Chapter 7 - Ventilation Flashcards
(39 cards)
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
