heating surface consists of thin corrugated metal plates stacked on top of each other. Channels are formed between the plates and the corner ports are arranged so that the two media (water and/or refrigerant, or both) flow through alternate channels, always in counter-current flow. These heat exchangers are up to 60% smaller than traditional shell & tube and coaxial type devices. This means they use less space and they weigh less.

BTU: British Thermal Unit. Quantity of heat required to raise the temperature of one pound(lb) of water one degree Fahrenheit(ºF).

Boiling point: of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. At this point, additional heat doesn’t increase the temperature.

Bubble point: is the temperature (at a given pressure) where the first bubble of the vapor is formed when heating a liquid consists of two or more components.


Calorie (cal): (definition in physics) is the amount of heat energy needed to raise the temperature of one gram(g) of water by one degree Celsius(ºC) (or one kelvin).

Capillary Tubing: A refrigerant metering device consisting of a small diameter tube which controls flow by restriction. They are carefully sized by inside diameter and length for each application. This device uses an internal diameter, length, and pressure drop to determine the capacity and it has a fixed regulation. The capillary tubing.

Capacitor: It is a device that stores and releases electrical energy. The capacitance(F) of a capacitor is critical for motor efficiency.

  • Run Capacitor: Run capacitors are designed for continuous duty and are energized the entire time the motor is running. Single-phase electric motors need a capacitor to energize a second phase winding. Therefore the capacitance is so critical. If the wrong run capacitor is installed, the motor will not have an even magnetic field. It causes the motor to become noisy and overheated. At the same time, it decreases its performance and increases energy consumption.
  • Start Capacitor: Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly. Start capacitors are designed for momentary use. Start capacitors stay energized long enough to rapidly bring the motor to 3/4 of full speed.

Cascade System: Arrangement in which two or more refrigeration systems, with different refrigerants that have different boiling points, are used in series (each refrigerant is passing through its refrigeration system). The evaporator of one machine is used to cool the condenser of another. These are usually used in ultra-low temperatures.

Check Valve: Device which permits fluid flow in only one direction.

Cold: the total or partial absence of heat.

Compression ratio:  is a ratio of the absolute discharge pressure (bar) to absolute suction pressure (bar).

COP (coefficient of performance): is an expression of efficiency and it is defined as the relationship between the power (kW) that is drawn out of the device as cold or heat, and the power (kW) that is supplied to the compressor.

Condenser: Part of a refrigeration system which receives hot, high-pressure refrigerant gas from the compressor and cools it down (condense it) to a liquid state.

Condensing Pressure (Pcond): Pressure of refrigerant inside condenser.

Condensing Temperature (Tcond): Temperature of refrigerant inside  condenser at which refrigerant vapor gives up its latent heat of vaporization and becomes a liquid.

Current(I): Transfer of electrical energy in a conductor using electrons changing position.


Defrost Cycle: Part of the refrigeration cycle in which the evaporator’s frost and ice accumulation are melted. The use of electric heaters or hot gas is the most common form. The defrost cycle also flushes any oil that is trapped in the evaporator back to the compressor.

Dehumidification: The removal of water vapor from air by cooling below the dew point.

Desuperheating: The process of removing heat from the superheated refrigerant.

Dew Point:  Temperature at which a refrigerant vapor first began to condense.

DC – Direct Current: Electric current in which the direction of the flow moves continuously in one direction. In a DC circuit, electrons emerge from the negative, or minus, pole and move towards the positive, or plus, pole.

Desuperheater Cooling (DSH): The refrigeration system discharge line goes to a DSH, which is a heat exchanger, where the gas reduces its temperature as it gives heat to air or water. In this way, the temperature of the gas is decreased flowing back to the compressor to cool and maintain a low discharge temperature.

Discharge Line: The high pressure, the hot gas line coming out of the compressor.

Drier filter: A component of a refrigeration system used to remove moisture and particles from the system.

Dry Bulb Temperature (DBT): Air temperature as indicated by an ordinary thermometer.

Dry Ice: Refrigeration substance made of solid carbon dioxide which changes directly from a solid to a gas (sublimates). Its subliming temperature is -78 ºC.



EER – Energy Efficiency Ratio: coefficient that shows how much cooling capacity the unit gives from the amount of energy it uses. It is a ratio of output cooling energy (in BTU) to electrical input energy (in Watt-hours).
Efficiency: Output of a device, system, or activity, by the necessary input energy to create this output. In a compressor, the efficiency would be the output work (cooling capacity), divided by the input energy (usually electrical).

Enthalpy: Enthalpy is a measure of heat in a substance. Scientists figure out the mass of a substance when it is under constant pressure. Once they have the mass, they measure the internal energy of the system, which is the enthalpy. They use the formula “H = U + PV.” H is the enthalpy value, U is the amount of internal energy, and P and V are the Pressure and the Volume of the system.

Evaporation: It is applied to the changing of a liquid to a gas state.

Evaporator: A component of a refrigeration system in which saturated refrigerant absorbs heat and turns into a gas (superheated).



Flash Gas: Spontaneous evaporation of some liquid refrigerant in the liquid line, which its presence reduces the efficiency of the refrigeration cycle and increases superheating at the evaporator. It is normally happening if the liquid line’s pipeline is too long, its diameter is small, and if there is a lack of subcooling. It also accrues if the amount of refrigerant in the system is low.

Flash Point: The lowest temperature at which vapors ignite when a source of ignition is present.

Floodback: happens when liquid refrigerant from the evaporator entering the compressor during system operation. It takes place if the evaporator coil freezes over(it may happen if the evaporator’s fan doesn’t work properly) if the refrigerant is overcharged, the superheat setting sets too low or using an unsuitable capillary tube, and if the expansion valve not functioning or not functioning properly. Floodback leads to compressor failure.

Flooded System: It is a refrigeration system in which liquid refrigerant fills most of the evaporator.

Foaming: Formation of foam in an oil-refrigerant mixture due to sudden evaporation of the refrigerant dissolved in the oil. This is most likely to occur when the compressor starts, and the pressure is suddenly reduced in the crankcase. It also happens because of the low level of oil, presence of air, and contamination in the system.



Gas: Vapor phase/stage of a substance.

Gauge pressure: is the pressure relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure and negative for pressures below it.

Glide Temperature: The temperature difference between the bubble point and the dew point temperatures.

Ground Wire: An electrical wire which safely conducts electricity from a structure into the ground.



Halogens: Substance containing fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and Tennessine (Ts).

HCFC: Acronym for Hydrochlorofluorocarbons which are halogenated compounds containing carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F). HCFC refrigerants like R22 because of its ODP(ozone depletion potential) has been prohibited in several countries.

Heat: is a form of energy as it spontaneously passes between a system and its surroundings, because of a difference in temperature between objects.

Heat Convection: is the transfer of heat using movement or flow of a fluid or gas.

Heat Exchanger: Any device that transfers heat between fluids.

Heat Load: Amount of heat for maintaining the temperature of a building at an acceptable range, measured in Btu or Watts.

Heat Pump: A heat pump is a reversible A/C system that is extracting heat from a cooler place and delivers heat to a warmer place. The heat delivered to the warmer place is, approximately, the sum of the original heat and the work that has been done by the heat pump. The greater temperature difference between the warm and cold environments requires greater amounts of work. In warm weather the heat pump acts like a traditional air conditioner, removing heat from the indoors and delivering heat to the outdoors. In cool weather, it removes heat from the outdoors and delivers heat to the indoors.

Heat Transfer: Movement of heat from one object to another one. Heat may be transferred by radiation, conduction, convection, or a combination of these three methods.

Hertz (Hz): Unit of frequency named after the German physicist Heinrich Rudolf Hertz. It equals the number of cycles per second.

HFC: is a group of refrigerants and it is an acronym for Hydrofluorocarbon that contains fluorine (F) and hydrogen (H) atoms. HFC refrigerants with high global warming potential such as R404A are phasing out in several markets.

High Side: The part of a refrigeration system between the compressor discharge line and the expansion system where the pressure of the refrigerant is high.

Horsepower (HP): is a unit of power that shows the rate at which work is done. Two common definitions that being used today are mechanical horsepower (1 HP = 746 W) and metric horsepower (1 PS = 736 W).

Hot Gas Bypass Defrosting (HGBD): Basically, this is a capacity regulator in a refrigeration system by entering hot, high-pressure refrigerant vapor from the discharge line directly to the low-pressure side of the system. In this way the compressor keeps working more while the evaporator is at the part-load condition. Also, hot gas bypass raises the suction temperature, which prevents frost forming.

Humidity: is the concentration of water vapor in the air. Presence of humidity in refrigeration system is inevitable, but it should be kept under the acceptable level. Excessive humidity in the system can causes several problems.

HVAC – heating, ventilation and air conditioning: is the technology of indoor and vehicular environmental comfort. Its goal is to provide acceptable thermal condition indoor.



Insulation: Any material with poor conductivity of heat or electricity, which is used to prevent the flow of the heat or electricity.

Inverter: It is a device for converting frequency. An inverter adjusts the speed of the compressor to control the refrigerant (gas) flow rate, consuming less current and power. An inverter has precise temperature control and as the temperature setting is attained, the unit adjusts its capacity to eliminate any temperature fluctuations.



kiloWatt: Unit of electrical power equal to 1000 watts.



Latent heat: Heat that is released or absorbed by matter as a change of state without a change in temperature.

Liquid injection Cooling (LI): The liquid injection is kind of a compressor motor cooling to keep the discharge temperature low because when working at low evaporation temperatures the flow of refrigerant that circulates is low and the discharge temperature is high.

Liquid Line: In the refrigeration system, it is the line containing the high pressure, subcooled liquid refrigerant starting from the outlet of the condenser till the inlet of the expansion valve.

Low Side: The part of the refrigeration system between the outlet of the expansion valve and the inlet of the compressor.

LRA – Locked Rotor Amps or Locked Rotor Current (A): It’s the current produced by the motor of the compressor under starting conditions when full voltage is applied, and the rotor is not spinning or rotating. It occurs instantly during startup.



MCC – Maximum Continuous Current (A): Maximum current at which the compressor’s motor can handle without tripping its protector or stopping the compressor.

Miscibility: is the two substances (Liquids) property to form a homogeneous mixture when adding together. For the lubricant (oil) to return to the compressor properly, the oil (PZ) and refrigerant (CO2) must be miscible (dissolve) in each other.

Moisture Indicator: A liquid line sight glass that has a visible indicator that changes color to determine the moisture content of the refrigerant.

Mollier diagram: Graph of refrigerant pressure, heat, and temperature properties.

Motor cooling: In the refrigeration system as the compressor motor continues to work, excess heat is produced. This heat reduces the compressor’s efficiency and it can lead to compressor breakdown. To cool the compressor motor, the basic method is by the suction gas (refrigerant). Other options are liquid injection (LI) cooling, desuperheating (DSH) cooling, and air cooling.

Motor poles: number of windings or number of magnetic poles sets in a motor, which is an even number (2,4,6,…).

Motor, PSC: A ”PSC motor” stands for “permanent split capacitor motor”. All single-phase motors have a starting problem, unlike three-phase motors. A PSC motor has a run capacitor connected between the run and starts the windings of the motor. The run capacitor creates a “phase shift” which is all that’s needed to achieve a little magnetic field rotation to start the rotor moving.

Motor, Shaded Pole: These motors have only one main winding and no start winding. Starting is accomplished through a design that uses a copper ring around a small portion of each motor pole. These “shades” that portion of the pole, causing the magnetic field in the ringed area to lag the field in the non-ringed portion. The reaction of the two fields initiates rotation. Since there is a lack of start winding, starting switch, or capacitor, the shaded pole motor is electrically very simple and inexpensive. Speed can be controlled by varying the voltage. These motors offer poor starting torque, typically 25 to 75 percent of rated load, and they are very low efficiency. These motors typically are up to 1/8 horsepower and have sleeve bearings.

Motor, Split Phase: The split-phase motor is mostly used for “medium starting” applications. It has start and run winding, both are energized when the motor is started. When the motor reaches about 75% of its rated full load speed, the starting winding is disconnected by an automatic switch.



Noncondensable gas: Gas which will not change into a liquid under the operating pressure-temperature conditions.



Ohm (Ω): A unit of measurement of electrical resistance named after the German physicist Georg Simon Ohm. One ohm is measurable when applying one volt of voltage causing a flow of one-ampere current.

Oil Separator: as refrigerants and refrigeration oils are miscible, some oil leaves the compressor anyway, which can lead to the oil foaming. Oil separator as it’s obvious from its name is a device used to separate and recover the oil carried by the refrigerant in the vapor phase at the compressor outlet of the refrigeration system. It ensures that the regulated oil return to the compressor crankcases.

Overload Protector: is a device that protects the unit from high temperature and/or high current, which is opening a circuit to stop the operation of the unit if dangerous conditions arise.



Pump Down: The act of closing the liquid line, with a solenoid valve, when the thermostat reaches its set point in a refrigeration system. The compressor then pumps the refrigerant into a receiver tank. Installation of pump down is an important way to prevent trapping liquid refrigerant in the evaporator, which may flood the compressor on the start-up or migrate to the compressor during the off cycle.



Radiant Heat: Transfer of heat by heat rays (see pic heat transfer).

Receiver: A cylinder (tank) connected to the condenser outlet for the storage of a liquid refrigerant in the system (see the previous pic).

Refrigerant: is a compound used in the refrigeration system that undergoes a phase transitions from a gas to a liquid and in reverse to transfer heat.

Refrigeration: Reducing and/or maintaining the temperature of an object or space below the ambient temperature. Refrigeration can be used for industrial, commercial, residential, and transport applications.

Refrigeration capacity: is the measure of the power of a cooling system indicating the amount of heat that can absorb, expressed in kcal/h or Watts.

Relative Humidity (RH): Humidity is a state of water vapors in the air and RH is the amount of moisture in a given volume of air as compared to the amount, which air is capable of holding, and measured as a percentage. If the RH is 30 percent, that means the air is holding 30 percent of the moisture that is capable of. As air temperature increases, the air’s capacity to hold moisture increases. If the air temperature rises and its moisture content (humidity) stays the same, then the RH decreases. This is what usually happens during winter in the buildings. The only way to re-establish the proper RH is to add moisture to the air (the function of a humidity).

Relay: is an electromagnetic switch used for turning on/off an electrical power of the circuit. It is usually applied to the device controlling power below 5kW, whereas the term ‘contactor’ is usually employed for the device above 5kW. The terms are often used interchangeably.

  • Start Relay: An electrical device that connects/disconnects the start windings of an electric motor.

Relief Valve or pressure relief valve (PRV): A safety device on a cooling system. It opens to release pressure when a dangerous pressure is reached. In the refrigeration system, it is usually installed on the receiver.

Reverse Cycle Defrosting (RCD): Method of heating the evaporator for defrosting ice buildup by using valves to move hot gas from the compressor into the evaporator.

RLA – Rated Load Amps (A): Nominal current of the compressor at nominal operating conditions.

Rotor: The rotating or turning part of a motor.

RPM (rpm, rev/min, r/min): Acronym for ”revolution per minute” is the number of turns in one minute.

Run Winding: The electrical winding of a motor that has current flowing through it during the normal operation of the motor.



Saturated Refrigerant: is the state of a refrigerant when it is a combination of gas and liquid and it will either condense or evaporate at this condition, which is its boiling point.

SEER (Seasonal energy efficiency ratio): is the cooling output during a typical cooling-season (in BTU) divided by the total electric energy input during the same period (in Watt-hour).

SEPR (Seasonal Energy Performance Ratio): It measures the seasonal energy efficiency of process chillers by calculating the ratio between annual cooling demand and annual energy input. It should be taken into consideration that the energy efficiency achieved at each outdoor temperature of an average climate is weighted by the number of hours observed for each of these temperatures.

Sensible heat: Heat that can be measured or felt. Sensible heat always causes a temperature rise.

Service Valve: is a valve used to separate one part of the refrigeration system from other parts (for adding/removing refrigerant, oil, for checking pressure, for vacuuming the system or replace part of the system).

Sight Glass: An indicator with a glass window, normally installed in the liquid line, and indicates the presence of gas bubbles. Some styles have a visible indicator that changes color to determine the moisture content of the refrigerant.

Single Phase: Producing, carrying, or powered by a single alternating voltage.

Slugging: Liquid slugging is a condition that occurs when the liquid refrigerant enters the compressor’s cylinder. This could be the result of flood back during the run cycle or migration during the off-cycle and it is a common cause of compressor breakdown

Solenoid Valve: An electromechanical valve (for use with liquid and/or gas) controls the flow of fluids by on/off an electrical current through a solenoid coil, which either closes or opens the valve. The most common use is when a normally closed valve is used on the liquid line for pump down.

Specific Heat: The quantity of heat, expressed in J/kgK, required to raise the temperature of 1 kg of a substance 1°C.

Start Winding: The electrical winding of a motor that has a current flowing through it while the motor is starting.

Stator: The stationary part of an electric motor.

Subcooler: is a component of the refrigeration system or section of condenser, in which the temperature of the condensed refrigerant liquid is reduced. This improves the system’s energy efficiency.

Suction Line: the low-pressure refrigerant gas line between the outlet of the evaporator and the inlet of the compressor.

Superheating: is a process during which, additional heat is given to the saturated refrigerant gas before it reaches the compression system.



Temperature: Temperature is the definition that gives physical meaning to the concept of heat. If an object is cold, we say it has a low temperature. If it is hot, we say it has a high temperature.

Thermostat: is a device that senses the temperature of an ambient and regulates it by switching the electrical current at a selectable setpoint.

Three Phase: Producing, carrying, or powered by three electrical circuits.

A ton of Refrigeration: Refrigeration effect equal to the melting of 1 ton of ice in 24 hours. It’s equal to 3,5 kW (12,000 Btu/h).

Transducer: it is a device that converts one form of energy into another, specifically when one of the quantities is electrical.

TXV – Thermostatic Expansion Valve: It is a component in refrigeration and air conditioning systems controlling the amount of refrigerant released into the evaporator. It is working by the difference between the current refrigerant temperature at the evaporator outlet and its saturation temperature at the current pressure, at a stable value, ensuring that the only phase in which the refrigerant leaves the evaporator is vapor, and, at the same time, supplying the evaporator’s coils with the optimal amount of liquid refrigerant to achieve the optimal heat exchange rate allowed by the evaporator. In addition, some thermal expansion valves are also specifically designed to ensure that a certain minimum flow of the refrigerant can always flow through the system.



Vacuum: is a space devoid of matter or in which the pressure is considerably below the atmospheric pressure. It is measured in units of pressure (Pa). A vacuum can be created by removing air from space or by reducing the pressure using a fast flow of fluid.

Vapor: The gaseous form of any substance.

Voltage: Electrical pressure which causes current to flow (V).

Volumetric efficiency: is a ratio of the amount of refrigerant gas entering the compressor (compressor inlet) divided by the amount of gas leaving the compressor (compressor outlet).



Watt (W): is the unit of power equal to one joule of energy per second named in honor of the English engineer James Watt.

Wet Bulb Thermometer: A device used for measuring relative humidity (RH). Evaporation of the moisture (part of the wet-bulb thermometer) decreases the temperature of the wet-bulb compared to the dry bulb temperature of the same air sample.

Wet Bulb Temperature (WBT): is measuring the amount of water vapor in the air and it indicates the lowest temperature obtained by evaporating water at constant pressure. WBT can be used along with the DBT to calculate the dew point or relative humidity.


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