American Commercial College of Texas Face Velocity of Laboratory Hoods Questions


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Velocity meters
• Measure linear velocity of a gas
• Also measure heat and pressure
• Types of velocity meters:
• anemometer
• pitot tubes: used in stack sampling
• mass flow meter: has two filaments (one at constant temperature
and one that gets heated)
• Cup anemometer – most widely used type. Consists of several metal
cups (usually three) attached to the ends of horizontal arms mounted on
a vertical shaft. Wind catching in the cups causes them to revolve. The
number of revolutions per minute corresponds to air velocity
Anemometers (cont)
Propeller anemometer – wind turns a propeller
instead of cups. Small propeller anemometers
are used to check the flow of air in ventilating
shafts as well as general air speed
Anemometers (cont)
Hot-wire anemometer contains an electrically
heated wire.
• When exposed to the wind, the wire loses heat.
• The wind speed is determined by the amount
of energy needed to keep the wire at a
constant temperature.
• Because of its accuracy, the hot-wire
anemometer is used mainly in experimental
• Care must be taken to not expose the wire to
harsh conditions (grit, etc.)
Pitot Tubes
• Consists of two pressure sensing tubes connected to
a manometer. Velocity pressure is determined and is
used to calculate air speed.
• Types:
• Standard – Type …works best in cleaner environment
• “S”(Stausscheibe) Type…can handle dirtier environment
Pitot Tube
P+: stagnation pressure (pressure air would have if brought to rest
without loss of mechanical energy)
Ps: static pressure (pressure if air is not moving)
Δp: dynamic pressure, which is the difference between P+ and Ps
Pitot Tube
• Inner tube measures P+
• Outer tube measures Ps
• Place tube parallel to
• Use monometer to measure Δp
Sampling with a Pitot tube
• Determine sampling locations within the duct
• Rectangular duct – divide into 16-64 equal subrectangles
• Circular duct – create concentric circles of equal area
• Average the velocities from all sampling locations
• Flow rate: Multiply the average velocity by the duct cross-sectional area


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