UL -- The Underwriters'
Laboratories, Inc.
U-LEATHER -- A flexible ring of U-section, used to pack the
glands of fluid power pistons or rams. The open side is the pressure side, and
the fluid is expanding the inner face against the ram, effectively seals it
against leakage. Also called U-PACKING.
ULLAGE -- The gas volume in an oil tank.
ULLAGE GAUGE -- The distance from the liquid surface to the
top of the tank.
ULTRASONIC CLEANING -- A cleaning process effective for small
crevices, blind holes, fine screw threads, etc., in which the part is immersed
in water or solvent vibrating at ultrasonic frequency.
ULTRASONIC TESTING -- A method of testing in which an
ultrasonic source is pressed against the part to be tested and the vibrations
are reflected from a discontinuity or another surface of the part, to generate
a signal in the receiver. The time lag of the echo is measured to determine the
distance to the discontinuity or the work-piece thickness.
UNIFLOW ENGINE -- A steam-engine in which the
steam enters through drop valves at the end of the cylinder and exhausts
through a piston-controlled belt of ports at the centre of the cylinder.
UNDERFEED STOKER -- A mechanical stoker in which the fuel is
fed automatically and progressively from below the fire, and gradually forced
up into the active zone, air being injected into the fuel bed just below the
combustion-level.
UNIT HEATER -- A heating device, usually located directly in
the space to be heated, consisting of a fan and a heat source.
UNIT WEIGHT -- In a steam boiler, the ratio of the mass of the
complete boiler to that of the hourly maximum steam production.
U-PACKING -- See U-LEATHER.
UPTAKE -- The duct that leads the flue
gases of a marine boiler to the base of the funnel.
USEFUL THERMAL OUTPUT -- The thermal energy made available for
use in any industrial or commercial process, or used in any heating or cooling
application, i.e., total thermal energy made available for processes and
applications other than electrical generation.
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