Glossary of Terms


Amphoteric Metal: A metal that is susceptible to corrosion in both acid and alkaline environments.

Anion: A negatively charged ion that migrates through the electrolyte (water) towards the anode under the influence of a potential gradient.

Anode: The electrode of an electrochemical cell at which oxidation occurs. Electrons flow away from the anode in the external circuit. Corrosion usually occurs and metal ions enter the solution at the anode.

Aerobic: In the presence of air or combined oxygen.

Blowdown: The process of discharging a significant portion of the aqueous solution (cooling water) to remove accumulated salts, deposits, and other impurities.

Cation: A positively charged ion that migrates through the electrolyte (water) toward the cathode under the influence of a potential gradient.

Conductivity: The capacity to conduct electricity in an electrolyte (water).

Corrosion: The deterioration of a material (usually a metal) that results from a reaction with its environment.

Corrosion Inhibitor: A chemical substance or combination of substances that, when present in the environment, prevents or reduces corrosion.

Corrosion Rate: The rate at which corrosion proceeds.

Corrosiveness (corrosivity): The tendency for an environment to cause corrosion.

Fouling: An accumulation of deposits on a surface.

Ion: An electrically charged atom or group of atoms.

Microbiocide: Any chemical that will kill microorganisms. Used synonymously with biocide and bactericide.

Oxidation: Loss of electrons by a constituent of a chemical reaction.

Pitting: Localized corrosion of a metal surface that is confined to a small area and takes the form of cavities called pits.

Rust: Corrosion products consisting of various iron oxides and hydrated iron oxides.

Scaling: The deposition of water-insoluble constituents on a metal surface.

Slime: An extracellular material produced by some microorganisms, characterized by a slimy consistency of various chemical compositions.


Alkalinity: A measure of the ability to neutralize hydrogen ion (acidity). In most cases the alkalinity is equal to the bicarbonate, carbonate, and hydroxide content of the water; it may also include phosphate species. Bicarbonate alkalinity breaks down in a boiler to carbonate, carbon dioxide, and hydroxide.

Amine: A chemical characterized by having as its primary functional group a nitrogen atom attached to a least one carbon atom; used for neutralizing acidity, especially carbon dioxide from the breakdown of alkalinity.

Blowdown: That portion of the boiler water which is periodically removed to maintain boiler cycles; blowdown is continuous and/or intermittent, often to a blowdown flash tank for heat recovery.

Boiler: In general, a piece of equipment consisting of a combustion chamber and a water-containing chamber for the purpose of converting water to steam.

Boiler feed pump: The feed pump that pressurizes the preheated feedwater for entry into the boiler section.

Carryover: Liquid water which exits the boiler. It carries all the dissolved solids that are present in the boiler water, which is generally undesirable. It is produced by poor water/steam separation in the steam drum due to any of a variety of causes.

Condensate: Condensed steam that may be returned to the feedwater loop of the boiler circuit.

Condensate Return: That fraction of steam that is returned as a condensate to the boiler, including extraction steam.

Cycles: The ratio of the concentration of a non-volatile dissolved species in the boiler liquid to that in the feedwater liquid; a measure of the degree of concentration.

Deaerator: A device for the mechanical removal of dissolved gases, especially oxygen, in boiler feedwater; functioning on the principle of reduced solubity of most dissolved gases as temperature increases, and providing for a mechanical means of removing the gases from the system.

Density: The mass of substance divided by its volume, expressed in the units of the measurement.

Dissolved solids: Non-volatile components (solutes) in a liquid phase which do not exist as a separate phase.

Drum Steam: The steam contained in the drum; i.e. saturated steam, also containing physically entrained water.

Expansion tank: A ballast tank which allows for volume changes in the water contained in a hot water heater (boiler), so as to prevent water hammer, and allow for the controlled flow of hot water.

Feedwater: Water which has been conditioned for entry into a boiler; usually chemically treated and heated to near boiling temperature at the throttle pressure of the boiler.

Hardness: Ions of polyvalent metal atoms which form insoluble species with carbonate, usually expressed as CaCO3 equivalent.

Heat Exchanger: A mechanical device for the transfer of heat energy from one phase to another, usually a shell and tube container in which both process and working liquids are circulated in separate spaces separated by a metallic conductor.

HVAC: Heating, Ventilation and Air Conditioning

Internal Treatment: Chemical treatment added to the boiler, as opposed to the feedwater or condensate; usually a precipitating treatment, chelant, all-polymer, combination, or coordinated phosphate.

Makeup: Any fresh water added to the boiler circuit to make up for losses due to blowdown or non-returned steam; usually added to the steam drum.

Oxygen Scavenger: A chemical substance that is capable of reacting with oxygen under feedwater conditions so as to reduce the level of dissolved oxygen below that achievable by mechanical deaeration.

Passivation: The process whereby a metal is converted to a passive phase, such as iron into magnetite.

Polymer: A chemical substance characterized by the presence of a large number of repeating units; in water treatment, polymers are used for dispersion or flocculation.

Steam: The gaseous phase of water.

Steam Drum: The boiler element consisting of a horizontal metal tube with risers, a downcomer, internal elements for chemical treatment addition, makeup water addition, level control, and a steam exit; used to promote efficient separation of the steam from the water in the boiler.

Steam Generator: A vessel in which steam is produced by a liquid/liquid or gas/liquid heat exchange, as opposed to direct firing.

Steam Quality: The attribute of steam that expresses the fraction of liquid moisture in the steam.


Cycle: A complete course of ion exchange operation, from backwash through brining, rinsing, and back to operation.

Effluent: The solution which emerges from an ion exchange column.

Elution: The stripping of adsorbed ions from an ion exchange material by the use of solutions containing other ions in relatively high concentrations.

Exhaustion: The state in which the resin is no longer capable of useful ion exchange; the depletion of the exchanger’s supply of available ions. The exhaustion point is determined arbitrarily in terms of: (a) a value in parts per million of leakage; and (b) the reduction in quality of the effluent water determined by some test parameter, such as conductivity.

Flow Rate: The volume of solution passing through a certain quantity of resin within a given time. Usually expressed in terms of gallons per minute per cubic foot of resin, or as gallons per minute per square foot of bed area.

Hydrogen Cycle: A complete course of cation exchange operation in which the active regenerant used is hydrogen-based, such as sulfuric acid.

Raw Water: Untreated water from wells or surface sources.

Regenerant: The solution used to restore the activity of an ion exchange. Acids are employed to restore a cation exchanger to its hydrogen form; brine solutions may be used to convert the cation exchanger to the sodium form.

Rinse: The operation which follows regeneration; a flushing out of excess regenerant solution.