Lagging - A covering, usually of insulating material, on pipe or ducts.
Laminar flow - Very smooth flow in which all the molecules are traveling in generally the same direction. For internal flows, it occurs at Reynolds numbers less than 2000.
Lift-off - This condition occurs when the fuel or fuel/air mixture velocity is too high, thus allowing the fuel to exit the stabilizing zone before it has achieved its ignition temperature.
Lining - The material used on the furnace side of a furnace wall. It is usually high-grade refractory tile or brick or plastic refractory material.
Live steam - Steam which has not performed any of the work for which it was generated.
Low draft switch - A control to prevent the burner operation if the draft is too low. Used primarily with mechanical draft.
Low fire start - The firing of a burner with controls in a low-fire position to provide safe operating condition during light off.
Low gas pressure switch - A control to stop the burner if gas pressure is too low.
Lower heating value - LHV, The theoretical heat the combustion process can release if the fuel and oxidant are converted to 100% efficiency to CO2 and H2O vapor.
Low-oil-temperature switch - A cold-oil switch; a control to prevent burner operation if the temperature of the oil is too low.
Low-water cutoff - A device to stop the burner on unsafe water conditions in the boiler.
Lug - As applied to boiler suspension, a steel eyepiece fitted and riveted or welded to the curvature of a boiler shell or drum and connected by a steel U-bolt or sling rod to overhead steel structure; used to support the weight of a boiler.
Makeup water - The amount of raw water necessary to compensate for the amount of condensate that is not returned in the feedwater supply to the boiler.
Manhole - An access opening to the interior of a boiler, elliptical and 11 in. by 15 in. or larger or circular 15-in. diameter or larger.
Manifold - A pipe or header for collecting a fluid from, or the distributing of a fluid to a number of pipes or tubes.
Manual reset device - A component of a control which requires resetting by hand to restart the burner after safe operating conditions have been restored.
Mechanical draft - The negative pressure created by mechanical means.
Micron - One millionth of a meter, or 0.000039 in. or 1/25400 in The diameter of dust particles is often expressed in microns.
Mixer - The part of a pre-mix burner (also gas-air mixer) which uses the kinetic energy of the high velocity fuel gas stream to draw in part or all of the air required by the burner for combustion.
Mud or lower drum - A pressure chamber of a drum or header type located at the lower extremity of a water-tube boiler convection bank which is normally provided with a blowoff valve for periodic blowing off of sediment collecting in the bottom of the drum.
Multifuel burner - A burner by means of which more than one fuel can be burned either separately or simultaneously, such as pulverized fuel, oil and gas.
Natural circulation - The circulation of water in a boiler caused by differences in density; also referred to as thermal or thermally induced circulation.
Natural gas - Gaseous fuel occurring in nature.
Net fan requirements - The calculated operating conditions for a fan excluding tolerances.
Noise - An undesirable sound.
Normal cubic meter - (Nm3) The quantity of a gas that is present in 1 m3 at the thermodynamic conditions of 1 atm and 0o C. For an ideal gas there are 22.41 Nm3 in 1 kmol.
Nozzle - A short flanged or welded neck connection on a drum or shell for the outlet or inlet of fluids; also a protecting spout for the outlet or inlet of fluids; also a projecting spout through which fluid flows.
NOx - Any combination of nitrogen and oxygen in a compound form. The most common in terms of environmental considerations is NO, which constitutes 90% of combustion NOx emissions, and NO2. All NO is eventually converted to NO2 in the atmosphere. Hence, most regulations are written to assume that the NOx which is emitted is in the form of NO2. Nox emissions are influenced by many factors, including furnace temperature, flame temperature, burner design, combustion air temperature, nitrogen content of liquid fuels, ammonia content of gas fuels, and other factors.
Oil block - Usually a monolithic block located at the center of a burner assembly. The oil block acts to stabilize the oil flame.
Oil burner - A burner that atomizes fuel oil and blows it into the combustion chamber in the form of a fine mist or vapor. Steam or mechanical motion plus air may be used as the operating medium.
Oil gun - The assembly of parts in a burner which provides atomized fuel oil mixture to the furnace for burning.
Oil tip - Part of the oil gun which discharges the atomized fuel oil mixture into the furnace through multiple openings. The hole pattern in the tip has a great effect on flame size and shape.
Operating control - A control to start and stop the burner; it must be set below the high limit control.
Orifice - (1) The opening from the whirling chamber of a mechanical atomizer or the mixing chamber of a steam atomizer thru which the liquid fuel is discharged. (2) A calibrated opening in a plate, inserted in a gas stream for measuring the velocity of flow.
Orifice discharge coefficient (Cd) - The ratio of the actual flow through an orifice to that of the theoretical or isentropic flow through an orifice. Basically this parameter is a measure of the orifice efficiency. Valves are dimensionless and range from 0.61 for a thin-plate orifice to 0.85 for thick plate square-edged orifices, and up top 0.90 - 0.95for tapered orifices.
Orsat - An instrument for determining the chemical analysis of flue gas.
Oxidation - Chemical combination with oxygen.
Oxygen attack - Corrosion or pitting in a boiler caused by oxygen.