NAPHTA - A volatile, flammable liquid hydrocarbon distilled from petroleum and used as a solvent or fuel.

NATURAL CIRCULATION - The circulation of a boiler caused by differences in density. Also referred as thermal or thermally induced circulation.

NATURAL CONVECTION - Movement of a fluid caused only by temperature differences (density changes).

NATURAL DRAFT COOLING TOWER - Cools water by moving air at low velocities.

NATURAL GAS - A highly compressible, highly expandable mixture of hydrocarbons having a low specific gravity and occurring naturally in gaseous form. Besides hydrocarbon gases, natural gas may contain quantities of nitrogen, helium, carbon dioxide, hydrogen sulfide and water vapor.

NATURALLY ASPIRATED - A term used to describe a diesel engine in which air flows into the engine by means of atmospheric pressure only.

NC - Normally closed contacts of a relay.

NEGATIVE CHARGE - The electrical potential which an atom acquires when it gains one or more electrons; a characteristic of an anion.

NET POSITIVE SUCTION - The difference between total pressure and vapor pressure in a fluid flow, expressed in terms of equivalent height or "head".

NEUTRAL - The circuit conductor that is normally grounded or at zero voltage difference to the ground.

NEUTRALIZATION NUMBER - An ASTM number given to quenching oils that reflect the oil’s tendency toward oxidation and sludging.

NEUTRALIZER - A substance that will combine with an acid or alkali chemically, thus removing the acidity or alkalinity.

NEUTRALIZING AMINES - Are amines used to neutralize the acid generated by the dissolution of carbon dioxide.

NEWTON - The unit of force in the metric system.  A Newton is the force required to accelerate an object of 1 kilogram mass to a velocity of 1 meter per second in 1 second.

NIPPLE - A short, threaded tubular coupling, used for making connections between pipe joints.

NITROGEN BLANKETING - Used with wet standby, where the space above the water level is filled with nitrogen at about 5 to 10 psig in order to keep the oxygen out.

NITROGEN DIOXIDE - Mildly poisonous gas (NO₂) often found in smog or automobile exhaust

NO - Normally open contacts of a relay.

NOBEL METAL - A chemically inactive metal, such as gold.

NO-FROST FREEZER - Low-temperature refrigerator cabinet in which no frost or ice collects on freezer surfaces or materials stored in cabinet.

NOISE - Any undesired sounds, usually of different frequencies, resulting in an objectionable or irritating sensation.

NOMINAL SIZE - A designated size that may be different from the actual size.

NOMINAL SIZE TUBING - Tubing measurement which has an inside diameter the same as iron pipe of the same stated size.

NON-CARBONATED HARDNESS - Hardness in water caused by chlorides, sulfates, and nitrates of calcium and magnesium.

NON-CODE INSTALLATION - Functional refrigerating system installed where there are no local, state, or national refrigeration codes in force.

NON-CONDENSABLES - Gaseous material not liquefied when associated water vapor is condensed in the same environment.

NON-DESTRUCTIVE INSPECTION (NDI) - A procedure such as ultrasonic or radiographic inspection, for determining the quality of a material without permanently altering anything.

NON-FERROUS ALLOY - Alloy containing less then 50 % iron.

NON-FROSTING EVAPORATOR - Evaporator which never collects frost or ice on its surface. Uses only thermostatic expansion valves.

NON-MECHANICAL REFRIGERATION - Those that obtain the required high and low pressure by some method other than a mechanical compressor.

NON-REACTIVE SILICA - Is a polymeric form of silica; thermally unstable which reverts to normal silica when heated. Difficult to detect, but may be present when boiler feedwater shows none, but boiler water reads silica.

NORMAL CARRYOVER - Refers to the carryover which occurs in any boiler operating under the best conditions.

NORMAL CHARGE - Thermal element charge which is part liquid and part gas under all operating conditions.

NORMALLY OPEN - (OR NORMAL CLOSED) The position of a valve, damper, relay contacts, or switch when external power or pressure is not being applied to the device. Valves and dampers usually are returned to a "normal" position by a spring.

NOZZLE GOVERNING -

NTU - Nephelometric turbidity unit. A light -interference analytical method to measure the turbidity of water.

NUCLEATE BOILING - The even boiling of water in which steam bubbles are formed within the boiler water gradually and are evenly distributed rather than being suddenly formed and erratically distributed.