DAMPING: (1) A characteristic of a system that results in dissipation of energy and causes decay in oscillations. (2) The negative feedback of an output rate of change.

DAY TANK: A fuel tank with the capacity to operate an engine for 24 hours. Also called SERVICE TANK.

DEAD CENTER: Either of the two positions when the crank and connecting rod are in a straight line at the end of the stroke.

DEAERATE: Process of removing dissolved oxygen.

DEAERATING FEED TANK (DFT): A unit in the steam-water cycle used to (1) free the condensate of dissolved oxygen, (2) heat the feedwater, and (3) act as a reservoir for feedwater.

DEGREE OF SUPERHEAT: The amount by which the temperature exceeds the saturation temperature.

DEHUMIDIFICATION: The mechanical process of removing water vapor from the air.

DENSITY: The weight per unit volume of a substance.

DENTAL COUPLING: A flexible coupling assembly, consisting of a set of external/internal gear teeth, that compensates for shaft misalign-ment between a driver and a driven machinery component.

DEPTH: The distance from the root of a thread to the crest, measured perpendicularly to the axis.

DESIGN PRESSURE (BOILER): The pressure specified by a manufacturer as a criterion in design. (In a boiler, it is approximately 103% of operating pressure.)

DESIGN TEMPERATURE: The intended operating temperature of the fresh water and lube oil at the engine outlet, at some specified rate of operation. The specified rate of operation is normal load.

DESUPERHEATED STEAM: Steam from which some of the superheat has been removed. DETONATION: Burning of a portion of the fuel in the combustion chamber at a rate faster than desired (knocking).

DIAL GAUGE OR INDICATOR: A pre-cision micrometer-type instrument that indicates the reading by a needle moving across a dial face.

DIAPHRAGM: A dividing membrane or thin partition.

DIESEL CYCLE (ACTUAL): Combustion induced by compression ignition, begins on a constant-volume basis and ends on a constant-pressure basis.

DIESEL CYCLE (TRUE): Combustion induced by compression ignition, theoretically occurs at a constant pressure.

DIESEL ENGINE: An engine using the diesel or semidiesel cycle of operation; air alone is compressed and diesel fuel is injected before the end of the compression stroke. Heat of compression produces ignition.

DIFFUSER: (1) A duct of varying cross sections designed to convert a high-speed gas flow into low-speed flow at an increased pressure. (2) A device that spreads a fluid out in all directions and increases fluid pressure while decreasing fluid velocity.

DIRECT CURRENT (d.c.): An electric current that flows in one direction only.

DIRECT DRIVE: One in which the drive mechanism is coupled directly to the driven member.

DIRECTIONAL CONTROL VALVE: A valve which selectively directs or prevents flow to or from desired channels. Also referred to as selector valve, control valve, or transfer valve.

DISPLACEMENT: The volume of air or fluid which can pass through a pump, motor, or cylinder in a single revolution or stroke.

DISTILLATE: The product (fresh water) resulting from the condensation of vapors produced by the evaporation of seawater.

DISTILLATION: The process of evaporating seawater, then cooling and condensing the resulting vapors. Produces fresh water from seawater by separating the salt from the water.

DISTILLING PLANTS: Units commonly called evaporators used to convert seawater into fresh water.

DOUBLE REDUCTION: A reduction gear assembly that reduces the high input rpm to a lower output rpm in two stages.

DOUBLE SUCTION IMPELLER: An impeller with suction inlet on each side.

DRAWING: (1) Illustrated plans that show fabrication and assembly details. (2) The original graphic design from which a blueprint may be made. Also called plans.

DRAWING NUMBER: An identifying number assigned to a drawing, or a series of drawings.

DRUM, WATER: A tank at the bottom of a boiler. Also called mud drum.

DUPLEX STRAINER: A strainer containing two separate elements independent of each other.

DYNAMIC PRESSURE: (1) The pressure of a fluid resulting from its motion, equal to one-half the fluid density times the fluid velocity squared. (2) In incompressible flow, dynamic pressure is the difference between total pressure and static pressure.