HVAC TERMS

Absolute Zero Temperature: Temperature at which all molecular motion ceases. (-459.67 F. and -273.15 C.) Absolute zero, according to current scientific thought, is the lowest temperature that could ever be. In fact, it's so low that we can never quite reach it, although research teams have come within a fraction of a degree.
Absorption Refrigeration: A system in which a secondary fluid absorbs the refrigerant, releasing heat, then releases the refrigerant and reabsorbs the heat. Ammonia or water is used as the vapor in commercial absorption cycle systems, and water or lithium bromide is the absorber.
Accumulator: Storage tank which receives liquid refrigerant from the evaporator and prevents it from flowing into the suction line and entering the compressor.
Across the line Start: Using one contactor to start a three phase motor.
Alternating Current - AC: Electric current in which the direction of the flow is constantly being reversed back and forth. In the USA it changes polarity from positive to negative 60 times per second. In other countries, the rate of alternation is often 50 cycles per second .
Alumina: A substance in a drier used to collect and hold moisture in a refrigeration system.
Ambient Temperature: Temperature of a fluid (usually air) which surrounds an object on all sides.
Ammonia: R-117, Chemical combination of nitrogen and hydrogen .
Amperage: The rate of flow of electricity through wire - measured in terms of amperes, very roughly analogous to gallons per minute flowing from a faucet.
Ampere: The unit of current. One ampere is the current flowing through one ohm of resistance at one volt potential. Analogous to gallons of water flow past a given point.
Brazed Plate Heat Exchangers: 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. The media are kept in the unit by a brazed seal around the edge of the plates. The contact points of the plates are also brazed to withstand the pressure of the media handled. These heat exchangers are up to 60% smaller than traditional shell & tube and coaxial type devices. This means they use less space and weigh less.
BTU: British Thermal Unit. Quantity of heat required to raise or lower the temperature of one pound of water one degree Fahrenheit. You might be able to imagine it this way. Take one gallon (8.33 pounds) of water and put it on your stove. If the water is 60 degrees F. and you want to bring it to a boil (212 degrees F.) then you will need about 1,200 BTUs to do this.
BTUH: British Thermal Unit per Hour. How many BTU's are used per hour.
Calorie: Quantity of heat required to raise or lower the temperature of 1 gram of water 1 degree Celsius.

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 particular application. This device uses it's internal diameter, length and pressure drop to determine it's capacity and has a fixed regulation. The capillary tubing is best suited in constant ambient conditions.
Capacitor: The simplest way to explain the mechanics of a capacitor would be to compare it to a battery. Both store and release electricity. Capacitors are charged with electricity, then releases its stored energy at a rate of sixty times per second in a 60 cycle alternating current system. The sizing is critical to motor efficiency just as sizing of batteries is critical to a radio. A radio that requires a 9V battery will not work with a 1.5V size battery. Thus, as the battery becomes weaker the radio will not play properly. A motor that requires a 7.5 mfd capacitor will not work with a 4.0 mfd capacitor. Much the same way, a motor will not run properly with a weak capacitor. This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings. Motor manufacturers spend many hours testing motor and capacitor combinations to arrive at the most efficient combination. There is a maximum of +10% tolerance in microfarad rating on replacement start capacitors, but exact run capacitors must be replaced. Voltage rating must always be the same or greater than original capacitor whether it is a start or run capacitor. Always consult manufacturers to verify correct capacitor size for the particular application. (source:Louisiana State University)
Capacitor - Run: Run capacitors are rated in a range of 2-80 microfarad (mfd). Run capacitors are also rated by voltage classification. The voltage classifications are 370V 440V and 480V. Capacitors with ratings above 80 microfarad (mfd) are starting capacitors. 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. This is why sizing is so critical. If the wrong run capacitor is installed, the motor will not have an even magnetic field. This will cause the rotor to hesitate at those spots that are uneven. This hesitation will cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat.(source:Louisiana State University)
Capacitor - Start: Starting capacitors are housed in a black plastic case and have a mfd range as opposed to a specific mfd rating on run capacitors. Start capacitors (ratings of 80 microfarad or higher) have three voltage classifications: 125V, 250V, and 330V. Examples would be a 35 mfd at 370V run capacitor and an 88-108 mfd at 250V 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 and are then taken out of the circuit. (source:Louisiana State University)
Cascade System: Arrangement in which two or more refrigeration systems are used in series. The evaporator of one machine is used to cool the condenser of another. These are used in ultra low temperatures.
Celsius Scale: The Celsius temperature scale (°C) was developed by Anders Celsius in 1742. The zero point of the Celsius scale is set to the temperature at which water freezes. The number 100 is set to the temperature at which water boils. The Celsius scale is the standard scale used around most of the world to measure air temperatures. The United States uses the Fahrenheit scale.
Centigrade Scale: The Celsius temperature scale (°C) was developed by Anders Celsius in 1742. The zero point of the Celsius scale is set to the temperature at which water freezes. The number 100 is set to the temperature at which water boils. The Celsius scale is the standard scale used around most of the world to measure air temperatures. The United States uses the Fahrenheit scale.
CFC: Acronym for Chlorofluorocarbon. Any of various halocarbon compounds consisting of carbon, hydrogen, chlorine, and fluorine, once used widely as aerosol propellants and refrigerants. Chlorofluorocarbons are believed to cause depletion of the atmospheric ozone layer.
CFM: Cubic feet per minute. Volume rate of air flow.
Check Valve: Device which permits fluid flow in only one direction.
Coefficient of Performance - COP: Ratio of work performed or accomplished as compared to the energy used. This ratio is calculated by dividing the total heating capacity provided by the heat pump, including circulating fan heat but excluding supplementary resistance heat (Btus per hour), by the total electrical input (watts) x 3.412.
Compound Refrigeration: An alternative to a cascade system, a Compound system uses two or more compressors connected in series in the same refrigeration cycle.
Compression Ratio: In refrigeration it is the ratio of the absolute low-side pressure to the absolute high-side pressure. To measure you would add 14.7 psi to the measured suction pressure and the head pressure. Then divide the high-side pressure by the low-side pressure.
Compressor: Pump of a refrigeration system which draws a low pressure on the cooling side of a refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle.
Compressor - Centrifugal: A pump which compresses the refrigerant gas through the centrifugal force created by rotors spinning at high speed. In his effort to improve mechanical air-conditioning systems, Willis Haviland Carrier (1876–1950) introduced the first practical centrifugal refrigeration compressor in 1922.
Compressor - Hermetic: A compressor which has the driving motor sealed inside the compressor housing. The motor operates in an atmosphere of the refrigerant. This style of compressor cannot be disassembled for maintenance. Common terms of describing a hermetic are; welded, can, pot and tin can.
Compressor - Multi-Stage: A compressor having two or more compressive steps. Discharge from each step is the intake pressure of the next in series.
Compressor - Open Drive: A compressor in which the crankshaft extends through the crankcase and is driven by an outside motor. Also called an external drive compressor.
Compressor - Reciprocating: A pump which uses a piston within a cylinder to provide compression. The principle of reciprocating mechanisms is hundreds of years old and the modern reciprocating compressor was undoubtedly driven from the research and development of the reciprocating steam engine.
Compressor - Rotary: A pump which uses vanes, eccentric mechanisms or other rotating devices to provide a pumping action. The first rotary compressor was introduced in 1957, permitting units to be smaller, quieter, weigh less, and more efficient than the reciprocating type.
Compressor - Screw: A screw compressor is a positive displacement machine that uses a pair of intermeshing rotors instead of a piston to produce compression. The rotors comprise of helical lobes affixed to a shaft. One rotor is called the male rotor and it will typically have four bulbous lobes. The other rotor is the female rotor and this has valleys machined into it that match the curvature of the male lobes.The principle of the screw compressor was first patented by Heinrich Krigar in Germany on March 24th 1878, patent number 4121. He modified and improved his designs later that year and lodged a second patent (number 7116) on August 16th 1878.
Compressor - Scroll: A scroll compressor is a positive displacement machine that uses the compression action provided by two intermeshing scrolls - one fixed and the other orbiting. The orbiting scroll basically wobbles inside the fixed scroll, it doesn't rotate it just wobbles on a cam on crankshaft. The rotating scrolls intermesh quite closely and some manufacturers mention that efficiency increases as the scrolls bed into each other, indicating an initial wear process. The principle of the scroll compressor was developed during the early 1900's and was patented for the first time in 1905. Although the theory for the scroll compressor indicated a machine potentially capable of reasonably good efficiencies, at that time the technology simply didn't exist to accurately manufacture the scrolls. It was almost 65 years later that the concept was re-invented by a refrigeration industry keen to exploit the potentials of scroll technology.
Compressor - Semi Hermetic: A hermetic compressor that can be disassembled for maintenance. Usually made of cast iron.
Compressor - Tandem: A refrigeration system where two compressors are piped together having a common suction and discharge. In a Copeland semi-hermetic application, the motor end bells are removed and replaced with a connecting chamber with a suction valve mounted. Since each compressor may be operated individually, the tandem provides simple capacity reduction and maximum power savings.
Compressor - Two Speed: A compressor with a two speed motor providing capacity reduction and power savings.
Condense: Action of changing a gas or vapor to a liquid.
Condenser: The part of the refrigeration system which receives hot, high pressure refrigerant gas from the compressor and cools it until it returns to a liquid state.
Condensing Pressure: Pressure inside a condenser at which refrigerant vapor gives up its latent heat of vaporization and becomes a liquid. This varies with temperature.
Condensing Temperature: Temperature inside a condenser at which refrigerant vapor gives up its latent heat of vaporization and becomes a liquid. This varies with the pressure.
Contactor: A type of magnetically-operated device used for repeatedly establishing and interrupting an electrical power circuit. It is usually applied to devices controlling power above 5kW, whereas the term 'relay' is ordinarily employed below 5kW. The terms are often used interchangeably.
Contactor - Definite Purpose:These contactors have been designed for specific applications where the operating conditions are clearly defined. The definite purpose contactors are usually rated for current alone and have less ability to handle inrush LRA current. They normally have a lower initial cost compared to general purpose contactors.
Contactor - General Purpose:These contactors are built for severe industrial use. They are usually designed for a minimum life of over 1,000,000 electrical cycles on most type of motor loads. General purpose contactors are listed by sizes that are generally related to motor horsepower groupings. They are also rated in current, a more useful rating for compressors. They usually conform to NEMA ratings.
Contactor - Resistive Rating:The resistive rating of a contactor is what the contactor has the capacity to withstand for brief surges of amp draw. Most contactors have a resistive rating as well as a motor rating. The resistive rating is higher in amperage value than the motor rating amperage value. This is because a resistive load is not called on to make and break motor current.
Convection Heat: Transfer of heat by means of movement or flow of a fluid or gas.
Coulomb: The quantity of electricity transferred by an electric current of one ampere in one second.
Crankcase Pressure Regulator - CPR: An outlet pressure regulator which maintains a predetermined maximum outlet pressure. Designed to prevent compressor motor overload. They were commonly used on low temp R-12 systems.
Cryogenics: Refrigeration which deals with producing temperatures of -250 F below zero and lower.
Cupronickel: An alloy of copper, nickel and strengthening impurities, such as iron and manganese. Cupronickel does not corrode in seawater because its electro negativity is adjusted to be neutral with regard to seawater. Because of this it is used for marine components, and sometimes for the propellers, crankshafts and hulls of premium tugboats, fishing boats and other working boats.
Current: Transfer of electrical energy in a conductor by means of electrons changing position.
Current Relay: A device which opens or closes a circuit. It is made to act by a change of current flow in that circuit.
Defrost Control: A control which initiates the defrost cycle in the refrigeration cycle.
Defrost Cycle: Part of the refrigeration cycle in which evaporator frost and ice accumulation is melted. The use of electric heat strips 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. There should be a minimum of four defrost cycles per 24 hours to help with oil return.
Desiccant: A substance used in a drier to collect and hold moisture in a refrigeration system. Common desiccants are activated alumina and silica gel.
Desuperheat: The process of removing heat from superheated refrigerant. This is commonly done with a TXV or liquid line solenoid valve which injects liquid refrigerant into the compressor body for motor cooling.
Dew Point: Temperature at which vapor (at 100 percent humidity) begins to condense and deposit as liquid.
Differential: The temperature or pressure difference between cut-in and cut-out temperature or pressure of a control.
Direct Current - DC: 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. Nevertheless, physicists define DC as traveling from plus to minus.
Discharge Line: The high pressure, hot gas line coming out of the compressor.
Discus Valve: A hockey puck type discharge valve which minimizes refrigerant re-expansion volume common in reed valves. It is typically 10% more energy efficient than reed compressor technology. The geometry of a traditional reed compressor does not allow all the discharge gas to exit when the piston is at top dead center. This leads to re-expansion volume, which is especially prevalent in low temperature applications. The discus compressor valve minimizes the gap between the piston and the discharge valve, making re-expansion virtually zero. This results in the highest possible efficiency.
Drier: A component of the refrigeration system with a substance used to remove moisture from the system. The first two numbers of a drier represent the cubic inch and the last number represents 1/8 of an inch. For example, 163-S indicates a 16 cubic inch capacity and 3/8 sweat. Without the "S" would represent a flare drier.
Dry Bulb Temperature: 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 -109 F (-78 C).
Electricity The flow of electrons in a circuit. The speed of electricity is the speed of light (approximately 186,000 miles per second ). In a wire, it is slowed due to the resistance in the material. Its pressure, or force, is measured in "volts," and its flow, or current, is measured in "amperes" or simply "amps." The amount of work it produces is measured in "watts" (amps X volts).
Electrolysis: Movement of electricity through a substance which causes a chemical change in the substance or its container.
Electromotive Force - EMF: The term used to describe anything which behaves like an electrical pump. Batteries, generators, thermoelectric devices, solar cells, and piezoelectric crystals all do the same job in an electrical circuit: they pick conduction charges up at points of low potential energy and lift them up to high potential energy. If we imagine that current is positive charge in motion, then an emf pumps the current from low voltage up to high voltage. Unit of measurement is the volt.
Energy Efficiency Ratio - EER: The energy efficiency rating (EER) of an air conditioner is its BTU rating over its wattage. For example, if a 10,000-BTU air conditioner consumes 1,200 watts, its EER is 8.3 (10,000 BTU/1,200 watts). You would like the EER to be as high as possible, but normally a higher EER is accompanied by a higher price.
Enthalpy: Enthalpy is a measure of heat in a substance. Scientists figure out the mass of a substance when it is under a constant pressure. Once they figure out the mass, they measure the internal energy of the system. All together, that energy 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 Pressure and Volume of the system.
Evaporator Pressure Regulator - EPR: An inlet pressure regulator which maintains a predetermined evaporator inlet pressure, regardless of sudden load or suction pressure changes. Commonly used on supermarket racks or multi-evaporator units maintaining different temperatures.
Evacuation: Removal of air and moisture from a refrigeration system. Blowing out lines with refrigerant will not remove trapped air or moisture in the system. Evacuation is the only means to remove contaminants in the system. Evacuating to 500 microns, or to the limits of the vacuum pump, is recommended.
Evaporation: Term applied to the changing of a liquid to a gas. Heat is absorbed in this process.
Evaporative Cooler: Alias, Swamp cooler. Nature's most efficient means of cooling is through the evaporation of water. Evaporative cooling works on the principle of heat absorption by moisture evaporation. The evaporative cooler draws exterior air into special pads soaked with water, where the air is cooled by evaporation, then circulated. Evaporative cooling is especially well suited where the air is hot and humidity is low.
Evaporator: A component of a refrigeration system in which saturated refrigerant absorbs heat and turns into a gas (superheated).
Expansion Valve - Automatic - AEV: The automatic expansion valve maintains a constant pressure in the evaporator by more or less flooding the evaporator surface, depending on the heat load of the cold chamber. The main disadvantage of this expansion valve is its relatively poor efficiency. It has been mainly used in applications where the cooling load is fairly constant and low evaporating pressures must be avoided. However, it is seldom used nowadays.
Expansion Valve - Balanced Port: In regular expansion valves the operating superheat varies due to an unbalance of pressure drop caused by changes of the head pressure and/or suction pressure. This unbalance can result in compressor flooding or evaporator starvation. The "Balanced Port" concept cancels the effect of this pressure unbalance, permitting the expansion valve to operate at a relatively constant superheat over a wide range of operation conditions.
Expansion Valve - Bi-Flow: This type TEV will meter refrigerant in either direction which makes it ideally suited for package heat pump applications. Bi-flow capability means reduced system complexity and resulting cost. (A single bi-flow TEV can replace 2 conventional TEV's and 2 check valves).
Expansion Valve - Electric - EEV: This type of valve is controlled by an electronic circuit which is often designed to allow the valve to control some aspect of system operation in addition to superheat at the outlet of the evaporator. For example, evaporator discharge air temperature or water temperature from a chiller could be monitored by the EEV's controller.
Expansion Valve - External Equalized: This type TEV senses the suction pressure at the outlet of the evaporator to control the metering of refrigerant. These valves are not affected by pressure drop across the evaporator, including the refrigerant distributors, and may be used for all refrigeration applications. The external equalizer type valve must be used on evaporators which use a refrigerant distributor. It provides no operational disadvantages with the respect to an internally equalized valve other than requiring an external equalizer be connected. An external equalizer does not provide some means of equalizing the high and low side pressures during the off cycle. A separate internal bleed hole is required. The equalizer connection on the valve, usually 1/4", must not be capped!
Expansion Valve - Heat Pump: This type of expansion valve has a special power element charge. This charge assist the outside coil of air-to-air heat pumps using R-22 in providing 0 degree superheat, or "wet gas", back to the windings of hermetic heat pump compressors during very low outdoor temperatures.
Expansion Valve - Internal Equalized: This type TEV senses the suction pressure at the valve outlet to control the metering of refrigerant. Internally equalized valves should be limited to single circuit evaporator coils having a pressure drop no greater than the equivalent of a 2 degree F saturated temperature change.
Expansion Valve - Pressure Limited - MOP: This type TEV has a power element charge that causes the TEV to close above a predetermined evaporator pressure, thereby restricting flow to the evaporator and limiting the maximum evaporator pressure at which the system can operate. It may not be used on conjunction with a crankcase pressure regulator CPR).
Expansion Valve - Thermostatic - TEV: The thermostatic expansion valve regulates refrigerant flow by maintaining a nearly constant superheat at the evaporator outlet. The effect of this type of regulation is, it allows the evaporator to remain as nearly fully active as possible under all load conditions.
Fahrenheit Scale: The Fahrenheit scale (°F) was the first widely used temperature scale. It was developed in the early 1700s by G. Daniel Fahrenheit. The zero point of the Fahrenheit scale is attained by mixing equal parts of water, ice, and salt. Fahrenheit set the number 32 at the freezing point of water. He set the boiling point of water to 212 on his scale.
Fan Cycle Control: A control or device on an air cooled condenser which cycles the condenser fan(s) by means of pressure or temperature to maintain head pressure in low ambient conditions of 50 F or lower. This is required to control liquid refrigerant feed to the evaporator. The flow characteristics of expansion valves and capillary tubing are proportional to the pressure differential across them. Care must be taken when making adjustments to prevent short cycling. Short cycling is most likely to occur at ambient temperatures of 30 to 70 degrees F. Any fan cycle that is less than 5 minutes is considered short cycling. Fans should never cycle more than 10-12 cycles per hour. Short cycling is normally caused by too close a differential in the control setting. It is recommended that the lead fan, on multiple fan units, be wired to run continuously. If more control is required, add a flooded condenser control along with the fan cycling control.
FLA - Full Load Amps: Changed in 1976 to "RLA - Rated Load Amps".
Flash Gas: Instantaneous evaporation of some liquid refrigerant in the evaporator which cools the remaining liquid refrigerant to desired evaporation temperature.
Flash Point: The lowest temperature at which a liquid gives off enough vapor to ignite when a source of ignition is present.
Flood Back: The results of liquid refrigerant from the evaporator entering the compressor. This situation will damage the compressor if preventive measures are not taken.
Flooded System: Type of refrigeration system in which liquid refrigerant fills most of the evaporator.
Foaming: Formation of a foam in an oil-refrigerant mixture due to rapid 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. In most circumstances a pump down solenoid valve and a crankcase heater would help prevent this. Compressor wear will be accelerated if nothing is done.
Freezer Burn: Condition applied to food stored in a freezer that has not been properly wrapped and has become hard, dry and discolored.
Freeze Up: Formation of ice in the TXV or any other control device which stops the flow of refrigerant. This happens when there is moisture in the system. Another form of freeze up is when frost forms on the evaporator and stops the airflow. This second condition will cause flood back.
Frost Back: A condition in which liquid refrigerant may be flowing from the evaporator into the suction line. It's usually indicated by sweating or frosting of the suction line.
Fusible Plug: A hollow plug of fitting filled with a metal with a low melting temperature. It is commonly used on a liquid receiver as a safety device to release pressure in case of a fire.
Gas: Vapor phase or stage of a substance.
Gas, Noncondensable: Gas which will not form into a liquid under the operating pressure-temperature conditions.
Ground Wire: An electrical wire which will safely conduct electricity from a structure into the ground.
Halogens: Substance containing fluorine, chlorine, bromine and iodine.
HCFC: Acronym for Hydrochlorofluorocarbons which are halogeneted compounds containing carbon, hydrogen, chlorine and fluorine. They have shorter atmospheric lifetimes than CFCs and deliver less reactive chlorine to the stratosphere where the "ozone layer" is found.
Head Pressure: The gauge pressure taken on a refrigeration system between the compressor discharge line and the metering device. The pressure may differ when when measuring the liquid pressure verses the hot gas pressure.
Head Pressure Control: A pressure operated control which opens an electrical circuit if the high side pressure rises above a desired setting.
Heat: Energy transferred from one body to another as the result of a difference in temperature. Heat flows from a hotter body to a colder body when the two bodies are brought together. This transfer of energy usually results in an increase in the temperature of the colder body and a decrease in that of the hotter body. A substance may absorb heat without an increase in temperature as it changes from one phase to another—that is, when it melts or boils. The distinction between heat (a form of energy) and temperature (a measure of the amount of energy) was clarified in the 19th century by such scientists as J.-B. Fourier, Gustav Kirchhoff, and Ludwig Boltzmann.
Heat Exchanger: Any of several devices that transfer heat from a hot to a cold fluid.
Heat of Fusion: Heat released from a substance to change it from a liquid state to a solid state. The heat of fusion of ice is 144 Btu per pound.
Heat of Respiration: Process of taking in air for oxygen and releasing it to dispose of carbon dioxide. Plants and vegetables produce this heat when stored in a cooler and this heat must be calculated in with the total Btuh load.
Heat Load: Amount of heat, measured in Btu or watts, which is removed during a period of 24 hours.
Heat Pump: A heat pump is a reversible A/C system that does mechanical work to extract heat from a cooler place and deliver heat to a warmer place. The heat delivered to the warmer place is, approximately, the sum of the original heat and the work done. Greater temperature differences between the warm and cold regions require 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 body or substance to another. Heat may be transferred by radiation, conduction, convection or a combination of these three methods.
Hertz - Hz: Unit of frequency. The number of hertz (abbreviated Hz) equals the number of cycles per second. The frequency of any phenomenon with regular periodic variations can be expressed in hertz, but the term is used most frequently in connection with alternating electric currents. It is named for the German physicist Heinrich Hertz, born Feb. 22, 1857, died Jan. 1, 1894.
HFC: Acronym for Hydrofluorocarbon.
High Pressure Control: A pressure operated control which opens an electrical circuit if high side pressure becomes too high.
High Side: The part of a refrigeration system between the compressor discharge line and the metering device.
Horsepower: Common unit of power, the rate at which work is done. In the English system, one horsepower equals 33,000 foot-pounds of work per minute—that is, the power necessary to lift a total of 33,000 lbs a distance of one foot in one minute. This value was adopted by James Watt in the late 18th century after experiments with strong dray horses and is actually about 50% more than the rate an average horse can sustain for a working day.
Hot Gas Bypass: Basically, this is a regulator in a refrigeration system which bypasses hot gas entering the the condenser into the the suction line or the evaporator inlet, to prevent the compressor suction pressure from falling below a desired setting. This is used when the load varies over a wide range.
Humidity: The concentration of water vapor in the air.
IAQ: Indoor air quality.
Insulation: Any material that is a poor conductor of heat or electricity, and that is used to suppress the flow of heat or electricity.
Kelvin Scale: The Kelvin temperature scale (K) was developed by Lord Kelvin in the mid 1800s. The zero point of this scale is equivalent to -273.16 °C on the Celsius scale. This zero point is considered the lowest possible temperature of anything in the universe. Therefore, the Kelvin scale is also known as the "absolute temperature scale". At the freezing point of water, the temperature of the Kelvin scale reads 273 K. At the boiling point of water, it reads 373 K.
Kilowatt: Unit of electrical power equal to 1000 watts.
Kingvalve: Liquid receiver service valve. This valve may be a horizontal or, more commonly, a vertical type.
Latent Heat: The heat given off or absorbed when a material melts or freezes, or boils or condenses. For example, when ice is heated, once the temperature reaches +32 F (0 C), it's temperature won't increase until all the ice is melted. The ice has to absorb heat in order to melt. But even though it's absorbing heat, it's temperature stays the same until all the ice has melted. The heat required to melt the ice is called the latent heat. The water will give off the same amount of latent heat when you freeze it.
Liquid Line: The line containing the high pressure, subcooled liquid refrigerant starting at the outlet of the condenser which continues to the inlet of the metering device.
Low Ambient Control: A control or device on an air cooled condenser which maintains head pressure in low ambient conditions of 50 F or lower, which is required to control liquid refrigerant feed to the evaporator. The flow characteristics of expansion valves and capillary tubing are proportional to the pressure differential across them. Commonly used controls are; fan cycling, flooded condenser valves, fan speed controls, power operated inlet air shutters or a water regulating valve on a water cooled system.
Low Pressure Control: A pressure operated control which opens an electrical circuit if the low side pressure falls below a desired setting.
Low Side: The part of a refrigeration system between the outlet of the metering device and the inlet of the compressor.ween the compressor discharge line and the metering device.
LRA - Locked Rotor Amps: It's the current you can expect under starting conditions when you apply full voltage. It occurs instantly during start up.
Maximum Fuse Size: 225% of RLA.
MBH Thousands of British Thermal Units (Btu). 82 MBH = 82,000 Btu.
MCC - Maximum Continuous Current: A motor protection value that is 156% of the established rated load current. Used to comply with U.L. and N.E.C. requirements that a motor compressor protection system will not permit a continuous current in excess of 156% of the rated load amps.
Megohm: A unit of resistance equal to one million ohms.
Metering Device:A metering device is installed at the inlet of the evaporator. It creates a pressure drop from the high side liquid refrigerant to the low side evaporator. The metering device can be an expansion valve, capillary tube, piston or even a manual valve.
Microfarad: The microfarad (symbolized µF) is a unit of capacitance, equivalent to 1/1,000,000 farad (0.000001 (10 to the -6th power).
Micron Gauge: Instrument for measuring vacuums very close to a perfect vacuum.
Miscibility: Liquids that are capable of being mixed. In order for oil to return to the compressor properly, the oil and refrigerant must be miscible (dissolve) in each other.
Modulating: Type of device or control which tends to adjust by increments (minute changes) rather than by either "full on" or "full off" operation.
Moisture Indicator: A liquid line sight glass that has a visible indicator which changes color to determine the moisture content of the refrigerant.
Motor, Four Pole: A 1725 RPM motor. A four-pole motor operating on the bench under no-load conditions runs at 1,800 RPM. When the motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor or moving part of the motor is restrained by the load from “catching up” to the field speed. The difference between the field speed of 1,800 RPM and the rotor speed of approximately 1,725 RPM is called the “slip.”
Motor, PSC: A "psc motor" stands for "permanent split capacitor motor." All single phase motors have a problem starting, unlike three-phase motors. A PSC motor has a run capacitor connected between the run and start 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. This “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 it lacks a 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 very low efficiency. These motors typically are up to 1/8 horsepower and have sleeve bearings.Motor, Six Pole: A 1175 RPM Motor. A six-pole motor operating on the bench under no-load conditions runs at 1,200 RPM. When the motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor or moving part of the motor is restrained by the load from “catching up” to the field speed. The difference between the field Speed of 1,200 RPM and the rotor speed of approximately 1,175 RPM is called the “slip.”
Motor, Split Phase: The split phase motor is mostly used for "medium starting" applications. It has start and run windings, 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.
Motor, Two Pole: A 3450 RPM Motor. A two-pole motor operating on the bench under no-load conditions runs at 3,600 RPM. When the motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor or moving part of the motor is restrained by the load from “catching up” to the field speed. The difference between the field speed of 3,600 RPM and the rotor speed of approximately 3,450 RPM is called the “slip.”
Motor Burnout: Condition in which the insulation on an electric motor has deteriorated due to overheating. If a compressor does burn out, the oil becomes extremely acidic. If all this acid is not removed when the compressor is replaced, the elevated acid levels will attack the new compressor and cause another compressor motor burn-out.
Mullion Evaporator Coil: An evaporator that mounts behind the frame member (mullion) between two doors. This type coil is a space saver and mounts vertically behind the mullion. This allows for more space for the shelves or drawers in an under counter reach in cooler.
NonFerrous: A group of metals and metal alloys which contain no iron.
Ohm: A unit of measurement of electrical resistance. One ohm exists when one volt causes a flow on one ampere.
Ohm's Law: The relationship between the volt, the ampere and the ohm discovered by German physicist George Simon Ohm in 1827. It is stated as follows: E = I x R, Where E = volts, I = amps and R = ohms.
Oil Separator: A device used to remove oil from gaseous refrigerant and return it to the compressor.
Overload Protector: A device, usually temperature or current operated, which opens a circuit to stop the operation of unit if dangerous conditions arise.
Ozone: A nearly colorless (but faintly blue) gaseous form of oxygen, with a characteristic odor like chlorine. Ozone is found in trace quantities in the Earth's atmosphere at all times, primarily in the stratosphere between heights of about 7 and 33 miles (the ozonosphere or ozone shield) where its production results from photochemical processes involving ultraviolet radiation. Its maximum concentration occurs between 13 and 17 miles. In the lower atmosphere, ozone is commonly formed as a product of electrical discharges through the air.
Ozone Layer: A region of the upper atmosphere, between about 10 to 20 miles in altitude, containing a relatively high concentration of ozone that absorbs solar ultraviolet radiation in a wavelength range not screened by other atmospheric components. Also called ozonosphere.
Part Wind Start: Using two contactors to start a three phase motor.
Peltier Effect: In 1834 French scientist Jean Peltier noted that when an electrical current is applied across the junction of two dissimilar metals, heat is removed from one of the metals and transferred to the other. This is the basis of thermoelectric refrigeration.
Potential Relay: A device which opens or closes a circuit. It is made to open oh high voltage and close on low voltage.
PSI: Acronym for "pounds per square inch".
PSIA: Acronym for "pounds per square inch absolute". Absolute pressure equals gauge pressure plus atmospheric pressure (14.7 psi).
PSIG: Acronym for "pounds per square inch gauge". The "G" indicates that it is gauge pressure and not absolute pressure.
Psychrometer: Also called a hygrometer, an instrument for measuring the relative humidity of air.
Psychrometric Chart: Shows the relationship between air volume, temperature, and relative humidity and is used to calculate specific humidity (gpp), dew point (wet bulb temp.), and vapor pressure.
Pump Down: The act closing off the liquid line, with a solenoid valve, when the thermostat is satisfied in a refrigeration system. The compressor then pumps the refrigerant into a receiver tank. It is important that a pump down be installed to prevent liquid refrigerant from trapping in the evaporator which may flood the compressor on start up or migrate to the compressor during the off cycle.
Radiant Heat: Transfer of heat by heat rays.
Receiver: A cylinder (tank) connected to the condenser outlet for storage of liquid refrigerant in the system.
Refrigerant: A refrigerant is a compound used in a heat cycle that undergoes a phase change from a gas to a liquid and back.
Refrigeration: The function of a refrigeration system is to remove heat from a place where it is not wanted (conditioned space) and reject it into another place where the heat is unobjectionable (outside the conditioned space). Refrigeration systems are closed systems; that is, they are sealed from the atmosphere.
Register: Combination grille and damper assembly covering an air opening or the end of an air duct.
Relative Humidity: Humidity is a state of (usually invisible) moisture in the air; relative humidity (RH) is the amount of moisture in a given volume of air as compared to the amount that it 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 it is capable of holding; as air temperature increases, so does the air's capacity to hold moisture; if the air temperature rises and its moisture content (humidity) stays the same, then the relative humidity becomes a lower percentage; when the temperature inside a building is raised, as so often is the case in the winter, the RH indoors will drop; the only way to re-establish the proper RH is to add moisture to the air (the function of a humidity).
Relay: A type of electro magnetically operated device used for repeatedly establishing and interrupting an electrical power circuit. It is usually applied to devices controlling power below 5kW, whereas the term 'contactor' is ordinarily employed
above 5kW. The terms are often used interchangeably.
Relay, Start: An electrical device which connects and/or disconnects the start windings of an electric motor.
Relief Valve: A safety device on a sealed system. It opens to release pressure when a dangerous pressure is reached.
Resistance: A material's opposition to the flow of electric current; measured in ohms.
Reverse Cycle Defrost: Method of heating the evaporator for defrosting ice buildup. Valves move hot gas from the compressor into the evaporator.
Reversing Valve: A component of a heat pump that reverses the refrigerant's direction of flow, allowing the heat pump to switch from cooling to heating or heating to cooling.
RLA: Acronym for "rated load amps". The maximum current a compressor should draw under any operating conditions. Often mistakenly called running load amps which leads people to believe, incorrectly, that the compressor should always pull these amps. You should never use the listed RLA to determine if the compressor is running properly or to condemn a compressor. The running amps of a compressor are determined by the evaporator temperature, condensing temperature and the line voltage.
Rotor: The rotating or turning part of a motor.
RPM: Acronym for "rounds per minute".
Run Time - Refrigeration Equipment:The amount of time a condensing unit is run per hour or per 24 hours. A cooler is normally sized for 16 hours and a freezer 18 hours.
Run Winding The electrical winding of a motor which has current flowing through it during the normal operation of the motor.
Saturated Refrigerant: The state of refrigerant when it is a combination of gas and liquid (bubbles) It will either condense or evaporate at this condition.
Seasonal Energy Efficiency Ratio - SEER: This is a rating system used to establish the efficiency level of cooling equipment. The higher the SEER rating, the less electricity the equipment uses and the more efficient it is. SEER is determined by dividing the cooling capacity, measured in BTU/h, of a continuously operating air conditioner by the electric power unit, measure in WATTs, of power consumed.
Sensible Heat:The heat absorbed or transmitted by a substance during a change of temperature which is not accompanied by a change of state.
Service Valve: A manually operated valve mounted on a refrigeration system, normally the compressor, which is used for checking the pressures.
Service Factor:Service factor is a multiplier that is applied to the motor’s normal horsepower rating to indicate an increase t of the moisture it is capable of holding; as air temperature increases, so does the air's capacity to hold moisture; if the air temperature rises and its moisture content (humidity) stays the same, then the relative humidity becomes a lower percentage; when the temperature inside a building is raised, as so often is the case in the winter, the RH indoors will drop; the only way to re-establish the proper RH is to add moisture to the air (the function of a humidity).
Relay: A type of electro magnetically operated device used for repeatedly establishing and interrupting an electrical power circuit. It is usually applied to devices controlling power below 5kW, whereas the term 'contactor' is ordinarily employed
in power output (or overload capacity) that the motor is capable of providing under certain conditions. Common values of service factor are 1.0, 1.15, and 1.25.
Shell and Tube Condenser: This type of condenser consists of a shell (a large tube) with a series of small tubes inside it. Water flows through the tubes and refrigerant flows through the shell. Heat is transferred from the refrigerant to the water.
Short Cycling:A compressor that starts and stops more frequently than it should. The results of short cycling is usually a burned overload or defective start components.
Sight Glass: An indicator with a glass window, normally installed in the liquid line, which indicates the presence of gas bubbles. Some styles have a visible indicator which changes color to determine the moisture content of the refrigerant.
Silica Gel: A substance used to collect and hold moisture in a refrigeration system.
Single Phase: Producing, carrying, or powered by a single alternating voltage.
Sling Psychrometer: The simplest hygrometer - a sling psychrometer - consists of two thermometers mounted together with a handle attached on a chain. One thermometer is ordinary. The other has a cloth wick over its bulb and is called a wet-bulb thermometer. When a reading is to be taken, the wick is first dipped in water and then the instrument is whirled around. During the whirling, the water evaporates from the wick, cooling the wet-bulb thermometer. Then the temperatures of both thermometers are read. If the surrounding air is dry, more moisture evaporates from the wick, cooling the wet-bulb thermometer more so there is a greater difference between the temperatures of the two thermometers. If the surrounding air is holding as much moisture as possible - if the relative humidity is 100% - there is no difference between the two temperatures. A chart is then used to check the differences for each degree of temperature so that the observer can find relative humidity easily.
Slugging: Liquid slugging is a condition which occurs when liquid refrigerant is allowed to enter the compressor cylinders. This could be the results of floodback during the run cycle or migration during the off cycle.
Solenoid Valve: An electromechanical valve (for use with liquid or gas) controlled by running (or stopping) 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 Gravity: The weight of a substance compared with the weight of an equal volume of pure water at 39 degrees Fahrenheit.
Specific Heat: The quantity of heat, expressed in Btu, required to raise the temperature of 1 lb of a substance 1°F.
Split System: A refrigeration or A/C system which places the condensing unit outside or away from the evaporator.
Start Winding: The electrical winding of a motor which has current flowing through it briefly while the motor is starting.
Stator: The stationary part of an electric motor.
Steam: Water in the vapor state, as when you boil water.
Subcooled Refrigerant: Cooling of liquid refrigerant below its condensing temperature.
Subcooler: A component of a refrigeration system or a section of the condenser in which the temperature of the condensed refrigerant liquid is reduced. This improves the energy efficiency.
Sublimation: The condition where a substance changes from solid to a gas without becoming a liquid, dry ice for example.
Suction Line: The low pressure refrigerant gas line between the outlet of the evaporator and the inlet of the compressor.
Superheat: The term used to describe the difference between the vapor point (ie. taking suction pressure and converting it to temperature using a pressure temperature chart) and the actual temperature of the refrigerant exiting the evaporator coil.
Superheated Refrigerant: The temperature of refrigerant vapor above its boiling point as a liquid for that refrigerant. For example; Any increase in temperature of steam above it’s boiling point (212 degrees) is called “superheat”. Steam at 220 degrees F is superheated by 8 degrees F.
Swamp Cooler: Slang name for an evaporative cooler. Nature's most efficient means of cooling is through the evaporation of water. Evaporative cooling works on the principle of heat absorption by moisture evaporation. The evaporative cooler draws exterior air into special pads soaked with water, where the air is cooled by evaporation, then circulated. Evaporative cooling is especially well suited where the air is hot and humidity is low.
TD: Temperature difference.
Temperature: Temperature is the property 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. It can also be observed that if a hot poker is plunged into cold water, the poker becomes cooler and the water becomes warmer. This means that the hot body gives up some of its heat to the cold body.
TEV: Acronym for "thermostatic expansion valve" which regulates refrigerant flow by maintaining a nearly constant superheat at the evaporator outlet. The effect of this type of regulation is, it allows the evaporator to remain as nearly fully active as possible under all load conditions.
Thermistor: A resistor whose resistance changes with temperature. Because of the known dependence of resistance on temperature, the resistor can be used as a temperature sensor.
Thermocouple: Two dissimilar metals connected at a point, that produce an electrical current when heated.
Thermodisk: A thermodisk is a temperature sensing device. When heat is applied to the disk, a bi-metal plate will bend. This will either make or break an electrical flow.
Thermodynamics: In general, the relationships between heat and other properties such as temperature, pressure, density, etc.
Thermoelectric Refrigeration:In 1834 French scientist Jean Peltier noted that when an electrical current is applied across the junction of two dissimilar metals, heat is removed from one of the metals and transferred to the other. This is the basis of thermoelectric refrigeration. Thermoelectric refrigeration uses a cold junction, a heat sink and a DC power source.The 
refrigerant in both liquid and vapor form is replaced by two dissimilar conductors. The cold junction (evaporator surface) becomes cold through absorption of energy by the electrons as they pass from one semiconductor to another, instead of energy absorption by the refrigerant as it changes from liquid to vapor. The compressor is replaced by a DC power source which pumps the electrons from one semiconductor to another. A heat sink replaces the conventional condenser fins, discharging the accumulated heat energy from the system. A thermoelectric cooling system refrigerates without use of of mechanical devices, except perhaps in the auxiliary sense, and without refrigerant.
Thermostat: A device used to switch electrical current at a selectable set point temperature.
Three Phase: Producing, carrying, or powered by a three electrical circuits.
Ton of Refrigeration:Refrigeration effect equal to the melting of 1 ton of ice in 24 hours. This may be expressed as 12,000 Btu/24 hour.
Transducer:Any device that converts one form of energy into another form of energy, specifically when one of the quantities is electrical. Thus, a loudspeaker converts electrical impulses into sound (mechanical impulses), a microphone converts sound into electrical impulses, a solar cell converts light into electricity, etc.Transformer: An electrical device that either raises or lowers the voltage of electricity.
Transformer, Buck and Boost: An electrical device that raises the voltage of electricity.
Transistor: Transistors are tiny electrical devices that can be found in everything from radios to robots. They have two key properties: 1) they can amplify an electrical signal and 2) they can switch on and off, letting current through or blocking it as necessary.
Tube in Tube Condenser: A water cooled condensing unit in which a small tube is placed inside a larger tube. Refrigerant passes through the outer tube and water through the inner tube, usually in a counter flow way which produces a better heat transfer.
TXV: A common slang term, used in the refrigeration industry, for a thermostatic expansion valve.
Ultraviolet:The invisible rays of the spectrum of light which are at its violet end. Sometimes abbreviated UV.
Vacuum:Vacuum is a pressure lower than atmospheric, except in outer space. Vacuums occur only in closed systems.
Valve Plate:A plate containing the suction and discharge valves that is mounted between the head and deck of a semi hermetic compressor.
Vapor:The gaseous form of any substance.
Vapor Barrier: A material with a high resistance to vapor movement, such as foil, plastic film, or specially coated paper, that is used to control condensation or prevent migration of moisture.
Vapor Lock:Rapid formation of vapor in lines, that causes a restriction in flow. Vapor formation begins to occur in lines 
when the medium reaches a temperature where the vapor pressure of the medium is equal to the pressure in the system.
Velocity:An object's speed and direction of motion.
Voltage:Electrical pressure which causes current to flow.
Volt:Basic unit of electrical potential. One volt is the force required to send one ampere of electrical current through a resistance of one ohm.
Water valve:A manual or electrical operated valve which provides a flow of water.
Watt: A watt is a unit of power equal to one joule of energy per second. The watt was named for the Scottish engineer and inventor James Watt (1736-1819).
Wax:An ingredient in some lubricating oils which may separate from the oil if cooled enough.
Wet Bulb:A device used in measuring relative humidity. Evaporation of moisture lowers the temperature of wet bulb compared to dry bulb temperature of the same air sample.
Wet Bulb Temperature:The lowest temperature that can be obtained by evaporating water into the air at constant pressure. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity.