Total Heat:
The latent heat of vaporization
added to the heat of the liquid with which it is in
contact.
Total Pressure:
The sum of the static and veloci-
ty pressures. It is also used as the total static
pressure over an entire area, that is, the unit
pressure multiplied by the area on which it acts.
Trap:
See Steam Trap, Thermostatic Trap, Float
Trap, and Bucket Trap.
Two-Pipe System (Steam or Water):
A heating
system in which one pipe is used for the supply
main and another for the return main. In a two-
pipe hot water system each heating unit
receives a direct supply of the heating medium.
Unit Heater:
A heating unit consisting of a heat
transfer element, housing, fan with motor, and
outlet deflectors or diffusers. It is usually sus-
pended from the ceiling and its heat output is
controlled by starting and stopping the fan by a
room thermostat. The circulation of the heating
medium (steam or hot water) is usually continu-
ous. It is used primarily for industrial heating.
Unit Pressure:
Pressure per unit area as Ibs. per
sq. in.
Up-Feed System (Hot Water or Steam):
A heat-
ing system in which the supply mains are below
the level of the heating units which they serve.
Vacuum Heating System (Steam):
A one- or
two-pipe heating system equipped with the nec-
essary accessory apparatus to permit the pres-
sure in the system to go below atmospheric.
Vapor:
Any substance in the gaseous state.
Vapor Heating System (Steam):
A two-pipe
heating system which operates at or near atmo-
spheric pressure and returns the condensation
to the boiler or receiver by gravity.
Velocity Pressure:
The pressure used to create
the velocity of flow in a pipe. It is expressed as a
unit pressure.
Ventilation:
Air circulated through a room for
ventilating purposes. It may be mechanically
circulated with a blower system or through
circulation with an open window, etc.
Vent Valve (Steam):
A device that permits air
to be forced out of a heating unit or pipe and
closes against water and steam.
Vent Valve (Water):
A device that permits air
to be forced out of a heating unit or pipe and
closes against water.
Warm Air Heating System:
A warm air heating
plant consists of a heating unit (fuel-burning fur-
nace) enclosed in a casing, from which the heat-
ed air is distributed to the various rooms of the
building through ducts. If the motive head pro-
ducing flow depends on the difference in weight
between the heated air leaving the casing and
the cooler air entering the bottom of the casing,
it is termed a gravity system. A booster fan may,
however, be used in conjunction with a gravityde-
signed system. If a fan is used to produce circu-
lation and the system is designed especially for
fan circulation, it is termed a fan furnace system
or a central fan furnace system. A fan furnace
system may include air washer, filters, etc.
Wet Bulb Temperature:
The lowest temperature
which a water-wetted body will attain when
exposed to an air current.
Wet Return (Steam):
That part of the return
main of a steam heating system which is com-
pletely filled with water of condensation.
Wet Saturated Steam:
Saturated steam contain-
ing some water particles in suspension.
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