HVAC Glossary of Terms | HVAC meaning
To assist you in comprehending the various air conditioning/cooling/HVAC industry phrases and abbreviations found on our website and literature, we have organized this glossary. Simply click on the alphabetical links below to navigate to the relevant section
A
Absolute Pressure: The sum of gauge pressure and atmospheric pressure. It is sometimes denoted with an “a” at the end, such as Bara or Psia.
Accumulator: A storage tank that receives liquid refrigerant from the evaporator and prevents it from flowing back into the compressor.
AC Fans: Various types of fans that are powered by an alternating current (AC) motor, providing air movement in heating, cooling, and ventilation systems.
ACIS™: Airedale Controls Integrated Systems (ACIS™) is a building energy management system (BEMS) that operates and optimizes various building services across multiple platforms and protocols. It is a type of BEMS.
ACRIB: The Air Conditioning and Refrigeration Industry Board is an organization located at www.acrib.org.uk.
Adiabatic Cooling: The process of reducing heat through a change in air pressure caused by volume expansion.
Air Conditioner: An appliance, system, or mechanism designed to control temperature, humidity, and air quality in a defined space.
Air-Cooled System: A type of air conditioning system that uses air as the transfer medium to reject heat from the refrigerant in the condenser. Typically, the air-cooled condenser is located and rejects waste heat to the outdoors.
Air Diffuser: An air distribution outlet or grill designed to direct and develop balanced airstreams.
Air Handling Unit (AHU): A central unit consisting of a fan or fans, heating and cooling elements, filter racks or chamber, dampers, humidifier, and other central equipment required to provide suitable ventilation and extract. More information can be found in our Air Handling Unit brochure.
Ambient: The normal atmospheric conditions of temperature and pressure.
Application: The location or site where an air conditioning/cooling system or unit is to be installed (applied).
ASHRAE: The American Society of Heating, Refrigerating, and Air-Conditioning Engineers is an organization located at www.ashrae.org.
Azeotrope/Azeotropic Mixture: A product resulting from the combination of two or three compounds that have identical vapor and liquid compositions. An azeotrope cannot be separated into its parts by distillation. Azeotropes will fractionate slightly and experience temperature glide outside of the identified azeotropic points.
Azeotropic Point: The temperature where a liquid mixture boils and produces a vapor with exactly the same composition as the liquid.
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B
Building Energy Management System (BEMS) is an automated control system that manages and supervises a building’s mechanical and electrical equipment, including lighting, ventilation, power, security, and fire systems. The system aims to improve performance, reduce operational costs, and facilitate decision-making. Another related term is ACIS™.
Blend refers to a mixture of two or more refrigerants with a specific ratio that can be separated by distillation. Regular blends may have a temperature glide of up to 10°C or more.
BREEAM, which stands for Building Research Establishment Environmental Assessment Methodology, is a widely used method for assessing, rating, and certifying the sustainability of buildings. More information can be found at www.breeam.org.
British Thermal Unit (BTU) is an imperial measurement of the quantity of heat needed to raise or lower the temperature of 1lb. of water by 1°F. This term is not commonly used except on some equipment imported from the Far East or the USA.
Bubble point refers to the pressure at which a refrigerant liquid starts to vaporize.
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C
Capillary tube: a small tube used to restrict flow, best used in constant conditions.
Carbon Usage Effectiveness (CUE): a ratio indicating the amount of CO2 emissions produced by a data center compared to the energy consumption of its IT equipment.
Cascade system: a refrigeration system with two or more circuits working in series.
Cassette unit: an air conditioning unit that fits into false or suspended ceilings, commonly used in commercial and residential applications.
Centrifugal compressor: a compressor that uses centrifugal force to compress gas.
CFC and CFCs: chlorofluorocarbons, gases that were widely used as refrigerants but have since been phased out due to their impact on the ozone layer and global warming.
CFM: cubic feet per minute, a measure of air flow in air conditioning systems.
Change of state: a change from one phase (solid, liquid, gas) to another.
Chilled water system: an air conditioning system that uses water or glycol solutions as a secondary cooling medium.
Chiller: a machine that removes heat from a secondary cooling medium via refrigeration.
CIBSE: Chartered Institution of Building Services Engineers.
CITB: Construction Industry Training Board.
Clean room: an environment with low levels of environmental pollutants, commonly used in manufacturing or scientific research.
Close control/Climate control: systems designed to maintain precise environmental conditions, also known as Precision Air Conditioning (PAC) and Precision Air Conditioning.
Coil: a component that provides heat transfer to air.
Cold aisle: a layout design for server racks in data centers that involves lining up server racks with cold air intakes and hot air exhausts.
Comfort air conditioning: systems designed to provide comfort to people and animals, not for close environmental control.
Commercial air conditioning: air conditioning for large buildings like hotels and hospitals.
Commissioning: a service that ensures equipment is set up correctly after installation for optimal performance.
Compressor: a component of refrigeration systems that raises the pressure of the refrigerant.
Condensate: moisture removed from the air during the dehumidification process in air conditioning.
Condenser: a vessel or air-cooled coil that removes the superheat of compression, condensing the refrigerant gas into a high-pressure liquid.
Condensing unit: a refrigeration/air conditioning system comprising a compressor, motor, and condenser heat exchanger.
Conduction: the transfer of heat through contact of two bodies of differing temperatures.
Constant air volume (CAV): a type of HVAC system where the supply air flow rate is constant but the supply air temperature is varied.
Controls: a control system that regulates a heating or air conditioning system.
Convection: the transfer of heat by changes of density in a fluid or gas.
Cooling tower: a device used to supply water for cooling condensers.
COP: coefficient of performance, the ratio of the refrigerant effect to the work by the compressor over a unit of time.
CRAC: Computer Room Air Conditioning unit.
CRAH: Computer Room Air Handling unit.
D
D is for Damper: which controls airflow in HVAC systems.
Data Centre: a facility for computer systems.
Decant/Receiver: stores refrigerant during maintenance.
Dehumidification removes moisture from conditioned air.
Density: is the weight of a unit volume of a substance.
Dew Point: is where air becomes saturated with moisture.
DX Systems: extract heat in the evaporator using refrigerant.
Downflow: HVAC systems discharge air downward.
Drop-in gases: can replace refrigerants without major system changes.
Ducts: are used to deliver and remove air in HVAC systems.
E
EC Fan
Electronically Commutated (EC) fans use brushless DC motors which include in-built electronics to convert the AC supply to DC without the need for a separate DC supply. EC fans provide a more efficient (up to 30%) means of airflow through Precision Air Conditioning (PAC) units with the additional benefit of variable speed control via an output signal from the unit controller. They are often used in HVAC systems for their energy efficiency and precise speed control.
Economizer
An economizer is a mechanical device used to reduce energy consumption by recycling energy produced within a system or optimizing environmental temperature differences to achieve efficiency improvements. It can be used in HVAC systems to pre-cool or pre-heat the air entering a cooling or heating system, respectively, using outside air instead of mechanically cooled or heated air.
EER
Energy Efficiency Ratio (EER) is a measure of system efficiency at a given set of rating conditions. It is a ratio calculated by dividing the cooling capacity in kW by the power input in kW. EER is often used to rate the efficiency of air conditioners and heat pumps.
Electronic Expansion Valve (EEV)
This performs the same function as the thermal expansion valve (TXV) but provides closer control as the DC stepper motor opens and closes the valve in response to an output from the controller; maintaining very close control of the evaporator superheat. EEVs are often used in refrigeration and air conditioning systems for their precise control of refrigerant flow.
Energy Technology List (ETL) / ECA Scheme
The Energy Technology List (ETL) is a list of products that meet certain energy-saving criteria established by the UK government. The Enhanced Capital Allowance (ECA) Scheme provides businesses with enhanced tax relief for investments in equipment that meets these criteria. The ETL/ECA Scheme aims to encourage the use of energy-efficient equipment and reduce greenhouse gas emissions.
Enthalpy
Enthalpy is a thermodynamic property of a substance defined as its total internal energy plus the product of its pressure and volume. It is a measure of the total heat content of a substance.
Entropy
Entropy is a thermodynamic property that describes the amount of thermal energy unavailable for conversion into work in a thermodynamic process. It is a measure of the disorder or randomness of a system.
Energy Reuse Effectiveness (ERE)
Energy Reuse Effectiveness (ERE) is a metric used to measure the efficiency of data center energy use. It is the ratio of energy emitted from the data center and reused elsewhere to total energy consumed.
Energy Reuse Factor (ERF)
Energy Reuse Factor (ERF) is a metric used to measure the efficiency of data center energy use. It is the ratio of the data center energy that is reused elsewhere in the facility to the total energy brought into the data center control volume (including IT, cooling, power, lighting, etc.).
ESEER
European Seasonal Energy Efficiency Ratio (ESEER) is a measure of equipment energy efficiency used in Europe. Rather than using a single condition, as with EER, the ESEER assumes four seasonal conditions for variable load performance of chillers and provides a suitable rating number for the equipment.
Evaporator coil
An evaporator coil is a heat exchanger that is usually located indoors as part of a split unit or within an air handler or duct system. Inside the coil, refrigerant evaporates as it absorbs heat from the air that passes over it, providing cooling.
Evaporating temperature
The evaporating temperature is the temperature at which a given refrigerant vaporizes within an evaporator. It is an important parameter in refrigeration and air conditioning systems, as it affects the cooling capacity and efficiency of the system.
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F
Flame Sensor
A device used in gas furnaces to detect the presence of a flame. If the flame sensor does not detect a flame, it will shut off the gas valve to prevent a gas leak.
Furnace
A device used for heating a building or other space. Furnaces can be powered by electricity, natural gas, propane, or oil.
Filter
A device used to remove impurities or particles from air or liquid. Filters are commonly used in HVAC systems to improve indoor air quality.
Fan
A device used to circulate air in an HVAC system. Fans can be used to move air through ductwork or to circulate air within a room.
Fresh Air Ventilation
The process of bringing outside air into a building to improve indoor air quality. Fresh air ventilation can be provided through natural ventilation (such as opening windows), mechanical ventilation (such as using an HRV or ERV), or a combination of both.
Fixed Plate Heat Exchanger
A heat exchanger that uses a series of fixed plates to transfer heat between two fluids. Fixed plate heat exchangers are commonly used in HVAC systems for heating and cooling applications.
Fault Code
A code that is displayed on an HVAC system’s control panel to indicate a problem or issue with the system. Fault codes can be used to diagnose and troubleshoot problems with HVAC systems.
Frequency Drive
A device used to control the speed of a motor in an HVAC system. Frequency drives can help to reduce energy consumption and improve system performance by allowing motors to run at variable speeds.
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G
Gauge pressure
The pressure reading obtained from a gauge that is typically calibrated to zero at atmospheric pressure (one bar absolute or 14.7 psi), allowing for negative pressure readings when below atmospheric pressure. This is often denoted with the letter “g” at the end of the pressure unit, such as Barg (bar gauge) or Psig (psi gauge).
Glide
The temperature range over which a refrigerant blend undergoes phase change, leading to the varying proportions of the blend’s components at different temperatures.
Glycol-Cooled System
An air conditioning system that employs a water/glycol mixture as a condensing medium. The glycol-cooled condenser is usually situated inside the air conditioner, alongside other refrigeration components. The water/glycol mixture is typically supplied to the unit from a drycooler or other appropriate source, and prevents the solution from freezing during winter operation.
GWP
Global Warming Potential is a measure of the contribution of a substance to global warming relative to carbon dioxide, where CO2 has a GWP of 1. Chlorofluorocarbons (CFCs) typically have a GWP of 5,000 to 10,000, while hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have GWP values of 1,000 to 4,000. Natural refrigerants and new hydrofluoroolefins (HFOs), such as R1234yf, usually have a GWP of less than 20.
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H
HCFC: Hydrochlorofluorocarbon is a type of refrigerant gas (e.g. R22) that has lower ozone depletion potential and global warming potential.
Heat capacity: The amount of heat required to raise the temperature of a given mass by 1 degree Kelvin.
Heat exchanger: A device designed to transfer heat between two physically separated fluids efficiently.
Heat pump: A mechanical device that absorbs low-grade heat from a lower temperature location and upgrades it to heat a space to a sufficient comfort level. Heat pumps can be air to air, air to water, water to water or ground to water.
Heat transfer: The exchange of thermal energy between two interacting media passing through a heat exchanger, such as refrigerants, air, and water.
Hemi-anechoic chamber: A room designed to completely absorb reflections of either sound or electromagnetic waves. Hemi-anechoic chambers have a solid floor that acts as a work surface for supporting heavy items such as large air conditioning units for testing sound levels.
HFC: Hydrofluorocarbon is a type of refrigerant gas (e.g. R134a) that has zero ozone depletion potential and high global warming potential.
HFO: Hydrofluoroolefin is a type of refrigerant gas (e.g. R1234ze) that has zero ozone depletion potential and low global warming potential.
High Performance Computing (HPC): Refers to the practice of aggregating computing power to solve large problems in science, engineering, or business that delivers much higher performance than one could get out of a typical desktop computer or workstation.
Hot aisle: A layout design for server racks and other computing equipment in a data center that involves lining up server racks in alternating rows with cold air intakes facing one way and hot air exhausts facing the other.
Humidification: The process of adding moisture to the air within a space.
HVAC: Heating, Ventilation, and Air Conditioning is the technology of indoor and vehicular environmental comfort that aims to provide thermal comfort and acceptable indoor air quality.
Hydrocarbon refrigerant (HC): A family of chemicals containing only carbon and hydrogen that are suitable for use as a refrigerant. Common examples include propane (R290), isobutane (R600a), and the CARE range. They have zero ozone depletion potential and very low global warming potential.
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I
Insulation
Insulation refers to the materials used to reduce the flow of heat, either to keep a space warmer or cooler, and to improve energy efficiency. Common insulation materials include fiberglass, mineral wool, and foam.
Inverter
An inverter is an electronic device that converts direct current (DC) to alternating current (AC). In HVAC systems, inverters are used in air conditioners and heat pumps to vary the speed of the compressor motor and improve energy efficiency.
Indoor air quality (IAQ)
Indoor air quality refers to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants.
Internet Service Provider (ISP)
An ISP is an entity that provides access to the Internet, enabling users to connect to websites and online services.
Inverter Compressor
An inverter compressor uses an inverter drive to regulate the speed of the compressor motor, allowing for variable capacity depending on demand.
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K
Kilowatt-hour (kWh)
Kilowatt-hour is a unit of energy equivalent to one kilowatt of power used for one hour. It is commonly used to measure energy consumption.
Kilowatt (kW)
Kilowatt is a unit of energy flow used to measure the cooling and heating capacity, as well as the power consumption, of an air conditioning system.
Kyoto Protocol
The Kyoto Protocol is an international agreement designed to regulate global warming by setting goals for reducing greenhouse gas emissions, which are believed to contribute to global warming. It was signed in Kyoto, Japan and is expected to be used as the framework for future HFC usage restrictions.
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L
Latent heat
Latent heat refers to the heat energy that is absorbed or released during a phase change of a substance, such as the melting of ice or the evaporation of water.
Leak detection
Leak detection refers to the process of identifying and locating leaks in refrigeration and air conditioning systems. Common methods include electronic leak detectors, ultraviolet dye, and bubble solutions.
Low Global Warming Potential (Low GWP)
Low GWP refers to refrigerant gases that have a lower impact on global warming than traditional refrigerants such as HCFCs and HFCs.
Latent Cooling Capacity
Latent cooling capacity is the amount of energy required to increase or decrease the moisture content of air during the air conditioning process, typically measured in kg/kg of dry air.
Low-Pressure Stage
The low pressure stage is the part of a refrigerant system where the temperature is at its lowest point. In a single-stage system, this is the area from the outlet of the expansion device to the compressor suction inlet.
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M
Mold
Mold is a type of fungus that can grow indoors in areas with high humidity or moisture. It can cause health problems and damage to buildings and materials.
Make Up Air Unit (MUA)
A MUA is an intake supply fan designed to replace air that has been exhausted from a building. It may be heated or cooled to minimize its impact on the building’s air conditioning systems, but is not intended to directly cool or heat the building.
Microchannel Coil
A microchannel coil is a type of heat exchanger used in air conditioning systems. It is constructed of parallel aluminum tubes that are mechanically brazed to enhanced aluminum fins, providing improved heat transfer and a lighter, corrosion-resistant coil.
Mineral Oil
Mineral oil is a lubricant frequently used with HCFC and HFC refrigerants. It has a low affinity for moisture and helps to keep the refrigerant compressor running smoothly.
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N
Noise pollution
Noise pollution refers to excessive or unwanted sounds that can cause annoyance, sleep disturbance, and other health problems. In HVAC systems, noise pollution can be caused by equipment such as fans, compressors, and motors.
N+1 Redundancy: A system resilience strategy that ensures availability in case of component failure. The system has at least one independent backup component for each component (N+1).
Natural Refrigerant: A refrigerant that does not contain fluorochemicals and is derived from natural sources, such as ammonia (R717), carbon dioxide (R744), propane (R290), and isobutene (R600a). They are an alternative to fluorochemical refrigerants due to their low direct Global Warming Potential (GWP) and Ozone Depletion Potential (ODP), but their commercial production is typically through man-made synthesis.
Near Azeotrope: A chemical product formed by combining two or more compounds that have nearly identical vapour and liquid compositions. Near azeotropes have a temperature glide of less than 2°C (according to ASHRAE 34 definitions).
Non-Condensable Gas: A gas in a refrigeration system that does not condense at the temperature and partial pressure that exists in the condenser. This leads to a higher head pressure and lower system efficiency. Air leakage is a common cause of non-condensable gas in refrigeration systems.
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O
Ozone Depletion Potential (ODP)
Ozone Depletion Potential refers to the potential of a substance to deplete the ozone layer in the atmosphere. Substances with high ODP, such as CFCs and HCFCs, are being phased out under the Montreal Protocol.
ODP: Ozone Depletion Potential is a measure of damage to the ozone layer relative to R11=1. CFCs have an ODP between 0.1 and 1, HCFCs between 0.01 and 0.1, while HFCs, HFOs, and natural refrigerants have an ODP of 0.
Oil Separator: A device designed to separate oil from the refrigerant in a refrigeration system.
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P
Packaged Unit: A self-contained air handling unit designed for outdoor installation, including all pre-assembled heating and cooling devices.
Precision Air Conditioning (PAC): An air conditioning system designed primarily for process cooling, such as data center equipment or manufacturing processes. These systems are reliable and have a high ratio of sensible-to-total cooling capacity (COP).
Pressure and Strength Tests: Tests performed to ensure the integrity of a system before evacuation and the addition of refrigerant.
Psychrometric Chart: A chart that represents factors relating to the condition of air and its state changes, based on 1kg of air.
PUE: Power Usage Effectiveness is a metric calculated by dividing a data center’s total energy consumption by the energy consumption of its IT equipment. A PUE value approaching 1.0 indicates 100% efficiency, where all energy is used by IT equipment only.
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R
Refrigerant
A refrigerant is a substance used in refrigeration and air conditioning systems to absorb and release heat energy, allowing for the transfer of heat from one location to another.
Refrigeration
Refrigeration refers to the process of removing heat from a space or substance, typically to lower its temperature for preservation or comfort. It is commonly used in refrigerators, air conditioners, and industrial processes.
R1234ze (HFO Refrigerant):
R1234ze is a fourth-generation refrigerant that is based on hydrofluoroolefin (HFO). It has a Global Warming Potential (GWP) lower than one, which is better than CO2, as rated by the International Panel for Climate Change (IPCC). It is seen as a replacement for R134a and is used in applications like the TurboChill™ that cools John Lewis.
R134a (HFC Refrigerant):
R134a is a haloalkane refrigerant with zero ozone depletion potential designed to replace R12. It is commonly used in automotive air conditioning, refrigerators, and chillers.
R22 (HCFC Refrigerant):
R22 is a hydrochlorofluorocarbon refrigerant gas found in most air conditioning equipment over 10 years old. It has ozone-depleting potential (ODP) if leaked into the atmosphere. The use of R22 for maintenance or repair has been banned since January 1st, 2015.
R407C (HFC Refrigerant):
R407C is a zeotropic hydrofluorocarbon refrigerant blend of difluoromethane (R32), pentafluoroethane (R125), and tetrafluoroethane (R134a). It was developed as a replacement for ozone-depleting R22.
R410A (HFC Refrigerant):
R410A is a third-generation hydrofluorocarbon refrigerant designed to replace earlier ozone-depleting substances. It is a zeotropic mixture of difluoromethane (R32) and pentafluoroethane (R125) used as a refrigerant in air conditioning applications.
Rack:
A rack is a metal frame used to hold various hardware devices such as servers, hard disk drives, modems, and other electronic equipment.
Reclaim:
Reclaim refers to the re-processing and upgrading of refrigerant by filtering, drying, distillation, and sometimes chemical treatment of the recovered refrigerant. The re-processed substance must meet specific quality standards, usually that of new refrigerant, through laboratory analysis. This process is typically done off-site at a re-processing or refrigerant manufacturing facility.
Recovery:
Recovery involves taking used gas out of a fridge system and filling it into packages. This process is performed by a refrigeration contractor and avoids releasing refrigerant into the environment, which is environmentally irresponsible and illegal in many countries. Recovery occurs during maintenance or when removing refrigerant permanently due to equipment decommissioning or retrofitting to a new gas.
Recycle:
Recycling refers to cleaning refrigerant by oil separation, distillation, and passing it through filter-driers to remove moisture, acidity, and particulate matter. This process improves the quality of recovered refrigerant before re-use, and it can be done on or off site.
Refrigerant:
Refrigerant is the fluid used for heat transfer within a refrigeration system. It absorbs heat at low temperature and pressure and transfers heat at high temperature and pressure. The refrigerant can be many materials, including fluorocarbon compounds, natural refrigerants such as ammonia, CO2, hydrocarbons, as well as other compounds such as water and air.
Refrigerant charge:
The refrigerant charge refers to the amount of refrigerant in kg in a closed system.
Refrigeration Cycle (Vapour compression cycle):
The vapour compression refrigeration cycle involves continuously circulating a constant volume of refrigerant gas in a closed system. By varying pressure and temperature within different parts of the system, the refrigerant absorbs waste heat from the conditioned space in the evaporator and rejects it through the condenser.
Relative Humidity (%rh):
Relative humidity
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S
Sensible heat
Sensible heat refers to the heat energy that can be sensed or measured by a thermometer, such as the temperature of a room or object.
Screw Compressor: A compressor that uses two synchronized screws to pressurize refrigerant vapor. It has a modulating performance band of 10 to 100%, making it suitable for use in chillers. Typical capacities range from 50 to 1000 kW.
Scroll Compressor: A type of compressor that uses two compliant scrolls, one fixed and one oscillating, to compress refrigerant as it passes between them. It is commonly used in air conditioning systems and can be fixed or variable capacity using digital unloading or inverter control.
Sensible Cooling Capacity: The amount of heat energy removed from the air without changing the moisture content of the air.
Sensible Heat Ratio (SHR): The ratio of sensible heat to total heat.
Server: A system that responds to requests across a computer network to provide or help provide a network service.
Split System: An air conditioning system consisting of an outdoor (condensing) unit and an indoor unit. It typically refers to smaller equipment, specifically air to air heat pumps.
Subcooling: The removal of heat from a liquid to a point lower than the saturation temperature at that pressure. This usually occurs in the condenser heat exchanger.
Superheating: The heating of a vapor to a temperature higher than the saturation temperature at that pressure. It usually occurs in the evaporator.
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T
Thermal comfort
Thermal comfort refers to the subjective experience of being comfortable in a space, taking into account factors such as temperature, humidity, and air movement.
Thermostatic Expansion Valve (TEV): A precision control device designed to regulate the rate at which liquid refrigerant flows into the evaporator, thereby controlling the evaporator superheat.
Thyristor: A semiconductor device used to switch electric currents and often used to control the output of electric heaters.
Turning Vane: A device inside mechanical ductwork used to smoothly direct air inside the duct where there is a change in direction, reducing resistance and turbulence.
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U
Upflow Unit: An air conditioning system that discharges air into the conditioned space via a top-mounted discharge plenum or overhead duct system.
Uninterruptible Power Supply (UPS): An electrical apparatus that provides emergency power to a load when the input power source fails.
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V
Ventilation
Ventilation refers to the process of bringing fresh air into a space and removing stale air, odors, and contaminants. It is an important aspect of indoor air quality and can be achieved through natural or mechanical means.
Vacuum Test: A test to check the gas tightness of a refrigeration system before charging it with refrigerant by drawing a vacuum on it.
Vapor: The gaseous form of a substance. In refrigeration terminology, it often refers to gases at near equilibrium with the liquid phase that do not follow gas laws and are in a gaseous state below the critical temperature.
Vapor Barrier/Seal: A barrier that prevents air, moisture, and contaminants from migrating through tiny cracks or pores in the walls, floor, and ceiling into a critical space. It is also used extensively on pipe insulation to prevent moisture ingress that may cause deterioration of the insulation or freezing in cold conditions.
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W
Water-Cooled System: A type of air conditioning system that utilizes water as a condensing medium in the cooling process. The water-cooled condenser is usually located inside the air conditioning unit alongside the other refrigeration components. Water is supplied to the unit through piping from a suitable source, such as a cooling tower.
Water Usage Effectiveness (WUE): A metric developed by The Green Grid, which evaluates the water utilized on-site for the operation of a data center. This includes water used for humidification, as well as water that evaporates on-site for energy production or cooling the data center and its support systems.
Watt: The watt is a derived unit of power in the International System of Units (SI) named after James Watt, a Scottish engineer (1736-1819). The watt is defined as joules per second and is used to measure the rate of energy conversion or transfer with respect to time.
Wet Bulb Temperature: The wet-bulb temperature is the temperature that a parcel of air would reach if it were cooled to saturation (100% relative humidity) by water evaporation, with the latent heat being supplied by the parcel.
