Power Engineering Dictionary

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SACRIFICIAL ANODES - Coupling of a more active metal to a structure resulting in a galvanic current flow through the corroding electrolyte.

SACRIFICIAL PROTECTION - Reduction of corrosion of a metal in an electrolyte by galvanically coupling it to a more anodic metal. A form of cathode protection.

SADDLE VALVE (TAP-A-LINE) - Valve body shaped so it may be silver brazed or clamped onto a refrigerant tubing surface.

SADDLE VALVE (TAP-A-LINE) - Valve body shaped so it may be silver brazed or clamped onto a refrigerant tubing surface.

SAFETY CAN - Approved container of not more than 5 gallon capacity. It has a spring closing lid and spout cover. It is designed to relieve internal pressure safely when exposed to fire.

SAFETY CONTROL - Device to stop refrigerating unit if unsafe pressure and/or temperatures and/or dangerous conditions are reached.

SAFETY PLUG - Device which will release the contents of a container before rupture pressures are reached.

SALINITY - A measure of the concentration of dissolved mineral substances in water.

SALT SPITTING - The ability of an anion exchanger to convert a salt solution to caustic; the ability of a cation exchanger to convert a salt solution to acid.

SATURATED AIR - When the air cannot hold any more moisture.                                                                                                                                                                                                  

SATURATED LIQUID - A liquid which is at saturation pressure and saturation temperature; in other words, a liquid which is at its boiling point for any given pressure.

SATURATED VAPOR - A vapor which is at saturation pressure and saturation temperature. A saturated vapor cannot be superheated as long as it is in contact with the liquid from which it is being generated.

SATURATION - Condition existing when substance contains all of another substance it can hold for that temperature and pressure.

SATURATION INDEX - The relation of calcium carbonate to the pH, alkalinity, and hardness of a water to determine its scale forming tendency.

SATURATION PRESSURE - The point, where at a given temperature a pure substance starts to boil.

SATURATION TEMPERATURE - The point, where at a given pressure a pure substance starts to boil.

SAYBOLT UNIVERSAL VISCOSITY - A commercial measure of viscosity expressed as a time in seconds required for 60 mL of a fluid to flow through the orifice of the standard Saybolt universal viscometer at a given temperature under specific conditions; used for the lighter petroleum products and lubrication oils.

SCALE - Surface oxidation, consisting of partially adherent layers of corrosion products, left on metals by heating or casting in air or in other oxidizing atmosphere. Also a deposit on a heat-transfer surface resulting from precipitation of salts present in water in contact with that surface, forming a hard, dense material.

SCALE CAUSING ELEMENTS - Calcium and magnesium elements forming scale.

SCALE REMOVAL - Waterside, removal of scale using either the mechanical, the water treatment or the acid cleaning process.

SCC - Stress corrosion cracking.

SCREENS - Equipment designed to prevent larger objects to enter water treatment system.

SCREW PUMP - Compressor constructed of two mated revolving screws.


SCRUBBER - An apparatus for the removal of solids from gases by entrainment in water.

SEAL, MAGNETIC - A seal that uses magnetic material, instead of springs or bellows, to provide the closing force.

SEAL, ROTARY - A mechanical seal that rotates with a shaft and is used with a stationary mating ring.

SECOND LAW OF THERMODYNAMICS - Heat will flow only from material at higher temperature to material at lower temperature.

SECONDARY REFRIGERANT - Chilled liquid-like water, which is circulated to distance units where air is to be cooled in individual rooms.

SECONDARY SYSTEM - A re-circulating water system that is a takeoff from a primary system; it does not circulate directly through the source of heat or cooling but only indirectly through a heat exchanger.

SECONDARY TREATMENT - Treatment of boiler feedwater or internal treatment of boiler water after primary treatment.

SEDIMENTATION - Gravitational settling of solid particles in a liquid system.

SEEBECK EFFECT - When two different adjacent metals are heated, an electric current is generated between the metals.

SENSIBLE HEAT - Heat which causes a change in temperature of a substance.

SENSIBLE HEAT - Sensible heat is any heat transfer that causes a change in temperature. Heating and cooling of air and water that may be measured with a thermometer is sensible heat. Heating or cooling coils that simply increase or decrease the air temperature without a change in moisture content are examples of sensible heat.

SENSIBLE HEAT FACTOR - The ratio of sensible heat to total heat.

SENSING DEVICE - A device that keeps track of the measured condition and its fluctuations so that when sufficient variation occurs it will originate the signal to revise the operation of the system and offset the change. Example: a thermostat "bulb". A sensing de vice may be an integral part of a controller.

SENSING ELEMENT - The first system element or group of elements. The sensing element performs the initial measurement operation.

SEPARATOR - A tank type pressure vessel installed in a steam pipe to collect condensate to be trapped off and thus providing comparatively dry steam to the connected machinery.

SEPARATOR - Device to separate one substance from another.

SEQUENCER - A mechanical or electrical device that may be set to initiate a series of events and to make the events follow in sequence.

SEQUESTER - To form a stable, water-soluble complex.

SEQUESTRANT - A substance that holds a mineral or metal in solution beyond its saturation point.

SERIES CIRCUIT - One with all the elements connected end to end. The current is the same throughout but the voltage can be different across each element.

SERVICE DROP - The overhead service wires that serve a building.

SERVICE SWITCH - Disconnect switches or circuit breakers. Purpose is to completely disconnect the building from the electric service.

SERVICE VALVE - Manually operated valve mounted on refrigerating systems used for service operation.

SET POINT - The value of the controlled condition at which the instrument is set to operate.

SETTLING BASIN - A containment design with external water treatment to settle sediments and to clarify.

SHAFT SEAL - Device used to prevent leakage between shaft and housing.

SHARP FREEZING - Refrigeration at temperatures slightly below freezing, with moderate air circulation.


SHED - To de-energize a load in order to maintain a kW demand set point.

SHED MODE - A method of demand control that reduces kW demand through shedding and restoring loads.

SHELL AND TUBE FLOODED EVAPORATOR - Device which flows water through tubes built into cylindrical evaporator or vice versa.

SHELL TYPE CONDENSER - Cylinder or receiver which contains condensing water coils or tubes.

SHELL-AND-TUBE - Designation of a heat exchanger having straight tubes encased inside a shell.

SHIELDED CABLE - Special cable used with equipment that generates a low voltage output. Used to minimize the effects of frequency "noise" on the output signal.

SHOCK DOSAGE - The feeding of treatment to a system all in one slug or dose rather than gradually (also called a slug dosage).

SHOCK FEEDER - A device which is used to add treatment to a system in an instantaneous manner.

SHORT CIRCUIT - A direct connection of low resistive value that can significant alter the behavior of an element or system.

SHORT CYCLING - Unit runs and then stops at short intervals; generally this excessive cycling rate is hard on the system equipment.

SHROUD, TURBINES - Also referred to as the sealing strip on turbine blades. Its purpose is to supply rigidity to the blades, lessen vibration and provide sealing between stages.

SHROUD, PUMPS - The front and/or back of an impeller.

SHUNT - A device to divert part of an electric current.

SIGHT GLASS - Glass tube or glass window in refrigerating mechanism. It shows amount of refrigerant or oil in system and indicates presence of gas bubbles in liquid line.

SILICA - Silicon dioxide (Si02), a mineral found naturally as quartz or in complex combination with other elements such as silicates.

SILICA GEL - Absorbent chemical compound used as a drier. When heated, moisture is released and compound may be reused.

SILICA, COLLOIDAL - Silica in colloidal form.

SILICA, VOLATILE - Silica carryover with steam.

SILT DENSITY INDEX - A measure of the tendency of a water to foul a reverse osmosis membrane, based on time flow through a membrane filter at constant pressure.

SIMPLE CYCLE - Referring to the gas turbine cycle consisting only of compression, combustion and expansion.

SINGLE PHASING - The condition when one phase of a multiphase (poly-phase) motor circuit is broken or opened. Motors running when this occurs may continue to run but with lower power output and over heating.

SINGLE SHAFT GAS TURBINE - A gas turbine arrangement in which the compressor and the gas turbine are all coupled to one shaft.

SINGLE STAGE COMPRESSOR - Compressor having only one compressive step between inlet and outlet.

SKIN CONDENSER - Condenser using the outer surface of the cabinet as the heat radiating medium.

SLIME - A soft, sticky, mucus-like substance, originating from a bacterial growth.

SLING PSYCHROMETER - Measuring device with wet and dry bulb thermometers. Moved rapidly through air it measures humidity.

SLUDGE - A deposit on a heat-transfer surface that does not have the hard, crystalline structure of a scale but is softer and less dense.

SLUG - A unit of measure for mass in the English system, which equals 14.6 kg in the SI system.

SLUGGING - Condition in which mass of liquid enters compressor causing hammering.

SLURRY - A water containing high concentration of suspended solids, usually over 5000 mg/L.

SLURRY EROSION - Material removal due to the combined action of corrosion and wear.

SODA ASH - A common water treatment chemical, sodium carbonate.

SODIUM CHLORIDE - Common table salt, used to produce a brine solution, used a secondary refrigerant.

SODIUM SULFITE (Na2S03) - A chemical used with water treatment to remove small amounts of oxygen.

SODIUM TRACER METHOD - A technique used to measure dissolved solids in steam to values as low as 0.001 ppm.

SODIUM ZEOLITE SOFTENING - The process of removing scale forming ions of calcium and magnesium and replacing them with the equivalent amount of sodium ions.

SOFT WATER - Water that is free of magnesium or calcium salts.

SOFTENING - The removal of hardness (calcium and magnesium) from water.

SOLAR HEAT - Heat created by visible and invisible energy waves from the sun.

SOLENOID VALVE - Electromagnet with a moving core. It serves as a valve or operates a valve.

SOLID ABSORBENT REFRIGERATION - Refrigeration system which uses solid substance as absorber of the refrigerant during the cooling part of the cycle and releases refrigerant when heated during generating part of cycle.

SOLID STATE HALOGEN LEAK DETECTOR - An electronic leak detector for all halogen. related refrigerants.

SOLUBLE IRON - Usually present in cooling water systems and can arise from metallurgical corrosion.

SOLU-BRIDGE - An electronic instrument used to measure conductivity of a water sample to determine the dissolved solids content.

SOUR GAS - A gaseous environment containing hydrogen sulfide and carbon dioxide in hydrocarbon reservoirs.

SOUR WATER - Waste water containing malodorous materials, usually sulfur compounds.

SPARGER - An extension into the bottom of a tank of a pipe which has a distribution nozzle on the end for mixing one fluid with another.

SPECIFIC CONDUCTANCE - Measures the ability of a water to conduct electricity. Conductivity increases with total dissolved solids and is therefore used to estimate dissolved solids present in the water.

SPECIFIC GRAVITY - The density of a substance compared to the density of a standard material such as water.

SPECIFIC HEAT - Ratio of quantity of heat required to raise temperature of a body 1 degree to that required to raise temperature of equal mass of water 1 degree.

SPECIFIC HEAT (Cp) - The ratio of the mount of heat required to raise a mass of material 1 degree in temperature to the amount required to raise equal mass of reference substance, usually water, 1 degree in temperature.

SPECIFIC HUMIDITY - Ratio of weight of vapor to the weight of gas in a unit volume of an air-water vapor mixture.

SPECIFIC VOLUME - Volume per unit mass of a substance.


SPLASH SYSTEM, OILING - Method of lubricating moving parts by agitating or splashing oil in the crankcase.

SPLIT-STREAM DEALKALIZER - Where the flow of water is divided through a parallel arrangement of hydrogen and sodium cat ion exchanger. The combined product being soft and low in alkalinity.

SPRAY CARRYOVER - Are referred to a mist or fog and are a degree of atomization of the boiler water and carried with the steam. This type of carryover is to be prevented by the drum internals.

SPRAY COOLING - Method of refrigerating by spraying expendable refrigerant or by spraying refrigerated water.

SPRAY MANIFOLD - A pipe, or extension of a water line, that has several openings fitted with nozzles which spray water.

SPRAY-COIL UNIT - A cooling circuit that sprays water over cooling coils through inflowing air to humidify or dehumidify that air, as required.

SPRAY-COOLING CIRCUIT - An open cooling-water circuit which sprays water and cools by evaporation, for example, a cooling tower, all evaporative condenser, an air washer, or a spray-coil unit.

Spread: The divergence of the air stream in a horizontal or vertical plane after it leaves the outlet.

STABILITY INDEX - An imperical modification of the saturation index used to predict scaling or corrosive tendencies in water systems.


STANDARD AIR CONDITIONS - Standard air density has been set at 0.075 Ib/cu ft. This corresponds approximately to dry air at 70F and 29.92 in Hg. In metric units, the standard air density is 1.2041 kg/m3 at 20C and at 101.325 kPa.

STANDARD CONDITIONS - The standard conditions referred to in environmental system work for air are: dry air at 70F and at an atmospheric pressure of 29.92 inches mercury (in Hg). For water, standard conditions are 68F at the same barometric pressure. At these standard conditions, the density of air is 0.075 pounds per cubic feet and the density of water is 62.4 pounds per cubic foot.

STATE - Refers to the form of a fluid, either liquid, gas or solid. Liquids used in environmental systems are water, thermal fluids such as ethylene glycol solutions, and refrigerants in the liquid state. Gases are steam, evaporated refrigerants and the air-water vapor mixture found in the atmosphere. Some substances, including commonly used refrigerants, may exist in any of three states. A simple example is water, which may be solid (ice), liquid (water), or gas (steam or water vapor).

STATIC HEAD - The pressure due to the weight of a fluid above the point of measurement.

STATIC SUCTION HEAD - The positive vertical height in feet from the pump centerline to the top of the level of the liquid source.

STATIC SUCTION LIFT - The distance in feet between the pump centerline and the source of liquid below the pump centerline.

STEAM - Water in vapor state.

STEAM DRUM - A pressure chamber located at the upper extremity of a boiler circulatory system, in which the steam is generated in the boiler and separated from the water.

STEAM JET REFRIGERATION - Refrigerating system which uses a steam venturi to create high vacuum (low pressure) on a water container causing water to evaporate at low temperature.

STEAM PURITY - Refers to all matter but water in the steam.

STEAM QUALITY - The percentage by weight of vapor in a steam and water mixture.

STEAM-ABSORPTION CONDENSER - That part of a steam-absorption machine in which the water refrigerant is condensed by cooling-tower water and returned to the evaporator or chiller.

STEAM-ABSORPTION MACHINE - A refrigeration or air-conditioning machine which uses, as a refrigerant, water evaporated by absorption in a brine regenerated by steam and condensed by cooling-tower water.

STICTION (STATIC FRICTION) - Resistance of start of motion.

STOICHIOMETRIC - The ratio of chemical substances reacting in the water that correspond to their combining weights in the theoretical chemical reaction.

Stratified Air: Unmixed air in a duct that is in thermal layers that have temperature variations of more than five degrees.

STRESS RAPTURE - A general type of damage referring to carbon steel tubing, when heated above 450C. Material will plastically deform (creep) and then rapture.

STUFFING BOX - That portion of the pump which houses the packing or mechanical seal, The stuffing box is usually referred to as the dry portion of the pump, and is located in back of the impeller and around the shaft.

SUBCOOLING - The difference between the temperature of a pure condensable fluid below saturation and the temperature at the liquid saturated state, at the same pressure.

SUBCOOLING - The process of cooling a liquid to a temperature below its saturation temperature for any given saturation pressure.

SUBLIMATION - A change of state directly from solid to gas without appearance of liquid.

SUBLIMATION - Condition where a substance changes from a solid to a gas without becoming a liquid.

SUCTION HEAD - The positive pressure on the pump inlet when the source of liquid supply is above the pump centerline.

SUCTION LIFT - The combination of static suction lift and friction head in the suction piping when the source of liquid is below the pump centerline.

SUCTION LINE - Tube or pipe used to carry refrigerant gas from evaporator to compressor.

SUCTION PRESSURE - Pressure in low-pressure side of a refrigerating system.

SUCTION PRESSURE CONTROL VALVE - Device located in the suction line which maintains constant pressure in evaporator during running portion of cycle.

SUCTION PRESSURE CONTROL VALVE - Device located in the suction line which maintains constant pressure in evaporator during running portion of cycle.

SUCTION SERVICE VALVE - Two-way manually operated valve located at the inlet to compressor. It controls suction gas flow and is used to service unit.

SUCTION SIDE - Low-pressure side of the system extending from the refrigerant control through the evaporator to the inlet valve of the compressor.

SULFATE - A compound, ion, or salt of sulfur and oxygen, such as sodium sulfate (Na2S04).

SULFITE DECOMPOSITION - Sodium sulfite, which is used as an oxygen scavenger, may decomposes with higher temperatures and concentration. The decomposition results in forming sulfur dioxide and thus leading to an acidic anhydride causing corrosion.

SULFONIC - A specific acidic group (SO3H) on which depends the exchange activity of certain cation adsorbents.

SULFUR DIOXIDE (SO2) - An old refrigerant.

SUN EFFECT - Solar energy transmitted into space through windows and building materials.

SUPERHEAT - The heat added to a fluid above its saturation point.

SUPERHEATED STEAM - Steam heated above its saturation temperature.


SUPERHEATED VAPOR - A vapor which is not about to condense.

SUPERHEATER - Heat exchanger arranged to take heat from liquid going to evaporator and using it to superheat vapor leaving evaporator.

SUPERHEATING - The process of adding heat to a vapor in order to raise its temperature above saturation temperature. It is impossible to superheat a saturated vapor as long as it is in contact with the liquid from which it is being generated; hence the vapor must be led away from the liquid before it can be superheated.

SUPERSATURATED SOLUTION - A solution of a salt or mineral with a concentration beyond the normal saturation point.

SURFACE BLOWDOWN - Removal of water, foam, etc. from the surface at the water level in a boiler.

SURFACE HEATING - The exterior surface of a heating unit. Extended heating surface (or extended surface), consisting of fins, pins, or ribs which receive heat by conduction from the prime surface. Prime surface: heating surface having the heating medium on one side and air (or extended surface) on the other.

SURFACE-SPRAY UNIT - A spray-coil unit.

SURFACTANT - A compound that affects interfacial tension between two liquids. It usually reduces surface tension.

SURFACTANTS - A wetting agent used to prevent fouling, mainly in water cooling systems.

SURGE - The sudden displacement or movement of water in a closed vessel or drum.

SURGE SURPRESSOR - A device that reduces harmonic distortion in line voltage circuits by clipping off transient voltages which are fed through the power lines from operating equipment.

SURGE TANK - Container connected to the low-pressure side of a refrigerating system which increases gas volume and reduces rate of pressure change.

SUSPENDED SOLIDS - Un-dissolved solids in boiler water.

SWAMP COOLER - Evaporative type cooler in which air is drawn through porous mats soaked with water.

SWELLING - The expansion of an ion-exchange which occurs when the reactive groups on the resin are converted from one form to another.

SYNERGISM - The combined action of several chemicals which produce an effect greater than the additive effects of each.

SYSTEM - A series of ducts, conduits, elbows, branch piping, etc. designed to guide the flow of air, gas or vapor to and from one or more locations. A fan provides the necessary energy to overcome the resistance to flow of the system and causes air or gas flow through the system. Some components of a typical system are louvers, grilles, diffusers, filters, heating and cooling coils, air pollution control devices, burner assemblies, volume flow control dampers, mixing boxes, sound attenuators, the ductwork and related fittings.

SYSTEM, CENTRAL FAN - A mechanical, indirect system of heating, ventilating, or air conditioning, in which the air is treated or handled by equipment located outside the rooms served, usually at a central location, and conveyed to and from the rooms by means of a fan and a system of distributing ducts.

SYSTEM, CLOSED - A heating or refrigerating piping system in which circulating water or brine is completely enclosed, under pressure above atmospheric, and shut off from the atmosphere except for an expansion tank.

SYSTEM, DUCT - A series of ducts, conduits, elbows, branch piping, etc. designed to guide the flow of air, gas or vapor to and from one or more locations. A fan provides the necessary energy to overcome the resistance to flow of the system and causes air or gas to flow through the system. Some components of a typical system are louvers, grilles, diffusers, filters, heating and cooling coils energy recovery de vices, burner assemblies, volume dampers, mixing boxes, sound attenuators, the ductwork and related fittings.

SYSTEM, FLOODED - A system in which only part of the refrigerant passing over the heat transfer surface is evaporated, and the portion not evaporated is separated from the vapor and recirculated.

SYSTEM, UNITARY - A complete, factory-assembled and factory-tested refrigerating system comprising one or more assemblies which may be shipped as one unit or separately but which are designed to be used together.

SYSTEMS CURVE - A graphic presentation of the pressure vs. volume flow rate characteristics of a particular system.

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