Chapter 7 - Ventilation Flashcards
Three methods to reduce or eliminate any hazard from the workplace
Engineering control
Administrative control
Personal protective equipment
Ventilation
Engineering control to reduce or eliminate the concentration of hazardous materials in the work place
Primary purposes for ventilation
1- maintain adequate oxygen supply
2 - control hazardous concentrations of chemicals
3 - remove odors
4 - control temperature and humidity
5 - remove contaminants at the source, before they enter the workforce
Three categories of ventilation
general, dilution, and local
General dilution
primarily used for comfort, such as temperature, humidity, and odor control, such as air conditioning or heating
Dilution Ventilation
system designed to dilute contaminants by mixing with fresh air
Components include, air exhaust, source of air sully, a duct system and a method to filter and temper incoming air
Used to control - contaminants of moderate toxicity, a large number of sources, intermittent exposures, or where emission sources are well distributed
Local (exhaust) ventilation systems
Designed to control contaminants at the source before mixing with breathing air occurs
Used to control - highly toxic substances, single source emissions, direct worker exposures
Air movement results from
differences in pressure
Difference in pressure can be attained by
heating or mechanical means
Temperature gradient
contributes to ventilation
Volumetric Air flow
Multiply velocity by cross-sectional area in which air flows
Q= V * A
Air moves from
area of higher pressure to area of lower pressure
Pressure created by fan is referred to as
static pressure
Static pressure on the downstream side of a fan is
Positive
Static pressure on the upstream side of a fan is
negative
Velocity pressure
the pressure in the direction of flow necessary to cause the air at rest to flow at a given velocity
Total pressure on a ventilation system
Equals the sum of static and velocity pressure
Equation for total pressure
TP = SP + VP Total pressure (in inches, water gauge) Static pressure (in inches, water gauge) Velocity pressure (in inches, water gauge)
Importance of airflow velocity
used to capture contaminants and overcome cross-drafts,
transportation of contaminants through the duct
balancing of the losses in the system
discharge of contaminant from the stack
When using dilution ventilation - flow rate of fresh air is determined by
contaminant generation
proper mixing
target final concentration
Parts of a local exhaust ventilation system
hood, duct air cleaning device fan stack
Canopy hood (local exhaust ventilation)
Used where hot gases and vapors are encountered workers do not work directly over source of emissions
Capture velocity
minimum velocity of hood- induced air necessary to capture the contaminant
Down draft hood
Used where contaminants heavier than air exist which are not being propelled away from the source of contaminant release
Draws air downward and away from the worker’s breathing zone
Enclosure hood
Hood encloses the contaminant source and the air is forced in an opposite direction (upward, downward, or backward, away from the worker)
Receiving Hood
Used at point of contaminant generation. - placed in close proximity to point of contamination generation
Two types of hood openings
Flanged opening - have a lip, designed to create a certain desired airflow
Plain openings- air movement is directly into the duct
Hood Entry Loss
represents the energy necessary to overcome the losses due to air moving through and into the duct
Exhaust Ducts
Used to convey contaminated air from the hood to the air cleaner and/or stack
Selection of exhaust duct size
based on minimizing friction loss, while maintaining an adequate transport velocity to keep particulate matter from settling out
When two or more ducts branch out
Exhaust volume must be properly proportioned between branches by
balancing the two branches by the proper sizing of ducts and fittings to ensure proper distribution
or blast gates are are slide gates that can be pushed into the duct to partially block airflow to lower the amount of air entering into that branch
Fans
generate the airflow volume of the system against airflow resistance presented by the system
axial or centrifugal
Air cleaning devices
remove contaminant from teh air stream
Ventilation Measurement Equipment
pitot tubes, rotating vane anemoometers, and thermal anemometers
Pitot tubes
inserted into duct system and connected to a manometer, used to measure various pressures within the system
Rotating vane anemometers
Used to measure airflow through large supply and exhaust systems
size should not exceed 5% of the cross-sectional area of the duct
thermal anemometers
digital instruments that measure the heat removed by an airstream as it passes over a probe, which allows for calibration to the velocity of the air stream at a given density
must be perpendicular to the air stream and should be maintained in that position until the measurement is stabilized
probe is very fragile
Pitot tube usage limited to velocities
at or below 600-800 fpm
Most efficient fan blades
backward, curved
