A Glossary of Commercial Air Handling, Piping, and Fan Technologies and Terms

You are currently viewing A Glossary of Commercial Air Handling, Piping, and Fan Technologies and Terms

There are a lot of commercial HVAC industry terms you should know to help you make the best decisions for your air handling and environment projects. We’ve put together a list to help our clients.

Absolute Zero

The zero point on the absolute temperature scale.


Refers to a cooling system that passes ambient air through the condensing unit in order cool and condense the refrigerant back into a liquid. 


The distribution or movement of air.

Air Plenum

The space between an inner and outer wall or between the cleanroom ceiling and roof deck. Air is supplied into the ceiling plenum and then flows through a HEPA or ULPA filter into the cleanroom and is returned through the double wall plenum back to the filters to be recirculated.

Air Shower

A hallway or room leading to the entrance of a cleanroom. In an air shower, high-velocity air blows off particles that could contaminate a cleanroom. Sometimes, air showers are combined with static removal equipment or HEPA filters to make them more effective in removing hair and other contaminants.


The process in which liquid is broken up into many very fine droplets increases the contact surface with the air, thus assisting evaporation. 


Microscopic living organisms that grow and multiply in warm, humid places.


Any liquid cooled by the refrigerant and used for the transmission of heat without a change in its state, having no flash point or a flash point above 150 F as determined by American Society for Testing and Materials Method D93.

BTU or British Thermal Unit

A British thermal unit is a unit of heat energy and refers to the heat required to raise the temperature of a pound of water from 59 F to 60 F The higher the Btu rating, the greater the heating capacity of the system.


British thermal units per hour.


Combined Annual Efficiency (CAE) is a measure of the amount of heat produced for every dollar of fuel consumed for both space and water heating.


Heat required to raise the temperature of 1 gram of water 1 C, actually from 4 C to 5 C.

Capacitor Motor

A single-phase induction motor with a main winding arranged for direct connection to a source of power and an auxiliary winding connected in series with a capacitor.


The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs; for cooling, in tons.

Capillary Tube

In refrigeration practice a tube of small internal diameter used as a liquid refrigerant flow control or expansion device between high and low sides; also used to transmit pressure from the sensitive bulb of some temperature controls to the operating element.

Carbon Monoxide

An odorless, colorless, tasteless, poisonous, and flammable gas produced when carbon burns with insufficient air.


A thermometric scale in which the freezing point of water is called zero deg and its boiling point 100 deg at normal atmospheric pressure (14.696 psi).

Central Chiller

A chiller is used to cool several processes or zones. The chiller can be water or air-cooled and have a stationary or portable configuration.

Central Utility Plant, CUP

The epicenter of the mechanical, electrical, and sometimes plumbing systems serves the building or many buildings on a site. The CUP is home to all major mechanical, electrical, and plumbing equipment.


Cubic Feet per Minute is a measurement that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air that is being forced through the ductwork by the system.

Change of Air

Introduction of new, cleansed or recirculated air to conditioned space, measured by the number of complete changes per unit time

Change of State

Change from one phase, such as solid, liquid, or gas, to another

Clean Room

A room or enclosure in which contaminants are controlled within specified limits. Air is directed through the room to control the airborne particle levels and, in some cases, temperature and humidity.

Coefficient of Performance, Compressor, Heat Pump

The ratio of the compressor heating effect (heat pump) to the rate of energy input to the shaft of the compressor, in constant units, in a complete heat pump, under designated operating conditions.


In a refrigeration circuit, it transforms cool, low-pressure refrigerant gas to hot, high-pressure refrigerant gas that is then condensed back into a liquid to be used again.


A change from the gaseous state of water (vapor) to the liquid state. It occurs when hot and humid air is cooled, losing its capacity to hold water vapor.


In heat exchange between two fluids, opposite direction of flow, coldest portion of one meeting coldest portion of the other.


A movable plate in the ductwork that regulates and modulates airflow. These are used to direct or modulate air, often used in zoning applications.

Defrosting Cycle

The portion of a refrigeration operation which permits the cooling unit to defrost.


The removal of water vapor from the air. This can be accomplished by cooling an air stream below its dewpoint temperature, causing the condensation of vapor, or by desiccant adsorption/absorption resulting in the removal of humidity from the air in the vapor phase.

Dewpoint Temperature

When condensation of water vapor in a space begins for a given state of humidity and pressure as the temperature of the vapor is reduced; The temperature corresponding to saturation (100 percent relative humidity) for a given absolute humidity at constant pressure.


The Department of Energy is a United States federal agency responsible for setting industry efficiency standards and monitoring the consumption of energy sources.


Air enters at the top of an air handler or furnace and is discharged vertically out the bottom.


A manufactured device containing a desiccant (water absorbing material) placed in the refrigerant circuit. Its primary purpose is to collect and hold within the desiccant all water in the system in excess of the amount which can be tolerated in the circulating refrigerant

Dry Air

Air not containing water vapor.

Dry-Bulb Temperature

The temperature of a gas or mixture of gases indicated by an accurate thermometer after correction for radiation.


A square or round conduit by which air is distributed from the air handler to the room or space.

Electronic Air Cleaner

An electronic device that filters out large particles and bioaerosols in indoor air.

Enthalpy (Heat Content)

The sum of the internal energy of a body and the product of its volume multiplied by the pressure.

External Equalizer

In a thermostatic expansion valve, a tube connection from a selected control point in the low-side circuit to the pressure sensing side of the control element such that the control-point pressure is transmitted to the actuating element (diaphragm or bellows). This connection provides a means for compensating for the pressure drop through accessories and the evaporator

Fan Coil

A unit includes a cooling and/or heating coil and a fan to move air through the ductwork to a room. Air filters and accessories to introduce outside ventilation air may also be included.

Flash Gas

The gas resulting from the instantaneous evaporation of refrigerant in a pressure-reducing device to cool the refrigerant to the evaporation temperature obtained at the reduced pressure


The formation of a foam or froth of an oil-refrigerant mixture due to rapid boiling out of the refrigerant dissolved in the oil when the pressure is suddenly reduced; This occurs when the compressor starts operating and, if large quantities of refrigerant were dissolved, large quantities of oil may boil out and be carried through the refrigerant lines.

Foul Gas (Noncondensable Gas)

Gas in a refrigerating system which does not condense at the temperature and partial pressure at which it exists in the condenser, and therefore imposes a higher head pressure on the system.


Failure of a refrigerant unit to operate normally due to formation of ice at the expansion device.

Freezing Point

Temperature at which a given liquid substance will solidify or freeze upon removal of heat; Freezing point for water is 32 F.

Geothermal Energy

A renewable energy source, using the heat of the earth to heat buildings.


This is added to the water in a chill water system to protect it from freezing. Can either be purchased with or without rust inhibitors. The freezing point of the fluid is dependent upon the concentration of the glycol. The higher the concentration of glycol, the lower the freezing point, but as the concentration goes higher, the heat transfer properties are reduced. This will reduce chiller capacity.

Heat Exchanger

Transfers heat from one fluid to another without the fluids coming into direct contact with one another.


High Efficiency Particulate Air. HEPA Filters are used to remove 99.9% of particles as small as 0.3 microns.

High Plume Dilution

Uses a discharge nozzle to entrain ambient air to dilute the exhaust and reduce exhaust contaminant concentrations. The addition of the ambient air increases the discharge wind band mass flow rate and velocity, resulting in greater nozzle discharge momentum which can push the diluted effluent high above the roofline.

Horizontal Flow

Air enters at the end or any side of the unit and is discharged horizontally out the other end or side.

Hot Gas Bypass

A valve that bypasses a portion of the hot gas leaving the refrigeration compressor. This bypassed gas does not enter the condenser and is reintroduced into the circuit after the expansion valve and before the evaporator.


Product or system used to increase the specific humidity of the air.


An automatic device used to maintain humidity at a fixed or adjustable set-point.


Stands for heating, ventilation, and air conditioning.


Materials whose cells absorb moisture, leading to a variation in their weight and dimensions. Hygroscopic materials always tend to reach equilibrium with the surrounding environment.


Integrated Energy Efficiency Ratio expresses cooling part-load EER efficiency for commercial unitary air conditioning and heat pump equipment on the basis of weighted operation at various load capacities.

Industrial Chiller

Chillers that are used for process cooling versus comfort cooling applications. The heat loads and leaving fluid temperatures may differ in process cooling applications compared to those found in comfort cooling applications. Some components, system pumps, and fluid volume requirements will be different.

Internal Equalizer

In a thermostatic expansion valve, an integral internal part of passage which provides exposure of the actuating element (diaphragm or bellows) to pressure leaving the valve.


Integrated Part-Load Capacity (IPLC) is the cooling capacity of the system operating at part-load conditions.


Integrated Part-Load Value (IPLV) is the efficiency performance factor at part-load cooling capacity. This performance is critical due to the higher quantity of operating hours under part-load conditions than at full load.

ISO 9000

A family of international standards for quality management and assurance.


Air humidification process without changes in temperature (dry bulb).

Laminar Flow

Unidirectional airflow. A cleanroom is filtered air flowing vertically or horizontally with uniform velocity in a single direction. Filtered air flows through your cleanroom and back to an air return, to be re-filtered and returned to the room. Laminar flow is a major part of keeping your cleanroom contaminant free.

Latent Heat

Change in enthalpy during a change of state; With pure substances, latent heat is absorbed or rejected at constant pressure.


Leadership in Energy and Environmental Design (LEED) is a green building rating system, with certification indicating a building’s sustainability through high efficiency and cost savings.

MERV Rating

The Minimum Efficiency Reporting Value (MERV) rating of a filter describes the size of the holes in the filter that allow air to pass through. The higher the MERV rating, the smaller the holes in the filter and the greater the efficiency.


A unit of measure equal to one-thousandth of a millimeter.

Modular Central Utility Plant (MCUP)

Modular Central Utility Plants are pre-manufactured modular buildings with central utility equipment. Produced offsite, they are often a more cost- and time-efficient solution.


Contamination in the air, or contamination generated from a process.


Replaceable filters remove large contaminants from the air before it’s cycled through HEPA or ULPA filters for final purification.

Pressure Drop

Static pressure loss in fluid pressure, as from one end of pipe to the other, due to friction or other causes.

Pressure Regulator, Evaporator (Back-Pressure Valve)

An automatic valve located between the evaporator outlet and compressor inlet that is responsive to its own inlet pressure or to the evaporator or refrigerator temperature and functions to throttle the vapor flow when necessary to prevent the evaporator pressure from falling below a selected value.

Pump Down (Refrigeration System)

The operation by which the refrigerant in a charged system is pumped into the liquid receiver.

Ratio of Compression

Ratio of absolute pressures after and before compression.

Refrigerating Cycle

A sequence of thermodynamic processes through which a refrigerant passes, in a closed or open system, to absorb heat at a relatively low temperature level and reject heat at a higher temperature level

Reverse Flow Chiller

A reverse flow is a portable water chiller without a tank. It is used in open-loop cooling applications where an external tank or trough is used. This tank or trough must gravity feed into the chiller pump, which sends water through the chiller and filter, back to the processor back to the tank or trough.

Scroll Compressor

A specially designed compressor that works in a circular motion, as opposed to up-and-down piston action.

Sensible Heat

Heat which is associated with a change in temperature; Specific heat exchange of temperature; In contrast to a heat interchange in which a change of state (latent heat) occurs.

Specific Heat

The ratio of the quantity of heat required to raise the temperature of a given mass of any substance one deg to the quantity required to raise the temperature of an equal mass of a standard substance (usually water at 59 F) one deg.

Standard Air

Dry air at a pressure of 760 mm (29.92 in) Hg at 21 C (69.8 F) temperature and with a specific volume of 0.833 m3/kg (13.33 ft3/lb).

Standard Flow Chiller

This is a portable water chiller with a tank. They are used in closed-loop cooling applications where all of the water pumped from the chiller returns to the machines under pressure generated by the chiller pump.

Split System Chiller

A chiller, combination heat pump or air conditioner with indoor components, such as a furnace or blower coil and separate outdoor condenser. Split systems should be matched for optimal efficiency.


Process of cooling refrigerant below condensing temperature, for a given pressure. Also cooling a liquid below its freezing point.


A device which senses temperature and humidity and adjusts a heating or cooling system to maintain desired levels.


The horizontal or vertical axial distance an air stream travels after leaving an air outlet before the maximum stream velocity is reduced to a specified terminal level

Ton of Refrigeration

A useful description of refrigerating effect equal to 12,000 BTU per hr (200 Btu per min).

Total Refrigerating Effect

The product of the weight rate of refrigerant flow and the difference in enthalpy of the entering and leaving refrigerant fluid, expressed in heat units per unit of time

ULPA Filters

ULPA stands for Ultra Low Penetration Air. ULPA Filters are used to remove 99.9% of particles as small as .1 microns.


Air handler or furnace configuration in which air enters at the bottom of the unit and is discharged vertically out the top.

Variable-Speed Motor

A variable-speed motor in a fan application provides a quiet, consistent airflow for enhanced comfort, efficiency, and humidity control. According to the Office of Energy Efficiency and Renewable Energy, a variable-speed motor running continuously at a half-speed may use up to 75% less power than a single-stage motor uses to move the same amount of air.


A system that exchanges stale, recirculated indoor air with fresh, filtered outside air.

Water Chiller

A refrigeration apparatus that produces cold water or water/glycol mix to cool industrial process equipment or provide comfort cooling for buildings.

Water Cooled

Absorbs heat from process water and transfers it to a separate water source such as a cooling tower, river, pond, etc. Water Cooled is usually reserved for large capacity applications, where the heat generated by an air-cooled water chiller creates a problem.

Water Vapor

A physical state of water, which can be defined as the aeriform state at a temperature lower than its own critical temperature.

Wet-Bulb Temperature

Thermodynamic wet-bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature; Wet-bulb temperature (without qualification) is the temperature indicated by a wet-bulb psychrometer constructed and used according to specifications


A way to increase comfort and energy efficiency by controlling when and where heating and cooling occurs.

Piping Terminology

Abrasion Resistance

Ability to withstand the effects of repeated wearing, rubbing, or scraping.

Acceptance Test

An investigation performed on an individual lot of a previously qualified product, by, or under the observation of, the purchaser to establish conformity with a purchase agreement.

Acrylic Resins

A class of thermoplastic resins produced by polymerization of acrylic acid derivatives.

Acrylonitrile-Butadiene-Styrene (ABS)

Plastics containing polymers and/or blends of polymers, in which the minimum butadiene content is 6%, the minimum styrene and/or substituted styrene content is 15%, and the maximum content of all other monomers is not more than 5%, and lubricants, stabilizers, and colorants.


A substance capable of holding materials together by surface attachment


The effect of time on materials.

Alkyd Resins

A class of thermosetting resins produced by condensation of a poly-based acid of anhydride and a polyhydric alcohol.


To prevent the formation of or remove stresses in plastic parts by controlled cooling from a suitable elevated temperature.

Beam Loading

The application of a load to a pipe between two points of support, usually expressed in pounds and the distance between the centers of the supports.

Bell End

The enlarged portion of a pipe that resembles the socket portion of a fitting and that is intended to be used to make a joint by inserting a piece of pipe into it.


Undesirable rounded elevation of the surface of a plastic, whose boundaries may be either more or less sharply defined. A blister may burst and become flattened.


To attach by means of an adhesive.


Showing evidence of thermal decomposition through some discoloration, distortion, or destruction of the surface of the plastic.

Burst Strength

The internal pressure required to break a pipe or fitting.

Butylene Plastic

Plastics based on resins made by the polymerization of butene or copolymerization of butene with one or more unsaturated compounds, the butene being the greatest amount of weight.


Chemically a carbohydrate, the source of the cellulose family of plastics.

Cellulose Acetate Butyrate

A class of resins made from a cellulose base; either cotton linters or purified wood pulp, by the action of acetic anhydride, acetic acid, and butyric acid.


A dispersion of “solution” of a plastic in a volatile solvent.

Chemical Resistance

The effect of specific chemicals on the properties of plastic piping with respect to concentration, temperature, and time of exposure.


The union or fusing together of fluid globules or particles to form larger drops or a continuous mass.

Cold Flow

Change in dimensions or shape of some material when subjected to external weight or pressure at room temperature.


A combination of ingredients before being processed or made into a finished product.

Compressive Strength

The crushing load at failure applied to a specimen per unit area of the resistance surface of the specimen.


A chemical reaction in which two or more molecules combine with the separation of water; The collection of water droplets from vapor onto a cold surface.


The product of simultaneous polymerization of two or more polymerizable chemicals known as monomers.


Fine cracks at or under the surface of a plastic.


The unit elongation of a particular dimension under load for a specific time following the initial elastic elongation caused by load application; Usually expressed in inches per inch per unit of time.


To change the properties of a polymeric system into a final, more stable, usable condition by the use of heat, radiation, or reaction with chemical additives.

Deflection Temperature

The temperature at which a specimen will deflect a given distance at a given load under prescribed conditions of test.


A deleterious change in the physical properties of a plastic evidenced by impairment of these properties.

Dielectric Constant

The ratio of the capacitance of a condenser having that material as dielectric to the capacity of the same condenser having a vacuum as dielectric.


The migration or wondering of the particles or molecules of a body of fluid matter away from the main body through a medium or into another medium.

Dimension Ratio

The diameter of a pipe divided by the wall thickness.


A free-flowing compound prepared without fluxing or addition of solvent.


An instrument that measures hardness; Measures the depth of penetration (without puncturing) of a blunt needle compressed on the surface for a short period of time.


That property of plastics materials by virtue of which they tend to recover their original size and like properties.


The capacity to take deformation before failure in tension, expressed as a percentage of the original length.


A dispersion of one liquid in another – possible only when they are mutually insoluble.

Environmental Stress Cracking

Cracks that develop when the material is subjected to stress in the presence of specific chemicals.


A compound formed by the reaction between an alcohol and an acid; Frequently used as plasticizers in rubber and plastic compounds.


Method of processing plastic in a continuous or extended form by forcing heat-softened plastic through an opening shaped like the cross-section of the finished product.


Method of forming plastic into a finished article by machining, drawing, cementing and similar operations.

Fiber Stress

The unit stress, usually in pounds per square inch (psi) in a piece of material that is subjected to an external load.


A relatively inert material added to a plastic to modify its strength, permanence, working properties, or other qualities or to lower costs.


The time a specimen will support a flame after having been exposed to a flame for a given period.

Flexural Strength

The pressure in pounds necessary to break a given sample; Applied to the center of the sample which has been supported by the end.


A process in which the shape of plastic pieces such as sheets or tubes is changed into a desired configuration.


A material or mixture prepared according to specifications and measurements.


To join two plastic parts by softening the material through solvents.

Graves Tear Strength

The force required to rupture a specimen by tearing it.


A comparative gauge of resistance to indentation.

Heat Distortion

The temperature at which a specimen will deflect a given distance with a given load.

Heat Joining

Making a piper joint by heating the edges of the parts to be joined so that they fuse and become essentially one piece.

Heat Resistance

The ability to withstand the effects of exposure to high temperature.

Hoop Stress

The tensile stress exerted circumferentially in the wall of the pipe when it contains a gas or liquid under pressure.

Hydrostatic Design Stress

The estimated maximum tensile stress in the wall of the pipes circumferential orientation due to internal hydrostatic pressure that can be applied continuously with a high degree of certainty that the pipe will not break.

Hydrostatic Strength

The hoop stress calculated by means of the ISO equation at which the pipe breaks due to an internal pressure build-up, usually with a time of 90 seconds.

Impact Strength

Resistance or mechanical energy absorbed by a plastic part when a load is suddenly applied.

Injection Molding

Method of forming a plastic to the desired shape by forcing heat-softened plastic into a relatively cool cavity where it rapidly solidifies (freezes).

ISO Equation

Shows the inter-relations between stress, pressure, and dimensions in pipe.


The location at which two pieces of pipe or a pipe and a fitting are connected together.


Compounds containing the carbonyl group (CO) to which is attached two alkyl groups; Commonly used as solvents for resins and plastics.

Light Stability

Ability of a plastic to retain its original color and physical properties upon exposure to sun or artificial light.

Longitudinal Stress

The stress imposed on the long axis of any shape.

Long-Term Hydrostatic Strength

The estimated tensile stress in the wall of the pipe in the circumferential orientation that when applied continuously will cause failure of the pipe at 100,000 hours.


A substance used to decrease the friction between solid faces sometimes used to improve processing characteristics of plastic compositions.

Modulus of Elasticity

The ratio of the stress per square inch to the elongation per inch due to this stress.

Molding, Compression

A method of forming objects from plastics by replacing the material in a confining mold cavity and applying pressure and usually heat.


The simplest repeating structural unit of a polymer.

Olefin Plastics

Plastics based on resins made by the polymerization of olefins or copolymerization of olefins with other unsaturated compounds, the olefins being in greatest amount by weight.

Organic Chemical

Chemicals derived from living organisms which contain the element carbon.

Phenolic Resins

Resins made by reaction of a phenolic compound or tar acid with an aldehyde.


A material that contains as an essential ingredient an organic substance of large molecular weight, is solid in its finished state, and at some state in its manufacture or in its processing into finished articles, can be shaped by flow.

Plastic Conduct

Plastic pipe or tubing used as an enclosure for electrical wiring.


A property of plastics and resins which allows the material to be deformed continuously and permanently without rupture upon the application of a force that exceeds the yield value for the material.

Plastic Pipe

A hollow cylinder of a plastic material in which the wall thickness is usually small when compared to the diameter and in which the inside and outside walls are essentially concentric.

Plastic Tubing

A particular size of plastics pipe in which the outside diameter is essentially the same as that of copper tubing.


A polymer prepared by the polymerization of butene-1 as the sole monomer.


A polymer prepared by the polymerization of ethylene as the sole monomer.


A product resulting from a chemical change involving the successive addition of a large number of relatively small molecules (monomer) to and from the polymer and whose molecular weight is usually a multiple of that of the original substance.


Chemical change resulting in the formation of a new compound whose molecular weight is usually a large multiple of that of the original substance.


A polymer prepared by the polymerization of propylene as the sole monomer.


A plastic based on resin made by polymerization of styrene as the sole monomer.

Polyvinyl Chloride

A synthetic resin which when plasticized or softened with other chemicals has some rubber like properties; Derived from acetylene and hydrochloric acid.


When expressed with reference to a pipe the force per unit area exerted by the medium in the pipe.


A chemical substance which is frequently added to plastic compounds to inhibit undesirable changes in the material, such as discoloration due to heat or light.

Stiffness Factor

A physical property of plastic pipe that indicates the degree of flexibility of the pipe when subjected to external loads.


The ratio of the amount of deformation to the length being deformed caused by the application of a load on a piece of material.


The mechanical properties of a plastic such as a load or weight carrying ability, and ability to withstand sharp blows; Strength properties include tensile, flexural, and tear strength, toughness, flexibility, etc.


When expressed with reference to pipe the force per unit area in the wall of the pipe of the pipe in the circumferential orientation due to internal hydrostatic pressure.


External or internal cracks in a plastic caused by tensile stresses less than that of its short-time mechanical strength.

Stress Relaxation

The decrease of stress with respect to time in a piece of plastic that is subject to an external load.

Styrene Plastics

Plastics based on resins made by the polymerization of styrene of copolymerization of styrene with other unsaturated compounds, the styrene being the greatest amount by weight.

Styrene-Rubber Plastics

Compositions based on rubbers and styrene plastics, the styrene plastics being in greatest amount be weight.

Sustained Pressure Test

A constant internal pressure test for 1000 hours.

Tear Strength

Resistance of a material to tearing.

Tensile Strength

The capacity of a material to resist a force tending to stretch it; In plastics testing, it is the load supported at the moment of rupture by a piece of test sample on being elongated.

Thermal Conductivity

Capacity of a plastic material to conduct heat.

Thermal Expansion

The increase in length of a dimension under the influence of an increase in temperature.


Forming with the aid of heat.


A plastic polymer which becomes more soft when heated and hard when cooled.


Plastic materials which undergo a chemical change and harden permanently when heated in processing.


Permitting the passage of light, but diffusing it so that objects on the other side cannot be clearly distinguished.


Any deviation from parallel flow in a pipe due to rough inner surfaces, obstructions, or direction changes.


A quantity expressing the magnitude of internal friction, as measured by the force per unit area resisting a flow in which parallel layers unit distance apart have unit speed relative to one another.


Property of liquids to pass away by evaporation.

Water Absorption

The percentages by weight of water absorbed by a sample immersed in water; Dependent upon area exposed and time of exposure.

Yield Point

The stress beyond which a material becomes plastic.

Yield Strength

The stress at which a plastic material exhibits a specified limit of permanent set.

Yield Stress

How much force needs to be applied to an object to cause it to change from elastic deformation to plastic deformation.

Fan Terminology

Belt Tube

Protect belts from the airstream and prevent any leakage from the fan housing.

Brake Horsepower (BHP)

The available power of an engine, assessed by measuring the force needed to brake it.

Fan Laws

Relate performance variables for any dynamically similar series of fans.

Fan Law 1

CFM varies as RPM.

Fan Law 2

SP varies as (RPM)2.

Fan Law 3

BHP varies as (RPM)3.

Fan Noise

Function of the fan design, volume air flow rate, total pressure, and efficiency.

Fan Performance

Air volume flow rate vs. Static pressure and Brake horsepower.

Fan Vibration

The structural response of a fan to excitations caused by impeller imbalance, unsteady aerodynamic forces, and drive torque pulsations.

Flow Conditions

Quantities like temperature and static pressure of the metered substance.

Flow Straightener (egg-crates)

Used in attempt to eliminate or reduce swirl or vertexflow in a duct.

Gap Size

Axial space between inlet nozzle and impeller; affects efficiency.

Gas Density

The mass of the gas occupying a certain volume at specified pressure and temperature.

Inlet Boxes

Used with centrifugal and axial fans to provide predictable inlet conditions and maintain stable fan performance; may be used to protect fan bearings from high temperature or corrosive gases.

Inlet Vortex

Spin in the airstream entering a fan inlet.

Mach Number

Dimensionless quantity, determines the ratio of flow velocity past a boundary to the local speed of sound.

Mechanical Efficiency

The effectiveness of a machine in transforming the power input to the device to power output.

Nozzle Inlet Chamber

A pipe or tube of varying cross sectional area used to direct or modify the flow of a fluid.

Outlet Diffusers

An expansion fitting between the fan outlet and the duct which allows the airstream to expand gradually.


A measure of the electric power that is needed to drive a fan relative to the amount of air that is circulated through the fan.

Revolutions per Minute (RPM)

The number of turns in one minute.

Reynolds Number

Dimensionless quantity, determines if a fluid flow is laminar or turbulent.

Rotational Speed

The number of turns of the object divided by time, specified as revolutions per minute (rpm) .

Static Pressure

The pressure of a fluid on a body when the body is at rest relative to the fluid.

Surface Roughness

Measure of the finely spaced micro-irregularities on a surface.


Any grouping of ductwork, filters, grilles, coils, etc.

System Effect

The derating or loss of capacity of a fan caused by poorly designed duct fittings at, or close to, the fan discharge and inlet.

System Effect Factor

The effect of the system configuration on the fan’s performance.

Total Fan Pressure

The pressure differential between the inlet and the outlet of the fan.

Turning Vanes

Will usually reduce the dynamic pressure loss through an elbow, but where a nonuniform approach velocity profile exists, such as at the fan outlet, the vanes may actually serve to continue the nonuniform profile beyond the elbow.

Volume Flow Rate

Volume of fluid that passes per unit time.

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