Tempering air - Air at a lower temperature added to a stream of pre-heated air to modify its temperature.
Tertiary air - Air for combustion supplied to the furnace to supplement the primary and secondary air.
Theoretical air - The quantity of air required for perfect combustion.
Theoretical draft - The draft which would be available at the base of a stack if there were no friction or acceleration losses from the stack.
Theoretical flame temperature - Same as "adiabatic temperature".
Therm - A unit of heat applied especially to gas. One therm = 100,000 Btu.
Thermal conductivity - The ability of a material to conduct heat, expressed as thermal power conducted per unit temperature and thickness. Metals and other thermal "conductors" have a large thermal conductivity. Refractories and other thermal "insulators" have a low thermal conductivity.
Thermal NOx - NOx formed via the Zeldovich mechanism. The rate-limiting step in this mechanism is the formation of the O radical. This occurs only at high temperatures (above about 2400o F.). Hence the term thermal NOx, since it is NOX produced in the highest temperature regions of the flame.
Thermo acoustical efficiency - Equal to the sound power level/heat release. A value used to characterize the amount of combustion noise emitted from a flame. Defined as the ratio of the acoustical power emitted from the flame to the total heat release of the flame. Approximately equal to 1 X 10-6 for premixed and turbulent flames and equal to 1 X 10-9 for diffusion and laminar flames.
Thermocouple - A temperature measuring instrument.
Throat - The neck portion of a passageway.
Through stay - A brace used in fire-tube boilers between the heads or tube sheets.
Tie plate - A plate, through which a bolt or tie rod is passed to hold brick in place.
Tie rod - A tension member between buckstays or tie plates.
Tile - A preformed, burned refractory, usually applied to shapes other than standard brick.
Time delay - A deliberate delay of a predetermined time in the action of a safety device or control.
Total air - The total quantity of air supplied to the fuel and products of combustion. Percent total air is the ratio of total air to theoretical air expressed as per cent.
Total pressure - The sum of the static and velocity pressures.
Total solids concentration - The weight of dissolved and suspended impurities in a unit weight of boiler water, usually expressed as ppm.
Tramp air - Any air that enters (infiltrates) the furnace through leaks. This air may be measured by the O2 analyzer and often contributes to the burning of the fuel.
Trap - A receptacle for the collection of undesirable material.
Treated water - Water which has been chemically treated to make suitable for boiler feed.
Trail-for-ignition - That period of time during which the programming flame failure controls permit the burner fuel valves to be open before the flame sensing device is required to detect the flame.
Trail for main flame ignition - A timed interval when with the ignition means proved, the main valve is permitted to remain open. If the main burner is not ignited during this period, the main valve and ignition means are cut off. A safety switch lockout follows.
Trail for pilot ignition - A timed interval when the pilot valve is held open and an attempt made to ignite and prove it. If the presence of the pilot is proved at the termination of the interval, the main valve is energized; if not the pilot and ignition are cut off followed by a safety lockout.
Try cock - One of three valves mounted on a boiler or water column within the visible range of the gauge glass and used to check the water level.
Tube - A hollow cylinder for conveying fluids.
Tube cleaner - A device for cleaning tubes by brushing, hammering, or by rotating cutters.
Tube plug - A solid plug driven into the end of a tube.
Turbulent burner - A burner in which fuel and air are mixed and discharged into the furnace in such a manner as to produce turbulent flow from the burner.
Turbulent flow - Characteristically random flow patterns that form eddies from large to small scales. For internal flows, it occurs at Reynolds numbers greater than 4000. Turbulence is integral to the mixing process between the fuel and air for combustion.
UHC - Any unburned hydrocarbon that is emitted in a combustion process. Also termed VOC (volatile organic compound).
Unaccounted for loss - That portion of a boiler heat balance, which represents the difference between 100 per cent and the sum of the heat absorbed by the unit and all the classified losses expressed as per cent.
Unburned combustible - The combustible portion of the fuel, which is not completely oxidized.
Unfired pressure vessel - A vessel designed to withstand internal pressure, neither subjected to heat from products of combustion nor an integral part of a fired pressure vessel system.
Use factor - The ratio of hours in operation to the total hours in that period.
Vane - A fixed or adjustable plate inserted in a gas or air stream used to change the direction of flow.
Vane control - A set of movable vanes in the inlet of a fan to provide regulation of airflow. Vane guide - A set of stationary vanes to govern direction, velocity and distribution of air or gas flow.
Valve, manual gas shutoff - A manually operated valve in a gas line for the purpose of completely turning on or shutting off the gas supply.
Valve, manual oil shutoff - A manually operated valve in the oil line for the purpose of completely turning on or shutting off the oil supply to the burner.
Valve, manual reset safety shutoff - A manually opened, electronically latched, electrically operated safety shut-off valve designed to automatically shut off fuel when de-energized.
Valve, motor driven reset safety shutoff - An electrically operated safety shut-off valve designed to automatically shut off fuel flow upon being de-energized. The valve is opened and reset automatically by integral motor device only.
Valve, fuel control - An automatically or manually operated device consisting essentially of a regulating valve and an operating mechanism. It is used to regulate fuel flow and is usually in addition to the safety shut-off valve. Such valve may be of the automatic or manually opened type.
Vapor - The gaseous product of evaporation.
Vapor generator - A container of liquid, other than water, which is vaporized by the absorption of heat.
Vaporization - The change from liquid or solid phase to the vapor phase.
Velocity pressure - The measure of kinetic energy of a fluid.
Velocity thermocouple - (suction pyrometer) a device for measuring furnace gas temperature. It is comprised of a thermocouple, which has been recessed into an insulating shroud, and a suction device such as an eductor, which aspirates large volumes of furnace gas through the shroud and past the thermocouple. The high velocity of a gas ensures good convective heat transfer to the thermocouple and surrounding furnace. The velocity thermocouple represents the most accurate means to measure flue gas temperature. Bare thermocouples are unacceptable for this purpose, being in error often by more than 100o F due to radiation losses.
Vent - An opening in a vessel or other enclosed space for the removal of gas or vapor.
Vertical firing - An arrangement of a burner such that air and fuel are discharged into the furnace, in practically a vertical direction.
Viscosity - Measure of the internal friction of a fluid or its resistance to flow.
Volatile matter - Those products given off by a material as gas or vapor, determined by definite prescribed methods.
Volume of air - The number of cubic feet of air per minute expressed at fan outlet conditions.
Vortex - (1) The swirling motion of a liquid in a vessel at the entrance to a discharge nozzle. (2) The point in a cyclonic gas path where the two spirals change general direction by 180o.