Engineering Glossary | EngineerHub

A

Absolute pressure

Pressure measured relative to a perfect vacuum. Absolute pressure equals gauge pressure plus local atmospheric pressure.

Absorbed dose

Mean energy imparted by ionizing radiation to matter per unit mass. The SI unit is the gray (Gy), equal to 1 J/kg.

ACH

Air changes per hour: ventilation airflow rate divided by room volume, expressed per hour. A value of 1 ACH means a volume of ventilation air equal to the room volume is supplied or removed each hour.

Activity (radioactivity)

Rate at which nuclear transformations occur in a radioactive material. The SI unit is the becquerel (Bq), equal to one transformation per second.

Adiabatic process

A thermodynamic process with no heat transfer across the system boundary; energy can still change through work.

Adsorption

Attachment of molecules to a surface. It differs from absorption, in which a substance enters the bulk of another material.

AGC

Automatic generation control: secondary power-system control that adjusts generator output to restore frequency and scheduled interchange after a disturbance.

Allowable stress

The stress permitted for a design after applying the relevant code, material basis, temperature effects and safety factors.

Ampacity

The current a conductor can carry under stated installation and thermal conditions without exceeding its permitted temperature.

Apparent power

Magnitude of complex AC power, measured in volt-amperes (VA). For single-phase systems S = Vrms·Irms; for balanced three-phase systems S = √3·VLL·IL.

ASME

American Society of Mechanical Engineers; publisher of widely used mechanical-engineering codes and standards, including pressure-equipment codes.

Atmospheric pressure

The absolute pressure exerted by the atmosphere. Standard sea-level atmospheric pressure is approximately 101.325 kPa.

AWG

American Wire Gauge: a logarithmic wire-size system commonly used in North America. A smaller AWG number means a larger conductor.

B

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Beam

A structural member primarily designed to resist loads transverse to its longitudinal axis through bending and shear.

Becquerel (Bq)

SI unit of radioactive activity. One becquerel equals one nuclear transformation per second (1 s⁻¹).

Bernoulli equation

An energy relation for steady fluid flow linking pressure, velocity and elevation, with extensions for pumps, turbines and losses.

Biot number

A dimensionless heat-transfer number comparing internal conduction resistance with surface convection resistance: Bi = hLc/k.

Boiling

Phase change from liquid to vapor at a heated surface or throughout a liquid when thermodynamic conditions permit vapor formation.

Bore

The internal diameter of a cylindrical component such as a pipe, bearing or cylinder.

BWR

Boiling water reactor: a light-water reactor in which water boils directly in the reactor vessel and the generated steam is sent to the turbine.

C

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CADR

Clean Air Delivery Rate: the volumetric rate of effectively cleaned air delivered by an air purifier, normally stated in CFM or m³/h.

Cantilever

A structural member fixed at one end and free at the other.

Capacitance

The ability of a component or system to store electric charge per unit voltage, measured in farads (F).

Cavitation

Formation and collapse of vapor cavities when local liquid pressure falls sufficiently low; it can cause noise, vibration, loss of performance and damage.

CFM

Cubic feet per minute: a US customary unit of volumetric airflow.

CHF

Critical heat flux: the limiting heat flux at which a boiling surface undergoes a boiling crisis. Depending on geometry and flow regime, this may occur by departure from nucleate boiling or dryout and causes a sharp deterioration in heat transfer.

Coefficient of friction

A dimensionless ratio relating friction force to normal force for contacting surfaces. Pipe-flow resistance uses distinct friction factors such as the Darcy or Fanning friction factor.

Column

A structural member primarily carrying axial compressive load and therefore susceptible to buckling.

Compressor

A machine that increases gas pressure by supplying mechanical work.

Condensation

Phase change from vapor to liquid, usually accompanied by release of latent heat.

Conduction

Heat transfer through a material due to a temperature gradient, commonly modeled with Fourier’s law.

Conductivity, electrical

A measure of how readily a material conducts electric current; it is the reciprocal of electrical resistivity.

Conductivity, thermal

A material property describing its ability to conduct heat, commonly denoted k and measured in W/(m·K).

Control rod

A neutron-absorbing component inserted into or withdrawn from a reactor core to control reactivity and power or to shut the reactor down.

Convection

Heat transfer between a surface and a moving fluid, commonly represented by q = hAΔT.

COP

Coefficient of performance: useful heating or cooling rate divided by the required power input, using consistent units; it is therefore dimensionless. The exact system boundary depends on the applicable rating method.

Core flow

Coolant mass or volumetric flow through a reactor core; in many reactor designs it strongly affects heat removal and, in BWRs, void fraction and power.

Corrosion allowance

Additional material thickness added to a design to account for expected loss of wall thickness during service.

Critical speed

A rotational speed near a natural frequency of a shaft–rotor system, at which resonance can produce high vibration.

Criticality

Condition of a nuclear chain reaction. A reactor is critical when the effective neutron multiplication factor keff = 1 and neutron population is steady from one generation to the next.

Current

Rate of electric charge flow, measured in amperes (A).

D

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Darcy friction factor

A dimensionless factor used in the Darcy–Weisbach equation to calculate pressure or head loss in internal flow.

Darcy–Weisbach equation

A general pipe-flow relation for head loss due to wall friction: hf = f(L/D)(v²/2g).

Decay heat

Heat generated after reactor shutdown by radioactive decay of fission products and actinides; it decreases with time but remains important for cooling.

Deflection

Displacement of a structural or mechanical component from its unloaded geometry under applied load.

Density

Mass per unit volume, commonly denoted ρ and measured in kg/m³.

Dew point

The temperature at which air becomes saturated with water vapor at its current moisture content and condensation can begin.

DNBR

Departure from nucleate boiling ratio: the ratio of the predicted heat flux for departure from nucleate boiling to the actual local heat flux at the same location; used as a PWR thermal margin.

Doppler feedback

Reactivity feedback caused by temperature-dependent broadening of nuclear resonance absorption peaks, generally negative in thermal reactors.

Dose rate

Radiation dose delivered per unit time. The quantity must be qualified, for example absorbed dose rate in Gy/s or effective dose rate in Sv/h.

Drag coefficient

A dimensionless coefficient used to relate aerodynamic or hydrodynamic drag force to dynamic pressure and reference area.

Dry bulb temperature

Ordinary air temperature measured by a thermometer shielded from radiation and moisture effects.

Duty

The required heat-transfer rate or process load handled by equipment such as a heat exchanger, boiler or condenser.

E

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ECCS

Emergency Core Cooling System: systems intended to provide or maintain reactor-core cooling following specified loss-of-coolant or inventory events.

EER

Energy Efficiency Ratio: cooling capacity in Btu/h divided by electrical power input in watts under specified rating conditions, expressed in Btu/(W·h).

Effective dose

Radiation-protection quantity equal to the tissue-weighted sum of equivalent doses to specified organs and tissues. Its SI unit is the sievert (Sv).

Efficiency

Useful output divided by input. Depending on the system it may refer to energy, power, hydraulic, thermal, electrical or mechanical conversion.

Emissivity

A dimensionless surface property, between 0 and 1, describing thermal-radiation emission relative to an ideal blackbody.

Enthalpy

A thermodynamic property H = U + pV; specific enthalpy is particularly useful for flow-energy balances and phase-change calculations.

Entropy

A thermodynamic state property. For a reversible process, dS = δQrev/T; according to the second law, the total entropy of an isolated system cannot decrease.

Equivalent dose

Radiation-protection quantity for a tissue or organ obtained by weighting absorbed dose by radiation type. Its SI unit is the sievert (Sv).

Euler buckling

Ideal elastic buckling of a slender column, with critical load proportional to EI/(KL)².

Evaporation

Phase change from liquid to vapor at a surface, possible below the normal boiling temperature.

F

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Factor of safety

A design margin commonly expressed as the ratio of a limiting or failure capacity to the corresponding working demand, using the convention defined by the applicable design method.

Fan laws

Similarity relationships linking fan flow, pressure and power to rotational speed and size for geometrically similar operation.

Feedwater

Water supplied to a boiler, steam generator or reactor vessel to replace steam production and maintain inventory.

Finite element analysis (FEA)

A numerical method that divides a continuum into discrete elements to approximate stresses, temperatures, flow or other field variables.

Flow coefficient Cv

A valve capacity coefficient widely used in US customary practice; it relates water flow in gpm to pressure drop and fluid specific gravity.

Flow coefficient Kv

A metric valve capacity coefficient relating water flow in m³/h to pressure drop in bar and fluid density.

Flow rate

Quantity of fluid passing a section per unit time, expressed as volumetric flow (for example m³/s) or mass flow (kg/s).

Fourier number

A dimensionless transient-conduction time parameter, Fo = αt/Lc².

Frequency

Number of cycles per second, measured in hertz (Hz). In AC grids, frequency reflects the balance between generation and demand.

Froude number

A dimensionless ratio comparing inertial and gravitational effects in free-surface flow.

G

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Gauge pressure

Pressure measured relative to local atmospheric pressure. Absolute pressure equals gauge pressure plus atmospheric pressure.

Gear ratio

Ratio between input and output gear speeds or tooth counts; it determines speed multiplication and torque transformation.

Generator

A machine that converts mechanical power into electrical power through electromagnetic induction.

GMR

Geometric Mean Radius: an effective conductor radius used in transmission-line inductance calculations.

Grashof number

A dimensionless group characterizing buoyancy relative to viscous forces in natural convection.

Gray (Gy)

SI unit of absorbed dose. One gray equals one joule of ionizing-radiation energy imparted per kilogram of matter (1 J/kg).

H

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Half-life

Time required for the activity or number of undecayed nuclei of a radionuclide to decrease to one-half of its initial value.

Head

Fluid energy per unit weight, commonly expressed as an equivalent height of fluid. Pump head is the energy added by a pump.

Head loss

Loss of mechanical energy of a flowing fluid due to friction, fittings, valves and other resistance.

Heat capacity

Energy required to raise the temperature of a body by one degree. Specific heat capacity is heat capacity per unit mass.

Heat exchanger

Equipment that transfers heat between fluids, usually without mixing them directly.

Heat flux

Heat-transfer rate per unit area, typically expressed in W/m².

Heat pump

A thermodynamic system that transfers heat from a lower-temperature source to a higher-temperature sink using external work; reversible systems can provide both heating and cooling.

Heat transfer coefficient

A proportionality coefficient h used in convection models to relate surface heat flux to a temperature difference.

Hertz (Hz)

SI unit of frequency equal to one cycle per second.

Horsepower

A unit of power. Mechanical horsepower is approximately 745.7 W; metric horsepower is approximately 735.5 W.

Humidity ratio

Mass of water vapor per unit mass of dry air, widely used in psychrometric calculations.

HVAC

Heating, Ventilation and Air Conditioning: systems that control indoor temperature, humidity, air movement and air quality.

Hydraulic diameter

Characteristic diameter used for non-circular ducts or passages, defined as four times flow area divided by wetted perimeter.

Hydraulic power

Power transferred to a fluid, commonly P = Δp·Q or ρgQH.

I

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ICS

Isolation Condenser System: a heat-removal system used in certain BWR designs that routes reactor steam to a condenser and returns condensate. Some designs rely on natural circulation and gravity rather than active pumps.

Impedance

Opposition to AC current combining resistance and reactance; represented as a complex quantity and measured in ohms.

Inductance

Electrical property by which changing current produces magnetic flux and an induced voltage, measured in henries (H).

Inertia, rotational

Resistance of a rotating body to angular acceleration. In power systems, rotating generator inertia temporarily buffers frequency changes.

Insulation

Material or arrangement used to reduce heat transfer, electrical current, sound transmission or another unwanted transport mechanism.

Irradiance

Radiant power incident on a surface per unit area, commonly W/m²; solar irradiance is important in photovoltaic and heat-gain calculations.

Isentropic efficiency

A comparison of actual performance with an ideal isentropic process between the same inlet state and outlet pressure. For turbines, efficiency is actual work divided by isentropic work; for compressors, it is isentropic work divided by actual work.

J

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Joint efficiency

A design factor accounting for weld or joint quality in pressure-equipment calculations, depending on construction and examination requirements.

K

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kVA

Kilovolt-ampere: 1,000 VA of apparent electrical power. Commonly used for transformers and generators.

kvar

Kilovolt-ampere reactive: 1,000 var of reactive power.

kW

Kilowatt: 1,000 watts of real power.

L

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L10 bearing life

Basic rating life at which 90% of a sufficiently large group of identical bearings are expected to survive under specified conditions.

Laminar flow

Ordered flow regime in which viscous effects dominate and fluid layers move relatively smoothly; in circular pipes it normally occurs at low Reynolds number.

Latent heat

Energy absorbed or released during a phase change without a corresponding temperature change at equilibrium pressure.

LMTD

Log Mean Temperature Difference: an effective average temperature driving force for a heat exchanger.

Load factor

Ratio of average load over a period to the maximum load during that period.

LOCA

Loss-of-coolant accident: a reactor event involving loss of primary coolant inventory through a break or leak.

M

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Mach number

Ratio of flow velocity to local speed of sound.

Makeup water

Water added to replace losses from evaporation, blowdown, leakage or process consumption.

Mass flow rate

Mass of fluid passing a section per unit time, commonly kg/s.

Mean effective pressure

A conceptual constant pressure that would produce the same work per cycle as an actual reciprocating engine cycle.

Moderator

Material used in a thermal nuclear reactor to slow neutrons, increasing the probability of fission in suitable fuel isotopes.

Moody chart

Chart relating Darcy friction factor to Reynolds number and relative roughness for fully developed pipe flow.

N

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Natural circulation

Fluid circulation driven by density differences rather than a mechanical pump, often caused by heating and cooling.

Natural frequency

Frequency at which a system tends to oscillate when disturbed and allowed to vibrate freely.

NPSH

Net Positive Suction Head: the total suction head at the pump reference datum above the liquid vapor-pressure head, representing cavitation margin as a length of liquid.

NPSHa

Net Positive Suction Head available from the actual system at the pump inlet.

NPSHr

Net Positive Suction Head required by a pump to meet a specified cavitation/performance criterion, normally supplied by the manufacturer.

NTU

Number of Transfer Units: a dimensionless heat-exchanger size/effectiveness parameter based on UA divided by the minimum heat-capacity rate.

Nusselt number

Dimensionless heat-transfer number comparing convective to conductive heat transfer across a fluid layer.

O

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Ohm’s law

Relationship V = IR between voltage, current and resistance for an ohmic element.

Overall heat-transfer coefficient

U-value for a heat exchanger or wall assembly that combines multiple conduction and convection resistances into one coefficient.

P

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Pascal (Pa)

SI unit of pressure equal to one newton per square metre.

Phase angle

Angular difference between periodic waveforms of the same frequency, such as AC voltage and current.

Pipe roughness

Characteristic surface roughness used with pipe diameter to determine relative roughness and friction factor.

Poisson’s ratio

Negative ratio of transverse strain to axial strain in a material under uniaxial elastic loading.

Power factor

Ratio of real power to apparent power in an AC system; for sinusoidal conditions it depends on the phase relationship between voltage and current.

Prandtl number

Dimensionless ratio of momentum diffusivity to thermal diffusivity, Pr = ν/α.

Pressure drop

Reduction in fluid pressure between two points due to friction, acceleration, elevation change, fittings or equipment.

Psychrometrics

Study of moist-air thermodynamic properties and processes such as heating, cooling, humidification and dehumidification.

Pump affinity laws

Relationships that approximate how pump flow, head and power change with rotational speed or impeller diameter.

PWR

Pressurized water reactor: a light-water reactor that keeps primary coolant at high pressure so it remains liquid while transferring heat to steam generators.

R

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R-value

Thermal resistance of a layer or assembly. Higher R-value means greater resistance to heat flow.

Radiation dose

A general term that should be qualified. Absorbed dose measures energy imparted by ionizing radiation per unit mass; equivalent and effective dose apply radiation and tissue weighting for radiation-protection purposes.

RCIC

Reactor Core Isolation Cooling: a steam-driven system in certain BWRs used to provide makeup water when the reactor is isolated from the main condenser.

RCS

Reactor Coolant System: the primary coolant pressure boundary and major connected components of a pressurized water reactor.

Reactance

Frequency-dependent opposition to AC current caused by inductance or capacitance; it is the imaginary component of impedance.

Reactive power

Oscillating AC power associated with electric and magnetic field energy storage, measured in var or kvar.

Reactivity

A measure of departure of a nuclear reactor from criticality, commonly ρ = (keff − 1)/keff. Positive reactivity corresponds to keff > 1 and negative reactivity to keff < 1.

Real power

Average power that performs net work or is converted to heat in an AC system, measured in watts (W). For sinusoidal single-phase systems P = Vrms·Irms·cosφ.

Relative humidity

Ratio of the water-vapor partial pressure to the saturation vapor pressure at the same temperature, usually expressed as a percentage.

Relative roughness

Pipe roughness divided by pipe diameter, ε/D; used with Reynolds number to determine turbulent friction factor.

Resistance

Opposition to electric current represented by the real part of impedance; for an ohmic element R = V/I and the SI unit is the ohm (Ω).

Reynolds number

Dimensionless ratio of inertial to viscous effects in fluid flow, commonly Re = ρvD/μ.

RHR

Residual Heat Removal: reactor systems used to remove decay and sensible heat during shutdown or accident conditions.

RMS

Root mean square: an effective magnitude for a varying signal. For sinusoidal voltage or current, RMS gives the equivalent heating value of DC.

RoCoF

Rate of Change of Frequency: how quickly grid frequency changes with time, usually expressed in Hz/s.

S

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Safety factor

See factor of safety: a margin between a limiting capacity and the applied design demand.

SCRAM

Rapid reactor shutdown by insertion of negative reactivity, typically through rapid insertion of control rods or another shutdown system.

Sensible heat

Heat transfer that changes a substance’s temperature without phase change.

SG

Steam generator: in a PWR, a heat exchanger that transfers heat from primary reactor coolant to secondary water to produce steam.

Shear stress

Stress acting tangentially to a surface or cross-section.

Sievert (Sv)

SI unit used for equivalent dose and effective dose in radiation protection. One sievert has the dimensions of 1 J/kg, with biological weighting included in the quantity definition.

SIL

Surge Impedance Loading: the natural loading of a transmission line at which reactive power generated by shunt capacitance approximately balances reactive power absorbed by series inductance.

Specific gravity

Dimensionless ratio of a substance’s density to the density of a specified reference substance, usually water for liquids and air for gases.

Specific heat capacity

Heat required to raise the temperature of unit mass of a substance by one degree, typically J/(kg·K).

Standing-wave ratio (SWR)

Ratio of maximum to minimum amplitude in a standing wave. In transmission systems, voltage standing-wave ratio is commonly used to indicate impedance mismatch.

Static pressure

Pressure associated with the thermodynamic state of a fluid, distinct from dynamic pressure associated with flow velocity.

Steam quality

Mass fraction of vapor in a saturated liquid–vapor mixture; quality 0 is saturated liquid and quality 1 is saturated vapor.

Strain

Deformation normalized by original dimension; axial engineering strain is ΔL/L.

Stress

Internal force per unit area within a material, commonly expressed in pascals or megapascals.

Strouhal number

Dimensionless number relating vortex-shedding frequency, characteristic length and flow velocity: St = fL/V.

Subcooling

Temperature margin by which a liquid is below its saturation temperature at the local pressure.

Superheat

Temperature margin by which a vapor is above its saturation temperature at the local pressure.

T

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Thermal diffusivity

Material property α = k/(ρcp) describing how rapidly temperature disturbances diffuse through a material.

Thermal expansion

Change in material dimensions with temperature, commonly estimated using the coefficient of thermal expansion.

Thermal resistance

Resistance to heat flow, analogous to electrical resistance; for a plane layer R = L/(kA).

Three-phase power

Electrical power delivered by three AC phases displaced by 120°. Balanced three-phase real power is √3·VLL·IL·PF.

Torque

Rotational moment of force, equal to force times perpendicular lever arm, measured in N·m.

Transformer

Static electromagnetic device that transfers AC electrical energy between circuits by induction, commonly changing voltage and current while maintaining frequency.

Turbulent flow

Flow regime with strong fluctuations and mixing; in pipes it generally occurs at sufficiently high Reynolds number.

U

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U-value

Overall heat-transfer coefficient of a building assembly or heat-transfer path. Higher U-value means more heat transfer for a given area and temperature difference.

Ultimate tensile strength

Maximum engineering tensile stress reached by a material during a tensile test before necking/fracture progression.

V

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Vacuum

Pressure lower than local atmospheric pressure. Engineering vacuum levels should be stated using absolute pressure or an unambiguous vacuum reference.

Vapor pressure

Equilibrium pressure of a vapor above its liquid at a given temperature. Cavitation risk increases when local pressure approaches vapor pressure.

Velocity head

Kinetic-energy head v²/(2g) in the Bernoulli equation.

Viscosity

Measure of a fluid’s resistance to deformation or shear. Dynamic viscosity is μ; kinematic viscosity is ν = μ/ρ.

Void fraction

Fraction of a two-phase flow volume occupied by the vapor or gas phase. In BWR cores, void fraction influences moderation, reactivity and coolant density.

Voltage

Electric potential difference between two points, equal to energy transferred per unit charge; measured in volts (V).

Voltage drop

Reduction in electrical potential along a conductor due to resistance and reactance under load.

Volumetric flow rate

Volume of fluid passing a section per unit time, such as m³/s, L/s or CFM.

W

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Wet bulb temperature

Temperature indicated by a properly ventilated wetted temperature sensor under specified conditions. It reflects evaporative cooling potential; the thermodynamic wet-bulb temperature corresponds to adiabatic saturation.

Work

Energy transferred by a force acting through a distance or by an equivalent generalized force–displacement pair.

X

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Xenon-135

A strong neutron absorber produced in reactor fission-product decay chains; its changing concentration can significantly affect reactor reactivity.

Y

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Yield strength

Stress at which a material begins significant permanent (plastic) deformation according to the specified test definition.

Young’s modulus

Elastic modulus relating axial stress to axial strain in the linear-elastic range: E = σ/ε.

Z

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Z-factor

Compressibility factor used to represent deviation of a real gas from ideal-gas behavior: pV = ZnRT.